Calcium-Molybdenum-Cobalt-2-Methylimidazole Trimetallic Organic Framework as a Saturable Absorber for Passive Mode-Locked Thulium–Holmium Doped Fiber Lasers

Calcium-Molybdenum-Cobalt-2-Methylimidazole Trimetallic Organic Framework as a Saturable Absorber for Passive Mode-Locked Thulium–Holmium Doped Fiber Lasers | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Calcium-Molybdenum-Cobalt-2-Methylimidazole Trimetallic Organic Framework as a Saturable Absorber for Passive Mode-Locked Thulium–Holmium Doped Fiber Lasers Harith Ahmad, Norazyyati Mokhtar, Nur Arina Mat Rusni, Muhammad Syamil Mohd Sa'ad, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8677115/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Apr, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Ultrafast fiber lasers operating in the 2 µm spectral region require robust and highly nonlinear saturable absorbers to achieve stable mode-locking. This study demonstrates that a calcium-molybdenum-cobalt-2-methylimidazole (CaMoCo-MIM), a trimetallic organic framework (TMOF), functions as an efficient saturable absorber (SA). Enabling stable and self-starting mode-locking pulse in a thulium–holmium-doped fiber laser (THDFL). The CaMoCo-MIM exhibits excellent nonlinear optical performance, achieving a high modulation depth of 36.2%, surpassing many previously reported MOF-based SAs. Stable soliton pulse generation was realised at a pump power of 307 mW, producing a central wavelength of 1895.21 nm, a 3 dB spectral bandwidth of 3.03 nm, and a pulse duration of 1.25 ps. A time-bandwidth product (TBP) of 0.316 indicates a nearly chirp-free pulse. Long-term operation over four hours confirmed robust stability with a constant repetition rate of 14.6 MHz. This TMOF establishes a high-modulation-depth SA in next-generation lasers. Physical sciences/Optics and photonics Physical sciences/Physics calcium-molybdenum-cobalt-2-methylimidazole trimetallic organic framework saturable absorber thulium-holmium doped fiber lasers mode-locking pulse Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Mode-locked fiber lasers have established themselves as among the most vital ultrafast lasers in contemporary photonics. Mode-locked fiber lasers serve as critical ultrafast sources, reliably producing periodic sequences of light pulses ranging from picoseconds to femtoseconds 1 . Such lasers can deliver high peak powers 2 while maintaining exceptionally stable, well-defined repetition rates 3 , making them indispensable for high-speed optical measurements and nonlinear optical applications. Mode locking enables the creation of coherent pulse trains with high temporal consistency by maintaining a stable phase relationship between the longitudinal modes within the laser cavity. Owing to their waveguide-based architecture, all-fiber mode-locked lasers exhibit significant advantages over traditional solid-state counterparts, inherent alignment-free operation, enhanced mechanical stability, and cost efficiency 4 . Due to these advantages, mode-locked fiber lasers have emerged as compelling platforms for both academic research and real-world applications. The gain medium is the primary factor that determines the operating wavelength of ultrafast fiber lasers. The emission bands of optical fibers doped with ytterbium, erbium, and thulium/holmium are around 1 µm, 1.5 µm, and 2 µm, respectively 5 . Recent years have seen a significant increase in research concerning 2 µm fiber lasers. Fiber lasers operating at 1 µm and 1.5 µm are already widely utilised in industrial processes and also in optical communications 6 . However, the 2 µm region is receiving increasing attention. This heightened interest arises from numerous inherent benefits, notably the protection against eye injury 7 , enhanced aqueous-phase absorption in biological tissues 8 , and elevated sensitivity for gas detection and environmental monitoring 9 . Furthermore, operation at longer wavelengths can alleviate specific nonlinear propagation effects in optical fibers, thereby supporting more stable pulse formation and higher pulse energies 10 . As a result, mode-locked fiber lasers have found expanding roles in applications such as precision micromachining, medical diagnostics and therapy, ultrafast spectroscopy, and remote sensing, underscoring their versatility and continued relevance as ultrafast laser platforms 11 – 13 . Thulium ions (Tm³⁺) and holmium ions (Ho³⁺) are excellent active dopants for fiber lasers operating around 2 µm. When both ions are incorporated into the same fiber, an efficient cross-relaxation and energy-transfer process significantly enhances pump utilization: laser diodes (LDs) operating near 1.55 µm can excite Tm³⁺ ions, which then transfer energy to Ho³⁺ ions via cross-relaxation and energy-transfer processes, overcoming the limited availability of efficient direct pump sources for Ho³⁺ and increasing the effective pump absorption and quantum efficiency 14 . This cooperative interaction improves system performance, reduces thermal load, and enables more compact laser designs. In addition, Tm³⁺-Ho³⁺ co-doped fibers exhibit a broader emission and gain bandwidth around 2 µm compared to singly doped fibers, allowing greater wavelength tunability and flexibility for mid-infrared applications 15 . These characteristics make Tm-Ho co-doped fibers a promising platform for high-efficiency and broadband 2 µm fiber lasers 16 . As nonlinear elements with intensity-dependent absorption, saturable absorbers (SAs) are often used to achieve passive mode-locking 17 . Ultrafast laser systems have widely used traditional SAs, such as semiconductor saturable absorber mirrors (SESAMs). However, their relatively low damage threshold 18 , limited operational bandwidth 19 , and high fabrication cost 20 have motivated the search for alternative SA. Recently, metal–organic frameworks (MOFs) have emerged as promising options due to the unique way their structures function. Metal-organic frameworks (MOFs) are crystalline architectures constructed from organic ligands coordinated to metal ions or clusters. It is possible to tailor these networks to display desired physical and chemical characteristics 21 . They have various practical applications due to their large surface area and intrinsic high porosity, such as gas storage 22 , gas separation 23 , catalysis 24 , chemical sensing 25 , and energy storage 26 . More recently, these advantageous features have sparked growing interest in exploiting MOFs for nonlinear optical applications, particularly for use as SAs in fiber laser systems 27 – 29 . The majority of reported MOF SAs have been demonstrated in erbium-doped fiber lasers operating at 1.5 µm. It occurred despite the rapid progress in the development of MOF-based nonlinear optical materials 30 . While these works have confirmed the feasibility of employing MOF for ultrafast pulse modulation, their extension to longer wavelengths has received comparatively limited attention. In particular, investigations of MOF-assisted mode-locking in the 2 µm spectral region remain scarce, especially in THDFL systems, where suitable SAs are still under active exploration 31 . This research gap is notable given that several MOF systems have exhibited broadband optical absorption extending into the near- and mid-infrared regions, suggesting their potential compatibility with 2 µm ultrafast laser operation 32 . Recent studies on zinc-, copper-, cobalt-, and nickel-based MOFs have demonstrated high modulation depths and strong third-order nonlinear optical responses, key parameters for achieving self-starting, stable passive mode-locking 33 . Furthermore, the inherently porous MOF architecture of MOF facilitates efficient thermal dissipation and moisture absorption, offering distinct advantages for managing heat accumulation and maintaining performance stability under high optical intensities 34 . Strong absorption near 2 µm has also been reported for selected MOF structures, reinforcing their suitability for thulium- and holmium-doped fiber laser systems 35 . In several instances, saturable absorbers derived from MOFs have demonstrated modulation depths exceeding those of conventional saturable absorbers, including topological insulators and transition-metal dichalcogenides, thereby mitigating spectral filtering-induced instabilities and facilitating scalable ultrafast performance 36 . However, existing studies have primarily focused on a narrow subset of MOF compositions and metal centers, leaving the nonlinear optical behavior, long-term stability, and wavelength adaptability of many alternative MOF systems insufficiently explored 37 . Improving the stability and pulse control of MOF-based SAs for the next generation of ultrafast fiber lasers needs ongoing study into new MOF designs and metal-ligand pairs that provide strong saturable absorption. This study investigates the application of calcium-molybdenum-cobalt-2-methylimidazole (CaMoCo-MIM), a trimetallic organic framework (TMOF), for passive mode-locking in a THDFL operating at approximately 2µm wavelength. A compact, fiber-integrated SA device was achieved by fabricating and depositing CaMoCo-MIM onto an arc-shaped optical fiber, enabling integration into the laser cavity. The CaMoCo-MIM exhibited an exceptionally high modulation depth of 36.2%, which is among the highest reported for TMOF-based SAs at ~ 2 µm. The resulting time bandwidth product (TBP) was approximately 0.315, close to the transform limit, enabling the generation of near-transform-limited soliton pulses with a duration of about 1.25 ps. It is one of the few experiments that shows a TMOF working as an SA in a 2 µm fiber laser setup. 2. Materials and Methods 2.1 Synthesis and Characterization of CaMoCo-MIM Saturable Absorber All trials were conducted using distilled water (DI). System Chemicals in Malaysia provided us with 99.00% pure calcium nitrate tetrahydrate (Ca(NO 3 ) 2 ·4H 2 O) and 99.50% pure ethanol, respectively. Acros Organics in the USA provided the ingredients, 2-methylimidazole (MIM) and cobalt nitrate tetrahydrate (Co(NO 3 ) 2 ·4H 2 O), with a purity of 99.00%. Sigma-Aldrich (USA) delivered 99.00% concentrations of ammonium bicarbonate and sodium molybdate dihydrate. All chemicals were utilised as packaged, with no further cleaning. The CaMoCo-MIM TMOF was prepared via a simple wet-chemical precipitation method. Metal precursors totaling 0.06 mol in molar quantity were dissolved within 60 mL of deionized water at a molar ratio corresponding to Ca:Mo: Co = 3:2:1. In a separate step, 50 mL of DI water containing 0.01 mol of MIM was added to the metal precursor solution while stirring continuously. At the same time, 0.01 mol of ammonium bicarbonate solution was added to the combined metal-organic solution after it was dissolved in DI water. To encourage framework formation, the resultant mixture was gently agitated for 4 days. The final product was obtained by centrifugation at 5000 rpm for 5 minutes, washed multiple times with ethanol and deionised water, and subsequently desiccated overnight at 80°C in an oven. The synthesis of CaMoCo-MIM is illustrated in Fig. 1 . The morphology of the CaMoCo-MIM material, as shown in Fig. 2 (a) , which was deposited on the arc-shaped fiber, was examined using a Coxem EM-30AX PLUS scanning electron microscope (SEM). At a magnification of 500x, the SEM picture shows that the CaMoCo-MIM SA was efficiently deposited along the arc-shaped fiber surface, resulting in an almost uniform coating. Upon increasing the magnification to 2000x, irregularly shaped nanoparticles become clearly visible. The presence of clustered nanoparticulates indicates that the drop-casting deposition technique promotes particle aggregation on the fiber surface. This aggregation may be further enhanced by the curvature of the arc-shaped region, which can influence local particle distribution during solvent evaporation. Despite the observed aggregation, the coating remains well adhered to the fiber, with no evidence of large-scale delamination. Elemental mapping in Fig. 2 (b) from further FESEM analysis verifies the existence of silicon (Si), cobalt (Co), gold (Au), oxygen (O), carbon (C), calcium (Ca), and molybdenum (Mo). The detection of Si and O is primarily attributed to the SMF-28 arc-shaped fiber host, which is composed of silicon dioxide (SiO₂) and serves as the substrate for CaMoCo-MIM SA deposition. The high carbon content, predominantly covering the upper surface of the arc-shaped fiber, originates from 2-methylimidazole (MIM) with the chemical formula C 4 H 6 N 2 . Meanwhile, the presence of Mo, Co, and Ca arises from their respective metal precursors used during synthesis. The detected Au signal is likely due to trace impurities during sample preparation. However, its atomic percentage is minimal, at only 0.35%, as shown in Fig. 2 (c). Carbon constitutes the highest atomic percentage at 57.68%, followed by O (29.87%), Si (7.13%), Mo (2.09%), Ca (1.97%), and Co (0.91%). Excluding the contribution from Au, the total atomic percentage amounts to 99.65%, indicating good compositional consistency of the deposited CaMoCo-MIM SA layer. Fourier-transform infrared (FTIR) spectroscopy was utilised to analyse the coordination environment of the metal nodes and 2-methylimidazole, as depicted in Fig. 3 . Typically, 2-methylimidazole (MIM) displays broad and prominent characteristic peaks within the range of 2200 to 3300 cm⁻¹, resulting from hydrogen bonding interactions between the pyrrole moiety and the pyridine nitrogen 38 . The lack of these peaks in the CaMoCo321-MOF spectrum suggests that the metal nodes are coordinated to the secondary amine groups of MIM 39 . Furthermore, the N—H stretching vibration, which is a noticeable characteristic peak of MIM at 1840 cm⁻³, is missing from CaMoCo-MIM-TMOF. It means that the deprotonation of the MIM ligand was complete during MOF formation 38 . These two observations confirmed that the metal-organic bonding exists at two different points. Subsequently, the characteristic peaks of MIM within CaMoCo-MIM-TMOF were identified at 1655 cm⁻¹, corresponding to the C = N stretching vibration in the imidazole ring, and within the range of 1400–1500 cm⁻¹, indicative of the stretching and bending modes associated with the imidazole ring 40 , 41 . Additionally, the observed peak at 773 may be attributed to the stretching vibration of the metal-oxygen-hydrogen bond 42 , 43 . A hydroxyl group from physically adsorbed water was identified as the source of the signal at 3430 cm⁻¹ 44 . The linear absorption characteristics of the MIM SA were analysed to assess its appropriateness for operation within the 2 µm mid-infrared wavelength range. For this investigation, the CaMoCo-MIM specimen was dispersed in deionised water to create a stable suspension. The sample was examined using an amplified spontaneous emission (ASE) source, with THDFL employed as the gain medium. Figure 4 shows a transmittance of about 38% at 1895.21 nm in the linear absorption spectra of the CaMoCo-MIM saturable absorber. This considerable absorption is attributed to the combined effects of the intrinsic electronic transitions within the CaMoCo-MIM framework and the baseline absorption of the deionized water solvent, which generally exhibits a strong absorption band near 1900 nm. To evaluate its saturable absorption characteristics, the nonlinear response of the CaMoCo-MIM SA was examined utilizing a 2-µm mode-locked fiber laser. A key characteristic of SA is the reduction in optical absorption with increasing incident laser intensity. This intensity-dependent absorption behaviour is clearly observed in the experimental result shown in Fig. 4 (b) . The nonlinear transmission data were fitted using the conventional SA model, expressed as 10,17 : $$\:A\left(I\right)={\alpha\:}_{o}\times\:\text{exp}\left(-\frac{I}{{I}_{sat}}\right)+{\alpha\:}_{ns}$$ 1 According to Eq. ( 1 ), \(\:{\alpha\:}_{o}\) stands for the depth of modulation, \(\:{I}_{sat}\) describes the intensity of saturation, and \(\:{\alpha\:}_{ns}\) represents the non-saturable loss. The data were analyzed for the SA’s individual parameters. By numerical fitting, the CaMoCo-MIM SA exhibited a high modulation depth ( \(\:{a}_{s}\) ) of 36.2%. This value is beneficial because it enables a self-starting mode-locked mechanism and ensures the generation of high-contrast, ultra-short pulses with considerable peak power. Additionally, the saturation intensity \(\:{I}_{sat}\) was measured at 79.7 MW/cm², demonstrating that the TMOF material exhibits strong power-handling capabilities and can withstand high-energy pulses before reaching its saturation limit. The non-saturable loss, denoted by the symbol \(\:{a}_{ns}\) , was measured at 8.01%. This loss is primarily attributable to scattering phenomena within the aqueous suspension and to the background absorption of the distilled water solvent. By combining these nonlinear features with a high modulation depth, it is demonstrated that the TMOF-based suspension functions as an efficient, high-performance SA suitable for applications involving steady mid-infrared lasers. 2.2 Fabrication of arc-shaped fiber and THDFL cavity setup Light was emitted into an SMF-28 fiber from a portable light source, creating an arc-shaped fiber. The optical power meter continuously measured the transmitted power. The fiber was shaped according to standard fabrication protocols and subsequently fixed onto a glass substrate using adhesive tape. Dimensional parameters of the fabricated fiber, including its length and curvature, were examined using an optical microscope interfaced with ToupView imaging software, in accordance with the method outlined in 45 . A 5-centimeter length of SMF-28 fiber was cleaned and securely fastened to a fiber holder before shaping. The exposed area was first rough-polished with 1000P silicon carbide (SiC) abrasive paper, and then fine-polished with a Thorlabs diamond film. After completing the process, the resulting fiber had an insertion loss of approximately 2.5 dB and an arc-shaped profile. The resulting arc-shaped fiber is depicted in Fig. 5 , along with a schematic diagram and an optical microscope image. Based on an arc length of 2.01 mm and a polishing depth of 61.97 µm, the residual fiber diameter within the polished area was determined to be 63.03 µm. The arc-shaped fiber was then covered with the TMOF layer using a drop-casting method, as previously described 46 . Figure 6 shows the enclosed space inside the THDFL where the arc-shaped fiber is integrated. The thulium-holmium-doped fiber was optically stimulated by two pump laser diodes, LD 1 and LD 2 , to gain enough amplification inside the cavity. Before being launched into the THDF, the light from both LDs was first routed via optical isolators (ISOs) to avoid back reflections. It was then coupled into the common ports of wavelength-division multiplexers (WDMs). WDM 1 was subsequently connected to ISO 3 via a 90:10 optical coupler. The optical coupler divides the circulating signal, diverting 10% for real-time monitoring and characterization, while maintaining the remaining 90% within the laser loop. The cavity was closed by recirculating the remaining optical signal through the saturable absorber (SA), the polarization controller (PC) for state-of-polarization tuning, and WDM 2 , thereby enabling stable laser operation. The overall cavity length was 14.67 m, consisting of 1.5 m of THDF, serving as the gain medium, and 13.17 m of standard single-mode fiber (SMF-28). Within the THDF, the group velocity dispersion (GVD) was − 0.05568 ps²/m and − 0.06403 ps 2 /m. for SMF-28, giving a net cavity dispersion − 0.9 ps², taking into consideration the various lengths. According to the computed net dispersion, the cavity was operating in negative dispersion. 3. Results and Discussion By integrating a CaMoCo-MIM SA and adjusting the polarisation controller within the THDFL cavity, stable mode-locked pulses were successfully generated with a consistent pump power of 307mW. The CaMoCo-MIM is essential for the initiation and sustained operation of ultrashort-pulse generation, as the cavity’s mode-locking capability depends on the saturable absorber modulation depth. Figure 7 (a) depicts the optical spectrum acquired through the optical spectrum analyser (OSA), characterised by a 3-dlB bandwidth of 3.03 nm and a central wavelength (λ₀) of 1895.21 nm. The laser cavity operates in a soliton-pulse regime when clearly defined Kelly sidebands are observed 47 , 48 . The temporal profile of a single pulse, shown by the purple curve in Fig. 7 (b) , is well-fitted by a sech² function. A weak pedestal is visible in the autocorrelation trace and is mainly due to the measurement system’s noise floor 49 . The pulse duration extracted from the fitting is 1.25 ps. An estimated time-bandwidth product of about 0.316 was obtained by integrating the pulse duration with the measured central wavelength and optical bandwidth. It indicates a negligible amount of chirp in the output pulses 50 . The oscilloscope trace in Fig. 7 (c) shows a set of pulses with a 68.4 ns time interval, giving a calculated repetition rate14.6 MHz. Figure 7 (d) displays the radio-frequency spectrum with a SNR of 38 dB, further demonstrating that the THDFL is stable when mode-locked with the CaMoCo-MIM SA. To test the stability of mode-locked operation over time, it was continuously monitored for 4 hours. Figure 8 (a) illustrates the evolution of the autocorrelation traces, while Fig. 8 (b) depicts the pulse duration as a function of time. The pulse profile remains consistently uniform throughout the observation period. The pulse duration was maintained between 1.18 ps and 1.25 ps, indicating that the laser’s temporal characteristics remain robust against environmental fluctuations. Applying radio-frequency (RF) spectroscopy to the mode-locked laser allowed for a more thorough evaluation of its frequency stability. Figure 8 (c) shows that the fundamental frequency peak remained sharp and clearly distinguishable throughout the four-hour measurement period. According to the data shown in Fig. 8 (d) , the fundamental frequency remained constant at 14.6 MHz. While the SNR fluctuated at approximately 6.1 dB, ranging from 32 dB to 39 dB, the mode-locking state remained self-starting and stable, with no signs of degradation or pulse breakdown. These findings confirm that the CaMoCo-MIM SA enables mode-locking with excellent stability and repeatability, supporting its suitability for real-world ultrafast applications. Figure 9 illustrates the connection between the mode-locked laser’s average output and incident pump power. As the pump is raised, the CaMoCo-MIM SA shows a consistent, linear rise in output power from 290 mW to 450 mW. At full pump power, the laser has a slope efficiency of 1.15%. This is due to the output coupler that samples only 10% of the intracavity power. Nevertheless, the linear trend validates a predictable power response. It confirms that the CaMoCo-MIM SA maintains its functional integrity without reaching a thermal-damage threshold or exhibiting power saturation over the investigated range. Table 1 compares the current work on MOF-derived materials. The specific surface areas (SAs) enumerated in the table encompass nickel-p-phenylenedicarboxylic acid metal–organic frameworks (Ni-MOF) 29 , copper-derived metal–organic frameworks (Cu-MOF) 51 , zinc-based metal–organic frameworks (Zn-MOF) 52 , nickel oxide micron polyhedral metal–organic frameworks (NiO-MOF) 53 , cobalt-oriented metal–organic frameworks (Co-MOF) 54 , and nickel-trimesic acid MOF 55 . Among the materials compared, the proposed CaMoCo-MIM exhibits the highest modulation depth, reaching 36.2%, which is significantly greater than those reported for Ni-MOF (14.25%), Zn-MOF (9.98%), Co-MOF (10.41%), Cu-MOF (5.7%), and nickel-trimesic acid MOF (4.1%). A high modulation depth indicates strong nonlinear optical absorption, which plays a crucial role in practical pulse shaping and self-starting mode-locking 56 . In the 2-µm spectral area, the CaMoCo-MIM operates at a central wavelength of 1895.21 nm and is especially compatible with THDFL devices. The improved nonlinear optical performance, enabled by its higher modulation depth, enables more effective pulse shaping and more reliable self-starting in mode-locked operation 56 . Regarding pulse characteristics, the CaMoCo-MIM-based laser generates near-transform-limited pulses with a TBP of 0.316, which closely approaches the theoretical value for sech²-shaped pulses, indicating minimal chirp and efficient intracavity dispersion control. The measured TBP is lower than the values reported for Ni-MOF of 0.318, NiO-MOF of 0.321, Co-MOF of 0.323, and Cu-MOF of 0.324, indicating that the CaMoCo-MIM SA supports the generation of high-quality, near-transform-limited ultrashort pulses. The resulting pulse width of 1.25 ps confirms ultrafast operation and aligns closely with the performance reported for other MOF-derived SA. Notably, despite exhibiting a significantly higher modulation depth, the CaMoCo-MIM SA does not induce excessive intracavity loss or pulse broadening, suggesting an efficient balance between nonlinear absorption and cavity dispersion. This favorable balance enables stable, high-quality ultrashort-pulse generation without compromising pulse duration. In terms of operational stability, the CaMoCo-MIM SA achieves an SNR of 38 dB, which is relatively high and corresponds to a linear value of 6309 times. The CaMoCo-MIM SA also delivered superior operational stability of 4 hours, outperforming Zn-MOF at 1.67 hours and the nickel-trimesic acid MOF at 1 hour, and matching the leading stability of Cu-MOF-based devices. This enhanced stability can be attributed to the effective integration of the CaMoCo-MIM with the fiber platform and its ability to withstand prolonged optical pumping without performance degradation. The result highlights the suitability of CaMoCo-MIM for sustained mode-locked operation and underscores its potential for practical deployment in ultrafast fiber laser systems. In addition, a laser efficiency of 1.15% is achieved, which is similar to that of other SAs based on MOFs that operate in the 2 µm range. Notably, this cavity samples only 10% of the output power. Another setup, such as Co-MOF, samples 30%, resulting in a slightly higher output efficiency of 2.36%. Hence, higher efficiency could be achieved by using a different coupler, such as a 70/30 coupler. However, the 90/10 coupler has been retained in this cavity to support stable pulse generation via the feedback mechanism. Overall, the CaMoCo-MIM SA offers a well-balanced combination of high modulation depth, good pulse quality, and stable operation, highlighting its potential for ultrafast fiber laser applications. Table 1 Comparison of recent works on MOF or MOF-derived materials for mode-locked fiber lasers. Materials Modulation Depth (%) Time Bandwidth Product (TBP) Pulse width (ps) Operation Wavelength (nm) SNR (dB) Maximum stability duration (Hour) Laser efficiency (%) Ref. Ni-MOF 14.25 0.318 1.3 1882 64 - - 29 Cu-MOF 5.7 0.324 1.08 1949 60 4 5.03 51 Zn-MOF 9.98 0.316 1.772 1906.75 51 1.67 1.33 52 NiO-MOF 18.98 0.321 0.766 1555 62.8 - - 53 Co-MOF 10.41 0.323 0.793 1558.9 60.5 - 2.36 54 nickel-trimesic acid MOF- 4.1 0.303 1.41 1951.6 51 1 - 55 CaMoCo-MIM 36.2 0.316 1.25 1895.21 38 4 1.15 This work 4. Conclusion This study demonstrates that a CaMoCo-MIM functions as an efficient SA, enabling stable, self-starting mode-locking in THDFL operating around 2 µm. The CaMoCo-MIM demonstrated exceptional nonlinear optical characteristics, specifically a modulation depth of 36.2%, exceeding that of other reported MOF-based SAs. At 307 mW of pump power, the laser generated a steady soliton pulse with a centre wavelength of 1895.21 nm, a 3 dB spectral bandwidth of 3.03 nm, and a pulse duration of 1.25 ps. A measured SNR of 38 dB and a calculated TBP of 0.316 both indicate the generation of stable, high-quality pulse trains, which closely align with the sech²-transform-limited value of 0.315. Extended stability assessments conducted over 4 hours confirmed dependable performance under environmental variations, maintaining a consistent repetition frequency of 14.6 MHz. The laser demonstrated a linear relationship between input and output power, with a slope efficiency of 1.15% and a maximum pump power of 450 mW. These results demonstrate that CaMoCo-MIM is a reliable and effective saturable absorber, with strong potential for integration into mid-infrared ultrafast fiber lasers and paving the way for advanced photonic applications. Declarations Conflicts of interest No conflicts of interest are associated with this study. Data availability The datasets analysed during this study are available from the corresponding author on reasonable request. Acknowledgements None Funding The authors acknowledge the funding from Universiti Malaya under the grant RU001-2025A. Author Contributions H. Ahmad – Supervision, Idea, N. Mokhtar – Running the experiment, drafting of the manuscript, N. A. M, Rusni – Sample preparation, drafting of the manuscript, M. S. M Sa’ad – Checking of manuscript, I. S. Imaduddin – Sample preparation and characterization, S. R. Majid - – Sample preparation and characterization, M. Z. H. Mayzan - Sample characterization, M. A. M. 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Separation and amplification of Kelly sidebands and main soliton pulse in a 2-µm ultrafast fiber chirped pulse amplifier. Infrared Phys. Technol. 127 , 104455. https://doi.org/10.1016/j.infrared.2022.104455 (2022). Fiehler, T., Saraceno, C., Steinmeyer, G. & Wittrock, U. Pitfall in autocorrelation measurements of laser radiation. Opt. Express . 32 , 36811. https://doi.org/10.1364/OE.533567 (2024). Ahmad, H., Loganathan, K., Yusoff, N. & Samion, M. Z. Optimizing a 1.9 µm mode-locked laser by controlling the concentration of reduced graphene oxide-magnesium oxide. Opt. Mater. (Amst) . 149 , 114993. https://doi.org/10.1016/j.optmat.2024.114993 (2024). Ahmad, H. et al. Cu-MOF as a saturable absorber for mode-locking at 2 µm and beyond. Sci. Rep. 15 , 1–11. https://doi.org/10.1021/acsami.2c10217 (2025). Ahmad, H. et al. Zn-MOF as a saturable absorber for thulium/holmium-doped fiber laser. Phys. Scr. 99 , 1–10. https://doi.org/10.1038/s41598-025-30422-6 (2024). Zhang, C. et al. Porous nickel oxide micron polyhedral particles for high-performance ultrafast photonics. Opt. Laser Technol. 146 , 107546. https://doi.org/10.1088/1402-4896/ad7896 (2022). An, M. et al. Co-MOFs as Emerging Pulse Modulators for Femtosecond Ultrafast Fiber Laser. ACS Appl. Mater. Interfaces . 14 , 53971–53980. https://doi.org/10.1016/j.optlastec.2021.107546 (2022). Murad, A. et al. Soliton emissions at 1.59 and 1.95 µm utilizing nickel-trimesic acid metal-organic framework saturable absorber. Opt. Quantum Electron. 56 , 1–13. https://doi.org/10.1007/s11082-024-07131-x (2024). Zheng, J. et al. Generation of high-energy self-mode-locked pulses in a Tm-doped fiber laser. Appl. Phys. Lett. 125 https://doi.org/10.1063/5.0217482 (2024). Additional Declarations No competing interests reported. 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University","correspondingAuthor":false,"prefix":"","firstName":"Mousa","middleName":"","lastName":"Hussein","suffix":""},{"id":587599668,"identity":"e4938aa2-fd46-4d09-8f35-aa7faaa13ee2","order_by":9,"name":"Shufeng Sun","email":"","orcid":"","institution":"Qingdao University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Shufeng","middleName":"","lastName":"Sun","suffix":""},{"id":587599669,"identity":"f4e27eb6-ceb0-4562-9283-90d03de7a921","order_by":10,"name":"Kavintheran Thambiratnam","email":"","orcid":"","institution":"International Islamic University Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Kavintheran","middleName":"","lastName":"Thambiratnam","suffix":""}],"badges":[],"createdAt":"2026-01-23 08:55:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8677115/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8677115/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-026-50198-7","type":"published","date":"2026-04-24T15:58:29+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":102241131,"identity":"1afc1190-7133-4d49-81ac-d4dfb1969143","added_by":"auto","created_at":"2026-02-09 16:57:27","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":91315,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis process of calcium-molybdenum-cobalt-2-methylimidazole (CaMoCo-MIM).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/f9834e13a235670a22dd9dae.png"},{"id":102241125,"identity":"6834e1b8-3034-4790-a5e2-93306eec2f16","added_by":"auto","created_at":"2026-02-09 16:57:26","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":466994,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FESEM morphology, (b) elemental mapping, and (c) EDX spectra of CaMoCo-MIM.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/b7f369e33e183e6f681a9433.png"},{"id":102297212,"identity":"fa3a1e42-ad91-4da8-9665-8c2d9d9a8b9f","added_by":"auto","created_at":"2026-02-10 10:26:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":37040,"visible":true,"origin":"","legend":"\u003cp\u003eFourier-transform infrared (FTIR) spectroscopy of CaMoCo-MIM\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/c2534fded6388f20f5e40c30.png"},{"id":102241122,"identity":"e8a9e989-2ad5-4ab2-97b8-0837ef518581","added_by":"auto","created_at":"2026-02-09 16:57:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":58783,"visible":true,"origin":"","legend":"\u003cp\u003e(a)\u003cstrong\u003e \u003c/strong\u003eLinear absorption spectrum of CaMoCo-MIM \u0026nbsp;SA and (b)Nonlinear absorption spectrum of CaMoCo-MIM SA\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/7dc81ba05ad587eae7142d16.png"},{"id":102241123,"identity":"f2702618-ecaf-4897-bd02-abf07073a6ef","added_by":"auto","created_at":"2026-02-09 16:57:26","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":128822,"visible":true,"origin":"","legend":"\u003cp\u003eThe arc-shaped fiber was captured using an optical microscope.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/2c1c8ef83707055c04460254.png"},{"id":102241126,"identity":"d773bad3-759b-4cd4-9cde-5d1fd863cd56","added_by":"auto","created_at":"2026-02-09 16:57:26","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":178128,"visible":true,"origin":"","legend":"\u003cp\u003eTHDFL cavity setup with arc-shaped fiber.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/656f58094bd8263169bb0e42.png"},{"id":102241129,"identity":"c164b7fa-ad9c-4cb8-b739-18bf1c1269a4","added_by":"auto","created_at":"2026-02-09 16:57:27","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":128844,"visible":true,"origin":"","legend":"\u003cp\u003eFundamental mode-locked (a) Mode-locked optical spectrum, (b) autocorrelation trace, (c) mode-locked pulse train, (d) RF spectrum of CaMoCo-MIM SA deposited onto the arc-shaped fiber.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/7280bf729353fca1aa8d2d1f.png"},{"id":102241130,"identity":"246bf0b2-7ccd-4136-a118-65b9b5e86278","added_by":"auto","created_at":"2026-02-09 16:57:27","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":90986,"visible":true,"origin":"","legend":"\u003cp\u003eStability analysis of the fundamental mode-locked fiber laser using CaMoCo-MIM SA over a four-hour duration. (a) Autocorrelation traces, (b) pulse duration as a function of time, (c) RF spectrum evolution, and (d) SNR and fundamental frequency stability.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/d3cc92a1d94b7acfe22f8ebc.png"},{"id":102241128,"identity":"67dccef3-e246-4a75-9201-d1ba572e41ed","added_by":"auto","created_at":"2026-02-09 16:57:27","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":36503,"visible":true,"origin":"","legend":"\u003cp\u003eOutput power versus pump power relation for CaMoCo-MIM SA.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/76ad756dbaae6f961c77eb2b.png"},{"id":107927761,"identity":"7dd2f99e-d116-49e9-bb0b-ed8f53f7f9e1","added_by":"auto","created_at":"2026-04-27 16:03:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1567984,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8677115/v1/8e300375-be22-43c7-abce-bcb0d13147df.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Calcium-Molybdenum-Cobalt-2-Methylimidazole Trimetallic Organic Framework as a Saturable Absorber for Passive Mode-Locked Thulium–Holmium Doped Fiber Lasers","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMode-locked fiber lasers have established themselves as among the most vital ultrafast lasers in contemporary photonics. Mode-locked fiber lasers serve as critical ultrafast sources, reliably producing periodic sequences of light pulses ranging from picoseconds to femtoseconds\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Such lasers can deliver high peak powers\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e while maintaining exceptionally stable, well-defined repetition rates\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, making them indispensable for high-speed optical measurements and nonlinear optical applications. Mode locking enables the creation of coherent pulse trains with high temporal consistency by maintaining a stable phase relationship between the longitudinal modes within the laser cavity. Owing to their waveguide-based architecture, all-fiber mode-locked lasers exhibit significant advantages over traditional solid-state counterparts, inherent alignment-free operation, enhanced mechanical stability, and cost efficiency\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Due to these advantages, mode-locked fiber lasers have emerged as compelling platforms for both academic research and real-world applications.\u003c/p\u003e \u003cp\u003eThe gain medium is the primary factor that determines the operating wavelength of ultrafast fiber lasers. The emission bands of optical fibers doped with ytterbium, erbium, and thulium/holmium are around 1 \u0026micro;m, 1.5 \u0026micro;m, and 2 \u0026micro;m, respectively\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Recent years have seen a significant increase in research concerning 2 \u0026micro;m fiber lasers. Fiber lasers operating at 1 \u0026micro;m and 1.5 \u0026micro;m are already widely utilised in industrial processes and also in optical communications\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, the 2 \u0026micro;m region is receiving increasing attention. This heightened interest arises from numerous inherent benefits, notably the protection against eye injury\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, enhanced aqueous-phase absorption in biological tissues\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, and elevated sensitivity for gas detection and environmental monitoring\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Furthermore, operation at longer wavelengths can alleviate specific nonlinear propagation effects in optical fibers, thereby supporting more stable pulse formation and higher pulse energies\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. As a result, mode-locked fiber lasers have found expanding roles in applications such as precision micromachining, medical diagnostics and therapy, ultrafast spectroscopy, and remote sensing, underscoring their versatility and continued relevance as ultrafast laser platforms\u003csup\u003e\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThulium ions (Tm\u0026sup3;⁺) and holmium ions (Ho\u0026sup3;⁺) are excellent active dopants for fiber lasers operating around 2 \u0026micro;m. When both ions are incorporated into the same fiber, an efficient cross-relaxation and energy-transfer process significantly enhances pump utilization: laser diodes (LDs) operating near 1.55 \u0026micro;m can excite Tm\u0026sup3;⁺ ions, which then transfer energy to Ho\u0026sup3;⁺ ions via cross-relaxation and energy-transfer processes, overcoming the limited availability of efficient direct pump sources for Ho\u0026sup3;⁺ and increasing the effective pump absorption and quantum efficiency\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. This cooperative interaction improves system performance, reduces thermal load, and enables more compact laser designs. In addition, Tm\u0026sup3;⁺-Ho\u0026sup3;⁺ co-doped fibers exhibit a broader emission and gain bandwidth around 2 \u0026micro;m compared to singly doped fibers, allowing greater wavelength tunability and flexibility for mid-infrared applications\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. These characteristics make Tm-Ho co-doped fibers a promising platform for high-efficiency and broadband 2 \u0026micro;m fiber lasers\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAs nonlinear elements with intensity-dependent absorption, saturable absorbers (SAs) are often used to achieve passive mode-locking\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Ultrafast laser systems have widely used traditional SAs, such as semiconductor saturable absorber mirrors (SESAMs). However, their relatively low damage threshold\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, limited operational bandwidth\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, and high fabrication cost\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e have motivated the search for alternative SA. Recently, metal\u0026ndash;organic frameworks (MOFs) have emerged as promising options due to the unique way their structures function. Metal-organic frameworks (MOFs) are crystalline architectures constructed from organic ligands coordinated to metal ions or clusters. It is possible to tailor these networks to display desired physical and chemical characteristics\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. They have various practical applications due to their large surface area and intrinsic high porosity, such as gas storage\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, gas separation\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, catalysis\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, chemical sensing\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e, and energy storage\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. More recently, these advantageous features have sparked growing interest in exploiting MOFs for nonlinear optical applications, particularly for use as SAs in fiber laser systems\u003csup\u003e\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe majority of reported MOF SAs have been demonstrated in erbium-doped fiber lasers operating at 1.5 \u0026micro;m. It occurred despite the rapid progress in the development of MOF-based nonlinear optical materials\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. While these works have confirmed the feasibility of employing MOF for ultrafast pulse modulation, their extension to longer wavelengths has received comparatively limited attention. In particular, investigations of MOF-assisted mode-locking in the 2 \u0026micro;m spectral region remain scarce, especially in THDFL systems, where suitable SAs are still under active exploration\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. This research gap is notable given that several MOF systems have exhibited broadband optical absorption extending into the near- and mid-infrared regions, suggesting their potential compatibility with 2 \u0026micro;m ultrafast laser operation\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Recent studies on zinc-, copper-, cobalt-, and nickel-based MOFs have demonstrated high modulation depths and strong third-order nonlinear optical responses, key parameters for achieving self-starting, stable passive mode-locking\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFurthermore, the inherently porous MOF architecture of MOF facilitates efficient thermal dissipation and moisture absorption, offering distinct advantages for managing heat accumulation and maintaining performance stability under high optical intensities\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Strong absorption near 2 \u0026micro;m has also been reported for selected MOF structures, reinforcing their suitability for thulium- and holmium-doped fiber laser systems\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. In several instances, saturable absorbers derived from MOFs have demonstrated modulation depths exceeding those of conventional saturable absorbers, including topological insulators and transition-metal dichalcogenides, thereby mitigating spectral filtering-induced instabilities and facilitating scalable ultrafast performance\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. However, existing studies have primarily focused on a narrow subset of MOF compositions and metal centers, leaving the nonlinear optical behavior, long-term stability, and wavelength adaptability of many alternative MOF systems insufficiently explored\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Improving the stability and pulse control of MOF-based SAs for the next generation of ultrafast fiber lasers needs ongoing study into new MOF designs and metal-ligand pairs that provide strong saturable absorption.\u003c/p\u003e \u003cp\u003eThis study investigates the application of calcium-molybdenum-cobalt-2-methylimidazole (CaMoCo-MIM), a trimetallic organic framework (TMOF), for passive mode-locking in a THDFL operating at approximately 2\u0026micro;m wavelength. A compact, fiber-integrated SA device was achieved by fabricating and depositing CaMoCo-MIM onto an arc-shaped optical fiber, enabling integration into the laser cavity. The CaMoCo-MIM exhibited an exceptionally high modulation depth of 36.2%, which is among the highest reported for TMOF-based SAs at ~\u0026thinsp;2 \u0026micro;m. The resulting time bandwidth product (TBP) was approximately 0.315, close to the transform limit, enabling the generation of near-transform-limited soliton pulses with a duration of about 1.25 ps. It is one of the few experiments that shows a TMOF working as an SA in a 2 \u0026micro;m fiber laser setup.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Synthesis and Characterization of CaMoCo-MIM Saturable Absorber\u003c/h2\u003e \u003cp\u003eAll trials were conducted using distilled water (DI). System Chemicals in Malaysia provided us with 99.00% pure calcium nitrate tetrahydrate (Ca(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;4H\u003csub\u003e2\u003c/sub\u003eO) and 99.50% pure ethanol, respectively. Acros Organics in the USA provided the ingredients, 2-methylimidazole (MIM) and cobalt nitrate tetrahydrate (Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;4H\u003csub\u003e2\u003c/sub\u003eO), with a purity of 99.00%. Sigma-Aldrich (USA) delivered 99.00% concentrations of ammonium bicarbonate and sodium molybdate dihydrate. All chemicals were utilised as packaged, with no further cleaning.\u003c/p\u003e \u003cp\u003eThe CaMoCo-MIM TMOF was prepared via a simple wet-chemical precipitation method. Metal precursors totaling 0.06 mol in molar quantity were dissolved within 60 mL of deionized water at a molar ratio corresponding to Ca:Mo: Co\u0026thinsp;=\u0026thinsp;3:2:1. In a separate step, 50 mL of DI water containing 0.01 mol of MIM was added to the metal precursor solution while stirring continuously. At the same time, 0.01 mol of ammonium bicarbonate solution was added to the combined metal-organic solution after it was dissolved in DI water. To encourage framework formation, the resultant mixture was gently agitated for 4 days. The final product was obtained by centrifugation at 5000 rpm for 5 minutes, washed multiple times with ethanol and deionised water, and subsequently desiccated overnight at 80\u0026deg;C in an oven. The synthesis of CaMoCo-MIM is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe morphology of the CaMoCo-MIM material, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e, which was deposited on the arc-shaped fiber, was examined using a Coxem EM-30AX PLUS scanning electron microscope (SEM). At a magnification of 500x, the SEM picture shows that the CaMoCo-MIM SA was efficiently deposited along the arc-shaped fiber surface, resulting in an almost uniform coating. Upon increasing the magnification to 2000x, irregularly shaped nanoparticles become clearly visible. The presence of clustered nanoparticulates indicates that the drop-casting deposition technique promotes particle aggregation on the fiber surface. This aggregation may be further enhanced by the curvature of the arc-shaped region, which can influence local particle distribution during solvent evaporation. Despite the observed aggregation, the coating remains well adhered to the fiber, with no evidence of large-scale delamination. Elemental mapping in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e from further FESEM analysis verifies the existence of silicon (Si), cobalt (Co), gold (Au), oxygen (O), carbon (C), calcium (Ca), and molybdenum (Mo). The detection of Si and O is primarily attributed to the SMF-28 arc-shaped fiber host, which is composed of silicon dioxide (SiO₂) and serves as the substrate for CaMoCo-MIM SA deposition. The high carbon content, predominantly covering the upper surface of the arc-shaped fiber, originates from 2-methylimidazole (MIM) with the chemical formula C\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eN\u003csub\u003e2\u003c/sub\u003e. Meanwhile, the presence of Mo, Co, and Ca arises from their respective metal precursors used during synthesis. The detected Au signal is likely due to trace impurities during sample preparation. However, its atomic percentage is minimal, at only 0.35%, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(c).\u003c/b\u003e Carbon constitutes the highest atomic percentage at 57.68%, followed by O (29.87%), Si (7.13%), Mo (2.09%), Ca (1.97%), and Co (0.91%). Excluding the contribution from Au, the total atomic percentage amounts to 99.65%, indicating good compositional consistency of the deposited CaMoCo-MIM SA layer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFourier-transform infrared (FTIR) spectroscopy was utilised to analyse the coordination environment of the metal nodes and 2-methylimidazole, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Typically, 2-methylimidazole (MIM) displays broad and prominent characteristic peaks within the range of 2200 to 3300 cm⁻\u0026sup1;, resulting from hydrogen bonding interactions between the pyrrole moiety and the pyridine nitrogen\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. The lack of these peaks in the CaMoCo321-MOF spectrum suggests that the metal nodes are coordinated to the secondary amine groups of MIM\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. Furthermore, the N\u0026mdash;H stretching vibration, which is a noticeable characteristic peak of MIM at 1840 cm⁻\u0026sup3;, is missing from CaMoCo-MIM-TMOF. It means that the deprotonation of the MIM ligand was complete during MOF formation\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. These two observations confirmed that the metal-organic bonding exists at two different points. Subsequently, the characteristic peaks of MIM within CaMoCo-MIM-TMOF were identified at 1655 cm⁻\u0026sup1;, corresponding to the C\u0026thinsp;=\u0026thinsp;N stretching vibration in the imidazole ring, and within the range of 1400\u0026ndash;1500 cm⁻\u0026sup1;, indicative of the stretching and bending modes associated with the imidazole ring\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. Additionally, the observed peak at 773 may be attributed to the stretching vibration of the metal-oxygen-hydrogen bond\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. A hydroxyl group from physically adsorbed water was identified as the source of the signal at 3430 cm⁻\u0026sup1;\u003csup\u003e44\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe linear absorption characteristics of the MIM SA were analysed to assess its appropriateness for operation within the 2 \u0026micro;m mid-infrared wavelength range. For this investigation, the CaMoCo-MIM specimen was dispersed in deionised water to create a stable suspension. The sample was examined using an amplified spontaneous emission (ASE) source, with THDFL employed as the gain medium. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows a transmittance of about 38% at 1895.21 nm in the linear absorption spectra of the CaMoCo-MIM saturable absorber. This considerable absorption is attributed to the combined effects of the intrinsic electronic transitions within the CaMoCo-MIM framework and the baseline absorption of the deionized water solvent, which generally exhibits a strong absorption band near 1900 nm.\u003c/p\u003e \u003cp\u003eTo evaluate its saturable absorption characteristics, the nonlinear response of the CaMoCo-MIM SA was examined utilizing a 2-\u0026micro;m mode-locked fiber laser. A key characteristic of SA is the reduction in optical absorption with increasing incident laser intensity. This intensity-dependent absorption behaviour is clearly observed in the experimental result shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e. The nonlinear transmission data were fitted using the conventional SA model, expressed as\u003csup\u003e10,17\u003c/sup\u003e:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:A\\left(I\\right)={\\alpha\\:}_{o}\\times\\:\\text{exp}\\left(-\\frac{I}{{I}_{sat}}\\right)+{\\alpha\\:}_{ns}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAccording to Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\alpha\\:}_{o}\\)\u003c/span\u003e\u003c/span\u003estands for the depth of modulation, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{I}_{sat}\\)\u003c/span\u003e\u003c/span\u003e describes the intensity of saturation, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\alpha\\:}_{ns}\\)\u003c/span\u003e\u003c/span\u003e represents the non-saturable loss.\u003c/p\u003e \u003cp\u003eThe data were analyzed for the SA\u0026rsquo;s individual parameters. By numerical fitting, the CaMoCo-MIM SA exhibited a high modulation depth (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{a}_{s}\\)\u003c/span\u003e\u003c/span\u003e) of 36.2%. This value is beneficial because it enables a self-starting mode-locked mechanism and ensures the generation of high-contrast, ultra-short pulses with considerable peak power. Additionally, the saturation intensity \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{I}_{sat}\\)\u003c/span\u003e\u003c/span\u003e was measured at 79.7 MW/cm\u0026sup2;, demonstrating that the TMOF material exhibits strong power-handling capabilities and can withstand high-energy pulses before reaching its saturation limit. The non-saturable loss, denoted by the symbol \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{a}_{ns}\\)\u003c/span\u003e\u003c/span\u003e, was measured at 8.01%. This loss is primarily attributable to scattering phenomena within the aqueous suspension and to the background absorption of the distilled water solvent. By combining these nonlinear features with a high modulation depth, it is demonstrated that the TMOF-based suspension functions as an efficient, high-performance SA suitable for applications involving steady mid-infrared lasers.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Fabrication of arc-shaped fiber and THDFL cavity setup\u003c/h2\u003e \u003cp\u003eLight was emitted into an SMF-28 fiber from a portable light source, creating an arc-shaped fiber. The optical power meter continuously measured the transmitted power. The fiber was shaped according to standard fabrication protocols and subsequently fixed onto a glass substrate using adhesive tape. Dimensional parameters of the fabricated fiber, including its length and curvature, were examined using an optical microscope interfaced with ToupView imaging software, in accordance with the method outlined in\u003csup\u003e45\u003c/sup\u003e. A 5-centimeter length of SMF-28 fiber was cleaned and securely fastened to a fiber holder before shaping. The exposed area was first rough-polished with 1000P silicon carbide (SiC) abrasive paper, and then fine-polished with a Thorlabs diamond film. After completing the process, the resulting fiber had an insertion loss of approximately 2.5 dB and an arc-shaped profile. The resulting arc-shaped fiber is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, along with a schematic diagram and an optical microscope image. Based on an arc length of 2.01 mm and a polishing depth of 61.97 \u0026micro;m, the residual fiber diameter within the polished area was determined to be 63.03 \u0026micro;m. The arc-shaped fiber was then covered with the TMOF layer using a drop-casting method, as previously described\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the enclosed space inside the THDFL where the arc-shaped fiber is integrated. The thulium-holmium-doped fiber was optically stimulated by two pump laser diodes, LD\u003csub\u003e1\u003c/sub\u003e and LD\u003csub\u003e2\u003c/sub\u003e, to gain enough amplification inside the cavity. Before being launched into the THDF, the light from both LDs was first routed via optical isolators (ISOs) to avoid back reflections. It was then coupled into the common ports of wavelength-division multiplexers (WDMs). WDM\u003csub\u003e1\u003c/sub\u003e was subsequently connected to ISO\u003csub\u003e3\u003c/sub\u003e via a 90:10 optical coupler. The optical coupler divides the circulating signal, diverting 10% for real-time monitoring and characterization, while maintaining the remaining 90% within the laser loop. The cavity was closed by recirculating the remaining optical signal through the saturable absorber (SA), the polarization controller (PC) for state-of-polarization tuning, and WDM\u003csub\u003e2\u003c/sub\u003e, thereby enabling stable laser operation.\u003c/p\u003e \u003cp\u003eThe overall cavity length was 14.67 m, consisting of 1.5 m of THDF, serving as the gain medium, and 13.17 m of standard single-mode fiber (SMF-28). Within the THDF, the group velocity dispersion (GVD) was \u0026minus;\u0026thinsp;0.05568 ps\u0026sup2;/m and \u0026minus;\u0026thinsp;0.06403 ps\u003csup\u003e2\u003c/sup\u003e/m. for SMF-28, giving a net cavity dispersion\u0026thinsp;\u0026minus;\u0026thinsp;0.9 ps\u0026sup2;, taking into consideration the various lengths. According to the computed net dispersion, the cavity was operating in negative dispersion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eBy integrating a CaMoCo-MIM SA and adjusting the polarisation controller within the THDFL cavity, stable mode-locked pulses were successfully generated with a consistent pump power of 307mW. The CaMoCo-MIM is essential for the initiation and sustained operation of ultrashort-pulse generation, as the cavity\u0026rsquo;s mode-locking capability depends on the saturable absorber modulation depth. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e depicts the optical spectrum acquired through the optical spectrum analyser (OSA), characterised by a 3-dlB bandwidth of 3.03 nm and a central wavelength (λ₀) of 1895.21 nm. The laser cavity operates in a soliton-pulse regime when clearly defined Kelly sidebands are observed\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. The temporal profile of a single pulse, shown by the purple curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e, is well-fitted by a sech\u0026sup2; function. A weak pedestal is visible in the autocorrelation trace and is mainly due to the measurement system\u0026rsquo;s noise floor\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. The pulse duration extracted from the fitting is 1.25 ps. An estimated time-bandwidth product of about 0.316 was obtained by integrating the pulse duration with the measured central wavelength and optical bandwidth. It indicates a negligible amount of chirp in the output pulses\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. The oscilloscope trace in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(c)\u003c/b\u003e shows a set of pulses with a 68.4 ns time interval, giving a calculated repetition rate14.6 MHz. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e displays the radio-frequency spectrum with a SNR of 38 dB, further demonstrating that the THDFL is stable when mode-locked with the CaMoCo-MIM SA.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo test the stability of mode-locked operation over time, it was continuously monitored for 4 hours. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e\u003cb\u003e(a)\u003c/b\u003e illustrates the evolution of the autocorrelation traces, while Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e\u003cb\u003e(b)\u003c/b\u003e depicts the pulse duration as a function of time. The pulse profile remains consistently uniform throughout the observation period. The pulse duration was maintained between 1.18 ps and 1.25 ps, indicating that the laser\u0026rsquo;s temporal characteristics remain robust against environmental fluctuations. Applying radio-frequency (RF) spectroscopy to the mode-locked laser allowed for a more thorough evaluation of its frequency stability. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e\u003cb\u003e(c)\u003c/b\u003e shows that the fundamental frequency peak remained sharp and clearly distinguishable throughout the four-hour measurement period. According to the data shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e, the fundamental frequency remained constant at 14.6 MHz. While the SNR fluctuated at approximately 6.1 dB, ranging from 32 dB to 39 dB, the mode-locking state remained self-starting and stable, with no signs of degradation or pulse breakdown. These findings confirm that the CaMoCo-MIM SA enables mode-locking with excellent stability and repeatability, supporting its suitability for real-world ultrafast applications.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e illustrates the connection between the mode-locked laser\u0026rsquo;s average output and incident pump power. As the pump is raised, the CaMoCo-MIM SA shows a consistent, linear rise in output power from 290 mW to 450 mW. At full pump power, the laser has a slope efficiency of 1.15%. This is due to the output coupler that samples only 10% of the intracavity power. Nevertheless, the linear trend validates a predictable power response. It confirms that the CaMoCo-MIM SA maintains its functional integrity without reaching a thermal-damage threshold or exhibiting power saturation over the investigated range.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e compares the current work on MOF-derived materials. The specific surface areas (SAs) enumerated in the table encompass nickel-p-phenylenedicarboxylic acid metal\u0026ndash;organic frameworks (Ni-MOF)\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, copper-derived metal\u0026ndash;organic frameworks (Cu-MOF)\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e, zinc-based metal\u0026ndash;organic frameworks (Zn-MOF)\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e, nickel oxide micron polyhedral metal\u0026ndash;organic frameworks (NiO-MOF)\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e, cobalt-oriented metal\u0026ndash;organic frameworks (Co-MOF)\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e, and nickel-trimesic acid MOF\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. Among the materials compared, the proposed CaMoCo-MIM exhibits the highest modulation depth, reaching 36.2%, which is significantly greater than those reported for Ni-MOF (14.25%), Zn-MOF (9.98%), Co-MOF (10.41%), Cu-MOF (5.7%), and nickel-trimesic acid MOF (4.1%). A high modulation depth indicates strong nonlinear optical absorption, which plays a crucial role in practical pulse shaping and self-starting mode-locking\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. In the 2-\u0026micro;m spectral area, the CaMoCo-MIM operates at a central wavelength of 1895.21 nm and is especially compatible with THDFL devices. The improved nonlinear optical performance, enabled by its higher modulation depth, enables more effective pulse shaping and more reliable self-starting in mode-locked operation \u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. Regarding pulse characteristics, the CaMoCo-MIM-based laser generates near-transform-limited pulses with a TBP of 0.316, which closely approaches the theoretical value for sech\u0026sup2;-shaped pulses, indicating minimal chirp and efficient intracavity dispersion control. The measured TBP is lower than the values reported for Ni-MOF of 0.318, NiO-MOF of 0.321, Co-MOF of 0.323, and Cu-MOF of 0.324, indicating that the CaMoCo-MIM SA supports the generation of high-quality, near-transform-limited ultrashort pulses. The resulting pulse width of 1.25 ps confirms ultrafast operation and aligns closely with the performance reported for other MOF-derived SA. Notably, despite exhibiting a significantly higher modulation depth, the CaMoCo-MIM SA does not induce excessive intracavity loss or pulse broadening, suggesting an efficient balance between nonlinear absorption and cavity dispersion. This favorable balance enables stable, high-quality ultrashort-pulse generation without compromising pulse duration. In terms of operational stability, the CaMoCo-MIM SA achieves an SNR of 38 dB, which is relatively high and corresponds to a linear value of 6309 times. The CaMoCo-MIM SA also delivered superior operational stability of 4 hours, outperforming Zn-MOF at 1.67 hours and the nickel-trimesic acid MOF at 1 hour, and matching the leading stability of Cu-MOF-based devices. This enhanced stability can be attributed to the effective integration of the CaMoCo-MIM with the fiber platform and its ability to withstand prolonged optical pumping without performance degradation. The result highlights the suitability of CaMoCo-MIM for sustained mode-locked operation and underscores its potential for practical deployment in ultrafast fiber laser systems. In addition, a laser efficiency of 1.15% is achieved, which is similar to that of other SAs based on MOFs that operate in the 2 \u0026micro;m range. Notably, this cavity samples only 10% of the output power. Another setup, such as Co-MOF, samples 30%, resulting in a slightly higher output efficiency of 2.36%. Hence, higher efficiency could be achieved by using a different coupler, such as a 70/30 coupler. However, the 90/10 coupler has been retained in this cavity to support stable pulse generation via the feedback mechanism. Overall, the CaMoCo-MIM SA offers a well-balanced combination of high modulation depth, good pulse quality, and stable operation, highlighting its potential for ultrafast fiber laser applications.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of recent works on MOF or MOF-derived materials for mode-locked fiber lasers.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterials\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eModulation Depth\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTime Bandwidth Product (TBP)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePulse width (ps)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eOperation Wavelength\u003c/p\u003e \u003cp\u003e(nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSNR (dB)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMaximum stability duration\u003c/p\u003e \u003cp\u003e(Hour)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eLaser efficiency (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eRef.\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi-MOF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e14.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.318\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1882\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003csup\u003e29\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCu-MOF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.324\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1949\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e5.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003csup\u003e51\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZn-MOF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.316\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.772\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1906.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003csup\u003e52\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNiO-MOF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e18.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.321\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.766\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1555\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e62.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003csup\u003e53\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCo-MOF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.323\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.793\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1558.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003csup\u003e54\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003enickel-trimesic acid MOF-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.303\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1951.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003csup\u003e55\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCaMoCo-MIM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e36.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.316\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1895.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eThis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study demonstrates that a CaMoCo-MIM functions as an efficient SA, enabling stable, self-starting mode-locking in THDFL operating around 2 \u0026micro;m. The CaMoCo-MIM demonstrated exceptional nonlinear optical characteristics, specifically a modulation depth of 36.2%, exceeding that of other reported MOF-based SAs. At 307 mW of pump power, the laser generated a steady soliton pulse with a centre wavelength of 1895.21 nm, a 3 dB spectral bandwidth of 3.03 nm, and a pulse duration of 1.25 ps. A measured SNR of 38 dB and a calculated TBP of 0.316 both indicate the generation of stable, high-quality pulse trains, which closely align with the sech\u0026sup2;-transform-limited value of 0.315. Extended stability assessments conducted over 4 hours confirmed dependable performance under environmental variations, maintaining a consistent repetition frequency of 14.6 MHz. The laser demonstrated a linear relationship between input and output power, with a slope efficiency of 1.15% and a maximum pump power of 450 mW. These results demonstrate that CaMoCo-MIM is a reliable and effective saturable absorber, with strong potential for integration into mid-infrared ultrafast fiber lasers and paving the way for advanced photonic applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflicts of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo conflicts of interest are associated with this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets analysed during this study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNone\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge the funding from Universiti Malaya under the grant RU001-2025A.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH. Ahmad – Supervision, Idea, N. Mokhtar – Running the experiment, drafting of the manuscript, N. A. M, Rusni – Sample preparation, drafting of the manuscript, M. S. M Sa’ad – Checking of manuscript, I. S. Imaduddin – Sample preparation and characterization, S. R. Majid - – Sample preparation and characterization, M. Z. H. Mayzan - Sample characterization, M. A. M. Lutfi – Running the experiment and data collection, Mousa Hussein – Review, editing and final draft, S. Sun – Review, editing and final draft, K. Thambiratnam - Review, editing and final draft.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFermann, M. E. \u0026amp; Hartl, I. Ultrafast fibre lasers. \u003cem\u003eNat. 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Lett.\u003c/em\u003e \u003cb\u003e125\u003c/b\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/5.0217482\u003c/span\u003e\u003cspan address=\"10.1063/5.0217482\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2024).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"calcium-molybdenum-cobalt-2-methylimidazole, trimetallic organic framework, saturable absorber, thulium-holmium doped fiber lasers, mode-locking pulse","lastPublishedDoi":"10.21203/rs.3.rs-8677115/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8677115/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUltrafast fiber lasers operating in the 2 \u0026micro;m spectral region require robust and highly nonlinear saturable absorbers to achieve stable mode-locking. This study demonstrates that a calcium-molybdenum-cobalt-2-methylimidazole (CaMoCo-MIM), a trimetallic organic framework (TMOF), functions as an efficient saturable absorber (SA). Enabling stable and self-starting mode-locking pulse in a thulium\u0026ndash;holmium-doped fiber laser (THDFL). The CaMoCo-MIM exhibits excellent nonlinear optical performance, achieving a high modulation depth of 36.2%, surpassing many previously reported MOF-based SAs. Stable soliton pulse generation was realised at a pump power of 307 mW, producing a central wavelength of 1895.21 nm, a 3 dB spectral bandwidth of 3.03 nm, and a pulse duration of 1.25 ps. A time-bandwidth product (TBP) of 0.316 indicates a nearly chirp-free pulse. Long-term operation over four hours confirmed robust stability with a constant repetition rate of 14.6 MHz. This TMOF establishes a high-modulation-depth SA in next-generation lasers.\u003c/p\u003e","manuscriptTitle":"Calcium-Molybdenum-Cobalt-2-Methylimidazole Trimetallic Organic Framework as a Saturable Absorber for Passive Mode-Locked Thulium–Holmium Doped Fiber Lasers","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-09 16:57:11","doi":"10.21203/rs.3.rs-8677115/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-04T07:31:53+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-02T07:43:02+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-01T03:23:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"83591775143199958093761323410240002777","date":"2026-02-09T21:46:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"189910771659895636573668380650950114621","date":"2026-02-08T03:16:47+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-05T02:56:06+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-04T05:57:20+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-02-02T10:00:19+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-01T05:11:16+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-02-01T04:40:17+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"95db6274-979e-478e-a510-a12bf747451f","owner":[],"postedDate":"February 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":62519224,"name":"Physical sciences/Optics and photonics"},{"id":62519225,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-04-27T16:01:57+00:00","versionOfRecord":{"articleIdentity":"rs-8677115","link":"https://doi.org/10.1038/s41598-026-50198-7","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-04-24 15:58:29","publishedOnDateReadable":"April 24th, 2026"},"versionCreatedAt":"2026-02-09 16:57:11","video":"","vorDoi":"10.1038/s41598-026-50198-7","vorDoiUrl":"https://doi.org/10.1038/s41598-026-50198-7","workflowStages":[]},"version":"v1","identity":"rs-8677115","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8677115","identity":"rs-8677115","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}