Investigation of longitudinal modes for different microchip Nd:YVO4/KTP green lasers

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Elsafty This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5868486/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Nowadays, single-frequency diode-pumped solid-state lasers play an important role in laser frequency standard and length metrology. In this paper, the longitudinal modes oscillation of three different low-cost microchips Nd:YVO4/KTP green lasers have been investigated. A short-cavity Nd:YVO 4 laser is used to explore Single Longitudinal Mode (SLM) without inserting any optical elements inside the cavity. The experiment confirms that SLM can occur by controlling the diode pumping current and laser crystal temperature at a wide range from (300 mA to 388 mA) and (18.5°C to 30°C), respectively. Microchip Nd:YVO4 laser Longitudinal modes Spatial Hole Burning (SHB) Diode-Pumped Solid-State Laser (DPSSL) Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Single-longitudinal-mode (SLM) lasers have widespread applications in fields such as lidar, coherent optical communications, precise interferometry, and length metrology. Nd:YVO 4 as an excellent gain medium is very popular in Diode-Pumped Solid-State (DPSS) Lasers to achieve the SLM operation owing to its high emission and high absorption cross-section[ 1 – 8 ]. Most solid-state lasers are based on optical resonators with a standing-wave operation. Hence Spatial Hole-Burning (SHB) inactive materials cause multi-longitudinal mode operation and consequently, the amplitude fluctuations of output power are observed. To obtain stable operation without power fluctuation single-mode operation is needed[ 9 ]. Therefore, to obtain a stable single mode, several methods for eliminating and limiting the SHB effect have been developed[ 9 ]. The volume Bragg Grating (VBG) method, which represents the action of the filter, contains multiple VBG that reflect a specific wavelength or band of wavelength and transmit only a single wavelength[ 10 ]. The Traveling Wave Cavity method, which is based on eliminating the laser beam to propagate in a single direction in the cavity has three different structures, Nonplanar Ring Cavity (NPRO) contains especially laser crystal coating which represents two functions, an output coupler, and a partially polarizing element to facilitate the laser beam propagating in unidirectional oscillation and prevent SHB operation, this method, unfortunately, it is not suitable for our laser module is used. Discrete Ring Cavity (unidirectional operation) It needs special cavity cutting and it uses an intracavity Faraday rotator and four or six-ring mirrors[ 11 ]. Twisted Mode Cavity is also based on inserting a polarizer into the laser resonator and two-quarter waveplates at the two ends of the gain medium also this method is not suitable for microchip internal cavity laser modules with a birefringent gain medium such as Nd:YVO 4 /KTP[ 12 , 13 ]. The short Cavity method is a simple method to select SLM by shorting the length of the cavity until it is larger than the gain linewidth of the medium and due to the inversely proportional between the cavity length (L) and longitudinal mode spacing(ΔƲ), only SLM reaches into oscillation threshold, although, in this method, it does not need to insert any optical element inside the laser resonator the problem lies on effective internal second-harmonic generation is single-mode difficult and the inability to cavity manufacture in our lab[ 13 ]. The Intracavity Etalon method involves inserting a Fabry-Perot Etalon (FPE) into the resonator, which is tuned to resonate with only one mode that has the highest transmittance. However, this method faces challenges in generating high power, requiring the addition of an amplifier[ 4 , 7 , 8 , 13 – 15 ]. The Seed Injection and Amplification method enhances power by injecting a seed signal and amplifying it. The Nonlinear Frequency Conversion method relies on nonlinear crystals made from high-quality materials, but its output mode quality is limited by the nonlinear properties of the crystal[ 13 ]. The Compound Cavity method uses multiple mirrors to form a laser cavity, altering the single longitudinal mode (SLM) selection condition by adjusting the cavity length and refractive index[ 13 ]. All the above techniques cannot be used in the state of our compact microchip Nd:YVO 4 /KTP laser because, all the laser components are in one package and we cannot insert any element in an internal laser cavity, in the present work, we used a simple SLM selection method, is based on a combination of two techniques, first is a digital electronic technique to control the number of longitudinal modes which the laser oscillates by using Proportional Integral Derivative ( PID) controller based on a feedback circuit to control the pumping diode laser pumping current and the Nd:YVO 4 laser crystal temperature and to obtain two orthogonally polarized longitudinal modes which are very easy to separate them into an SLM operation by the second technique which is based on a polarization selection technique by using a simple polarizer element. This technique is very simple and suitable for internal DPSS commercial laser, which doesn’t need inserting any optical element inside the cavity or need any specific optical elements manufacturing, we will use the microchip laser as one package and control the laser current and temperature. Additionally, two optical systems are employed to investigate the longitudinal oscillation modes at 1064 and 532 nm. 2. Theory 2.1 Construction and Transition of a Microchip Nd:YVO 4 /KTP Laser Nd:YVO 4 laser crystals are directly connected with KTP frequency doubling crystals and are employed to produce large quantities of green laser pointers. Because they are produced in large quantities, these hybrid crystals are incredibly affordable. This design's simple adjustment and subsequent attainment of laser activity is a significant benefit. Due to the crystal's facets' great reflectivity, the crystal itself functions as a proper resonator. Therefore, no more resonator modifications are required. Figure 1 (a) depicts the structure of a microchip Nd:YVO 4 /KTP laser which consists mainly of a pumping diode laser at the wavelength of 808 nm and two crystals bonded together are the laser gain crystal (Nd:YVO 4 ), and a frequency-doubling crystal, potassium titanyl phosphate (KTP). While the crystals are coated to only let light at 532 nm (green) depart the cavity and to keep the fundamental lasing wavelength at 1064 nm (NIR) from resonating in the cavity, some traces of 1064 nm are still visible in the green output light[ 2 ]. We have taken advantage of this advantage by using optical bandpass filters in our test systems. Figure 1 (b) depicts the atomic energy level diagram of the transition of Nd:YVO 4 /KTP laser. According to the detailed study of the spectroscopic properties of the Nd:YVO 4 crystal, the 1064 and 1066 nm lasings come from transitions between energy levels with stark splitting, as shown in Fig. 1 (b). The left side shows the transitions within the Nd:YVO 4 crystal between different energy levels. As long as the appropriate choices and adjustments are made, you can get a very rich laser output. Shen et al.[ 7 ] revealed that the ratio of the stimulated-emission cross section among the 4F3∕2 → 4I 9∕2, 4F3∕2 → 4I 11∕2, and 4F3∕2 → 4I 13∕2 transitions is a significant factor having influence on multiwavelength operations in Nd lasers. Among these transitions, 4F3∕2 → 4I 11∕2 has a higher fluorescence branching ratio and is the most effective one to generate laser wavelengths in the 1.06 µm band. A further spectroscopic study of this crystal has revealed that there are five or six emission bands within the 4F3∕2 → 4I 11∕2 transition resulting from stark splitting, as shown on the right side of Fig. 1 (b). The 1064 nm radiation comes from the R1 → Y2 transition and the 1066 nm radiation from the R2 → Y2 transition. Normally, laser emissions at 1066 nm cannot compete successfully with other emissions due to their relatively small stimulated emission cross section and their difficulty suppressing parasitical oscillations, as shown in previous work using the traditional method[ 16 ]. 2.2 Generation Green laser beam at 532 nm One of the most significant wavelength standards in the visible range has been demonstrated to be the 532 nm system. Nd:YVO 4 crystal characteristics were first reported 50 years ago[ 17 ]. They are optically pumped and have significantly greater absorption coefficients than other solid-state laser crystals, including Nd:YAG crystals. Consequently, they are more effective. After being pumped by a laser diode with an operational wavelength of 808 nm, Nd:YVO 4 emits single-mode infrared laser light at 1064 nm. Since crystals are employed with low output powers (below 20 mW), the applied pumping power is also rather modest, averaging no more than around 350 mW. An integrated nonlinear KTP crystal is part of the microchip design. The material is designed to generate a second harmonic signal since it is \(\:{\text{ꭓ}}_{2}\) nonlinear. A brief explanation is given as follows: Considering an isotropic medium and ignoring for the time being any potential material property dispersion, the induced polarisation at low light \(\:\overrightarrow{P}\left(t\right)\) of a light wave that is moving through a dielectric material can be expressed as $$\:\overrightarrow{P}\left(t\right)=\:{\text{Ɛ}}_{0\:}.\text{ꭓ}\:.\:\overrightarrow{E}\left(t\right)\:\:\:$$ 1 \(\:{\text{Ɛ}}_{0\:}\) is the vacuum permittivity, \(\:\text{ꭓ}\) is the susceptibility of the material, and \(\:\overrightarrow{E}\) is the wave's electric field vector. Every field and material property also depends on the spatial coordinate, but for the sake of simplicity, we limit the examination to a single point in space. Higher light levels, such as those found in our laser crystal, cause the dipoles of the laser material to no longer react linearly to the electric field oscillation. Higher-order terms can no longer be disregarded. A power series representation can be used to depict the induced polarisation following a Taylor series expansion: $$\:\overrightarrow{P}\left(t\right)=\:{\text{Ɛ}}_{0\:}.{\text{ꭓ}}_{1}.\:\overrightarrow{E}\left(t\right)+\:{\text{Ɛ}}_{0\:}.{\text{ꭓ}}_{2}.\:{\overrightarrow{E}}^{2}\left(t\right)+\:{\text{Ɛ}}_{0\:}.{\text{ꭓ}}_{3}.\:{\overrightarrow{E}}^{3}\left(t\right)\:+\dots\:\:$$ 2 The susceptibilities \(\:{\text{ꭓ}}_{n}\) in our anisotropic system are tensors, as opposed to scalar quantities \(\:\text{ꭓ}\) in isotropic media. They believe that the polarization and crystallographic direction affect the light's ability to propagate in nonlinear media. As a result, anisotropic materials like KTP are the only ones where the nonlinear second-order susceptibility, \(\:{\text{ꭓ}}_{2}\) , is visible. In non-centrosymmetric materials, the lowest-order nonlinear coefficient needs to be taken into account. We obtain the second-order part of Eq. ( 2 ) using the assumption that the driving electric field is a monochromatic wave \(\:\overrightarrow{E}\left(t\right)={\overrightarrow{E}}_{0}.\text{cos}\omega\:t\) that oscillates at the frequency. $$\:{\overrightarrow{P}}_{2}=\:{\text{Ɛ}}_{0\:}.{\text{ꭓ}}_{2\:}.\:{{\overrightarrow{E}}_{0}}^{2}\left(t\right)\:.\text{cos}\left(\omega\:t\right)\:\:$$ 3 This can also be expressed using trigonometric relations as $$\:{\overrightarrow{P}}_{2}=\:\frac{1}{2}{\text{Ɛ}}_{0\:}.{\text{ꭓ}}_{2\:}.\:{{\overrightarrow{E}}_{0}}^{2}+\:\frac{1}{2}{\text{Ɛ}}_{0\:}.{\text{ꭓ}}_{2\:}.\:{{\overrightarrow{E}}_{0}}^{2}.\text{cos}\left(2\omega\:t\right)\:\:$$ 4 We can see a linear and square nonlinear interaction oscillating twice the frequency using equations ( 2 ) and ( 4 ). This indicates that both laser beams with frequencies \(\:{\omega\:}\) and \(\:2{\omega\:}\) exit the cavity after passing through the KTP crystal, the infrared wave at 1064 nm, which was produced inside the Nd:YVO 4 crystal, and the second harmonic wave at 532 nm (green), which was created inside the KTP crystal[ 2 , 18 ]. The first term in Eq. ( 4 ) describes the effect of optical rectification, but it is not discussed in further detail here. Whereas the considerations above only justify the emergence of light that oscillates at twice the pump laser frequency, rendering the process efficient requires proper phase matching. This indicates that these partial waves with various frequencies must constructively interfere with one another This constructive interference cannot be achieved in any dispersive media with frequency-dependent permittivity. However, this can be achieved in birefringent materials, such as our anisotropic crystal, by considering plane waves as the solution for the spatial dependence of the fields at both frequencies, for this to work, the second harmonic field’s wave vector must be twice as large as the pump field’s. Phase matching in the anisotropic KTP crystal necessitates cutting the crystal in a particular direction utilizing the ordinary and extraordinary axis material dispersion, type-II phase matching is typically used in KTP crystals, where the pump and second harmonic signals are polarized orthogonally to each other. Sasaki et al[ 3 ] found that the output power of the second harmonic generation in Nd:YVO 4 /KTP systems reach a maximum if the KTP crystal acts as a λ/2 phase retardation plate for the fundamental wavelength, this is the case when type-II phase matching is used.. However, these aspects do not need to be considered specifically, since the crystals that can be purchased are already optimized to maximize the efficiency of the second harmonic generation[ 8 ]. Having one laser with two wavelengths, which are also related to each other, makes this DPSS laser an interesting object for different metrological applications[ 2 , 5 , 6 ]. 2.3 Laser Cavity Modes 2.3.1 Longitudinal modes The optical characteristics of laser are dependent on the cavity modes, so, we will discuss the properties of laser cavity modes associated with longitudinal (frequency-dependent). For achieving the lasing process, threshold gain must satisfy the two conditions in the two equations, (4) for a simple case and (5) for a general case. Simple Case Requirement can be satisfied as follows, consider the round-trip pass of the beam through the amplifier, and assume that, the gain is uniform and experiences an exponential growth of (e gƲ 0 2L ) for an around trip through the amplifier. It will experience a loss at each mirror of 1-R, where R is the mirror reflectivity. Hence the beam will be reduced by the factor R after is reflected from each mirror. The minimum round steady-state requirement for threshold lasing is that the gain exactly equals the loss[ 19 , 20 ]. In this case, the beam will remain unchanged after it makes one trip through the medium. When the two mirrors have the same reflectivity R, the threshold is $$\:{g}_{th}=\frac{1}{2L}\text{ln}\frac{1}{{R}^{2}}$$ 4 In the general case, if the two mirrors have different reflectivity R1 and R2, we will allow fractional losses a1 and a2 at the Brewster windows or any other region in the path of the beam other than inside the amplifier. We also include a possible distributed loss (α) within the gain medium. The threshold equation for the around trip becomes $$\:{g}_{th}\frac{1}{2L}\text{ln}[\frac{1}{{R}_{1}{R}_{2}\left(1-{a}_{1}\right)\left(1-{a}_{2}\right)}\:+\alpha\:\:]\:\:$$ 5 Longitudinal mode frequency separation \(\:\varDelta\:\upsilon\:\) , where c is the speed of light, d is resonator length, η is the refractive index of the resonator medium, is $$\:\varDelta\:\upsilon\:=\frac{c}{2\eta\:d}\:\:\:\:\:$$ 6 The number of longitudinal modes (n) can be determined by knowing three factors: the resonator length, laser bandwidth, and the type of broadening (homogenous or inhomogeneous) present. $$\:n=\frac{2\eta\:d\text{ʋ}}{c}\:\:=\frac{2\eta\:d\:}{\lambda\:}\:\:\:\:\:$$ 7 Where ʋ is laser mode frequency and \(\:\lambda\:\) is laser wavelength. Thus, the number of longitudinal modes of the laser depends on the resonator length, the refractive index of the resonator medium, and the laser wavelength or frequency[ 19 , 20 ]. Because each laser longitudinal mode has a slightly different frequency to obtain laser frequency stabilization, a single longitudinal mode should be obtained (single frequency of laser). Figure 2 : Resulting laser cavity modes when a gain bandwidth of a laser amplifier is combined with resonances of a two-mirror laser cavity[ 19 , 20 ]. 2.3.2 Spatial Hole Burning (SHB) Spectral Hole Burning (SHB) is a distortion of the gain shape of the lasing medium caused by the saturation effect of standing (stationary) waves. The act of selectively bleaching a material's absorption spectrum at a certain frequency to produce a spectral hole or enhanced transmission at that frequency is known as "hole burning". Two basic requirements must be met for the phenomenon to be observed, the spectrum is in homogeneously and the material undergoes, after light absorption, a modification that changes its absorption spectrum, typical materials include dye molecules dissolved as shown in Fig. 3 [ 9 , 20 ]. The Spatial hole-burning effect causes the laser to operate in multi-longitudinal modes. This effect can make it difficult to achieve single-frequency laser operation with standing waves laser cavities because the lasing medium experiences stronger gain saturation, the effective broadening of the gain spectrum, when spatial hole burning occurs in a saturable absorber section, this effect tends to stabilize single frequency operation, hole burning can reduce laser efficiency, SHB effect can be eliminated by using ring shape cavity or spatial polarization states in a linear cavity[ 9 ]. 3. Experimental setup As shown in Fig. 4 , the experimental setup consists of a low-cost commercial microchip (Nd:YVO 4 /KTP) green laser, which emits (1064 nm and 532 nm). Oscillating longitudinal modes for three different low-cost Nd:YVO 4 /KTP are investigated, namely and we switched the laser for each experiment. To maintain the temperature of the laser crystal during the experiment and hence obtain moderate wavelength stability, the laser is mounted on a heat sink that is fixed on a Peltier element (TEC thermoelectric cooler). A temperature sensor (AD590) is fixed at the heat sink and connected with the Peltier element to a PID temperature controller (TED 200 C). A current controller (LDC 220) is used to regulate the laser current to the specific values needed for conducting the required tests during the experiment. A Glan Thompson polarizer is used to filter the two orthogonally polarized longitudinal modes hence obtaining the SLM operation and investigating the number of longitudinal modes at different polarization angles. A dichroic mirror is used to separate the two wavelengths, 532 nm, and 1064 nm, it also, helps to maintain the output power, instead of using a simple beam splitter. The first NIR beam is directed toward the optical spectrum analyzer (OSA), model (AQ6370C), which works in the wavelength range 600–1700 nm and with a resolution of 0.02 nm to detect the longitudinal oscillating modes (at 1064 nm) of the microchip Nd:YVO 4 /KTP laser. The green beam should follow exactly the 1064 nm beam from the phase-matching condition of the second harmonic generation process. The second green beam is directed into the Confocal Fabry Perot interferometer (FPI) model of Thorlabs, (SA200-5B), with a Free Spectral Range (FSR) of (1.5 GHz) to investigate the same longitudinal modes of the Nd:YVO 4 laser at 532 nm. A power meter (Newport 1835-C) is used to measure the laser output power at various oscillation modes and polarization rotation angles. 4. Results and discussion The purpose of this work is to analyze the mode structure of the DPSS microchip laser. One of the goals of the investigation is to determine if the lasers always output SLM or if side modes would appear under certain conditions for the environment or laser. We investigate the number of longitudinal modes oscillation for three different Nd:YVO 4 /KTP green laser modules. The first laser (code: Ram) : As shown in Fig. 5 , the obtained mode structure of the first laser (code: Ram) using OSA (left figures- red lines) and the confocal FPI (right figures- blue lines), at 1064 and 532 nm, respectively. Figure 5 (a) and (b) depict that the laser operates in 3 longitudinal modes at a laser pumping current of 370.2 mA and a laser temperature of 22.97 ℃, the output power of the laser at green is 18.75 mW. Figure 5 (c) and (d) show that the laser operates in two longitudinal modes at a laser pumping current of 370.2 mA and a temperature of 22.97 ℃ with a polarization degree of 235°, the output power of the laser at green is 12.75 mW, and we found these two modes are polarized orthogonally. Thus, we can select one mode by using an external polarizer. The second laser (model: Pointer 303) : Figure 6 (a) and (b) depict the obtained mode structure of the second laser (model 303) at 1064 nm by using the OSA (left figures- red lines) and Confocal FPI (right figures- blue lines). The laser operates in two longitudinal modes at a laser pumping current of 370.1 mA and a temperature of 22.99 ℃, the output power of the laser at green is 13.5 mW. Figure 7 (c) and (d) depict the obtained mode structure of the second laser (model 303) laser at 1064 nm using the OSA and at 532 nm using Confocal FPI, the laser operates in SLM at a laser pumping current of 370.1 mA and a laser temperature of 22.99 ℃, the output power of the laser at green is 15.11 mW at a polarization degree of 264 ° . Additionally, the second laser operates in SLM at a pumping current of 353.4 mA and laser temperatures of 24.30 ℃ as shown in Fig. 7 (a), and in two longitudinal modes at a pumping current of 364.3 mA and a laser temperature of 22.88 ℃, as shown in Fig. 7 (b). The laser operates in two modes at 1064.4080, 1064.9560 nm, and a single at 1063.8800 nm at a pumping current of 200 mA and a laser temperature of 23 ℃. It can be operated in a single mode at different two wavelengths of 1064.2720 nm and 1066.5920 nm at a pumping current of 200 mA and a laser temperature of 23 ℃ as shown in Fig. 7 (d). It can be operated in three modes at 1064.1920, 1064.5240 nm, and 1064.3400 nm and another single mode at 1066.5760 nm as shown in Fig. 7 (e). Also, it operates at two longitudinal modes at 1064.1920 and 1064.5200 nm and a single mode at 1066.5680 nm As shown in Fig. 7 (f). Referring to section (2.1)'s explanation and interpretation of the 1046 nm and 1066 nm transition's appearance. It is necessary to regulate the laser current and temperature to stabilize the cavity length. Furthermore, we found the SLM operation can be achieved by using this laser (model-303) at different ranges of current and temperature. of (24.1 to 24.5 ℃) and a pumping current range of (350.6 to 380 mA). The laser output power of SLM operation is 15 mW at 532 nm and 1 mW at 1064 nm. The third laser (code:US) Figure 8 shows the spectra structure of longitudinal modes of the third laser (code: US) using OSA (left figure) and the Confocal FPI (right figure), respectively. The laser operates at two longitudinal modes at a pumping current of 380.6 mA and a laser temperature of 22.98 ℃. The output power of the laser at 532 nm is 44.69 mW. Also, it can be operated at two longitudinal modes at a pumping current range of 300 to 380.6 mA at a laser temperature of 23℃. Unfortunately, these two modes are not orthogonally polarized, so we can’t separate them into a single mode. 5. Conclusion In this paper, three different low-cost Nd:YVO 4 /KTP green lasers have been characterized experimentally, in terms of oscillating longitudinal modes, polarization, and output powers. The number of oscillating longitudinal modes is controlled by controlling the diode-laser pumping current and laser temperature. Two different optical systems, an Optical Spectrum Analyzer with a resolution of 0.02 nm and a Confocal Fabry Perot Interferometer with an FSR of 1.5 GHz are used to investigate the oscillating modes at the two wavelengths of 532 and 1064 nm at the same time. A simple method is also demonstrated to obtain stable SLM operation at 1064.86 nm, by adjusting the current and temperature over a wide range from (300 mA to 388 mA) and (18.5°C to 30°C). The method can be achieved at a low cost by using commercial Nd:YVO 4 crystals without any requirements of inserting optical elements in the resonator, the laser can operate in a stable SLM mode with the output power of the laser at green is 7.01 mW. Declarations Competing interests The author has no competing interests to declare that are relevant to the content of this article. Funding The equipment used in this research is supported by the National Institute of Standards (NIS), Egypt. Author Contribution The author has accepted full responsibility for the content of this manuscript and has given their approval Acknowledgment The author is grateful to Dr. Haitham Mohamed (NIS) for his unwavering support. References A.S. Elsafty, O. Terra, M. Sobee, A.M. El Sherbini, T.M. El Sherbini, Low-Cost Single Frequency DPSS Nd:YVO4 Laser for Length Metrology, in: CLEO Sci. Innov. CLEOS I 2023, 2023. https://doi.org/10.1364/CLEO_AT.2023.JTh2A.84. A.S. Elsafty, O. Terra, M. Sobee, A.M.E. 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Elsafty","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+0lEQVRIiWNgGAWjYDACCSDmAbMSGx8wGIAYjA1Ea2k2IFVLApsEUe7in92d+OANQ528wfHktuqCgjoG+fbDDQwffuGx5M7ZzYZzGA4bbjjzsO32DIPDDAZnEhsYZ/bhseZG7jZpHoYDjBtuJLbd5jE4wGAgwdjAzNuDW4f8jdztv3kY6uxBWop5DIAOmwHU8hePFgOgLcw8DMyJIC3MPAbMQHuBWhh+4NZieCN3s+Qcg8PJM888bJbmMTjMA/LLwd4G3FrkbuRu/PCmos6273j6w888f+rk5NuPP3zw4w8e70Och2CC4+gAYxshLZiAoC2jYBSMglEwggAAQFhVZ86ub1YAAAAASUVORK5CYII=","orcid":"","institution":"National Institute of Standards (NIS)","correspondingAuthor":true,"prefix":"","firstName":"Ahmed","middleName":"Saudi.","lastName":"Elsafty","suffix":""}],"badges":[],"createdAt":"2025-01-20 20:08:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5868486/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5868486/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":74979987,"identity":"004bb7cc-5321-490d-841c-4f7c8ad89a5f","added_by":"auto","created_at":"2025-01-29 04:33:00","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":45437,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The structure of the microchip Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP green laser, AR: Anti Reflection, HR: High Reflection[2], (b) Transitions within Nd:YVO\u003csub\u003e4\u003c/sub\u003e crystal (left) between different energy levels, (right) from energy level \u003csup\u003e4\u003c/sup\u003eF\u003csub\u003e3∕2 \u003c/sub\u003eto \u003csup\u003e4\u003c/sup\u003eI \u003csub\u003e11∕2\u003c/sub\u003e with stark splitting resulting in 1064 and 1066 nm emissions[16]\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/6bcd276ec06a44970456bd65.jpg"},{"id":74980011,"identity":"a9245dfd-570d-46b4-8802-7218f28aca9c","added_by":"auto","created_at":"2025-01-29 04:33:00","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":40501,"visible":true,"origin":"","legend":"\u003cp\u003eResulting laser cavity modes when a gain bandwidth of a laser amplifier is combined with resonances of a two-mirror laser cavity[19,20].\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/27d3ddee2be8a73f955211a5.jpg"},{"id":74979988,"identity":"5af9498a-868b-4f81-8e04-48a4b5e8a8a0","added_by":"auto","created_at":"2025-01-29 04:33:00","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":30453,"visible":true,"origin":"","legend":"\u003cp\u003eSpectral hole burning effect[9,20].\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/651ea76308b3b23cc8417d58.jpg"},{"id":74981083,"identity":"0f762140-ebb1-4122-a225-d2afcb5bfa83","added_by":"auto","created_at":"2025-01-29 04:41:00","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":52202,"visible":true,"origin":"","legend":"\u003cp\u003eLongitudinal-modes analysis (at 1064 and 532 nm), thermal control. PE: Peltier element, HS: Heat sink, P: Polarizer, F1: 1064 nm pass filter, F2: 532 nm pass filter, OSA: Optical spectrum analyzer, PD: Photodetector, FPI: Fabry Perot Interferometer (FPI).\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/b8498e7eae94c630f427face.jpg"},{"id":74980001,"identity":"070e25e7-3e81-47e3-ae91-250eba1410cc","added_by":"auto","created_at":"2025-01-29 04:33:00","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":183784,"visible":true,"origin":"","legend":"\u003cp\u003eOptical spectra of oscillating longitudinal modes of the microchip green laser measured (first laser code: Ram) using an optical spectrum analyzer (OSA) (left figures) and scanning (FPI) (right figures). (a and b) Three longitudinal modes at a polarization degree of 235°. (d) and (f)) Two orthogonally polarized longitudinal modes at a polarization degree of 235°. (e) SLM at 1064.8600 nm at a polarization degree of 145°.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/d276a7389582e2dea44de6ef.jpg"},{"id":74979993,"identity":"c93c827a-ab25-4a1a-aa2e-c10eaee9efc7","added_by":"auto","created_at":"2025-01-29 04:33:00","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":119442,"visible":true,"origin":"","legend":"\u003cp\u003eOptical spectra of oscillating longitudinal modes of the second microchip green laser measured (second laser model: pointer 303) using (OSA) and (FPI). (a) and (b) Two longitudinal at 1064.2800 and 1064.7880 nm. (c) and (d) SLM operation at a polarization degree of 264°.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/32c56b2a41e839a831ff3d9b.jpg"},{"id":74980017,"identity":"87cf6b8e-96a4-4cc2-85dc-05e7eb21edaa","added_by":"auto","created_at":"2025-01-29 04:33:01","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":183949,"visible":true,"origin":"","legend":"\u003cp\u003eOptical spectra of oscillating longitudinal modes of the (second laser-model: pointer 303). (a) SLM at a laser pumping current of 353.4 mA and laser temperature of 24.30 ℃. (b) Two longitudinal modes at a laser pumping current of 364.3 mA and laser temperature of 22.88 ℃. (c) Single mode at 1063.8800 nm and two modes at 1064.4080 and 106.9560 nm at a laser pumping current of 200 mA and laser temperature of 23 ℃. (d) Single mode at two different wavelengths 1064.2720 nm and 1066.5920 nm at laser pumping current of 162 mA and laser temperature of 23.7 ℃. (e) Three longitudinal modes at 1064.1920 nm, 1064.5240 nm, and 1064.3400 nm, and single mode at 1066.5760 nm at a laser pumping current of 185 mA and laser temperature of 23.3 ℃. (f) Two longitudinal modes at 1064.1920 nm, 1064.5200 nm, and single-mode at 1066.5680 nm at a laser pumping current of 192 mA and laser temperature of 22.92 ℃.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/af9ac5be1eecb003fea50a50.jpg"},{"id":74981085,"identity":"6db73525-2944-4496-97de-4816d5e0c5e1","added_by":"auto","created_at":"2025-01-29 04:41:00","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":75747,"visible":true,"origin":"","legend":"\u003cp\u003eOptical spectra for two longitudinal modes for the third laser (code: US) at a range of pumping current of (300 to 380.6 mA) and a laser temperature of 23℃.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/4eac58ef5e104c52f4435e85.jpg"},{"id":75156153,"identity":"dd173d0f-0426-416c-8f64-26781024d949","added_by":"auto","created_at":"2025-01-31 10:38:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1223153,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5868486/v1/b5cae0a5-926c-49de-8ffa-bd400fc1d21e.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Investigation of longitudinal modes for different microchip Nd:YVO4/KTP green lasers","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSingle-longitudinal-mode (SLM) lasers have widespread applications in fields such as lidar, coherent optical communications, precise interferometry, and length metrology. Nd:YVO\u003csub\u003e4\u003c/sub\u003e as an excellent gain medium is very popular in Diode-Pumped Solid-State (DPSS) Lasers to achieve the SLM operation owing to its high emission and high absorption cross-section[\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6 CR7\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMost solid-state lasers are based on optical resonators with a standing-wave operation. Hence Spatial Hole-Burning (SHB) inactive materials cause multi-longitudinal mode operation and consequently, the amplitude fluctuations of output power are observed. To obtain stable operation without power fluctuation single-mode operation is needed[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Therefore, to obtain a stable single mode, several methods for eliminating and limiting the SHB effect have been developed[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The volume Bragg Grating (VBG) method, which represents the action of the filter, contains multiple VBG that reflect a specific wavelength or band of wavelength and transmit only a single wavelength[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The Traveling Wave Cavity method, which is based on eliminating the laser beam to propagate in a single direction in the cavity has three different structures, Nonplanar Ring Cavity (NPRO) contains especially laser crystal coating which represents two functions, an output coupler, and a partially polarizing element to facilitate the laser beam propagating in unidirectional oscillation and prevent SHB operation, this method, unfortunately, it is not suitable for our laser module is used. Discrete Ring Cavity (unidirectional operation) It needs special cavity cutting and it uses an intracavity Faraday rotator and four or six-ring mirrors[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Twisted Mode Cavity is also based on inserting a polarizer into the laser resonator and two-quarter waveplates at the two ends of the gain medium also this method is not suitable for microchip internal cavity laser modules with a birefringent gain medium such as Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The short Cavity method is a simple method to select SLM by shorting the length of the cavity until it is larger than the gain linewidth of the medium and due to the inversely proportional between the cavity length (L) and longitudinal mode spacing(ΔƲ), only SLM reaches into oscillation threshold, although, in this method, it does not need to insert any optical element inside the laser resonator the problem lies on effective internal second-harmonic generation is single-mode difficult and the inability to cavity manufacture in our lab[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The Intracavity Etalon method involves inserting a Fabry-Perot Etalon (FPE) into the resonator, which is tuned to resonate with only one mode that has the highest transmittance. However, this method faces challenges in generating high power, requiring the addition of an amplifier[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe Seed Injection and Amplification method enhances power by injecting a seed signal and amplifying it. The Nonlinear Frequency Conversion method relies on nonlinear crystals made from high-quality materials, but its output mode quality is limited by the nonlinear properties of the crystal[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The Compound Cavity method uses multiple mirrors to form a laser cavity, altering the single longitudinal mode (SLM) selection condition by adjusting the cavity length and refractive index[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAll the above techniques cannot be used in the state of our compact microchip Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP laser because, all the laser components are in one package and we cannot insert any element in an internal laser cavity, in the present work, we used a simple SLM selection method, is based on a combination of two techniques, first is a digital electronic technique to control the number of longitudinal modes which the laser oscillates by using Proportional Integral Derivative ( PID) controller based on a feedback circuit to control the pumping diode laser pumping current and the Nd:YVO\u003csub\u003e4\u003c/sub\u003e laser crystal temperature and to obtain two orthogonally polarized longitudinal modes which are very easy to separate them into an SLM operation by the second technique which is based on a polarization selection technique by using a simple polarizer element. This technique is very simple and suitable for internal DPSS commercial laser, which doesn\u0026rsquo;t need inserting any optical element inside the cavity or need any specific optical elements manufacturing, we will use the microchip laser as one package and control the laser current and temperature. Additionally, two optical systems are employed to investigate the longitudinal oscillation modes at 1064 and 532 nm.\u003c/p\u003e"},{"header":"2. Theory","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Construction and Transition of a Microchip Nd:YVO\u003csub\u003e4\u003c/sub\u003e /KTP Laser\u003c/h2\u003e \u003cp\u003eNd:YVO\u003csub\u003e4\u003c/sub\u003e laser crystals are directly connected with KTP frequency doubling crystals and are employed to produce large quantities of green laser pointers. Because they are produced in large quantities, these hybrid crystals are incredibly affordable. This design's simple adjustment and subsequent attainment of laser activity is a significant benefit. Due to the crystal's facets' great reflectivity, the crystal itself functions as a proper resonator. Therefore, no more resonator modifications are required.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) depicts the structure of a microchip Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP laser which consists mainly of a pumping diode laser at the wavelength of 808 nm and two crystals bonded together are the laser gain crystal (Nd:YVO\u003csub\u003e4\u003c/sub\u003e), and a frequency-doubling crystal, potassium titanyl phosphate (KTP). While the crystals are coated to only let light at 532 nm (green) depart the cavity and to keep the fundamental lasing wavelength at 1064 nm (NIR) from resonating in the cavity, some traces of 1064 nm are still visible in the green output light[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. We have taken advantage of this advantage by using optical bandpass filters in our test systems. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b) depicts the atomic energy level diagram of the transition of Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP laser.\u003c/p\u003e \u003cp\u003eAccording to the detailed study of the spectroscopic properties of the Nd:YVO\u003csub\u003e4\u003c/sub\u003e crystal, the 1064 and 1066 nm lasings come from transitions between energy levels with stark splitting, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The left side shows the transitions within the Nd:YVO\u003csub\u003e4\u003c/sub\u003e crystal between different energy levels. As long as the appropriate choices and adjustments are made, you can get a very rich laser output. Shen et al.[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] revealed that the ratio of the stimulated-emission cross section among the 4F3∕2 \u0026rarr; 4I 9∕2, 4F3∕2 \u0026rarr; 4I 11∕2, and 4F3∕2 \u0026rarr; 4I 13∕2 transitions is a significant factor having influence on multiwavelength operations in Nd lasers. Among these transitions, 4F3∕2 \u0026rarr; 4I 11∕2 has a higher fluorescence branching ratio and is the most effective one to generate laser wavelengths in the 1.06 \u0026micro;m band. A further spectroscopic study of this crystal has revealed that there are five or six emission bands within the 4F3∕2 \u0026rarr; 4I 11∕2 transition resulting from stark splitting, as shown on the right side of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The 1064 nm radiation comes from the R1 \u0026rarr; Y2 transition and the 1066 nm radiation from the R2 \u0026rarr; Y2 transition. Normally, laser emissions at 1066 nm cannot compete successfully with other emissions due to their relatively small stimulated emission cross section and their difficulty suppressing parasitical oscillations, as shown in previous work using the traditional method[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Generation Green laser beam at 532 nm\u003c/h2\u003e \u003cp\u003eOne of the most significant wavelength standards in the visible range has been demonstrated to be the 532 nm system. Nd:YVO\u003csub\u003e4\u003c/sub\u003e crystal characteristics were first reported 50 years ago[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. They are optically pumped and have significantly greater absorption coefficients than other solid-state laser crystals, including Nd:YAG crystals. Consequently, they are more effective. After being pumped by a laser diode with an operational wavelength of 808 nm, Nd:YVO\u003csub\u003e4\u003c/sub\u003e emits single-mode infrared laser light at 1064 nm. Since crystals are employed with low output powers (below 20 mW), the applied pumping power is also rather modest, averaging no more than around 350 mW. An integrated nonlinear KTP crystal is part of the microchip design. The material is designed to generate a second harmonic signal since it is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{ꭓ}}_{2}\\)\u003c/span\u003e\u003c/span\u003e nonlinear. A brief explanation is given as follows:\u003c/p\u003e \u003cp\u003eConsidering an isotropic medium and ignoring for the time being any potential material property dispersion, the induced polarisation at low light \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\overrightarrow{P}\\left(t\\right)\\)\u003c/span\u003e\u003c/span\u003e of a light wave that is moving through a dielectric material can be expressed as\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\overrightarrow{P}\\left(t\\right)=\\:{\\text{Ɛ}}_{0\\:}.\\text{ꭓ}\\:.\\:\\overrightarrow{E}\\left(t\\right)\\:\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{Ɛ}}_{0\\:}\\)\u003c/span\u003e \u003c/span\u003e is the vacuum permittivity, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{ꭓ}\\)\u003c/span\u003e\u003c/span\u003e is the susceptibility of the material, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\overrightarrow{E}\\)\u003c/span\u003e\u003c/span\u003e is the wave's electric field vector. Every field and material property also depends on the spatial coordinate, but for the sake of simplicity, we limit the examination to a single point in space.\u003c/p\u003e \u003cp\u003eHigher light levels, such as those found in our laser crystal, cause the dipoles of the laser material to no longer react linearly to the electric field oscillation. Higher-order terms can no longer be disregarded. A power series representation can be used to depict the induced polarisation following a Taylor series expansion:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\overrightarrow{P}\\left(t\\right)=\\:{\\text{Ɛ}}_{0\\:}.{\\text{ꭓ}}_{1}.\\:\\overrightarrow{E}\\left(t\\right)+\\:{\\text{Ɛ}}_{0\\:}.{\\text{ꭓ}}_{2}.\\:{\\overrightarrow{E}}^{2}\\left(t\\right)+\\:{\\text{Ɛ}}_{0\\:}.{\\text{ꭓ}}_{3}.\\:{\\overrightarrow{E}}^{3}\\left(t\\right)\\:+\\dots\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe susceptibilities \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{ꭓ}}_{n}\\)\u003c/span\u003e\u003c/span\u003ein our anisotropic system are tensors, as opposed to scalar quantities \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{ꭓ}\\)\u003c/span\u003e\u003c/span\u003e in isotropic media. They believe that the polarization and crystallographic direction affect the light's ability to propagate in nonlinear media. As a result, anisotropic materials like KTP are the only ones where the nonlinear second-order susceptibility, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{ꭓ}}_{2}\\)\u003c/span\u003e\u003c/span\u003e, is visible. In non-centrosymmetric materials, the lowest-order nonlinear coefficient needs to be taken into account. We obtain the second-order part of Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) using the assumption that the driving electric field is a monochromatic wave \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\overrightarrow{E}\\left(t\\right)={\\overrightarrow{E}}_{0}.\\text{cos}\\omega\\:t\\)\u003c/span\u003e\u003c/span\u003e that oscillates at the frequency.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{\\overrightarrow{P}}_{2}=\\:{\\text{Ɛ}}_{0\\:}.{\\text{ꭓ}}_{2\\:}.\\:{{\\overrightarrow{E}}_{0}}^{2}\\left(t\\right)\\:.\\text{cos}\\left(\\omega\\:t\\right)\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThis can also be expressed using trigonometric relations as\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:{\\overrightarrow{P}}_{2}=\\:\\frac{1}{2}{\\text{Ɛ}}_{0\\:}.{\\text{ꭓ}}_{2\\:}.\\:{{\\overrightarrow{E}}_{0}}^{2}+\\:\\frac{1}{2}{\\text{Ɛ}}_{0\\:}.{\\text{ꭓ}}_{2\\:}.\\:{{\\overrightarrow{E}}_{0}}^{2}.\\text{cos}\\left(2\\omega\\:t\\right)\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWe can see a linear and square nonlinear interaction oscillating twice the frequency using equations (\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and (\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e4\u003c/span\u003e). This indicates that both laser beams with frequencies \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\omega\\:}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:2{\\omega\\:}\\)\u003c/span\u003e\u003c/span\u003e exit the cavity after passing through the KTP crystal, the infrared wave at 1064 nm, which was produced inside the Nd:YVO\u003csub\u003e4\u003c/sub\u003e crystal, and the second harmonic wave at 532 nm (green), which was created inside the KTP crystal[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe first term in Eq.\u0026nbsp;(\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e4\u003c/span\u003e) describes the effect of optical rectification, but it is not discussed in further detail here. Whereas the considerations above only justify the emergence of light that oscillates at twice the pump laser frequency, rendering the process efficient requires proper phase matching. This indicates that these partial waves with various frequencies must constructively interfere with one another This constructive interference cannot be achieved in any dispersive media with frequency-dependent permittivity. However, this can be achieved in birefringent materials, such as our anisotropic crystal, by considering plane waves as the solution for the spatial dependence of the fields at both frequencies, for this to work, the second harmonic field\u0026rsquo;s wave vector must be twice as large as the pump field\u0026rsquo;s. Phase matching in the anisotropic KTP crystal necessitates cutting the crystal in a particular direction utilizing the ordinary and extraordinary axis material dispersion, type-II phase matching is typically used in KTP crystals, where the pump and second harmonic signals are polarized orthogonally to each other. Sasaki et al[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] found that the output power of the second harmonic generation in Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP systems reach a maximum if the KTP crystal acts as a λ/2 phase retardation plate for the fundamental wavelength, this is the case when type-II phase matching is used.. However, these aspects do not need to be considered specifically, since the crystals that can be purchased are already optimized to maximize the efficiency of the second harmonic generation[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Having one laser with two wavelengths, which are also related to each other, makes this DPSS laser an interesting object for different metrological applications[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Laser Cavity Modes\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Longitudinal modes\u003c/h2\u003e \u003cp\u003eThe optical characteristics of laser are dependent on the cavity modes, so, we will discuss the properties of laser cavity modes associated with longitudinal (frequency-dependent). For achieving the lasing process, threshold gain must satisfy the two conditions in the two equations, (4) for a simple case and (5) for a general case. Simple Case Requirement can be satisfied as follows, consider the round-trip pass of the beam through the amplifier, and assume that, the gain is uniform and experiences an exponential growth of (e\u003csup\u003egƲ\u003c/sup\u003e\u003csub\u003e0\u003c/sub\u003e\u003csup\u003e2L\u003c/sup\u003e) for an around trip through the amplifier. It will experience a loss at each mirror of 1-R, where R is the mirror reflectivity. Hence the beam will be reduced by the factor R after is reflected from each mirror. The minimum round steady-state requirement for threshold lasing is that the gain exactly equals the loss[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In this case, the beam will remain unchanged after it makes one trip through the medium. When the two mirrors have the same reflectivity R, the threshold is\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:{g}_{th}=\\frac{1}{2L}\\text{ln}\\frac{1}{{R}^{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn the general case, if the two mirrors have different reflectivity R1 and R2, we will allow fractional losses a1 and a2 at the Brewster windows or any other region in the path of the beam other than inside the amplifier. We also include a possible distributed loss (α) within the gain medium. The threshold equation for the around trip becomes\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$\\:{g}_{th}\\frac{1}{2L}\\text{ln}[\\frac{1}{{R}_{1}{R}_{2}\\left(1-{a}_{1}\\right)\\left(1-{a}_{2}\\right)}\\:+\\alpha\\:\\:]\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eLongitudinal mode frequency separation \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:\\upsilon\\:\\)\u003c/span\u003e\u003c/span\u003e, where c is the speed of light, d is resonator length, η is the refractive index of the resonator medium, is\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$$\\:\\varDelta\\:\\upsilon\\:=\\frac{c}{2\\eta\\:d}\\:\\:\\:\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe number of longitudinal modes (n) can be determined by knowing three factors: the resonator length, laser bandwidth, and the type of broadening (homogenous or inhomogeneous) present.\u003cdiv id=\"Equ8\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ8\" name=\"EquationSource\"\u003e\n$$\\:n=\\frac{2\\eta\\:d\\text{ʋ}}{c}\\:\\:=\\frac{2\\eta\\:d\\:}{\\lambda\\:}\\:\\:\\:\\:\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere ʋ is laser mode frequency and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\lambda\\:\\)\u003c/span\u003e\u003c/span\u003e is laser wavelength. Thus, the number of longitudinal modes of the laser depends on the resonator length, the refractive index of the resonator medium, and the laser wavelength or frequency[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Because each laser longitudinal mode has a slightly different frequency to obtain laser frequency stabilization, a single longitudinal mode should be obtained (single frequency of laser).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e: Resulting laser cavity modes when a gain bandwidth of a laser amplifier is combined with resonances of a two-mirror laser cavity[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Spatial Hole Burning (SHB)\u003c/h2\u003e \u003cp\u003eSpectral Hole Burning (SHB) is a distortion of the gain shape of the lasing medium caused by the saturation effect of standing (stationary) waves. The act of selectively bleaching a material's absorption spectrum at a certain frequency to produce a spectral hole or enhanced transmission at that frequency is known as \"hole burning\". Two basic requirements must be met for the phenomenon to be observed, the spectrum is in homogeneously and the material undergoes, after light absorption, a modification that changes its absorption spectrum, typical materials include dye molecules dissolved as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Spatial hole-burning effect causes the laser to operate in multi-longitudinal modes. This effect can make it difficult to achieve single-frequency laser operation with standing waves laser cavities because the lasing medium experiences stronger gain saturation, the effective broadening of the gain spectrum, when spatial hole burning occurs in a saturable absorber section, this effect tends to stabilize single frequency operation, hole burning can reduce laser efficiency, SHB effect can be eliminated by using ring shape cavity or spatial polarization states in a linear cavity[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Experimental setup","content":"\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the experimental setup consists of a low-cost commercial microchip (Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP) green laser, which emits (1064 nm and 532 nm). Oscillating longitudinal modes for three different low-cost Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP are investigated, namely and we switched the laser for each experiment.\u003c/p\u003e \u003cp\u003eTo maintain the temperature of the laser crystal during the experiment and hence obtain moderate wavelength stability, the laser is mounted on a heat sink that is fixed on a Peltier element (TEC thermoelectric cooler). A temperature sensor (AD590) is fixed at the heat sink and connected with the Peltier element to a PID temperature controller (TED 200 C). A current controller (LDC 220) is used to regulate the laser current to the specific values needed for conducting the required tests during the experiment. A Glan Thompson polarizer is used to filter the two orthogonally polarized longitudinal modes hence obtaining the SLM operation and investigating the number of longitudinal modes at different polarization angles. A dichroic mirror is used to separate the two wavelengths, 532 nm, and 1064 nm, it also, helps to maintain the output power, instead of using a simple beam splitter. The first NIR beam is directed toward the optical spectrum analyzer (OSA), model (AQ6370C), which works in the wavelength range 600\u0026ndash;1700 nm and with a resolution of 0.02 nm to detect the longitudinal oscillating modes (at 1064 nm) of the microchip Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP laser. The green beam should follow exactly the 1064 nm beam from the phase-matching condition of the second harmonic generation process. The second green beam is directed into the Confocal Fabry Perot interferometer (FPI) model of Thorlabs, (SA200-5B), with a Free Spectral Range (FSR) of (1.5 GHz) to investigate the same longitudinal modes of the Nd:YVO\u003csub\u003e4\u003c/sub\u003e laser at 532 nm. A power meter (Newport 1835-C) is used to measure the laser output power at various oscillation modes and polarization rotation angles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Results and discussion","content":"\u003cp\u003eThe purpose of this work is to analyze the mode structure of the DPSS microchip laser. One of the goals of the investigation is to determine if the lasers always output SLM or if side modes would appear under certain conditions for the environment or laser. We investigate the number of longitudinal modes oscillation for three different Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP green laser modules.\u003c/p\u003e \u003cp\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eThe first laser (code: Ram)\u003c/span\u003e:\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the obtained mode structure of the first laser (code: Ram) using OSA (left figures- red lines) and the confocal FPI (right figures- blue lines), at 1064 and 532 nm, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) and (b) depict that the laser operates in 3 longitudinal modes at a laser pumping current of 370.2 mA and a laser temperature of 22.97 ℃, the output power of the laser at green is 18.75 mW. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c) and (d) show that the laser operates in two longitudinal modes at a laser pumping current of 370.2 mA and a temperature of 22.97 ℃ with a polarization degree of 235\u0026deg;, the output power of the laser at green is 12.75 mW, and we found these two modes are polarized orthogonally. Thus, we can select one mode by using an external polarizer.\u003c/p\u003e \u003cp\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eThe second laser (model: Pointer 303)\u003c/span\u003e:\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) and (b) depict the obtained mode structure of the second laser (model 303) at 1064 nm by using the OSA (left figures- red lines) and Confocal FPI (right figures- blue lines). The laser operates in two longitudinal modes at a laser pumping current of 370.1 mA and a temperature of 22.99 ℃, the output power of the laser at green is 13.5 mW. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(c) and (d) depict the obtained mode structure of the second laser (model 303) laser at 1064 nm using the OSA and at 532 nm using Confocal FPI, the laser operates in SLM at a laser pumping current of 370.1 mA and a laser temperature of 22.99 ℃, the output power of the laser at green is 15.11 mW at a polarization degree of 264\u003csup\u003e\u0026deg;\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAdditionally, the second laser operates in SLM at a pumping current of 353.4 mA and laser temperatures of 24.30 ℃ as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a), and in two longitudinal modes at a pumping current of 364.3 mA and a laser temperature of 22.88 ℃, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b). The laser operates in two modes at 1064.4080, 1064.9560 nm, and a single at 1063.8800 nm at a pumping current of 200 mA and a laser temperature of 23 ℃.\u003c/p\u003e \u003cp\u003eIt can be operated in a single mode at different two wavelengths of 1064.2720 nm and 1066.5920 nm at a pumping current of 200 mA and a laser temperature of 23 ℃ as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d). It can be operated in three modes at 1064.1920, 1064.5240 nm, and 1064.3400 nm and another single mode at 1066.5760 nm as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(e). Also, it operates at two longitudinal modes at 1064.1920 and 1064.5200 nm and a single mode at 1066.5680 nm\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(f).\u003c/p\u003e \u003cp\u003eReferring to section (2.1)'s explanation and interpretation of the 1046 nm and 1066 nm transition's appearance. It is necessary to regulate the laser current and temperature to stabilize the cavity length. Furthermore, we found the SLM operation can be achieved by using this laser (model-303) at different ranges of current and temperature. of (24.1 to 24.5 ℃) and a pumping current range of (350.6 to 380 mA). The laser output power of SLM operation is 15 mW at 532 nm and 1 mW at 1064 nm.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eThe third laser (code:US)\u003c/span\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the spectra structure of longitudinal modes of the third laser (code: US) using OSA (left figure) and the Confocal FPI (right figure), respectively. The laser operates at two longitudinal modes at a pumping current of 380.6 mA and a laser temperature of 22.98 ℃. The output power of the laser at 532 nm is 44.69 mW. Also, it can be operated at two longitudinal modes at a pumping current range of 300 to 380.6 mA at a laser temperature of 23℃. Unfortunately, these two modes are not orthogonally polarized, so we can\u0026rsquo;t separate them into a single mode.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eIn this paper, three different low-cost Nd:YVO\u003csub\u003e4\u003c/sub\u003e/KTP green lasers have been characterized experimentally, in terms of oscillating longitudinal modes, polarization, and output powers. The number of oscillating longitudinal modes is controlled by controlling the diode-laser pumping current and laser temperature. Two different optical systems, an Optical Spectrum Analyzer with a resolution of 0.02 nm and a Confocal Fabry Perot Interferometer with an FSR of 1.5 GHz are used to investigate the oscillating modes at the two wavelengths of 532 and 1064 nm at the same time. A simple method is also demonstrated to obtain stable SLM operation at 1064.86 nm, by adjusting the current and temperature over a wide range from (300 mA to 388 mA) and (18.5\u0026deg;C to 30\u0026deg;C). The method can be achieved at a low cost by using commercial Nd:YVO\u003csub\u003e4\u003c/sub\u003e crystals without any requirements of inserting optical elements in the resonator, the laser can operate in a stable SLM mode with the output power of the laser at green is 7.01 mW.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe author has no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThe equipment used in this research is supported by the National Institute of Standards (NIS), Egypt.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThe author has accepted full responsibility for the content of this manuscript and has given their approval\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003eThe author is grateful to Dr. Haitham Mohamed (NIS) for his unwavering support.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA.S. Elsafty, O. Terra, M. Sobee, A.M. El Sherbini, T.M. El Sherbini, Low-Cost Single Frequency DPSS Nd:YVO4 Laser for Length Metrology, in: CLEO Sci. Innov. CLEOS I 2023, 2023. https://doi.org/10.1364/CLEO_AT.2023.JTh2A.84.\u003c/li\u003e\n\u003cli\u003eA.S. Elsafty, O. Terra, M. Sobee, A.M.E. Sherbini, T.M.E. Sherbini, Spectral characteristics of a microchip Nd:YVO4 laser, J. Opt. 52 (2023) 1717\u0026ndash;1723. https://doi.org/10.1007/s12596-022-00968-z.\u003c/li\u003e\n\u003cli\u003eK.M.A. J.Z. Sotor, G. Dudzik, A.J. Antonczak, Single- longitudinal mode, monolithic, green solid-state laser, Appl. Phys. B Lasers Opt (2011).\u003c/li\u003e\n\u003cli\u003eJ. Sotor, G. Dudzik, K. Abramski, A compact single-longitudinal mode microchip laser operating at 532 nm, Photonics Lett. Pol. 6 (2014) 2\u0026ndash;4. https://doi.org/10.4302/plp.2014.1.02.\u003c/li\u003e\n\u003cli\u003eand F.-L.H. 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Cambridge university press, (2004).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Microchip Nd:YVO4 laser, Longitudinal modes, Spatial Hole Burning (SHB), Diode-Pumped Solid-State Laser (DPSSL)","lastPublishedDoi":"10.21203/rs.3.rs-5868486/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5868486/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNowadays, single-frequency diode-pumped solid-state lasers play an important role in laser frequency standard and length metrology. In this paper, the longitudinal modes oscillation of three different low-cost microchips Nd:YVO4/KTP green lasers have been investigated. A short-cavity Nd:YVO\u003csub\u003e4\u003c/sub\u003e laser is used to explore Single Longitudinal Mode (SLM) without inserting any optical elements inside the cavity. The experiment confirms that SLM can occur by controlling the diode pumping current and laser crystal temperature at a wide range from (300 mA to 388 mA) and (18.5\u0026deg;C to 30\u0026deg;C), respectively.\u003c/p\u003e","manuscriptTitle":"Investigation of longitudinal modes for different microchip Nd:YVO4/KTP green lasers","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-29 04:32:55","doi":"10.21203/rs.3.rs-5868486/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"791edb5d-6d61-43f4-91ea-a720c5c3871a","owner":[],"postedDate":"January 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-01-31T10:38:31+00:00","versionOfRecord":[],"versionCreatedAt":"2025-01-29 04:32:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5868486","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5868486","identity":"rs-5868486","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}