Integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator

Integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator | 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 Integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator Yonghui Tian, Zhen Han, Liheng Wang, Yong Zheng, Pu Zhang, Yongheng Jiang, and 10 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4743222/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 Fourier domain mode-locked optoelectronic oscillator (FDML OEO) is a crucial component for the upcoming sixth-generation (6G) communication era, as it can break the limitation of mode building time in the conventional OEO and generate high-quality frequency-tunable microwave signals or waveform such as linearly chirped microwave waveform (LCMW) for millimeter-wave applications thanks to its ultra-low phase noise. However, most FDML OEOs reported thus far are discrete and their operating bandwidth are limited, which makes it difficult to meet the real applications’ requirements. Here, we propose and demonstrate the first integrated tunable FDML OEO with the tunable frequency range of 3-42.5 GHz in the lithium niobate on insulator (LNOI) photonic integrated circuit platform. As examples, we demonstrate the generation of LCMW, quadratic-chirp signal, and triangle waveform with the center frequency covering millimeter-wave band using the proposed FDML OEO and the phase noise can be maintained as low as -93dBc/Hz at 10 KHz. The FDML OEO provides a promising solution for the compact and effective signal generation solution, which breaks the bandwidth limitations and facilitates the realization of extensive applications in the field of radio frequency (RF), including high-precision microwave photonic radar, next-generation wireless communication, and unmanned autonomous driving systems. Physical sciences/Optics and photonics/Applied optics/Microwave photonics Physical sciences/Optics and photonics/Applied optics/Integrated optics Lithium niobate on insulator (LNOI) Integrated microwave photonics (IMWP) Fourier domain mode-locked (FDML) Optoelectronic oscillator (OEO) Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction To meet the requirements of 6G applications in future, large-capacity wireless communication and high-resolution microwave photonic radar systems are desired to operate at higher frequencies and wider bandwidths, in which the microwave source plays a key role in generating high frequency and large bandwidth radio frequency (RF) signals. [ 1 – 4 ] Microwave photonic (MWP) is an attractive technology for generating high frequency and large bandwidth radio frequency (RF) signals due to its intrinsic broadband operation, low transmission loss over long distances, and anti-electromagnetic interference. [ 5 , 6 ] Various schemes based on MWP technology have been investigated as a suitable microwave source, such as optical beat frequency, external modulation, and optoelectronic oscillator (OEO). [ 7 – 10 ] Among these approaches, the OEO is a simple and cost-effective candidate for generating high-frequency microwave signals, owing to its low phase noise and broadband tunability. [ 11 ] Recently, various OEO-based microwave sources have been reported. [ 12 – 21 ] However, the maximum frequency tuning rate of conventional OEO is limited by the mode building time of the OEO, which results in the inability of conventional OEOs to generate the fast frequency tunable microwave signals, such as linearly chirped microwave waveform (LCMW) often used as radar and wireless communication signals for their strong anti-jamming ability, high resolution, and excellent signal-to-noise ratio. [ 22 ] The Fourier domain mode-locked (FDML) [ 23 ] optoelectronic oscillator is an attractive solution for generating fast frequency tunable microwave signals. The key principle is to simultaneously oscillate thousands of longitudinal modes in the Fourier domain, to generate a stable periodic LCMW signal directly in the oscillation loop. Although a substantial quantity of solutions based on discrete optical components shows optimization in aspects, such as bandwidth, quality, and waveform of the generated signal, [ 24 – 32 ] these systems are bulky, complex, expensive, and limited bandwidth, which impose restrictions on their deployments in 6G era. Therefore, a highly integrated FDML OEO capable of generating high-frequency broadband signals is becoming an urgent pursuit. Lithium niobate on insulator (LNOI) is an excellent photonic integrated circuit platform candidate to realize the integrated FDML OEO. On the one hand, LNOI have excellent characteristics of low loss, high stability, wide transparency window, and outstanding linear elector-optical effect, which meet the requirements for achieving high-performance OEO systems. [ 33 – 35 ] On the other hand, the LNOI is compatible with inexpensive wafer-scale fabrication techniques, enabling mass commercialization. [ 36 , 37 ] In recent years, a number of optical devices and systems with unprecedented performance metrics have been reported in LNOI, [ 38 – 44 ] paving the way for the LNOI platform to be applied to various integrated microwave photonic applications in the future. Due to the chemically stable nature of LNOI, the etching of lithium niobate (LN) has still faced with some challenges. The etchless of LN scheme based on the silicon nitride (Si 3 N 4 ) loaded LNOI is a satisfactory option for integrated photonic circuits. [ 45 ] In this contribution, we propose and demonstrate the first integrated tunable FDML OEO in silicon nitride-loaded lithium niobate on insulator (Si 3 N 4 -LNOI). Benefit from our optimization design, the FDML OEO system show a record-breaking frequency tuning range from 3 to 42.5 GHz. As proof of concept, we demonstrate the generation of LCMW, quadratic-chirp signal, and triangle waveform with the center frequency covering the millimeter-wave band, and the phase noise below − 93dBc/Hz at 10 KHz. The integrated tunable FDML OEO has the key advantages of compactness, wideband and stability, and shows great potential for a wide range of practical applications like radar and unmanned autonomous driving systems, which require high quality microwave sources in the upcoming 6G era. 2. Results 2.1 Principles and designs The schematic of the proposed FDML OEO system with the size on chip only 9 mm×1 mm is illustrated in Fig. 1 (a), which includes a phase modulator (PM) (ⅱ), a tunable high-quality factor (Q-factor) micro-ring resonator (MRR) (ⅲ), and grating couplers (ⅳ). The detailed working principle of the FDML OEO system is illustrated in Fig. 1 (b) and explained in the following: Light from a continuous wave (CW) laser is coupled into the chip through a gating coupler and routed to the PM, which generates double sidebands with π phase difference. After that, the resonance notch of the tunable MRR filters parts of one sideband of the spectrum while introducing a π phase shift. Then the processed optical signal is routed to a photodetector (PD), which converts the optical signal to the RF signal with a frequency equal to the spacing between the notch and the carrier wave. Finally, the RF signal is amplified by an electric amplifier (EA) before feeding back to the PM to complete the OEO loop. When the overall gain of the opto-electronic loop exceeds the loss, a stable single-mode oscillation would occur (see Note S1, Supporting Information). [ 46 ] In the proposed FDML OEO system, the fast-tuning microwave photonic bandpass filter (MBPF) (shown in Fig. 1 (b)) is used for frequency tuning, which is driving by periodic signals with the frequency tuning period or its multiple be synchronized with the round-trip time of the OEO loop. The process produces a quasi-stationary operation, which break the maximum frequency tuning rate limited by the characteristic time constant or mode building time for up oscillation in a new oscillation mode in the cavity, facilitates the generation of high-quality time-frequency signals that meet the needs of future applications (see Note S2, Supporting Information). [ 26 ] The micrograph of PM is shown in Fig. 1 (c). Thanks to the Pockels effect of LN, the bandwidth of the achieved PM meets the requirement by using optimized traveling wave electrodes. The widths of optical waveguide, ground, and signal electrodes are chosen as 1 µm, 100 µm, and 50 µm, respectively. The gap between ground and signal electrodes is designed to be 6 µm (see Note S3, Supporting Information). These parameters are designed to achieve group refractive index matching between light and electricity, while matching the 50 Ω resistor to reduce retroreflection, thereby increasing the electro-optic bandwidth of the PM. Unlike the Mach-Zehnder intensity modulator (MZM), there is no need to consider bias stability control when using PMs in OEO systems, avoiding the requirement for complex thermal modulation or feedback systems, facilitating high volume ultra-compact integration. The Q-factor of the MRR directly affects the sharpness of the spectral cropping and the free spectra range (FSR) of the MRR determines the range of available spectra, which influence the phase noise and the bandwidth of the signal generated by the OEO, respectively. Therefore, it is imperative to maximize the Q-factor and the FSR of the MRR. Here, we demonstrate a micro-racetrack resonator with compact Euler-curve bends as shown in Fig. 1 (d), which can improve the Q-factor of the MRR while maintain a satisfactory FSR. To reduce the impact of the waveguide sidewalls roughness on the optical transmission loss, we widen the optical waveguide to 4.5µm to support multi-mode transmission while pushing the supported fundamental mode away from the waveguide sidewall, which can vastly improve the Q-factor of the MRR. Simultaneously, the use of Euler-curve bends and 200 µm adiabatic tapers to prevent the excitation of high-order optical modes, thus ensure that only single-mode is present in the MRR. Compared with pure circular bends of the same crosstalk, Euler-curve bends have a shorter length which in turn results in wider FSR. [ 47 ] Fig. 1 (e) illustrates a titanium (Ti) micro-heater, which utilizes the thermos-optic effect to tune the wavelength shift and in turn for tuning the frequency of the generated signal. 2.2 Experimental results Characterization of key devices In order to facilitate the characterization of the on-chip PM in the OEO, we measured the MZM composed of two PMs with the same parameters. Figure 2 (a) shows the electro-optic S 21 response of the modulator. One can see that the 3 dB electro-optic bandwidth is about 78 GHz (from 0.3 GHz), which meets the bandwidth requirement of the desired OEO system. As the key component in the system, a tunable MRR plays a decisive role in the spectral response of the OEO. The transmission spectrum of the high-Q MRR measured by an high resolution optical spectrum analyzer (OSA, APEX A2687) is presented in Fig. 2 (b), which allowed us to extract 3 dB bandwidth of 12 pm, corresponding to a loaded Q-factor of 1.3×10 5 . The MRR’s Q-factor is affected by the close proximity of the heater electrode and the rough waveguide sidewall which can be further improved in the future. The measured FSR of the MRR is 87 GHz, providing sufficient spectral range for microwave signals generation by the OEO system. Figure 2 (c) shows the spectrum tuning of the MRR by applying different electrical power to the micro-heater. The resonant wavelength of the high-Q MRR redshifts, when the voltage applied to the Ti micro-heater is increased from 0 V to 15 V, exhibiting the desired broadband tunability. Frequency-tunable MBPF The performance of the on-chip microwave photonic bandpass filter is evaluated, using the experimental setup shown in Fig. 3 . A CW light with a power of 13 dBm generated by an external tunable laser (TL, Santec TSL-570) is sent to the polarization controller (PC, Thorlabs-FPC561) and then routed to the chip. The vector network analyzer (VNA, Keysight N4372E) provides a frequency swept RF signal with a power of -5 dBm to the PM, by using a high-speed microwave GSG probe (T-PLUS 110 GHz). A 50 Ω matched resistor is used to reduce the reflection of microwave signals at the end of the PM. The recovered RF signals from the lightwave component analyzer (LCA, Keysight N4372E) is sent back to the VNA to acquire the frequency response of the integrated MBPF. In order to verify the frequency tunability, the center frequency of the notch band of the filter can be continuously tuned by applying various power to the micro-heater generated by the multichannel power supply (MPS, GWINSTEK GDP-3303S), as shown in Fig. 4 (a). With the voltage applied to the micro-heater is varied, the center frequency is continuously ranged, while the rejection ratio of the filter does not change with frequency to remain at 10 dB. Figure 4 (b) depicts the stability of the filter’s 3 dB bandwidth with the change in frequency, and multiple measurements result in the filter with a 3 dB bandwidth of approximately 3.1 GHz. Integrated tunable FDML OEO The experimental setup for measuring the performance of the FDML OEO is shown in Fig. 5 . The optical signal and electrical signal are amplified by an erbium-doped fiber amplifier (EDFA, KEOPSYS CEFA-C-PB-LP-SM-23-MSA1-B201-FA-FA) and an electrical amplifier (EA, SHF S807C) in order to achieve stable single mode oscillations in the experiment. The frequency of the generated signal can be tuned by varying the voltage applied to the micro-heater by an arbitrary waveform function generator (AFG, Tektronix 3102C). In the OEO system, the amplified signal is equally divided into two paths by an electrical coupler (EC). In the one path, the single is guided to the input of the PM to realize the OEO loop. The other path routes the signal to an electrical spectrum analyzer (ESA, Keysight N9030B) for real-time monitoring and analysis of the signals by the OEO. Frequency tunability of the integrated OEO is demonstrated as shown in Fig. 6 (a), the superimposed spectrums of the generated microwave signal, which shows a wideband frequency range from 3 to 42.5 GHz is obtained, covering S, C, X, K u , K, and K a bands. We can also see that the amplitude of the signal is lower at higher frequencies, which can be attributed to the frequency dependent attenuation of the OEO system cables and instrumentations. The tuning range of the OEO is limited by the FSR of the high-Q MRR and the gain provided by the amplifier in the loop. A wider frequency tuning range of more than 60 GHz is possible by using a MRR with a wider FSR and cascade multiple amplifiers to provide ample gain. We also demonstrated the side mode suppression ratio (SMSR) of the microwave signals generated by the OEO as shown in Fig. 6 (b), a satisfactory SMSR of 48 dB is achieved at an oscillation frequency of 8.8 GHz. Figure 6 (c) shows the generated microwave waveform in the time domain, which is measured by high-speed real-time oscilloscope (OSC, Keysight UXR0594AP) with a sampling rate of 256 GSa/s. The inset in Fig. 6 (c) show a real-time capture of a section of the generated waveform. Figure 6 (d) displays the RF spectrum of the OEO signal at around 23.76 GHz with a span of 20 MHz, and it can be concluded that the separation between two adjacent modes is 4.27 MHz, which is determined by the length of the OEO experimental link (see Note S4, Supporting Information) and also an essential indicator of the formation of the FDML. Figure 6 (e) and (f) show the phase noise with a frequency tuning step of 5 GHz from 15 GHz to 35 GHz and the measured phase noise at a 10 KHz offset frequency of different microwave signal frequencies, respectively. It is clear to see that the phase noise values are maintained around − 93 dBc/Hz at the offset frequency of 10 KHz, which verifies the most important advantage of the OEO to have a stable phase noise with different oscillation frequencies. The phase noise is mainly determined by the Q-factor, which can be further improved by increasing the Q-factor of the MRR. Additionally, according to the Yao-Maleki phase noise model, the use of kilometer-scale fiber-optic coils in the OEO loop can significantly improve the phase noise of the generated RF signal (see Note S5, Supporting Information). [ 48 ] A stable time-frequency signal is generated when the AFG applies a periodic drive signal to control the MBPF to synchronize its frequency tuning period or a multiple thereof with the round-trip time of the OEO loop, which is the key to formation of the FDML OEO. By varying the waveform function of the AFG output voltage, the type of signal generated can be controlled. As shown in Fig. 7 , we demonstrate high-quality time-frequency waveform generation based on the FDML OEO with examples of LCMW, quadratic-chirp signal, and triangle waveform. The duration and peak value of the applied voltage function are contingent upon the performance of the AFG, which consequently affects the bandwidth and period of the generated signal. To intuitively demonstrate the flexibility and reconfigurability of the device, images of various types of signals generated by the FDML OEO measured with real-time OSC are shown in Fig. 8 . Designing and varying the output voltage function of the AFG to modify the waveform, center frequency, and the frequency scanning range of the generated signal. Figure 8 (a) shows the generated chirp signals with center frequencies from 12 to 30.5 GHz, which exhibits a favorable linear shape. The fluctuations that appear in the figure are possibly due to the change in the link state during the experiments. The quadratic chirp signals with center frequencies from 14 to 36 GHz are illustrated by Fig. 8 (b). Figure 8 (c) shows the real-time frequency distribution of triangle wave signals, which center frequencies from 4 to 18.5 GHz. The applied periodic voltages could not meet the experimental requirements to obtain satisfactory results limited by the AFG performance. However, this problem can be solved by using an AFG with a longer voltage cycle time and higher peak voltage. 3. Discussion In this work, the first integrated tunable FDML OEO in the LNOI platform is proposed and experimentally demonstrated, achieving a high compactness, wideband tunability, and reconfigurability. The system is designed and fabricated with a tunable range from 3 GHz to 42.5 GHz, due to the design of the phase modulator diminishes the V π . The phase noise is maintained at -93 dBc/Hz at 10 KHz in all realizable bands. Importantly, we demonstrate a wide range of proven time-frequency generating capabilities that meet the demand for high-frequency, integrated, flexible, and tunable microwave sources for the 6G era. Table 1 Comparison of previous FDML OEOs Refs. Platform Integrated Components Tuning range Phase noise ECL GST [24] - Discrete 6.5 ~ 7.5 GHz -104 dBc/Hz at 10 KHz 2800 m FMCW [25] - Discrete 0.4.~15 GHz - 1030 m LCMW [26] - Discrete 0 ~ 18 GHz -134 dBc/Hz at 10 KHz 4500 m LCMW [27] - Discrete 8.3 ~ 11.7 GHz - 4500 m Dual-chirp [28] - Discrete 4 ~ 19 GHz - 4500 m LCMW [29] - Discrete 15 ~ 18 GHz -113.89 dBc/Hz at 10 KHz 4000 m LCMW [30] - Discrete 9.3 ~ 12.7 GHz -89.5 dBc/Hz at 10 KHz - Phase-coded [31] SOI MRR 4 ~ 19 GHz - 16730 m LCMW [32] SOI MRR 7.2 ~ 13.2 GHz - 11500 m LCMW This work LNOI PM + MRR 3 ~ 42.5 GHz -93 dBc/Hz at 10 KHz 46.8 m LCMW, Q-chirp, Triangle *ECL: Equivalent circuit length; GST: Generated signal types; FMCW: Linear frequency-modulated continuous wave; Q-chirp: quadratic-chirp Comparison of our work with the previously reported FDML OEOs is shown in Table 1 , where one can see that we demonstrated a record-breaking tuning range. Notably, from the preceding section the adjacent mode spacing of the OEO is 4.27 MHz, which corresponds to an equivalent circuit length of approximately 46.8 m, mostly from the optic fiber patch cords, EDFA, and PD devices used. For fair comparison, the length of each OEO fiber loop is also provided in Table 1 . Although our FDML OEO does not reach the level of classical discrete systems in terms of phase noise metrics, according to the Yao-Maleki phase noise model it is known that the phase noise of an OEO is related to the length of the fiber used, and if we use a 4500 m fiber in the loop, the phase noise can be theoretically drop to -132.66 dBc/Hz at 10 KHz. [ 48 ] 4. Conclusion In summary, we demonstrate an integrated tunable FDML OEO in LNOI with ultra-wideband tunable range, through on chip PM and high-Q MRR and the size of the integrated chip is only 9 mm×1 mm. An unprecedented frequency tuning range from 3 to 42.5 GHz is achieved, and the phase noise is maintained around − 93 dBc/Hz at 10 KHz. Furthermore, multiple time-frequency signal generation with center frequencies up to the millimeter-wave levels also be achieved, including LCMW, quadratic-chirp signal, and triangle waveform signals. The entire system’s total insertion loss is 15.38 dB including coupling loss (see Note S6, Supporting Information). It is expected that by hybrid integrating the laser, amplifiers, and photodetector onto the LNOI chip, the phase noise can be further reduced, which can meet the requirements for practical applications in the 6G era. [ 49 – 52 ] The demonstrated compact OEO with ultra-wideband tunable ranges in this work can already meet the urgent needs of the millimeter-wave applications including high-precision millimeter-wave radar for autonomous driving, wireless transmission with large communication capacity, unmanned autonomous driving systems, and electronic warfare systems. 5. Methods Fabrication of photonic chip The proposed integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator is fabricated by electron beam lithography (EBL) and inductively coupled plasma (ICP) etching processes. We procured the wafers from NanoLN and specifically opted X-cut LNOI, to harness the superior electro-optical tensor component γ 33 of LN, thereby enhancing the performance of Y-propagating modulators. A Si 3 N 4 thin film is deposited onto the surface of LNOI through reactive sputtering. Then, electron beam lithography (EBL) is used to pattern the resist, and inductively coupled plasma (ICP) etching is used to form waveguide and grating coupler structures. The electrodes are subsequently fabricated through precise techniques, including direct laser writing, electron beam evaporation deposition, and lift-off processes. Declarations Data Availability Statement All the data supporting the findings in this study are available in the paper and Supplementary Information. Additional data related to this paper are available from the corresponding authors upon request. Conflict of Interest The authors declare no conflict of interest. Funding This work was supported by National Natural Science Foundation of China (NSFC) (62075091, 62205135), Key Research and Development of Gansu Province (22YF7GA008, 23YFGA0007), Natural Science Foundation of Gansu Province (23JRRA1026, 22JR5RA493). Author contributions Z. H. and Y. T. conceptualized this research., Z. H., L. W. and P. 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Laser Photon Rev 15:2100030 Op de Beeck C, Mayor FM, Cuyvers S et al (2021) III/V-on-lithium niobate amplifiers and lasers. Optica 8:1288–1289 Zhang X, Liu X, Ma R et al (2022) Heterogeneously integrated III–V-on-lithium niobate broadband light sources and photodetectors. Opt Lett 47:4564–4567 Additional Declarations There is NO Competing Interest. Supplementary Files SupportingInformation.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4743222","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":330689363,"identity":"c489058b-8136-4a8a-bfef-1bd5acff7864","order_by":0,"name":"Yonghui Tian","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxUlEQVRIiWNgGAWjYDACZiBmbLCRAXN4iNHBA9GSxkOCFgawlsMkaLFnZ372mHfHeR75GQmMD962McibE3YYm7kx75nbPIwzEpgN57YxGO5sIKiFwUyat+02D7NEAhuQwZBgcICgFvZvQJXneNgkEth/E6mFB2TLAR4eoC3MxGk5zFMmObctmUeC52Gz5JxzEoYbCGlh7z++TeJtm52cfHvywQ9vymzkCdoCAkyQ6GBsABISRKgHqf1BnLpRMApGwSgYqQAA/vQyF0/pxkMAAAAASUVORK5CYII=","orcid":"","institution":"Lanzhou University","correspondingAuthor":true,"prefix":"","firstName":"Yonghui","middleName":"","lastName":"Tian","suffix":""},{"id":330689364,"identity":"ef0437f7-0b5d-4ff7-a878-491c28d580e2","order_by":1,"name":"Zhen Han","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Zhen","middleName":"","lastName":"Han","suffix":""},{"id":330689365,"identity":"e4b839ca-e511-4430-b4d7-b4ac09d9f881","order_by":2,"name":"Liheng Wang","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Liheng","middleName":"","lastName":"Wang","suffix":""},{"id":330689366,"identity":"4c8965b5-e446-4213-8a0c-58c26a36dae5","order_by":3,"name":"Yong Zheng","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Yong","middleName":"","lastName":"Zheng","suffix":""},{"id":330689367,"identity":"dfa91c98-af3d-4992-bfe5-ee6c5e8fc387","order_by":4,"name":"Pu Zhang","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Pu","middleName":"","lastName":"Zhang","suffix":""},{"id":330689368,"identity":"d302ee38-09b0-43f6-9ebd-70b59428e944","order_by":5,"name":"Yongheng Jiang","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Yongheng","middleName":"","lastName":"Jiang","suffix":""},{"id":330689369,"identity":"0b28ce7d-6844-4be7-8119-8a7c7c09baa8","order_by":6,"name":"Huifu Xiao","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Huifu","middleName":"","lastName":"Xiao","suffix":""},{"id":330689370,"identity":"e8a368e1-0361-4cf3-b9d2-0c137499d784","order_by":7,"name":"XuDong Zhou","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"XuDong","middleName":"","lastName":"Zhou","suffix":""},{"id":330689371,"identity":"b2f9ef57-2446-4cd4-ac6d-ed0219f294bc","order_by":8,"name":"Mingrui Yuan","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Mingrui","middleName":"","lastName":"Yuan","suffix":""},{"id":330689372,"identity":"2c194ef1-0167-40bd-b27b-baf8abc484a2","order_by":9,"name":"Mei Xian Low","email":"","orcid":"","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Mei","middleName":"Xian","lastName":"Low","suffix":""},{"id":330689373,"identity":"a16d9c2c-1465-4d4c-ad2f-77f2b9ac1926","order_by":10,"name":"Aditya Dubey","email":"","orcid":"https://orcid.org/0000-0003-4240-0478","institution":"Royal Melbourne Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Aditya","middleName":"","lastName":"Dubey","suffix":""},{"id":330689374,"identity":"528b267d-60fe-47e5-ad00-a3cb85d7525e","order_by":11,"name":"Thach Nguyen","email":"","orcid":"https://orcid.org/0000-0002-8409-5638","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Thach","middleName":"","lastName":"Nguyen","suffix":""},{"id":330689375,"identity":"e189bd2b-69d5-451f-81ec-8e8924f2cc93","order_by":12,"name":"Andreas Boes","email":"","orcid":"https://orcid.org/0000-0001-8443-3396","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Andreas","middleName":"","lastName":"Boes","suffix":""},{"id":330689376,"identity":"a01e9bc5-13e6-4fd0-8d2f-3e5ed16e808d","order_by":13,"name":"Qinfen Hao","email":"","orcid":"","institution":"Institute of Computing Technology Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Qinfen","middleName":"","lastName":"Hao","suffix":""},{"id":330689377,"identity":"f1c307fa-3061-49a0-ae34-5dfe8239039f","order_by":14,"name":"Guanghui Ren","email":"","orcid":"https://orcid.org/0000-0002-9867-8279","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Guanghui","middleName":"","lastName":"Ren","suffix":""},{"id":330689378,"identity":"57d601a7-40a2-4a74-9781-3bb6cb1747c6","order_by":15,"name":"Arnan Mitchell","email":"","orcid":"https://orcid.org/0000-0002-2463-2956","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Arnan","middleName":"","lastName":"Mitchell","suffix":""}],"badges":[],"createdAt":"2024-07-15 13:31:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4743222/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4743222/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":60977112,"identity":"574dd217-7787-4628-859b-0d4b68e3c4a3","added_by":"auto","created_at":"2024-07-24 08:27:16","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2994246,"visible":true,"origin":"","legend":"\u003cp\u003eIntegrated tunable FDML OEO in thin film lithium niobate (a) Schematic diagram of the integrated FDML OEO chip layout. (ⅰ) Shows the spectrogram of the input CW laser, and (ⅱ)~(ⅳ) show the cross sections at different locations of the chip. (b) The operation principle of the tunable FDML OEO, including the microwave photonic bandpass filter. (Ⅰ) Different colors are used to represent the sidebands generated by PM with π phase difference. (Ⅱ) The pink color at the resonant wavelength indicates the π phase shift introduce by the over-coupled MRR. (c), (d) and (e) show an optical microscope image of the PM, high Q-factor Euler MRR, and Ti micro-heater next to the MRR .\u003c/p\u003e","description":"","filename":"image1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/3d40ae68bc52c97608f910fa.jpg"},{"id":60977109,"identity":"fa65559a-d663-46bc-9fca-720f4090268d","added_by":"auto","created_at":"2024-07-24 08:27:15","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2483141,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterization of key devices on the LNOI chip (a) The electro-optic S\u003csub\u003e21 \u003c/sub\u003eresponse of the MZM. (b) Measured transmission spectrum of the tunable high-Q MRR. (c) Measured optical spectra with various voltages applied to the Ti micro-heater.\u003c/p\u003e","description":"","filename":"image2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/71b33da5c75aba4087bcca6e.jpg"},{"id":60977072,"identity":"6cac1403-e5c3-45ea-b87e-85edbc53426d","added_by":"auto","created_at":"2024-07-24 08:27:15","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":580942,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental setup to the measurement of the bandpass response of integrated MBPF.\u003c/p\u003e","description":"","filename":"image3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/9e9b98704299891deb735d3e.jpg"},{"id":60977115,"identity":"dc42cdef-7ab3-4c54-b484-e1351bda428e","added_by":"auto","created_at":"2024-07-24 08:27:16","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1352576,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental results for the integrated MBPF. (a) Measured RF responses of the bandpass filtering at various center frequencies. (b) The 3dB bandwidths of the filter with the change in RF frequencies.\u003c/p\u003e","description":"","filename":"image4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/4d4733c33682709635bddd47.jpg"},{"id":60978005,"identity":"dd654476-28c4-49b3-822d-e4770427c12c","added_by":"auto","created_at":"2024-07-24 08:35:16","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":785896,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental demonstration of the integrated tunable FDML OEO.\u003c/p\u003e","description":"","filename":"image5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/c76705a47fa3cde96784f2ef.jpg"},{"id":60978798,"identity":"4d3c431e-7be5-4c78-b652-5667f36994de","added_by":"auto","created_at":"2024-07-24 08:43:16","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":3805213,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental results of the integrated tunable FDML OEO. (a) Superimposed spectrum of frequencies from 3 to 42.5 GHz. (b) Electrical spectrum of the produced 8.8 GHz microwave signal. The SMSR is about 48 dB. (c) Temporal waveform of the RF signal, the inset shows a section of the waveform. (d) RF spectrum with a span of 20 MHz. (e) Phase noise with a frequency tuning step of 5 GHz from 15 GHz to 35 GHz. (f) Measured phase noise at a 10 KHz offset frequency for different microwave signal frequencies. All phase noise values are maintained around -93 dBc/Hz at the offset frequency of 10 KHz.\u003c/p\u003e","description":"","filename":"image6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/208d1db931496c6ad1b78c79.jpg"},{"id":60978797,"identity":"2edd7ec9-b818-415b-a601-a1459c83f8c3","added_by":"auto","created_at":"2024-07-24 08:43:15","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":5195530,"visible":true,"origin":"","legend":"\u003cp\u003eSuperimposed spectrum of time-frequency signals. (a) LCMW: center frequencies from 12 to 30.5 GHz. (b) Quadratic chirp signals: center frequencies from 14 to 36 GHz. (c) Triangle signals: center frequencies from 4.2 to 18.5 GHz.\u003c/p\u003e","description":"","filename":"image7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/a1532fae754ff94173aa4bb8.jpg"},{"id":60977113,"identity":"a6cafc6c-65b4-4cf8-aa4a-1496abfe76d9","added_by":"auto","created_at":"2024-07-24 08:27:16","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":7938456,"visible":true,"origin":"","legend":"\u003cp\u003eReal-time frequency distribution of various signals. (a) Chirp signals. (b) Quadratic chirp signals. (c) Triangle signals.\u003c/p\u003e","description":"","filename":"image8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/d5df1ffe8d42eb520520aad4.jpg"},{"id":69476123,"identity":"adef7d40-8992-4046-a78e-74d252e5734a","added_by":"auto","created_at":"2024-11-20 19:28:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":25613218,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/238e7d2e-8c68-431f-94d7-2823e6869fb9.pdf"},{"id":60978003,"identity":"f0e3234a-5e35-479b-8967-0bfdf04e1fd7","added_by":"auto","created_at":"2024-07-24 08:35:15","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1775608,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4743222/v1/fe1ea3f7e7cf4b32c60ce0e4.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eTo meet the requirements of 6G applications in future, large-capacity wireless communication and high-resolution microwave photonic radar systems are desired to operate at higher frequencies and wider bandwidths, in which the microwave source plays a key role in generating high frequency and large bandwidth radio frequency (RF) signals.\u003csup\u003e[\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e Microwave photonic (MWP) is an attractive technology for generating high frequency and large bandwidth radio frequency (RF) signals due to its intrinsic broadband operation, low transmission loss over long distances, and anti-electromagnetic interference.\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e Various schemes based on MWP technology have been investigated as a suitable microwave source, such as optical beat frequency, external modulation, and optoelectronic oscillator (OEO).\u003csup\u003e[\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e Among these approaches, the OEO is a simple and cost-effective candidate for generating high-frequency microwave signals, owing to its low phase noise and broadband tunability.\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e Recently, various OEO-based microwave sources have been reported.\u003csup\u003e[\u003cspan additionalcitationids=\"CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR20\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e However, the maximum frequency tuning rate of conventional OEO is limited by the mode building time of the OEO, which results in the inability of conventional OEOs to generate the fast frequency tunable microwave signals, such as linearly chirped microwave waveform (LCMW) often used as radar and wireless communication signals for their strong anti-jamming ability, high resolution, and excellent signal-to-noise ratio. \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe Fourier domain mode-locked (FDML)\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e optoelectronic oscillator is an attractive solution for generating fast frequency tunable microwave signals. The key principle is to simultaneously oscillate thousands of longitudinal modes in the Fourier domain, to generate a stable periodic LCMW signal directly in the oscillation loop. Although a substantial quantity of solutions based on discrete optical components shows optimization in aspects, such as bandwidth, quality, and waveform of the generated signal,\u003csup\u003e[\u003cspan additionalcitationids=\"CR25 CR26 CR27 CR28 CR29 CR30 CR31\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e these systems are bulky, complex, expensive, and limited bandwidth, which impose restrictions on their deployments in 6G era. Therefore, a highly integrated FDML OEO capable of generating high-frequency broadband signals is becoming an urgent pursuit.\u003c/p\u003e \u003cp\u003eLithium niobate on insulator (LNOI) is an excellent photonic integrated circuit platform candidate to realize the integrated FDML OEO. On the one hand, LNOI have excellent characteristics of low loss, high stability, wide transparency window, and outstanding linear elector-optical effect, which meet the requirements for achieving high-performance OEO systems.\u003csup\u003e[\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e On the other hand, the LNOI is compatible with inexpensive wafer-scale fabrication techniques, enabling mass commercialization.\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e In recent years, a number of optical devices and systems with unprecedented performance metrics have been reported in LNOI,\u003csup\u003e[\u003cspan additionalcitationids=\"CR39 CR40 CR41 CR42 CR43\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e paving the way for the LNOI platform to be applied to various integrated microwave photonic applications in the future.\u003c/p\u003e \u003cp\u003eDue to the chemically stable nature of LNOI, the etching of lithium niobate (LN) has still faced with some challenges. The etchless of LN scheme based on the silicon nitride (Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e) loaded LNOI is a satisfactory option for integrated photonic circuits.\u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e In this contribution, we propose and demonstrate the first integrated tunable FDML OEO in silicon nitride-loaded lithium niobate on insulator (Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e-LNOI). Benefit from our optimization design, the FDML OEO system show a record-breaking frequency tuning range from 3 to 42.5 GHz. As proof of concept, we demonstrate the generation of LCMW, quadratic-chirp signal, and triangle waveform with the center frequency covering the millimeter-wave band, and the phase noise below \u0026minus;\u0026thinsp;93dBc/Hz at 10 KHz. The integrated tunable FDML OEO has the key advantages of compactness, wideband and stability, and shows great potential for a wide range of practical applications like radar and unmanned autonomous driving systems, which require high quality microwave sources in the upcoming 6G era.\u003c/p\u003e"},{"header":"2. Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Principles and designs\u003c/h2\u003e \u003cp\u003eThe schematic of the proposed FDML OEO system with the size on chip only 9 mm\u0026times;1 mm is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (a), which includes a phase modulator (PM) (ⅱ), a tunable high-quality factor (Q-factor) micro-ring resonator (MRR) (ⅲ), and grating couplers (ⅳ). The detailed working principle of the FDML OEO system is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b) and explained in the following: Light from a continuous wave (CW) laser is coupled into the chip through a gating coupler and routed to the PM, which generates double sidebands with π phase difference. After that, the resonance notch of the tunable MRR filters parts of one sideband of the spectrum while introducing a π phase shift. Then the processed optical signal is routed to a photodetector (PD), which converts the optical signal to the RF signal with a frequency equal to the spacing between the notch and the carrier wave. Finally, the RF signal is amplified by an electric amplifier (EA) before feeding back to the PM to complete the OEO loop. When the overall gain of the opto-electronic loop exceeds the loss, a stable single-mode oscillation would occur (see Note S1, Supporting Information).\u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/sup\u003e In the proposed FDML OEO system, the fast-tuning microwave photonic bandpass filter (MBPF) (shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b)) is used for frequency tuning, which is driving by periodic signals with the frequency tuning period or its multiple be synchronized with the round-trip time of the OEO loop. The process produces a quasi-stationary operation, which break the maximum frequency tuning rate limited by the characteristic time constant or mode building time for up oscillation in a new oscillation mode in the cavity, facilitates the generation of high-quality time-frequency signals that meet the needs of future applications (see Note S2, Supporting Information).\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eThe micrograph of PM is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (c). Thanks to the Pockels effect of LN, the bandwidth of the achieved PM meets the requirement by using optimized traveling wave electrodes. The widths of optical waveguide, ground, and signal electrodes are chosen as 1 \u0026micro;m, 100 \u0026micro;m, and 50 \u0026micro;m, respectively. The gap between ground and signal electrodes is designed to be 6 \u0026micro;m (see Note S3, Supporting Information). These parameters are designed to achieve group refractive index matching between light and electricity, while matching the 50 Ω resistor to reduce retroreflection, thereby increasing the electro-optic bandwidth of the PM. Unlike the Mach-Zehnder intensity modulator (MZM), there is no need to consider bias stability control when using PMs in OEO systems, avoiding the requirement for complex thermal modulation or feedback systems, facilitating high volume ultra-compact integration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Q-factor of the MRR directly affects the sharpness of the spectral cropping and the free spectra range (FSR) of the MRR determines the range of available spectra, which influence the phase noise and the bandwidth of the signal generated by the OEO, respectively. Therefore, it is imperative to maximize the Q-factor and the FSR of the MRR. Here, we demonstrate a micro-racetrack resonator with compact Euler-curve bends as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (d), which can improve the Q-factor of the MRR while maintain a satisfactory FSR. To reduce the impact of the waveguide sidewalls roughness on the optical transmission loss, we widen the optical waveguide to 4.5\u0026micro;m to support multi-mode transmission while pushing the supported fundamental mode away from the waveguide sidewall, which can vastly improve the Q-factor of the MRR. Simultaneously, the use of Euler-curve bends and 200 \u0026micro;m adiabatic tapers to prevent the excitation of high-order optical modes, thus ensure that only single-mode is present in the MRR. Compared with pure circular bends of the same crosstalk, Euler-curve bends have a shorter length which in turn results in wider FSR.\u003csup\u003e[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (e) illustrates a titanium (Ti) micro-heater, which utilizes the thermos-optic effect to tune the wavelength shift and in turn for tuning the frequency of the generated signal.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Experimental results\u003c/h2\u003e \u003cp\u003e \u003cb\u003eCharacterization of key devices\u003c/b\u003e \u003c/p\u003e \u003cp\u003eIn order to facilitate the characterization of the on-chip PM in the OEO, we measured the MZM composed of two PMs with the same parameters. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a) shows the electro-optic S\u003csub\u003e21\u003c/sub\u003e response of the modulator. One can see that the 3 dB electro-optic bandwidth is about 78 GHz (from 0.3 GHz), which meets the bandwidth requirement of the desired OEO system.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs the key component in the system, a tunable MRR plays a decisive role in the spectral response of the OEO. The transmission spectrum of the high-Q MRR measured by an high resolution optical spectrum analyzer (OSA, APEX A2687) is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (b), which allowed us to extract 3 dB bandwidth of 12 pm, corresponding to a loaded Q-factor of 1.3\u0026times;10\u003csup\u003e5\u003c/sup\u003e. The MRR\u0026rsquo;s Q-factor is affected by the close proximity of the heater electrode and the rough waveguide sidewall which can be further improved in the future. The measured FSR of the MRR is 87 GHz, providing sufficient spectral range for microwave signals generation by the OEO system. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e (c) shows the spectrum tuning of the MRR by applying different electrical power to the micro-heater. The resonant wavelength of the high-Q MRR redshifts, when the voltage applied to the Ti micro-heater is increased from 0 V to 15 V, exhibiting the desired broadband tunability.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFrequency-tunable MBPF\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe performance of the on-chip microwave photonic bandpass filter is evaluated, using the experimental setup shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. A CW light with a power of 13 dBm generated by an external tunable laser (TL, Santec TSL-570) is sent to the polarization controller (PC, Thorlabs-FPC561) and then routed to the chip. The vector network analyzer (VNA, Keysight N4372E) provides a frequency swept RF signal with a power of -5 dBm to the PM, by using a high-speed microwave GSG probe (T-PLUS 110 GHz). A 50 Ω matched resistor is used to reduce the reflection of microwave signals at the end of the PM. The recovered RF signals from the lightwave component analyzer (LCA, Keysight N4372E) is sent back to the VNA to acquire the frequency response of the integrated MBPF.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn order to verify the frequency tunability, the center frequency of the notch band of the filter can be continuously tuned by applying various power to the micro-heater generated by the multichannel power supply (MPS, GWINSTEK GDP-3303S), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a). With the voltage applied to the micro-heater is varied, the center frequency is continuously ranged, while the rejection ratio of the filter does not change with frequency to remain at 10 dB. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (b) depicts the stability of the filter\u0026rsquo;s 3 dB bandwidth with the change in frequency, and multiple measurements result in the filter with a 3 dB bandwidth of approximately 3.1 GHz.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eIntegrated tunable FDML OEO\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe experimental setup for measuring the performance of the FDML OEO is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The optical signal and electrical signal are amplified by an erbium-doped fiber amplifier (EDFA, KEOPSYS CEFA-C-PB-LP-SM-23-MSA1-B201-FA-FA) and an electrical amplifier (EA, SHF S807C) in order to achieve stable single mode oscillations in the experiment. The frequency of the generated signal can be tuned by varying the voltage applied to the micro-heater by an arbitrary waveform function generator (AFG, Tektronix 3102C). In the OEO system, the amplified signal is equally divided into two paths by an electrical coupler (EC). In the one path, the single is guided to the input of the PM to realize the OEO loop. The other path routes the signal to an electrical spectrum analyzer (ESA, Keysight N9030B) for real-time monitoring and analysis of the signals by the OEO.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFrequency tunability of the integrated OEO is demonstrated as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (a), the superimposed spectrums of the generated microwave signal, which shows a wideband frequency range from 3 to 42.5 GHz is obtained, covering S, C, X, K\u003csub\u003eu\u003c/sub\u003e, K, and K\u003csub\u003ea\u003c/sub\u003e bands. We can also see that the amplitude of the signal is lower at higher frequencies, which can be attributed to the frequency dependent attenuation of the OEO system cables and instrumentations. The tuning range of the OEO is limited by the FSR of the high-Q MRR and the gain provided by the amplifier in the loop. A wider frequency tuning range of more than 60 GHz is possible by using a MRR with a wider FSR and cascade multiple amplifiers to provide ample gain. We also demonstrated the side mode suppression ratio (SMSR) of the microwave signals generated by the OEO as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (b), a satisfactory SMSR of 48 dB is achieved at an oscillation frequency of 8.8 GHz. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (c) shows the generated microwave waveform in the time domain, which is measured by high-speed real-time oscilloscope (OSC, Keysight UXR0594AP) with a sampling rate of 256 GSa/s. The inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (c) show a real-time capture of a section of the generated waveform.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (d) displays the RF spectrum of the OEO signal at around 23.76 GHz with a span of 20 MHz, and it can be concluded that the separation between two adjacent modes is 4.27 MHz, which is determined by the length of the OEO experimental link (see Note S4, Supporting Information) and also an essential indicator of the formation of the FDML. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (e) and (f) show the phase noise with a frequency tuning step of 5 GHz from 15 GHz to 35 GHz and the measured phase noise at a 10 KHz offset frequency of different microwave signal frequencies, respectively. It is clear to see that the phase noise values are maintained around \u0026minus;\u0026thinsp;93 dBc/Hz at the offset frequency of 10 KHz, which verifies the most important advantage of the OEO to have a stable phase noise with different oscillation frequencies. The phase noise is mainly determined by the Q-factor, which can be further improved by increasing the Q-factor of the MRR. Additionally, according to the Yao-Maleki phase noise model, the use of kilometer-scale fiber-optic coils in the OEO loop can significantly improve the phase noise of the generated RF signal (see Note S5, Supporting Information).\u003csup\u003e[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eA stable time-frequency signal is generated when the AFG applies a periodic drive signal to control the MBPF to synchronize its frequency tuning period or a multiple thereof with the round-trip time of the OEO loop, which is the key to formation of the FDML OEO. By varying the waveform function of the AFG output voltage, the type of signal generated can be controlled. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, we demonstrate high-quality time-frequency waveform generation based on the FDML OEO with examples of LCMW, quadratic-chirp signal, and triangle waveform. The duration and peak value of the applied voltage function are contingent upon the performance of the AFG, which consequently affects the bandwidth and period of the generated signal.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo intuitively demonstrate the flexibility and reconfigurability of the device, images of various types of signals generated by the FDML OEO measured with real-time OSC are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. Designing and varying the output voltage function of the AFG to modify the waveform, center frequency, and the frequency scanning range of the generated signal. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (a) shows the generated chirp signals with center frequencies from 12 to 30.5 GHz, which exhibits a favorable linear shape. The fluctuations that appear in the figure are possibly due to the change in the link state during the experiments. The quadratic chirp signals with center frequencies from 14 to 36 GHz are illustrated by Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (b). Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (c) shows the real-time frequency distribution of triangle wave signals, which center frequencies from 4 to 18.5 GHz. The applied periodic voltages could not meet the experimental requirements to obtain satisfactory results limited by the AFG performance. However, this problem can be solved by using an AFG with a longer voltage cycle time and higher peak voltage.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Discussion","content":"\u003cp\u003eIn this work, the first integrated tunable FDML OEO in the LNOI platform is proposed and experimentally demonstrated, achieving a high compactness, wideband tunability, and reconfigurability. The system is designed and fabricated with a tunable range from 3 GHz to 42.5 GHz, due to the design of the phase modulator diminishes the V\u003csub\u003eπ\u003c/sub\u003e. The phase noise is maintained at -93 dBc/Hz at 10 KHz in all realizable bands. Importantly, we demonstrate a wide range of proven time-frequency generating capabilities that meet the demand for high-frequency, integrated, flexible, and tunable microwave sources for the 6G era.\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 previous FDML OEOs\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" 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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRefs.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePlatform\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eIntegrated Components\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTuning range\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePhase noise\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eECL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eGST\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[24]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.5\u0026thinsp;~\u0026thinsp;7.5 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-104 dBc/Hz at 10 KHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2800 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eFMCW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[25]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.4.~15 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1030 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLCMW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[26]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026thinsp;~\u0026thinsp;18 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-134 dBc/Hz at 10 KHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4500 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLCMW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[27]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8.3\u0026thinsp;~\u0026thinsp;11.7 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4500 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eDual-chirp\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[28]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4\u0026thinsp;~\u0026thinsp;19 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4500 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLCMW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[29]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15\u0026thinsp;~\u0026thinsp;18 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-113.89 dBc/Hz at 10 KHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4000 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLCMW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[30]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiscrete\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.3\u0026thinsp;~\u0026thinsp;12.7 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-89.5 dBc/Hz at 10 KHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ePhase-coded\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[31]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSOI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMRR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4\u0026thinsp;~\u0026thinsp;19 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e16730 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLCMW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e[32]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSOI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMRR\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.2\u0026thinsp;~\u0026thinsp;13.2 GHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e11500 m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLCMW\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eThis work\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eLNOI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003ePM\u0026thinsp;+\u0026thinsp;MRR\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e3\u0026thinsp;~\u0026thinsp;42.5 GHz\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e-93 dBc/Hz at 10 KHz\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e46.8 m\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003eLCMW, Q-chirp, Triangle\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003e*ECL: Equivalent circuit length; GST: Generated signal types; FMCW: Linear frequency-modulated continuous wave; Q-chirp: quadratic-chirp\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eComparison of our work with the previously reported FDML OEOs is shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, where one can see that we demonstrated a record-breaking tuning range. Notably, from the preceding section the adjacent mode spacing of the OEO is 4.27 MHz, which corresponds to an equivalent circuit length of approximately 46.8 m, mostly from the optic fiber patch cords, EDFA, and PD devices used. For fair comparison, the length of each OEO fiber loop is also provided in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Although our FDML OEO does not reach the level of classical discrete systems in terms of phase noise metrics, according to the Yao-Maleki phase noise model it is known that the phase noise of an OEO is related to the length of the fiber used, and if we use a 4500 m fiber in the loop, the phase noise can be theoretically drop to -132.66 dBc/Hz at 10 KHz.\u003csup\u003e[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn summary, we demonstrate an integrated tunable FDML OEO in LNOI with ultra-wideband tunable range, through on chip PM and high-Q MRR and the size of the integrated chip is only 9 mm\u0026times;1 mm. An unprecedented frequency tuning range from 3 to 42.5 GHz is achieved, and the phase noise is maintained around \u0026minus;\u0026thinsp;93 dBc/Hz at 10 KHz. Furthermore, multiple time-frequency signal generation with center frequencies up to the millimeter-wave levels also be achieved, including LCMW, quadratic-chirp signal, and triangle waveform signals. The entire system\u0026rsquo;s total insertion loss is 15.38 dB including coupling loss (see Note S6, Supporting Information).\u003c/p\u003e \u003cp\u003eIt is expected that by hybrid integrating the laser, amplifiers, and photodetector onto the LNOI chip, the phase noise can be further reduced, which can meet the requirements for practical applications in the 6G era. \u003csup\u003e[\u003cspan additionalcitationids=\"CR50 CR51\" citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]\u003c/sup\u003e The demonstrated compact OEO with ultra-wideband tunable ranges in this work can already meet the urgent needs of the millimeter-wave applications including high-precision millimeter-wave radar for autonomous driving, wireless transmission with large communication capacity, unmanned autonomous driving systems, and electronic warfare systems.\u003c/p\u003e"},{"header":"5. Methods","content":"\u003cp\u003e \u003cb\u003eFabrication of photonic chip\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe proposed integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator is fabricated by electron beam lithography (EBL) and inductively coupled plasma (ICP) etching processes. We procured the wafers from NanoLN and specifically opted X-cut LNOI, to harness the superior electro-optical tensor component γ\u003csub\u003e33\u003c/sub\u003e of LN, thereby enhancing the performance of Y-propagating modulators. A Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e thin film is deposited onto the surface of LNOI through reactive sputtering. Then, electron beam lithography (EBL) is used to pattern the resist, and inductively coupled plasma (ICP) etching is used to form waveguide and grating coupler structures. The electrodes are subsequently fabricated through precise techniques, including direct laser writing, electron beam evaporation deposition, and lift-off processes.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eData Availability Statement\u003c/h2\u003e\n\u003cp\u003eAll the data supporting the findings in this study are available in the paper and Supplementary Information. Additional data related to this paper are available from the corresponding authors upon request.\u003c/p\u003e\n\u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by National Natural Science Foundation of China (NSFC) (62075091, 62205135), Key Research and Development of Gansu Province (22YF7GA008, 23YFGA0007), Natural Science Foundation of Gansu Province (23JRRA1026, 22JR5RA493).\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eZ. H. and Y. T. conceptualized this research., Z. H., L. W. and P. Z. performed the simulation and chip layout of the device, M. X, G. R. and A. B. contributed to the fabrication. Z. H., Y. Z., P. Z., Y. J, and X. Z. assisted in static and high-speed measurement. Z. H. wrote the original draft. All the authors reviewed and revised the manuscript, and Y. T. supervised the full project.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe authors acknowledge the facilities, and the scientific and technical assistance, of the Micro Nano Research Facility (MNRF) and the Australian Microscopy \u0026amp; Microanalysis Research Facility at RMIT University. The research was in part carried out at the RMIT Micro Nano Research Facility (MNRF) in the Victorian Node of the Australian National Fabrication Facility (ANFF-Vic). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHecht J (2016) The bandwidth bottleneck. Nature 526:139\u0026ndash;142\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGhelfi P, Laghezza F, Scotti F et al (2014) A fully photonics-based coherent radar system. 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Opt Lett 47:4564\u0026ndash;4567\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"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":"Lithium niobate on insulator (LNOI), Integrated microwave photonics (IMWP), Fourier domain mode-locked (FDML), Optoelectronic oscillator (OEO)","lastPublishedDoi":"10.21203/rs.3.rs-4743222/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4743222/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFourier domain mode-locked optoelectronic oscillator (FDML OEO) is a crucial component for the upcoming sixth-generation (6G) communication era, as it can break the limitation of mode building time in the conventional OEO and generate high-quality frequency-tunable microwave signals or waveform such as linearly chirped microwave waveform (LCMW) for millimeter-wave applications thanks to its ultra-low phase noise. However, most FDML OEOs reported thus far are discrete and their operating bandwidth are limited, which makes it difficult to meet the real applications\u0026rsquo; requirements. Here, we propose and demonstrate the first integrated tunable FDML OEO with the tunable frequency range of 3-42.5 GHz in the lithium niobate on insulator (LNOI) photonic integrated circuit platform. As examples, we demonstrate the generation of LCMW, quadratic-chirp signal, and triangle waveform with the center frequency covering millimeter-wave band using the proposed FDML OEO and the phase noise can be maintained as low as -93dBc/Hz at 10 KHz. The FDML OEO provides a promising solution for the compact and effective signal generation solution, which breaks the bandwidth limitations and facilitates the realization of extensive applications in the field of radio frequency (RF), including high-precision microwave photonic radar, next-generation wireless communication, and unmanned autonomous driving systems.\u003c/p\u003e","manuscriptTitle":"Integrated ultra-wideband tunable Fourier domain mode-locked optoelectronic oscillator","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-24 08:27:11","doi":"10.21203/rs.3.rs-4743222/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":"f1d8b3d1-f13b-42a6-bf47-89f853381039","owner":[],"postedDate":"July 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":35007543,"name":"Physical sciences/Optics and photonics/Applied optics/Microwave photonics"},{"id":35007544,"name":"Physical sciences/Optics and photonics/Applied optics/Integrated optics"}],"tags":[],"updatedAt":"2024-11-20T19:20:32+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-24 08:27:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4743222","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4743222","identity":"rs-4743222","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}