image: Fig. 1 A. The schematic of the FDML OEO utilizing a 1 km-long single mode fiber (SMF), which is realized by a TFLN photonic integrated chip (PIC) consisting of a Mach-Zehnder modulator (MZM), an electrically tunable add-drop micro-ring resonator (MRR), and two beam splitters. The oscillating RF frequency is given by beating the un-modulated optical carrier from the upper arm and the modulation sideband selected by the MRR at the PD. As the MRR passband is swept by an applying periodic driving signal, the frequency of the generated RF signal also sweeps accordingly to give out the LCMW, with a frequency tuning range equal to half of the MRR's free spectral range (FSR) from FSR/2 to FSR. B. Measured phase noise curves of the 40, 50, 60 and 65 GHz oscillations of the OEO, as compared with that of a 50 GHz signal from a commercial microwave source (Keysight E8257D). C. Real-time frequency distribution of the LCMW signal, showing an unprecedented scanning bandwidth of 30 GHz, with a linearity greater than or equal to 0.9992.
Credit: Xinlun Cai et al.
Linearly chirped microwave (LCMW) signals, whose frequency varies linearly with time, have extensive applications in radar, ranging and sensor systems, enabling precise measurement of target distance and velocity. A large scanning bandwidth and high chirp rate enhance the ranging resolution and measurement range, while high linearity improves signal accuracy, and low phase noise strengthens interference resistance. However, traditional electronically generated LCMW methods, such as voltage-controlled oscillators or digital frequency synthesizers, face challenges in improving frequency and bandwidth due to relatively low speed of electronic devices. Although optically generated LCMW methods offer advantages in high frequency and wide bandwidth, their high-frequency phase noise tends to deteriorate. A Fourier-domain mode-locked optoelectronic oscillator (FDML OEO) is capable of generating LCMW with low phase noise and large TBWP at high frequencies by utilizing low-loss long optical fibers or high-Q optical resonators as energy storage element. The key element to realize the FDML OEO is a fast frequency-scanning filter, which is driven by a periodic signal with a tuning period equal to the round-trip time of oscillation RF signals circulating in the OEO loop. This process will stimulate a large number of longitudinal modes simultaneously with fixed phase relationship in the Fourier domain, to force a periodically repeated and stable chirped oscillation directly in the loop. Several FDML OEOs have been demonstrated in silicon on insulator platforms or silicon-nitride-lithium-niobate platforms with thermal-optic frequency-scanning filters. However, the slow and nonlinear nature of the thermo-optic effect restricts the scanning bandwidth, chirp rate and linearity of the LCMW signals.
“The ultra-fast linear electro-optic effect of thin-film lithium niobate (TFLN) enables an ultra-fast and highly-linear frequency-scanning MRR filter. This approach overcomes the limitations of traditional thermally-tuned MRR filters, including slow tuning and nonlinear frequency response, thereby enabling the generation of LCMW signals with large scanning bandwidth, high linearity, and a high chirp rate," said Rui Ma, a postdoctoral researcher at Sun Yat-sen University, China.
In a new paper published in Light: Science & Applications, a team of researchers, led by Prof. Xinlun Cai from Sun Yat-sen University, China, and Prof. X. Steve Yao from Hebei University, China, and NuVison Photonics, Inc, USA, have demonstrated an FDML OEO fabricated on a TFLN platform deploying an electrically tuned ultrafast frequency-scanning filter, thanks to the high speed Pockels effect in TFLN. Record-breaking high radiofrequency oscillations up to 65 GHz are achieved, with a phase noise more than 14 dB less at 50 GHz than that of a high-performance commercial signal source at an offset frequency 10 kHz away from the carrier. A LCMW with an unprecedented scanning bandwidth of 30 GHz, corresponding to an impressive chirp rate of 5.7 GHz/μs and a large time-bandwidth product of 159054, is successfully generated by the FDML OEO.
Journal
Light Science & Applications
Article Title
V-band ultra-fast tunable thin-film lithium niobate Fourier-domain mode-locked optoelectronic oscillator