John E. Bowers, Chao Xiang, Warren Jin, Lin Chang, University of California, Santa Barbara; Bohan Li, Lue Wu, Heming Wang, Boqiang Shen, Kerry J. Vahala, California Institute of Technology

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A key ingredient here is the ultra-high-Q silicon nitride (SiN) ring resonators fabricated at a CMOS foundry [1]. The silicon nitride waveguide has a high aspect-ratio SiN core (100 nm thick) surrounded by buried 14.5-um thick thermal oxide cladding and top deposited upper cladding of 2-um thickness. Over 20 h of annealing at 1,150 °C is employed to minimize the residual hydrogen content of the deposited SiN and SiO2 films. The fabricated silicon nitride ring resonators reported here have different FSRs of 5.4 GHz, 10.8 GHz and 30 GHz. Statistic measurement of the entire 200-mm-diameter wafer unveils that wafer-scale high-yield ultralow loss waveguides are achieved. The nominal intrinsic Q factors are over 200 million for 30 GHz ring resonators (with finesse over 42,000), and around 100 million for 5 GHz ring resonators that employ a narrower SiN waveguide core. We first use these ultra-high-Q ring microresonators to reduce the laser linewidth of an InP distributed feedback (DFB) laser. The InP DFB laser has significant amplified spontaneous emission noise and the laser intrinsic linewidth is on the order of 100 kHz. By placing the laser chip and SiN microresonator chip in close proximity, the laser output is butt coupled to the SiN microresonator bus waveguide. Tuning of the laser gain current shifts the lasing wavelength. When the lasing wavelength coincides with one of the ring resonances, intracavity feedback from the ring resonator is fed back into the laser chip, thus locking the DFB laser output and reducing the laser linewidth. This locking process also depends on the relative phase condition between the forward and backward light, thus the air gap between the two chips is precisely controlled. The DFB laser noise is reduced by 50 dB and the white noise floor is down to below 1 Hz2 /Hz for locking with 5.4 GHz FSR ring resonators. By taking the output from the drop port of the microresonator, the minimum fundamental linewidth achieved is 1.2 Hz, more than an order of magnitude better than any previously reported integrated lasers. The 50 dB frequency noise reduction, however, is still limited by the thermorefractive noise of the microresonators. In our most recent work using a spiral resonator with 140 MHz FSR and lower thermorefractive noise, 70 dB noise reduction is achieved, which results in fundamental linewidth as low as 40 mHz [2]. The InP DFB laser provides over 30 mW power in the SiN bus waveguide, far exceeding the parametric oscillation threshold of the ultra-high-Q microresonator. As a result, we also observed mode-locked Kerr comb generation in the microresonators. The comb formation also relies on the laser self-injection locking. The laser-microresonator compound system creates an operation point where dark pulses are created without the requirement for extra dispersion engineering provided by avoided mode crossings. Photodetection of the generated comb signal on a photodetector generates a high-purity microwave beatnote. At an offset frequency of 10 kHz, the phase noise approaches -100 dBc/Hz for a Kerr comb with 10.8 GHz FSR, and is 129 dBc/Hz at 100 kHz. For a Kerr combs with 5.4 GHz FSR, the phase noise is -114 dBc/Hz at 10 kHz and is -140 dBc/Hz at 100 kHz. For the 10.8 GHz FSR microresonator, we also obtained a comb with a repletion rate of 43.2 GHz. The high coherence of the self-injection locked laser pump line is transferred to the comb lines, resulting a multi-wavelength narrow-linewidth laser source. The microwave oscillator based on the pump laser, microcombs generation and photodetection can be further integrated on a same chip, through heterogeneous integration [3]. Our work paves the way for a fully integrated photonics-based microwave oscillator with low noise. The oscillator frequency can be easily increased to > 100 GHz and beyond by reducing the microresonator diameter.