Injection locked low noise chip-based silica solitonmicrowave oscillator published in IEEE Journal of Selected Topics in Quantum Electronics

Low-noise microwaves are invaluable for applications spanning from radar to high-speed wireless communications. Deriving microwaves from narrow linewidth lasers via optical frequency division (OFD) constitutes one of the most promising methods to synthesize ultralow-noise microwaves. OFD used to rely upon bulky systems. Recent work has aimed at generating low-noise microwaves from miniaturized systems. Optical frequency mircocombs generated from chip-scale microresonators attract lots of attention in this endeavor. Nevertheless, the noise purification in OFD is limited by the bandwidth of the servo and actuator. Beyond the locking bandwidth (at high offset frequencies), the derived phase noise still follows the free-running microcomb itself. Hence, it is critical to achieve low phase noise for free-running microcombs at high offset frequencies. Recent collaboration work between Tsinghua University (led by Chengying Bao) and KAIST (led by Hansuek Lee), published in the special issue on frequency combs, Wei et al, IEEE J. Sel. Top. Quant. Electron. 30, 2900308, reports on low noise microwave synthesis from an on-chip silica wedge microresonator.

Silica microresonators are well-suited for low noise microwave synthesis, due to its large mode volume and low Kerr nonlinearity. However, the phase noise of free-running soliton microcombs is still largely impacted by fluctuations of spectral center frequency of microcavity solitons. Raman effect of the soliton pulse can shift the center frequency of the pulse towards lower frequencies (longer wavelengths), which is known as soliton self-frequency shift (SSFS). The SSFS is sensitive to pulse width change. Therefore, pump frequency noise can penetrate into phase noise, as pump frequency detuning determines the pulse width. Fortunately, it has been shown that dispersive wave emission can induce spectral recoil and balance the SSFS, making the center frequency change immune to pulse width. This balanced state can be known as a quiet point. Leveraging quiet point, the collaborating group reached a single-sideband phase noise as low as -143 dBc/Hz at an offset frequency of 100 kHz for a 10 GHz microwave signal. This is about 10 dB lower than one of the best electric oscillators, i.e., Rohde & Schwarz SMA100B with B711 low noise option. Moreover, we show that the phase noise at higher offset frequencies still has room for improvement, as it is not limited by the shot noise yet. This is confirmed the measuring the phase noise of the microwave signal using different optical powers. The low phase noise at high offset frequencies can be important when using microcombs to synthesize high frequency microwaves for Radar and wireless communication applications.

The free-running microcomb enjoying quiet point noise quenching still has relatively high phase noise at low offset frequencies. The authors then suppressed the low-offset phase noise by phase modulating the pump for injection locking. Previous work of injection locking in MgF₂ and Si₃N₄ microcavities has shown that high-offset phase noise can be deteriorated by injection locking, as the modulation signal can have higher noise than the soliton. In the published work, the injection in the silica platform is observed to be fairly weak and does not impact the high-offset phase noise. Although the locking is weak, it improves the long-term stability of the microwave signal over 4 orders of magnitude.

The published work further confirms the great potential in generating ultralow noise microcombs in the silica platform. The injection locked microcomb takes advantage of the microwave signal to improve the long-term stability, while the soliton dynamics purifies the microwave signal at the high-offset phase noise (Fig. 1). In this way, the demonstrated silica soliton microwave signal reduces the integrated timing jitter in short time scale and is less impacted by the long-term drift, which can be useful for high-speed signal processing. More details about the collaboration work can be found in   https: // ieeexplore. ieee.org / document /10587044.

Reference

Image Caption: Illustration of a silica wedge microresonator soliton-based low noise microwave generator. Phase modulation of the pump laser leads to injection locking that suppresses the long-term drift of the solitons (variations in the blue curve), while the short-term noise of the injected microwave signal (fluctuations in the green curve) is purified by the soliton dynamics. Thus, a low noise microwave can be synthesized (yellow curve).