A whispering gallery mode resonator (WGMR) is an optical device that can confine light in both spatial and temporal dimensions with total reflection. Owing to its high-Q and small mode-volume characteristics, a WGMR is a powerful and reliable platform for various photonic applications such as microlasers, optomechanics, biosensing, laser stabilization, and nonlinear optics.
The WGMR-based self-injection locking technology uses resonant backscattering inside the resonator to purify the spectrum, which significantly reduces the laser linewidth and has been extensively used for laser stabilization. Because no electronic feedback control is required in this scheme and a WGMR can achieve a Q-factor of over 109 with a micron-scale, the technique can achieve a sub-hertz linewidth with a compact size. In addition, the WGMR with a wide transparency window can be integrated with lasers operating in various wavelength bands to achieve narrow-linewidth lasing.
In a high-Q WGMR, the photon’s lifespan can be prolonged from nanoseconds to microseconds, which improves light-matter interaction, making WGMRs well-suited for the acquisition of ultralow-threshold nonlinear optical effects. Numerous nonlinear effects, including second-harmonic generation, four-wave mixing (FWM), stimulated Raman scattering, and stimulated Brillouin scattering, have been theoretically investigated and experimentally demonstrated in WGMRs. In particular, optical frequency combs (OFCs), namely microcombs, based on four-wave mixing in a WGMR have attracted substantial attention and have been generated in many integrated phonic platforms. However, the conventional generation of microcomb requires narrow-linewidth pumps, which is challenging for on-chip integration. The WGMR-based self-injection locking is a viable alternative scheme for integrated microcomb.
The study introduces important parameters of WGMRs, followed by the recent research progress on self-injection locked laser based on the WGMR, nonlinear optical effects, and high-level integration of microcombs using self-injection locking.
First, four key parameters of WGMRs are summarized, namely resonance wavelength, free spectral range (FSR), quality factor, and mode volume.
WGMRs are excellent candidates for achieving a substantial linewidth reduction owing to their high-Q property. In 2015, researchers presented a 1550-nm semiconductor laser with sub-hertz instantaneous linewidth, which is stabilized by WGMR-based self-injection locking technique. The typical structure of a narrow-linewidth self-injection-locked laser with a high-Q WGMR is presented in Fig. 1. The scheme utilizes resonant Rayleigh scattering in the resonator, due to the internal and surface inhomogeneities. In backscattered light, a fraction of the light is reflected to the laser with the frequency of the selected high-Q WGMR. Owing to the fast optical feedback, a notable depression in laser frequency noise is achieved, which corresponds to the linewidth of the laser. In 2017, researchers proposed a theoretical model for a WGMR-based self-injection locking method, as shown in Fig. 2. The analysis indicated that the degree of linewidth reduction scales with the square of Q. Moreover, a detailed theoretical model illustrated the importance of five structural parameters that may improve linewidth reduction. The five parameters were the backscattering efficiency, the phase delay of feedback light, laser-microresonator frequency detuning, coupling regimes, and the optical path length between the laser and resonator. Self-injection locking is based on the WGMR technology and has been widely employed in lasers from UV to mid-infrared bands, as shown in Table 1. However, the linewidths of a self-injection locked laser in the UV and mid-IR bands require further improvement.
The nonlinear characteristics of high-Q WGMRs have been systematically investigated. The threshold power of the nonlinear effect is proportional to the mode volume and inversely proportional to the square of Q. Therefore, a laser with low output power, ranging from microwatts to milliwatts, can generate nonlinear effects inside a WGMR. Moreover, high-performance photonic applications have been obtained based on an integrated nonlinear WGMR, as shown in Fig. 5. In recent years, FWM-based microcombs have attracted increasing interest, and researchers have demonstrated microcombs on various nonlinear material platforms, such as Si3N4,silica, Hydex, silicon, AlN, SiC, and AlGaAs, with a full list provided in Table 2. Owing to low propagating loss, wide transparency windows, and a CMOS-compatible foundry(CMOS, complementary metal oxide semiconductor), WGMRs based on the Si3N4 platform have been extensively investigated. In addition, WGMRs based on AlGaAs have achieved significant improvements in Q factors in recent years. AlGaAs with high refractive and high nonlinear refractive indices is another ideal platform for obtaining an integrated microcomb.
The generation of microcombs pumped by on-chip lasers is both of paramount importance and full of challenges. The WGMR-based self-injection locking technique can achieve a fully chip-based micro-comb. In this case, the WGMR provides optical feedback to narrow the linewidth of the laser and serves as nonlinear material to generate microcomb. To date, a considerable number of studies have demonstrated fully chip-based OFCs based on laser diodes and high-Q WGMRs, as shown in Fig. 6. To determine the nonlinear dynamics of self-injection locking, researchers have investigated the self-injection locking process by considering nonlinear interactions.
A WGMR with a high Q and small volume is an excellent candidate for an ultranarrow linewidth laser with low-threshold nonlinear effect excitation. Furthermore, a high level of integration of microcombs can be achieved by combining both the self-injection locking method and low-threshold FWM. Finally, further developments can be made in self-injection-locking parameter optimization and Q-factor improvements.
