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The ultra-narrow linewidth diode lasers self-injection locked to high-Q crystalline microresonators are available commercially, providing the linewidth below 1 kHz for various wavelengths and enabling microcomb generation. Here we demonstrate a technique that allows applying this approach for photonic integrated chips containing microresonators and significantly narrow the laser diode (Fabry-Perot or DFB) linewidth due to self-injection locking to the silicon nitride (SiN) microresonator with high Q-factor. Considered laser diodes are CMOS-compatible, as well as integrated microresonators made of silicon nitride. This makes it possible to realize in the future large-scale production of laser devices based on microresonators.
We stabilized the InGaAsP/InP Fabry-Perot (FP) laser diode (Seminex, 100 mW, 1535 nm, 20 nm spectral width) and the DFB laser (Nolatech Company, 1550 nm, output power up to 20 mW) by different silicon nitride microresonators with Q-factor higher than one million (LIGENTECH Company). Microresonators with different free spectral ranges (1 THz, 150 GHz, 35 GHz) allowed observing the different regime of operation, single frequency, and multi-frequency, when different laser diode lines are suppressed. The spectral linewidth of each locked line was better than 20 kHz (limited by Ref. laser).
The developed technique allowed us to integrate different types of laser diodes with high-Q SiN microresonators and developed a fully integrated optical frequency comb source. We measured spectral characteristics (spectral linewidth, phase noises) of free running and locked states, the stabilization coefficient, and the locking range and compare these values to the theoretical estimations. We discuss requirements for the optical frequency comb generation in such systems and demonstrate measured spectra of optical combs. Also, we discuss possible applications of such system, operating in multi-frequency locking or comb regimes, and demonstrate the application for spectroscopy measurements.
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