Self-Injection Locked Silica External Cavity Narrow Linewidth Laser

A silica-based external cavity narrow linewidth laser is fabricated and experimentally demonstrated based on a commercial silica platform. An add-drop microring resonator and a 2×2 directional coupler in the silica chip form a wavelength-dependent and self-injection-locked external cavity. The linewidth-suppressing effect of the external cavity is theoretically calculated based on the chirp reduction theory, which shows that a long external cavity length at the resonant wavelength can be used to suppress the noise of a free-running laser. In the self-injection locked state, the measured intrinsic linewidth of the fabricated external cavity laser is 27 Hz, which is greatly reduced compared to the 49.7-kHz linewidth of the free-running state. The frequency stability is also measured to be improved by an order of magnitude from 4.52 × 10 ⁻⁸ to 9.07 × 10 ⁻¹⁰ at 1 s averaging time. The proposed narrow-linewidth laser shows the potential for massive production and can be used in various applications such as integrated microwave photonics, LIDAR, high-resolution metrology, and satellite communications.