Narrow linewidth VCSEL self-injection-locked to a microresonator with a two-fiber coupler

A narrow linewidth vertical-cavity surface-emitting laser (VCSEL) self-injection-locked to a microresonator consisting of a microsphere and a two-fiber coupler is experimentally demonstrated. A relative π/2 phase difference between the fields of two tapered fibers modifies the original rejection bands of the single-tapered fiber-coupled microsphere resonator into passbands without quality-factor (Q-factor) deterioration, which can serve as an external cavity providing wavelength-dependent optical feedback. The microsphere, with a diameter of ~347 μm, is coupled with two tapered fibers that have the same waist diameter of ~3 μm. The full width at half maximum (FWHM) of the passband is 94.28 MHz, corresponding to a Q-factor of 2.12 million. As the VCSEL is self-injection-locked to the microsphere resonator, the instantaneous linewidth is narrowed from 596 kHz to 13.59 kHz. Moreover, the frequency noise of the VCSEL is reduced by more than 30 dB in the frequency range beyond 10 kHz. The self-injection-locked VCSEL, featuring high coherence and low frequency noise, can be utilized as a high-performance laser for high-speed, short-reach fiber-optic communication systems and high-resolution spectroscopy.