Hybrid integrated narrow linewidth semiconductor laser based on the distributed feedback from an external deformed microcavity

Abstract: Optical microcavities with rotational symmetry have been widely used for narrowing linewidth and reducing frequency noise, however, the narrow but wavelength dependent optical feedback restricts the narrow linewidth laser works only at some discrete wavelength matching the resonance of the microcavity. Here, we demonstrate a narrow linewidth semiconductor laser with continuous wavelength tunability by hybrid integrating a DFB laser chip with a deformed microcavity fabricated on a 220 nm SOI wafer. The deformed microcavity with vortex radius demonstrates the unique characteristics of unidirectional energy storage, wavelength self-adaptivity, and self-focusing of the Rayleigh scattering based distributed feedback. In addition, the strength of Rayleigh scattering is also significantly enhanced by the high numerical aperture silicon waveguide. The optical feedback signal measured by the optical frequency domain reflectometry (OFDR) shows that the deformed microcavity can effectively lengthen the equivalent propagation distance without wavelength dependence. With the wavelength self-adaptive optical feedback from the deformed microcavity, the intrinsic linewidth of a DFB laser diode is narrowed to 525 Hz and the side mode suppression ratio (SMSR) is improved to 76 dB in a maximum allowable continuous wavelength tuning range of 2.25 nm. The frequency noise and relative intensity noise (RIN) are reduced to 2.98 Hz2 /Hz and -148.74 dB/Hz at the offset frequency of 1 MHz, respectively. The work demonstrated here paves a new way for integrated tunable narrow linewidth lasers, which are of crucial importance in high-speed communication and high-precision spectroscopy