Narrow Linewidth DFB Laser Diode Self-Injection Locked to a Packaged Spherical Whispering-Gallery-Mode Resonator in Add-Drop Configuration

A method for narrowing the linewidth of a DFB laser diode by self-injection locking it to a packaged spherical whispering-gallery-mode (WGM) resonator in add-drop configuration is proposed and experimentally demonstrated. The WGM resonator in add-drop configuration employs a silica microsphere with a diameter of 330 μm as the cavity and two tapered optical fibers with waist diameters of 1.8 μm to excite and collect the WGMs, respectively, by which a passband with a full width at half maximum (FWHM) of 1.47 GHz at the drop port can be achieved. By introducing the resonant feedback into the lasing cavity of the DFB laser diode, the instantaneous linewidth is narrowed by two orders of magnitude, i.e., from 116 kHz to 355 Hz. The white noise of the DFB laser diode is suppressed by more than 20 dB for frequencies higher than 10 kHz. In comparison with the weak feedback originating from Rayleigh backscattering in the same spherical WGM resonator, the resonant feedback from the WGM resonator in add-drop configuration is relatively strong, resulting in a higher linewidth narrowing ratio and frequency noise suppression ratio for such a packaged WGM resonator with a moderate quality factor (Q-factor) on the order of 105. The method demonstrated here not only provides an effective solution for narrowing the linewidth of a DFB laser diode but also significantly reduces the stringent requirements on both device fabrication and coupling between the lasing cavity and the external microcavity.