Kilohertz linewidth VCSEL with an external high Q-factor intrinsic optical fiber Fabry-Pérot cavity

Importing optical feedback from an external cavity has been widely used for linewidth narrowing in semiconductor lasers. Unlike edge-emitting laser diodes with anti-reflection coating, the reflectivity of the output mirror of a vertical-cavity surface-emitting laser (VCSEL) is usually higher than 99%, meaning that stronger optical feedback is preferred for narrow linewidth operation. Here, we leverage the inherently circularly symmetric optical field distribution and the strong feedback from an external high Q-factor intrinsic optical fiber Fabry-Pérot (FP) cavity to significantly enhance the feedback efficiency and narrow the linewidth of a VCSEL to the level of kilohertz. The FP cavity with a loaded Q-factor on the order of 10⁶ consists of two identical fiber Bragg gratings (FBGs). Experimental results show that the measured linewidth of a VCSEL, extracted from the short-delay self-heterodyne spectrum, can be narrowed from 2.5 MHz to 2.1 kHz under optimized locking conditions by the resonant feedback from an intrinsic FP cavity with a loaded Q-factor of 8.74 × 10⁶. The white frequency noise is reduced to ∼263 Hz²/Hz, corresponding to a calculated intrinsic linewidth of ∼789 Hz. In addition, another FP cavity with an enlarged free spectrum range (FSR) and a loaded Q-factor of 4.8 × 10⁶ is used to narrow the linewidth without the simultaneous excitation of the two polarization modes. The measured linewidth and white frequency noise of the VCSEL with such an external FP cavity are 4.7 kHz and ∼251 Hz²/Hz, respectively. These results provide a proof-of-concept demonstration that a high-Q intrinsic optical fiber FP cavity can serve as an efficient external feedback element for kilohertz-level linewidth narrowing of VCSELs, while further stabilization and packaging are required for practical applications.