Sub-100-Hz DFB Laser Injection-Locked to PM Fiber Ring Cavity

Low-noise lasers are invaluable tools in applications ranging from precision spectroscopy and displacement measurements to the development of advanced optical atomic clocks. Although all these applications benefit from reduced frequency noise, some also demand a low-cost and robust design. In this work, we introduce a novel and practical concept of a simple, single-frequency laser that utilizes a ring fiber cavity for self-injection locking of a standard semiconductor distributed feedback (DFB) laser. Contrary to previously reported solutions, our laser configuration is fully spliced from polarization-maintaining (PM) single-mode optical fiber components. This results in an adjustment- and maintenance-free narrow-band laser operation with significantly enhanced stability against environmental noise. Importantly, continuous-wave (CW) single-frequency laser operation is achieved through self-injection locking, while the low-bandwidth active optoelectronic feedback serves exclusively to maintain this regime. Operating with output powers of ∼8 mW, the proposed fiber configuration narrows the natural Lorentzian linewidth of the emitted light to ∼ 75 Hz and ensures phase and intensity noise levels of less than – 120 dBc/Hz (> 10 kHz) and – 140 dBc/Hz (> 30 kHz), respectively. Furthermore, the thermally stabilized laser exhibits a frequency drift of less than ∼ 0.5 MHz/min with a maximal frequency walk-off of < 8 MHz. We believe that translating this laser design to integrated photonics in the near future could dramatically lower costs and reduce the footprint in various applications, including ultra-high-capacity fiber and data center networks, atomic clocks, and microwave photonics.