Assessment of a sub-MHz linewidth fiber Bragg grating external-cavity InGaN laser diode

Oxxius narrow-linewidth near-UV laser sources today

This 2021 SPIE paper reports a sub-MHz linewidth InGaN fiber Bragg grating laser delivering single-frequency emission around 400 nm with a side-mode suppression ratio approaching 50 dB — achieved without anti-reflection coating on the diode facet. The fiber Bragg grating, with a bandwidth close to 30 pm, selects a single longitudinal mode from the otherwise multimode InGaN edge-emitting diode, yielding a few mW of spectrally pure output from a compact, robust assembly.

This work is part of a sustained Oxxius research effort on narrow-linewidth near-UV diode laser sources, complementing the parallel JNOG 2021 study on the same FBG external-cavity architecture. The applications motivating this development — Doppler-free spectroscopy, atom cooling and trapping, interferometry, and quantum sensing — all require sub-MHz integrated linewidth and high spectral purity at wavelengths where compact solid-state alternatives are limited. The results presented here validate the FBG external-cavity approach as a manufacturable, low-cost path to coherent near-UV emission.

Why sub-MHz linewidth matters at 400 nm

Achieving sub-MHz linewidth from an InGaN diode laser is non-trivial. The Henry linewidth enhancement factor of GaN-based diodes is typically higher than in conventional III-V materials, making mode control and noise reduction more challenging. The FBG feedback scheme described here addresses this by providing strong, wavelength-selective optical feedback that forces single-mode operation and reduces phase noise — without the mechanical complexity of a diffraction grating external cavity. The resulting source combines the spectral purity of a DPSS laser with the electrical efficiency and wavelength agility of a diode-based architecture. This research directly informs the narrow-linewidth diode laser capabilities available through Oxxius’s Single Frequency Laser product line.

Abstract

Narrow linewidth laser diodes (LDs) emitting in the near-UV (NUV) are gaining attention for applications ranging from spectroscopy to atom cooling and interferometry or other applications requiring high spectral purity. InGaN edge-emitting LDs can exhibit a power of hundreds of mW in an unstable multimode regime detrimental to aforementioned uses. In this paper we report on a compact and robust design based on a low-cost blue LD, a beam shaping optical system and a fiber Bragg grating (FBG) acting as a wavelength selective reflector. One longitudinal mode of the non-antireflection coated laser diode is selected by a close to 30 pm bandwidth FBG allowing a few mW output power around 400 nm and a sidemode- suppression-ratio approaching 50 dB exceeding our last published results. Our previous studies showed that a single-frequency regime with a sub-MHz integrated linewidth and an estimated …