Narrow linewidth near-UV InGaN laser diode based on external cavity fiber Bragg grating

Oxxius narrow-linewidth near-UV laser sources today

Published in Optics Letters in 2021, this paper demonstrates a narrow linewidth near-UV InGaN laser diode at 400 nm achieving 16 kHz intrinsic linewidth and sub-MHz integrated linewidth through self-injection locking via a fiber Bragg grating. The narrowband FBG — fabricated specifically for the near-UV range — selects a single longitudinal mode from the otherwise multimode InGaN edge-emitting diode, yielding 44 dB side-mode suppression ratio and mW-level output power in a compact, low-cost assembly requiring no anti-reflection coating on the diode facet.

This Optics Letters paper is the peer-reviewed journal companion to the SPIE 2021 conference paper on the same architecture, providing the detailed frequency noise analysis that confirms sub-MHz integrated linewidth — a result that places this FBG-based diode source in the same coherence class as much more complex external cavity diode laser (ECDL) systems. The self-injection locking mechanism is inherently passive, requiring no active feedback electronics, which significantly simplifies integration into compact instruments.

Applications requiring near-UV coherence

A narrow linewidth near-UV InGaN laser diode with sub-MHz linewidth opens access to a class of measurements previously limited to bulky or expensive laser systems: Doppler-free atomic spectroscopy, interferometric sensing, quantum key distribution, and coherent Rayleigh scattering. At 400 nm, the source is well-suited for exciting transitions in calcium, magnesium, and several molecular species relevant to environmental sensing and fundamental physics. Oxxius’s Single Frequency Laser product line builds on this research to deliver coherent near-UV emission in production-ready, OEM-compatible modules.

Abstract

We realize a fiber Bragg grating InGaN-based laser diode emitting at 400 nm and demonstrate its high coherency. Thanks to the fabrication of a narrowband fiber Bragg grating in the near-UV, we can reach single-mode and single-frequency regimes for the self-injection locked diode. The device exhibits 44 dB side-mode suppression ratio and mW output power. Detailed frequency noise analysis reveals sub-MHz integrated linewidth and 16 kHz intrinsic linewidth. Such a narrow linewidth laser diode in the near-UV domain with a compact and low-cost design could find applications whenever coherency and interferometric resolutions are needed.