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

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 intrinsic linewidth of 16 kHz was possible by a carefully engineered external cavity. We will study the influence of the cavity length with different fiber types (SM or PM). Assessment will focus on the linewidth and a detailed intensity and frequency noise analysis of the emission. We will also investigate for the first-time the stability of several types of UV-FBG submitted to tens of mW of 400 nm light guided into the fiber core. This work demonstrates state-of-the-art performances by connecting low-cost components and opens the way to the fabrication of highly coherent laser sources that could meet the markets for the NUV applications.