The development of precision laser spectroscopy around 420 nm for gas sensing, atomic clocks and laser cooling is slowed down by the lack of compact narrow linewidth laser sources allowing lab-to-market technology transfer. In the infrared (IR) part of the spectrum, the laser diode technology is mature to address those kind of specifications but for shorter wavelengths there are still technological issues. Commercial blue laser diodes have a wide multimode optical spectrum. To improve the frequency noise performances, the use of an external cavity has been proven to favor single mode behavior. Nevertheless, opto-mechanical instabilities of the external cavity limit the laser linewidth to a few MHz. To overcome this issue, we propose a compact and low-cost all-fiber-based locking setup for frequency noise suppression of a 420 nm external-cavity diode laser. This versatile and compact optical reference allows to reduce the laser frequency noise up to 40 dB associated with a linewidth reduction from 850 kHz to 20 kHz. To our knowledge this is the first demonstration of such a stabilization scheme in this wavelength range. The originality of our work is to point out that actual performances of fiber based photonic components around 420 nm, limit the noise reduction efficiency of such optoelectronic feedback loop scheme. This simple locking scheme might be implemented for a large range of wavelengths and can be integrated on a small footprint for embedded applications requiring narrow linewidth blue laser diodes.
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.
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