Towards advanced UV laser sources for quantum and metrology
Leveraging Exail’s expertise in Fiber Bragg Grating (FBG) mirror manufacturing, a research collaboration(1) led by Institut FOTON has unveiled a compact coherent and performant fiber grating laser operating below 400 nm in the single-mode emission regime. Featuring a small footprint compatible with standard butterfly packages, low frequency noise, and fiber-coupled output, this FBG laser source stands as a prime candidate for integration with photonic platforms tailored for quantum information processing and metrology.
Laser diodes are crucial components in modern photonic devices due to their good performance in terms of linewidth, optical power and frequency noise, particularly in the C-band (1550 nm). However, applications such as compact optical atomic clocks and portable quantum sensors require UV-range laser diodes with small footprints and low frequency noise to manipulate atoms or ions. Commercially available UV laser devices usually rely on external cavities with diffraction gratings, which lack the compactness and mechanical robustness needed for use outside controlled laboratory environments.
FBGs offer a promising solution by relocating the external cavity into the optical fiber. This design combines low propagation losses with wavelength-selective mirrors, providing tailored reflectivity and bandwidth adjustments to optimize the power-finesse balance. Exail’s extensive experience in designing and photoinscribing FBG mirrors using its Talbot interferometer has previously demonstrated(2) highly selective and stable FBG filters across a broad wavelength range from 375 nm to 2200 nm.
In a recent breakthrough published in Applied Physics Letters, researchers at Institut FOTON (Lannion, FR) have developed a miniature UV laser diode emitting at 399.6 nm, stabilized by optical feedback from an Exail-manufactured FBG. The external fiber cavity, acting as a spectral filter, greatly enhances the spectral purity of the initially multimode laser. This configuration narrows the linewidth to 14 kHz and delivers milliwatt-level output power, with a side-mode suppression ratio (SMSR) of 40 dB, thanks to the narrowband FBG’s efficiency. By eliminating the need for beam-shaping lenses, the laser’s total footprint was reduced to a compact 0.5 x 2 cm², making it ideal for integration into a butterfly package.
This innovative FBG laser significantly improves linewidth performance, outperforming previous compact external cavity semiconductor lasers by at least two orders of magnitude. Such precision is critical for enhancing the locking performance of compact optical atomic clocks. Additionally, the laser output power, reaching up to 1.8 mW at 399.6 nm, is up to 100 times higher than other compact UV lasers, overcoming the limitations of integrated optics in this spectral range, which often suffer from high propagation and coupling losses.
The combination of compactness, wavelength versatility, and mW-level output power makes this FBG laser an optimal candidate for hybrid integration with photonic platforms designed for quantum information processing.
1 Institut FOTON (Univ. Rennes /CNRS), Oxxius, Exail and Ecole Polytechnique Fédérale de Lausanne (EPFL). This noteworthy work is the result of R&D efforts funded by the Brittany region and Lannion Trégor Communauté, supporting innovation in photonics technologies.
2 A. Congar and al., Narrow linewidth near-UV InGaN laser diode based on external cavity fiber Bragg grating, Opt. Lett. (2021).
References:
Sub-20 kHz low-frequency noise near ultraviolet butt-coupled fiber Bragg grating external cavity laser diode, R. Kervazo and al., Appl. Phys. Lett. 125, 161102 (2024) / https://doi.org/10.1063/5.0235240
