Watt-class injection-locked diode laser system at 399 nm for atomic physics

TL;DR

This paper addresses the challenge of producing Watt-class, narrow-linewidth blue/near-UV laser light for atomic physics. It demonstrates a seeded, injection-locked multimode diode system in which a seed of $5.5\ \text{mW}$ injects into a $1.2\ \text{W}$ follower to generate light at $398.9\ \text{nm}$ with an inherited linewidth broadened by $3.94(6)\ \text{kHz}$. Active stabilization maintains the injection lock for more than a day, and heterodyne and ytterbium-beam spectroscopy confirm the lock and spectral purity. The approach provides a compact, cost-effective path to Watt-class UV light using readily available components and is adaptable to other wavelengths in the visible/near-UV spectrum.

Abstract

We demonstrate an injection-locked 399 nm laser system with up to 1 W output power and a locked power fraction of 0.57. The system consists of a high power, multimode diode laser that is seeded by 5 mW from a single-mode external cavity diode laser. The locked high-power laser inherits the frequency agility and linewidth of the seed laser with 3.9 kHz broadening. With active stabilization, the injection lock can be maintained for more than a day. We verify the utility of this system for atomic physics by performing spectroscopy of an ytterbium atomic beam.

Figures (4)

• Figure 1: Schematic design of the injection-locked laser system. Faraday isolators (A) and (B) prevent the follower laser from destabilizing the seed laser and prevent back-reflections from the system output from destabilizing the follower laser. An initial aspheric lens (C) with focal length $13.9\ \text{mm}$ collects the highly divergent output of the follower diode. Telescopes (D) and (E) circularize and collimate the beam. A scanning Fabry-Perot interferometer (FPI) and laser spectrum analyzer (LSA) monitor the follower beam and provide feedback to the current controller through a computer program. To stabilize the injection lock, a water-cooled manifold maintains the follower laser at $20\ \text{C}$.
• Figure 2: (A) Normalized power spectra of the seed laser (blue curve) and follower laser (orange, green, and purple curves) measured by the LSA at several follower currents. The unlocked follower spectrum (purple curve) is much broader ($2\ \text{THz}$) than shown. (B) Transmission of the follower laser through the FPI with follower current 1.2 A (blue curve) and 0.45 A (orange curve). (C) Normalized spatial intensity profiles of the follower laser at the system output with 39 mW output power (left) and 1000 mW output power (right). Because the follower is a multimode laser, its profile significantly widens and changes shape as the current is increased.
• Figure 3: Heterodyne characterization of the seed and follower lasers. (A, B) Heterodyne spectra of seed and follower laser, respectively, with a reference laser. Each data set is collected over a time $T_\text{obs}$, and the plotted spectra represent the averages of several such data sets collected during a $1$ s time interval. In (B), the photodiode response to the bare follower beam is subtracted. At this time scale, the seed and follower peaks have coherence linewidths of 8 MHz. (C) Heterodyne spectrum of seed laser with follower laser, obtained by averaging data sets collected during a $10$ s time interval. The peak has a coherence linewidth of $\Gamma_c = 3.94(6)\ \text{kHz}$ and a full width at half maximum $\Gamma_\text{FWHM} = 1.42\ \text{kHz}$. (D) Frequency noise power spectral density of seed-reference heterodyne signal (orange curve), follower-reference heterodyne signal (green curve), and seed-follower heterodyne signal (blue curve).
• Figure 4: Transmission spectroscopy of a cold beam of ytterbium with (A) the seed and (B) the follower lasers. The yellow curves are pseudo-Voigt fits with Gaussian widths and amplitudes shown. The Lorentzian widths are fixed to the literature value of $29\ \text{MHz}$Kroeze2025. The resonances are shifted off their literature values in a manner consistent with a small Doppler shift arising from non-orthogonality between the lasers and atomic beam. Adjusted $R^2$ values for each fit are also shown.