Sub-4 kHz Linewidth Distributed Feedback Lasers at 778.1 nm Wavelength for Two-Photon Rb Spectroscopy

Abstract

Short-wavelength GaAs/AlGaAs-based semiconductor lasers are of interest for emerging quantum technologies such cold atom systems. However, such technologies require high-power, stable single-mode, and narrow linewidth sources at specific target wavelengths, e.g. 780.24 nm for D2 87Rb transitions. Several material properties, such as propagation losses, self-heating, and far-field emission pattern, are critical for laser performance, including maximum power output before damage, coupling efficiency into optical components, and linewidth, in accordance with the Schawlow-Townes formula. In our work, an aluminum-free active area is designed with InGaAsP quaternary compounds and sandwiched between AlGaAs cladding layers. The presence of an asymmetrical mode expander into both p- and n-doped AlGaAs cladding shapes mode profile to achieve a Gaussian-like emission pattern. The asymmetrical mode expander configuration reduces propagation losses, modal confinement within the active region, and decrease the vertical beam divergency. A 3 mm-long phase-shifted laterally-coupled DFB laser is fabricated by reactive ion etching of the reported epilayer material, with weak coupling grating that contributes to narrow the laser linewidth. The AR/HR laser device exhibits single-mode emission at the wavelength of 778.1 nm with a tunability of 0.8 nm, power output of 48 mW, SMSRs exceeding 35dB, and vertical beam divergence of 20.5. Instantaneous Lorentzian linewidth, measured by homodyne technique with automated noise characterisation tool, proved as narrow as 3.67 kHz. The laser is employed to probe the 87Rb two-photon transitions employing a heated Rb cell in retroreflection configuration. Using an acousto-optical modulator for frequency scan, the laser resolved hyperfine levels for the 87Rb two-photon transition.