External cavity diode laser in Littrow configuration with narrowband emission at 448 nm for NO₂ gas detection

An external cavity diode laser (ECDL) in Littrow configuration with narrowband emission at 448 nm is presented. This wavelength coincides with a strong absorption cross-section line of the NO₂ gas molecule in the blue spectral region. The ECDL is based on a commercially available multi-longitudinal modes GaN Fabry-Perot laser diode. Longitudinal mode selection is performed using a reflective holographic grating. The Littrow angle is fixed at 43.2 deg relative to the laser diode axis and the diffraction grating is at 3 cm from its output facet. Maximum tuning range of 3.8 nm is achieved with a linewidth of about 0.01 nm at the rated current. The lowest linewidth is obtained with the grating lines oriented parallel to the plane of the p-n junction of the laser diode. The slope efficiency of the ECDL is 0.35 (mW/mA), which is 30% lower than that of the laser diode. The maximum output power achieved by the ECDL is about 40 mW, which is about 75% of that of the laser diode. Maximum power efficiency is also obtained with the polarization direction of the laser diode parallel to the grating grooves. An excellent power stability is achieved over 2 h of continuous operation, with <1% fluctuation.