Properties of 62x nm red-emitting single-mode diode lasers

Abstract

Single-mode lasers in the spectral region between 620 nm and 630 nm are still realized using complex laser systems, such as ring-dye laser or using non-linear frequency shifted lasers, when used in applications such as laser cooling of beryllium ions or spectroscopy on rare earth elements. Direct emitting AlGaInP based diode lasers offer a much simpler approach to this wavelength range, but so far lack a suitable beam quality and spectral purity. Recently distributed Bragg reflector (DBR) ridge waveguide lasers (RWL) were developed for the 630 nm to 640 nm region. Building on this knowledge CAMFR simulations were performed to find suitable grating periods and duty cycles to obtain emission wavelengths below 630 nm. The grating itself was then introduced by stepper lithography and reactive ion etching into the laser structure. The manufactured DBR-RWLs show laser emission at 628.5 nm and 626.5 nm at a temperature of 15°C with threshold currents below 150 mA. The spectral emission shows single-mode operation with side mode suppression ratios > 20 dB. Two DBR-RWLs with the shorter wavelength were packaged into sealed TO-3 housings to provide a small-sized non-condensing environment with temperatures down to -25°C. When cooled internally to about 0°C, an emitted power of more than 50 mW was measured at a wavelength of 626.0 nm. At this operation point a diffraction-limited single longitudinal mode was observed that allowed a heterodyne measurement where a spectral width below 1 MHz was obtained. These new diode lasers have the potential to drastically miniaturize existing set-ups for quantum information processing.