Reliable High-Power DBR Laser Diodes at 1178 nm
Laser diodes emitting in the yellow spectral range are key components for applications like laser cooling of sodium atoms and time-resolved fluorescence spectroscopy. However, this yellow spectral region is currently not accessible directly with laser diodes. One solution to realize such sources anyhow is based on high-brilliance laser diodes with a highly strained InGaAs quantum well (QW) emitting at 1178 nm and subsequent non-linear frequency conversion .
Recently, watt-level DBR-tapered laser diodes (Fig. 1) with a highly strained quantum well emitting at 1178 nm were introduced by the FBH . Based on them, reliability and optical output power could be further improved by optimizing the laser structure. The new laser diodes show an optical output power of about 3.5 W, a nearly diffraction-limited beam with an M²1/e² value smaller than 1.5, and a narrow-band single-mode emission, which is indispensable for efficient frequency doubling in a nonlinear crystal.
The power current characteristics up to a taper current of 7 A are shown in Fig. 2. The threshold current was less than 150 mA and the slope efficiency around 0.7 W/A. The low series resistance of less than 100 mOhm resulted in a record conversion efficiency of more than 45% at 2.5 W output power. The spectrum at 2.5 W shows a single peak at 1178.0 nm with a side-mode suppression of more than 30 dB.
At the previously demonstrated power level of 1 W the improved lasers show no degradation. Thus, a constant power step-stress test was performed to estimate the reliability of the new design. The optical output power was then successively increased to 2.5 W, before degradation behavior became visible (see Fig. 3). This result demonstrates that stable operation of 2 W to 2.5 W can be maintained for more than 5,000 h.
These lasers will allow the construction of rugged, miniaturized yellow-emitting laser modules with an output power of several hundred milliwatts, suitable for biomedical applications and fundamental research.
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 K. Paschke, G. Blume, O. Brox, F. Bugge, J. Fricke, D. Feise, J. Hofmann, H. Wenzel, and G. Erbert, "Watt-level continuous-wave diode lasers at 1180 nm with high spectral brightness", Proc. SPIE 9348, Photonics West, San Francisco, USA, Feb. 07-12, 93480X (2015).