Improvement of diode lasers by better understanding the influence of external feedback
High-power frequency-stabilized laser diodes such as distributed Bragg reflector (DBR) tapered diode lasers enable a steadily growing number of applications and scientific experiments. Advances in the fields of integrated surface gratings and chip technology in the last years made further improvements possible. Meanwhile, these devices emit several watts of diffraction-limited output power and simultaneously feature a narrow spectral linewidth of < 75 fm (20 MHz) at an emission wavelength of 1067 nm. These specifications pave DBR tapered diode lasers the way for an even wider field of more demanding applications. However, even low and often unavoidable external feedback can disturb the emission properties of the diode laser.
Unavoidable external optical feedback is a common problem in optical setups. The influence of external optical feedback on the emission properties of a laser can vary from small changes in emission wavelength, linewidth and output power to coherence collapse and device damage. These effects have been studied thoroughly for low-power laser diodes and with this study for the first time for high-power frequency-stabilized laser diodes. The measurement setup shown in Fig. 1 was realized at the FBH in cooperation with the Technical University of Denmark to study and quantify the effects of external feedback on DBR tapered diode lasers. This work is supported by the European Commission within MIB project (667933-2).
By moving the external feedback mirror along the optical axis, the feedback phase φ can be changed. The alteration of the emission wavelength in dependence of the feedback phase is shown in Fig. 2 for four different feedback powers. At very low feedback power ratios of µAT = -105 dB the DBR tapered laser is undisturbed and shows a random wavelength shift < 200 fm. With increasing feedback power ratio µAT = -85 dB the emitted frequency shifts mode jump free with the feedback phase. Further increase of the feedback power leads to mode jumps if the feedback phase is changed, as can be seen at µAT = -75 dB. The results are in good agreement with the theoretical prediction (black line) of the Lang Kobayashi equations for low feedback power ratios. In Fig. 3 a) the maximum frequency shift and b) the mode spacing is plotted over the feedback power ratios. The latter is in good agreement for the complete measured feedback power ratio range with the theoretical predictions and the maximum frequency shift up to a feedback power ratio of µAT = -65 dB. Even at feedback power ratios of µAT = -45 dB the maximum measured wavelength shift is well below 10 pm for the tested DBR tapered diode laser.
An investigation based on this measurement method was presented at Photonics West 2019 . In the paper, the influence of external feedback on a DFB laser is used as a reference and compared to previous publications in this field. Furthermore, the influence of low-power external feedback on the emission properties of a DBR tapered diode is presented.
The developed measurement setup is a necessary and powerful tool to study and quantify the effects of external feedback on high-power diode lasers. Based on these measurements, a new generation of high-power diode lasers with reduced sensitivity to external feedback could be developed.
 C. Zink, M. Christensen, M. T. Jamal, A. K. Hansen, M. Maiwald, O. B. Jensen, B. Sumpf, G. Tränkle, "Investigation of controlled external feedback on the properties of low and high-power frequency stabilized diode lasers", Proc. SPIE 10939, Photonics West, San Francisco, USA, 10939J (2019).