Wavelength stabilization and feedback sensitivity of multi-kW diode laser stacks

Abstract

In the first part of this paper, we report on a fiber-coupled wavelength-stabilized high power diode laser system with an optical output power of more than 4 kW at a wavelength of 969 nm. The spectrum is locked over the entire working range with a FWHM of less than 0.5 nm. More than 90% power is encircled within 0.7 nm. The radiation is coupled into a fiber with a core diameter of 1000 µm. Additionally, we show the way for further power scaling up to 8 kW by using diode laser bars that enable higher injection current and output power. In the second part of this paper, we report on the impact of external back-reflections on the diode laser either by long term operation with VBGs or by short term operation in an experimental set-up based on a partially reflective mirror. For our standard set-up with VBG, we do not observe any feedback induced failures or degradation for operating times up to 3,000 hours. However, as expected, we do observe failures with increased feedback. In accordance with the literature, misalignment towards the p-clad can be very critical at wavelengths greater than 960 nm. The feedback-induced failure-rate increases exponentially with the injection current which make high power diode laser bars with high filling factors a very robust solution for external wavelength stabilization.