Coherent and spectral beam combining in thulium doped fiber laser and amplifier array

Sponsor: Czech Science Foundation

Řešitel: Pavel Honzátko, Ph.D.

From: 2011-01-01

to: 2013-12-31

The aim of the project was investigation of techniques for coherent and spectral combining of beams to achieve high radiance from thulium doped fiber (TDF) lasers. The photodarkening observed in early experiments with TDF lasers  was connected with long wavelength pumping and it seems to be insignificant for pumping at 793 nm used in state-of-art TDF lasers. The main problem is thus thermal management.

Compared to long wavelength pumped YDF lasers, thermal load is much higher for TDF lasers due to higher quantum defect. Nevertheless, it is possible to overcome the heat management problems and increase the TDF source radiance at 1900 nm spectral region beyond the limits of single laser by coherent or spectral beam combining.

Our team was first who reported the coherent beam combination of a pair of thulium-doped fiber lasers performed at moderate powers. We built two constituent laser (Fig.1) providing a power of more than 10 W at a wavelength of 2000 nm with a slope efficiency of more than 0.5.

Fig. 1: Constituent lasers

Then a compound Fox–Smith resonator was created using the PM coupler. When incoherent coupling occurs, output power should be distributed evenly between the output ports, as shown in Fig. 2.

Fig. 2: Incoherent combining                                                Fig. 3: Coherent combining

In our configuration, idler output is terminated with angle-polished connector to suppress the reflection (Fig. 3). The compound resonator has only one output reflector in this way. The obtained laser power was more than 20 W due to a constructive interference at the output of the laser while it was 3% of this value at the idler output where destructive interference occured.  

We believe that beam combining is a possible way how to increase the radiance of the fiber lasers in the future.

1. P. Honzatko, Y. Baravets, F. Todorov, P. Peterka, and M. Becker, “Coherently combined power of 20 W at 2000 nm from a pair of thulium-doped fiber lasers,” Laser Phys. Lett. 10 (2013) 095104, doi:10.1088/1612-2011/10/9/095104.