High-Brightness Laser Diodes with External Feedback

Low-index quantum-barrier single-pass tapered semiconductor optical amplifiers for efficient coherent beam combining

“…From this comparative study it seems that the monolithically integrated Ge-absorber region deteriorated the beam quality of the tapered amplifier. This conclusion contradicts previous results obtained in both broad area lasers [18,26] and in free-standing tapered lasers including absorbing regions located on either side of the stripe, that have proven to improve the beam quality of such devices [19].…”

“…As reported in [19], free-standing tapered lasers with highly reflective rear facets can show improved beam quality and higher η CL when reverse bias absorber sections are included on either side of the ridge section, to suppress intensity in backwards propagating modes. Even when low facet reflectivities are used, the residual backwards propagating field has been predicted to play a role in limiting beam quality [26], especially when it induces bleaching around the ridge waveguide section, hindering its effectiveness as a mode filter. We therefore assess the benefit here of similar absorber sections on the beam quality, resulting coherent combining efficiency and phase noise, monolithically implemented here using highly absorbing Ge-layers that are designed to resonantly absorb any guided mode, using the designs process reported in [27], which has successfully suppressed higher order lateral modes and improved beam quality in high power broad area lasers.…”

“…The RW section is actually critical as its role consists in both pre-amplification of the low-power input signal, and in mode filtering. One known explanation for beam quality degradation effects in tapered lasers is the influence of parasitic round-trip effects in the amplifier leading to a dysfunction of the RW mode filter [24][25][26]. Such parasitic round trip effects can be effectively suppressed by implementing absorber regions on either side of the ridge [16].…”

“…Alternatively, DBR gratings can be implemented in the ridge section (to provide the rear-facet optical feedback for round-trip operation) and then anti-reflection coating applied to the rear facet, which strongly suppresses the parasitic-backwards propagating field. It is proposed that the backwards propagating field remains a significant limit to beam quality in single passtapered amplifiers [26]. In this case, the backwards propagating amplified spontaneous emission (ASE) field and the field reflected at the output facet (AR coating R F = 0.3%) have the potential to excite higher-order modes in the ridge, for degraded overall beam quality.…”