Abstract

Recent theoretical calculations predict that spectral hole-burning will cause single-longitudinal-mode (SLM) lasers to have more strongly damped relaxation resonances and smaller modulation bandwidths than multiple-longitudinal-mode (MLM) lasers of similar cavity structure.¹ A typical 100-μm long VPR-BH laser² emits two to three longitudinal modes for output powers of 4-9 mW. Using a short-external-cavity configuration consisting of a 70% reflector positioned near the front facet, the emission spectrum of a VPR-BH laser was stabilized in a single longitudinal mode with a mode suppression ratio of >17 dB. The small signal frequency response for reflector-stabilized SLM operation was measured with an automatic network analyzer and compared to the frequency response of the same laser without an external reflector. As shown In Fig. 1, stabilizing the output in a single longitudinal mode suppresses the relaxation resonance and reduces the 3-dB modulation bandwidth from 16 to 14.5 GHz at a bias power of 8 mW. The feedback from the external mirror has a negligible effect on the laser photon lifetime and relaxation resonance frequency,³ so that the damped frequency response under SLM operation can be attributed entirely to the effect of nonlinear gain associated with spectral hole-burning.⁴ The external cavity technique, therefore, allows direct comparison between the effects of nonlinear gain In SLM and MLM lasers of identical structure.

© 1987 Optical Society of America