Asymmetric Gain in a Vertical-Cavity Surface-Emitting Laser

Abstract

The gain spectrum of a semiconductor can be locally modified by a strong nearly resonant laser field through population pulsations.1,2 Rayleigh-gain, an asymmetric modification of the gain profile of an atom, has been investigated previously.3,4 Laser diode longitudinal modes have been used to show gain asymmetry1,5, but the mode spacing (75 GHz5) has limited the observation to the periphery of the curve. Mapping the more closely spaced gain peak and dip is important because they can significantly modify the output spectrum of a laser (creating new lasing lines at the new gain peak and extinguishing the original lasing with the gain dip). Pump/probe experiments in traveling wave amplifiers (TWA)6 have shown the gain to be asymmetric, but the gain peak and dip were broadened because the injected signal grew in power (from ≃ 87 μW to 4.4mW6) during propagation. Using a high Q vertical-cavity surface-emitting laser (VCSEL) as the gain medium we have intracavity powers at the injected frequency that do not grow with propagation and are much higher than the TWA experiments (up to 75 mW in a 5 μm spot), pushing the gain peak as far as 36.5 GHz from the injected frequency. Rather than using longitudinal modes to probe discretely the gain asymmetry, when the gain extrema are within 10 GHz of the injected frequency we directly observe localized dips and peaks in the continuous ouput spectrum of the VCSEL. As increased injected power pushes the gain extrema out further, we observe modification of the line shape and pushing of the lasing. We find that the detuning of the gain extrema from the injected frequency increases linearly with the intracavity intensity at the injected frequency (in contrast with relaxation oscillations and other effects that scale with the field level). Understanding local modifications of the cw gain profile is of crucial importance to elucidate instabilities in semiconductors lasers.