Pump-blocking and intensity noise in semiconductor lasers

Semiconductor lasers are particularly well suited for the implementation of pump-noise suppression since it is possible to drive them with a current the noise of which is well below shot noise. Indeed, if the quantum efficiency of the carrier-to-photon conversion is high enough, the electron statistics of the pumping can be transferred to the light emission, yielding a reduction of the intensity noise in the laser. In this simple picture, the maximal amount of squeezing is equal to the quantum efficiency. However, experimental results on intensity noise reduction by pump-noise suppression are usually above this limit. This discrepancy suggests that additional noise sources must be involved. We have investigated the recently proposed "pump-blocking" effect. If the carrier number rises above its stationary value, more of the injected carriers find occupied quantum states, and therefore less of them are allowed into the active layer. These excess carriers are a source of noise and hinder the pump-noise suppression. They also give rise to a voltage noise across the laser diode that we have experimentally measured.