Observation of amplitude squeezing in a constant-current-driven semiconductor laser.
Electromagnetic fields with photon-number fluctuation reduced below the standard quantum limit have been generated in a constant-current\char21{}driven semiconductor laser. The generation is based on a new principle of high impedance suppression for pump-amplitude fluctuation in a highly saturated laser oscillator. The observed noise level is 7.3% (31% after correction for detection quantum efficiency) in power below the standard quantum limit in the entire frequency range between 350 and 450 MHz.