The quantum noise limits of a semiconductor laser with dispersive loss are examined theoretically and experimentally. Using optical feedback from Cs vapor as a dispersive loss element we demonstrate almost 6 orders of magnitude reduction in the linewidth, to a 44 Hz level, and 1.9 dB amplitude noise squeezing below the standard quantum noise limit.
The quantum mechanical limits to the fundamental noise performance of semiconductor lasers are reviewed. Recent advances in pushing the laser noise below theses limits are then discussed with emphasis on pump suppression, electronic feedback, and correlation techniques such as optical feedback. It is found that narrow-linewidth semiconductor lasers with sub-shot- noise photon statistics are within the reach of current technology.
Amplitude-squeezed states are generated from a room-temperature semiconductor laser using a combination of pump suppression and dispersive optical feedback. The laser amplitude noise is found to be sensitive to extremely weak feedback levels, of the order of 10-8 of the output power. a reduction of the noise from 2% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL under optimal feedback conditions is obtained. A single mode theory is presented but is found to be inadequate in explaining the measured dependence of the noise reduction on the feedback power. A multimode theory including asymmetrical cross-mode nonlinear gain is proposed to explain this discrepancy.
Optical feedback from an external cavity containing an element of dispersive loss was used to reduce the amplitude noise of a semiconductor laser. At feedback levels of Pfb/Pout approximately equals 10+-2), a maximum amplitude noise reduction of 16 decibel was measured close to threshold but the potiential for reduction was reduced considerably at higher injection currents as the laser noise approached the shot noise limit. In addition, the threshold current decreased and the linewidth was reduced to 10 kilohertz. The relaxation oscillation peak in the amplitude noise spectrum was also found to be dramatically suppressed and we find evidence that the relaxation resonance can be moved to much higher frequencies using optical feedback techniques.
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