Quantum noise reduction in semiconductor lasers using dispersive optical feedback

This thesis describes the phase and amplitude noise properties of semiconductor lasers subjected to weak, dispersive optical feedback. In the first half, experiments demonstrating reductions in the laser frequency noise power spectrum and spectral linewidth by several orders of magnitude are presented. Weak optical feedback is applied to the laser by an external cavity containing an atomic vapor. The presence of the vapor adds to the dispersion of the cavity and simultaneously locks the laser to a fixed frequency reference. The role of 1/f frequency noise in limiting the effectiveness of this linewidth reduction technique is investigated and 1/f noise is found to be the dominant contribution to the linewidth under strong optical feedback conditions.;Experimental investigations of the effects of optical feedback on the amplitude squeezing in a semiconductor laser are then discussed. The low frequency squeezing in a room-temperature device is increased from 3% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL with optimal feedback. The experimental results are found to agree poorly with the single-mode model and a multi-mode model including the effects of asymmetrical cross-mode non-linear gain is developed to explain the discrepancy. Finally, amplitude squeezing as large as 29% below the SQL, corresponding to 41% below the SQL at the output facet, is measured from a room temperature commercial semiconductor laser with no external modifications.;An electronic feedback scheme utilizing FM sideband locking is then implemented alongside the optical feedback, and an additional reduction in the low frequency 1/f frequency noise power spectrum by over two orders of magnitude is obtained. With both systems operating simultaneously, the spectral linewidth is narrowed from its free-running value of about 20 MHz to 1.4 kHz. Excellent absolute frequency stability is also achieved. Noise in semiconductor lasers with weak optical feedback is developed. Large reductions in the amplitude noise are expected close to threshold and an enhancement of the amplitude squeezing in a pump-suppressed semiconductor laser at moderate pump rates is predicted. Measurements on a laser biased near threshold are then described and a reduction in the low frequency amplitude noise power spectrum by 7 dB is obtained.