Characteristics of intensity noise reduction of optical pulses using variable spectral filters and optical fibers

Noise reduction of optical pulses using variable spectral filters and optic al fibers in the anomalous dispersion region is investigated numerically an d experimentally. In the numerical analysis, quantum noise evolution of the optical pulse is calculated using a quantized nonlinear Schrodinger equati on and linearization approximation. It is shown that when the optical spect rum is in the broadening process, photon-number squeezing can be obtained u sing a band-pass filter. On the other hand, when the optical spectrum is in the narrowing process, a correlated pulse pair is generated using a band-r ejection filter and photon number squeezing can be also observed. In the ex periment, the characteristics of noise reduction of optical pulses are inve stigated using a passively mode-locked fiber laser at 1.55 mum and a variab le spectral filter. Noise distribution of optical pulses changes periodical ly as the fiber input power is changed. Intensity noise of the fiber laser is reduced by as much as 23 dB using the band-pass filter and is slightly r educed below the shot noise level. Using the band-rejection filter, the noi se reduction by the correlated pulse pair is confirmed. The characteristics of the experimental results are almost in agreement with those of the nume rical ones.