Noise reduction of optical pulses using variable spectral filters and optical fibers in the anomalous dispersion region is investigated numerically and experimentally. In the numerical analysis, quantum noise evolution of the optical pulse is calculated using a quantized nonlinear Schrödinger equation and linearization approximation. It is shown that when the optical spectrum is in the broadening process, photon-number squeezing can be obtained using 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-rejection filter and photon number squeezing can be also observed. In the experiment, the characteristics of noise reduction of optical pulses are investigated using a passively mode-locked fiber laser at 1.55 µm and a variable spectral filter. Noise distribution of optical pulses changes periodically 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 reduced below the shot noise level. Using the band-rejection filter, the noise reduction by the correlated pulse pair is confirmed. The characteristics of the experimental results are almost in agreement with those of the numerical ones.

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