The performance of an optical ASK receiver is analysed with the consideration of optical channel noise. Three noises are considered: the LO-channel noise, the signal-channel noise, and the shot noise. Their statistical properties and spectral distributions are investigated. Using correlation functions and a conditional probability scheme, we prove that the noises are independent Gaussian distributions. Through spectral analysis, the effect of optical filter bandwidth on the receiver performance is examined. The study reveals that the receiver performance degrades if a wide bandwidth optical filter is employed. In addition, there may exist an optimum amplifier gain to obtain a maximum signal-to-noise ratio when a high gain receiver preamplifier and a wide bandwidth optical filter are employed. The bit error probability is also evaluated. The results show that systems with little laser phase noise are subject to degradation by optical channel noise than those with larger phase noise.