1) The document discusses different types of noise that can affect optical fiber communication systems, including thermal noise, dark current noise, and quantum noise. 2) It provides equations to calculate the mean square value of thermal noise current in a resistor, as well as expressions for shot noise due to dark current and quantum noise on the photocurrent. 3) The document examines noise sources in different components of an optical receiver, such as the photodetector and amplifier. It provides equations to calculate the signal-to-noise ratio for different receiver configurations, including those using p-i-n photodiodes and avalanche photodiodes.