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An understanding of the origins of noise is required for a receiver's performance to be accurately characterized. The amount of noise present in a receiver will be the primary factor that determines the receiver's sensitivity. In this chapter, we will first review the definitions and analysis techniques needed to understand the effects of noise on a receiver's performance. The noise sources that are commonly found in an optical receiver are then discussed, including noises that are of optical as well as electrical origin. Our goal is to develop equivalent circuit models that will accurately describe the noise performance of an optical receiver. Once we have established an acceptable model, we will derive expressions for the noise-density and total noise present in the receiver.
5.1 Fundamentals of Noise Analysis
Noise, defined in the broadest practical terms, is any signal present in the receiver other than the desired signal. Another widely used definition is that noise is any unwanted disturbance that masks, corrupts, reduces the information content of, or interferes with the desired signal. We note an important exception to these definitions. Distortion effects that arise within the receiver circuitry are not included in our definition of noise. Receiver distortion is a circuit design problem, not a noise problem.
The sources of noise present in a receiver circuit can be broadly divided into two classes. The first are intrinsic noise sources arising from fundamental physical effects in the optoelectronic and electronic devices used to construct the receiver. The second are coupled noise sources arising from interactions between the receiver circuitry and the surrounding environment. Examples of intrinsic noise sources are the thermal-noise found in resistors, electronic shot-noise and thermal-noise in transistors, and the quantum shot-noise inherent in photodetection. These noise sources are found in all optical receivers. Coupled noise originates in solar, cosmic, or atmospheric disturbances, nearby electrical transmission lines, power supplies, fast switching logic circuitry, etc. Coupled noise that degrades receiver performance is often termed interference or circuit cross-talk. The amount of coupled noise present in a receiver is dependent on the exact physical location and orientation of the circuitry with respect to the surrounding electromagnetic fields. In a well designed receiver with adequate shielding, coupled noise can usually be made to be negligible when compared to intrinsic noise [1].
There is also a type of noise-like disturbance, called microphonics, that can arise from mechanical vibrations causing small changes in receiver components that have a critical mechanical alignment. Most optical receivers are subject to some amount of mechanical vibrations, whether from power supply cooling fans, nearby road-traffic, or motion of a vehicle containing the receiver. Microphonics can occur in an optical receiver where there is a tuned circuit whose resonant frequency is sensitive to a mechanical spacing. As the receiver vibrates, the resonant frequency varies and the receiver response becomes time varying. Microphonics can also occur if there are substantial co-aligned reflections from both the photodetector surface and from the incoming fiber. This allows a resonant cavity similar to a Fabry-Perot interferometer to form [2].
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