Radio over Fiber for Wireless Communications: From Fundamentals to Advanced Topics

Chapter 4An Improved Expression for Relative Intensity Noise

From the previous chapter, it is obvious that the optical link noise plays an important role in deciding the cumulative SNR of a Fi-Wi link. A directly modulated ROF link is mainly subjected to shot, RIN, and thermal noise. The thermal noise is completely independent of the signal power. The shot noise increases linearly with the mean optical power and is insensitive to the modulation index provided the modulating signal has zero mean. However, the relative intensity noise has a different behavior and needs special attention. That is the focus of this chapter.

The RIN is conventionally considered to be proportional to the square of the mean optical power. This is true under small signal conditions. Nevertheless, many experiments [2, 3, 5–6] have shown that the RIN also increases with the modulation index that reflects the power of the stochastic modulating signal . Way [3] observed this dependency, and mentioned a dynamic RIN under direct modulation conditions.

Therefore, accurate characterization of the RIN is important. ...