TABLE OF CONTENTS We saw in Chap. 1 that many silicon photodiodes exhibit detection performance in their region of best sensitivity within about 30 percent of the ideal responsivity and limited by fundamental quantum processes r( A/W) = 0.807 ?( m). This quantum limited performance does not mean, however, that detectors do everything we want, or even less that detection systems based on them are perfect. The detector might be operating as well as possible but still not well enough. All that really counts for your system is the final measurement signal-to-noise ratio (S/N). We have spent some time looking at the signal (photocurrent), how to maximize it, and how to measure it in a useful bandwidth. Now we must look at the fundamental sources of noise. All detection systems are limited, at least, by shot noise and thermal (or Johnson) noise.
