174 CHAPTER 4. OPTICAL RECEIVERS
Figure 4.23: Measured receiver sensitivities versus the bit rate forp–i–n(circles) and APD (tri-
angles) receivers in transmission experiments near 1.3- and 1.55-μm wavelengths. The quantum
limit of receiver sensitivity is also shown for comparison (solid lines).
4.7 Receiver Performance
The receiver performance is characterized by measuring the BER as a function of the
average optical power received. The average optical power corresponding to a BER
of 10−^9 is a measure of receiver sensitivity. Figure 4.23 shows the receiver sensitivity
measured in various transmission experiments [96]–[107] by sending a long sequence
of pseudorandom bits (typical sequence length 2^15 −1) over a single-mode fiber and
then detecting it by using either ap–i–nor an APD receiver. The experiments were
performed at the 1.3- or 1.55-μm wavelength, and the bit rate varied from 100 MHz
to 10 GHz. The theoretical quantum limit at these two wavelengths is also shown in
Fig. 4.23 by using Eq. (4.5.25). A direct comparison shows that the measured receiver
sensitivities are worse by 20 dB or more compared with the quantum limit. Most of
the degradation is due to the thermal noise that is unavoidable at room temperature
and generally dominates the shot noise. Some degradation is due to fiber dispersion,
which leads to power penalties; sources of such penalties are discussed in the following
chapter.
The dispersion-induced sensitivity degradation depends on both the bit rateBand
the fiber lengthLand increases withBL. This is the reason why the sensitivity degra-
dation from the quantum limit is larger (25–30 dB) for systems operating at high bit
rates. The receiver sensitivity at 10 Gb/s is typically worse than−25 dBm [107]. It
can be improved by 5–6 dB by using APD receivers. In terms of the number of pho-
tons/bit, APD receivers require nearly 1000 photons/bit compared with the quantum