4.7. RECEIVER PERFORMANCE 175
Figure 4.24: BER curves measured for three fiber-link lengths in a 1.55-μm transmission exper-
iment at 10 Gb/s. Inset shows an example of the eye diagram at the receiver. (After Ref. [110];
©c2000 IEEE; reprinted with permission.)
limit of 10 photons/bit. The receiver performance is generally better for shorter wave-
lengths in the region near 0.85μm, where silicon APDs can be used; they perform
satisfactorily with about 400 photons/bit; an experiment in 1976 achieved a sensitivity
of only 187 photons/bit [108]. It is possible to improve the receiver sensitivity by using
coding schemes. A sensitivity of 180 photons/bit was realized in a 1.55-μm system
experiment [109] after 305 km of transmission at 140 Mb/s.
It is possible to isolate the extent of sensitivity degradation occurring as a result
of signal propagation inside the optical fiber. The common procedure is to perform
a separate measurement of the receiver sensitivity by connecting the transmitter and
receiver directly, without the intermediate fiber. Figure 4.24 shows the results of such a
measurement for a 1.55-μm field experiment in which the RZ-format signal consisting
of a pseudorandom bit stream in the form of solitons (sequence length 2^23 −1) was
propagated over more than 2000 km of fiber [110]. In the absence of fiber (0-km
curve), a BER of 10−^9 is realized for− 29 .5 dBm of received power. However, the
launched signal is degraded considerably during transmission, resulting in about a 3-
dB penalty for a 2040-km fiber link. The power penalty increases rapidly with further