4.4. RECEIVER NOISE 155
(a)
(b)Figure 4.15: (a) Epitaxial-layer structure and (b) frequency response of an OEIC receiver mod-
ule made using a waveguide photodetector (WGPD). (After Ref. [73];©c2000 IEEE; reprinted
with permission.)
the photodiode. The fiber ferrule was directly laser welded to the package wall with a
double-ring structure for mechanical stability. The resulting receiver module withstood
shock and vibration tests and had a bandwidth of 10 GHz.
Another hybrid approach makes use of aplanar-lightwave-circuitplatform con-
taining silica waveguides on a silicon substrate. In one experiment, an InP-based OEIC
receiver with two channels was flip-chip bonded to the platform [76]. The resulting
module could detect two 10-Gb/s channels with negligible crosstalk. GaAs ICs have
also been used to fabricate a compact receiver module capable of operating at a bit rate
of 10 Gb/s [77]. By 2000, fully packaged 40-Gb/s receivers were available commer-
cially [79]. For local-loop applications, a low-cost package is needed. Such receivers
operate at lower bit rates but they should be able to perform well over a wide tempera-
ture range extending from−40 to 85◦C.
4.4 Receiver Noise
Optical receivers convert incident optical powerPininto electric current through a pho-
todiode. The relationIp=RPinin Eq. (4.1.1) assumes that such a conversion is noise
free. However, this is not the case even for a perfect receiver. Two fundamental noise
mechanisms, shot noise and thermal noise [80]–[82], lead to fluctuations in the current
even when the incident optical signal has a constant power. The relationIp=RPinstill
holds if we interpretIpas the average current. However, electrical noise induced by
current fluctuations affects the receiver performance. The objective of this section is to
review the noise mechanisms and then discuss the signal-to-nose ratio (SNR) in optical
receivers. Thep–i–nand APD receivers are considered in separate subsections, as the
SNR is also affected by the avalanche gain mechanism in APDs.