Page 125 - Optical Communications Essentials

Photodiodes and Receivers

Photodiodes and Receivers 115

and detection of signals. In electric circuits noise is caused by the spontaneous
fluctuations of current or voltage. Intuitively one can see that if the received
signal level is close to the noise level, it is difficult to get a good reproduction of
the original signal that was sent. As an analogy, if people are talking to one
another in a noisy room, it is hard to understand what others are saying if their
voice levels are only a little bit higher than the background noise level.
The signal-to-noise ratio SNR (also designated by S/N) at the output of an
optical receiver is defined by

SNR = signal power from photocurrent (7.5)
photodetector noise power amplifier noise power+
r
The noises in the receiver arise from the statistical nature of the randomness of
photon-to-electron conversion process and the electronic noise in the receiver
amplification circuitry. Section 7.4.2 describes what these are.
To achieve a high SNR, the numerator in Eq. (7.5) should be maximized
and the denominator should be minimized. Thus, the following conditions
should be met:

1. The photodetector must have a high quantum efficiency to generate a large
signal power.
2. The photodetector and amplifier noises should be kept as low as possible.
For most applications it is the noise currents in the receiver electronics that
determine the minimum optical power that can be detected, since the photodi-
ode quantum efficiency normally is close to its maximum possible value.
The sensitivity of a photodetector in an optical fiber communication system
is describable in terms of the minimum detectable optical power. This is the
optical power necessary to produce a photocurrent equal to the total noise cur-
rent or, equivalently, to yield a SNR of 1.

7.4.2. Noise sources
The main noises associated with pin photodiode receivers are quantum or shot
noise and dark current associated with photodetection, and thermal noise
occurring in the electronics.
Shot noise arises from the statistical nature of the production and collection
of photoelectrons. It has been found that these statistics follow a Poisson
process. Since the fluctuation in the number of photocarriers created is a fun-
damental property of the photodetection process, it sets the lower limit on the
receiver sensitivity when all other conditions are optimized.
The photodiode dark current arises from electrons and holes that are ther-
mally generated at the pn junction of the photodiode. This current continues
to flow through the bias circuit of the device when no light is incident on the pho-
todetector. In an APD these liberated carriers also get accelerated by the electric
field across the device and therefore are multiplied by the avalanche mechanism.

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