Abstract: In optical communication systems, sensitivity is a measure of how weak an input signal can get before
the bit-error ratio (BER) exceeds some specified number. The standards body governing the application
sets this specified BER. For example, SONET specifies that the BER must be 10 -10 or better.
Gigabit Ethernet and Fibre Channel specifications require a BER of 10 -12 or better. This BER is the
foundation for determining a receiver's sensitivity. In the design of an optical receiver, such as a small
form factor optical transceiver module, it is vital that the module be capable of converting and shaping the
optical signal while meeting or surpassing the maximum BER. Ultimately, the influence of noise on the
signal will determine the sensitivity of the system. The portion of the receiver that contributes the most
noise is the optical-to-electrical conversion provided by the photodetector and the transimpedance
amplifier (TIA). More often than not, designers will use a combination of simple estimation and "rules of
thumb" to predict performance and select components. This discussion presents a reliable method for
estimating the receiver's sensitivity.
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