Detectors
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The detector must be able to accept highly attenuated power - down in the nanowatt levels. Subsequent stages of the receiver amplify and reshape the electrical signal back into its original shape. Some detector packages have a preamplifier built in to boost the signal immediately.
Detectors operate over a wide range of wavelengths and at speeds that are usually faster than the LEDs and lasers. The most important figure of merit for a detector is its sensitivity:
what is the weakest optical power it can convert without error?
The signal received must be greater than the noise level of the detector. Any detector has a small bit of fluctuating current running through it; this minuscule current is noise - spurious, unwanted current.
The minimum power received by the detector must still be enough to ensure that the detector can clearly distinguish between the signal and the underlying noise. This is expressed as a signal-to-noise ratio or a bit error rate.
SNR is a straightforward comparison of the signal level and the noise level, while bit error rate is a more statistical approach to determining the probability of noise causing a bit to be lost or misinterpreted.
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