Whats the NEP? How is it related to the minimum detectable signal?
The noise-equivalent power (NEP) is used as a figure of merit for the weakest optical signal that can be detected. It is the root mean square of the optical power that will produce a signal-to-noise ratio (SNR) of 1 in a 1-Hz bandwidth. We measure this by first measuring the voltage noise at the output of our detectors. This linear spectral density has units of V•Hz-1/2. We convert this to an equivalent optical noise by dividing by the responsivity (A/W) and the transimpedance gain (V/A). This yields a NEP with units of W•Hz-1/2. The minimum optical power defined as the optical power required to produce a SNR of 1 for the entire measurement bandwidth (B) can be expressed as: As shown in the above relationship, limiting your measurement bandwidth with band-pass filters significantly reduces the amount of noise in your measurement. Often the NEP is frequency-dependent. For example, the NEP for the Model 1801 is 3.3 pW•Hz-1/2 from DC to 10 MHz, and 30 pW•Hz-1/2 from 10 MHz to more than 30
