Amplitude Spectral Density from a Fast Fourier Transformation
The noise equivalent power (NEP) is a metric which expresses the minimum detectable power per square root bandwidth of a given detector. In other words, NEP is the input signal power that results in a signal-to-noise ratio SNR = 1 per a 1 Hz output bandwidth, the bandwidth here is a signal power bandwidth. The product of NEP and SNR for photonic systems is the total signal power per bandwidth.
To measure PSD of both signal and noise you need not provide &quot;a pulsed illumination&quot; in the measurement setup. The light source is 'on' for all the time of measurement with light, and 'off' for all the time of the noise measurement. Temporal dynamics is the result of both optical noise of the light source (photon noise) and electrical noise of the photodetector and the data acquisition circuit as these noises vary in the time domain. In the frequency domain, you measure the spectral distribution of these noises, and not of the light flashes. You need a fast ADC for these measurements; for measurements in the high-frequency end of your noise spectral range (tens or hundreds MHz) you may require an instrument called electrical spectrum analyzer.
For instructions on building a measurement setup, ask the suppliers of your photodetectors and lab instruments. To begin the process, I recommend you to study the guides of optical instrument manufacturers: NEP – Noise Equivalent Power by Thorlabs and Measuring the Noise Equivalent Power of a photodetector by Koheron.