In my system, I need to measure the amplitudes and phases of four lasers, each of a different, known wavelength, chopped at a 50% duty cycle via TTL at frequencies spaced evenly (or not) around some fundamental frequency f0. The lasers are attenuated by some analyte medium, and the resulting laser power (which is measured by a single detector) tells me what I need to know about the medium.
The signals are recovered by performing I/Q demodulation. The application is similar to the one in this question. The signals' individual frequencies can be controlled by me, but should not be more than 6-7 Hz away from f0.
In single-signal applications of my setup, I've used the Exact Blackmann window, which is known to be good for single-frequency measurements, at least according to National Instruments. My question is, is there a more robust choice for recovering four extremely close signals?
Other details: sample rate is 8192 or 16384 Hz for two seconds, f0 is in the neighborhood of 1230 Hz. Typically I have the frequencies evenly spaced, at f0±6 and f0±2, so they're all 4 Hz apart. The input signal is demodulated and windowed four times, once at each of the frequencies of the lasers. Followup question: could spacing them unevenly help?