  We present a field programmable gate array (FPGA)-based system that counts
photons from laser-induced fluorescence (LIF) on a laboratory plasma. This is
accomplished with FPGA-based up/down counters that demodulate the data, giving
a background-subtracted LIF signal stream that is updated with a new point as
each laser amplitude modulation cycle completes. We demonstrate using the FPGA
to modulate a laser at 1 MHz and demodulate the resulting LIF data stream. This
data stream is used to calculate an LIF-based measurement sampled at 1 MHz of a
plasma ion fluctuation spectrum.
