I have just started experimenting again at 461 THz ( 650 nm ):
This is a plot of an SSB carrier after optical detection ( at IF ) and after upconverting to VHF for baseband demodulation.
And this is a plot of an FM carrier, modulation 1750 Hz, deviation 3.5 kHz, after optical detection and upconversion but before baseband demodulation.
The plots were taken during high ambient light conditions, no optical filtering and of a laser carrier scattering off an uneven wall. What you see in these images is the RF spectrum right at the output of the Burr-Brown OPT210 optodetector device, only upconverted to the 144 MHz IF. This is what the actual signal looks like as it goes into the 2 meter multimode rig for demodulation. The carrier visible at the left of the gridded window is the local oscillator carrier leakage through the upconverter mixer. This signal represents the DC spectrum component in the laser modulation baseband.
These spectrum plots were made on an Anritsu MS2661B spectrum analyser. The plots were then saved onto a SRAM PCMCIA card and transported to my laptop for editing. Pretty neat.
Yes, I am already thinking of how to modulate and unleash the tens of milliwatts available from my 632.8 nm HeNe laser....a commercial modulator would be nice, but I guess I'll have to revert to a water tank modulator or something else this crude.
I have also located some PMT ( Photo Multiplier Tubes ) ( 41 kb ), but I cannot locate the data for two of the models and the third model is very narrowly sensitive in the visible blue spectrum only.... According to fellow hams experience, PMT's achieve 10 - 20 dB better SNR ratios compared to semiconductor devices ( 59 kB ) at room temperatures.
Something in Finnish...