I am currently using a Photo-diode to attempt to view the absorption of Rb at about 780nm. I'm using a signal generator to oscillate the frequency of the laser so that it scans over the resonances of the atoms. I've placed a photo-diode after the cell so that I can see the absorption, however I'm getting a constant signal in my scope.
I'm definitly catching the beam in the diode, as the signal in my scope drops to zero when I block the beam. I'm definitly crossing resonance, as I can see it in an IR viewer. I don't believe I'm saturating the diode, since I found the saturation point, and turned the power down significantly.
My circuit is very simple, and is set up as attached. Does anyone have any idea what I might be doing wrong? thanks in advance

 

If you have a constant level when illuminated and a definite drop to zero in the dark, it really sounds like the photodiode is saturated, or else the associated amp may have too much gain.

What happens when you block most of the illumination?

 

I've tried blocking out almost all the power, but I'm still not seeing anything. I am not actually using an amplifier right now, so that can't be the problem. Thanks though

 

I would want to do some serious control of the laser illumination. Those critters produce lots of photons. It sounds like saturation of the photodiode.

For a cheapie device to try, check TAOS (Texas Advanced Optical Sensors) for their line of light to voltage converters. Mouser carries them, and for just over a buck you can get one in TO-92 that has response from IR to UV. I don't have my catalog, but I think response is below 780 nm.