Beam modulation (unintentional)

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Updated: 2005-03-28 by HoloM (the god)
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Jonathan

Beam modulation (unintentional)

Post by Jonathan »

Has anyone encountered radio interference problems with their laser diode? I just switched to a constant curent driver and ended up getting radio reception. I was able to actually hear a radio signal, music and all, through the headphones, which I use to monitor beam disturbances.

The beam is diverted just before the shutter, through a Michelson interferometer, to a solar cell. The solar cell is connected to the phono jack of an amplifier, and it provides a very good audio check on when mode hopping is occurring. But this surprised me - getting what appears to be a modulated laser beam from radio interference. Without the headphones and solar cell it would have gone unnoticed.

I understand that shielded wire is needed (I was using plain copper wire to hook up the driver). But is there anything else that can be done?


John K

Beam modulation (unintentional)

Post by John K »

My sugestion would be to, shorten any wire leads, ground where you can, use shelding, use filtering, or move the equipment to another spot if you can. Ferrite beads over over the wires may work. Make sure the signal isn't comming from the amp.
Joe Farina

Beam modulation (unintentional)

Post by Joe Farina »

That's a surprising and interesting discovery.

When I tried (unsuccessfully) to stablize the interferometer fringes from a laser diode by active temperature control, the next thing I was going to try was careful shielding of the system -- but I never got around to it. (I had none originally.)

It seems that the issue of shielding can get rather involved, and require considerable thought. There is a book (that I copied portions of) called "Noise Reduction Techniques in Electronic Systems" (Second Edition, 1988) by Henry W. Ott. (It's over 400 pages long.)

During those periods of RF interference, it would be interesting to know what your interferometer was doing (if you were watching).
Jonathan

Beam modulation (unintentional)

Post by Jonathan »

Thanks for those tips. I did check, and the signal is most definitely getting to the amp via the beam.
Jonathan

Beam modulation (unintentional)

Post by Jonathan »

The beam that hits the solar cell is also the beam that emerges from the interferometer. I usually just monitor the fringes by holding a white card partially in the beam path, and letting the rest hit the solar cell. The throw is several feet from the table so I don't need an expanding lens to see the fringes, and it doesn't overfill the cell.

Doing this shows up the correlation between the audio and visual symptoms of mode hopping, multi-mode behaviour and linewidth fluctuations, etc. For example, discrete mode hops are clicks. Fringe washouts are static sounds, sometimes quite loud.

In regards to the radio signal, the fringe movements did show correlation to the audio reception. The periods of clearest reception, where you could make out the music and words, were accompanied either by partial contrast loss or full fringe washout. In fact surges of both, in and out in unison, were common.

I'm beginning to think that the business of RF interference may be more of problem to wavelength stability than expected. Could it be that much of our difficulty, in getting active temperature control and a handle on current levels to "find the mode hop free zones", is due to RF noise?

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