Simplest way to modulate optical beam - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 03, 2020, 08:01:45 pm
content: Hey!I'm itching fora project where I can use my newly aquired lock-in amplifier, and my EPR project is coming along mighty slow..I was considering some kind of optical spectroscopy, maybe circular dichroism or something similarly exotic. which brings me to my question:What's the easiest parameter to modulate in an optical beam?A Photoelastic modulator/ birefringence mod. as used in CD spectroscopy would be cool, but likely very difficult.I know THE simplest solution is a chopper wheel, but a circle with holes and a motor is just so... dull.Any ideas?

author: dmendesf
author_rank: Frequent Contributor
timestamp: October 03, 2020, 08:54:13 pm
content: LCDs?

author: chris_leyson
author_rank: Super Contributor
timestamp: October 03, 2020, 09:11:20 pm
content: Modulate it directly if it's a laser diode otherwise LCDs as used in projectors, but don't expect to find data sheets or a source of parts. You could try modulating a small lens along its normal axis with a voice coil but you might need a second lens modulated in the opposite direction to re-focus the beam. Not easy.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2020, 09:22:06 pm
content: The motor and disc with holes is quite easy. An extra LED /Phototransistor pair could create the reference signal. A motor from an old cassette player has a relatively stable frequency. Another simple variant is a relatively slow running DC fan.Just a frequency generator and LED is even simpler but limited to the LED source.A LCD as a chopper can be tricky with temperature dependent transmission.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 03, 2020, 09:27:09 pm
content: Thanks for the advice! Salvaging a cassette deck and just cutting a chopper disc of cardboard would be easy, but I was considering things other than amplitude to modulate: wavelenght, polarization, etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2020, 09:53:55 pm
content: Wavelength modulation is rather difficult, as it would need some spectrometer. The CDs give some polarization, but also quite some scattering. True diffraction gratings foils are also available not that expensive and should give a considerably better wavelength separation. The modulation depends a lot on the range.For modulation of the polarization one could use an LCD with 1 filter in front, though it can be difficult to find one of sufficient size. It could also have some amplitude modulation, as not only polarization is changing.Another option would be mechanically rotating a filter foil, e.g. using some belt drive and a relatively large ball bearing and some tube to have a relatively large open center.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 03, 2020, 10:37:45 pm
content: That could work, having a belt driving a polarizing filter or waveplate.A lot of wierd modulators exists based on flexing or inducing strain in some special optical material, often with quarz actuators, but I think this is out of the question for DIY use, unless one buys a complete unit.I've seen mirrors mounted on galvanometer coils, that can jitter, I guess one could do the same with diffraction grating.Please note that when I day CD I mean 'Circular Dichroism' - not using a cd for diffraction, this very confusion makes CD spectroscopy VERY hard to research

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 04, 2020, 12:47:14 am
content: May I present to you the worlds most janky optical chopper- a sharpied CD* glued to a 12v fan spindle! It is going to be interesting to see if this is enough to get lock-in detection. I made 8 slots to get a higher frequency, since fans are not that fast. I estimate 20 hz with the eight slots.*an actual compact disk-not circular dichroism

author: Zero999
author_rank: Super Contributor
timestamp: October 04, 2020, 09:24:59 am
content: A speaker with a mirror glued to the front of it would theoretically produce some FM, due to the Doppler effect, but it would be tiny and quite difficult to detect.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: October 04, 2020, 10:05:57 am
content: What about reflective diffraction grating in the same setup?

author: thinkfat
author_rank: Unknown
timestamp: October 04, 2020, 10:13:01 am
content: https://www.youtube.com/watch?v=MUdro-6u2Zg?t=1296

author: chris_leyson
author_rank: Super Contributor
timestamp: October 04, 2020, 10:40:33 am
content: Nice one thinkfat, I had forgotten about retroreflectorsCorner cube or glass sphere will do the trick, a flat piece of glass normal to the beam will reflect 50% of the light back, cheaper than a 1st surface mirror. I would avoid diffraction gratings. EDIT: Forgot to add, CCD camera chips make useful reflective gratings.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 04, 2020, 10:25:17 pm
content: With a bundle of optical fiber you can try to get frequency modulation in the same way as a ring laser gyroscope.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 05, 2020, 01:19:56 am
content: Quote from: Zero999 on October 04, 2020, 09:24:59 amHave it effectively do AM by wiggling the beam back and forth with a barrier partially obstructing it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 05, 2020, 03:57:05 am
content: The mirror FM thing works, I did it as a lab project back in school. But there are lots of details to work out and lots of bandwidth needed.Back on the simpler side of things, how about a galvo mirror to do AM. Slightly more sophisticated than a chopper and with faster frequency changes and potentially more control on AM waveform.