Are there any optical filters which filter the signal's frequency and not based on the wavelength of the light? So what I mean is, if I have a modulated/pulsating light signal riding on a large DC offset, is there some way I can filter out the DC offset using optics alone? I've tried searching the internet for this but this is obviously hard since I don't know which keywords to use... "optical filters" are always based on filtering the spectrum of light, not on the temporal signal.

Would appreciate any help and/or discussions/debates about this, thanks a lot!

  • $\begingroup$ It sounds as if you want the optical equivalent of a demodulator. Is that a fair summary? $\endgroup$ – John Rennie Aug 22 '14 at 6:16
  • $\begingroup$ At which frequency is the AC part? $\endgroup$ – a.j. Aug 22 '14 at 8:12
  • $\begingroup$ Thanks for your response! The frequency of the AC part is low, like ~1-2 Hz. Demodulator can be regarded as a fair summary. Do you chaps have something in mind which can do this? $\endgroup$ – derbedhruv Aug 22 '14 at 8:20
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    $\begingroup$ In this range you want to filter the signals 'behind' your detector electronically or digitally. E.g, use sth like that, thinksrs.com/products/SR600.htm though im not sure how well the filters work to separate DC from 1Hz $\endgroup$ – a.j. Aug 22 '14 at 9:10
  • $\begingroup$ Thanks for the reply. Yeah, digital filtering is what I'm doing at present. I wanted to know if there's some optical component which does this sort of thing through some inherent material property.. $\endgroup$ – derbedhruv Aug 22 '14 at 9:19

You can use a reverse saturable absorber in such a way that the threshold of the RSA must be such that the RSA transmits only the AC part and acts as an opaque object for the DC signal.

  • $\begingroup$ It's called a "saturable transmitter" :-) . I gotta say this is a cool idea, but finding something which switches at a specified DC level may be tough. $\endgroup$ – Carl Witthoft Aug 22 '14 at 12:16
  • $\begingroup$ Thank you so much for your response! Could you please give some examples/links to such materials? Also, if the material saturates with increasing intensity, then won't it eat up the AC component? $\endgroup$ – derbedhruv Aug 22 '14 at 12:49
  • $\begingroup$ Hmmm... rethinking: you want the "gate" to open only above a power threshold, so in fact you want a standard "saturable absorber" as used in passive Q-switching (search "nickel acetate"). However, unless your DC component is very close to the saturation edge, I fear it'll be hard to implement. $\endgroup$ – Carl Witthoft Aug 22 '14 at 12:57
  • $\begingroup$ This material ideally acts like a switch which is controlled by intensity.When the intensity exceeds a threshold value then the transmittance of the material reaches 100% and transmits all signals above the threshold and below threshold it is opaque.So if you place your DC intensity level at threshold then only the AC part will be transmitted. But finding such an ideal material is difficult.If you would like to see more check Non_linear Optics-RSA,SA materials and Z-Scan techniques. CdS,ZnO nanparticles made into thin films have shown some appreciable non-linear effects. $\endgroup$ – Arun_phy_07 Aug 26 '14 at 12:11
  • $\begingroup$ @Arun_phy_07, I would like to harness your idea as an electro-optic shutter for airplane cockpit windows. Could you please provide an URL or link to an article about your idea? Thank you. $\endgroup$ – Frank May 27 '16 at 5:31

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