downconverting optical frequencies...
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From: fisico32 on 18 Jul 2010 09:41 hello forum, I know that is it is possible to downconvert high frequencies to lower frequencies.... what is the highest frequency we can downconvert and why? How is it done? ( I don't think the issue exists in the reverse direction: we can convert any low frequency signal to a high frequency one) thanks fisico32
From: Tim Wescott on 18 Jul 2010 11:57 On 07/18/2010 06:41 AM, fisico32 wrote: > hello forum, > > I know that is it is possible to downconvert high frequencies to lower > frequencies.... > > what is the highest frequency we can downconvert and why? How is it done? > > ( I don't think the issue exists in the reverse direction: we can convert > any low frequency signal to a high frequency one) To respond to your title, yes, there are nonlinear optical materials that will mix two laser beams to a longer wavelength signal. I don't know how far above the visible this sort of thing works -- it is, AFAIK, a chemical bond thing, which would stop responding to light somewhere between far infrared and X-ray. But -- AFAIK -- the phase noise in even a really good laser beam is huge, so you'd have to have exceptionally high data rates to maintain enough phase correlation over a bit interval to make such a technique worthwhile. I think it's still in the experimental stage, and will be for quite a while. (The reason for all the "AFAIK" qualifications above is because I have friends to chat with who've done work on this, but no direct experience, or even articles read) -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: glen herrmannsfeldt on 18 Jul 2010 12:37 Tim Wescott <tim(a)seemywebsite.com> wrote: (snip) > To respond to your title, yes, there are nonlinear optical materials > that will mix two laser beams to a longer wavelength signal. > I don't know how far above the visible this sort of thing works -- it > is, AFAIK, a chemical bond thing, which would stop responding to light > somewhere between far infrared and X-ray. I would expect the limit to be somewhere before vacuum UV, which is the frequency where all dielectrics start absorbing. Otherwise, you need a crystal with the appropriate non-linearities. It is techniques like this that allow for optical frequency counting, and the reason for defining the speed of light. Until that point, time measurments and distance measurements had about the same relative uncertainty, around 1e-8. Optical frequency counting moved the relative uncertainty of time measurements down to 1e-15 or so. Defining the speed of light allows for the same uncertainty for distance (defined in terms of speed and time.) > But -- AFAIK -- the phase noise in even a really good laser beam is > huge, so you'd have to have exceptionally high data rates to maintain > enough phase correlation over a bit interval to make such a technique > worthwhile. I think it's still in the experimental stage, and will be > for quite a while. -- glen
From: HardySpicer on 18 Jul 2010 18:46 On Jul 19, 1:41 am, "fisico32" <marcoscipioni1(a)n_o_s_p_a_m.gmail.com> wrote: > hello forum, > > I know that is it is possible to downconvert high frequencies to lower > frequencies.... > > what is the highest frequency we can downconvert and why? How is it done? > > ( I don't think the issue exists in the reverse direction: we can convert > any low frequency signal to a high frequency one) > > thanks > fisico32 Yep and people have even worked on optical PLLs. In theory if you pass a laser through a special crystal that doubles the frequency you should be able to get X-rays off it but I don't believe you do as the freq response is seldom high enough in the material. Hardy
From: steveu on 18 Jul 2010 21:37
>hello forum, > >I know that is it is possible to downconvert high frequencies to lower >frequencies.... > >what is the highest frequency we can downconvert and why? How is it done? > > ( I don't think the issue exists in the reverse direction: we can convert >any low frequency signal to a high frequency one) One crude downconvert with real world applications is to shine two lasers on a silicon photosensor. I think most types of silicon sensor work OK for this. The modes of a laser are typically hundreds of megahertz apart. If you use gratings to pick out a single mode from each of the lasers, you will see the difference frequency in the output of the sensor. Steve |