Wavefront modification Log Out | Topics | Search
Moderators | Register | Edit Profile

NIR Discussion Forum » Bruce Campbell's List » Equipment » Wavefront modification « Previous Next »

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 147
Registered: 9-2001
Posted on Tuesday, August 28, 2007 - 8:10 am:   

I finally got a chance to read the article that Mike referred us to. I see I was mistaken before: the reference to the "perfect" retroreflector was to a phase-conjugate mirror, which is also the term I couldn't remember previously.

\o/
/_\
Top of pagePrevious messageNext messageBottom of page Link to this message

Michael C Mound (mike)
Intermediate Member
Username: mike

Post Number: 20
Registered: 7-2007
Posted on Tuesday, August 28, 2007 - 2:44 am:   

Dave,

Yes, I know, the program is a bit obscure. Best bet short of downloading a trial version of the rather obscure file extensions would be to view them directly from the Web page. The link is

http://www.zemax.com/kb/articles/195/1/How-To-Model-Corner-Cube-Retroreflectors/Page1.html

and one other related paper:

http://www.turpion.org/php/paper.phtml?journal_id=qe&paper_id=476

You can view the modeling pages one by one.

BR,

Mike
Top of pagePrevious messageNext messageBottom of page Link to this message

David W. Hopkins (dhopkins)
Senior Member
Username: dhopkins

Post Number: 118
Registered: 10-2002
Posted on Monday, August 27, 2007 - 7:55 am:   

Mike and Howard,

Mike, What program opens the .ses and .zmx files? I cannot seem to use the files you sent.

Howard, I looked up the article, as I just am always behind in reading the New Scientist. The authors state that the technique works on human teeth, daisy petals, egg shells and white paint, but best on paint. That suggests that the diffusing layer cannot be too thick. Therefore, the light cannot suffer a lot scattering. However, I should think that there are a lot of scattering events even in a thin layer of paint. Pretty amazing that it should work.

However, it would certainly surprise me if the technique could be controlled well enough to obtain a focus as good as a fine lens. But for some uses in NIR optics, this might be a technique to remember....

Best regards,
Dave
Top of pagePrevious messageNext messageBottom of page Link to this message

Michael C Mound (mike)
Intermediate Member
Username: mike

Post Number: 19
Registered: 7-2007
Posted on Monday, August 27, 2007 - 2:37 am:   

Howard, et al.,

Perhaps this was the reference (or one like it) that you recalled on retroreflectors.

Mike
application/x-zip-compressedRetroreflectors
corner_cube.zip (10.8 k)
Top of pagePrevious messageNext messageBottom of page Link to this message

Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 142
Registered: 9-2001
Posted on Sunday, August 26, 2007 - 9:42 pm:   

Dave - more or less, I think, except that you've got it backward: they would have to be generating the "map" somehow, from measurements on the light coming through backward, using either computer calculations or optical calculations. Another way to do it, is to reverse the light itself, without necessarily measuring it, using something akin to retroreflectors. I recall reading, a long time ago, about a technique to make a phase-sensitive reflecting material; it was a "phase-matched something-or-other" as I recall. It becomes essentially a perfect retroreflector under the right circumstances.

The problem with an ordinary mirror is that it doesn't reverse either the direction or the phase of the incident light, and you'd have to be able to do both. At least if my guess is correct. An ordinary retroreflector reverses only the direction of the light.

\o/
/_\
Top of pagePrevious messageNext messageBottom of page Link to this message

David W. Hopkins (dhopkins)
Senior Member
Username: dhopkins

Post Number: 117
Registered: 10-2002
Posted on Sunday, August 26, 2007 - 2:09 pm:   

Howard,

This seems to be saying that they can map the path of numerous rays through the various materials, so that they can select those rays that go where they want for their input. That would mean that the material is not "randomly", but that they are employing the materials as small faceted mirrors. So, why not use mirrors?

Thanks for following up on this.

Best regards,
Dave
Top of pagePrevious messageNext messageBottom of page Link to this message

Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 141
Registered: 9-2001
Posted on Friday, August 24, 2007 - 6:50 pm:   

(In the interim, Bruce sent me a copy of the New Scientist article. Here are my comments to Bruce after having read it: )

Here's what it looks like to me. In order to get that spot of light on the other side of the sample, the incident light has to be modified in just the right way to compensate for the scattering of the sample. Typically, in optics that sort of thing is usually done by putting the light through the sample in the opposite direction, maybe with some sort of optical processing or maybe not. But when the light comes through the sample having traversed it "backwards" so to speak, it contains the signature of the sample in it, and then it's a matter of reversing this optical signature when you send the light through the sample again, "forwards".

Taking this one more step, if you could come up with a method to impress the same signature on the light some other way, by some sort of modifications of it before it goes into the sample, then knowing what you had to do to get the light through the sample and form the spot will tell you what the sample had to be like. Kinda like we do for a calibration: we know that different combinations of components in a sample give different spectra, so by measuring the spectra we back-calculate to figure out what was in the sample. This is all conceptual and I have no idea about how you might actually go about doing any of that with light, but it seems to me that something like that has to be going on.

\o/
/_\
Top of pagePrevious messageNext messageBottom of page Link to this message

Howard Mark (hlmark)
Senior Member
Username: hlmark

Post Number: 140
Registered: 9-2001
Posted on Friday, August 24, 2007 - 2:30 pm:   

Bruce - sounds interesting. Would it be possible for you to scan the article and send it to me? I'd be particularly interested in whether the materials involved in this rather specialized material is transparent in the NIR. Sometimes when these optics guys get through making a material that meets their somewhat exotic needs, for their somewhat exotic uses, you wind up with something that has no use whatsoever for any other rational application.

In any case, I'll forward your message to a buddy of mine, who is an optical physicist, and see what his comments are.

\o/
/_\
Top of pagePrevious messageNext messageBottom of page Link to this message

Bruce H. Campbell (campclan)
Moderator
Username: campclan

Post Number: 107
Registered: 4-2001
Posted on Friday, August 24, 2007 - 1:28 pm:   

In the Aug 18-24 edition of New Scientist, on page 24, an article describes how modification of the wavefront of an illumination beam (probably in the visible region) allows a layer of paint to focus the beam on a spot with an intensity of about 1000 times that of the unmodified beam.

The article gives a reference: Optical Letters, vol 32, p2309 with authors Allard Mosk and Ivo Vellekoop of Twente University in Enschede, The Netherlands.

This appears to me to be a way to increase the depth of penetration of an NIR beam. Futher, if the NIR beam could be rastered, detection limits could be lowered for inhomogeneous powders.

Finally, modifying the wavefront may have implications in understanding the scattering phenomenon.

Bruce

Add Your Message Here
Posting is currently disabled in this topic. Contact your discussion moderator for more information.