fringe locking circuit

These are all of the old posts from the first two years of the forum. They are locked.
Updated: 2005-03-28 by HoloM (the god)
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Joe Farina

fringe locking circuit

Post by Joe Farina »

If anyone wants to experiment with fringe locking, the actual circuit isn't that complicated.

I was obliged to design this circuit myself, and since I don't know much about electronics, it's probably quite primitive, but I can verify that it works.

The basic idea is to continuously change one of the path lengths (usually the reference beam) so that it "mimics" the behavior of the other one. Fringe locking is excellent for correcting slow fringe drift. (One experienced photoresist holographer told me that he does overnight exposures without any problem when using his fringe locker.)

Fringe locking is not a total solution, though, and it's not particularly easy to use in practice. If anyone wants to stabilize the fringes on an interferometer, then the following circuit can be used as a starting point.

Take an ordinary 741 op-amp (which costs about a dollar) and connect a photodiode to each of the inputs (connect the positive ends of the diodes to the plus (+) and minus (-) inputs of the op-amp). So you will have a total of two photodiodes going into the the op-amp to provide a "differential signal." Between each of the photodiodes and the op-amp, install a 10K resistor. Between the (+) input of the op-amp and one of the 10K resistors, install a 1M resistor, with the other end going to ground (connect the other ends of the photodiodes to ground also). Now, install a 1M potentiometer across the "top" of the op-amp (one end connected between the (-) input of the op-amp and one of the the 10K resistors, and the other end connected to the output end of the op-amp). This allows you to vary the "gain" of the system. (Anyone familiar with electronics will probably recognize this as a standard differential amplifier circuit, which is exactly what it is.) The ouput from the op-amp goes to your speaker or piezo element, on which the mirror is mounted. Along the output line, between the connection made by the 1M potentiometer and the speaker, install a 1K potentiometer. This provides adjustable "damping." The other end of the speaker goes to ground. Then apply 18 volts across the op-amp (I used two 9 volt batteries) and the circuit is complete.

It took me a while to find a good transducer to provide the mirror movement. The one which provided the best results for me was obtained from Radio Shack and was called a "Micro Speaker." These are small speakers which have a clear plastic membrane (not a paper one). This membrane is quite rigid (not flimsy). I epoxied the mirror to this plastic membrane.

To test the fringe locker, set up an interferometer, and use the speaker-mounted mirror in one of the paths. If you place the photodiodes in the fringe pattern (I used a rigid mounting for the diodes which also allowed them to be moved), and then start fiddling with the "gain" and "damping" of the system, a "lock" should result at some point. (In the method I used, I had the diodes on either side of a single bright fringe.)

I won't guarantee success, but it worked for me.

For further reading, see:

http://xmission.com/~ralcon/whylock.html

http://www.xmission.com/~ralcon/genlock.html





Colin Kaminski

fringe locking circuit

Post by Colin Kaminski »

Thank you Joe! If you don't mind I will draw up your circuit and show it on my web page for those of us who have a hard time reading a description and making a circuit.
Colin Kaminski

fringe locking circuit

Post by Colin Kaminski »

Joe was kind enough to send me a sketch of his fringe locker circuit. I made some notes on it and posted it to my web page. (Look at the bottom of my page.)

http://home.pacbell.net/colinsk/holo/

I am going to re-write my page soon.

Thank you Joe!


Colin Kaminski

fringe locking circuit

Post by Colin Kaminski »

Joe,

Is this the speaker you used?

http://www.radioshack.com/category.asp? ... 000&Page=1

Thank you.
Joe Farina

fringe locking circuit

Post by Joe Farina »

No, and the one I used does not appear to be listed on that website for some reason. If my memory serves me correctly, it wasn't very long ago that I saw some of these speakers at my local Radio Shack (the type that I used). It has a clear, solid plastic membrane which is fully exposed on the front of the speaker. The Radio Shack part number on the package that I bought was 273-092. On the front of the package it says: "8-Ohm Mini Speaker, 1 1/8" (29mm) diameter" and on the back: "Rated input 0.10W, Maximum input 0.20W, Frequency range 300-8000Hz, Resonant frequency 650Hz +/- 130, Impedance 8 ohms +/- 20%, Sound pressure level 88 +/- 3dB."

The Radio Shack part number I cited above may be outdated. In a RadioShack.com catalog that I have, a speaker with nearly identical specifications to the one I used (it appears to be the same one) has the catalog number of 910-3093.

Probably, a wide variety of speakers will provide some kind of mirror movement, but the one described above gave the best results among several that I tried.
Colin Kaminski

fringe locking circuit

Post by Colin Kaminski »

Thank you Joe, my Radio Shack has three 273-092 in stock. I'll give it a spin this weekend.
Jean Dufrasne

fringe locking circuit

Post by Jean Dufrasne »

Just an idea (maybe wrong or incomplete).
Can we use this method to 'synchronize' 2 laser diodes ?
Is it possible if we use 2 stabilized diode in frequency (of course, same freq. for both) ?

Jean

Jeffrey Schneider

fringe locking circuit

Post by Jeffrey Schneider »

There is an article "An Inexpensive Fringe Stabilizer For Long Exposure Holography" by S.P. McGrew in volume 1 of "Proceedings Of The International Symposium On Display Holography" (July 12-16, 1982).
In the abstract he says that device "provides a relatively simple way to phase-lock two lasers together".
The article states that the laser's high reflector mirror is mounted on the device speaker so this would probably rule out it's use on laser diodes unless you had an external mirror diode system.

Colin Kaminski

fringe locking circuit

Post by Colin Kaminski »

Here is some more information on External Cavity Diode laser systems.

http://www.phys.uconn.edu/research/ultr ... ab/diodes/

I have a paper somewhere that describes phase locking two diodes but I can't find it at the moment. I am certain I posted a link to it on Frank's forum if you are patient enough to look for it.
Colin Kaminski

fringe locking circuit

Post by Colin Kaminski »

This is a good question. Normally when I read about locking two lasers they stabilize one then lock the other to it. I haven't really studied it but I think they use "injection seeding" (unless I am completely mistaken).

It is harder with diodes because with most diodes we do not have access to the cavity. The hard part with diodes would be to get the initial rough frequency alignment. Once you are close enough to produce measurable fringes then I think it could be done like any other diode frequency stabilization scheme. I am only guessing.

I hope by the time I am ready for 100mw; diodes will have progressed far enough that I can just buy one and not have to sync two diodes. And at the rate I get to projects I think I won't have any worries.



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