fringe locking circuit
Posted: Mon Jul 22, 2002 9:37 am
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
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