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Hello. A few hours ago, I tried out the circuit for fringe locking being suggested here. Somewhere I read that the mirror is to be mounted on the speaker, so this was what I did:

1. The speaker is protected (?) by a cylindrical something having a hole in its middle on top of it. It is too small to put the 1x1cm(0,4x0,4 inch) mirror in it and there ought to be a 90° angle of incidence to get the point in and out.

2. So I cut off the top and saw a white circle mounted on a shiny surface. I removed the circle and I - occasionally - broke two of three connections, which were also holding the surface in position under the surface. That made me think about something: How can you use a glass mirror on such a tender spot ? How does this work ? The mirror will cut off all connections for sure - it is far too heavy for that thin metallic surface.

I have not assembled my fringe locker yet as I am hoping to make some holograms before I go back into testing. I have two of the speakers Joe recommends and they look pretty stable. I plan to glue the mirror to the center of the cone with silicon.

The mirror I have is part number: NT43-854

from Edmund:

http://www.edmundoptics.com/IOD/Display ... uctid=1743

It is small, light and inexpensive. It also will reflect 96.5% of the light at 650nm.

It is very small compared to the speaker and I don't think I will have to modify it at all.

Even if the speaker works somehow, how can I test the circuit using a DMM ? What should it do ? Change current or voltage ( both in which areas ?) and when ?
And all other questions that aim at checking out the function of the circuit
Can you or someone else help me with this problem ?

Perhaps because I have been looking at a Michelson interferometer for months, I would test it in a Michelson interferometer. I am under the impression that it is important to have the two sensors straddling a single fringe. This would be easy in a interferometer.

If you set the DMM to voltage and read the voltage to the speaker leads you can see the corrections the locker is making. Better yet would be an oscilloscope.

Everything I read suggests the fringe locker signal should be used to diagnose the problems as opposed as the final solution to the problems.

But... don´t photodiodes work due to the photoeletrical effect that states there is a change in current which is directly proportional to incoming light energy E = h*f - W ?
A very general question i know...

Thilo, We are beyond my knowledge but it is something like: the bias resistor converts the current from the diode into a voltage source so the op amp can amplify it and send it to the speaker. I do know the speaker is looking for a changing voltage.

That´s an understandable explanation of the phenomenon. Thanks.