Since the other thread ("replacement for 741') is dragging on, I wanted to post a description of a fringe-locker circuit. But first, let me say that this forum is incredible. I'm not used to such an active forum with so many knowledgeable participants. Thank you!
Years ago, I wanted to try fringe-locking. Commercial fringe-lockers are in the 2k range, so I wanted to build my own. Even though I did a lot of reading, there wasn't a single schematic (or even a patent) for a fringe-locker. With the help of a friend and some guesswork, I managed to build a working circuit. I was really surprised and happy, the cost in parts was probably under $100. This circuit is shown on the holography forum, and as far as I know, it's the only circuit diagram available for a fringe-locker, on-line or off.
When a certain kind of hologram is made (called a split-beam hologram) laser light in split into two beams. These beams are eventually re-combined at the hologram to record an interference pattern in the holographic emulsion. The path lengths of each beam need to remain exactly the same for the length of the exposure, even a variation of one micron will degrade the hologram. (Holographers do everything they can to use rigid mounts, vibration isolation tables, draft exclusion, temperature control, etc.) to minimize this, and these are known as "passive" methods to keep the path lengths exactly the same.
But sometimes for long exposures, "active" means are needed, and this is where a fringe-locker comes in. Exactly how the fringe-locker is used is difficult to explain, but a "secondary" interferometer (with fringes) is included in the setup which "mimics" the actual fringes which form in the holographic emulsion. If this secondary interferometer is locked in place, then the hologram fringes will also be locked, to a certain extent at least.
The photodiodes shown above are placed in the fringes of the secondary interferometer. These fringes are large, alternating bright and dark, with a width of maybe 1/4 inch for each fringe. The photodiodes straddle a bright fringe, or a number of fringes. The differential amplifier circuit drives a transducer (small speaker with a mirror on it) to move back and forth over small distances. This mirror is placed in one the beam-paths. After turning the locker on, the potentiometers are "tuned" and the system will lock.
The circuit sends a singal to the transducer which increases (or decreases) the path of one beam, so that it matches the other exactly. It actually works. But sometimes my locker will "lose" its lock, and this may be because the speaker cannot move far enough to make the necessary corrections. I think commercial fringe-lockers use a piezo stack for the transducer, but they typically require 100 to 150 volts, and I don't know how to modify the above circuit for this. I have only had a brief opportunity to test my homemade locker, but want to build a couple more. I thought the 741 might be too ancient, that's why I asked about it.