How puzzling all these changes are! I’m never sure what I’m going to be from one minute to another!
Alice in Wonderland
The most common cause of failure of a hologram is movement of the primary fringes during exposure. The problem of fringe stability is particularly serious when working with low-power lasers, or with very large holograms, or with nonsilver materials such as photoresist, where exposures even with powerful lasers may be several minutes.
The answer is an active fringe stabilizer. This works on the servomechanism principle (servo for short). The elements of a servo system are an error detector, an amplifier, and an output to correct the error. In the case of the fringe stabilizer, the ...