Excerpt from HMP 7 Chapter 6
Optics and Equipment
Many of the newly introduced silver-halide recording materials are several times less sensitive than the Agfa emulsions they are intended to replace. This results in longer exposure times being used, which greatly increases the probability of unwanted vibration during the mastering and copying processes.
This chapter describes a system whereby unwanted motion can be detected and compensated for, so that high quality holograms can be recorded in spite of longer exposure times. This feedback-based system can also help holographers using other recording materials in a wide variety of research or production situations.
Fringe Stabilizers
by Jeff Odhner (Odhner Holographics)
In order to make a good hologram, every optical component that is directly involved in the holographic recording process must be immobilized while the emulsion is being exposed. To be more accurate, it is the interference pattern which makes up the hologram (and represents the optical path difference between the reference and object beams) that must be held perfectly still. The dark and light lines on this interference pattern are referred to as fringes. Fringe movement on the holographic recording material is the primary cause of failure to produce a hologram.
No matter how good the recording setup is; no matter how expensive the components are; there will always be some amount of fringe movement during the exposure time when using a CW laser. Anything that causes a change in the effective optical path length between the reference and object beam will cause the fringes to move. Culprits include: air currents, temperature gradients, thermal drift, vibrations, laser frequency shifts, scene motion, and component settling. In practice, many of these variables are uncontrollable past a certain point. Due to the fact that recent supply problems have forced holographers to switch to less sensitive emulsions (which means longer exposure times) the problem of fringe movement becomes even more important to solve. One solution that has proven effective is called active fringe stabilization.
What is a Fringe Stabilizer?
A fringe stabilizer senses a movement of the dark and light lines (fringes) in the interference pattern that makes up the hologram and then quickly reacts by changing the effective optical path length of either the reference or object beam to compensate for that fringe movement. If this optical path difference is compensated for fast enough, the fringes will effectively remain stationary. A good fringe locker, when properly set up, can stabilize to within 1/100th of a fringe for exposure times exceeding an hour, although a tenth of a fringe stability is usually more than sufficient.
How Does a Fringe Stabilizer Work?
A fringe stabilizer works on the principle of a closed loop feedback and control system. A sensor detects the fringe movement and converts this into a proportional voltage (fringe error). This voltage is fed into a control system which changes the effective optical path length of the reference or object beam until the fringe error (movement) disappears. (See figure 6.1.)
A fringe stabilizer consists of three basic components:
1. A Phase Detector which senses phase difference changes between the reference beam and the object beam (manifested by fringes).
2. Electronics to generate a control signal which is an error voltage proportional to the fringe movement.
3. A Phase Modulator which changes the relative phase of the two beams in proportion to the error voltage.
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