Blocking Un-Wanted Beams and Stray Light
Stray light becomes an ever more complicated problem as the setups contain more optics. Blocking unwanted light is a requirement of making good holograms. There are many kinds of unwanted light.
Stray Light from Optics
Stray light from optics comes in many forms. Painting the edges of lenses with black paint is a good start at taking care of un-wanted light.
The best way to stop unwanted light is to look through the plate holder and place a black card in the way of any light getting to the plate that is not from the main beams. There are special papers available but black poster board will work in a pinch. It is important to pay special attention to the collimation mirror.
Back-Reflections Re-Entering the Laser
Back-reflections into a laser can be a problem, as the reflecting surface can form together with the laser cavity mirrors an unstable resonator configuration. This can lead to a chaotically fluctuating laser output with many extra unwanted longitudinal modes und intermittend mode jumps, which can easily ruin a hologram. Particularly prone to generate such effects are spatial filters, even more so if they use metallic and not blackened pinholes. Especially laser diodes are very sensitive to even minute back reflection.
The least expensive solution is to slightly mis-allign all lenses and other reflecting surfaces so their backscatter misses the laser.
A more expensive solution is to pass the light through a polarizing cube beam splitter followed by a 1/4 wave plate. Any light that is reflected back towards the laser will then be deflected 90 degrees off the polarizing cube beam splitter and so won't enter the laser any more. However, this involves circularly polarized light and this is often not desirable in holographic applications.
The most expensive solution to avoid back reflections is to use a Faraday Isolator.
Beams that Represent a Safety Hazard
Building your optical bench either above or below eye level is the best way to keep the beam safe. Also building side above the laser plane can add a level of safety. Having a laser with adjustable power allows you to lower the power for alignment and raise the power for exposures is also possible. For very high power lasers that have no adjustable power, using a variable beam splitter to a beam dump can be used.
For pulsed lasers a alignment laser can be aimed to be co-incident with the pulsed beam for alignment. If this laser is chosen as a color the film is not sensitive to, it can also be used as a safe light for loading film.
A beam dump allows one to dispose of the beam without any back reflections and without having it go somewhere that might fog the film.
A beam dump consists of a black box with a small opening. Inside the box is a cone that diffuses the light to the sides. When designing your own make sure that the power density on the face of the cone (laser power/area) does not exceed the threshold of damage of the material of the cone.
A different beam dump can be made from a glass shade 12 welding filter cut it in half lengthwise. Bounce the beam into the middle of the two plates mounted in parallel so it enters at a 45 degree angle. At each point the glass absorbs much of the light and after only a few bounces the beam is dissipated.