Flat mirrors are used in holography whenever the beam needs to be turned. For most holography there is no need for precision mirrors but it is very important the the mirror be coated with a reflection coating on the front side (Known as a "front surface" mirror). Small concave mirrors are available that can be used as a beam expander. Cleaning Mirrors
Silvered mirrors have the advantage that they are inexpensive but their damage threshold is usually low. Their reflectivity is only about 85-90% in non enhanced aluminum coatings.
Silvered mirrors almost always have a protective overcoating and can also have an enhanced coating to increase the reflectivity to better than 90% in the visible range. Enhanced coatings are cost advantagous compared to buying a more powerful laser.
Dielectric mirrors have the highest reflectivity and damage threshold available. They are more sensitive to reflection angle than silvered mirrors and are not often used at angles greater than 50 deg. Reflectivities can exceed 98%.
They are created by coating the surface with coatings of alternating index of refraction at 1/2 wavelength thicknesses. The larger the difference in index of refraction and the more coatings, the higher efficiency the mirror will have. This is the mirror of choice inside of laser cavities because of their high efficiency.
Damage threshold is about 100mj/cm^2.
10.1" f 4.5 parabolic mirror in a home made mount.
Some holographers prefer collimation lenses rather than mirrors but the cost is lower for a mirror so many use a mirror. The ideal mirror is a spheroid figure. Telescopes have a paraboloid figure. Since there are many used telescope mirrors available many holographers choose to use them. Since a telescope suffers from the aberration called coma it is best to keep the reflected angle as small as possible.
- A disposable shower cap makes a good cover for your collimation mirror when it is not in use.
- For a collimated beam the pin hole of the spatial filter (or focal point of the expanding lens) needs to be the diameter times the focal ratio away from the mirror. For the mirror above 49.5 inches.
Finding the Focal Point of a Collimator when f stop or focal point is unknown
- If the f stop or the focal length of a collimator is not known, here is a way to find out where to place the pinhole to get a collimated beam. Measure the diameter of your collimator. Now draw a circle on a white piece of paper with that diameter. Place the paper about 2 or 3 meters (actually as far away as possible) away from the collimator such that the reflected raw laser beam bounces off the collimator and hits directly in the center of the circle on the paper. Now take your lens and place it in the beam prior to the collimator and close to it. Keep an eye on the spot being projected onto the paper with the circle drawn on it. Now start moving the lens away from the collimator, up stream of the laser beam while watching the spot on the paper. As you move back the beam will get bigger and bigger until there is a point where it remains the same size. Mark the distance the lens focal point is from the collimator. Now continue back until the spot becomes smaller. Mark this distance. The focal point of the collimator will be approximately in the center of these two marks.
- For a final adjustment, place the lens focal point at the found focal point. Take the white paper with the circle on it and move it toward the collimator. If the spot becomes smaller the beam is diverging and the lens needs to be moved further away from the collimator. If the spot becomes larger when moving toward the collimator the beam is converging and the lens needs to be moved closer to the collimator. Repeat this procedure until the spot remains the exact same size while moving from that very far away point all the way up to very close to the collimator. When you find the focal length (which is from the focal point of the lens to the center of the collimator, write it down directly on the back or side of the collimator for future reference thus needing to do this procedure once for that mirror.
- It is best to use a low power lens that allows this procedure to be carried out such that the beam is never expanded past the edge of the collimator. If, while moving the lens away from the collimator the beam expands past the edges of the collimator and the reflected spot has not yet stopped expanding, stop at this point and note the size of the reflected spot on the paper. It should be the same size or larger then the drawn circle (it is impossible that the reflected spot is smaller or the reflected spot would have already stopped expanding and would have started getting smaller). If the spot size is the same size of the circle you are at the approximate focal length. If the spot is large then the drawn circle, continue moving the lens away from the collimator unit the spot is the same size as the drawn circle and this will be your approximate focal length. At this point perform the procedure for Final Adjustment above.
Mirrors Used for Polarization Rotation
This image is self explanitory on how to rotate the polarization of the laser beam with two front surface steerable mirrors. This was taken directly from Kaveh's Thesis with his permission.