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Laser Beam Adjustment

Inexpensive lasers usually have lens which are pressed into place, and are not adjustable. More sophisticated lasers and laser modules have lens systems which are fitted into threaded collars, which enables adjustment of the beam. Depending upon the laser and the manufacturer, opening the laser module and adjusting the lens may void the manufacturer's warranty. Always check with the manufacturer before making such adjustments.

Unless there is need to do optical experiments, laser communication, or point out distant objects, there is little need to have a collimated beam. When such a need arises, the simplest way to get there is to aim the laser at a wall or object at least fifty meters away and adjust the lens system until the beam diameter is as small as possible. If it is necessary to get the smallest possible dot over very long distances, one person (the one in charge of measuring beam width) could be positioned where the beam will hit, while another person can do the adjusting of the lens system. By communicating via radios or cell phones, the measurement person can inform the adjuster when the beam is at its smallest diameter. As discussed in an earlier tutorial, all laser beams diverge with distance, so don't expect the beam to remain the same size as it was when it emerged from the laser. When doing these kinds of precise adjustments, it is imperative that the laser be solidly mounted on a heavy, vibration-free object. It is also imperative that the measurement person be wearing high quality laser goggles.

Long distance beam adjustments are best done when the air's humidity level is quite low. Also, in the case of diode lasers, at long distances the beam may appear oval in shape, rather than the round shape it appears near the laser. This is because the beam produced by most diode lasers is flat, like a ribbon, rather than round as is the case with a gas laser.

This oval shape becomes more pronounced with distance, unless a very sophisticated lens system is used. Be very cautious when adjusting lasers outdoors, and do it only in areas where there is no possibility of the beam striking people or cars or airplanes. Just as when using a firearm, make sure that the laser is aimed at a large backstop (such as a windowless building or hillside) so that it will do no harm to people in the distance that you cannot see.

REDUCING LASER BEAM INTENSITY

In many situations, such as optical experiments, cave exploration , etc., it may be necessary to reduce or attenuate the output power of a laser. Some lasers have a variable power control, but many do not. For these fixed-output lasers, the simplest way to attenuate the power is to place an optical filter of some kind in line with the beam. Lens from sunglasses, welding goggles, welding helmets, or photographic filters might be used, as well as dichroic filter gels, such as those used in stage lighting. Bear in mind that plastic optical filters and dichroic gels work well with low powered lasers, but if a laser's power is quite high, these plastic filters can deform or even melt. Heat-resistant glass filters are the safest choice when attenuating beams possessing substantial power levels. Two excellent sources for gel filters are:

http://www.rosco.com/
http://www.leefilters.com

When variable beam attenuation is needed, one simple way to achieve it is to use a polarizing filter (such as those used on cameras) or even polarized sunglasses. By simply placing the polarizer in line with the beam, and then rotating it, the power of the beam can be varied over a wide range. Bear in mind that if the laser is of substantial power, and the polarizer is plastic, it may deform or melt. A glass polarizer should be used with higher powered lasers. Here are some links about polarization:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html#c2
http://www.3dlens.com/polarizingfilm.htm
http://polarization.com/shop/catalog/index.html
http://www.corning.com/docs/specialtymaterials/pisheets/Pi201.pdf

If the diameter of a laser's beam is not important, the laser's effective power (per square millimeter of beam cross-sectional area) can be attenuated by expanding the diameter of the beam with an auxiliary lens. Although the total power of the beam does not change appreciably, the level of power per square millimeter goes down. When passing a laser beam through a lens of any kind, remember that a certain percentage of the light will be lost because of absorption in the lens elements and because of reflections at each air/glass interface (where the beam enters and exits the lens). The amount of loss will be different for different colors of light, but in general, for single-component, uncoated glass lens, the total loss will be about 8% (4% for each side of the lens). For uncoated plastic lens, the loss could be even more. The fact that all lens absorb a small amount of the light passing through them, which is then converted to heat, is the main problem with using plastic lens in laser systems. In more powerful Class IIIb or greater lasers, enough heat may build up in a plastic lens to cause it to deform, thereby changing the focal point and beam shape. With low to moderate power lasers this effect is seldom a problem. The positive side of using plastic lens is increased durability and reduced cost. For more information on lens, check out the following links:

http://www.canon.com/technology/s_labo/light/003/03.html
http://www.physicsclassroom.com/Class/refrn/U14L5a.html
http://physics.bu.edu/~duffy/PY106/Lenses.html

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