Laser Characteristics

Laser Characteristics

Thousands of books and papers have been written on the characteristics of laser light.  The purpose of this little chapter is not to write another book, but to give a brief overview for the benefit of our visitors and customers who have not studied laser physics.  If you wish to learn more, just click on the links at the end of this chapter.

Unfortunately, thanks to sci-fi movies and other fiction, the general public has been led to believe that a laser beam will remain a tight beam and travel on forever.  In reality, thanks to the laws that govern this universe, laser beams spread like any other light - just not so quickly.  By using various lens and optical systems, the light exiting a laser can shaped into a “tool” which can be used to accomplish a useful purpose.  For the most part, laser beams are shaped into one of three categories:

A -- Tightly collimated, somewhat parallel-sided beams which are useful in laser communication systems, as a means of pointing at distant objects, and for optical experiments such as ray tracing.
B --  Expanding (diverging) beams, similar to a flashlight beam, which are useful for optical experiments, effective power reduction, or area illumination.
C -- Beams which converge to a focus (focal point), and are useful for concentrating the entire energy of the beam into a small area.

To explain the difference, here are some examples of what can be done: A 100 mW laser focused at two centimeters from the lens might be able to easily light matches, burn wood, or melt plastic (at two centimeters), but at thirty meters the beam would be so large in diameter and of such low intensity as to barely be detectable, and would certainly pose no eye hazard.  This is the same effect as seen with a convex (magnifying) lens in sunlight.  At the focal point of the lens, enough energy may be concentrated to ignite paper, melt plastic, or even boil water; but beyond the focal point, the beam diameter increases and the energy level (per square millimeter) becomes so low that the heat cannot even be felt in one’s skin. 

The same 100 mW laser, with a collimated beam, might have no effect on a match or piece of wood at any distance, yet pose an eye hazard hundreds of meters away.  This is because, at long distances, a well collimated beam will still have a relatively small diameter and a substantial amount of power per square millimeter of beam cross sectional area. 

Note that for a given power level, the smaller in diameter a laser beam is, the more energy there will be (per square millimeter), whether the beam size is the result of the lens system or the nature of the laser diode itself.

Visualize an X, the two lines which make up the X being the outer edges of a laser beam.  Where the lines cross is the focal point of the beam.  If the focal point is very close to the laser’s lens, the lines cross at a sharp angle, and the focal point becomes very short in length, therefore concentrating all of the laser’s energy in tiny, somewhat spherical area.  If the laser’s lens is moved closer to the diode producing the light, the focal point moves further from the laser and its shape changes from spherical to oblong, and less energy is available at any point along its length.  That’s why lasers always seem more powerful when closely focused.

A collimated laser beam resembles an = sign, rather than an X.  The sides of the beam remain relatively parallel for some distance and never come together to a focus point. 

Because of diffusion caused by dust and moisture in the atmosphere, plus the very nature of photons, the most perfectly collimated beam will eventually grow larger in diameter (diverge), and given enough distance will effectively fade away.  As a collimated beam emerges from a well designed lens system, the larger in diameter beam is, the less it will diverge over distance.  The downside is that the larger beam diameter will have less energy (per square millimeter) for doing power demonstrations like lighting matches and popping balloons.  For more knowledge on this fun subject, see:

http://www.ph.ed.ac.uk/~wjh/teaching/optics-lab/Beam/beam_expand.pdf
http://www.physics.gatech.edu/gcuo/lectures/3803/OpticsI21DiffractionII.ppt
http://www.mellesgriot.com/products/optics/gb_2_1.htm
http://en.wikipedia.org/wiki/Gaussian_beam
http://www.rp-photonics.com/collimated_beams.html