Measuring Laser Power

Measuring Laser Power

It is often important to determine laser power output levels, and the optimal way to do this is by using accurately calibrated laser power meters.  There are a number of laser power meters available commercially, but if you can’t justify the cost for one, you might consider building your own, via methods described in the repair FAQ page.  If you only need to determine which of two lasers (of the same wavelength) is most powerful, or whether a laser power modification has made any changes, you can get some idea by using a silicon solar cell hooked to a volt meter.  The meter readings will only be comparative, and will give no indication of the actual laser power, but even this might be better than knowing nothing at all.  Any kind or size of silicon photovoltaic cell should do for this purpose.  Just cover its entire surface with some opaque material, leaving only a small opening for the laser beam to shine through.  Hook a voltmeter to the leads from the solar cell, put on your laser goggles, and fire away.  Here are some links related to power meters and measurement:

http://www.coherent.com/Lasers/index.cfm?fuseaction=show.page&id=250&loc=830
http://www.repairfaq.org/sam/laserioi.htm#ioilpm3

When instrumentation is not available, some idea of a laser’s output power may be gleaned by doing simple tests, such as popping balloons, lighting matches, or cutting electrical tape and comparing the results with those produced by lasers of known power levels.  When reporting the results of such simple tests, it does little good to state that a laser will pop a balloon at a given distance without clearly specifying the voltage powering the laser, the distance from the laser’s lens to the focal point of the laser beam, the exact characteristics of the target, and so on.

In order for laser users to gain understanding about such things as power levels, it is important that those who post reports on various laser forums carefully document their experiments and results.  Here are some suggestions of how to inexpensively do this:

A -- Accurate volt meters are readily available and fairly inexpensive.  Always tell the total voltage powering a laser module, because the laser’s output power can be greatly affected by the voltage of its power supply.  An ammeter is also useful in order to measure the current being drawn by the module.  When using laser modules powered by batteries, it is good to record the battery voltage both before and after the experiment has been conducted.
B -- Accurate thermometers are readily available and inexpensive.  Always tell the approximate temperature of the environment in which the laser is being used.  Laser diodes put out less power as their temperature increases, so it is important to have some idea of what the ambient temperature is, in order to make accurate comparisons.
C -- In lasers with beams that have been focused, instead of collimated, always tell the distance of the beam’s focal point from the end of the laser’s lens.  There are a number of ways to quickly determine this distance. 

If the focal point is very close to the laser, one can often see this point by viewing the beam in a darkened area, particularly if the air contains some smoke or moisture.  Viewing the beam as it passes through a block of clear plastic is an even better way to find the focal point.  Plus, by passing the laser beam through clear plastic, its approximate size and shape can be seen - whether it is round or flat or oval - and is a helpful way to visually teach laser principals.  The plastic block used should be extremely clear (preferably acrylic) with flat, highly polished ends and sides, and at least several centimeters in length and thickness.

Another simple, and less expensive, way to find the focal point is to view the beam through a very thin sheet of translucent white plastic.  The little plastic rulers given away at trade shows and other places are excellent for this purpose because such rulers are only about a millimeter in thickness and have a scale printed on one side.  In order to use them, simply mount the laser in a stationary position, and place the ruler (with the scale facing away from the laser) into the laser beam, preferably near the end of the laser.  As the beam passes through the translucent plastic, it will create a bright spot who’s diameter can be measured by using the scale printed on the ruler.  As the ruler is moved away from the laser, the diameter of the beam will be seen to increase, decrease, or remain unchanged (in the case of a collimated beam).  If the beam is found to be decreasing in diameter as the ruler is moved further from the laser, it will at some point decrease to its smallest diameter and then begin increasing in diameter again as the ruler is moved even further away.  The point at which the beam is smallest in diameter is the laser’s focal point, and also the point of highest temperature anywhere along the length of the beam.  It is at this point where lasers will be most able to pop balloons, light matches, etc.

It should be obvious to all that it is imperative that quality laser goggles be worn when doing such measurements because the only way to accurately read the scale is to have your eyes very near the beam itself.  Remember the earlier warning that laser goggles are not designed to look directly into a laser beam, so do not risk your vision by doing so.  Note also that what is read on the ruler’s scale is not the actual diameter of the laser beam.  In reality the beam is much smaller, but because it becomes diffused as it passes through the plastic sheet, it appears quite large.  Also note that because of diffusion, the thicker the plastic, the larger in diameter the beam will appear to be.  If it becomes necessary to adjust the focal point of a laser module, remember that the further the lens is from the laser, the closer to the laser the beam’s focal point will be, and vise versa.

D -- Always describe as completely as possible the nature of the target used in the experiment or demonstration.  When using things like balloons as targets, remember that the greatest heating effects occur in colors which are contrasting to the color of the laser.   For instance, when demonstrating a red laser, a teacher might find it impossible to pop a red balloon, whereas a green balloon might be popped easily.  This is because the red balloon transmits most of the red light, while the green balloon tends to absorb it, thus heating quickly and popping.  The reverse is also true: A green laser will more readily pop red balloons than green ones.  Both color lasers might be able to easily pop black balloons because the black color readily absorbs both frequencies of light.  Balloons of any color can be made easier to pop by darkening a small portion of their surface with a black Sharpie marker, or something similar.  It is also well to note that the areas near the ends of a balloon have somewhat thicker rubber than the sides.  Although this thick area is darker and passes less light, its thickness also requires more heat to pop the balloon.  Even though the sides of a balloon are thinner and more transparent than the ends, it is normally requires less energy to pop a balloon by firing at its side.

When describing the effects of balloon-popping demonstrations and experiments, it is very important to tell what size a balloon is, its color, where the laser struck its surface, and (when possible) what brand balloon is used.  That way other people can more accurately replicate the experiment, and/or compare one laser to another.  The same care in documenting holds true when doing demonstrations of lighting matches, burning paper, cutting electrical tape, etc.  It’s a waste of time to say that a given laser will light a match, without first specifying the color and brand of the match, the color and power level of the laser, the distance of the focal point, etc.  It is also important to specify how the laser or module is mounted during the experiment.  Often a given demonstration of power cannot be accomplished unless the laser is held very still by being secured to a tripod or other stationary object.