06 November 2013

The complex and important science of measuring the power of lasers

Accurately measuring the power in a laser beam is a difficult proposition. There are nearly as many different kinds of meters as there are lasers, and none of them are cheap. If you tried to use a detector designed for a continuous-wave milliwatt 330nm nitrogen UV laser, instead for measuring a pulsed multi-kilowatt 10um IR CO2 laser, you probably would end up setting off the smoke alarm. A collaboration of researchers from NIST and Colorado-based Scientech have recently come up with a new way to measure power for this kind of high-energy laser. The solution, they say, is to simply weigh the beam.

The standard method to measure the power in a beam is to use it to heat up a detector. That power can then be compared to the equivalent amount of electrical power that would cause an equivalent temperature change. The detector usually has a special coating to absorb the beam energy, and will take few moments to integrate the signal. A device known as an integrating sphere basically a hollow reflective ball with an input port, and an output port is sometimes placed in the beamline to smooth out irregularities. A uniform front is then presented to the detector, which greatly increases the accuracy.

50-kilowatt CO2 laser

The problem with higher power levels, is that any device used to control the beam (like a mirror, lens, or absorber) tends to heat up and introduce distortions. The common-sense solution to this problem is to just do what grandma might do when she wants to sample the tomato sauce. Rather than lift the whole steaming pot to their mouth, any reasonable cook siphons off a small sample stream, in parallel, with a spoon. The same thing is done for the laser system by taking just a small snippet of the beam, either continuously with something like a partially reflecting mirror, or transiently by popping up a mirror into the path as needed.

Current methods of measuring laser power are… large

A high-power laser can cut stuff like sheet metal at incredible speeds; 100 inches per minute may be nothing for a thin part. In order to do this, the power needs to be precisely controlled so that the part is cleanly cut, but the power used is not overkill. Any method of measurement that steals precious beam energy on a percentage basis, is stealing money. Researchers have long appreciated that there is more to a beam than heat. For example, the so-called radiation pressure of a beam has been proposed as a possible propulsive source for spacecraft. A large sail would capture the beam, and the craft could be accelerated to high speeds. Radiation pressure has recently been used to levitate small objects, too.

To see how the NIST researchers got up to actually weighing a light beam, let’s take a simple example. What is the total energy in the water jet of a power washer? A simple question like that is tough to answer because it is nearly impossible to simultaneously, and independently, measure all the physical forms in which that energy is deposited. A five HP (horsepower) compressor might be be used to pressurize the water jet, but we would probably be lucky to measure one horsepower in the actual jet itself with some kind of force detector.

We might further imagine increasing the total energy in our water jet by pre-heating the water with a propane burner, as industrial washers do. The additional ionic strength of the hot water may be great for degreasing or removing soot, but to measure the effect, some other kind of detector would probably be needed. Another kind of energy might be an intrinsic potential energy. For example, the jet itself could carry chemical energy, as in the kerosene of a flame thrower, which might then be ignited on impact to deliver an even greater punch. The energy that you require in an industrial laser depends on what you are cutting. Since different materials absorb best at different frequencies of light, this energy will depend on the intrinsic properties of the beam like wavelength.

The Scientech researchers found that the momentum of a high-powered laser could be used as a reliable stand-in for its power at the cut. By bouncing the entire beam off of a mirror cantilevered on a sensitive lab balance, they could have their beam, and measure it too. The mirror absorbed less than 0.3% of the beam and did not require any auxiliary cooling to preserve it. When the researchers used their power scale on a 100 kilowatt CO2 laser, they measured a resulting force of about 0.7 millinewtons, which is the equivalent of 70 milligrams of mass.

Laser cutters of this magnitude work with a large diameter beam because the energy can be better handled by the system optics when it is diffuse. Only at the point of the cut is the beam then focused to high intensity. The mirror used on the balance was a gold-coated silicon wafer and was necessarily quite large around eight inches in diameter. The researchers are now working to make their device as small and portable as possible. A smaller mirror might also be more accurate because it would be less affected by any air currents which would introduce error.

The use of force measurements to gauge power could potentially take the high end laser cutting industry by storm. It may never lead to the much-needed redefinition of the kilogram standard, but for accurately and quickly measuring high power lasers, it may be the perfect tool.

Research paper: dx.doi.org/10.1364/OL.38.004248 – “Use of radiation pressure for measurement of high-power laser emission”


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