It is true that the success of these laser systems is highly dependent on how well the source laser performs within the system that they are integrated into. Currently, fiber lasers are primarily used in directed energy systems due to their inherent high quality and stability. However, several additional factors must be taken into consideration when designing these systems. Even though the laser source may be stable, the components that are part of the laser system are susceptible to degradation and failure because they are made of physical matter. In addition, the environment in which these laser systems are deployed can be harsh, affecting long-term performance. Beam delivery components, protective housings, and beam shaping optics, can all degrade over time, from normal use or from laser thermal effects. Knowing how these variables affect the overall system's performance is crucial to its success.
There are several external environmental factors that can affect the system's ability to neutralize targets. Only the correct amount of power with an accurately positioned beam will be effective in protecting the warfighter soldiers and sailors. Measuring and compensating for changes in laser power and beam position on the target are critical to understanding how to compensate for these losses due to environmental factors. One of the most critical variables is laser power density.
Whether you have a laser pointer operating at a few milliwatts, a laser being used for micro welding pulsing at a few Joules of energy per pulse, or a laser being used to disable drones with 10's or even 100's of kilowatts of continuous-wave power, the performance of that laser can be characterized by its Power Density (expressed as Watts per square centimeter – W/cm²).
Power Density is defined as the amount of laser light (expressed in Watts for average or continuous-wave power) with respect to the beam size at which the laser is doing its work (expressed in a unit of area such as cm2), commonly known as a "spot size." Too high a Power Density, due to a relatively higher laser power or relatively smaller spot size, will usually cause the laser to overwork or uncontrollably damage objects in its beam path that aren't meant to be damaged. Conversely, too low a Power Density, due to a relatively low laser power level or a relatively larger spot size, will cause the laser to not do any work at all. Understanding the laser's Power Density as it relates to the material being processed is vital to knowing how the overall system will perform.
The top of the Power Density equation (W/cm2) represents the amount of laser light being applied. This is achieved by measuring the laser's average or continuous-wave power (or pulse energy for pulsed lasers). For Directed Energy lasers, this means 10's or even the 100's of kilowatts of power. While you may think that this is difficult to determine, there are devices that will measure laser powers that high. Conventional water-cooled thermopile sensors can measure from 1kW to 30kW of laser power with ±3% NIST-traceable accuracy. In addition, the 120K-W power measurement system mentioned earlier uses calorimeter measurements of differences in water temperature to measure laser average or continuous-wave power with ±5% NIST-traceable accuracy from 10 kW to 120 kW and beam sizes up to 200 mm in diameter.
The bottom of the Power Density equation is a measurement of the laser's beam profile. Beam Profiling is a collection of laser characteristics that correlate to the laser's quality and usually consist of the size and shape of the laser beam, as well as the spatial or energy distribution across the beam. The quality of the laser can also be measured by its caustic measurements such as Rayleigh Length, M2, Beam Parameter Product (BPP), K value, among others, which are an indication of the laser's ability to focus. When a laser source is developed, the manufacturer includes one or more of these ratings in the product specifications to illustrate the quality of the laser being purchased. For example, an M2 rating for a single mode Gaussian laser with a value of 1 would indicate the highest-quality laser possible.
The days are quickly coming when military personnel will no longer be put in harm's way to defend our country. High-powered Directed Energy laser systems have proven to be an efficient and effective way to neutralize threats. But changes always occur within a laser system, whether because of multi-kilowatt laser thermal effects or the impact of the surrounding environment. Understanding how these changes affect the overall system performance can only be achieved through measurement of the system's performance. Tools are now available to perform these measurements and, as a result, improve these system designs. Our fighting men and women deserve the best weapons we can give them.