What You Need to Know to Do It Right
Most people working with lasers today are trying to do something with the light beam, either as the raw beam or, more commonly, modified with optics. Whether it is printing a label on a part, welding a precision joint or repairing a retina, it is important to understand the nature of the laser beam and its performance. Laser beam profiling provides the tools to characterize the laser and know precisely what the beam is doing at the point of the work and if the optics are having the desired effect. Lasers and laser applications come in many varieties, varying in power density, wavelength, depth-of-focus, beam size, pulse duration and myriad other parameters. It is this variety that makes lasers so useful for interacting with and manipulating many different materials and media. But, it is also this variety that adds complexity to the beam profiling process.
Beam profilers come in many types, each with its own advantages and challenges. The basic types can be narrowed into a couple of categories: array-based or camera-based profilers and mechanical scanning apertures, knife edges, and other devices. Today the camera-based and scanning aperture profilers are the leading techniques for most applications. Camera-based systems are generally silicon CCD or CMOS devices, although there are applications using arrays with pyroelectric detectors and microbolometers for detection of longer wavelength lasers. Scanning aperture systems combine a moving slit and single-element photo detector (Figure 1).
Other more specialized instruments, such as the Photon Goniometric Radiometer far field profiler (Figure 2), use a scanning pinhole aperture to measure divergent sources in the far field.
Over the past few years the technology of camera arrays has improved dramatically with reduced pixel sizes, high dynamic range digital interfaces, and electronic exposure and gain controls to vastly increase the usefulness of these devices as laser profilers. With the introduction of CMOS cameras, the costs have also come down significantly. Speed and jitter control, along with high dynamic range electronics, have also improved the performance of the scanning aperture instruments, allowing them to achieve submicron precision for both pointing and beam size measurements. The availability of USB2 and Firewire (IEEE1394) interfaces for both these types of profilers has also increased their ease-of-use and convenience for connecting to both laptop and desktop computers.
Choosing a Profiler
The nature of the lasers to be measured and the requirements for the measurements are the most important criteria for selecting the type and model of profiler best suited to the user’s needs. There are basically four questions that need to be answered to determine the type of laser beam profiler to use.
The first is what wavelength(s) do you intend to measure. The answer to this question determines the type of detector needed, and what the most cost-effective approach may be.
For the UV and visible wavelength range from 250nm up to the very near infrared at around 1100nm, the silicon detector has the response to make these measurements. For these wavelengths there are the largest number of cost-effective solutions including CCD and CMOS cameras and silicon detector-equipped scanning aperture systems. Which of these is the best will be determined by the answers to the other three questions. UV beams from 190nm to 250nm can be measured with CCD and CMOS arrays, but the energetic photons at these wavelengths will damage the arrays. For more than occasional use, specialized UV conversion plates, which convert the UV light to visible wavelengths, should be employed.
For the near infrared, from 1100 to 1700nm, the choices become less abundant. In the lower end of this range from 1100-1200nm the CCD and CMOS cameras may still work, but above 1200nm InGaAs, pyroelectric, or microbolometer arrays become necessary. These are quite expensive – five to ten times the cost of the silicon detectors. Scanning slit systems equipped with germanium detectors are still quite reasonably priced, within a few hundred dollars of their silicon-equipped cousins. At the mid- and far-infrared wavelengths the pyroelectric arrays and scanning slits with pyroelectric detectors provide viable alternatives, again the best approach being determined by the answers to the subsequent questions.