The second question is what beam width or spot size does one wish to measure. This question determines the profiler type. Arrays are limited by the size of their pixels. At the current state-of-the-art, pixels are at best around 5μm for silicon arrays, and considerably larger with InGaAs and pyroelectric arrays. This means that a UV-NIR beam should be larger than 50μm in diameter to ensure that enough pixels are lit to make an accurate measurement.
InGaAs pixels are at best 30μm, limiting the minimum measurable beam size to around 250μm; pyroelectric array pixels are even larger at 85μm, meaning the beams need to be at least a half a millimeter to yield accurate results. Scanning slit profilers can measure with better than 2% accuracy beams that are four times the slit width or larger, putting the minimum beam sizes at around 4μm without magnification. Those investigators who want to measure their beams directly without additional optics may find this to be an advantage.
The third question is the power of the beam. This determines the need for attenuation, beam splitting, as well as the detector type. Array detectors, especially silicon CCD and CMOS cameras, will always need attenuation when measuring lasers. Scanning slit type profilers can measure many beams directly without any attenuation, due to the natural attenuation of the slit itself. The slit only allows light to the detector as the slit passes through the beam, and then it only lets in a fraction of the total light. Arrays allow the entire beam to impact the detector, leading to detector saturation unless the beam is severely attenuated.
Lasers with powers above 100mW can be measured with the pyroelectric detector-equipped scanning slit profiler. Properly designed, these profilers can measure up to kilowatts of laser power. High power lasers in the hundreds to thousands of watts can also be profiled using the spinning or scanned wire techniques.
CW or Pulsed?
The final question is whether the laser is continuous wave (CW) or pulsed. Lasers that operate pulsed at a repetition rate of less than 1-2 kHz can only be profiled with an array. Scanning apertures simply cannot make these measurements effectively. CW and lasers with repetition rates above 2 kHz can be measured with scanning slits, provided the combination of the repetition rate and the beam size are sufficient to provide enough laser pulses during the transit time of the slits through the beam to reconstruct a good profile. Pulsed beams have other considerations, particularly those with ultra short pulses, concerning power levels and pulse energy damage thresholds.
A Few More Questions
Besides these four questions about the physical nature of the laser to be measured, there are a few more that need to be asked. How and where a profiler is to be used is also an important consideration in the equation. Camera arrays provide a true two-dimensional image of the beam and will show fine structure and hot spots, which a slit may integrate out. Slit-based profilers will generally provide a more accurate beam size measurement and can measure very small beams directly. Accurate collimation or focus control requires the highest beam size accuracy. Checking the laser for hot spots, uniformity or beam shape may dictate that the 2d profile is more important than the absolute size measurement accuracy.
For a profiler to be used on a factory floor for a manufacturing step, ease-of-use, high throughput, and reproducibility become paramount. In this case the profiler requiring the least “fiddling” is generally the best fit. For any process that uses or works with CW or high frequency pulsed lasers, the scanning slit will have the advantage of measuring the beam directly, even at its focus point, without additional attenuation optics. The dynamic range of these systems is also broad enough to measure both the focused and the unfocused beam without changing the level of attenuation. Camera arrays, on the other hand, may not be able to handle both levels of power density without adjustment. This means that for applications where the beam needs to be brought into focus or measured in various points along its propagation, such as with the measurement of M2 or focusing an optical system, the slit profiler will tend to be much more convenient (Figure 3).
People who are new to the idea of profiling are often looking for a one-size-fits-all profiler. Although one profiler may be able to measure several different lasers, a truly universal solution simply does not exist. With all the various wavelengths, beam sizes, and power levels it is impossible to make one instrument to do it all. Even the optics – lenses, attenuators, etc. – must have antireflective coatings that are specific to the wavelengths of use. The art of attenuation can also be very tricky. It is possible to introduce error into the measurement simply by using the wrong type of attenuators.