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Collimation Measurements Are Easier Than You Think!
NanoScan gives real-time, accurate, high-precision results
A common misconception about laser beam collimation angle measurements is that it must involve at least two or more beam size measurements along the beam path. For this reason, collimation measurements are often viewed as complicated and time consuming, and real time laser beam collimation angle measurements are considered out of the question.
In fact, a single beam size measurement is all that is required to determine laser beam collimation, which greatly simplifies this measurement. Thus, real-time optical alignment can be performed to determine best collimation. In addition, the measurements are quite simple to perform and require no special training. Unlike with most measurement shortcuts, high-precision collimation measurements can be performed with exceedingly high resolution, higher than alternative techniques.
All that is required for these accurate measurements of collimation is a test lens and a scanning-slit beam profiler, such as the NanoScan. A CCD camera profiler or other camera beam profiler that uses optics results in a change of the optical path length, since the optics have a different index of refraction from that of air and thus cannot be used with this technique.
Measuring laser beam collimation with a plano-convex lens and
Photon NanoScan scanning-slit laser beam profiler.
The laser beam profiler is positioned such that it measures beam size at the lens's focal distance from the center of the lens. From lens theory, the angle of collimation is determined by the equation:
Θ= Df /f
where Θ is the angle of collimation, Df is the beam size measured at the focal length, and f is the focal length of the lens. Once the beam size is measured at the focal length of the lens, simply dividing this measured beam size by the lens focal lengths determines the laser beam collimation angle. The beam profiler remains fixed, and active alignment is easily performed in real time. This level of simplicity, speed, and functionality is simply not possible with techniques involving multiple beam profile positions.
As an example, suppose a collimation angle of 100 microradians or less is required. Using a standard 200 mm focal length lens, the beam from the collimating optic is directed through the test lens and is measured by the laser beam profiler. The desired level of collimation, 100 microradians, is achieved when the beam profiler measure a beam size at the focal point of:
(100 microradians) (200 mm) = 20 microns.
Some basic considerations are required for the lens. The lens diameter should be approximately twice the diameter of the incoming beam, and the focal lens should be at least 10 times its diameter. Standard off-the-shelf lenses from reasonable lens vendors will have the necessary lens quality.
