A common misconception is that beam collimation angle measurements must involve a least two or more beam size measurements along the beam path. For this reason, collimation measurements are often viewed as complicated and time-consuming. Real-time collimation angle measurements are considered out of the question.
In fact, beam collimation can be determined through a single beam diameter measurement, allowing for real-time optical alignment. In addition, the measurements are quite simple to perform and require no special training. Unlike most measurement shortcuts, high-precision collimation measurements are performed with exceedingly high resolution — higher than alternative techniques.
A test lens and a scanning slit or knife-edge beam profiler are all that is required for these collimation measurements. Since the beam size measurement is taken at the focal length of the lens, measurement requires a free-space scanning slit or knifeedge beam profiler. A CCD-based profiler, or other camera-type profiler that uses optics, results in a change of the optical path length, since the optics have a different index of refraction than air, and cannot be used with this technique.
The beam profiler is positioned such that it measures beam size at the lens 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 divergence angle determines the 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, consider verifying a beam collimation angle of 100 micro radians. 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 beam profiler. The desired level of collimation, 100 micro radians, is achieved when the beam profiler measures a beam size at the lens focal point of (100 micro radians)( 200 mm) = 20 microns.
Some basic considerations are required for the lens. The collimating lens diameter should be approximately twice the diameter of the incoming beam, and the focal length should be at least ten times its diameter. Standard off-the-shelf lenses from reasonable lens vendors will have the necessary lens quality.