Allen Cary, Director of Marketing, Ophir-Spiricon
Laser pointing accuracy and pointing stability are defined as an angular value, usually in milli- or microradians (mr or μr). Pointing accuracy is simply whether the laser points where it is supposed to. Pointing stability is a measure of how much the beam position drifts from the ideal target over time. Stability can be affected by a number of factors, internal and external to the laser itself, including physical motion, heat buildup, cavity instability, air currents, and many more.
There are actually two components to laser pointing and laser pointing stability, linear and angular, which combine to affect the beam’s position. The linear component comes from the horizontal and vertical motion of the laser, perpendicular to the axis of propagation. Call this X and Y. This is the motion that one sees when the laser is raised or lowered, translated right or left, or is subjected to some type of vibration. This motion is normally 1:1, that is, if the laser is raised 1mm, the spot moves 1mm unless there is a magnification lens in the optical path¹.
The angular component is the tip or tilt of the laser beam altering the path from the parallel. We call this dθx or dθy. It is possible to design a fairly simple measurement system to separate these two components of pointing. This can be very useful if the measurement’s purpose is to determine the stability of pointing in an optical system. It is good to know whether instability is coming from linear motion of the laser or from angular perturbations.
In order to measure the linear and angular pointing, the measurement system will comprise two detectors. The first is set up as a near field profiler imaging the front aperture of the laser via a beam splitter. The other detector in the far field is set up with a lens of a known focal length (e.g., 1 meter) with the detector positioned at the geometric focus. With this system any linear motion of the laser will be detected by the near field profiler, but will not appear in the far field due to the f-theta properties of the lens. Angular changes, on the other hand, will be detected by the laser beam profiler.