Image Processing Reference
No matter how the expanded laser beam is created, when collimated, the imper-
fections of the collimating optic are imprinted on the wavefront. Figure 5.2 shows a
singlet lens used to provide collimation; however, a better wavefront will be cre-
ated using a doublet or achromatic lens. Introducing an adjustable iris after the lens
creates a sharper outer edge to the beam.
Any of the methods described above are suitable for generating the plane wave-
front used to feed the optical system, but the question arises as to whether the beam is
collimated. Several tools are available to test whether a beam is collimated; that is, the
beam is neither expanding nor contracting. These include holographic collimation
checkers and shear plates, but the simplest approach is to measure the diameter of the
beam at several points from the lens, including a point near the lens and in a region of
the beam past the distance for the system being developed (Fig 5.4). By adjusting the
position of the collimating lens, a location is found where the beam diameter is constant
at the measured points. The laser beam is now collimated and ready for use.
5.4.2 Wavefront sensor
The choice of wavefront sensor for use in a tip-tilt system depends strongly on the
bandwidth needed and the amount of light available from the light source. The
model image-stabilization system is driven by a laser that provides significant
amounts of light to the sensor. Also, the system responds to a slowly varying source
of tilt so that the bandwidth requirements are very low.
Figure 5.4 Illustration of the instrument setup to test the collimation of a beam.