NanoModeScan: Which Scan Heads Not to Use
The NanoScan 9mm/5µm (aperture/slit) is the best scan head for measuring M-Squared with NanoModeScan. We do not, however, recommend using the smallest scan heads (3.5mm aperture, 1.8μm slit) for M2 measurement, because they are subject to a distortion effect called vignetting: Scanning slits are usually thought of as two dimensional, but they also have thickness, based on the material that they are cut into. Since this material is generally about 13μm thick, what we think of as a slit is actually more like a tunnel (over seven times thicker than it is wide). As a result of this configuration, if a laser beam is either too wide or incident on an angle, part of the beam can be cut off. To avoid this, we make sure to use a slit that is wide enough to allow for this. The smallest 1.8μm slit can be subject to vignetting when the laser beam width exceeds about 2mm.
This small-aperture head is fine for many narrow-beam applications, but there are two reasons to avoid using it in the NanoModeScan. First, M2 measurement is highly dependent on accurate beam measurements, which can be slightly incorrect if vignetting occurs. Second, even a fairly narrow beam near focus could diverge in the far field to a size greater than 2mm. Thus, if you can be sure that your laser beam will be within about 2mm, even in the far field, you could use even the smallest NanoScan head for M2 measurements. Nevertheless, this isn't usually necessary, as the waist of the caustic generated for the test space is normally over 100µm.