In reality they don't do the same thing. The m²-200s is a fully automated system that does the measurement reproducibly with no intervention apart from the initial alignment of the laser beam. This makes it very popular with laser manufacturers doing the same measurement on similar lasers. It provides consistent results in a final test environment. There are some shortcomings though. Since it is a CCD based system, it is confined to the UV-Vis and very near infrared wavelength ranges (~266-1100nm). It is also not too good for the lower UV range below 350nm because continuous use in this range tends to destroy the CCD imager relatively rapidly. For this reason, the NanoModeScan may be a better choice for this as well as for infrared sources above 1100nm. Because the NanoScan is available with silicon, germanium or pyroelectric detectors, the NanoModeScan can be used with any wavelength. Additionally the very wide dynamic range of the slit scanning beam profiler, there is no need for the adjustable attenuation. This makes the measurement considerably faster than the m²-200s.
Both the m²-200s and the NanoModeScan use a method that scans the detector through the beam caustic. The m²-200s does this by varying the path length of the beam and successively focusing different parts of the beam path on the fixed CCD imager. The NanoModeScan does this by moving the scanhead itself though the beam caustic. In either case the process consists of multiple beam measurements over a period of time from 20 seconds to several minutes. This requires that the beam be completely stable or the results will be meaningless. Here is where the m²- 1780 differs from either of these approaches. In this system the beam caustic is divided into 10 "slices" and simultaneously measured on the CCD. This allows the whole caustic to be measured at once and the m² value to be determined for every frame of the acquisition. This means that even an unstable beam, or one in the midst of being adjusted can be measured with direct and immediate feedback. It also allows the measurement of the m² of a single pulse of a pulsed or even single shot laser.
In addition to the M² parameter, several other ISO parameters are calculated by all of these instruments. They include:
- Laser beam waist size and location
- Divergence angle
- Rayleigh range
The m²-200s uses a pair of mirrors mounted to a motion stage to vary the path length of the laser beam caustic. This successively focuses different positions in the beam caustic onto the CCD, making the beam size measurements. From these beam measurements the propagation parameters are determined using a fit algorithm.