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Instantaneous measurement of M2 beam propagation ratio in real-time

The M2 propagation ratio has become the principle measurement of laser beam quality used by both laser manufacturers and laser users to ensure the performance of their lasers. There are several recognized methods of determining this value, but until recently it has always involved making a number of time for these measurements. This has effectively precluded the M2 measurement of laser with single shot or very low repetition rates. It has also prevented the measurement of the variation of M2 in lasers that may not have a stable propagation or the real-time observation of the effects of cavity adjustments on the propagation factor. The instrument that we will describe in this paper makes instantaneous M2 measurements that allow all of the above. By capturing the entire caustic in one camera frame, the M2 value can be determined for each data acquisition, allowing single shot beams to be measured or real-time adjustments to continuous wave or pulsed lasers of any frequency to be observed.

The M2 propagation factor is defined according to the following formula:

Which when rearranged, demonstrates the significance of the value.

The ability to focus the laser beam to the smallest waist size, or diffraction limited spot size, is a function of M2 times the wavelength constant 4λ/∏ and the divergence angel Θ. When the M2 value approaches 1, then only the wavelength and divergence angle affect this waist size. For this reason many laser makers strive to build laser and laser systems with M2 value as close to 1 as possible. M2 has thus become a specification of laser quality.

The ability to make real-time measurements of this parameter provides the fastest possible measurement and allows for the measurement of a single beam pulse in low repetition rate lasers and even some high repetition rate lasers with rates to 50KHZ.

We will show the results of this new measurement technique for a number of lasers and compare these results, where possible, with those obtained by other recognized methods and instruments.