M2 Measurement Solutions

Photon has two dedicated ModeScan instruments for automating the measurement of M2. Our newest is the ModeScan 1780 that measures the M2 Beam Propagation Ratio and all associated ISO 11146 parameters instantaneously in real time at video rates to over 20Hz. The measurement technique, patented by Photon Inc., uses 10 reflective surfaces to form simultaneous images of the propagating beam at 10 locations on the 12-bit FireWire BeamPro Model 2512 CCD array camera. With all ten measurement positions acquired at once, the instrument is suitable for measurement of both CW and pulsed lasers down to single-shot rates. Beam diameters are obtained with NIST-traceable accuracy to better than 2%, which translates to M2 measurements with accuracy to better than 4%. The ModeScan 1780 is suitable for the measurement of lasers with wavelength of 250 to 1100nm.

For lasers outside these wavelengths Photon has the NanoModeScan, which is also a dedicated, fully automated turnkey M2 system using the NanoScan slit profiler to make the measurements either according to the ISO 11146 Method or a fully automated Rayleigh Method. The ISO method requires that the user input ten measurement points along the axis of propagation, five around the waist and five measurement points that are at least two Rayleigh ranges distal to the test lens. Once these have been set up, the system can make the entire M2 measurement in as little as 20 seconds. The dedicated NanoModeScan software reports the k-factor (1/M2), the actual beam waist diameter and location, and the beam's Rayleigh range in addition to the value for M2 for each axis.

The NanoModeScan can also determine the M2 using the Rayleigh Method as a fully automated process. By selecting the Rayleigh method, the ModeScan will find the waist and then the Rayleigh points for both axes. It will then report the propagation parameters as with the ISO method. This method is fully automated and requires no intervention from the operator, other than the initial set up and input of the wavelength and lens parameters. It does take longer than the ISO method to make the measurements since it must find the actual waist and Rayleigh points on its own. The time required for the entire measurement is dependent on the laser being measured, but is usually no more than 2 or 3 minutes. It has the advantage of being fully automated and it can be left to run while the operator does some other task.

Rayleigh Method for M2 Measurement

M2 is a measurement of the propagation of a laser and it quantifies the quality of the laser beam. A perfect TEM00 Gaussian laser beam will have an M2 of 1.0. This is the theoretical "best" laser propagation. It means that in the lens equation:

If you solve for the minimum focus spot,

the minimum beam size possible, d0, will be dependent on the wavelength, λ, and the divergence angle, λ. The M2, being 1, will have no effect on the value of d0. This is the reason that most laser manufacturers want to have a laser with the M2 value close to 1.0.

The Rayleigh Method uses a test lens and some means of translating the profiler along the axis of propagation. The profiler finds the location of the waist, formed by the test lens, and measures its diameter. The profiler is then moved to the point proximal to the test lens where the diameter is . This location is recorded and the profiler is then moved to the point distal to the test lens with the same diameter. These points are called the Rayleigh points. The distance between the Rayleigh points and the d0 values are then plugged into the formula to obtain the M2 value:

where:

• 2Zr is the distance between the Raleigh points
• d0 is the waist diameter
• λ is the wavelength of the laser

Manual M2 Measurement

Photon's scanning slit and camera-based profilers all have an M2 "Wizard" standard in their software packages. It was called the k-factor wizard in legacy products and is now called the M2 Wizard.

The M2 Wizard walks the user through these steps and calculates the M2 when it is done. The following screen captures show the 5 steps in the NanoScan M2 Wizard.

• ModeScan 1780
• NanoModeScan
• FireWire BeamPro