NanoModeScan

The NanoModeScan combines the flexibility and speed of the NanoScan with dedicated M² measurement hardware and software. The NanoModeScan provides an automated measurement of M2 using either the ISO 11146 or the Rayleigh method.

By adding the capabilities of the NanoScan to the ModeScan, the range of possible measurable lasers is greatly expanded and the speed of the measurements dramatically improved. The NanoScan's software controlled variable scan speed allows the measurement of both CW and kHz pulsed lasers with any NanoScan scan head, covering the entire wavelength range from UV to FIR. The NanoScan's rapid beam finding and autoranging speed up the total M² measurement to ~20 seconds for CW lasers.

Both 200mm and 400mm lenses are available to generate the proper artificial waist for the laser source under test. For ease of alignment, there is an entrance iris on the optical axis of the NanoModeScan and a precision alignment stage for horizontal and vertical positioning.

The ISO 11146 Method
The ISO 11146 method for measuring the propagation of a laser source calls for the measurement of the beam diameter for at least 10 positions through the waist created by a test lens inserted in the beam path. Five locations should be within ±1 Rayleigh range of the artificial waist and at least five more points beyond two Rayleigh ranges from this waist. These measurements are then used to compute the laser propagation parameters. Once points are selected properly, the ISO Method is the fastest measurement method and best for volume testing of lasers.

The Rayleigh Method
The ISO method requires the user to manually select the measurement points, and changing one or two of the selected points can yield different M² values. The Rayleigh method is completely automated, selecting its own measurement points based on mapping the Rayleigh range of the beam waist. This method is fully discussed in the Photon Application Note Fast M²(k-factor) Measures with Photon Beam Profilers. In addition, the Rayleigh method can yield more -consistent results for M² values for lasers that are not exactly like those for which the ISO standard was written, such as fiber lasers, lensed diode lasers, and VCSELs.

The NanoScan Difference
With the NanoScan-equipped NanoModeScan, all scan heads can measure pulsed beams with repetition frequencies down to 3-4kHz. Measuring pulsed beams in discussed in the application note Measuring Pulsed Beams with a Slit-Based Profiler. The silicon and germanium detectors will measure less than a milliwatt of power. The pyroelectric detector-equipped NanoScan head can analyze higher power lasers at all wavelengths. The increased dynamic range of the NanoScan enhances the signal to noise ratio of the system and allows a much broader range of laser powers to be analyzed with one instrument setup.

Real-Time Divergence Measurement
By monitoring the divergence angle T, it is possible to make a measurement that will be directly proportional to M². This enables the adjustment of the laser performance in real time at the NanoScan's rapid update rate (up to 20Hz). To use this feature, the scan head is moved to a position one geometric focal length from the test lens. Divergence is the beam diameter divided by the focal length, and the measured divergence is equal to M times the embedded divergence.

Therefore when the beam diameter at this location is minimized, the divergence is at its minimum and the M2 of the laser should then be optimized. After this real-time adjustment, the full M² measurement can be done to generate the required parameter values. This method makes the NanoModeScan an even more valuable tool for the final setup of lasers on the manufacturing floor by decreasing the time it takes both to adjust the laser system and to make the measurements required for quality control documentation.