Proper alignment of the MS-1780 is essential for its operation, and this can occasionally seem difficult to achieve. Here is a simple method for making sure the system is properly aligned to get the “first light” conditions that will allow easy fine tuning for experimental results.
In order for the system to work properly the laser beam needs to enter the optical center of the aperture on the horizontal axis of the instrument. In order to be able to fine-adjust the beam it is useful to use steering mirrors to direct the beam. These can be mirrors or quartz plate front surface reflectors if attenuation of the input beam is desired. For ease of alignment the MS-1780 and the laser should be set up at the same height.
Fig 1: Two pinhole alignment tool:
A very simple method of ensuring the beam is aligned to the center of the optical path of the MS-1780 is to use the two pinhole alignment tool. This mounts to the front C-mount attachment of the MS-1780’s gimbal mount. By adjusting the input beam to align with the two pinholes, the central alignment to the instrument’s optical plane is assured. Once this is done, the alignment tool should be removed¹, and the rest of the tuning procedure completed.
Fig 2: Alignment tool mounted on MS1780 gimbal mount
Aligning the beam is accomplished by steering it until the beam passes through both apertures on the alignment tool. This ensures that the beam is centered and running straight into the ModeScan box.
Fig 3: Beam centered on pinhole apertures
Once this is done, the alignment tool should be removed. A “first-light” condition should be seen on the MS-1780 software and the rest of the alignment procedure described in the MS1780 manual can be done. (Chapter 4.11 describes alignment in detail)
Fig 4: First Light seen on the software screen
Fig 5: Auto exposure brings beams to proper intensity level
Once the beams are on the screen, minor adjustments of the input will align each beam to its appropriate box. Calibration should then be done and at that point measurement is possible.
Fig 6: M2 Measurement
Using this method it should be possible to have the system set up and measuring M2 parameters within a few minutes.
¹ Leaving the alignment tool on the instrument will create apertures that will distort the propagation of the beam and result in erroneous M2 values. Be sure to remove it before making experimental measurements.