Self-calibrating, real-time M-square measurement system

Abstract

M-square measurements since the inception of the ISO 11146-1 measurement standard of 1996 has been one that has been difficult even for a seasoned veteran of such measurements. Variations of more than 10% are not uncommon for the same measurement tool on the same laser being measured. Much of the variation comes from alignment, the motion involved (time averaged based), complex attenuation techniques which often include variable neutral density filters and the type of sensors employed. Moreover, setup times for the instrument can take hours and the measurements themselves many minutes. Measurement of a laser or a laser systems' M-square should be as simple as measuring the power of the laser. In that one aligns the laser to the device; put the device in self calibration mode; make a measurement. In 2012 the authors developed a passive optical design that provided real-time M-square measurement of a laser or laser system but nevertheless still required calibration of the key optics within the system: a Fabry-Perot etalon pair and their spacing in order to obtain an accurate M-square result. Using existing data from the sensor along with a simple ray tracing technique, the etalon spacing can be determined with high accuracy through the deconvolution of the data from the sensor; thereby eliminating a separate time consuming calibration. The key calibration information can now be obtained in a fraction of a second without any effort on the part of the user.