Transverse mode analysis of a laser beam with a non-optical technique

摘要

Quality evaluation of a laser beam is a subject of interest to both designers and users of lasers. A well known method is to measure the M2 second-moment from the longitudinal evolution of beam width determined from intensity profile monitored by a CCD camera. This standard procedure is time consuming, and costly. It is therefore difficult to be implemented for checking, for instance, each VCSEL diode moving out of the assembly line of a high volume production factory. In this paper, we propose an alternative fast method allowing to separate single transverse mode from multiple transverse modes oscillation. This method is based on an electronic analysis (locking amplifier) of the local slope of the output laser characteristic, i.e. laser output power versus pumping intensity. The only optoelectronics component used is a cheap photodiode with a large sensitive area (active diameter 5mm) for measuring the laser output power. Correlation between M~2 and local slope variations, as the pumping intensity is increased, has been experimentally demonstrated with different lasers : Nd:YVO_4 and VCSEL.