Measurement of beam waist for an optical cavity based on Gouy phase

Optical cavities have a variety of applications. However, a highly accurate and efficient method for measuring cavity beam waist size is urgently needed. In this article, we propose a simple approach to deduce the optical cavity beam waist size in terms of Gouy phase using a round-trip matrix for n-mirror cavities. Since Gouy phase can be easily and accurately obtained, this approach provides an alternative way of measuring the beam waist size. Experimentally, the effectiveness of this new method has been demonstrated in a 2-mirror cavity and a 4-mirror planar cavity. The results of beam waist size measurement show fairly good agreement with theoretically predicted values. This method is much simpler, and more convenient than other beam waist size measurement methods, e.g. methods based on pinhole photodiode or CCD.