Low cost plane parallel plate lateral shearing interferometer for infrared laser beam diagnostics

Abstract

Few cost-effective methods exist for measuring the wave front of mid-and long-wave infrared beams that span 3-5 and 8-12 m, respectively. One obvious need within the infrared laser community is the ability to measure the degree of collimation of an infrared laser beam, e.g. that formed by a beam expanding telescope used with a CO 2 laser or a system of lenses used to collimate a Quantum Cascade Laser (QCL). An ideal approach for this type of metrology is the use of a lateral shearing interferometer (LSI). An LSI uses various methods to displace ("shear") a beam with respect to itself to create an interferogram that can subsequently be used for diagnosing the wavefront quality of the beam. Since this type of interferometer is of the common path variety, it is insensitive to vibration making it ideal for field applications, where vibration isolation may neither be possible nor practical. In this paper we present and demonstrate, through laboratory measurements and computer ray tracing simulations, a low-cost LSI using a single commercial off-the-shelf uncoated ZnSe window in conjunction with an infrared camera. This plane parallel plate LSI configuration was used to interactively collimate a LWIR beam and also provide quantitative transmitted wavefront error data using static fringe analysis software. We also present a self-contained review of the theory of lateral shearing interferometry, including the necessary design equations as applicable to this LSI configuration, to enable researchers to construct a similar beam diagnostic shearing interferometer from readily available components.