Dynamic laser beam stabilization for terahertz systems

Abstract

One of the great difficulties of carrying out Terahertz experiments is the alignment of the laser beam that is used to generate the Terahertz radiation. High precision and stability of the beam direction is required over long time periods. Manual adjustments must be made regularly due to the undesired fluctuations of the laser beam direction. A dynamic laser beam alignment system actively reduces misalignment from vibration sources, thermal gradients and mechanical creep. Commercial automatic systems for laser beam stabilization exist but are expensive especially when they must be employed in multiples. We have designed a system that is straightforward and inexpensive using a simple mirror actuation device based on the concept of thermal expansion. Nichrome wire coils surrounding four aluminium rods are used in the mirror actuator. By varying the current through these coils the amount of expansion caused by the dissipated heat in the rods can be controlled. Our circuit consists of a number of stages including a novel feedback design that prevents the rod from ever reaching its maximum length. Our hardware implementation is able to automatically compensate laser beam pointing in real-time and over extended periods of time.