Interferometry is a general term used to describe the techniques that use light waves to study small differences in displacement. By allowing the constructive/destructive interference of two laser beams, one probe and one reference (usually one beam split in two), an interference pattern can be achieved that demonstrates small differences in distance travelled between the two beams. The technique is commonly used to detect defects, surface flatness and uniformity in wafers, study surface quality and roughness, analyse air flows in wind tunnels and metrology applications.
Since the technique relies on the very small difference in path difference between the two beams to generate the interference patterns, it is essential that the light is from a single highly stable wavelength source having a narrow linewidth, (and therefore a long coherence length) to ensure the two beams are in phase with each other for the entire length of the test platform. As the length of sample and reference beam increases, the divergence of the beam becomes an important factor as the beam energy is dispersed causing longer exposure times and decreased resolution.
The torus laser is ideal for interferometry as it is a single longitudinal mode (SLM) laser with a linewidth below 1 MHz and includes an active mode locking mechanism that ensures wavelength drift and mode hop is eliminated. The long coherence length and low divergence of the torus, allow them to be used for large paths with high resolution.