Control of laser mirror spacing and angular orientation is very important for laser frequency and mode stability. Interferometric methods have been used to measure such small displacements. For example, using a Mach-Zehnder interferometer, a stable He-Ne laser, and a frequency counter, submicrometer displacement can be measured. Because the Mach-Zehnder interferometer is very sensitive to mechanical vibration and misalignment, this technique is rather difficult, and expensive equipment, such as stable lasers and fast counters, is needed.
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