MachZehnderinterferometers

Mach-Zehnder interferometers are optical devices that split a light beam into two separate paths, then recombine them to produce interference. They are built from two beam splitters and two mirrors (in free-space configurations) or their equivalents in fiber or integrated photonics. The input beam is divided at the first beam splitter into an upper and a lower arm, which are redirected by mirrors and then sent to the second beam splitter where they interfere. The resulting intensities at the two output ports depend on the relative phase accumulated along the two arms.

The operating principle relies on phase differences between the two paths. Any change in the optical path length or the refractive index along one arm alters the phase difference, shifting the interference pattern at the outputs. For ideal 50/50 beam splitters, the output intensities follow I1 ∝ 1 + cos(Δφ) and I2 ∝ 1 − cos(Δφ), where Δφ is the phase difference between the arms. Real devices may include loss, dispersion, and unequal splitting, which modify these relations but the basic interference behavior remains.

Variants of the Mach-Zehnder interferometer include free-space, fiber-based, and integrated photonic implementations. In fiber and on-chip versions, waveguides replace free-space paths, enabling compact, stable, and scalable sensors and modulators. Applications span precision metrology, refractive-index sensing, temperature sensing, optical communication, and quantum optics, where single-photon or entangled-photon interference is used for state characterization and quantum information processing.

Historically, the device was introduced by Ludwig Mach and Ludwig Zehnder in the late 19th century and has since become a fundamental tool in optics for measuring phase shifts with high sensitivity.