What Is the Mach-Zehnder Interferometer?

The Mach-Zehnder interferometer is a device that is used for making precise optical measurements. It can demonstrate interference by splitting a light beam and measuring the phase shifts between the two. More than a century ago, the device was created by the prominent physicists Ludwig Zehnder and Ludwig Mach. A versatile diagnostic tool, the Mach-Zehnder interferometer is used to illustrate examples in quantum physics, aerodynamics, and plasma physics. Air flows around aerodynamic structures, and temperature changes, pressure, and density in gaseous mediums can be observed.

The basic components of the interferometer are a light source, two beam splitters, two mirrors, and two detectors. The beam splitter is most often a half-silvered mirror that refracts a part of the light beam and reflects the rest. Light from a light source, typically a laser, falls onto a beam splitter, which splits the light into two beams of equal intensity. The beams travel in different directions and hit the two mirrors. The phase of each light beam is changed by its contact with the mirror surface.

The beams are recombined in the second beam splitter, and detectors assist in the study of the phase differences in the light paths. An alternative arrangement has the recombined beams pass through a positive lens, causing the beams to focus at a single point. If all the reflecting surfaces are aligned in such a way that they are absolutely parallel, no interference fringes are produced when the beams recombine. If the angles of the mirror surfaces differ even slightly, however, then the recombined beams produce interference fringes. The interference fringe pattern produced by the Mach-Zehnder interferometer will show dark and bright lines that vary in intensity.

The device is extremely sensitive and can even act as an accurate thermometer. For instance, a cell filled with water could be placed in the path of one of the split beams, while another filled with air could be placed in the other path. The refractive index of fluids like water depends on temperature, and if the water in the cell experiences even a slight temperature change, the effect is seen in the resulting fringe pattern. It's possible to measure very minute changes in water temperature with the Mach-Zehnder interferometer.

It's important to have an understanding of optics when using a Mach-Zehnder interferometer to make accurate measurements. When light falls on a surface, the reflected light shifts by exactly one half a wavelength if the material on the other side of the surface possesses a higher refractive index. If the refractive index of this material is lower, then the there is no phase change in the reflected beam. When light travels from one medium into another, there is no phase change either, but the direction of the beam changes because of refraction.

The Mach-Zehnder interferometer can also be used to study the refractive index of gases and even check objects for flatness. Measurement of optical inaccuracies in a plate or surface can be also conducted with the aid of the interferometer. Some scientists also use the interferometer in flow-visualization applications by employing the technique of light discrimination to observe changes.