Editing Mach–Zehnder interferometer (section)

== Design ==
[[File:Mach Zehnder interferometer alternate candle images.svg|thumb|350px|Figure 1. The Mach&ndash;Zehnder interferometer is frequently used in the fields of aerodynamics, plasma physics and heat transfer to measure pressure, density, and temperature changes in gases. In this figure, we imagine analyzing a candle flame. Either output image may be monitored.]]
[[File:Mach-Zender interferometer fringe localization.svg|thumb|Figure 2. Localized fringes result when an extended source is used in a Mach–Zehnder interferometer. By appropriately adjusting the mirrors and beam splitters, the fringes can be localized in any desired plane.]]

The Mach&ndash;Zehnder interferometer is a highly configurable instrument. In contrast to the well-known [[Michelson interferometer]], each of the well-separated light paths is traversed only once.

If the source has a low [[Coherence (physics)|coherence length]] then great care must be taken to equalize the two optical paths. White light in particular requires the optical paths to be simultaneously equalized over all [[Wavelength|wavelengths]], or no [[Wave interference|fringes]] will be visible (unless a monochromatic filter is used to isolate a single wavelength). As seen in Fig.&nbsp;1, a compensating cell made of the same type of glass as the test cell (so as to have equal [[Dispersion (optics)|optical dispersion]]) would be placed in the path of the reference beam to match the test cell. Note also the precise orientation of the [[beam splitter]]s. The reflecting surfaces of the beam splitters would be oriented so that the test and reference beams pass through an equal amount of glass. In this orientation, the test and reference beams each experience two front-surface reflections, resulting in the same number of phase inversions. The result is that light travels through an equal optical path length in both the test and reference beams leading to constructive interference.<ref name="Zetie">{{cite web|url=http://www.cs.princeton.edu/courses/archive/fall06/cos576/papers/zetie_et_al_mach_zehnder00.pdf|title=How does a Mach–Zehnder interferometer work?|last=Zetie|first=K. P.; Adams, S. F.; Tocknell, R. M.|publisher=Physics Department, Westminster School, London|access-date=8 April 2012}}</ref><ref name="Ashkenas1950">{{cite thesis|last=Ashkenas|first=Harry I.|url=https://thesis.library.caltech.edu/1483/|title=The design and construction of a Mach–Zehnder interferometer for use with the GALCIT Transonic Wind Tunnel. Engineer's thesis|date=1950|publisher=[[California Institute of Technology]]|doi=10.7907/D0V1-MJ80|type=engd}}</ref>

Collimated sources result in a nonlocalized fringe pattern. Localized fringes result when an extended source is used. In Fig.&nbsp;2, we see that the fringes can be adjusted so that they are localized in any desired plane.<ref name=HariharanBasics2007>{{cite book |last=Hariharan |first=P. |title=Basics of Interferometry |date=2007|publisher=Elsevier Inc. |isbn=978-0-12-373589-8}}</ref>{{rp|18}} In most cases, the fringes would be adjusted to lie in the same plane as the test object, so that fringes and test object can be photographed together.