**1. Introduction**

Mach‐Zehnder interferometry (MZI) was invented by Ludwig Zehnder in 1891 [1] and was sub‐ sequently refined by Ludwig Mach in the year after [2]. The techniques were invented taking advantages of the interference phenomena of light to measure the phase difference between the two light beams in which one is varied by the presence of a sample. As illustrated in **Figure 1**, the basic structure of the interferometry is composed of one beam splitter (half mirror) and two

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

reflecting mirrors. The coherent light beam from a laser after collimation was divided into two beams: the reference beam and the test beam on which the sample is interposed. These two beams serve as two arms, the reference and the test arms of the interferometry. The presence of an object on the test arm will result in the difference in optical path length, thereby changing the interference pattern of the laser at the half mirror [HM2]. The fringe patterns can be moni‐ tored and recorded either along the direction of the reference beam or the test beam. Compared to other interferometers like Michelson, the separation of the two arms of MZI can provide a wide application due to large and freely accessible working space though the optical alignment is relatively difficult. Taking advantage of this spacious working place, MZI has been utilized for various experiments: electron interferometer functioning in high magnetic field [3], flow visualization and flow measurements [4], for sensing applications [5]. Furthermore, optoflu‐ idic Mach‐Zehnder interferometer for sensitive, label‐free measurements of refractive index of fluids was also developed [6]. The unique structure of Mach‐Zehnder has also been utilized for optical communication as a modulator [7]. On the other hand, a lot of efforts have been made to fabricate microscale optical systems including Mach‐Zehnder interferometer modulators using polymeric materials [8, 9]. In this chapter, we focus on studies on the local deformation in poly‐ meric systems undergoing photocuring by ultraviolet (UV) light. Since the polymer mixture undergoes transition from liquid to solid by the reaction and at the same time phase separation takes place, the deformation (shrinkage and/or swelling) would affect the phase separation pro‐ cess and the resulting morphology. Mach‐Zehnder interferometry would be useful to monitor the extent of deformation in the nanometer scales during the reaction.

**Figure 1.** Basic unit of Mach‐Zehnder interferometry (MZI).
