Acknowledgements

6. Discussions and summary

218 Holographic Materials and Optical Systems

PDM, which is explained in Section 4.

We have proposed phase-shifting interferometry selectively extracting wavelength information as a novel multiwavelength imaging technique. In this technique, not only multiwavelength images but also the information of 3D space are simultaneously captured by the combination with holography. The technique is characterized as phase-division multiplexing (PDM) of wavelengths, and wavelength information is separately extracted in the space domain from the information of multiple wavelength-multiplexed images. 2π-PDM is the technique to analytically and completely solve the system of equations with 2N holograms against 2N + 1 variables contained in each hologram. An experimental demonstration was conducted and clear color 3D imaging ability was successfully shown. Note that detailed analyses against both the experimen-

As future works, constructions of three-color digital holography and multidimensional holography systems are important to realize full-color 3D imaging and multidimensional holographic sensing. Figure 12 shows an example of the required holograms in three-wavelength 2π-PDM [38] and numerical results for theoretical validation. Phase shifts indicated in Figure 12(a) mean that three-color 3D imaging with 2π-PDM is capable, when a spatial light modulator or wave plates are used as phase shifter(s) as described in Ref. [38]. Also, a combination of a piezo and a wave plate or a spatial light modulator will be applicable as another implementation. Figure 12(b)–(i) shows the results of a numerical simulation for three-

Figure 12. (a) An example of holograms required for three-wavelength 2π-PDM and (b)–(i) its numerical results. (b) Amplitude and (c) phase distributions of the assumed object wave and (d) one of three-wavelength-multiplexed phaseshifted holograms. Reconstructed amplitude images at the wavelengths of (e) 640 nm, (f) 532 nm, (g) 473 nm, and (h) phase image at 640 nm. (i) Color synthesized image obtained from (d)–(f). In the results, wave plates are assumed as phase shifters as described in Ref. [38]. The image-reconstruction procedure is in the same manner of dual-wavelength 2π-

tal demonstration and the theory in 2π-PDM were reported in Ref. [38].

We appreciate Kris Cutsail-Numata for checking the English grammar in this chapter of the book. One of the authors would like to sincerely thank Shu Tahara for encouragement. This research was supported by Japan Science and Technology Agency (JST), PRESTO, Konica Minolta Science and Technology Foundation, The Okawa Foundation, Research Foundation of Tokyo Institute of Technology, the Japan Society for the Promotion of Science (JSPS), MEXT-Supported Program for the Strategic Research Foundation at Private Universities (from 2013 to 2018), and Research Foundation for Opt-Science and Technology.
