1. Introduction

Holography [1–4] is a technique to record a wavefront of an object wave by utilizing interference of light as well as reconstruct a three-dimensional (3D) image of an object. The medium containing the information of an interference fringe image is called a "hologram", which contains both the amplitude and phase information of an object wave. 3D image information is reconstructed using a hologram and diffraction theory. One of the most remarkable features in holography is that 3D motion-picture recording of any ultrafast physical phenomenon can be achieved, even for light propagation in 3D space [3]. Digital holography [5–8] is a technique to record a hologram digitally using an image sensor, and reconstruct both the 3D and quantitative phase images of an object using a computer or spatial light modulator. This technique has been researched for not only the observation of ultrafast phenomenon, but also for microscopy [9, 10], quantitative phase imaging [11, 12], and multimodal imaging [13, 14].

In recent years, there has been an increase in demand for multispectral imaging techniques. Multiwavelength information helps us to perceive, analyze, and recognize an object such as body tissue or a tumor. Wavelength of light has the ability to clarify color and material distributions of an object [15], visualize the localization and dynamics of molecules with Raman scattering [16, 17], and analyze the health of human skin [18]. In digital holography, the information of multiple wavelengths and 3D space is obtained by recording waves with multiple wavelengths that are irradiated from light sources, called multiwavelength/color digital holography [19, 20]. Multiwavelength digital holography has the ability for not only color 3D imaging [19, 20], but also dispersion imaging [21] and 3D shape measurement with a wide range by using multiwavelength phase unwrapping [22], due to the recording of quantitative phase information with multiple wavelengths. Temporal division [23–25], spatial division [26–28], and space-division multiplexing [19, 20, 29], which are generally adopted for multiwavelength imaging in an imaging system, can be merged into digital holography to record multiple wavelengths. In general imaging systems, wavelength information is temporally or spatially separated when recording image(s), as shown in Figure 1(a)–(e). However, holographic techniques make it possible to record multiwavelength/ color information using a monochromatic image sensor and to reconstruct it from wavelengthmultiplexed image(s). In holography, multiple wavelength information is obtained also by utilizing temporal frequency-division multiplexing (Figure 1(f)) [30, 31] and spatial frequency-division multiplexing (Figure 1(g)) [32, 33]. In these techniques, Fourier and inverse Fourier transforms are required to separate wavelength information. In the former, many wavelength-multiplexed images and an image sensor with a high frame rate are needed. In the latter, the spatial bandwidth available for recording an object wave at a wavelength is restricted as the number of wavelengths is increased.

Since 2013, we have presented a novel multiwavelength imaging technique utilizing holography and wavelength-multiplexed images [34–39]. The presented technique gives phase-shifting interferometry [40–51] the function to extract wavelength information such as wavelength dependencies of amplitude, phase, and polarization state selectively from wavelength-multiplexed phase-shifted holograms. It is especially important to record not only phase images but also amplitude distributions of object waves at multiple wavelengths in order to achieve multicolor and multispectral 3D imaging of multiple objects. By making use of holography for multiwavelength imaging, 3D space information is simultaneously captured. In this chapter, we explain the proposed technique, phase-shifting interferometry selectively extracting wavelength information: phase-division multiplexing (PDM) of multiple wavelengths and two-step phase-shifting interferometry-merged phase-division multiplexing (2π-PDM).

1. Introduction

206 Holographic Materials and Optical Systems

is increased.

Holography [1–4] is a technique to record a wavefront of an object wave by utilizing interference of light as well as reconstruct a three-dimensional (3D) image of an object. The medium containing the information of an interference fringe image is called a "hologram", which contains both the amplitude and phase information of an object wave. 3D image information is reconstructed using a hologram and diffraction theory. One of the most remarkable features in holography is that 3D motion-picture recording of any ultrafast physical phenomenon can be achieved, even for light propagation in 3D space [3]. Digital holography [5–8] is a technique to record a hologram digitally using an image sensor, and reconstruct both the 3D and quantitative phase images of an object using a computer or spatial light modulator. This technique has been researched for not only the observation of ultrafast phenomenon, but also for microscopy [9, 10], quantitative phase imaging [11, 12], and multimodal imaging [13, 14]. In recent years, there has been an increase in demand for multispectral imaging techniques. Multiwavelength information helps us to perceive, analyze, and recognize an object such as body tissue or a tumor. Wavelength of light has the ability to clarify color and material distributions of an object [15], visualize the localization and dynamics of molecules with Raman scattering [16, 17], and analyze the health of human skin [18]. In digital holography, the information of multiple wavelengths and 3D space is obtained by recording waves with multiple wavelengths that are irradiated from light sources, called multiwavelength/color digital holography [19, 20]. Multiwavelength digital holography has the ability for not only color 3D imaging [19, 20], but also dispersion imaging [21] and 3D shape measurement with a wide range by using multiwavelength phase unwrapping [22], due to the recording of quantitative phase information with multiple wavelengths. Temporal division [23–25], spatial division [26–28], and space-division multiplexing [19, 20, 29], which are generally adopted for multiwavelength imaging in an imaging system, can be merged into digital holography to record multiple wavelengths. In general imaging systems, wavelength information is temporally or spatially separated when recording image(s), as shown in Figure 1(a)–(e). However, holographic techniques make it possible to record multiwavelength/ color information using a monochromatic image sensor and to reconstruct it from wavelengthmultiplexed image(s). In holography, multiple wavelength information is obtained also by utilizing temporal frequency-division multiplexing (Figure 1(f)) [30, 31] and spatial frequency-division multiplexing (Figure 1(g)) [32, 33]. In these techniques, Fourier and inverse Fourier transforms are required to separate wavelength information. In the former, many wavelength-multiplexed images and an image sensor with a high frame rate are needed. In the latter, the spatial bandwidth available for recording an object wave at a wavelength is restricted as the number of wavelengths

Since 2013, we have presented a novel multiwavelength imaging technique utilizing holography and wavelength-multiplexed images [34–39]. The presented technique gives phase-shifting interferometry [40–51] the function to extract wavelength information such as wavelength dependencies of amplitude, phase, and polarization state selectively from wavelength-multiplexed phase-shifted holograms. It is especially important to record not only phase images but also amplitude distributions of object waves at multiple wavelengths in order to achieve multicolor and multispectral 3D imaging of multiple objects. By making use of holography for multiwavelength imaging, 3D space information is simultaneously captured. In this

Figure 1. Multiwavelength imaging systems. (a) Temporal division, spatial division with (b) multiple image sensors and a prism and (c) a stacked image sensor, space-division multiplexing with (d) a color image sensor and (e) a grating, (f) temporal frequency-division multiplexing, and (g) spatial frequency-division multiplexing.
