2. Principle of HOE

#### 2.1. Basic concept of HOE

The HOE is an optical element (such as a lens, filter, beam splitter, or diffraction grating), i.e., produced by using holographic imaging process or principles [4]. Figure 1 shows the basic concept of HOE. The two beams from the laser are interfering in recording materials. One beam is the object beam reflected or scattered from the object, and another beam is reference beam. The object beam and the reference beam intersect and interfere with each other to record an interference pattern in recording materials. This interference pattern records the information of the object. When the object is a lens, the interference pattern reconstructs the optical element, which has function of lens as shown in Figure 1.

Generally, HOEs are classified into two main types, thin and volume HOEs. In the case of a thin HOE, the efficiency is low due to the incident light beams that are diffracted by grating in various directions, and the diffraction efficiency is changed so much when the incident angle is changed. Then, for the volume HOEs, the incident light beams are diffracted by grating only at the particular angle, so the high diffraction efficiency can be achieved. Also, HOEs can be classified into transmission and reflection types depending on the geometry of the recording, as shown in Figure 2.

Figure 1. HOE principle: (a) recording and (b) reconstruction.

Figure 2. HOE classification: (a) transmission HOE and (b) reflection HOE.

technique. HOEs can be a mirror, lens, or directional diffuser, because it can implement various functions on a single material according to high diffraction efficiency and narrow-band frequency characteristics. Therefore, the HOEs are widely applied in many fields such as a hologram memory, holographic projection screen, holographic printer, 3D head-mounted display (HMD),

In this chapter, first, the principle of HOE, the basic concept of recording and reconstruction for HOE, and the characteristic of the HOE are described in detail. Then, several examples of applications of HOE such as waveguide and wedge-shaped waveguide-based HMD, HOE

The HOE is an optical element (such as a lens, filter, beam splitter, or diffraction grating), i.e., produced by using holographic imaging process or principles [4]. Figure 1 shows the basic concept of HOE. The two beams from the laser are interfering in recording materials. One beam is the object beam reflected or scattered from the object, and another beam is reference beam. The object beam and the reference beam intersect and interfere with each other to record an interference pattern in recording materials. This interference pattern records the information of the object. When the object is a lens, the interference pattern reconstructs the optical

Generally, HOEs are classified into two main types, thin and volume HOEs. In the case of a thin HOE, the efficiency is low due to the incident light beams that are diffracted by grating in various directions, and the diffraction efficiency is changed so much when the incident angle is changed. Then, for the volume HOEs, the incident light beams are diffracted by grating only at the particular angle, so the high diffraction efficiency can be achieved. Also, HOEs can be classified into transmission and reflection types depending on the geometry of the recording,

and so on.

2. Principle of HOE

100 Holographic Materials and Optical Systems

2.1. Basic concept of HOE

as shown in Figure 2.

lens array, and solar concentration are reviewed.

element, which has function of lens as shown in Figure 1.

Figure 1. HOE principle: (a) recording and (b) reconstruction.

In a transmission HOE, the object beam and the reference beam are on the same side of the recording material. The diffracted beam emerges on the opposite side from the incident beam; the beam goes through the entire thickness of the materials. In a reflection HOE, the object beam and the reference beam are on the different side of the recording material. The diffracted beam is on the same side as the incident beam. The fringes due to interference between the object beam and the reference beam are perpendicular to the grating plane for transmission gratings or parallel for reflection gratings.

HOEs have some characteristics as follows [5]. Multiple holograms can be recorded on a single recording material; spatially overlapping elements are possible. General optical elements are obtained by surface processing. However, HOEs are obtained by recording interference patterns from two coherent light beams on high-resolution photosensitive materials.

Therefore, it is easy to fabricate and duplicate, and it is possible to enter mass production. The production and functioning of HOEs are based on the implementation of the diffraction and interference of light; it is easy to utilize in the narrow bandwidth. Axial synthesized holograms serve the functions of standard, power, and compensating optical elements. The direction of the diffracted beam is determined by fringe of interference pattern on the surface, while the efficiency of diffracted beam is determined by the direction of interference pattern and refractive index in its inner structure. These characteristics and interactions provide both advantages and disadvantages for any particular application.

The characteristics of the recording material have significant effects on the many applications and the development of holography. The properties of ideal holographic material should be good light sensitivity, flat spatial frequency response, bright hologram, no haze, no absorption, no shrinkage or detuning, industrially available, fast hologram formation, unnecessary postprocessing, and stability (environmental and light). To fabricate the HOE, it is necessary to understand the optical characteristics of the recording material.

Typically, different materials, such as silver halide emulsion [6], dichromated gelatin [7], photoresist [8], photorefractive [9], or photopolymer [10], are used for manufacturing of HOEs. A photopolymer is one of the hologram recording materials, which has high diffraction efficiency, low cost, and excellent signal-to-noise ratio [10, 11]. Furthermore, it does not require any chemical or wet processing after recording the holograms. Because of such advantages, the photopolymer has been used widely in several research fields, which include optical elements [12, 13], holographic storage [14], holographic display [15], etc.

Bayer MaterialScience is developing its photopolymer to be easy to handle, with high diffraction efficiency, polychromatic, durable, and customizable. This material will be simple to expose, with no wet or heat processing. The ease of use and simple processing requirement allow these materials to be amenable to mass production of holographic optical elements [16–19].

Bayfol HX film 102 consists of a four-layer stack of a backside cover film of the substrate, the substrate itself, the light-sensitive photopolymer, and a protective cover film as shown in Figure 3. A polycarbonate (PC) substrate with a thickness of 175 2 μm and polyethylene (PE) are used as backside cover foil and protective cover foil, which are both 40 μm in thickness. The protective cover film can be removed. The photopolymer layer itself has a thickness (d) of 16.8 μm.

Figure 3. Bayfol HX 102 film structure.

Bayfol HX 102 photopolymer can be used to manufacture reflection and transmission volumephase holograms with appropriate laser light within the spectral range of 440–660 nm.

In Figure 4, the basic product characteristics are depicted. The transmission spectrum of the unrecorded photopolymer film was recorded after removal of the protective cover film. In this material, the dye-related absorption peaks are located at 473, 532, and 633 nm, with associated transmittance of 56%, 45%, and 31%, respectively.
