Toward a compact, full-color holographic printer

Three-dimensional content and devices are becoming more prevalent, and demand for static 3D image prints for medical imaging and security applications is expected to increase. One of the most natural ways to view 3D without external glasses is by holography. A hologram records in two or three dimensions the interference pattern formed when coherent light coming directly from a point source (the reference beam) meets light from the same source that has bounced off an object (the object beam). The holographic information is recorded as an interference grating of coherent light, and the diffraction efficiency of the grating is important for bright images. Large diffraction efficiency can be obtained by applying Bragg diffraction of thick hologram gratings.¹ However, existing holographic printers are complex, bulky, and heavy.² They require many different optical elements, such as complex optical lenses, mirrors, and diaphragms divided by air spaces (see Figure 1), which require precise, labor-intensive, mechanical tuning. We developed a new technique to simplify and shrink a full-color holographic printer without sacrificing image quality.

Figure 1. Conventional scheme for holographic printer. SLM: Spatial light modulator.

Holographic optical elements (HOEs) can act as diffraction gratings, mirrors, and lenses. In previous work, we used ‘waveguide hologram’ (WGH) technology to make a microhologram recording in a single color.³ WGH uses total internal reflection on the optical media boundary for the fine separation of diffracted and non-diffracted beams.⁴ We reduced the holographic printer's size, but only for single-color holograms. We have developed a way to combine red, green, and blue beams to record a full-color 3D image in a WGH-based printer.

The principal scheme for integrating and applying the holographic optical elements comprises a total internal reflection plate using WGH and the holographic optical element (HOE) for shaping the reference beam (see Figure 2). The signal beam element is composed of a combined HOE (H1), an illumination HOE (H2), and a holographic Fourier lens. H1 is designed to split the coherent laser output into signal and reference beams. It transforms the signal beam into uniform illumination of the plane of H2.

Figure 2. Proposed holographic printer based on waveguide hologram (WGH) technology. HOE: Holographic optical element.

In our analysis, the signal beam was diffracted by H1 and propagated inside the waveguide substrate, where the incident beam angle was designed to provide total internal reflection within the substrate (see Figure 3). H1 also expands and shapes the beam such that a rectangular image is formed in the exact location of H2. In this way, H2 diffracts a rectangular transformed beam into the collimated beam, illuminating SLM in Figure 2.

Figure 3. Replacement of conventional optics using waveguide hologram, showing the ‘H1’ and ‘H2’ holograms (the combined HOE and the illumination HOE, respectively).

H2 can be recorded simultaneously with H1 for optimal illumination beam shaping and for the required phase distribution in the plane of the spatial light modulator (so that it can form the virtual phase mask in the predefined plane). The holographic forming of the illumination system enables the compensation of possible overall distortions or aberrations of the signal beam. The holographic lens performs the Fourier transformation of the modulated signal beam in the plane of the light-sensitive material. Our approach forms the reference beam with the corresponding light distribution in the plane of the optical recording media and a predefined angle of incidence.

We implemented WGH with combined illumination HOEs (see Figure 3). H1 was about 2×2mm, and H2 corresponds to the 12×10mm dimensions of the SLM. To obtain perfect separation of the beams inside the glass substrate, the angles of beam propagation (inside the waveguide) were more than 55°. We used the conventional Fourier objective lens and reference beam forming elements as the first stage results. We used our holographic printer to print a green 10×10cm 3D hologram (see Figure 4). The printing speed was 0.23cm²/sec, which is about 10min for D4 (102×136mm) size printing. The hogel-to-hogel uniformity was 90% (a hogel is the holographic equivalent of a pixel).

Figure 4. Recorded hologram samples (single color).

Figure 5. Proposed multi-stacked RGB waveguide.

We constructed a multi-stacked red, green, and blue (RGB) WGH to function as a full-color optical head (see Figure 5). The RGB beam paths do not cross each other, so this structure minimizes the crosstalk for H1. H2 then aligns the RGB beams to form a full-color beam that illuminates a digital signal source (either LCD or liquid crystal on silicon, LCOS). The actual implementation of the multi-stacked RGB WGH is shown in Figure 6. We used our WGH-based setup to record holograms and examined their quality (see Table 1). In each case, comparable quality was achieved in terms of sharpness, area uniformity, gray level, and hogel-to-hogel uniformity with reduced hogel size of 400×400μm². Compared to the conventional optics-based structure, our RGB WGH-based one required a twentieth of the volume, a tenth of the number of components, while offering three times the optical efficiency and comparable holographic quality. Both general consumers and professionals, such as photographers and medical imagers, will benefit.

Figure 6. Separated RGB beams by multi-stacked RGB WGH (left) and side-view of the separate RGB beam (right).

In summary, for a glasses-free 3D experience anywhere any time, it is critical to make a compact, integrated holography device. Our WGH-based printer has several advantages. First, it uses various integral optical elements, replacing separate and optical elements that require adjustment and making the device more robust. Second, the geometrical form of the integral elements enables us to make a smaller, flatter device. We are now building a functioning full-color compact hologram printer, fully integrated with RGB waveguide hologram technology.