The coherent backlight unit (BLU) using a holographic optical element (HOE) for full-color flat-panel holographic display is proposed. The HOE BLU consists of two reflection type HOEs that change the optical beam path and shape by diffraction. The illumination area of backlight is 150 mm x 90 mm and the thickness is 10 mm, which is slim compared to other conventional coherent backlight units for holographic display systems. This backlight unit exhibits a total efficiency of 8.0% at red (660 nm), 7.7% at green (532 nm), and 3.2% at blue (460 nm) using optimized recording conditions for each wavelength. As a result, we could get a bright full color hologram image.
© 2017 Optical Society of America
Holographic display is considered as an ultimate technique for displaying 3D images because it can provide realistic images without eyestrain and fatigue [1, 2]. This is due to reconstruct the wavefront of object by diffraction whereas conventional 3D displays use only binocular disparity for depth perception in space. Therefore, holographic display has attracted much attention recently.
Generally, the holographic display system consists of two parts as Fig. 1. One is high resolution Spatial Light Modulator (SLM) displaying holographic information and the other is backlight unit (BLU) for illuminating the SLM.
The holographic display is the system based on the diffraction optical system, so the light source should have coherent characteristics. This coherent light such as LED or Laser should be expanded to a proper size by using lens system to illuminate the SLM. However, because it is difficult to use for a TV set or a tablet due to the bulky size of a conventional optical system, it is necessary to make a slim BLU for using a large size of SLM.
Although a lot of research for reducing the thickness of BLU have been reported by using waveguide hologram , waveguide surface grating , edge-illuminated hologram , compact beam expander , reflection HOE  and so on, they are still in research state with small size and mono color reconstruction only. Therefore, in order to make a slim flat-panel holographic display system for consumer application, it is necessary to develop a large size slim optical element and color reconstruction.
In this paper, we propose a slim BLU structure for a large flat panel holographic display using a commercial transmission type SLM with 5.5 in. size, 1920 x 1080 full HD resolution.
2. Concept and design
2.1 Slim BLU concept and design
The optical system of the conventional holographic display consists of light source such as LD or LED and a large field lens necessary to illuminate the SLM. Therefore, the whole system becomes bulky as described in Fig. 1. In order to reduce the whole system volume, it is necessary to replace this bulky field lens to a slim lens component as shown in Fig. 2. In this concept, the light sources such as LD or LED can be used as separate or integrated component with slim lens.
One of the solutions of slim lens is holographic optical element (HOE). Therefore, we proposed HOE BLU as slim BLU.
2.2 Principle and application of HOE
HOE is the optical element made by holographic method. This means that the HOE can have optical functions by the grating structure created by interference of the reference and signal beams . Since this interference fringe pattern can be recorded in a thin medium, HOE is one of the solutions for slim and flat optical elements. This interference fringe patterns act as a complicated diffraction grating, allowing the incident light to be diffracted in the desired direction and shape. It means that HOE transforms the specific wavefront to a desired wavefront by diffraction.
There are two applications of HOE. One is a non-focusing element such as beam splitter, mirror, beam deflector etc. and the other is a focusing element such as a spherical or a cylindrical lens. It is also possible to combine these two types HOE.
For example, when the interference pattern between plane reference wave and spherical signal wave is recorded like Fig. 3(a), a complicate volumetric grating is recorded inside the recording medium. When this complicated volume grating is reconstructed by conjugated reference beam, it functions as an off-axis reflective lens as can be seen at Fig. 3(b). This is an example of HOE application combined with non-focusing and focusing elements.
The diffraction efficiencies of transmission and reflection type volume grating can be calculated by the following Kogelnik’s equation, respectively .
When a refractive index modulation is fixed, the diffraction efficiencies of transmission and reflection type volume grating according to the Bragg angle can be calculated and plotted as shown in Fig. 4.
In the case of the transmission type volume grating, the diffraction efficiency is more sensitive to the Bragg angle of incident light than the reflection type. That is, the diffraction efficiency may vary greatly even with a small change in incident angle, especially at large angle required for slim structure. This means that a reflection type volume grating can maintain efficiency at large angles, whereas the transmission type volume grating has a very narrow window for correct optical properties in real device properties. Therefore, we determined to apply the reflection type volume grating structure to the HOE.
Considering the slim structure and the device characteristics, we proposed HOE BLU with two reflection type HOEs. The incident light beam is transformed to the collimation beam which has a very small diffraction angle by HOE 1 (H1) in order to illuminate the whole display as shown at Fig. 5(a). Then, this collimation beam diffracted by H1 is incident onto the HOE 2 (H2) at a very large angle with respect to the normal as shown in Fig. 5(b). Because this H2 is designed to have both mirror and lens functions at the same time, finally we can get a focused beam of small diameter at a distance as shown in Fig. 5(c). As a result, the conventional large optical backlight system can be replaced by 2 HOEs, so we can implement slim BLU for holographic display.
3. Experimental results
3.1 Color HOE recording
Since the HOE is a hologram with an optical function, a holographic recording process is required for HOE fabrication. In particular, the HOE BLU requires a color hologram recording process. In order to make color HOE, we used RGB color lasers whose wavelengths are 660 nm (Red), 532 nm (Green) and 460 nm (Blue). These RGB Laser beams are combined by two dichroic mirrors to make white beams for reference and signal beam as illustrated Figs. 6(a) and 6(b).
For H1 recording, we design the recording setup as shown in Fig. 6(a). The RGB beams are divided into two parts by a beam splitter and each of the beams is combined by a dichroic mirror to finally form two white beams. This white beam acts as a signal and a reference beam. The signal beam is expanded by the 20x objective lens and collimated by lens and then this collimated signal beam is incident to the photopolymer at normal angle. The reference beam is diverged by cylindrical lens and is incident to the photopolymer with the angle of 7.5°. Then the interference pattern is recorded in photopolymer and H1 is fabricated.
H2 recording setup is shown in Fig. 6(b). To act as a HOE BLU, H2 must be connected to H1. Therefore, the H2 recording set up should consider the beam shape reconstructed by H1. In Fig. 6(b), to use the same beam as the beam reconstructed by H1, the reference beam is collimated by cylindrical lens. And the diverged beam by 20x objective lens is used as a signal beam, which finally acts as a lens. In order to use the diffracted beam by H1 as the incident light of H2, the recording angle of H2 between the signal beam and the reference beam should be 82.5° with respect to the normal. As a result, H1 and H2 are connected and function as HOE BLU.
As a recording medium, we used the photopolymer (Bayfol HX TP, Covestro AG). The thickness of this photopolymer is 16 μm and ± 2 μm variation. This photopolymer has a high resolution spatial frequency of > 4000 lp/mm and can be recorded within the visible spectral range of 440 nm to 680 nm. And this photopolymer has also low shrinkage by using optimized chemistry concept. Therefore, this is a recording material optimized for hologram exposure .
In case of large size hologram recording such as H2 in our design, due to the limited laser power (especially blue laser), the power density on recording material is reduced to 0.1 mW/cm2 at each color, while the exposure energy of photopolymer requires about tens of mJ/cm2 , so the total exposure time is tens of minutes. This long exposure time is a major cause of low diffraction efficiency due to vibration, laser power fluctuations, etc. Therefore we should find a way to reduce the recording time. Using a high power laser is the best way, but there are limitations to the available laser, so we have to solve it by optimizing the exposure method and conditions.
Generally, to record RGB color hologram, single or multi-layer of recording materials and sequential as well as simultaneous exposures of RGB laser beams can be considered . Among these methods, we chose RGB simultaneous exposure method to avoid any vibration problem during serial exposure. And we investigated the effect on diffraction efficiency of Reference to Signal (RS) beam intensity ratio. To do that, we recorded the test HOE sample with the same exposure time at single color and measured the efficiency as Fig. 7. This result shows that the highest efficiency can be obtained at RS ratio 1.
With these improved exposure conditions, recording time was reduced to less than 400 s.
3.2 Diffraction efficiency
The diffraction efficiency according to the refractive index modulation at the fixed Bragg angles can be plotted as Fig. 8. This figure shows that the maximum diffraction efficiency can be obtained at Δn = 0.03.
The diffraction efficiencies of H1 and H2 were measured to determine the optical properties of the fabricated HOE. The diffraction efficiency can be evaluated by measuring the intensity ratio of the diffracted beam to the incident beam. However, since H1 has a characteristic of being diffracted within the glass substrate, the diffraction efficiency of H1 can be calculated indirectly by Eq. (1) after measuring the transmitted beam intensity as following Fig. 9.Table 1.
When we look at the measurement results, we can see that the efficiency of the HOE itself and total efficiency is lower than expected.
The reason why the efficiency of HOEs is low can be explained as follows. Theoretically, the diffraction efficiency of the HOE can be up to 100% but the measured efficiencies are lower. There are two reasons for low efficiency. First, as mentioned in Section 3.1, long exposure time due to low power density causes noise factors such as vibration and laser beam intensity variation, resulting in low diffraction efficiency. And the second reason is by RGB simultaneous exposure. Generally, photopolymer has free radicals associated with the absorption of light energy. In the case of color recording, these limited free radicals are used competing with each other by the RGB beams, which reduces the amount of light energy that can be absorbed compared to monochromatic exposure, resulting in lower efficiency .
Next, to see why the total efficiency is low, we analyzed each light beam intensity according to the light flow as shown in Fig. 10.
The intensities considering the reflection at each interface and diffraction by HOEs can be calculated as follows.Table 2 shows the parameter values and calculation result at green color.
We can see that light path and intensity change at each interface due to reflection. Therefore, the total diffraction efficiency differs from the value simply multiplied by H1 and H2.
As a result, 12.89% efficiency is expected, but the measured result is 7.7%, which has about 5.19% difference. This difference can be estimated as the scatterings, absorptions and reflections by each HOE. Actually, as shown in Fig. 11, it can be seen that undesirable beams are seen around the HOE BLU.
To illuminate the 5.5-inch panel, we actually recorded H1 and H2 of 150 mm x 10 mm and 150 mm x 90 mm size, respectively. As intended, it could be seen that the incident light converges to one focus as Fig. 12(b). The size of the focus is about 2 mm x 1 mm, and we can get the color image shown in Figs. 12(c)–12(f) near the focus. To characterize the optical characteristics of the reconstructed image by HOE, we measured the uniformity and signal to noise. The results are 60.54% and 1: 100, respectively.
By combining the SLM and HOE BLU, we could make the flat-panel holographic display system as Fig. 13.
When Computer Generated Hologram (CGH) image which was designed to be viewed at a distance of 50 cm in front of the panel was inputted through the PC, image was reconstructed as shown as Fig. 14(a). This reconstructed image looks uneven due to low diffraction efficiency uniformity of HOE and other defects from dusts etc., but it can be seen clearly at a distance of 50 cm in front of the panel. On the other hand, at the panel position, the image is very blurred as you can see at Fig. 14(b). This means that the hologram is correctly reconstructed at the intended position of the image, and that any depth representation is possible according to the CGH design.
We proposed a new coherent BLU using HOEs for holographic display system. Using the multi-functional HOEs, we were able to make 150 mm x 90 mm x 10 mm size, slim and high efficiency BLU with efficiencies of 8%, 7.7% and 3.2%, in the RGB, respectively. This total efficiency can be increased when we reduce the noises and use a glass substrate with a refractive index similar to photopolymer.
We have been able to make a slim holographic display system in combination with a 5.5 in. flat panel SLM and successfully demonstrate a holographic image. We expect that this slim HOE BLU will be applied to holographic display for consumer electronics such as TV set and tablets in near future.
Authors appreciate the support by Covestro AG (formerly Bayer Materials Science AG) for providing the photopolymer Bayfol HX photopolymer used for recording HOE.
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