This paper reports thin, transparent, and soft displays based on polymer waveguides that are compliant with curvilinear interfaces. In order to prove a feasibility of optical waveguide for a flexible display, we suggest the waveguide fabricated by a multi-step lithography process using two photo-curable pre-polymers with different refractive index. The displays are composed of light sources, polymer waveguides, and scatter patterns. The light signal propagating through the waveguides forms images of the scatter patterns by deflecting the light signals to outer surface. The scatter patterns are configured to a seven-segment. The seven-segment design with a switching methodology of the light sources contributes to selectively representing all decimal numbers from 0 to 9 by combination of activated segments. For a large area display based on the proposed methodology, a single light source interconnected to multi-waveguide section is integrated with a QWERTY key pad design. The display shows high transparency and flexibility without visual distortion.
© 2014 Optical Society of America
A key component of integrated optics is optical waveguides which can confine and transfer light in one or two dimensions. The optical waveguides have been widely used in optical communication devices such as directional couplers, optical modulators, laser diode cavities, and optical filters [1–3] and sensing modules for manipulation of optical beams and signals [4,5] that are required for more compact and highly sensitive systems.
The waveguides have also been considered as attractive components for technological advances in display systems. Various waveguide technologies have been proposed for backlight in the display. Planar waveguides based on an optical fiber contribute to simplifying the structure of the backlight in liquid crystal display . Wedge-shaped design leads to reduction of the thickness of light-guide in the display .
Recently, two-dimensional waveguides using optical fibers and polymer waveguide have also been investigated for the backlights . The study for the polymer waveguide based backlight suggests a capability of the waveguide for flexible display applications. Due to the benefits from soft and highly flexible nature in the polymer waveguide, the waveguide technologies have been used for head-mounted displays and projection displays [9, 10]. However, prior studies for application of the waveguide to display systems still have focused on supplementary components instead of a main visual unit.
In this paper, we propose a novel thin-film visual display based on polymer waveguides. The waveguide based displays are fabricated by using two photo-curable pre-polymers as core and clad materials, which have different refractive index. The display is large (size: 60 mm ☓ 110 mm for QWERT keypad), very thin (thickness: 60μm), highly transparent (optical transmittance: 90%), and flexible. For feasibility tests of the proposed approaches, a seven-segment display and a large area keypad have been demonstrated.
2. Optical waveguide display: a seven-segment display
Figure 1(a) shows an illustrated unit of a proposed optical waveguide based seven-segment display, which is composed of a light source, a polymer waveguide, and a scatter pattern. The waveguide is designed to be a structure of two-dimensional core imbedded into a clad. Since the core retains higher refractive index than the clad, the waveguide structure allows the light propagating through the core to be mostly confined in the core. In order to scatter the light at a specific area, a scatter pattern designed to a hemispherical dot array is formed into the bare core (Fig. 1(b)).
The working principle of the proposed optical waveguide display is as follows: coupling the light from the source to the core of optical waveguide, propagating the light to the scatter pattern through the core, and emitting the light to the air at the scatter pattern. In order to form images of decimal numbers on the display, several units of the waveguide display should be integrated. A seven-segment display can be an example of the waveguide display to represent the decimal numbers. Figure 2 shows a schematic diagram of the waveguide display based on seven-segment structure. Each segment consists of a light source, an optical waveguide, and a scatter pattern. A combination of activated segments using a switching methodology of the light sources allows for selectively representing decimal numbers ranging from 0 to 9. As an example of the seven-segment display, an decimal number, “3”, can be represented by activating channels for the segments, “1, 2, 4, 6, and 7” (Fig. 2).
In order to show colored images on the display, laser diodes (LDs) and light emission diodes (LEDs) can be feasible candidates for the light sources. In this study, Red LDs (wavelength: 630 nm) are used for the light sources since the LDs, which retains relatively lower diverse angles in vertical and horizontal directions than LEDs, is favorable to confine and transfer the light easily when an optical waveguide is coupled to a thin core. Moreover, for an aspect of coupling to optical waveguide, the LDs show higher efficiency than the LED.
The optical waveguides structure for the seven-segment display was fabricated by a multi-step photolithography process using two photo-curable liquid pre-polymers with different refractive index (Fig. 3). Bisphenol A ethoxylate diacrylate and tetra(ethylene glycol) diacrylate supplied from Sigma-Aldrich were used as a core and a clad material, respectively.
To construct the waveguide structure, a bottom clad (thickness: 15 μm) and a core layer (thickness: 17 μm) were sequentially formed on a silicon wafer by a continuative process of spin-casting and UV cross-linking in nitrogen environment. The core layer became a narrow rectangular core with width of about 50 μm through a reactive ion etching (RIE) process. An additional clad layer (thickness: 32 μm) covering the core, was formed on the structure by the same process that was used for the bottom clad and core layer. Scatter patterns designed to a hemispherical dot array were established onto the surface of the core by an additional RIE process.
The refractive indices of the polymer materials for clad and core were measured by a prism coupler method  which was the most popular technique for characterizing optical properties of a polymer film. For the tests, thin film specimens (thickness: about 4 μm) were prepared on a silicon substrate. The refractive indices of the core and clad film against transverse electric field-polarized light at a wavelength of 630 nm are 1.5647 and 1.5031, respectively.
Figure 4(a) shows the mask layout of optical waveguide for seven-segment display. Each segment is a combination of optical waveguide and scatter pattern. Figure 4(b)-4(d) are photographs showing geometry of the fabricated optical waveguide and scatter pattern. The width of waveguides is 50 μm and the bending radius of curved section in the waveguide is 1 mm. A diameter and height of the hemispherical dots for the scatter pattern is about 20 μm and 15 μm, respectively. The hemispherical design of the dots allows for radial scattering of the light signal into the air. To evaluate optical characteristics of the fabricated waveguides, light propagation losses were measured by the cutback method . For the tests, the waveguides with different lengths ranging from 4 to 28 mm in a 4 mm step were prepared and then the light insertion losses of the waveguides were measured by using a single-mode fiber for the input and a multi-mode fiber for the output. The input and output fibers were precisely aligned and butted against the waveguides. Figure 5 shows the insertion losses of the waveguides with different length. Based on a linear fitting of the length dependent insertion loss data, the propagation loss was estimated as 0.6 dB/cm (wavelength: 630 nm).
Finally, the fabricated waveguides were pigtailed to a fiber arrayed block with seven channels so that the lights from laser diodes (wavelength: 630 nm, power: about 40 mW) can be coupled into each core of the waveguides. Figure 6 shows a demonstration of seven-segment display, which represents individual decimal numbers using a switching methodology of the light sources. The scatter causes a diminution of the brightness along the guide since it reduces intensity as going further from the launch point. This phenomenon can be solved by controlling the amount of scattering strength which depends on the size and the space of dots composing scatter patterns.
3. Large area display: a QWERTY keypad
For a large area display based on the proposed waveguide technology, a waveguide is designed to be a structure, which is composed of a multi-waveguide section coupled to a single light source and a planar guide section. The multi-waveguide consists of an input light-guide and multiple narrow light-guides. For a QWERTY keypad display, scatter patterns are designed to twenty-six alphabetic letters with two arrow symbols by modulating their layout through arrangement of hemispherical scatter elements and controlling depth of the elements with physical position. Figure 7 shows a working principle of the waveguide display. The display forms images by continuative steps of spreading out a light signal of the source to planar guides through the multi-waveguide, propagating the light signals from the planar guides to scatter pattern, and forming images of the pattern by deflecting the light signal to outer surface.
The waveguide and scatter patterns were fabricated by using the same photo-curable polymers and procedure described in section 2.2. The structure was fiber-pigtailed to a laser diode and then peeled off from a silicon wafer substrate. The size of core interconnected to the light source is 2 mm wide and 18 μm thick. Although a single-mode fiber with small diameter is used as a light source, the multi-waveguide design, which diverge the input light signal into multiple narrow light-guides, enables the light signal to be spread into a large area in the planar guiding section. In scatter pattern, the spherical dots were fabricated by controlling their depth to be exponentially decreased as position of the letters became more distant from the light source since intensity of the light signal could be exponentially attenuated according to length of the light-guide. The depth control of the spherical dot allows for large area display with consistent light intensity among the letter images. Figure 8 shows a demonstration of a large area visual display with size of 60 mm☓110 mm for a QWERTY keypad. The polymer waveguide based display is very thin (thickness: 60 μm), highly transparent (optical transmittance: 90%), and flexible.
In this paper, we propose a novel thin-film visual display based on polymer waveguides. The visual display is composed of light sources, polymer waveguides, and scatter patterns. The polymer waveguides are designed to seven-segments or multi-waveguide. The waveguide structures are fabricated by a multi-step photolithography process using two photo-curable liquid pre-polymers with different refractive index. The seven-segment design with a switching methodology of the light sources contributes to selectively representing all decimal numbers from 0 to 9 by combination of activated segments. For a large area visual display based on the proposed methodology, a single light source interconnected to multi-waveguide section is integrated with a QWERTY key pad design. The visual display with the size of 60 mm ☓ 110 mm provides uniform light intensity for images and it is very thin (thickness: 60 μm), highly transparent (optical transmittance: 90%), and flexible. In addition, this approach has advantages in commercialization of the devices with high market competitiveness due to low cost of the starting materials and scalability of the process.
This work reported here was supported by the Creative Research Center Program of ETRI and the Pioneer Program of Korea National Research Foundation (NRF-2013M3C1A3059575).
References and links
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