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Fiber-optic color synthesizer Y. Jeong, D. Lee, Jhang W. Lee, and K. Oh Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju, 500-712, South Korea Received February 21, 2006; revised April 20, 2006; accepted April 24, 2006; posted April 26, 2006 (Doc. ID 68318) Full-color synthesis was achieved, for what we believe is the first time, utilizing a novel 3 ⫻ 1 hard polymerclad fiber coupler along with red, green, and blue (RGB) LED primaries. By using RGB LEDs that are coupled to three input ports, the device rendered full color from the output port with a circular emitting pixel of 135 m in diameter with an extended color gamut. The proposed fiber-optic color synthesizer can provide a compact waveguide solution for the beam scanning display and the tunable pure white source for LED backlighting. © 2006 Optical Society of America OCIS codes: 060.2350, 060.1810, 330.1720, 230.3670.
With rapid progress in ubiquitous networks, research in display devices is converging in the direction of small form factor, high-resolution, and low power consumption. For handheld devices, there are practical limits; small size displays cannot produce highly discernible images because of the resolution limit,1,2 and yet large size displays with a high enough resolution require a suitable distance from the view position. To cope with these rather contradictory difficulties, light beam scanning display technologies are being intensively developed both experimentally3,4 and theoretically.5 Among those attempts, virtual retinal display (VRD) combined with the scanning photonic system (SPS) have drawn significant attention because the technologies can eliminate display screens, providing a high potential for miniaturization. Thus far conventional light beam scanning display technologies have been based on bulk optics for color generation and light delivery. Recently a compact design based on optical fibers was reported,6 where a fiber scanner using dual bimorph piezoelectric actuators was implemented, yet the system only rendered a monochromatic display. In the perspective of the miniaturization and enhancement of efficiency, LED backlit systems for LCD have been widely attempted to exploit potential advantages such as a wide color gamut, tunable white points, high dimming ratios,7,8 and flexible design capability in light guides.9 Conventional white LEDs, however, do not provide a full-red color component, and the further development toward compact solutions for pure white generation is still required for rich color rendering. Furthermore, the fact that LEDs serve both as a light source and a heat generator has been a major technical challenge in LCD display with long-term reliability. In this Letter, we propose a novel fiber-optic color synthesizer (FOCS) that could be applied both in microscanning displays and in backlight systems based on LED color sources. Using commercial surface mount device (SMD) red, green, and blue (RGB) LEDs, we have achieved a full-color synthesis by introducing a novel fused taper 3 ⫻ 1 hard polymer-clad fiber (HPCF) coupler, along which an efficient color mixing occurs over the fused waveguides. The device has a high potential not only in optical engine minia0146-9592/06/142112-3/$15.00
turization for SPS but also in new white light for LED backlit systems. It also provided a fundamental solution to separate the heat source, RGB LEDs, from pure white output from the optical fiber, to contribute to flexible design in LCD backlights. HPCFs have played a major role in short-reach optical networks owing to their large core diameters, large numerical apertures, and subsequent easy interconnections in the optical links. Figure 1(a) shows a typical HPCF structure, where the core is composed of 200 m pure silica with 15 m thick low-index polymer cladding. Typical numerical apertures range from 0.37 to 0.48. In a homemade fiber fusion and tapering system, three bare strands of HPCFs were fused together, and equal power splitting in the entire visible range was achieved by using an optimized fabrication condition. The 3 ⫻ 3 HPCF coupler showed an average insertion loss of 5.56 dB for each port and a low excess loss of 0.78 dB measured at 635 nm. With optimal heating temperature and tapering conditions, the coupling zone adiabatically became a perfect circular cross section with a diameter of 135 m, where the most uniform mode coupling is achieved. After fabrication of the 3 ⫻ 3 coupler, we cleaved the coupling zone at the midpoint of the waist to make a 3 ⫻ 1 coupler for the RGB LED inputs, as shown in Fig. 1(b), where the photographs of the cross sections at the coupling zone and the end face of the HPCF are shown in the inset. Note that the cleaved end face at the coupling zone serves, in
Fig. 1. (Color online) (a) HPCF structure and (b) schematics of 3 ⫻ 1 HPCF coupler. Two photographs show the end face of the coupling zone and the HPCF. © 2006 Optical Society of America
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Fig. 2. (Color online) Three-dimensional diagram of the FOCS assembly.
fact, as the emitting pixel of the synthesized color. The area of the circular output pixel can be flexibly controlled in the tapering process, and the output beam pattern could be also tailored. The dimensions of the packaged 3 ⫻ 1 coupler slab was as listed; the taper-zone length of 10 mm, width of 1 mm, and thickness of 0.4 mm. These form factors are not fully optimized, yet the whole FOCS system has much smaller space requirements in comparison with bulklensed equivalents. Further miniaturization is being pursued by the authors. In Fig. 2, a schematic of the proposed FOCS system is illustrated. The FOCS consists of RGB LEDs and their driving electronics along with fiber optics. The electronics control the LEDs with 8 bits of resolution obtained by a digital-to-analog converter (DAC). The lights out of the RGB LED primaries were directly butt coupled to input ports of the 3 ⫻ 1 HPCF coupler by using a silicon V block. The bare SMD LEDs have a viewing angle of 130° for the x axis and 140° for the y axis without a lens. Owing to the limitation of HPCF numerical aperture, the butt-coupling efficiency from the bare SMD LED to the fiber was 35%, which can be further improved with proper optics such as a domed lens LED with a smaller radiation angle and a higher numerical aperture HPCF. At the circular output pixel, the FOCS emits the color beam with a viewing angle of 23° along with a uniform flattop intensity distribution, which could have a high potential in micro-SPS. Before we investigate the color synthesis, we characterized the RGB LEDs and their transmission characteristics along the HPCF. The output spectra of the LED primaries are overlaid in Fig. 3(a). The peak intensities were located near 634.2, 528.5, and 443.8 nm, respectively, for RGB LEDs. For these LED inputs, the intensity profiles at the output of the HPCF were measured at a screen 15 mm away from the end face, and their charge-coupled device (CCD) images are summarized in Fig. 3(b) for different launching conditions. The top plot of Fig. 3(b) describes the output intensity profiles through the HPCF for RGB LEDs for the maximum power coupling conditions. Note that the profiles show a flattop distribution for all RGB colors. The LED to the HPCF launching conditions, however, did have a significant influence over the output intensity distribution as shown in the middle and bottom plots of Fig. 3(b). Here we used a green LED for a reference input.
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As the axial gap between the HPCF and the LED increased, while maintaining the center launching condition, the output beam size decreased as shown in the middle plot. The off-centered launchings resulted in ring-shaped outputs as shown in the bottom plot. For guiding high multimodes, the ray tracing concept is well matched to understand these phenomena with the diverse radiation of LEDs. In the off-centered case, the ray launched within the limited angle traces the helix path, and as a result, the ring pattern appears. The launching condition, therefore, is an important factor both for the beam profile and for the coupling efficiency for the FOCS. In this study, we chose the center-to-center launching along with the butt coupling to maximize the coupling efficiency and the color rendering capability. The HPCF is a highly multimode waveguide especially in the visible range, which allows an efficient mixing of colors in the coupling zone. Figure 4(a) shows the measured locus of synthesized colors from the proposed device over the CIE 1931 chromaticity diagram. By controlling the RGB LED power levels, the proposed color synthesizer provided flexible color control over a wide gamut shown as the solid curve.
Fig. 3. (Color online) (a) Normalized electroluminscence intensity of the RGB light sources; (b) photoimage at the HPCF end face with various launching conditions.
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Fig. 4. (Color online) Measured CIE 1931 chromaticity diagram for (a) the extended color gamut and (b) covering white zone by the proposed FOCS.
It is noted that the proposed FOCS covers a much wider color gamut than a conventional CRT display and LCD,10 which experimentally confirmed the strong potential of the FOCS for microprojection display optical engines. Scanning microelectromechanical system (MEMS) mirror systems and sequentially coded color-imaging techniques optimized for the FOCS could provide a new type of 2D microprojection color displays, which is being investigated by the authors. Having confirmed the wide color gamut, we have focused on the issue of pure white generation out of the FOCS; the results are shown in Fig. 4(b). With proper adjustment of RGB driving currents, we could cover the entire white zone in the CIE diagram. It is well known that a conventional white LED does not provide a pure white but a mixture of a blue and a yellow with a weak red component by a composite phosphor. When the proposed FOCS is applied along with high-luminescent RGB LED primaries, pure and bright white sources can be readily obtained. The synthesized pure white from the circular end face of the FOCS output port showed the photometric brightness of 20,526 Cd/ m2 for input RGB LEDs with brightnesses of 20,339, 31,442, and 12,000 Cd/ m2, respectively. Consequently, the coupling efficiency from the three inputs to the circular emitting pixel was 32.18%. High brightness is attributed to a small diameter of 135 m and a welldefined emitting area with an optical quality surface in the proposed device, and further enhancement is being pursued by the authors with the improvement of fiber to LED or laser diode coupling. It is also of note that the FOCS can flexibly locate the input and the output by changing the input HPCF length and the taper length. In this experiment, we used the taper-zone length of 10 mm, and the further elongation of the HPCF length and the taper-zone length could effectively enhance the heat dissipation from
RGB LEDs, which will inherently separate the whitelight source from the heat sources, RGB LEDs. In summary, the compact fiber-optic full-color synthesizer has been experimentally demonstrated by using the novel 3 ⫻ 1 HPCF-fused taper coupler. With high-temperature fusion technology, a circular emitting surface with a diameter of 135 m was achieved along with 35% LED to fiber coupling efficiency. With RGB primaries at the three input ports of the device, the proposed FOCS provided flexible color control over the full RGB color gamut, whose area exceeded that of the LCD and the CRT. The device also provided a pure white color index with a brightness of 20,526 Cd/ m2 that could be directly applied to the LED-based backlight system. This work was supported in part by the Brain Korea-21 Information Technology Project, Ministry of Education, Korea, and the Korea Science and Engineering Foundation (KOSEF) through the Ultrafast Fiber-Optic Networks Research Center at Gwangju Institute of Science and Technology in Korea. K. Oh’s e-mail address is
[email protected]. References 1. M. B. Spitzer, P. M. Zavracky, J. Crawford, P. Aquilino, and G. Hunter, “Eyewear platforms for miniature displays,” in Proceedings of the Society for Information Display (2001), paper 17.5, pp. 258–261. 2. M. van Waldkirch, P. Lukowicz, and G. Tröster, Opt. Express 11, 3220 (2003). 3. J. S. Kollin and M. Tidwell, in Proc. SPIE 2537, 48 (1995). 4. T. M. Lippert, V. Andron, and T. E. Sanko, “Overview of light beam scanning technology for automotive projection displays,” in Eighth Annual Symposium on Vehicle Display, Detroit, Michigan, Session 1.1 (2001), pp. 15–16. 5. G. C. de Wit, Appl. Opt. 38, 5587 (1997). 6. E. J. Seibel, S. S. Frank, M. Fauver, J. CrossmanBosworth, J. R. Senour, and R. Burstein, “Optical fiber scanning as a microdisplay source for a wearable low vision aid,” in Proceedings of the Society for Information Display (2002), paper P-37, pp. 338–341. 7. G. Harbers and C. Hoelen, “High performance LCD backlighting using high intensity red, green and blue light emitting diodes,” in Proceedings of the Society for Information Display (2001), paper LP-2, pp. 702–705. 8. Y. Martynov, H. Konijn, N. Pfeffer, S. Kuppens, and W. Timmers, “High-efficiency slim LED backlight system with mixing light guide,” in Proceedings of the Society for Information Display (2003), paper 43.3, pp. 1259–1261. 9. X. Yang, Y. Yan, and G. Jin, Opt. Express 13, 8349 (2005). 10. D. R. Jenkins, D. C. Beuzekom, G. Kollman, C. B. Wooley, and R. Rykowski, in Proc. SPIE 4295, 176 (2001).
