An integrated optical projection display technique utilizing three-color-mixing waveguides and grating-light-valve devices is demonstrated. This new projection display system comprises an optical lens, a microscanner, a grating light valve, and a 3×1 planar waveguide device. The planar waveguide device is fabricated using a SU-8 negative photoresist process, which is suitable material for rapid prototyping of integrated optical circuits. It works as a three-color-mixer and is successfully used for color image generation. The intensity of color for each pixel in the display picture is tuned by groups of movable ribbons comprising a grating light valve and image generating diffraction gratings. This study also demonstrates a surface-micromachined optical scanner using four stress-actuated polysilicon plates to raise a horizontal mirror. The electrostatically-driven mirror can be used for scanning projection display applications. Experimental data show that the optical scanner has a mirror scanning angle up to ±15° using an operating voltage of 25 V. A sub-millisecond switching time (<900 μs) and an optical insertion loss of 0.85 dB is achieved for multi-mode waveguides. The development of the proposed integrated-optical could be promising for an image generation system.
©2006 Optical Society of America
The rapid growth of the display and projection system industry has made substantial impacts on consumer electronics for decades. Not only do display and projection systems dominate the domestic entertainment market, but they have also been integrated into automobiles and mobile communications. Some conventional display technologies such as liquid-crystal displays  and plasma displays  have become popular recently. Alternatively, the grating light valve (GLV) displays, which rely on MEMS (micro-electro-mechanical-systems) technology to control and to direct light from semiconductor lasers to form a TV picture on a screen, have also attracted considerable interest . Similarly, other projection display techniques using MEMS technology such as the Digital Light Processor™ (DLP) chip , real-time color micro display  and laser display technology using a three-color mixing device , have also been promising and are ready for commercialization.
Through suitable arrangement of a number of channel waveguides, the use of integrated optics enables color and intensity modulation [7, 8]. A three-color-mixer device combining three colors of light (red, green and blue), is definitely a crucial component in any projection display system . This study reports on a new three-color-mixer using MEMS technology. Micromachined optical components have several advantages over their conventional counterparts including higher accuracy, higher switching speed, lower power consumption, and the capacity to be mass-produced at low cost. They could be realized using mature microfabrication technology [10, 11]. More importantly, the compactness of the micromachined optical elements makes the development of a miniaturized optical system feasible.
In this study, we also report a new projection display system comprising a three-color-mixing device, an optical lens, a GLV device and a microscanner. The 3×1 planar waveguide using a SU-8 material involves simple and low-cost processing steps when compared to silica-based materials. The GLV device presented in this paper is designed for optical modulation. Analytical models for the gratings are developed using optical diffraction theory. The microscanner is driven electrostatically and can be used for scanning projection display applications.
2. System architecture and three-color-mixing waveguides
Figure 1 schematically illustrates the new projection display system with integrated optical three-color-mixing waveguides, a GLV device, an optical lens, and a microscanner. The 3×1 planar waveguide provides red, green and blue colored lights and the mixing of which forms the free color. The free color light emits from the out-port of the planar waveguide device and is then collimated using a ball lens (with a diameter of 300 μm). The projection display picture is created by groups of movable ribbons that make up a GLV for optical intensity modulation. Finally, the microscanner is used to project the light beam to a screen. The light sources used in the experiment include a He-Ne laser (632.8 nm, red), diode-pumped lasers (532 nm, green) and an argon-ion laser (488 nm, blue), which are coupled into the waveguides via three multi-mode fibers.
This study used SU-8 negative photoresist as a core material of the 3×1 planar waveguides. This material has several excellent characteristics for optical applications, including high transmittance for light from the visible to the near-IR range (as shown in Fig. 2), a high refractive index after hard-baking and a low polymer volume shrinkage rate of 7.5% following the curing process. These properties make SU-8 a promising candidate for the core material of waveguides. A low-cost fabrication process for a multi-mode 3×1 planar waveguide device using the SU-8 thick photoresist is developed in this study. First, a 90-μm-thick SU-8 film (MICROCHEM Corp., SU-8 50) was spin-coated on the glass as the core layer and soft-baked at 65°C for 10 min followed by 95°C for 30 min. The waveguide patterns were defined by standard photolithography process using an UV light source. Following the exposure process, the SU-8 film was then baked at 65°C for 3 min, with a further baking at 95°C for 10 min. The patterns were developed for 10 min followed by hard baking on a hot plate at 180°C for l hour. The development process thus enables the formation of 3×1 core structures with a thickness of 90 μm and a width of 90 μm. Such a SU-8 core has a refractive index of 1.60 at 670 nm. In this study, the waveguiding properties of these integrated optical waveguide structures are calculated using a numerical method based on finite difference simulation. Figure 3 shows a scanning electron micrograph of a waveguide based on the SU-8 structures. Note that the new waveguides use SU-8 structures without any further coated cladding, which can significantly simplify the fabrication process.
The optical insertion loss of the SU-8 waveguides was measured using a conventional cutback method. Figure 4 shows the optical loss of the multi-mode SU-8 waveguide device is measured to be 3.5 dB/cm for light at a wavelength of 632.8 nm. The optical loss of the waveguide is caused mainly due to the natural attenuation of the light propagating in the SU-8 core material. This work also realized a prototype of a full color display by additive mixing of red, green and blue light colors and explored display characteristics through integrated-optical components using a 3×1 planar waveguide device. The measured intensity of the 3×1 planar waveguide mixing device (at a ratio of blue/green/red of 1: 0: 1) is shown in Fig. 5. Correspondingly, a full color display could be achieved by adding red, green and blue light colors in various combinations and intensities.
3. Grating light valve (GLV) devices
The GLV device is an optical MEMS device made by using the Multi-User MEMS Processes (MUMPs) available through MEMSCAP Inc. The fabrication process involves standard surface micromachining steps . Briefly, polysilicon is used as the structural layer; phosphosilicate glass (PSG) is used as the sacrificial layer, and silicon nitride is used to electrically isolate polysilicon from the silicon substrate. Finally, a layer of gold is deposited to provide the microstructures with probing, wire-bonding, electrical routing and high reflectivity mirror surfaces. Hydrofluoric acid (HF) is used to release the structures. Finally, the chip is dried using a supercritical CO2 process, or by heating the released chip after rinsing.
The electrostatically-driven GLV shown in Fig. 6 can be moved perpendicularly to the plane of the substrate to change the phase relationship between light reflected off the grating lines and the substrate. The dimensions of each grating line are 2 μm wide and 220 μm long with a 2-μm gap. The GLV device is coated with a layer of gold in their reflective top layer. The movable gratings are attached to the substrate by flexure beams which provide a restoring mechanical force. The electrostatic gratings have 0.75 μm deep dimples in the upper electrode surfaces to prevent stiction of the gratings after the release etch process, and to prevent the upper electrodes from contacting and shorting to the lower electrodes during actuation. The electrostatic gratings were designed to modulate optical intensity by shifting power from the zero diffracted order to the ± 1st diffracted orders. Figure 6 shows a diffraction pattern for a grating with an active area of 380 μm × 240 μm and 2 μm line/space dimensions. The angular separation and the number of diffracted orders agree with optical diffraction theory to within 3 %. When voltage is applied, the grating is moved toward the substrate, thus changing the phase difference between the light reflected from the top of the grating and light reflected from the substrate visible between the grating lines. The vertically actuated, surface-micromachined polysilicon GLV have been designed for optical switching and modulation applications. The moving grating achieves an 18.1 dB contrast ratio between the maximum and minimum intensity of the first diffracted order with a driving voltage of only 3 V at modulation rates up to 110 kHz , making it an excellent candidate for an optical switch or modulator.
4. Surface-micromachined optical scanner
Figure 7 shows an SEM image of an electrostatically-driven microscanner with a mirror scanning angle of up to ±15°, with a mirror diameter of 320 μm. The mirror is coated with a gold layer to serve as a reflecting surface. It is well known that a curved mirror surface tends to distort reflected light, causing scattering and insertion loss. The current device overcomes this limitation by introducing a layer of PSG between two polysilicon layers. The resulting three-layer sandwich stack structure has a total thickness of 4.25 μm, which ensures sufficient rigidity and therefore prevents the gold layer from physically curving. The sandwiched mirror structure has effectively improved the optical insertion loss and the flatness of the reflection area. The scanner is fabricated using the MUMPs process as described in Section 3. This study successfully demonstrated a surface-micromachined optical scanner using four stress-actuated polysilicon plates to raise a horizontal mirror. The optical scanner has a mirror scanning angle up to ±15° with a driving voltage of 25 V and a switching time of 900 μs. The optical insertion loss of the scanner is 0.85 dB for multi-mode waveguides.
It has been demonstrated that the blue/green/red light with a ratio of 1: 1: 0 can be mixed to produce cyan colored light and the blue/green/red light with a ratio of 0: 1: 1 can be mixed to produce yellow colored light. The light output from the planar waveguide device was collimated through a ball lens and a GLV directed to the microscanners. Figure 8 shows two scanning images of the cyan and yellow light with an exposure for two seconds. With this capability, a projection display system can be realized.
This work demonstrates a new and integrated optical three-color-mixing waveguides in a projection display system. The 3×1 planar waveguide structures were designed to produce a three-color-mixing device for a projection display. The simple SU-8 waveguide structure appears to be a promising approach, and is an excellent candidate for integrated optics. This projection display system comprises the 3×1 planar waveguide device, an optical lens, a GLV device and a microscanner. The electrostatically-driven GLV device was designed for optical modulation. The new microscanner using four stress-actuated polysilicon plates to raise a horizontal mirror was also demonstrated. The devices and the display system are extensively characterized. The development of the proposed technology could be promising for projection display applications.
This work was supported by the Ministry of Education in Taiwan under the MOE Program for Promoting Academic Excellence of Universities (A-91-E-FA08-1-4) and the National Science Council in Taiwan (NSC 95-2218-E-150-002). Access to major fabrication equipment from the Center for Micro/Nano Technology Research, National Cheng Kung University is greatly appreciated.
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