1. Introduction

Ever-growing deployment of hand-held mobile smart IT devices has recently accelerated the development of efficient and convenient touch panel technology [1–3]. Flat panel display technology has become indispensable as a way to present various types of information any place, any time. Owing to the diversification of video presentation methods, the size of flat panel displays is increasing for use in applications such as video street signage, projection screen replacement, and realistic 3D media display. For such large-size displays, touch screen technology is also important. However, the touch panel technology that is applied to smart phones and notepads, which is based on electrical capacitance measurement, is not suitable for large-size displays.

For use as large-size touchpads, optical multi-touch screens have been receiving considerable attention because their sizes are easily expanded [4]. Monitoring scattered light from a frustrated total internal reflection (FTIR) enables rapid detection of the multi-touch of fingertips, although this requires a camera with fast image processing [5]. The FTIR signal has been detected in a planar waveguide structure with an array of light sources and detectors, which requires significant computational power for analyzing the optical pattern [6]. Detection of the shadow caused by the obstructing object was also widely investigated and has been used for developing robust commercial touch panels [7–9].

Optical waveguide technology based on polymer material has been maturing to provide various devices for modern optical communication system [10,11]. The flexibility of polymer waveguides was used for demonstrating unique optical devices, such as fast optical modulators and strain-induced tunable lasers [12,13]. Recently, a polymer waveguide device was adopted for measuring the force applied on a surface [14,15]. In the present work, a flexible polymer waveguide is incorporated to realize an optical touch panel device. A polymer waveguide matrix is formed to sense the touch signal, and then, the touch on the surface of the device introduces the loss of the optical signal propagating through the waveguide matrix. The loss is due to a change in the mode-coupling strength between the channel-guided mode and a planar-waveguide mode. Owing to the high sensitivity of the coupling strength, the touch panel operates with a small force comparable to the pressure of a pencil writing. To overcome the problem of waveguide pattern visibility, an index-matching liquid is inserted to remove the air gap. Using a 4 × 4 matrix waveguide configuration, we demonstrate the functionality of this device in multi-level pressure-detecting touch panels.

2. Operating principle

The proposed optical touch panel is based on a directional coupling of the guided modes in two waveguides, as shown in Fig. 1 . For a preliminary demonstration, a 4 × 4 matrix-type channel waveguide array is prepared on a substrate. Then, the channel waveguide matrix is covered by a planar waveguide of plastic film. To prevent initial coupling, a spacer pattern is inserted between the channel waveguide and the planar waveguide. When pressure is applied to the top of the plastic substrate, the planar waveguide is bent so as to make contact with the channel waveguide. Then, the light signal initially launched at the channel waveguide is coupled into the planar waveguide. Once the light is coupled into the planar waveguide, it no longer comes back to the channel waveguide. By monitoring the output power remaining in the channel waveguide, one can detect a touch on the surface.

 

Fig. 1 Schematic of the proposed optical touch panel consisting of channel waveguides in a 4 × 4 matrix configuration and flexible planar waveguides covering the channel waveguides with an initial gap maintained by the spacer pattern. When pressure is applied, the planar waveguide is bent to touch the channel waveguide and cause evanescent field directional coupling.

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In order to design the waveguide geometry, two-dimensional beam propagation method simulation was performed. Depending on the waveguide thickness and the radius of curvature produced by the applied pressure, the directional coupling efficiency was calculated. The waveguide structure is shown in Fig. 2(a) . The 30-μm-thick planar waveguide with a bending part was placed above the straight waveguide. Both waveguides should be made of the same polymer for the efficient power coupling. The refractive index of the polymer is 1.562 for 1550 nm wavelength, and the cladding has a refractive index contrast of 0.005. The radius of curvature was fixed at 10 mm, while the gap distance was a variable. As shown in Fig. 2(b), the propagating light confined on the channel waveguide was suddenly coupled into the planar waveguide at the point of a small gap.

 

Fig. 2 Design results based on BPM simulation: (a) waveguide structure and design parameters defined, (b) intensity profiles exhibiting the directional coupling into the planar waveguide for a gap distance of 0, and d of 6 μm, and (c) output power remaining on the channel waveguide as a function of gap distance for various channel waveguide thicknesses.

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Power coupling efficiency was dependent on the gap distance as well as the channel waveguide thickness, as shown in Fig. 2(c). Because a thinner channel waveguide produces a deeper evanescent field, the sensitivity to the gap distance was increased for the thinner waveguide. When the gap distance was larger than 5 μm, there was no initial coupling.

3. Fabrication procedures

The proposed plastic optical touch panel was fabricated by following well-established polymer waveguide fabrication procedures, as illustrated in Fig. 3 . The arrayed channel waveguide was formed on a glass wafer with an UV-curable polymer material (CO-156) available from ChemOptics Co. The core layer polymer was spin-coated on a glass wafer to have a 5 μm thickness. It was cured for 10 min in a UV chamber and then hard-baked at 160°C for 30 min. To form a rectangular core waveguide, a thick photoresist (AZ9260) was used to define the waveguide pattern with a width of 8 μm, and then, the core layer was etched to the bottom using oxygen plasma. Etching parameters are used for core etching are O₂ pressure of 100 mTorr, RF power of 100 W, and an etch rate of 0.5 μm/min. To maintain the initial gap between the two waveguides, an SU-8 photoresist was used to define the spacer pattern with a thickness of 10 μm. The waveguide made of CO-157 material typically exhibits the propagation loss of about 1.0 dB/cm for 1550 nm wavelength.

 

Fig. 3 Fabrication procedure of the plastic optical touch panel completed by attaching the two parts: a glass substrate with a channel waveguide and a planar waveguide coated on a flexible plastic substrate.

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The planar waveguide was formed with the CO-156 polymer of 30-μm thick on a 500-μm thick PMMA film. The film was turned upside down over the channel waveguide matrix and was adhered by the edges using a UV curable epoxy. At this moment, an index-matching liquid was inserted to fill the gap between the two waveguides in order to improve the transparency of the touch panel and enhance the evanescent field. Before using the refractive index liquid, the waveguide pattern was slightly visible to the naked eye, as shown in Fig. 4(a) . However, when the liquid fills the gap, the channel waveguide pattern becomes invisible to the naked eye, as shown in Fig. 4(b). To find the oil with proper refractive index, a series of index matching oil with a different refractive index was prepared, and then the propagation loss of the curved channel waveguide was measured with the oil covers the core as the cladding. The oil with the refractive index closest to that of the core was found in this way. The transparency of the touch panel was once again compared with different angle, as shown in Figs. 4(c) and 4(d). The sample in Fig. 4(c) was filled with water, while the sample in Fig. 4(d) was filled with index-matching liquid. One can clearly observe that the waveguide pattern is invisible on the sample with index-matching liquid.

 

Fig. 4 Transparency of optical touch panel (a) before liquid insertion with the waveguide pattern visible and (b) after the index-matching liquid is injected, and the waveguide pattern becomes invisible; for a different viewing angle, the transparency (c) when water is filling up the gap, and (d) when the index-matching liquid has filled the gap and no waveguide pattern is visible. The distance between the waveguides is 0.5 mm, and the size of glass wafer is 3 x 3 cm².

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4. Measurement of the performance

The optical touch panel device was characterized using a 1550-nm laser source. For coupling into the four-channel waveguide, a V-groove of a single-mode waveguide array was pigtailed on the edge of the touch-panel device. For output power monitoring, a CCD camera was used to observe the variation in the output mode power. Initially, the mode picture was obtained, as shown in Fig. 5(a) . The waveguides located at the outside exhibited slightly lower output power, which was not critical in the device operation monitoring the power variation. In the other mode images shown in Fig. 5, one can observe a significant output power change on each channel when the corresponding channel waveguide was touched one by one.

 

Fig. 5 Waveguide-mode pictures of (a) initial guided light, and (b)–(e) show the output power decreasing when a pressure is applied on a corresponding waveguide. The distance between waveguides at the output is 250 μm.

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For a quantitative measurement of a touch-induced optical power change, a multi-mode fiber array was pigtailed on the output port of the device. A certain amount of force was applied on points P₁ and P₂ in Fig. 6(a) using a force gauge. The pressure-applying tip attached to the gauge was round with a curvature radius of about 0.5 mm. By increasing the applied force gradually, we observed a monotonic decrease in the output power until a maximum force of 1.2 N, and an output power change of 13 dB was obtained, as shown in Fig. 6(b). The minimum force of 0.3 N was required to initiate the change of output power, and it could be decreased by reducing the spacer height to enhance the sensitivity. The temporal response of the device was measured, as shown in Fig. 6(c). For the external force increased to 0.7 N on point P₁, the optical power decreased by about 9 dB. After about 60 s, the force increased to 1.0 N, and the output power decreased by 8 dB. The output power quickly recovered to the initial value as soon as the contact force disappeared. The abrupt change of output power observed just after the force application is due to the manual operation of force application. It was hard to adjust the force to 0.7 N or 1.0 N immediately, and causes some delay as noticed in the data.

 

Fig. 6 Output power variation measured for (b) applied force on P1, as shown in (a); (c) the instantaneous output power measured when P1 was pressed with increasing force in two steps; and (d) the power variation when P1 and P2 were pressed step-by-step.

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Detection of multi-point touch is important in many touch-panel applications. Hence, we observed the response for touching the two points P₁ and P₂ simultaneously as shown in Fig. 6(d). For P₁ touched at first by 1 N, the output power decreased by 10 dB. Then, maintaining the force on P₁, another 1.5 N was applied to P₂, the power further decreased. As P₂ was released, the power recovered by one step, and as P₁ was released, it recovered to its initial value. The two-step power change occurred because the force applied on P₁ was not enough to radiate the guided mode completely. Thus, we demonstrated that the proposed device is useful for detecting multiple-point contacts simultaneously.

5. Conclusion

A novel optical touch-panel device was proposed and demonstrated based on directional coupling between channel waveguides fabricated on a glass substrate and a planar waveguide fabricated on a flexible plastic substrate. With 4 × 4 matrix configuration, the output optical power change was proportional to the applied force, and the point of applied force was detectable by monitoring the output power distribution. The transparency of the optical touch panel was improved by incorporating an index-matching liquid whose refractive index was close to that of the waveguide core material. The device was capable of applied pressure measurement as well as multi-touch detection and was sensitive to the applied force of 0.3 N, which can be improved by controlling the evanescent field depth and the initial gap distance.