Digital liquid lens for fast scanning technologies

A robust lens, with rapid focus response and high diopter range, is used in a prototype touchless fingerprint sensor.
31 December 2015
C. W. Gary Tsai

Variable-focus liquid lenses have recently been exploited in compact camera modules to enhance autofocusing capabilities.1–3 Widespread adoption of these lenses in the manufacture of cellphone camera modules (CCMs) currently looks unlikely, however, because an alternative technology—voice coil motors—is already dominant in the CCM market. To find other suitable applications for variable-focus liquid lenses, we have recognized the need to tailor the lenses to unique device specifications.

One example application of variable-focus liquid lenses is for 2D barcode readers, in which the lenses have millisecond response times and several million operation cycles.4 Most liquid lens devices operate in analog mode because of specific design limitations, where hysteresis is pertinent in the responses. The set focus essentially depends on the amplitude of the applied voltage (i.e., similar to a mass–spring mechanical system). The focus response is therefore initially fast, but slow near to the target because of reduced driving forces. A problematic phenomenon, however, is that response times for small focus changes are greater than for larger focus changes.

To enable further applications, we have been developing liquid lenses (see Figure 1) with focal range greater than 40 diopters, tens of millions of operation cycles, and digital switching times of 5ms. Based on these lenses, we have developed a prototype touchless fingerprint sensor. The liquid lens that we report here has a robust double O-ring mechanical structure—see Figure 2(a)—and operates in digital mode.

Figure 1. A digital liquid lens with nine digital steps in its focus range.

Figure 2. (a-1) The concentric annulus electrode structure used in the digital liquid lens setup. (a-2) Cross section of a digital liquid lens. (a-3) Applying voltage, V(ϕ), and voltage with a πlagging phase, V(ϕ–π), during the focus-switching period. (a-4) Applying voltage during the focus-holding period. (b) Model of voltage application. Vholding: Holding voltage. Vdriving: Driving voltage. Tholding: Holding time. Tdriving: Driving time. (c) Relationship between applied voltage and velocity (V) of a tri-phase line of oil droplets. r: Radius of oil droplets. t: Time.

With our digital liquid lens, we are able to overcome the problematic response time issue associated with conventional analog devices. We achieve this by applying an over-scale voltage at the beginning, and by switching back to the setting voltage when the focus is close to the target value.5 This method, however, is complex, and difficult to implement in practical applications. Therefore, we have devised a simple digital operation mode with which we achieve fast response times6 by applying an alternating voltage into the liquid during the focus-switching period. We apply another alternating voltage—with a π lagging phase—into the electrodes, from the initial location of a tri-phase line of oil droplets that leads to the target electrode. The tri-phase interface then reaches the target electrode, which needs only the holding voltage to maintain the position of the oil droplet at the target focus. We intentionally set a high applied voltage so that we obtain a fast focus response with a low holding voltage. This prevents failure in the dielectric layer, as shown in Figure 2(b). The pattern of the high-driving and low–holding voltage gives rise to a fast switching speed and a long operation cycle as the optimal driving pattern. The advancing speed of the tri-phase line versus the amplitude of the applied voltage—see Figure 2(c)—indicates that the focus switching time can be less than 3ms in the digital liquid lens when the over–driving voltage is 30V root mean square.

As well as achieving a fast focusing response with our liquid lens, we are also able to achieve auto-centering in the optical axis and no hysteresis in the focus loop. Furthermore, our liquid lens enables fabrication variance. This is in contrast to analog lenses, in which the focal length is altered (for example) by changing the thickness of the insulation layer in the substrate. Our lens benefits from an easy-control algorithm and interfaces with custom-designed integrated circuits.

To achieve a reliable seal during the assembly process, we used a double O-ring mechanical structure for the liquid lens packaging (see Figure 3).7 If the seal were to fail during operation, the liquids inside the lens would leak out and evaporate, and thus generate an air bubble in the lens. An outer O-ring with a small diameter absorbs the thermal volume expansion of the liquid if the ambient temperature increases, and vice versa (see Figure 3). Without this outer O-ring, the liquid pressure would increase because of volumetric thermal expansion. The leak between the inner O-ring (with a larger diameter) and the glass would therefore increase, as would the length of the cycle time (under the high ambient temperature). We have empirically calculated that a liquid lens with a single O-ring would exhibit a bubble soon after 24 hours at 65°C, whereas the liquid lens with a double O-ring could last more than two weeks under the same conditions.

Figure 3. Schematic of the double O-ring structure of the liquid lens. (a) Liquid volume expansion caused by environmental temperature increase. (b) Liquid volume contraction caused by temperature decrease.

We have used fast z-axis scanning of our digital liquid lens to develop a prototype touchless fingerprint sensor.8 The fast focus response and high diopter range of our lens enable scanning of the focal plane—with a 2 million-pixel (2M) CCM—from 3 to 1cm, in 40ms. We can therefore resolve the limited depth of field problem in macro photography. To use our device, one holds a finger above the sensor to capture a fingerprint image. The sensor, with a scanned picture, and recognized features of a fingerprint are shown in Figure 4. These results indicate that the macro photograph picture area is larger than the dimension of the sensor.

Figure 4. A touchless fingerprint sensor based on z-axis scanning of the digital liquid lens. (a) The prototype module. (b) Captured color picture of a fingerprint. (c) Recognized fingerprint picture with marked minutiae.

We have developed a liquid lens that has a digital operation mode and a double O-ring structure. We have realized a touchless fingerprint sensor prototype with this lens. Based on this work, it is possible that fast-focus scanning technology could be introduced to facial recognition cameras, focus stacking in microscopy (i.e., multiple images taken at different focus distances to obtain an image with a greater depth of field), and macro 3D photography.3, 9 Ultimately, our digital liquid lens could be embedded into the autofocus of camera arrays.10 We are continuing to improve our digital liquid lens, with a particular focus on its reliability and manufacturability. Our next step will be to seek partners to develop new applications of our lens.

C. W. Gary Tsai
Lustrous Electro-Optics Inc.
Hsinchu, Taiwan

C. W. Gary Tsai founded Lustrous Electro-Optics Inc. after receiving his PhD from National Tsing Hua University in 2010. His aim is to commercialize his PhD research work, which involved the development of a digital liquid lens and a liquid iris.

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9. M. Martínez-Corral, P.-Y. Hsieh, A. Doblas, E. Sánchez-Ortiga, G. Saavedra, Y.-P. Huang, Fast axial-scanning widefield microscopy with constant magnification and resolution, J. Display Technol. 11, p. 913-920, 2015.
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