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The reflective method is utilized in the optical zoom function of a thin camera for the advantage of folding the optical path. An ionic polymer metal composite deformable mirror used in a reflective zoom system achieves large deformations to change optical power with a low bias voltage. Polydimethylsiloxane is used as a buffer layer to improve surface roughness. The surface roughness of this layer is about 17 nm. The optical focusing power of the deformable mirror reaches 73.8 m−1 diopters with 3 volts. A complete reflective camera module is fabricated using two ionic polymer metal composite deformable mirrors in the zoom function. The zoom ratio is about 1.6 × .
© 2015 Optical Society of America
1. Introduction
In recent years, a built-in camera is a necessary component for many commercial portable devices, such as cell phones, tablets and notebooks. Such cameras need to be smaller than ever, following trends in design and quality. The conventional auto-focus image system is composed of a series of lenses which are shifted by a mechanical motor system. However, the space required for shifting the lens makes it difficult for these systems to be made smaller. To solve this problem, many alternative optical components have been studied actively for the last decade. Using deformable mirrors in reflective optics is one such method.
The reflective method has the advantage that the light path can be folded to miniaturize the camera module [1]. Deformable mirrors are also widely applied in adaptive optics [2–4] and have recently been made with microelectromechanical system (MEMS) technology. However, a MEMS deformable mirror needs a relatively high operating voltage (over 100 V); therefore, it is not convenient for portable devices. For this reason, ionic polymer metal composite (IPMC) is used as a material for deformable mirrors. IPMC can achieve large deformations under low voltages (usually less than 5 V) [5,6]. It also has characteristics like low weight, no noise, and flexibility. Therefore, it is widely used in applications such as actuators, artificial muscle and biomimetic sensors [7,8].
The structure of IPMC comprises an ionic polymer membrane between metal electrodes on both sides. The ionic polymer membrane, Nafion®, is the actuation backbone of IPMC. The metal electrode layer, which is usually formed by a platinum electroless plating process, adheres on both sides of the ionic polymer membrane. The profile is shown in Fig. 1. By diffusion in the ion exchange process, the ionic polymer membrane can absorb water molecules and several kinds of cation, such as H+, Li+, Na+, or K+ [9]. Among these cations, Li+ ions provide IPMC with the best performance in force generation. IPMC can be actuated by a bias voltage. When the bias voltage is applied to both sides of the electrode, the anion group, which is the structure of the polymer membrane, is fixed. However, the cations are easily attracted to the cathode. At the same time, water molecules are also dragged to the cathode by the moving cations because of their polarity. Many water molecules gathered at the cathode cause volume expansion of the ionic polymer membrane. The unbalanced strain in the IPMC makes it bend toward to the anode side [10–12].
Fig. 1 The cross-sectional profile of IPMC deformable mirror structure. (a) IPMC deformable mirror with no bias voltage applied. (b) IPMC deformable mirror with bias voltage applied.
In the fabrication of IPMC, an electroless plating process for generating the electrode layer is the conventional method for effectively adhering metal ions to the polymer surface. Before the electroless plating process, however, surface roughening of the ionic polymer membrane is necessary to strengthen the adhesion between the metal and polymer layer. This can make the IPMC more durable and achieve better actuation performance [13,14]. However, the coarse surface caused by roughening would cause poor light reflection due to the scattering effect. Many micro-valleys spread across the IPMC surface would be an obstacle for making a deformable mirror. To solve this problem, polydimethylsiloxane (PDMS) is adopted as a buffer layer, filling these micro valleys. Then a silver layer is deposited on the PDMS as a reflective surface. Silver, which has good reflectivity in visible wavelengths and low activation, is an excellent material for the reflective surface.
In this paper, the fabrication process is described in Section 2. The performance of the IPMC deformable mirrors is discussed in Section 3. A compact camera module with optical zoom capability using two IPMC deformable mirrors is discussed in Section 4. The conclusions are provided in the last section.
2. Fabrication process
2.1 IPMC fabrication
The fabrication of conventional IPMC can be roughly divided into three major steps: pretreatment, the initial compositing process, and the surface electrode plating process. After completing these steps, the IPMC can be placed into a LiOH solution for ion exchange.
The pretreatment process includes roughening and cleaning the ion polymer membrane, which is made from Nafion 117. In the roughening process, 1000 Cw sandpaper or sandblasting is used to roughen both sides of the Nafion membrane. This is shown in Fig. 2(a).
Fig. 2 IPMC fabrication process. (a) Roughening process. (b) Pt+ ion replacement process. (c) Initial compositing process. (d) Surface electrode plating process. (e) Li+ ion exchange process.
The initial compositing process aims to metalize the inner surface of the ionic polymer membrane by chemical reduction. The first step is ion replacement which is shown in Fig. 2(b). The reaction is shown in Eq. (1) [15].
The initial compositing process makes a foundation for the electrode. There is an approximately 10 μm platinum particle layer formed underneath the membrane surface which is shown in Fig. 2(c). The primary reaction is shown in Eq. (2).The surface electrode plating process is also a chemical reaction. The procedure is similar to the initial compositing process, and a platinum particle layer of about 10 μm is formed in the end. In contrast to the initial compositing process, this process lets the platinum particles gradually stack up upon the surface and form a platinum layer – see Fig. 2(d). This process effectively increases the conductivity of the IPMC electrode. After these processes, the IPMC is immersed into a 0.1 M LiOH solution for ion exchange, as shown in Fig. 2(e).
2.2 Surface treatment
The IPMC is first flattened on a hotplate by a heavy load at 60°C for 4 hours to release internal stresses produced in the fabrication process. After that, IPMC is cut into the appropriate shape by a craft knife. With the assistance of an acrylic model, the redundant electrode is scraped or cut off. The shape of the conductive region corresponds to our deformable mirror. Then the processed IPMC is fixed on the glass by double-sided tape and heat-resistant tape to form a flat surface.
To improve the rough surface, PDMS is used as a buffer layer to fill micro-valleys on the IPMC surface. The PDMS is composed of a 10:1 weight ratio of polymer and curing agent. These two elements are mixed uniformly and then put in a vacuum desiccator, at around 20 to 30 Torr for 1.5 hours, to remove air bubbles. The appropriate amount of PDMS gel is coated on the IPMC membrane by a spin-coater at 1500 rpm for 50 seconds. In our experience, the spin method alone cannot achieve a flat surface. The coarse IPMC surface causes the PDMS gel to be uneven. For this reason, the platen method is used. A soft and smooth plastic slice is carefully placed on the PDMS gel. The PDMS gel tightly contacts the plastic slice and forms a flat plane. The plastic material has poor adhesion to PDMS, and is easily stripped off after the PDMS is cured. A piece of glass is placed on the plastic sheet and lightly fixed in place by tape. The structure is shown in Fig. 3(a). It is then placed on a hotplate at 40°C for 16 hours to cure the PDMS. After that, the upper piece of glass and plastic sheet are removed carefully. The remaining structure is placed in a vacuum desiccator at room temperature for two days. After the PDMS has thoroughly changed from liquid gel to solid film, a flat PDMS layer remains coated on the IPMC surface.
Fig. 3 (a) Structure of PDMS application using platen method. (b) Photo of fabricated IPMC deformable mirror image.
When the buffer layer is complete, the next step is to form a reflective layer. The treated IPMC is processed using electron beam evaporation. A titanium layer (60 Ả) and a silver layer (600 Ả) are deposited onto the PDMS surface. The titanium layer enhances the adhesion between the polymer and metal layers. The silver layer has good reflectivity and is therefore used as the reflective surface. Figure 3(b) shows the finished sample.
3. Experimental results and device characterization
3.1 Surface roughness
It is well known that if the surface roughness of the interface is below a certain threshold, incident photons will regard the surface as a flat plane. The ideal maximum surface roughness is regarded as one tenth of an incident ray’s wavelength in applications of geometric optics [16]. In other words, if this system is generally applied to the visible wavelength region (400 nm ~700 nm), the surface roughness should be less than 40 nm.
A probe-type surface analyzer is used to measure the surface roughness of both the conventional IPMC and the surface-treated IPMC. The result for conventional IPMC is shown in Fig. 4(a). Ra is the average roughness. The surface is extremely uneven with many micro-valleys because of the roughening process; the roughness measurements reach up to 270 nm. This is much higher than the ideal maximum roughness of 40 nm. Figure 4(b) is a surface image taken by scanning electron microscope (SEM). The platinum ions stack messily and roughly with narrow cracks and small holes. Obviously, this would cause significant scattering and be a poor optical reflective surface.
Fig. 4 Surface roughness of conventional IPMC. (a) Measured by probe-type surface analyzer. (b) SEM image.
Figure 5(a) is the result for the surface-treated IPMC. The scales for Fig. 4 and Fig. 5 are made the same for comparison. The surface is flat with no cracks or valleys because the PDMS layer covers the uneven surface uniformly. The average roughness is about 17 nm which is better than the ideal roughness. Figure 5(b) is an image of the surface-treated IPMC with its PDMS coating and silver layer. Also, it is obvious that the surface is smoother than the conventional IPMC. Clearly, using PDMS as a buffer layer to cover IPMC is effective in reducing surface roughness.
Fig. 5 Surface roughness of surface-treated IPMC. (a) Measured by probe-type surface analyzer. (b) SEM image.
3.2 Surface reflectivity
Reflectivity is the most important index for a mirror used in an optical system. The performance directly affects image quality. Two kinds of IPMC are deposited with silver layers (500 Ả) for this measurement. A Jasco V670 UV-Visible IR spectrometer is used to determine the reflectivity. Wavelength range is from 300 nm to 800 nm, covering the visible spectrum. Figure 6 shows the results of specular reflectivity. The conventional, unsmoothed IPMC has only about 0.5% specular reflectivity across the whole range. The light is scattered almost entirely due to the excessive surface roughness. The surface-treated IPMC has about 90% reflectivity in the visible spectrum, which demonstrates that the PDMS layer provides a flat surface for optical reflection. The rapid drop of reflectivity in the ultraviolet wavelength is due to the properties of silver; strong surface plasma absorption gives rise to this phenomenon.
3.3 Center displacement
IPMC has the characteristic that it can be operated at low voltage. A deformable mirror made of IPMC can achieve relatively high optical power compared to other focus-changing techniques [1–5]. This advantage of the IPMC deformable mirror has the potential to be of benefit for miniature zoom systems made for portable devices.
The deformable mirror is made elliptical in shape to fit our zoom module; 6.6 mm in the sagittal direction and 5.6 mm in the tangential direction. In this way it can provide different optical power in two directions to correct astigmatism aberration. The center displacement of the deformable mirror can be measured by optical microscope. First, the optical microscope is used to get a clear image of the deformable mirror surface. The image becomes blurred when a bias voltage induces surface deformation. The stage of the optical microscope is adjusted to make the image clear again, and the adjustment distance, which corresponds to the magnitude of surface deformation, is recorded.
Figure 7 shows the center displacement versus differential voltage of the IPMC deformable mirrors. Each point is the average of three values. The displacement in Fig. 7(a) is approximately quadratic function of introduced voltages as predicted by Branco et al. [17]. In this work, we use laser displacement sensor (Keyence LK-G80) to determine the dynamic response of a surface improved IPMC by applying sinusoid wave of 3 volts at different frequency. The result is shown in Fig. 7(b). There is a peak value, which indicates the resonant frequency of the IPMC. We found that the resonant frequency was approximately at 180 Hz. The bias voltage is increased to only 3 V in order to maintain a flat reflective surface. The electrolysis of water begins when the voltage is more than 1.5 V. When the voltage is above 3 V, bubbles are produced that would cause the PDMS layer to peel off of the IPMC and destroy the reflective surface. The shortage of water molecules also rapidly degrades the performance of IPMC. The figure shows the results for the two types of deformable mirror. The surface-treated mirror is a conventional IPMC deformable mirror coated with about 35 μm of PDMS buffer. Open-loop force and position responses of an IPMC are not repeatable, and hence closed-loop precision control is of critical importance to ensure proper functioning, repeatability and reliability [18].
Fig. 7 (a) Center displacement of conventional IPMC and surface-treated IPMC. (b) dynamic response of surface-improved IPMC.
The bias voltage causes cations and water molecules to be attracted to the cathode. At higher voltages, more water molecules gathering at the cathode cause more volume change and thus larger displacements. The center displacement of the surface-treated IPMC is lower than that of the conventional IPMC under the same bias voltage; this shows that the elasticity of PDMS has little effect on the actuation of the deformable mirror.
The relationship between optical power and center displacement are shown in the Fig. 8 and equations below. The focal length is given by f, P is the optical power, rc is the radius of curvature, h is displacement, and D is the diameter of the circular area. By measuring the center displacement, the optical power can be obtained.
Table 1 shows the optical power of the surface-treated IPMC deformable mirror. The center displacement is 145 μm at 3 V. Although there is a single value of displacement, the different diameters make the optical power different in each direction. The values for optical power are 53.2 m−1 and 73.8 m−1. The biconic effect can be easily achieved by the ellipse shape.
Table 1. Data for IPMC Deformable Mirror under 3 V.
Table 2 shows the properties of different focus-changing techniques. Obviously, the IPMC technique achieves high optical power change at low applied voltage compared to the other techniques.
Table 2. Comparison of Different Techniques for Optical Power Change.
A white-light interference microscope can determine the pattern of deformation. The IPMC deformable mirror covered by a PDMS layer is placed in the instrument and a bias voltage is applied. The pattern is shown in Fig. 9(a). The IPMC deformable mirror pulls down into a concave ellipse pattern. Figure 9(b) is the cross-sectional profile of the center area.
Fig. 9 (a) Deformable mirror center image by white-light interference microscope. (b) Cross-sectional profile.
4. Optical zoom system
The optical zoom system produced in the present work is intended for use in portable devices [Fig. 10]. Compact system size is the design goal. Using the reflective method, the camera module size can be reduced by folding the light path. It is also effective in avoiding chromatic aberration. The camera module performs the functions of changing focus and zooming. It contains five elements, including two deformable mirrors and three reflective solid mirrors. The first (solid) mirror and deformable mirror 1 (DM1) are divergent with negative (N) power. The second and third solid mirror and deformable mirror 2 (DM2) are convergent with positive (P) power. This system uses a configuration of the “NPNPP” type. The first mirror with negative power has the function of receiving optical rays from large angles. The thickness of this optical zoom module is only 9.2 mm, which is thinner than most conventional optical zoom cameras that use mechanical motors.
Deformable mirrors change optical power by deformations of their reflective surfaces. The first one is responsible for changing the field of view and the second one is responsible for compensating the focal length. At the wide end, the first deformable mirror is actuated to convex and the second deformable mirror is actuated to concave. This results in the shortest effective focal length of 4.61 mm; the field of view is 52°. However, at the tele-end, the deformable mirrors are actuated to be flat and provide the longest effective focal length of 9.22 mm; the field of view is 27°. The simulation result shows the zooming ratio can reach to approximately 2 × . Figure 11 shows the surface contours of two deformable mirrors when the system is at the wide end. Table 3 lists parameters of three mirrors and two IPMC deformable mirrors in optical zoom system.
Table 3. Optical Zoom System Specification.
Figure 12 shows the image simulation results of two configurations. ZEMAX is used to simulate the quality of the image. The parameters for the simulation are a 5 megapixel image sensor with 1.4 μm x 1.4 μm pixel size. Figure 12(a) is the image at the tele end and Fig. 12(b) is the image at the wide end. Figure 13 shows the structure of the optical zooming module after packaging. Unlike traditional camera systems in which lenses are mounted on tubes in sequence, the five mirrors are fixed in a folding path to narrow the space.
To verify the zooming function and performance of the camera module, we set up an optical experiment using a CCD sensor to receive the image. First, we use a 3D printer to fabricate the shelf holding the camera module and the CCD sensor. A piece of paper printed with “NTU” is fixed on a transparent acrylic plate and placed in front of the camera module. A light source is placed behind the transparent acrylic plate at an appropriate distance. The experiment is conducted in a dark room to avoid interference from other light sources. Although the distance from the paper to the camera module is fixed, the zoom function can be implemented by actuating the two deformable mirrors.
In the first step, we actuate the first deformable mirror to get a clear image. By adjusting the system focal length, the image gradually changes from blurriness to clarity. In this situation, a large image of the word “NTU” appears in the image sensor. In the next step, we apply voltage to the second deformable mirror. Along with the difference in optical power, the field of view also changes. Then we adjust the first deformable mirror to get a clear image again. It can be seen that word “NTU” appears as a smaller image. The zoom ratio is approximately 1.6 × . Finally we release both applied voltages to let the deformable mirror return to its original state.
In this experiment, the zoom ratio does not reach 2 × because we only applied the bias voltage to 3.2 V. Higher bias voltage would cause intense electrolysis which would make the PDMS layer peel off from the IPMC. Ways to improve this are to replace the LiOH solution with another solution possessing a higher electrolysis voltage, or to find a firmer material as buffer layer. The experiment image is not as clear as the simulation results [Fig. 14]. We propose two possible reasons. One is the misalignment of objects combination, such as the distance from the image plane to the CCD sensor. The other is aberrations caused by imperfect mirror surfaces. The deforming surface might be slightly different from the simulation. Theoretically speaking, it is possible to correct induced aberrations by means of adaptive optics [3,4,21]. A video of the zoom function in action is also provided with this paper.
5. Conclusions
In summary, because traditional optical zoom systems need space for shifting lenses, they are not suitable for slim, portable devices. Using the reflective method and deformable mirrors can address this problem. A reflection module can be fabricated in miniature because of the folding light path. It also can reduce the degree of chromatic aberration. The deformable mirror changes optical power using a bias voltage. This feature allows the space to be used more effectively.
IPMC is used to make a deformable mirror. The advantage of IPMC is that it can be operated at low voltage and achieves relatively high changes in optical power. However, surface roughening is necessary in the fabrication process. To improve the surface, PDMS is coated on the IPMC surface as a buffer layer. PDMS can fill the micro-valleys and create a flat plane. The surface roughness of conventional IPMC is up to 270 nm, which is too rough for optical reflection. The surface-treated IPMC has a roughness of only about 17 nm, which is lower than one tenth of the visible wavelength region. In the measurement of specular reflectivity, the surface-treated IPMC achieves up to 90% reflectivity. This shows that a silver layer deposited on PDMS provides good reflection.
The change in optical power of an IPMC deformable mirror achieves about 73.8 m−1 with only 3 V. Compared to other focus-changing techniques, IPMC displays better performance in this regard, which shows its potential for use in portable devices. Finally, IPMC deformable mirrors are built into a camera module to verify the zoom function. The experimental results show that a zoom ratio is achieved of approximately 1.6 × .
Acknowledgments
The authors would like to gratefully acknowledge the funding supports from National Taiwan University 104R7747 and Ministry of Science and Technology 101-2628-E-002-019-MY3.
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