We report an electrically controlled optofluidic zoom system which can achieve a large continuous zoom change and high-resolution image. The zoom system consists of an optofluidic zoom objective and a switchable light path which are controlled by two liquid optical shutters. The proposed zoom system can achieve a large tunable focal length range from 36mm to 92mm. And in this tuning range, the zoom system can correct aberrations dynamically, thus the image resolution is high. Due to large zoom range, the proposed imaging system incorporates both camera configuration and telescope configuration into one system. In addition, the whole system is electrically controlled by three electrowetting liquid lenses and two liquid optical shutters, therefore, the proposed system is very compact and free of mechanical moving parts. The proposed zoom system has potential to take place of conventional zoom systems.
© 2017 Optical Society of America
Zoom imaging devices are essential tools in many areas such as industry, medical science, military, even our daily life. And people even more favor compact imaging systems, which means small overall size and low cost. Adaptive lenses such as liquid lenses and liquid crystal lenses can make image system compact [1–5]. For liquid crystal lenses [3,4], the refractive index can be tuned by applying voltages, thus the focal length is tuned without moving parts. However, polarization dependence and the restricted aperture can lead to lower efficiency and lower resolution. Fortunately, liquid lenses are polarization-independent and provide spherical interface, which is very similar to a glass lens. Many zoom lens systems based on liquid lenses are proposed [6–10]. In previous publications, elastomeric membrane liquid lenses and electrowetting liquid lenses are two common lenses which are used in a zoom lens system. For example, a fluidic adaptive zoom lens with high zoom ratio is proposed . Driven by liquid pressure, the system can achieve a large zoom ratio. Another zoom lens system for laparoscope is proposed based on elastomeric membrane liquid lenses . The system can also achieve a large zoom tuning range from 3.2 mm to 12.9 mm in simulation. However, these zoom lens systems have two obvious disadvantages: gravity effect and mechanical drive, which make them difficult to obtain high resolution image and need complex driving unit. Maybe, the electrowetting liquid lens is an ideal way to realize zoom image system due to direct voltage actuation, and its two-phase structure can help to eliminate gravity effect. The image quality of a liquid lens based on electrowetting is as good as that of a conventional glass lens. However, the electrowetting lens still has one disadvantage: low optical power. For example, zoom objectives using electrowetting lenses is proposed [8-9]. The objective can achieve good image quality and continuous zoom change. However, due to low optical power of electrowetting lenses, the zoom ratio of these kinds of zoom system is very small. More recently, a three-phase electrowetting zoom lens is proposed . The zoom lens can acquire tuning focal length range and is compact. However, the tuning range of focal length is still very limited. Therefore, only if both the zoom range is largely increased and the imaging quality is improved, can the zoom systems be widely applicable and take place of the conventional glass zoom systems.
In this paper, we propose and experimentally demonstrate an electrically optofluidic zoom system which can achieve a large continuous zoom change and high-resolution image. The proposed zoom system consists of an optofluidic zoom objective and a switchable light path which are controlled by two liquid optical shutters. It can achieve a very large tunable focal length range from 36mm to 92mm, and the zoom ratio is over 2.6 × . In this tuning range, the proposed zoom system can correct aberrations dynamically, thus the image resolution is high. In addition, the proposed zoom system is free of moving parts, compact, thus potentially low cost.
2. Device structure
The proposed zoom system is shown in Fig. 1(a). The proposed zoom system consists of two main parts: an optofluidic zoom objective and a switchable light path. The optofluidic zoom objective is composed of three liquid lenses and two glass lenses, and the switchable light path is composed of two beam splitters, two reflectors and two liquid optical shutters. The optofluidic zoom objective can vary its focal length by changing the interfaces of the three electrowetting liquid lenses. The switchable light path has two light paths switched by the two liquid optical shutters. When the system need short focal length, the three liquid lenses vary the interfaces to obtain the focal length. Then the short light path turns on while closing the long light path by applying voltages on the liquid optical shutter 2, as shown in Fig. 1(b). In this state, the proposed system has short focal length and wide field of view (FOV), which can be used as a camera. When the liquid optical shutter 2 is turned on while keeping the liquid optical shutter 1 closed, as shown in Fig. 1(c), the proposed zoom system can vary the interfaces for long focal length and narrow FOV, which can be used as a telescope. The longest focal length of the short channel is the same as the shortest focal length of the long channel. Thus, the whole system has continuous zoom change.
A conceptual model of the proposed zoom system is shown in Fig. 2. We take infinite object distance for analysis. The effective focal length f and back focal distance L can be obtained as follows:.
We see that f is determined by L and h' . h' is the function of the power of the three liquid lenses. In previous optofluidic zoom system [6–10], L is fixed. Therefore, the tuning range of f is only determined by power of liquid lenses. We know that the power of an electrowetting liquid lens is very limited, so the tuning range of f is very small. However, in the proposed zoom system, there are two optical paths, which means L is also tunable. Therefore, the tuning range of f is increased a lot.
In this system we eliminate moving parts that would typically be associated with a varying L by using two interchangeable optical paths. For the proposed system, liquid optical shutters are the essential parts to switch the light paths. The device structure and operating mechanism of our shutter are depicted in Fig. 3. Two glass substrates are placed together to form a chamber. The bottom substrate is composed of two glass stripes coated with indium-tin oxide (ITO) electrode. The right ITO glass stripe is coated with a Teflon layer. A small amount of opaque liquid is filled in the cell. Silicone oil is filled in the surrounding of the opaque liquid. In the voltage-off state, the opaque liquid keeps in one corner in the cell and the light channel is open, as shown in Fig. 3(a). When a voltage is applied to the electrodes, the opaque liquid moves toward opposite direction due to electrowetting and covers the whole substrate. Then the light channel is closed, as shown in Fig. 3(b).
In the proposed zoom system, both liquid lenses  and liquid optical shutters  are electrowetting-actuated devices. The working principle is based on Young–Lippmann equation, the relationship of the contact angle θ and the applied voltage U can be described as follows :
3. Design and fabrication
We designed and simulated the proposed zoom system in Zemax. Since there are two light paths, we simulated two configurations. The 2D layout is shown in Fig. 4. Figure 4(a) shows the layout of short focal length mode, and the back focal distance is 40mm. Figure 4(b) shows the layout of long focal length mode, and the back focal distance is changed to 84mm.
The radii (r1, r2, r3) were varied to produce the desired focal lengths. To get the desired focal length, we first optimized the radii of the three liquid lenses to get the solution for each focal length in Zemax. The merit function is constructed based on the three requirements.12]. f3(r1, r2, r3) is included in the Default Merit Function in Zemax . The three functions are included in Zemax. In the optimization process, we use Default Merit Function added with desired focal lengths f0 and we set back focal distance L0 fixed.
After optimization, the optimized radii converted to applied voltages according the product data book provided by Varioptics . Then the optimized voltages were applied to the three electrowetting liquid lenses. In fact, because of fabrication error, we usually need slightly adjust each voltage until the image is clearest. Figure 5 shows several solutions in the tuning range, and corresponding voltage is also shown in this figure. From the figure, we see that the whole focal length can be tuned from 36mm to 92mm, which is realized by the two modes together. For short focal length mode, the tuning range is 36mm to 60mm. And, for long focal length mode, the tuning range is larger (60mm to 92mm). In fact, for short focal length mode, the tuning range can be designed as long as that of long focal length mode. However, for short focal length mode, the FOV is larger, so that the aberration is much more difficult to correct than that of long focal length mode. Therefore, in our design, we shorten the focal length tuning range of short focal length mode to reserve the ability to correct aberrations.
We also evaluate the optical property of the proposed zoom lens system. The key parameters are shown in Table. 1.
To get high-resolution image, the aberrations must be corrected when zooming. Figure 6 shows the blur spots of the proposed system in the focal length tuning range. From the spot diagram, we see that the diameter of the ray traced spot is relatively small and close to that of Airy disk. The corresponding MTFs are shown in Fig. 7. We see that the MTF curve at each focal length is also close to that of the diffraction limit. We further magnified special line pair, as shown in Fig. 7, to evaluate the image quality. All the angular resolution of the magnified line pairs (for example 82cycles/mm at f = 36mm, 64cycles/mm at f = 44mm and so on) are 1.34′, which means 1mm line width target at 5m object distance. From the figure, we see that in such a large tuning range, the MTF is not degenerated when zooming. And, the values of the worst MTF curves in this range can be kept over 0.2. Based on both spot diagrams and MTFs, we can conclude that the image quality is good in the tuning range and the most aberrations are corrected. We also see that not all aberrations are corrected completely. For example, for large FOV (12°) at f = 36mm, the blue spots are larger than the other spots, which indicates chromatic aberration is still not corrected completely. The main reason is that the main power of liquid lens is used to get short focal length, so the ability to correct aberrations becomes weaker. However, the overall image quality is good enough to achieve high resolution.
To assemble the proposed zoom system, we design and fabricate all the elements except the liquid lenses. The liquid lens is a commercialized electrowetting liquid lens Arctic 39N0 produced by Varioptics . The effective aperture of the liquid lens is ~3.9mm. The fabricated device is shown in Fig. 8(a). All the fabricated elements are shown in Figs. 8(b) - 8(c). The beam splitter is a dielectric layer. The transmittance and reflectance are 50% and 50%, respectively. The reflector is a glass substrate coated with silver film, and the reflectance is 95%. The glass lenses are made of material SF2 and FK1 in glass data of SHOTT . The refractive index and Abbe number of SF2 are 1.647 and 33.89, respectively. The refractive index and Abbe number of FK1 are 1.486 and 81.81, respectively. For the fabricated eletrowetting liquid optical shutter, the conductive liquid is the solution of ink doped with NaCl (ρ∼1.08 g/cm3). Then the surrounding of the water was filled with silicone oil (ρ∼1.08 g/cm3). The dimension is 15mm × 15mm × 5mm. For the whole device, the dimension of the optofluidic zoom objective is Φ25mm × 24mm, and the dimension of the switchable light path is 48mm × 41 mm × 37 mm.
4. Experiment and discussion
Before evaluating the imaging performance of the proposed zoom system, we first test the electro-optical property of the liquid optical shutter. In the experiment, an circular aperture behind the liquid optical shutter is used to model the aperture of the light path in the proposed zoom system. The diameter of the circular aperture is 5.5mm. When we applied 70 V voltage on the shutter, the black liquid spread and covered the whole aperture due to electrowetting effect, thus the light channel was closed. The recorded images of dynamics of the liquid actuation is shown in Fig. 9. From Figs. 9(a)-9(d), the light channel can be gradually closed. The time used to make the aperture close is ~270ms. When the voltage is off, the opaque liquid recovers to its original place as shown in Figs. 9(e)-9(h). The light channel is reopened. The time used to make the aperture reopen is ~2s. From the experiment, we can conclude that the fabricated liquid optical shutter can switch the light channel on and off with only voltage actuation.
To evaluate the optical performance of the proposed zoom system, we built a scene which consists of a resolution target chart and colorful toy. The objective distance is 5m. CMOS camera was used to record the image. The pixel size in the CMOS is 2.2μm × 2.2μm. The resolution is 2592 × 1944. We first turned off the long channel by applying 70V voltage (AC 1000Hz) on the liquid optical shutter 2. Then the calculated voltages were used to apply on the three liquid lenses for controlling radii of the three interfaces. The captured pictures are shown in Figs. 10(a)-10(d). For this mode, the FOV is relatively large. The whole scene can be observed. Besides, the colorful object “Baby bell” can be continuously magnified. When it is magnified to 1.5 × , it is difficult to get larger magnified image because the optical power of the liquid lenses almost reaches its limitation. Then, we turned on the long channel by removing the voltage on the liquid optical shutter 2 while applying 70V voltage (AC 1000Hz) on the liquid optical shutter 1. After 2s response time, the long channel is open. Thus, the device switches to long focal length mode. We recalculated voltage data and applied the voltage on the three liquid lenses. The captured image is shown in Figs. 10(e)-10(h). The object is further magnified. Finally, we see that only the center of scene can be observed as shown in Fig. 10(h). Comparing Fig. 10(a) with 10(h), the object is magnified to 2.6 × . In the whole experiment, the focal length tuned in a very large range, approximately from 36mm to 92mm.
In each subfigure of Fig. 10, we magnified the observed target bar whose line width is 1mm. In Zemax simulation (Fig. 7), the MTF at each focal length is equally good. In our experiment, we see that all the target bars can be clearly observed. However, the magnified image for long focal length mode are better. There are two main reasons. First, for long focal length mode, the FOV is smaller, which results in less aberrations. Second, the image plane is CMOS. When we use long focal length mode, the magnification is larger than that of short focal length mode. Thus, the image of long focal length mode takes up more pixels than that of short focal length mode. Therefore, the digital magnified target is clearer. On the whole, image quality is equally good in the whole tuning range. From the experiment, we can conclude that the proposed zoom system can achieve a large tunable focal length range from 36mm to 92mm with high-resolution image. Besides, only the voltage is used to zoom the focal length and switch the optical shutters, and no moving parts are used. Thus, the proposed zoom system is very compact and easy to operate.
For the proposed zoom system, the focal length tuning range is largely improved, but the light efficiency is decreased. For short channel, there are two beam splitters, so the transmittance decreases to 25%. For long channel, there are another two reflectors, so the transmittance is lower (22%). However, the aperture is relatively large, so light efficiency is acceptable in practical use as shown in Fig. 10. Besides, we can increase the exposure time to compensate the loss of light efficiency.
The tuning range of the focal length can be further improved. There are two ways. On one hand, if we use liquid lenses with larger power, the tuning range can be increased. However, to improve the optical power of the liquid lens, we must find liquids with larger refractive index. On the other hand, we can use more light channels, for example, three channels. In this case, the tuning range can be increased largely, but light efficiency will be further decreased, which need more exposure time or larger aperture to compensate the light loss.
To conclude, we report an electrically optofluidic zoom system which can achieve a large continuous zoom change and high-resolution image. The zoom imaging system consists of an optofluidic zoom objective and a switchable light path which are controlled by two liquid optical shutters. The proposed zoom system can achieve a large tunable focal length range from 36mm to 92mm. The zoom ratio is 2.6 × . And in this tuning range, the zoom system can correct aberrations dynamically. Due to large zoom ratio, the proposed imaging system can be used as a zoom camera or a zoom telescope. The whole system is electrically controlled by using three electrowetting liquid lenses and two liquid optical shutters. Therefore, the proposed system is very compact and free of mechanical moving parts.
National Natural Science Foundation of China (61535007 and 61505127), and the Equipment Research Program in Advance of China (JZX2016-0606/Y267).
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