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Abstract
In this paper, a high stability liquid lens with optical path modulation function is designed and fabricated. The liquid lens has an outer chamber and an inner chamber, and the inner chamber has a structure with three annular anchoring layers. This structure can limit the sliding of the three-phase contact line under electrowetting effect and anchor the position of contact angle with a limited distance. The feasibility of this structure is verified by simulation and practice. The zoom imaging, contact angle, focal length and response time of the liquid lens are analyzed. The structure with three annular anchoring layers provides six anchored precision optical path modulation gears, and the optical path difference can be changed by mechanical hydraulic control, up to 1.17 mm. Widespread applications of the proposed liquid lens are foreseeable such as microscopic imaging and a telescope system, etc.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Liquid lenses are widely used in microscope, telescope, camera, laparoscope and other fields because of their continuous zoom and fast response capabilities [1–9]. At present, liquid lenses can be roughly divided into three categories: (1) Mechanical force liquid lenses are hydraulically driven by elastic membrane, piston or servo motor, changing the curvature of the liquid-liquid (L-L) interface or liquid-membrane interface [10–15]; (2) Under the action of external non-uniform electric field, the dielectric liquid lenses are driven by dielectric force, which changes the curvature of liquid droplets, thus realizing the change of focal length [16–23]; (3) Electrowetting liquid lenses change the contact angle by driving the three-phase contact line to slip with the electrowetting effect. Thus, the focal length can be varied by changing the curvature of the liquid boundary [24–27]. Among the three types of liquid lenses, electrowetting liquid lenses can realize continuous zooming without mechanical movement, with fast zooming speed, low cost and light weight. Thus, electrowetting liquid lenses have been commercialized in 2002 by B. Berge, et al.
In recent years, electrowetting liquid lens has been gradually developing towards the application of functional integration. In 2016, an optical shutter with a high extinction ratio based on electrowetting lens was proposed. By controlling the smooth interface of the liquid lens, the proposed optical shutter can make the transmitted and reflected beams have high extinction ratios of 55 dB and 66 dB, respectively [24]. In 2017, a multifunctional liquid lens based on electrowetting driving for high performance miniature camera was developed. This device was composed of an electrowetting driving liquid lens and liquid iris, and the integration of zoom function and variable aperture function were verified by the experiments [25]. In 2019, a fast controllable confocal acoustic microscope based on electrowetting zoom lens was proposed. The microscope was equipped with electrowetting photoacoustic synchronous zoom liquid objective lens, which could promote horizontal slice imaging of samples with irregular surfaces or multilayer structures at different depths [26]. In 2021, a method for measuring the field curvature of the target lens with electrowetting liquid lens was proposed. An electrowetting liquid lens was used to replace the laser, telecentric system or microscope in the traditional optical path. The testing system only needed the target lens and the electrowetting liquid lens [27]. This method not only reduces the cost but also simplifies the experimental process.
In 2021, an optofluidic phase modulator based on electrowetting liquid lens was proposed [28]. The modulator consists of an inner chamber and an outer chamber. By applying different voltages to the modulator, the transparent sheet fixed between the L-L interface moves up and down, resulting in the change of optical path length. In recent years, our group has also done related works. In 2018, we proposed an optofluidic lens with movable L-L interface and adjustable focus [29]. This optofluidic lens consists of an inner chamber and an outer chamber. The L-L interface of the inner chamber provides zoom imaging function. The outer chamber is driven by electrowetting to change the position of the L-L interface of the inner chamber. In 2019, our group proposed an optofluidic variable optical path modulator [30]. This device consists of two main chambers which are communicated with each other to form a closed-loop fluid system. When a voltage is applied to the device, the L-L interface in one chamber can move up and down under the action of electrowetting to adjust the optical path in the other chamber. Therefore, electrowetting lens with optical path modulation capability has become a research hotspot. However, there are some issues existing in the above devices. Firstly, the repeatability of the sliding range within the three-phase contact line driven by electrowetting is poor, which would affect the imaging quality. Secondly, the position of the L-L interface is roughly driven by a whole sheet of electrode according to electrowetting effect. That is to say, the voltage control accuracy under this design is relatively low.
In this paper, we propose a high stability liquid lens with optical path modulation function, which can solve the above problems. The proposed device has a double-layer structure consisting of an outer chamber and an inner chamber which are connected. The inner chamber has a special microstructure, which is called annular anchoring layers (AAL). It can precisely limit the sliding range of the three-phase contact line driven by electrowetting. The proposed device also has the capability to modulate the optical path difference driven by mechanical hydraulic pressure. Compared with our group’s previous research [29–30], an AAL is adopted in this work. This AAL can realize a high repeatability of large sliding range within the three-phase contact line, and achieves the effect of accurately anchoring the contact angle. This work can also provide a method to eliminate the problems of advance/lag contact angle. Furthermore, we stack three AALs together, which can solve the problem of optical path modulation error caused by inaccurate displacement of L-L interface. Experiments show that the proposed liquid lens has both high stability zoom imaging function and accurate optical path modulation function. Therefore, the proposed liquid lens will promote the diversified development of the electrowetting liquid lenses, and has a wide range of applications, such as microscopy imaging and telescopic system, etc.
2. Mechanism and structure
2.1. Mechanism of high stability
The structure of the high stability liquid lens with optical path modulation function is shown in Fig. 1. The cross-section of the proposed device is shown in Fig. 1(a), and it has a sleeve structure with an outer chamber and an inner chamber. The outer chamber is composed of a polymethylmethacrylate (PMMA) tube with an upper micropore and a lower micropore that allows the two liquids to be injected or discharged. The outer chamber is directly cemented with an ITO glass and a glass cover sheet. Three AALs are made of PMMA, which are called first/second/third AAL from bottom to top. Each AAL is coated with a hydrophobic layer, a dielectric layer, and an electrode layer for electrowetting actuation. The inner chamber is composed of four PMMA rings and three AALs. The diameter of the AAL is slightly smaller in diameter than the other four PMMA rings for forming a truncated geometric structure. In order to facilitate the flow of liquid, two rectangular holes are designed at the bottom of the inner chamber. The inner chamber is directly cemented with the ITO glass.
Fig. 1. High stability function mechanism of the proposed liquid lens. (a) Schematic structure of the proposed liquid lens. (b) The three-phase contact line located at point A, U=0 V. (c) Three-phase contact line located at point B, U = V1. (d) The three-phase contact line located at point C, U = V2.
The mechanism of the proposed high stability structure is shown in Figs. 1(b)–1(d). Taking the first AAL as an example, in the initial state, the applied voltage U=0 V. The three-phase contact line is located at point A, as shown in Fig. 1(b). The applied voltage starts to rise (U = V1). Since the effect of electrowetting, the three-phase contact line slides up to point B, as shown in Fig. 1(c). When the applied voltage continues to increase (U = V2), theoretically, the three-phase contact line should have slipped to point D. However, it is difficult for the three-phase contact line to step over this geometric structure and finally anchor at point C, as shown in Fig. 1(d). Therefore, the width of the geometric structure determines the sliding distance of the three-phase contact line. That is to say, the AALs can limit the slip of three-phase contact line and anchor the boundary of the L-L interface. Thus, the proposed structure can realize the high stability of the liquid lens.
2.2. Mechanism of optical path difference modulation function
The boundary of the L-L interface can move at equal intervals among the first, second and third AALs, and change the optical path after moving up or down, as shown in Fig. 2. In the initial state, the boundary of the L-L interface is located at the lower end of the first AAL, as shown in Fig. 2(a). In this state, the distance between the center of the L-L interface and the ITO glass is l1, and the distance between the center of the L-L interface and the cover sheet is l2. NaCl solution Liquid-1 is at the bottom and the refractive index is n1, and silicone oil Liquid-2 is at the top and the refractive index is n2. In this case, the optical path at the optical axis is n1l1+n2l2; Then, NaCl solution Liquid-1 is injected into the lower micropore and silicon oil liquid-2 is discharged from the upper micropore. The volume of the NaCl solution changes, which makes the L-L interface boundary rise to the upper end of the third AAL, as shown in Fig. 2(b). The rising distance of the L-L interface is x, and the optical path at the optical axis is changed to be n1(l1+x)+n2(l2-x). The optical path difference (OPD) can be expressed as:
On the contrary, when silicone oil Liquid-2 is injected into the upper micropore and NaCl solution Liquid-1 is discharged from the lower micropore, which makes the boundary of the L-L interface move down. In this case, the OPD can also be changed.Fig. 2. Working mechanism of optical path modulation of the proposed liquid lens. (a) Initial state. (b) L-L interface moving up.
2.3. Simulation of the high stability structure
We use the computational fluid dynamics module (CFD) in COMSOL Multiphysics to simulate the proposed geometric structure of anchoring the three-phase contact line slip. Physical field of two-phase flow level set is used for simulation. As shown in Fig. 3, the conductive liquid in the blue part is NaCl solution (dynamic viscosity 1.5 mPa·s, density 1.160 g/cm3, refractive index 1.3516). The non-conductive liquid in the red part is silicone oil (dynamic viscosity 50 mPa·s, density 1.087 g/cm3, refractive index 1.5082). The initial contact angle is 136°, and the interfacial tension between NaCl solution and silicone oil is 50 mN/m. The thickness of the dielectric layer is 3 μm and the relative dielectric constant is 2.65. A truncated geometric structure with a width of 0.2 mm is constructed on the inner wall of a cylinder with a diameter of 8 mm. According to Young-Lippmann equation, the relationship between the contact angle θ and the applied voltage U can be described as follows:
where ε is the dielectric constant of the insulating layer, d is the thickness of the dielectric layer, γ12 is the interfacial tensions of silicon oil/NaCl solution, and θ0 is initial contact angle.Fig. 3. Simulation results of the proposed geometric structure of anchoring the three-phase contact line slip. (a), (b) and (c) are the simulations of L-L interface shape of the liquid lens with applied voltage of 0 V, 50 V and 60 V, respectively. (d), (e) and (f) are the simulations of pressure distribution at the yellow rectangular area in (a), (b) and (c) with applied voltage of 0 V, 50 V and 60 V, respectively.
The location of the three-phase contact line and the pressure distribution are simulated when the applied voltage is 0 V, 50 V and 60 V. When the applied voltage is 0 V and 50 V, the three-phase contact line does not move to the boundary of the geometric structure, as shown in Figs. 3(a)–3(b). The pressure is uniformly distributed over the L-L interface, as shown in Figs. 3(d)–3(e). When the applied voltage is 60 V, the three-phase contact line moves to the position of the geometric boundary, as shown in Fig. 3(c). The pressure is concentrated at the corner of the geometric structure, as shown in Fig. 3(f). When the applied voltage continues to rise, the three-phase contact line remains at the same position. We can draw a conclusion that the three-phase contact slip driven by the electrowetting effect will be anchored when encountering a truncated geometry, which is caused by the interfacial tension between the two liquids. Therefore, the proposed liquid lens has a high stability not only when it is actuated by the driving voltage, but also when it is used in a slightly oscillating environment.
3. Fabrication
The fabrication procedure of the proposed liquid lens is shown in Fig. 4. In order to observe the L-L interface and the position of the three-phase contact line, all components of the proposed device are highly transparent materials. The ITO film (In2O3, thickness ∼200 nm) is used as the electrodes and coated with a dielectric layer (dielectric constant ∼2.56, thickness ∼3 μm) and the hydrophobic layer (thickness ∼30 nm). Then, the coated electrode is cemented with three PMMA rings (2 mm height, 8 mm inner diameter) by glue UV-339 for forming the three AALs, as shown in Fig. 4(a). The inner chamber is prepared by connecting four PMMA rings (2 mm height, 9 mm inner diameter) with the three AALs, as shown in Fig. 4(b). The inner chamber and outer chamber (12 mm height, 13 mm inner diameter) are bonded to ITO glass whose size is 25 mm × 25 mm. Then, the NaCl solution (density ∼1.160 g/cm3, refractive index ∼1.3516) is injected first and then the silicone oil (density ∼1.087 g/cm3, refractive index ∼1.5082) is injected. During the process of liquid injection, the volume of the two liquids is controlled so that the boundary of L-L interface is located at the required position. Finally, the inner chamber and outer chamber are sealed with the cover sheet, as shown in Figs. 4(c)–4(d). The actual PMMA parts and electrodes of the prepared device are shown in Fig. 4(e). The top view and side view of the actual fabricated inner chamber are shown in Figs. 4(f)–4(g). The complete high stability liquid lens with optical path modulation function is shown in Fig. 4(h).
Fig. 4. Fabrication procedure of the proposed liquid lens. (a) Hydrophobic layers, dielectric layers, electrodes and PMMA rings bonded together by glue UV-339 to form three AALs. (b) Four PMMA rings and three AALs cemented together by glue UV-339 for forming the inner chamber. (c) Inner chamber cemented with ITO glass. (d) Outer chamber, cover sheet and ITO glass cemented together. (e) Some components of the proposed liquid lens. (f) Top view of the inner chamber. (g) Side view of the inner chamber. (h) Complete device of the proposed liquid lens.
4. Experiment, results and discussion
4.1. Anchoring effect of AAL
The anchoring effect of the proposed liquid lens with high stability is tested, as shown in Fig. 5. To facilitate the observation of the L-L interface and its position, blue ink is added to the NaCl solution, as shown in Figs. 5(a)–5(b). The anchoring effect is tested in the first AAL. By changing the volume of the liquids and controlling the distance between the boundary of the L-L interface and the upper end of the first AAL, different anchored contact angles under applied voltage of 90 V are shown in Fig. 5(c). Finally, the anchored contact angle and threshold voltage under three-phase contact line slip distance is drawn, as shown in Fig. 5(d). When the slip distance of the three-phase contact line is 0 mm and 0.4 mm, the anchored contact angles are 136° and 127°, respectively, as shown in Figs. 5(a)–5(b). When the slip distance is under ∼0.2 mm, the threshold voltages are ∼35V. When the slip distance reaches to ∼1 mm, the threshold voltage is ∼90 V. It can be seen that this truncated geometry has a good anchoring effect in controlling the three-phase contact line, and eliminating the problem of poor repeatability of the slip range within the three-phase contact line.
Fig. 5. Anchoring effect of the AAL on three-phase contact line under applied voltage of 90 V. (a) L-L interface shape when the three-phase contact line slip distance is 0 mm. (b) L-L interface shape when the three-phase contact line slip distance is 0.2 mm. (c) Slipping distance diagram of the three-phase contact line. (d) Anchored contact angle and threshold voltage under the three-phase contact line slip distance.
The L-L interface and zoom imaging results of the liquid lens are shown in Fig. 6. In order to observe the shape and position of the L-L interface, blue ink is added in the NaCl solution. The boundary of the L-L interface at the upper end of the geometry during actuation (0-80 V) is shown in Fig. 6. The boundary of the L-L interface in the middle of the geometry under applied voltage of 0 V is shown in Fig. 6. The boundary of the L-L interface in the middle of the geometry and under applied voltage of 80 V is shown in Fig. 6. The boundary of the L-L interface in the lower end of the geometry during actuation (0-80 V) is shown in Fig. 6(d). It can be seen from Figs. 6(a), 6(b) and 6(d), the boundary of the L-L interface is still anchored during the actuation process, regardless of whether the boundary position of the L-L interface is at upper or lower end of the geometric structure. The top view of the imaging results in Fig. 6 also shows no significant changes. It can be seen that the truncated geometry has the function of anchoring the contact angle at both the upper boundary and the lower boundary, and is relatively stable. As shown in Fig. 6(b) and 6(c), the zoom imaging results are relatively obvious under applied voltage of 0 V and 80 V.
Fig. 6. L-L interface shape and imaging results of the proposed liquid lens. (a) Boundary of the L-L interface located at point A during actuation from 0-80 V (see Visualization 1). (b) Boundary of the L-L interface located at the point B when the applied voltage is 0 V (see Visualization 2). (c) Boundary of the L-L interface located at the point C when the applied voltage is 80 V (see Visualization 2). (d) Boundary of the L-L interface located at the point D during actuation from 0-80 V (see Visualization 3).
We have added three visualizations to demonstrate the anchored dynamic process during actuation. Visualization 1 shows the dynamic process when the liquid-liquid interface is anchored at point A. Visualization 2 shows the dynamic process when the liquid-liquid interface is slipping within point B and point C. Visualization 3 shows the dynamic process when the liquid-liquid interface is anchored at point D. From Visualization 1 and Visualization 3, we can see that when applied voltage of 80 V, the liquid-liquid interface can be change little. While in Visualization 2, the curvature of the liquid-liquid interface can be changed. That is to say the three-phase contact line can be anchored by the ALL structure.
The optical properties of the proposed liquid lens under the applied voltage of 90 V are shown in Fig. 7. Firstly, we put the proposed device on the platform of the contact angle measuring instrument (Type of JCY-2, Shanghai Fangrui Instruments Co., Ltd., China). This contact angle measuring instrument contains a CCD camera and a measurement software. Then, by controlling the injection and discharge of the two liquids, the boundary of the L-L interface is just located in the middle of the first AAL, and thus the characteristics of the proposed liquid lens are obtained. When we applied voltages on the liquid lens, the contact angle will be changed accordingly. The shape of the contact angle would be captured by the CCD camera and the degrees will be measured by the software. Last, we collated a series of data to produce Fig. 7(a). In the initial state, the contact angle is 136° and the applied voltage increases from 0 V. The contact angle remains at 136° until the applied voltage reaches 40 V. Therefore, the threshold voltage of the liquid lens is 40 V. When the applied voltage rises to 90 V, the contact angle is 90°. When the applied voltage increases to 120 V, the contact angle becomes 56°.
Fig. 7. Optical properties of the proposed liquid lens. (a) Changes of the contact angle under different applied voltages. (b) Response time of the contact angle under the applied voltage of 90 V. (c) Setups for measuring the focus lengths of the proposed liquid lens. (d) Experimental and theoretical changes of the focal lengths under different applied voltages. (e) Zoom ratio when applied different voltages.
To measure the response time of the contact angle under applied voltage of 90 V, we used the software to break up the videos which are captured by the contact angle measuring instrument into each frame images. Then, we use the measurement software to analysis and process the images. Finally, the response time of the contact angle under the applied voltage of 90 V is shown in Fig. 7(b). The contact angle starts to become smaller when the applied voltage reaches to 90 V at 2.5 s in an instant. At 8.5 s, the contact angle reaches 90° and remains stable without changing for 3 seconds. When the applied voltage is withdrawn at 11.5 s, the contact angle recovered to 133° at about 25 s, and the actuation process is relatively slow. The contact angle can not be completely recovered in a short time because of the effect of charge accumulation. In order to facilitate the observation of phenomena, silicone oil with high viscosity is used. Silicone oil with low viscosity can be used to shorten the response time of the liquid lens.
There are some methods to measure the focal lengths such as employing the Shack-Hartmann [31]. In our experiment, the focal length is measured by the testing system which consists of a collimator (Type of F1000, Shanghai Precision Instrument co., Ltd., China), a solid lens, a CCD camera and the proposed liquid lens. When the proposed liquid lens works as a positive lens, the light source can be focused on the CCD camera. When the proposed liquid lens works as a negative lens, a solid lens should be added before the liquid lens for focusing, as shown in Fig. 7(c). The negative focal length can be calculated by the following equation:
where f is the focal length of the liquid lens, fs is the focal length of the solid lens, fL is the focal length of the testing system, and D is the distance between the liquid lens and the solid lens. We have added details in the revised manuscript. According to Young-Lippmann equation we can derive the theoretical relationship between focal length and the driving voltage. In the initial state, the focal length is ∼-35.5 mm. When the applied voltage increases from 40 V, the focal length starts to change. When the applied voltage increases to 90V, the focal length is infinite. As the applied voltage continues to increase, the focal length changes from negative to positive. When the applied voltage is 120 V, the focal length is ∼45.5 mm.The zoom ratio is also depicted in Fig. 7(e). The largest zoom ratio of can be reached to ∼0.98 when the driving voltage is 80 V.
4.2. Optical path modulation
By injecting NaCl solution into the lower micropore and discharging an equal volume of silicone oil from the upper micropore, the L-L interface is controlled to rise gradually, as shown in Fig. 8. In the initial state, the boundary of the L-L interface is located at the lower end of the first AAL. When we inject 75.4 μL of NaCl solution and discharge 75.4 μL of silicone oil, the boundary of the L-L interface rises to the upper end of the first AAL. According to the above process, 377.0 μl of NaCl solution is finally injected and 377.0 μl of silicone oil is discharged. The boundary of the L-L interface rises to the upper end of the third AAL. Therefore, the proposed device has six anchored precision optical path modulation gears, which could also eliminate the optical path modulation error caused by the unstable repeatability of the boundary of the L-L interface. In addition, the liquid lens also has the function of continuous optical path modulation between adjacent gears.
Fig. 8. Positions of L-L interface when modulating the optical path. (a) L-L interface anchored at lower end of the first AAL. (b) L-L interface anchored at upper end of the first AAL. (c) L-L interface anchored at lower end of second AAL. (d) L-L interface anchored at upper end of second AAL. (e) L-L interface anchored at lower end of third AAL. (f) L-L interface anchored at upper end of third AAL.
The change of optical path difference (OPD) with the injection volume and the injection speed of the NaCl solution is shown in Fig. 9. As shown in Fig. 9(a), with the increase of the injected volume, the L-L interface rises and the OPD also increases in an approximate linear relationship. There are six precise optical path modulation gears, and the OPD can finally reach to 1.17 mm. When the L-L interface moves up, the response time of the OPD changing with the injection speed is shown in Fig. 9(b). When the injection speed of the NaCl solution is 301.6 μL/s, 150.8 μL/s and 75.4 μL/s, the response time to reach the maximum OPD of 1.17 mm are 1.25 s, 2.5 s and 5 s, respectively. When the L-L interface moves down, the response time of the OPD changing with the extraction speed is shown in Fig. 9(c). When the extraction speed of the NaCl solution is -301.6 μL/s, -150.8 μL/s and -75.4 μL/s, the response time to reach the maximum OPD of -1.17 mm are 1.25 s, 2.5 s and 5 s, respectively.
Fig. 9. Changes of OPD with injection volume and injection speed of the NaCl solution. (a) Changes of OPD at different injection volume. (b) OPD response time for moving up. (c) OPD response time for moving down.
4.3. Applications
The application of the proposed high stability liquid lens with optical path modulation function in imaging is shown in Fig. 10. The setup for imaging application is shown in Fig. 10(a). The optical path is arranged according to the sequence of the object, the proposed liquid lens, solid lens, and the CMOS camera. When the boundary of the L-L interface is located at the lower end of the first AAL, the image captured by CCD camera is shown in Fig. 10(b), and the image is very blurred. The L-L interface moves up by the injection and extraction of the two liquids. When the boundary of the L-L interface is located at the upper end of the third AAL, the imaging result is shown in Fig. 10(c), and the image is very clear and the words “BUAALIQUID” are clearly visible.
Fig. 10. Imaging application of the proposed device. (a) Setup for imaging application. (b) Image result when the L-L interface boundary is at the lower end of the first AAL. (c) Image result when the L-L interface boundary is at the lower end of the third AAL.
The application of the proposed high stability liquid lens with optical path modulation function in interferometry is shown in Fig. 11. The interference optical path is set as the Michelson interference optical path for optical path modulation application, as shown in Fig. 11(a). The He-Ne laser (λ=632.8 nm) illuminates the polarizers and the beam expander. Then, the laser beam is divided into two laser paths by a beam splitter (BS), and one laser path passes through the proposed liquid lens. Then, the two laser paths pass through the mirror reflectors respectively, and pass through the BS again. The two laser paths are combined to produce interference fringes, and finally the interference fringes are captured by the CMOS camera. When the L-L interface is located at the lower end of the first AAL, the interference fringes are shown in Fig. 11(b). Then the L-L interface moves up by the injection and extraction of the two liquids. When the boundary of the L-L interface is located at the upper end of the third AAL, the interference fringes is shown in Fig. 11(c). From the two interferograms, we can see the obvious changes of fringe spacing.
Fig. 11. Optical path modulation application of the proposed device. (a) Setup for optical path modulation application. (b) Optical path modulation result when the L-L interface boundary is at the lower end of the first AAL. (c) Optical path modulation result when the L-L interface boundary is at the lower end of the third AAL.
5. Discussion
In the optical path modulation function, we only depend hydraulic actuation to achieve this function. Thus, during the actuation process, the driving voltage is zero. If we want to realize the tunable zoom function and optical path modulation function simultaneously. We should apply voltage and hydraulic pressure at the same time. However, this might make the operation more complex and also bring some instability to the device. Therefore, we need to balance the functions and device stability.
From Visualization 1 and Visualization 3, we can see that the liquid-liquid interface is hardly changed at all when applied voltages. That is to say, the focus ability is almost lost at the boundaries anchoring layers. While, when the liquid-liquid interface is slipping within point B and point C, the dynamic change is no different from that of a traditional liquid lens. The focus ability would not be affected when inserted the anchoring layers. At the same time, the demerits of the anchoring layers are that it will make the production process to be more complicated. For example, we need to plating the dielectric layer and the electrode at a very small width. The three anchoring layers can also result in poor sealing and difficulty in alignment, which can seriously affect imaging quality.
We used Zemax to simulate the MTF when the liquid lens is working at different AALs. From Fig. 12, we can see that when the liquid lens is working at first ALL, the spatial frequency is 6 mm/lp @MTF>0.2. When the liquid lens is working at second ALL, the spatial frequency is 20 mm/lp @MTF>0.2. While, when the liquid lens is working at third ALL, the spatial frequency is ∼3.2 mm/lp @MTF>0.2. Thus, the proposed liquid lens should be work at second ALL for zoom imaging application. Optical path modulation function and focus function can be worked at all ALLs.
Fig. 12. Simulation of the MTF when the liquid lens working at different AALs. (a) Working at first AALs. (b) Working at second AALs. (c) Working at third AALs.
6. Conclusion
In this paper, a high stability liquid lens with optical path modulation function is designed and fabricated. The proposed liquid lens is driven by a mixture of electrowetting effect and mechanical hydraulic. The proposed device has an outer chamber and an inner chamber, and the inner chamber has a structure with three annular anchoring layers. This structure can limit the slip of the three-phase contact line under electrowetting effect and anchor the contact angle. The simulation and practical verification show that the structure can overcome the contact angle error caused by the poor repeatability within the three-phase contact line, and solve the problems of advance angle and lag angle. The zoom imaging, contact angle, focal length and response time characteristics of the proposed device are analyzed. The structure with three annular anchoring layers provides six anchored precise optical path modulation gears, and the optical path difference can be continuously tuned among adjacent gears. The optical path difference can be changed by mechanical hydraulic up to 1.17 mm. The proposed liquid lens can be applied in the field of imaging and phase modulation.
Funding
National Natural Science Foundation of China (61927809, 61805169).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
No data were generated or analyzed in the presented research.
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