1. Introduction

An adaptive liquid lens has a tunable liquid interface, which allows its focal length to change without any moving parts. Thus, the system based on the adaptive liquid lens is usually compact. Various driving methods and structures have been proposed [1–12]. Among these devices, the most promising device is the one with two features: wide focal length tuning range and zoom ability. Especially, until now, there is no liquid lens which can realize zooming with only one liquid-liquid (L-L) interface. The liquid lens with air-liquid (A-L) interface usually has large power and wide focal length tuning range. However, it suffers from serious gravity effects. The liquid lens with L-L interface can eliminate the gravity effect by choosing two liquids with the same density, while the optical power and focal length tuning range are rather limited. One common approach is employing a refractive or diffractive solid lens to increase the power. For example, a liquid tunable diffractive/refractive hybrid lens has been proposed [13]. The hybrid lens consists of a Fresnel lens and a PDMS membrane lens. The power is increased, while the tuning range is still limited. Besides, due to A-L interface, it suffers from gravity effect. Similar hybrid lenses have been proposed for spectral imaging applications [14,15]. These devices utilize high chromatic dispersion of the lens to axially separate and discretely collect only the focused images at different narrow wavelength bands. However, gravity effect and large focal length cannot be achieved by using these devices. In our previous work, we employ a refractive lens to help an electrowetting lens obtain high power, and the gravity effect is also eliminated [16]. However, the tuning range focal length is not widened. To realize zooming, usually several liquid lenses need to be used. Thus, the lens system are still bulky To further miniaturize the zoom system, one liquid lens with zoom ability is proposed. An all-liquid dual-lens optofluidic zoom lens is proposed [17], which has two L-L interface in one cavity. Another displaceable and focus-tunable electrowetting optofluidic lens is proposed [18]. The L-L interface can move and deform, thus it can realize zooming. However, for these liquid lenses, at least two voltages are needed to realize zooming. Therefore, the operation is still complex. Therefore, novel methods to increase the focal length tuning range are still desirable, and realizing zoom by one L-L interface is also desired to largely reduce the system size and simplify driving mechanisms.

We propose a zoom liquid lens employing multifocal Fresnel zone plate. The proposed lens has two optical surfaces: L-L interface and Fresnel zone plate. The Fresnel zone plate is designed to have multifocal points and an increased depth of focus. Combining the two optical surfaces together, the proposed lens has two obvious advantages. Due to increased depth of focus, the proposed lens can realize zooming using only one tunable L-L interface, which is not available for conventional liquid lens. Thus, it is possible to remove conventional zooming mechanisms from cameras. Besides, the focal length tuning range is also increased, and a lens system based on the proposed lens can simultaneously collect two images with different magnifications.

2. Device structure and theoretical analysis

The cross-sectional cell structure and the operating mechanism of our adaptive lens are depicted in Fig. 1. The proposed lens has two optical surfaces: L-L interface and Fresnel zone plate as shown in Fig. 1(a). The L-L interface is formed by two immiscible liquids (oil and conductive liquid). The L-L interface is a tunable interface which can be driven by electrowetting effect according to Young–Lippmann equation [1]. The Fresnel zone plate is fabricated on the window glass of the proposed lens. The Fresnel zone plate has many designed ring-shaped blocks on a planar plate, as shown in Fig. 1(b). The Fresnel zone plate has two main conjugate focal lengths. In our design, for the two main conjugate focal lengths, the focal length has an enhanced depth of focus (DOF) in a range from f_min to f_max, as shown in Fig. 1(b). Therefore, the two optical surfaces both can be treated as tunable surfaces. In this way, we can optimize the two variables f_p and f_L to satisfy the following equations, where f_p is the focal length of the Fresnel zone plate, f_L is the focal length of the L-L interface.

 

Fig. 1. Schematic cross-sectional structure and operating mechanism of of the proposed adaptive lens: (a) Schematic of the proposed lens. (b) Schematic of FZP. (c) Zoom state 1. (d) Zoom state 2.

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Besides, due to two main conjugate focal lengths of the FZP, the proposed lens has two additional extended focal length tuning range. The two additional optical power (D=1/f) of the proposed lens can be expressed as follows:

3. Fabrication and experiment

In the fabrication process, we use lithography to create the lens pattern in the resist (SU-8). The diameter of the FZP is 5mm. The type of the FZP is phase-correcting. The number of phase steps is 1. The circles of the pattern are 100. The designed wavelength is 532nm. The designed positive and negative focal lengths are 70mm and −70mm, respectively. The lens pattern is fabricated on a window glass with a diameter of 9 mm. The material of the glass is BK7. The fabricated FZP is shown in Fig. 2(a). Figure 2(b) shows the optical microscope images of a fabricated FZP with 10× magnification and 20× magnification, respectively. The other L-L interface is formed by using a conductive liquid and an oil. The conductive liquid is NaCl solution, and its density is ∼1.09g/cm³ (refractive index n₁=1.38). The oil is silicon oil and its density is ∼1.09g/cm³ (refractive index n₂=1.50). The tilted sidewall of the proposed lens is coated with Teflon as the hydrophobic layer. The thickness of the coated hydrophobic and insulating layer is measured to be ∼3µm. Electrowetting effect happens on the sidewall to change the shape of the L-L interface according to Young–Lippmann equation [1]. The relationship of the contact angle θ and the applied voltage U can be described as follows:

 

Fig. 2. Fabrication of the proposed lens: (a) Fabricated FZP. (b) Microscope images of a fabricated FZP with 10× and 20× magnification. (c) Fabricated multifocal zoom lens. (d) Image quality at different focal lengths.

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The fabricated zoom liquid lens is shown in Fig. 2(c). The material of the main frame is aluminum. The whole size of the fabricated adaptive lens is 12mm (Diameter)×7mm (Height). The effective aperture is 5mm. For the fabricated FZP, we also measured its DOF. The f_min is measured to be ∼66.8mm, and f_max is measured to be ∼73.6mm, as shown in Fig. 2(d). In the tuning range, the resolution target can be seen clearly and image quality is very good, and the DOF is enhanced to ∼6.8mm, which is not available for conventional refractive lenses. Besides, we measured light intensity along the optical axis. The diffraction efficiency of the FZP at zeroth diffraction order is ∼7.5%, and that at 1st diffraction order is ∼15.8%.

4. Characterizing the performance of the proposed lens

4.1 Focal length tuning range

Due to two focal lengths of the FZP, the proposed lens has two additional focal length tuning range. When one focal length is used for imaging, the others are undesirable, which acts as stray light to degrade the imaging quality, as shown in Figs. 3(a). The measured focal length vs the applied voltage is shown in Fig. 3(b). We also magnify the interesting area in the high-power area, as shown in Fig. 3(c). When the voltage increases from 0 V to 65V, the focal length can tune in the range (-∞ to −34.5mm) and (17.5 to ∞). The minimum f-number can reach 3.5 and −6.9, respectively.

 

Fig. 3. Focal length tuning range of the proposed adaptive lens. (a) Different focal length (b) Measured focal length vs the applied voltage. (c) Magnified interesting area.

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4.2 Zoom range and imaging quality

To evaluate the zoom range and imaging quality, we characterize the imaging resolution using the resolution test chart as the target object. The measurement configuration is shown in Fig. 4(a). The light source is white light, which is located on the focal point of the collimator tube. The white light passes through a filter, and the wavelength of is 532nm. The collimated light then passes through the proposed lens and images on the CMOS camera. The distance of the proposed lens and CMOS is ∼89.5mm, and the distance of the proposed lens and glass lens is ∼59mm. The focal length of the glass lens is 65mm. A dynamic video showing the zoom range is also included, see Fig. 4(b) (Visualization 1). When a voltage (25V) is applied on the proposed lens, a clear image appears on the CMOS. The height of the image is measured to be ∼1.87mm, as shown in Fig. 4(b). When the voltage increases, the image becomes out of focus. However, when the voltage increases to 45V, another clear smaller image appears on the CMOS. The height of the image is measured to be ∼1.54mm, as shown in Fig. 4(c). When we further increase the voltage, the smallest image appears on the CMOS at 67V. The height of the image is measured to be ∼1.26mm, as shown in Fig. 4(d). In the whole process, the back focal length keeps fixed. The magnification is ∼1.48×, which is even larger than that of conventional three liquid lens system (see Ref. [19]). In the test, the unit of Number 9 can be distinguished, and the corresponding line width is ∼100µm. The angular resolution is 1′23″. Therefore, it can largely reduce the system size and simplify driving mechanisms. In the zoom process, there are only three clear images appearing on the CMOS. The main reason is that there are only three solutions for the two variables f_p and f_L which satisfy both Eqs. (1) and (2).

 

Fig. 4. Measurement configuration of the proposed adaptive lens. (a) Setup of the experiment. (b) Zoom 1, U=25 V (Visualization 1). (c) Zoom 2, U=45 V. (d) Zoom 3, U=67 V.

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4.3 Application in an image acquisition zoom system

The proposed lens is suitable to be used in a double image acquisition zoom system. To demonstrate the use of our proposed lens for practical imaging, a double image acquisition zoom system is established using our proposed lens. There are two obvious features comparing with conventional zoom system. Firstly, the double image acquisition zoom system can obtain two images simultaneously, which means it can act as two lens systems. One acts as a wide-angle lens, the other acts as long focal length lens. Therefore, we can not only observe a scene with wide field of view, but also see the detailed information clearly. Secondly, for each focal length tuning range, the proposed lens can realize zooming with only one L-L interface, which means one voltage is adequate to zoom the image. The setup is shown in Fig. 5(a). The whole objective consists of the proposed lens and a telescope objective as shown in Fig. 5(b). A beam splitter (BS) creates two light channels. When a voltage of ∼40V is applied on the device, two images are obtained simultaneously by the two cameras. The image of the short focal length is captured by CMOS camera 1, and the image of the long focal length is captured by CMOS camera 2. The two images are shown in Figs. 5(c)–(e). For the short focal length, the field of view (FOV) is relatively large and magnification is small. For the long focal length, the FOV is small, and magnification is large. The detailed information can be seen clearly, as shown in Fig. 5(d). The image of the short focal length can still be zoomed by changing the voltage. When the voltage gradually increases to ∼60V, another clearly image appears on the CMOS camera 1, as shown in Fig. 5(e). The FOV becomes larger, and the magnification becomes smaller. From the experiment, we can see that the double image acquisition zoom system can not only obtain two images simultaneously, but also realize zooming using one L-L interface, which is not available for conventional zoom system. With such a proposed lens, it is possible to remove conventional zooming mechanisms from cameras, thereby reducing cost, weight, and associated complexity.

 

Fig. 5. Double image acquisition zoom system using one proposed adaptive lens. (a) Setup of the zoom system. (b) Telescope objective consisting of the proposed adaptive lens. (c) Image taken at 45 V for CMOS camera 1. (d) Image taken at 45 V for CMOS camera 2. (e) Image taken at 60 V for CMOS camera 1.

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5. Conclusion

In conclusion, we demonstrate a zoom liquid lens employing Fresnel zone plate. The proposed lens has two optical surfaces: liquid-liquid (L-L) interface and Fresnel zone plate. The proposed lens can not only increase the focal length tuning range, but also realize zooming using only one L-L interface. The shortest positive and negative focal length are ∼17.5mm and ∼-34.5mm, and the diameter is 5mm. The zoom ratio of the proposed lens reaches ∼1.48×. Our results confirm that the proposed lens has widespread applications in imaging system.