Liquid lens with adjustable focus for ultrasonic imaging

In the ultralight or ultrathin applied domain of zoom lens, the traditional glass / plastic lens is limited for manufacture technology or cost. Therefore, a liquid lens was put forward to solve the problems. The liquid zoom lens has the merits of lower cost, smaller volume, quicker response, lower energy consumption, continuous zoom and higher accuracy. In liquid zoom lens the precise focal length is obtained by the contact angle changing to affect the curvature radius of interface. In our works, the relations of the exerted voltage, the contact angle, the curvature radius and the focal length were researched and accurately calculated. The calculation of the focal length provides an important theoretical basis for instructing the design of liquid zoom lens.

We numerically and experimentally realize broadband acoustic focusing lenses by employing units of phase manipulation composed of different numbers of cavity structures immersed in air, which is different from other units of phase manipulation by adjusting the structural parameters. The phase delays of transmitted units could cover a whole 2π range by using six cavity structures. Based on these units, we design a transmitted acoustic lens with different focal lengths, and the working bandwidth could reach about 2.1 kHz. Besides, the influence of air attenuation on the focusing performance and of the inclined angles of the channels in the cavity structure on the phase delays are discussed in detail. Moreover, we realize a transmitted acoustic lens with two types of units, a reflected acoustic lens and a three-dimensional transmitted acoustic lens. Compared with other acoustic lenses, the proposed acoustic lenses have the advantages of broad bandwidths, high focusing performance, easy fabrication, and high robustness, which have great potential applications in airborne acoustic focusing devices.

A liquid acoustic lens with variable focal length is described for photoacoustic microscopy. This liquid lens takes advantage of the elastic and deformable lens interface to tune its focal length in a pneumatical manner. The curvature of the lens interface as well as the dependant focal length was characterized as a function of the infusion volume of the liquid. Experiments were carried out to demonstrate the zooming ability of this liquid acoustic lens. Targets embedded at different depths were photoacoustically imaged without performing mechanically axial scanning.

An experimental setup used to measure the important optical properties of electrowetting liquid lens is proposed. The simple and precise method of measuring dynamic responses and focal lengths of liquid lens under different excitation signals is based on Gaussian beam transmission theory. The measurement method can be widely used in all kinds of zoom lens systems. The device is simple and economical, and also has the advantages of convenient operation, high measurement precision and wide range measurement. This work provides a new way to study the dynamic response of electrowetting liquid lens and the the mechanism of electrowetting liquid lens. The fabrication process and some relevant noticeable points for the homemade liquid lens are introduced. The testing device of dynamic process of lens consists of a He-Ne laser, an electrowetting lens, a circular diaphragm, a phototube, a digital storage oscilloscope and a computer. The change of the focal length of liquid lens due to the applied voltage will affect the flux detected by the photoelectric receivers. It is proved according to Gaussian beam transmission theory that the light flux received by the phototube changes with time, which represents the relationship between the focal length and time and the dynamic characteristics of the liquid lens. Therefore, the intensity of output signal of photoelectric receiver reflects the focal length of liquid lens. A dynamic changing process of the focal length of a self-regulating varifocal liquid lens based on electrowetting technology is tested under alternating current signal. It shows that the focal length of the liquid lens changes with the corresponding amplitude and polarity of the sine voltage. In one cycle, 4 peak signals of 50 Hz appear in turn, and the peak amplitude increases with the increase of voltage. Peaks 1 and 2 are caused by the voltage polarity, while peaks 3 and 4 by the oscillation modes. This is due to the fact that the liquid surface changes with time in the spherical shape under low voltage, but it will generate new oscillation mode when the amplitude is high.

Laser communication (lasercom) can enable more efficient links across larger distances compared with radio frequency (RF) systems. However, lasercom systems are typically point-to-point connections that would have difficulty interacting with several concurrently active spatially diverse users, where RF systems can more easily support such scenarios. Lasercom pointing, acquisition and tracking (PAT) systems have traditionally relied on mechanical beam steering devices, such as fast steering mirrors (FSMs) or gimbals, both of which are subject to potential mechanical failure. In this work we investigate an alternative steering solution using liquid lenses. Liquid lenses are tunable lenses that can non-mechanically alter focal length based on an applied voltage or current. A series of liquid lenses, one on-axis to control beam divergence, and one each offset in the x and y-axes to steer, could be used to achieve laser pointing control. Currently available commercial off the shelf (COTS) liquid lenses are based on electrowetting (manufactured by Corning [1]) or pressure-driven (manufactured by Optotune [2]) operation. In this work, we analyze the suitability of both types of liquid lenses for use in a space-based multiple access lasercom terminal. Early liquid lens technology first surfaced in 1995 with the control of the shape of an oil droplet through electrowetting [3]. The technology then started to become commercially available with the founding of Varioptic in 2002. However, there is limited data on liquid lens survivability and operation in a space-like environment. Through vacuum testing, we have found that electrowetting-based liquid lenses not only survive, but continue to operate nominally in a very low-pressure environment. The pressure-driven liquid lenses appeared to have issues initially in vacuum testing, with gas bubbles forming in the lens aperture during pump-down. However, after extended exposure to vacuum of approximately two weeks, the gas bubbles diffuse through the lens membrane, and the lenses operate in vacuum. Steering transfer functions were developed both in ambient and in vacuum conditions for both lens types, and in each case, the differences between the two curves were largely negligible. The electrowetting lenses provide a steering range of 2.7°, both in and out of vacuum, with an approximate slope of 0.046°/V. In testing the Optotune lenses, the steering was limited by the camera detector size, but for a range of −92 mA to 144 mA on the steering lens, the lenses provided for approximately 8.6° of steering with a slope of 0.0367°/V. These steering ranges can be extended to near hemispherical coverage with the addition of a diffuser and wide-angle fisheye lens [4]. Maximum hysteresis error, the difference in steering angle response when increasing lens voltage or current as opposed to decreasing lens voltage or current, was identified at 0.02° for the Corning lenses and 0.05° for the Optotune lenses. A Zemax beam quality analysis was conducted to see how transmit gain would be affected by refraction through the liquid lenses. Through this analysis, the worst-case link penalties were determined to be −0.5 dB for the Corning lenses at _0.8° steering and −0.4 dB for the Optotune lenses at _1.0° steering. Thus, we see that liquid lenses are likely good candidates for space applications and may perform well in nonmechanical beam steering. We discuss next steps in environmental testing as well as optical layout and control approaches for using liquid lenses in PAT systems for a nanosatellite based optical antenna.

With the continuous development of micro-unmanned system technology, the application requirements of small, lightweight and range-measuring are put forward for the mounted vision module. First of all, this paper proposes a monocular ranging method based on small liquid lens focusing, the position of the equivalent principal plane of the imaging system and the relationship between the equivalent focal length and the voltage of the liquid lens are deduced. Then, combined with the image focusing algorithm and the Gaussian optical imaging theory, establishing a nonlinear mapping model between the distance information of the target and the control voltage value of the liquid lens. Finally, the monocular ranging experiment is designed with the optical imaging system of liquid-solid lens combination as the hardware platform. The results show that the monocular ranging error based on the liquid adjustable focus lens can be controlled within 1mm, so the method proposed in this paper is feasible and can meet the visual ranging requirements of micro-unmanned systems.

The liquid lens has a broad application prospect in zoom and image systems for its quick corresponding speed, small volume and low consumption. However, the range of focal length is small, because the refractive index of conductive liquid has a little difference to that of non-conductive liquid in the liquid lens. In order to increase the zoom range of the liquid lens, a composite lens made up of a liquid lens and a solid lens is presented in this paper. The focal length variation property of the composite lens has been investigated, and the corresponding composite lens parameters are optimized. The zoom of composite lens with different slope solid interfaces has been studied.

In this paper, we design, fabricate and characterize a new electromagnetically actuated variable-focus liquid lens which consists of two polymethyl methacrylate (PMMA) substrates, a SU-8 substrate, a polydimethylsiloxane (PDMS) membrane, a permanent magnet and a planar electromagnetic actuator. The performance of this liquid lens is tested from four aspects including surface profiling, optical observation, variation of focal length and dynamic response speed. The results shows that with increasing current, the optical chamber PDMS membrane bulges up into a shape with a smaller radius of curvature, and the picture recorded by a charge-coupled device (CCD) camera through the liquid lens also gradually becomes blurred. As the current changes from −1 to 1.2 A, the whole measured focal length of the proposed liquid lens ranges from −133 to −390 mm and from 389 to 61 mm. Then a 0.8 A square-wave current is applied to the electrode, and the actuation time and relaxation time are 340 and 460 ms, respectively. The liquid lens proposed in the paper is easily integrated with microfluidic chips and medical detecting instruments due to its planar structure.

This paper describes a liquid lens whose focal point can be varied in both the axial and radial directions by acoustic radiation force. The liquid lens consists of a lens cell, two immiscible liquids with different refractive index (water and silicone oil), and an annular PZT transducer with four-divided electrodes. The oil-water interface acting as the lens surface can be deformed by the acoustic radiation force, and the focal point can changed three-dimensionally and the lens acts as a variable-focus lens. The displacement angle in the radial direction was approximately 3 degrees when the input voltage was 45 V at 1.9 MHz. By exciting with AM signals, the hemispherical water droplet oscillated and the focus scanning in the axial and radial directions could be achieved.

3D focusing acoustic lens optimization method using multi-factor and multi-level orthogonal test designing theory

According to the principle of dielectrophoresis, an aspherical double-liquid lens based on parallel plate electrodes is designed. In comparison with the liquid lenses based on patterned-electrodes, the aspherical double-liquid lens structure uses continuous electrodes, which have the advantages of simpler processing, lower cost, easier realization and more practicability. The droplet in the dielectric electrophoretic liquid lens is polarized in the electric field and moves towards the higher electric field intensity under the action of the dielectrophoretic force. With the change of applied voltage, the dielectrophoretic force varies, thus the contact angle of the droplet at the liquid-liquid interface is changed. Firstly, the models of aspherical double-liquid lenses under different voltages are established with Comsol software, and the data of interfacial profile are derived. Then using Matlab software, the derived interface surface data are fitted by polynomial, and the aspherical coefficients are obtained. Finally, the optical models are built with Zemax software, and the variation range of focal length and root mean square (RMS) radius of aspherical double-liquid lens under different voltages are analyzed. In order to further study the characteristics of aspherical double-liquid lens, it is compared with spherical double-liquid lens model. The liquid material, cavity structure and droplet volume of spherical double-liquid lens are consistent with those of aspherical double-liquid lens. The corresponding spherical double-liquid lens model is established by using the Zemax software, the range of focal length and RMS radius of spherical double-liquid lens under different voltages are obtained. The results show that the focal length variation range of aspherical double-liquid lens is larger than that of spherical double-liquid lens, and the imaging quality of the former is better than that of the latter. The experimental preparation of the designed aspherical double-liquid lens device is carried out, and its focal length and imaging resolution are measured. When the operating voltage is in a range of 0–280 V, the focal length varies from 54.2391 to 34.5855 mm, which is basically consistent with the simulation result. The feasibility of the liquid lens structure is verified experimentally. The imaging resolution can reach 45.255 lp/mm. The designed aspherical double-liquid lens based on the parallel plate electrode can provide a new scheme for the high-quality imaging of liquid lens and its application, and can expand the application scope of liquid lens.

A dielectric liquid lens driven using a chevron-patterned electrode is prepared. The bending angle of the electrode is 90°. This newly designed electrode can effectively and symmetrically deform the shape of the liquid lens, which in turn leads to a variable focus. For a 3-mm-diameter lens, a driving voltage changed from 0 to 52 V can vary its focal length from ~19.3 to ~4.9 mm. Using a 40-Vrms pulse voltage to impact the lens, the dynamic response time is ~6.7 s. Using a step voltage to impact the liquid lens, the response time can be largely reduced. Multiple lenses or a microlens array can be driven at the same time. Due to in-plane actuation, the driving voltage is insensitive to the size of the liquid lens.

Most existing stereoscopic head mounted displays (HMDs), presenting a pair of stereoscopic images at a fixed focal distance, lack the ability to correctly render the naturally coupled accommodation and convergence cues. Psychophysical studies have shown that such displays may cause many adverse consequences such as visual fatigue, diplopic vision, degraded oculomotor response, and depth perception errors. In this paper, we present a see-through HMD with addressable focal planes utilizing a novel active optical element - a liquid lens. The element, with a varying optical power from -5 to 20 diopters, is able to address the focal distance of the HMD from infinity to the near point of the eye. A monocular prototype was built from off-the-shelf elements and experimental results are presented to validate the proposed designs. We also describe both subjective and objective measurements of the accommodation responses of the viewer to the focal distances presented by the prototype.

Investigation on liquid medium integration for piezoelectric MEMS tunable liquid lenses

This paper proposes a large depth-of-field (DOF) imaging method, which uses an image acquisition system equipped with a liquid lens. This paper also establishes the relationship between the focusing distance and the current value to estimate the object distance. The proposed method firstly uses a liquid lens to capture multi-focus images. Then the different objects in the scene are segmented by image processing algorithm. Then we apply an effective blur estimation algorithm on each object to determine the clearest image from all multi-focus images and obtain the corresponding current value. Finally, the clear images of different objects are combined to obtain a large DOF image, and meanwhile, the distances of different objects can be estimated, respectively. Experimental results show that the proposed method can effectively achieve large DOF imaging and measure approximate object distances.