Tunable-focus lenses already exist in the nature. For instance, the human eye is a single-lens system with tremendously wide tunable focus range. The primary tuning mechanism is shape change, as controlled by the muscles in the eye, as shown in Fig.
1-1. As the incident light passing by the lens, it will focus on the retina, and the retina creates an image signal of the visual world for the brain. In order to mimic this characteristic, there are some practical methods reported, including conventional tunable-focal lenses, liquid lens, and liquid crystal lens, described in the following section.
Fig. 1-1 Imaging theory of the human eye
1.2.1 Conventional tunable-focal lenses
The conventional tunable-focal lenses are composed of several pieces of individual lens, and most of the conventional lenses are made of glass or other transparent material. These lenses can reconfigure the transmitted light beam. Adjusting the distance (d) between the lenses by mechanical movement varies the focal length [1], as shown in Fig. 1-2. However, it needs some volume to let the lenses adjusting which can accomplish the tunable-focal function. Therefore, the issues of this structure are bulky and heavy. As the small size and light weight requirements increase, the other technologies have been developed.
3
Fig. 1-2 Conventional tunable-focal lenses
1.2.2 Liquid lens
Similar to the glass lens, a liquid lens focuses the incident light based on the changing of surface profile. According to the operation method, liquid lenses can be classified into two types. The first type is mechanical lens which the focal length is controlled by pumping liquid. The second type is electro-wetting lens which the focal length is controlled by applying different external voltage.
1.2.2.1 Mechanical liquid lens technology
This kind of liquid lens is fill with only one kind of fluid and sealed with the elastic membrane. Pumping liquid in the chamber which changes the curvature of the liquid profile can create different focal length, as shown in Fig. 1-3 [2]
Fig. 1-3 Structure of liquid lens: (a) no focusing state, (b) focusing state
4
1.2.2.2 Electro-wetting liquid lens technology
Electro-wetting liquid lens is filled with two different kinds of fluid substrates, such as water and oil which have different density, dielectric constant and viscosity. The voltage applied to the substrate can induce different electrical force to vary the curvature of fluids profile which leads to different focal length, as shown in Fig. 1-4 [3].
Fig. 1-4 Structure of electro-wetting lens: (a) no focusing state (b) focusing state
1.2.3 Liquid crystal lens
In the last decade, many advances have been made in the area of Liquid crystal materials which have been widely used for the display and other electro-optic devices due to their physical properties such as birefringence and effect by the electric field.
The way of applied voltage which can generate no-uniform electric field distribution will vary the liquid crystal director, which denote the optical phase and path will be changed.
A number of studies have investigated the LC lens which can be traced back to
5
1979, a paper was published by Susumu Sato that has been the subject of much discussion ever since.[4] The structure could change the focal length by adjusting the applied voltage. The suitable liquid crystal distribution can generate and control the optical path difference between the center and the edge of the cell. The optical function could be the same to the convex or concave lens. Since the electric field distribution is very essential for the liquid crystal director, how to design the electrode structure and applying the voltage to form appropriate electric field distribution has been the subject of active research. The liquid crystal lens can be classified into:
inhomogeneous and homogeneous electric field approaches. Following the approach and characteristic of each type will be introduced.
1.2.3.1 Inhomogeneous electric field approaches Inhomogeneous liquid crystal distribution
In order to generate a inhomogeneous electric field, the structure need to deposit the indium-tin-oxide (ITO) electrode on a concave substrate, as shown in Fig. 1-5 [4].
In the voltage-off state, the effective refractive index of liquid crystal is , which the incident light can be focused. In the voltage-on state, the inhomogeneous electric field reorients the liquid crystal and the effective refractive index become which have long focal length. The advantages of this structure are: lower operation voltage and shorter initial focal length. However, the issue of this structure is higher optical aberration.
6
Fig. 1-5 Operation mechanism of the inhomogeneous LC distribution structure (a) Voltage-off and (b) Voltage-on state
Monomer and liquid crystal mixed
By applying the inhomogeneous electric field, an adaptive lens using electrically induced LC and monomer, which have different refractive indexes, concentration redistribution was proposed [5]. The electric field makes the LC molecules to diffuse towards the high electric field region and the liquid monomer towards the lower electric field region, as shown in Fig. 1-6. An inhomogeneous electric field can generate the non-uniform distribution of LC and monomer, as shown in Fig. 1-6 (c).
Therefore, a gradient refractive index lens has been obtained. The advantage of this structure is lower optical aberration, but its response time is slow.
Fig. 1-6 Operation mechanism of monomer and LC mixed structure (a) voltage-off state, (b) voltage-on state, (c) side-view of the whole structure in the voltage-on state
7
1.2.3.2 Homogeneous electric field approaches
The other kind of liquid crystal lens is controlling the LC director distribution by designed electrode shape, as shown in Fig. 1-7. Appling different voltages to the outer and inner electrodes can generate a spatial non-uniform and symmetrical electric field in the LC layer. It can also behave as convex or concave lens by controlling the applied voltage. For instance, applying voltage higher than the voltage , the LC director in the outer position will follow the electric field direction perpendicular to the electrode, and the incident polarized light beam will see which is smaller than . Therefore, the LC lens has a gradient refractive index distribution and behaves as conventional convex lens. However, the most critical issue of this structure is the LC director will disorder at the connecting electrode position, and generate a poor image quality.
Fig. 1-7 Operation mechanism of the homogeneous electric field structure (a) side-view and (b) top-view