Liquid crystal lens array for a 3D endoscope

Liquid crystals (LCs) have unique electro-optical characteristics that make them useful for studying and fabricating practical optical devices. Recently, the properties of LC lenses have received much research attention. They are thin, tunable in focus, and electrically switchable to 2D/3D modes. Rather than controlling light by varying the thickness and shape of a polished material, in an LC lens system the refractive index of the LC is tuned by applying a voltage.

LC lenses are thus suitable for enhancing endoscope imaging systems, which usually have a solid lens a few millimeters in diameter, but for which it is an advantage to have a smaller lens with a short focal length with 3D image capabilities. It is also an advantage for the device to work at low voltage (i.e., less than 20V) so that it can be used in an integrated circuit, which cannot tolerate high voltages. The advantages of 3D endoscopy are to improve the speed and accuracy of surgery. 3D imaging also helps surgeons learn to control the instrument more quickly.¹ Numerous types of LC lenses have been proposed, such as a 3D sensing system,² a 3D imaging system,³ and an optical tunable LC lens.^4–6 These systems have been built for research and are not in clinical use.

There are several challenges to making a better LC lens. For instance, photopolymerized LC lenses have short focal lengths but require high applied voltage⁷ and have a complicated structure that is costly to manufacture.⁸ An optical system for a 200μm aperture size based on an LC lens array has been developed, but suffers from distortion. Although this can be reduced, it is impossible to extract a 3D image as a result.⁹ Other LC lens systems use low applied voltage,^10,11 a patterned lenticular LC lens,^12–14 and a liquid lens actuated by LC pistons,¹⁵ but none combines low-voltage operation with a small aperture and 3D imaging capability.

We have developed a small lens based on an LC lens array with a hexagonal arrangement, small inactive region, and low applied voltage.¹⁶ It shares the image with seven lenses instead of nine lenses in a commonly used arrangement of 3 × 3 lenses and produces better image quality. Unlike some previous methods,^17,18 we do not use the patterned electrode as the lens. Instead, we use the electrode pattern to achieve a better electric field distribution.

The fabrication process and operating principles of our LC lens array are schematically illustrated in Figure 1. We first take a glass substrate and coat it with AZ 40XT photoresist with a spin coating machine at 2000rpm for 25 seconds: see Figure 1(a). This is then gently baked for 5 minutes at 60°C and 5 minutes at 80°C to avoid bubbles in the resist film, and finally for 7 minutes at 130°C. As the photoresist reaches the softening point, it curves: see Figure 1(b). Next we coat the photoresist with indium tin oxide (ITO, a transparent conductor) and etch the pattern, before adding a layer of the UV-curable adhesive Norland Optical Adhesive NOA81, a photosensitive conductive prepolymer with good optical performance over the substrate, photoresist, and ITO: see Figure 1(c). Finally, we solidify this layer by polymerizing it with UV light : see Figure 1(d). The electrode layout is shown in Figure 2.

Figure 1. Fabrication of the curved circular electrode. AZ 40XT: Photoresist. ITO: Indium tin oxide. NOA81: UV-curable Norland Optical Adhesive that acts as an insulating layer.

Figure 2. Indium tin oxide (red) on the curved surface of the photoresist (silver). The horizontal gray area with the orange border represents a planar section. PVA: Poly(vinyl alcohol).

The LC lens structure is shown in Figure 3. It consists of two layers of ITO as electrodes, two glass substrates of thickness 550μm, a 35μm-thick layer of NOA81 as an insulating layer, an LC layer with cell gap of 30μm, and two alignment layers of poly(vinyl alcohol) (PVA) that are mechanically buffed to align the LC directors (direction of preferred orientation of the LC molecules). The rubbing directions of the two alignment layers are antiparallel, and the lens diameter for each one is 350μm.

Figure 3. Schematic of the liquid crystal (LC) lens array structure.

The device was filled with nematic LC material E7 (Merck) with the following properties: extraordinary and ordinary dielectric constants ε_p=19.0 and ε_s=5.2 of the LC, respectively, and the birefringence refractive index of the LC, Δn, is 0.225. The effective dielectric constant of the LC, ε_lc, is 9.74. The LC and glass layers (d_lc and d_g) are 30 and 550μm thick, respectively. The glass dielectric constant, ε_g, is 7.75, the dielectric constant of NOA81 ε_ins∼5 at a frequency of 1kHz, and the insulator thickness, d_ins, is 35μm.

We measured the focusing properties of the LC lens, the image quality, interference patterns, and focal length under different applied voltages. Figure 4 shows the experimental setup we used to probe the voltage-dependent focal length of the lens. When a voltage (V) is applied to the electrodes, the LC layer experiences an inhomogeneous electric field because the top ITO electrode is spherical in shape. Within the LC layer, the electric fields at the border E_b and center E_c can be calculated as follows:

Figure 4. Experimental setup.

Substituting the values above into Equations 1 and 2 gives E_c=33.33V and E_b=10.18V. Although the electric field is lower at the border than at the center, the electric field between two electrodes is a superposition of the border electric field of more than one ring, which in practice reduces the electric field gradient from the center to the border. The curved electrodes avoid stack overflow of the electric field at the center of the lens and permit shorter focal lengths. Such electrodes make it possible to work on a wider range of applied voltages, which gives more precise control over focusing. As can be seen in Figure 5, the electric field increases rapidly with voltage, and so the system requires only low voltage, which is an advantage for an integrated optical system.

Figure 5. Electric field at the lens border (E_b) and lens center (E_c) versus voltage.

To evaluate the optical properties of this hexagonal LC lens array, we recorded the interference fringes between the ordinary and extraordinary rays (that is, rays polarized perpendicular and parallel to the plane of incidence, respectively) from a helium-neon (He-Ne) laser beam (λ=632nm) that pass through the LC lens placed between two crossed polarizers. Figure 6 shows the recorded interference fringes at various applied voltages. When the voltage is applied, the appearance of the interference fringes changes as the LC directors reorient themselves in the applied electric field. The retardation difference of two adjacent constructive or destructive interference rings indicates a phase change of 2π, and the variation in phase retardation induced by the applied voltage shows how the electrical field alters the lens properties. The curvature of the phase profile of this LC lens array gradually boosts as voltage increases.

Figure 6. The interference patterns in LC lenses for applied voltages 0–5V.

We investigated the imaging properties of this LC lens when it was placed 2mm from the objective lens of a side-view endoscope with a 20×30mm test photograph placed 57mm from the LC lens. We took images of the test photograph in dim light as shown in Figure 7. Since there are just seven lenses we use computer control and image processing to take photos of an object from different viewing angles and combine them to make a 3D image of the object. It can be seen that the LC lens focuses and defocuses on the test photograph as the applied voltage varies.

Figure 7. The image focusing/defocusing of the LC lens at V_rms of (a) 4, (b) 3, and (c) 2V.

In summary, we have proposed a new LC lens array using hexagonally arranged curved circular electrodes. As the applied voltage V is varied from 0 to 10V_rms, the focal length continuously changes from 2.5cm to infinity. A small cell gap makes the response time fast, and it is easy to fabricate and very attractive for a wide range of applications that need a miniature lens and short focal length. We are now working to apply this concept to an endoscope for clinical use and also to cell phone camera lenses.