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Abstract
Highly efficient transmission-type beam deflectors that have high angular resolution have been widely used for various applications. Continuously tunable beam deflectors have also been needed for many purposes. An indium-tin-oxide (ITO), widely used for transparent electrodes, was placed on the upper and lower glass substrate. The ITO layer on the lower substrate was patterned by the contact mask aligner for relatively wide input and output pad compared to main grating ITO patterns in the active area. These input and output pads on the lower substrate are connected to each driving integrated circuit (IC), which has 360 channels for continuous control. A small pixel pitch of grating patterns of 6 μm (the electrode width is 3 μm with a 3 μm spacing) was developed, and the maximum diffraction angle is calculated and measured at 2.541° with a wavelength of 532 nm. A minimal cell gap of 2.5 μm was applied for the full phase modulation by using a high-birefringence liquid crystal. A driving module for continuous beam steering is also developed and applied to the beam deflector system. A diffraction efficiency of about 50.9% is observed at an angle of diffraction about 2.541°.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
Optical systems for diffracting a beam of a specific wavelength to a desired direction still have been developed and researched for a long time. The system of a beam steering is structurally classified into two categories. One is mechanical structure, and the other is nonmechanical structure. In contrast to a huge size system of mechanical structures, nonmechanical-type beam steering systems have a lot of advantages, including a compact system size, less weight, robust reliability, and low power consumption compared to the mechanical type.
With these advantages, nonmechanical-type beam steering systems have been widely used for various applications, such as optical communications [1], phased array beam steering system [2], diffractive lens systems [3,4], waveguide-based electro-optics [5], and so on. In the system of a nonmechanical beam steering, the material for changing the refractive index is positively necessary to change the phase of an optical beam [6].
Among a lot of candidates of materials, liquid crystals have been widely used to change the refractive index, due to the advantage of a low driving voltage, mature fabrication, robust reliability, and low costs. The liquid crystals have various birefringences according to customized configurations, and many types of liquid crystal have been researched [7–9]. Especially, high-birefringent liquid crystals have attracted much interest due to their capability to reduce the cell gap.
Although, the beam steering devices that have fixed periodic prism have many applications with its accuracy of shaping, the electrically variable phase prisms using liquid crystals have many advantages in terms of availability. As conventional prisms refract the direction of propagation of the light, the light is also deflected according to the variation in the refractive index of a liquid crystal [10–12]. The liquid crystals can be reoriented when an electric field is applied across them, and they change the direction of the light that passes through the medium due to its birefringence. For doing this, the ITO electrode is required to steer an angle because the periodically applied electric fields from electrodes change the liquid crystal movement. Each patterned-ITO electrode on the lower substrate can be addressed individually, and this allows us to manipulate the shape of the grating grooves. The blazed diffraction grating, a special type of diffraction grating, can be made from a blazed phase modulation profile, and the incident beam by use of a beam deflector can be steered to the desired angle [13,14].
The main purpose of this study is to develop a continuously tunable transmission-type beam deflector using liquid crystals with high angular resolution. In addition, the driving system module that was adopted with driving IC was also developed to steer the incident beam effectively.
2. BEAM DEFLECTOR SYSTEM DESIGN
Figure 1 shows a whole continuously tunable transmission type of the beam deflector system, which includes the beam deflector cell and the driving system module. To control each channel independently, commercial driving IC was implemented to the system, and the driving control was determined by the graphic user interface (GUI). The specific input steering data could be transferred from PC to driving board, and transferred data is finally delivered to driving IC through the microcomputer (MICOM) and the field programmable gate array (FPGA). At the final stage of the beam deflector driving module, each channel of driving IC generates the voltage in accordance with data from the driving board, the voltage from the channel of driving IC was applied to the patterned ITO electrode placed on the lower substrate.
For the data communication between the PC and the driving module, a universal serial bus (USB) interface was adopted in the system in Fig. 1, and a chip on film (COF) type of driver IC was connected to the ITO pad on the lower substrate by using the anisotropic conductive film (ACF) bonding. The driver IC has a total of 360 channels, and the maximum driving voltage of a driver IC is . The beam deflector system in Fig. 1 has two driver ICs for a total of 720 channels on the lower substrate.
A. Steering Angle and Spatial Resolution
Figure 2 describes the principle of the liquid crystal deflector in detail. A phase grating is created by a combination of optically transmissive material that has a periodic differential refractive index and the modulation of incident light as it penetrates that material. When an external voltage is applied to the upper and lower electrodes, the effective refractive index of liquid crystal is changed across the neighboring electrode. As part of that change, the phase differences across the liquid crystal cell were observed according to the electric field profile. It deflects the transmitted lights optically like the prism. The number of channels is and the pixel pitch is . We designed the beam deflector system so that the refractive index difference , and the cell gap , can modulate a phase larger than for satisfying the phase matching condition.
The shifts are all the same from a phase point of view, and the cut down basis on the was applied to create a sawtooth phase profile.
As described in Eq. (1), the maximum steering angle and the smallest steering angle are determined by the channel number of the prism and the cell size. Steering angle can be derived as follows:
where is the wavelength of the light, is total channel number for one unit prism that is defined by , is the total number of channels, and is the total number of unit prisms. is the pixel pitch that is 6 μm of the electrode and width is 3 μm with a 3 μm spacing.The total channel number, , is decided as 720 according to drive IC. In Eq. (1), the maximum steering angle is 2.541° at the wavelength of 532 nm when the total number of unit prisms, , is 360. On the other hand, the smallest angle, which could be also called angular resolution, is 0.007° when the total number of unit prisms, , is 1.
Phase delay is caused by a result of the different speeds between the ordinary and extraordinary waves in birefringent media as follows:
where is the wavelength of the light, birefringence is defined as the difference between the extraordinary () and ordinary () refractive indices. The minimum value of the blazed grating depth is phase delay that corresponds to the first diffraction order. A high birefringence of liquid crystal material (), and , are 1.847 and 1.537, as used for a minimal cell gap while preserving the cell gap for the full phase modulation, and this was sandwiched between two substrates.The homogeneous-aligned configuration was applied to maximize the index of modulation in Eq. (2), when is 532 nm in the previous assumption. The maximum phase delay in this system is in Eq. (2). The minimum required thickness for the beam deflector to have a full phase change is determined by the birefringence of the liquid crystal, and the cell gap was set to 2.5 μm.
B. Bottom ITO Glass Design and Fabrication
Figure 3 shows the layout of a patterned ITO in the lower substrate. The ITO with 2000 Å was deposited on the lower glass substrate, and it is patterned using photolithography to give the periodic electrodes with a grating structure.
The photolithographic method is the most common process for fabricating patterned ITO, and the desirable minimum critical dimension size for lines and spaces was used for the contact mask aligner.
The cross section of the patterned-ITO electrode in the active area on the lower substrate is shown in Fig. 4. From this image, the linewidth of ITO on the lower substrate is about 3 μm, and a space is about 3 μm, which corresponded to the grating fill factor (duty cycle) of 0.5.
3. BEAM DEFLECTOR DRIVING MODULE DESIGN
The beam deflector driving module was designed for changing numbers of a prism within an active area to steer the incident beam to the desired direction with a high speed.
A block diagram of a beam deflector driving module is shown in Fig. 5(a), where two driving IC have 360 output channels each. Data from driving board is transferred to memory in driving IC and stored for the next frame. Figure 5(b) shows signal timing diagram in driving IC in Fig. 5(a).
AC voltage should be applied to the liquid crystals for its reliability. For this reason, the beam deflector system has two types of gamma. POL signal in Fig. 5(b) is for selection of voltage between negative and positive gamma. Each of the 720 channels can generate a different output voltage independently.
Electrical specification is shown in Table 1. Pixel driving voltage is for patterned-ITO electrodes on the lower substrate, so that electrodes have a peak-to-peak voltage () of 20 V, which reaches a top peak of 20 V and a bottom peak of 0 V. Reference voltage for the ITO electrode on the upper substrate is from 0 V to max 10 V. When a reference voltage of 10 V is applied to the ITO electrode on the upper substrate, a negative gamma is defined from 0 V to 10 V, and a positive gamma voltage is defined from 10 V to 20 V. The maximum data loading speed is 30 Hz from PC to beam deflector cell, and this data loading speed is defined as the maximum steering speed within the beam deflector driving module. Total power consumption is about 500 mW including two driving ICs and other components in the beam deflector system.
Table 1. Electrical Specification of Beam Deflector Driving Module
4. EXPERIMENTS
Phase delay of the beam deflector cell according to applying voltage between the lower and the upper electrodes as a function of the voltages from 0 V to 10 V is shown in Fig. 6. Phase delay was measured by using the charge-coupled device (CCD) camera, which was used to utilize the difference of interference and diffraction of the light from different two paths.
Diffraction efficiency is defined as the ratio of intensity of a diffracted beam, to intensity of a nondiffracted beam, . Figure 7 depicts the diffraction efficiency of the beam deflector, and it was measured as a function of various values of the steering angle. The diffraction efficiency curves demonstrate that values start from 100%. However, it decreases gradually, and 50.9% is measured at the maximum steering angle of 2.541°.
Figure 8 shows the image and video clip of the steered beam produced by the beam deflector system. It was captured at a distance of 50 cm from the green light source at a wavelength of 532 nm and indicates units for measuring a plane angle including degree.
Fig. 8. Captured images of a steered beam (see Visualization 1) with (a) steering angle zero, (b) steering angle 1°, and (c) the maximum steering angle 2.541°.
Figure 8(a) shows the beam spot at steering angle zero () without an external applied voltage. Figure 8(b) shows the steered beam spot at 1°, and the maximum steered beam at 2.541° was shown in Fig. 8(c). The recording of a change in a constant beam steering angle is shown in Visualization 1 in Fig. 8.
The noise of high-order diffraction is shown in Visualization 1, since the shape of a blazed phase profile generated in the device is not the same as the ideal one. This mismatch is derived from the fringe effect caused by the adjacent electrode and liquid crystals.
5. CONCLUSION
A blazed transmission grating for nonmechanical steering was investigated in this paper. The fringe effect caused by a microscale patterned-ITO electrode was diminished by the low cell gap with using the relatively high birefringence of liquid crystal.
From the minimum 0.007° to the maximum , the steering angle over the 720-electrode arrays is demonstrated, and the diffraction efficiency is also measured as a function of an angular deflection.
The demonstrated efficiency is dramatically decreased to 50.9% of the initial value for the maximum steering at 2.541°. This is because the distorted blazed grating shape gives rise to an increase in the amount of scattering and noise parameters. This problem could be solved by use of a driving algorithm and a novel structure with a necessity to decrease in pixel dimension for future beam steering systems.
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