Stressed Liquid-Crystal Optical Phased Array for Fast Tip-Tilt Wavefront Correction

Kent State University From the SelectedWorks of Philip J. Bos December 20, 2005 Stressed Liquid-Crystal Optical Phased Array for Fast Tip-Tilt Wavefront Correction Bin Wang Guoqiang Zhang Anatoliy Glushchenko John L. West Philip Bos, Kent State University - Kent Campus, et al. Available at: https://works.bepress.com/philip_bos/38/

Stressed liquid-crystal optical phased array for fast tip-tilt wavefront correction Bin Wang, Guoqiang Zhang, Anatoliy Glushchenko, John L. West, Philip J. Bos, and Paul F. McManamon A liquid-crystal optical phased-array technology that uses stressed liquid crystals provides a new type of tip-tilt wavefront corrector. It demonstrates a very fast time response 10 khz and high beam-steering efficiency 91%. The new technology presented here will allow for a nonmechanical, high-speed correction with simple device construction. 2005 Optical Society of America OCIS codes: 160.3710, 230.3720, 220.1000, 230.2090, 230.5480. 1. Introduction It is well known that ground-based astronomers optical observation has been limited by the distortion of the Earth s atmosphere. It is important to smooth out the millisecond time-scale distorting effects of the atmospheric turbulence by using adaptive optics systems, which are able to adaptively cancel out or at least minimize atmospheric distortion in real time. When a wavefront experiences a turbulent atmosphere, tip-tilt distortion accounts for 85% of all the aberrations induced on the wavefront. Therefore tiptilt distortion is the primary concern of any adaptive optics system and it becomes critical to find a simple and effective way to perform tip-tilt correction. 1 3 Although devices to provide this correction are available, a much faster correction speed 10 khz is required in fast-moving and aero-optical systems. 4 It has been more than 15 years since liquid-crystal (LC) devices began to be used for wavefront control. 5,6 There are many advantages in using LC spatial light modulators, such as low cost, low power consumption, no moving parts involved, and device compactness. However, there are two main drawbacks of nematic LC devices: their polarization dependence and slow response time. 7 One can overcome the first problem B. Wang (wangbin@lci.kent.edu), G. Zhang, A. Glushchenko, J. L. West, and P. J. Bos are with the Liquid Crystal Institute, Kent State University, Kent, Ohio 44242. P. F. McManamon is with AFRL/SNJ Building 620, 2241 Avionics Circle, Wright-Patterson Air Force Base, Ohio 45433-7304. Received 22 June 2005; revised 10 August 2005; accepted 21 August 2005. 0003-6935/05/367754-06$15.00/0 2005 Optical Society of America by incorporating a quarter-wave plate into a device used in reflection mode 8 or using two orthogonal devices of identical LC materials connected in tandem. 9 Generally, nematic LC-material-based devices have response times of dozens to hundreds of milliseconds. There are practically four methods for improving the slow switching speeds of nematic LCs, including use of a transient nematic effect, 10 a pi-cell configuration, 11 dual-frequency LC materials, 12 14 and LC polymer composites. A ferroelectric LC-based binary half-wave shifter has also been tried 15 for high-speed wavefront correction. However, to achieve large optical phase shifts (several wavelengths) for wavefront corrections, one must increase the thickness of the LCs in all these devices. Unfortunately, the increase in thickness will give rise to a longer response time, which is proportional to the square of the thickness. Therefore it is critical to utilize LC materials that are able to provide not only large optical phase shifts but also short response times (several kilohertz). Recently we developed fast-switching stressed liquid crystals (SLCs), 16 which overcome the inherit problem of conventional LC materials. SLCs decouple the switching speed and the cell thickness such that the increase of the SLCs thickness will not slow down their response time. SLCs can provide a large optical phase shift in a fraction of a millisecond. In addition, SLCs have a linear response between the phase shift and the applied voltage, which greatly simplifies the design of driving electronics. We have fabricated a SLC-based optical phased-array tip-tilt corrector, which can provide a 3.1 m phase shift in 0.1 ms 10 khz in reflection mode. First we describe the fabrication and characterization of the SLC optical phased-array tip-tilt corrector 7754 APPLIED OPTICS Vol. 44, No. 36 20 December 2005

Fig. 1. Schematic drawings of a stressed LC cell: (a) after polymerization, (b) after shearing, (c) after an electric field has been applied. in Sections 2 and 3. The performance of this tip-tilt corrector is described in Section 4. We conclude in Section 5. 2. Fabrication of Stressed Liquid Crystals The LC polymer composites have been extensively studied over the past decade because of their intriguing physics and their potential applications in novel, fast-switching LC devices. The LC polymer network created by thermal or UV curing of a LC monomer mixture strongly affects the electro-optical properties of LC devices. Depending on the concentration of the monomer in the LC mixture and on details of the phase separation process, a variety of LC network structures can be created. Introducing stress to the LC polymer composite has led to improved electro-optical characteristics. 17 21 The SLC materials comprised interconnected microdomains of a LC dispersed in a stressed polymer matrix. There are four major benefits of introducing shearing stress into the SLC systems: (1) LC alignment is along the shearing direction, which reduces light scattering and eliminates the alignment layer and rubbing process; (2) there exists a linear relationship between the phase shift and the applied voltage; (3) the phase shift is dependent on shearing strength; and (4) the extremely fast response time is independent of device thickness. Generally speaking, the fabrication of SLCs consists of two steps: polymerization and shearing. Figure 1 shows a schematic drawing of a stressed LC cell. Figure 1(a) represents a LC cell network structure after polymerization. Figure 1(b) represents the network structure after a shearing force has been applied. The LC directors inside the network are aligned along the shearing direction. Figure 1(c) shows the LC directors aligned along the electric field direction when an electric field is applied (for LC 0). The SLC material that we used for tip-tilt correctors is a mixture of the LC 5CB, monomer RM82, and optical adhesive NOA65 at a weight ratio of 90:2:8. The photoinitiator is 0.2% of the whole mixture. One Fig. 2. Structure of the SLC tip-tilt corrector with a 24 strip interdigitally patterned ITO bottom substrate and a nonpatterned ITO top substrate. The width of an ITO strip is 412 m; a line gap is 5 m. of the tip-tilt corrector substrates has 24 interdigitally patterned indium tin oxide (ITO) electrodes with a pitch of 417 m (ITO, 412 m; line gap, 5 m), and the other one has a uniform unpatterned ITO coating, as shown in Fig. 2. The cell thickness is controlled by 40 m fiber spacers placed outside the pixel area. The mixture of the LC material is sandwiched between two substrates, and then the cell is placed into a UV lamp chamber and undergoes UV polymerization. The temperature of the chamber is 50 C, and the UV intensity is 20 mw cm 2. Polymerization takes an hour. The cell shows strong scattering after polymerization. However, it becomes transparent after 80 m shearing distance is applied. The shearing direction is perpendicular to the stripes of the patterned electrodes. Inside a SLC, each LC domain is surrounded by other randomly oriented LC domains, which causes major light scattering. The light scattering that results from a refractive-index mismatch between a LC and a polymer matrix is less significant because the polymer s dimension is much smaller than the LC domain s dimension and the wavelengths of the incident light. When the LC domains are aligned in the same direction, the light scattering of the film is reduced dramatically, as the mismatch of refractive index between LC domains disappears. Figure 3 shows the transmission spectra before and after the shearing of the LC device. A fully cured NOA65 cell of the same substrates is used as a reference to correct for reflection loss. The scattering is significantly reduced after shearing because the LC directors inside the polymer network are aligned along the shearing direction. Figure 3 also indicates 20 December 2005 Vol. 44, No. 36 APPLIED OPTICS 7755

Fig. 3. (Color online) SLC tip-tilt corrector transmittance before and after shearing referenced to the transmission of a NOA65 cell to correct for reflection loss. that the transmission of the SLC cell decreases at shorter wavelengths, because the interconnected polymer domain sizes are comparable to the wavelength of light; thus scattering takes place. The device is glued to the sheared state to retain shearing alignment. 3. Electro-Optical Characterizations of a SLC Device The electro-optical characterization setup of the SLC tip-tilt corrector is shown in Fig. 4. A near-ir laser with 1.55 m wavelength serves as a light source. The device s shearing direction is 45 with respect to the transmission axes of a pair of crossed polarizer and analyzer. The patterned electrodes are connected, so the SLC acts as a single-pixel device for this measurement. The measured device switching speeds are shown in Fig. 5. The switching speeds for voltage on and off are approximately 55 and 30 s, respectively, for a half-wave phase shift in the transmissive mode. They are much faster than other nematic LC devices, which switch the same amount of phase shift. The measured phase shift as a function of voltage is shown in Fig. 6. The linear phase shift region is Fig. 5. (Color online) Measured switching times of the SLC tip-tilt corrector. 1.55 m and V 200.0 V. roughly from 67.0 and 191.0 V, which agrees with the linear fitting. The linearity of the phase shift allows the tip-tilt corrector that drives the electronics to be obtained by use of a simple resistor network. Figure 7 shows the transmission spectra of the SLC device when it is switched to the on and off states. These results are also referenced to transmission of a NOA65 cell. One can clearly see that the transmission loss of the SLC itself is minimal in near-ir region. One can obtain a large phase shift of a SLC device by increasing the cell thickness, which has little influence on the SLC device s switching speed. The trade-off is the proportionally increased driving voltage. Figure 8 demonstrates the measured phase shift versus applied voltage for a 540 m thick SLC cell. 4. Characterizations of the Performance of a Tip-Tilt Corrector A. Steering Angle and Drive Methods The SLC tip-tilt corrector is based on optical phasedarray beam-steering technology. 22,23 Figure 9(a) shows Fig. 4. Schematic drawing of the SLC tip-tilt corrector s electrooptical characterization setup. Fig. 6. Measured retardation of the SLC tip-tilt corrector as function of voltage. The linear range is roughly 67.0 191.0 V. 7756 APPLIED OPTICS Vol. 44, No. 36 20 December 2005

Fig. 7. (Color online) Measured transmission spectra of the SLC tip-tilt corrector referenced to a NOA65 cell. that, when no voltage is applied to the SLC device (left), the optical phase profile has a rectangular shape (right), and the incident laser beam will not change its propagation direction. Figure 9(b) shows that, when a linear voltage ramp is applied (left), the optical phase profile is a triangle or prism (right), and the beam is steered away from its incident direction. From Fig. 6 we know that there is a linear phase shift region from 67.0 to 191.0 V. Therefore a serial resistor network connected to the interdigitally patterned ITO electrodes can easily provide a linear voltage ramp. Setting two-end voltage V H (high voltage) and V L (low voltage) to 191.0 and 67.0 V, respectively, yields the linear voltage ramp. The steering angle is governed by the expression sin nd L, (1) where n n e n o, n o and n e are LC ordinary and extraordinary refractive indices, respectively, d is the LC cell thickness, and L is the bottom width of the Fig. 8. Measured phase shift of a 540 m thick SLC device as function of voltage in the transmission mode. Fig. 9. (Color online) Schematic drawings of the beam-steering effect of a LC cell in several voltage driving conditions. Left, LC director configurations; right, corresponding optical phase profiles. indicates the beam polarization direction and 1 indicates the beam propagation direction. (a) No voltage applied. (b) linear voltage ramp applied. Left, low voltage; right, high voltage. (c) Linear voltage ramp applied. Left side, high voltage; right side, low voltage. triangle phase profile. Therefore optical phase shift nd and L determine steering angle. For our SLC tip-tilt corrector, L and nd are approximately 10,000 and 2.0 m, respectively. Thus the steering angle is 0.0115. When we connected V L to the left end and V H to the right end, we obtained the phase profile shown in Fig. 9(b) such that an incident beam is steered to the left. If we flip V L and V H, the triangle phase profile has a different slope, as depicted in Fig. 9(c). Therefore the incident beam is steered to the right. Operating this device between the states of Figs. 9(b) and 9(c) doubles the steering angle to 2. The steering angle can be further doubled by operation of the device in reflection mode, because nd is doubled. These considerations are adopted in our device design, which we discussed in Subsection 4.B. B. Beam Profile and Steering Efficiency The experimental setup for measuring the beam profile and steering efficiency is shown in Fig. 10(a). A laser beam 1.55 m passes through a polarizer, 20 December 2005 Vol. 44, No. 36 APPLIED OPTICS 7757

Fig. 11. (a), (b) Beam profiles from a reflected reference cell in the Z and Y directions. (c) SLC steered and nonsteered beam profiles in the Z direction. To facilitate comparison of the beam intensity, the two peaks of the beams are aligned. (d) SLC steered and nonsteered beam profiles in the Y direction. Fig. 10. (Color online) (a) Schematic drawing of the setup for beam profile and switching speed measurements. (b) Three possible positions to which the beam can be steered. whose transmission axis is in the z direction. Then the beam goes through a beam expander (BE) and reaches the reflective SLC tip-tilt corrector. The reflected beam first passes through a beam compressor (BC) and is received by a photodetector. By employing the beam expander and the beam compressor we further increase the steering angle. Figure 10(b) is a side-view simplified version of Fig. 10(a), which is focused only on the three positions of the reflected beam. There are three possible positions on the z axis for a reflected beam. Position P0 corresponds to the state when no voltage is applied to the SLC device; P1 and P2 correspond to the states when the two voltage ramps shown in Figs. 9(b) and Fig. 9(c) are applied to the device. A detector can be moved to any of these three positions by a micrometer translation stage. First, a reflective cell filled with fully cured NOA65 adhesive replaces the SLC device shown in Figs. 10(a) and 10(b), which is used as a base reference to check the beam profile and steering efficiency. The detector is placed at position P0. A 15 m pinhole is attached to the detector. By moving the detector in Z and Y directions and recording the readings of the detector across the whole beam, we obtain the beam profile from the reflected reference cell. The beam profiles in the Z and Y directions are shown in Figs. 11(a) and 11(b). Then the reference cell is replaced by the SLC tip-tilt corrector. Repeating the same beam profile measurement made for the reference cell, we have plotted the beam profiles with and without the voltage ramp applied in the Z and Y directions in Figs. 11(c) and 11(d), respectively. Figure 11(c) shows the beam profiles in the steering and nonsteering cases in the Z direction. The bottom horizontal axes show the beam width and the position of the nonsteered beam and the top horizontal axes show the beam width and the position of the steered beam in the Z direction. The two peaks plotted are aligned to facilitate comparison of their peak intensities. Similarly, Figure 11(d) shows the beam profiles for steered and nonsteered cases in the Y direction. For both Figs. 11(c) and Fig. 11(d) the measured intensities of the steered and nonsteered peaks are quite similar. The measured beam steering efficiency is 91%. C. Switching Speed of the SLC Tip-Tilt Corrector The SLC tip-tilt corrector s switching speed at room temperature as measured by an oscilloscope is shown in Fig. 12. The waveform at the top is the time response of the SLC tip-tilt corrector, and here we call it a switching curve. The waveform at the bottom is a driving waveform applied to one end of the device. The driving waveform applied to the other end of the device is opposite that shown in Fig. 12 and is not shown here. Therefore the device has low voltage on one end and high voltage on the other when it steers the beam. The driving waveform s base frequency is 10 khz. The amplitudes of the waveform are 67.0 and 191.0 V, respectively. The switching curve is obtained with the setup shown in Fig. 10. The switching curve at a low voltage level, point A (Fig. 12), and from D to E indicates that the beam is steered away from the detector placed at position P1 in Fig. 10(b); the switching curve at a high voltage level, from B to 7758 APPLIED OPTICS Vol. 44, No. 36 20 December 2005

Fig. 12. Measured response time of the SLC tip-tilt corrector. The waveform at the top is the time response of the SLC device; the waveform at the bottom is the driving waveform. The base frequency of the driving waveform is 10.0 khz, and amplitudes are 67.0 and 191.0 V, respectively. C, indicates that the beam is steered into the detector placed at position P1 in Fig. 10(b). The rise time from point A to point B indicates how fast the beam is steered into the detector, and the fall time from C to D indicates how fast the beam is steered away from the detector. The beam size is 2.8 mm (Fig. 11), and the detector s diameter is 1.5 mm. The measured rise and fall times are 100 s, much faster than those of conventional nematic LC devices switching the same amount of phase shift. 5. Conclusions A fast-switching tip-tilt corrector based on stressed liquid crystal optical phase arrays has been fabricated. 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