Lifa Hu

Phase-only liquid-crystal spatial light modulator for wave-front correction with high precision

We introduce a novel parallel-aligned liquid-crystal (LC) spatial light modulator (SLM) that has been designed to operate in a phase-only mode for wave-front correction. We measured and analyzed theoretically the electro-optic characteristics of the LC SLM and obtained a peak-to-valley value of 0.07049lambda(lambda= 0.6328microm) after correction. A Strehl ratio of 0.989 indicates the approximate upper limit of an aberrated wave front that the LC SLM can correct when it is used in an adaptive optical system.

Adaptive optics imaging system based on a high-resolution liquid crystal on silicon device

An adaptive optics imaging system is introduced in this paper. A high resolution liquid crystal on silicon (LCOS) device was used as a phase only wave front corrector instead of a conversional deformable mirror. The wave front aberration was detected by a Shack-Hartmann (SH) wave front sensor, which has lambda/100 rms wave front measurement accuracy. Under this construction 0.09lambda (lambda=0.6328microm) Peak to Valley correction precision was reached. Further more, some low frequency hot convection turbulence induced by an electric iron was compensated in real time at the same precision. The Modulation Transfer Function (MTF) of this system was also measured before and after wave front correction. Under the active correction of LCOS, the system reached the diffraction limited resolution approximately 65l p/mm on the horizontal direction. All of this showed the ability of using this device in high resolution, low temporal turbulence imaging system, such as retinal imaging, to improve the resolution performance.

Open-loop correction of horizontal turbulence: system design and result

Adaptive optics systems often work in a closed-loop configuration due to the hysteretic and nonlinearity properties of conventional deformable mirrors. Because of the high-precision wavefront generation and nonhysteretic properties of liquid-crystal devices, the open-loop control becomes possible. Open-loop control is a requirement for advanced adaptive optics concepts. We designed an open-loop adaptive optics system with a liquid-crystal-on-silicon wavefront corrector. This system is simple, fast, and can save much more light compared to conventional liquid-crystal-based closed-loop systems. The detailed principle, construction, and operation are discussed. The 500 m horizontal turbulence correction experiment was done using a 250 mm telescope in the laboratory. The whole system can reach a 60 Hz correction frequency. Evaluation of the correction precision was done at closed-loop configuration, which is 0.2 lambda (lambda=0.633 microm) in peak to valley. The dynamic image under open-loop correction got the same resolution compared to closed-loop correction. The whole system reached 0.68 arc sec resolution capability at open-loop correction, which is slightly larger than the systems diffraction-limited resolution of 0.65 arc sec.

Investigation of optical testing with a phase-only liquid crystal spatial light modulator

We illustrate that the phase-only liquid crystal spatial light modulator (LC SLM) can be used for optical testing. The large phase change and the phase modulation precision are discussed. The computer generated holograms (CGH) method is used to acquire the significant phase modulation. And the phase modulating characteristics of the LC SLM are measured. It shows the phase modulation depth is more than 2pi and the modulation precision is down to 1/14lambda (PV) and 1/100lambda (rms) (lambda=632.8nm). In order to verify this method, the former surface of a convex lens is tested by ZYGO interferometer. The parallel straight fringes are obtained. It is shown that PV is 1/3lambda and rms is 1/20lambda after compensated by the LC SLM.

Liquid Crystal based adaptive optics system to compensate both low and high order aberrations in a model eye

Based on a simple eye model system, a high resolution adaptive optics retina imaging system was built to demonstrate the availability of using liquid crystal devices as a wave-front corrector for both low and high order aberrations. Myopia glass was used to introduce large low order aberrations. A fiber bundle was used to simulate the retina. After correction, its image at different diopters became very clear. We can get a root mean square (RMS) correction precision of lower than 0.049lambda (lambda=0.63mum) for over to 10 diopters and the modulation transfer function (MTF) retains 511p/mm, which is nearly the diffraction limited resolution for a 2.7mm pupil diameter. The closed loop bandwidth was nearly 4 Hz, which is capable to track most of the aberration dynamics in a real eye.

Organic dual-wavelength distributed feedback laser empowered by dye-doped holography

An organic dual-wavelength distributed feedback (DFB) laser empowered by a dye-doped holographic polymer dispersed liquid crystal (HPDLC) grating is reported for the first time. The dual-wavelength laser operates at 586.6 nm and 670.2 nm simultaneously in one laser beam via the seventh and eighth Bragg orders of a one dimensional (1D) HPDLC grating with a special period. We theoretically and experimentally demonstrate the mechanism of our dual-wavelength DFB laser and investigate the threshold and spectral properties of the output laser. Our organic dual-wavelength DFB laser shows the advantages of simple configuration, one-step fabrication, cost-effectiveness and the potential application to be integrated in all optical networks.

Preliminary use of nematic liquid crystal adaptive optics with a 2.16-meter reflecting telescope.

A nematic liquid crystal adaptive optics system (NLC AOS) was assembled for a 2.16-m telescope to correct for atmospheric turbulence. LC AOS was designed and optimized with Zemax optical software. Second, an adaptive correction experiment was performed in the laboratory to test the performance of the NLC AOS. After the correction, the peak to valley (PV) and root mean square (RMS) of the wavefront were down to 0.2 lambda (lambda=633 nm) and 0.05 lambda, respectively. Finally, the star of Pollux (beta Gem) was tracked using the 2.16-m Reflecting Telescope, and real time correction of the atmospheric turbulence was performed with the NLC AOS. After the adaptive correction, the average PV and RMS of the wavefront were reduced from 11 lambda and 2.5 lambda to 2.3 lambda and 0.6 lambda, respectively. Although the intensity distribution of the beta Gem was converged and its peak was sharp, a halo still existed around the peak. These results indicated that the NLC AOS only partially corrected the vertical atmospheric turbulence. The limitations of our NLC AOS are discussed and some proposals are made.

Enhancement of pump efficiency for an organic distributed feedback laser based on a holographic polymer dispersed liquid crystal as an external light feedback layer

We report a low threshold, high energy conversion organic distributed feedback (DFB) laser based on a holographic polymer dispersed liquid crystal (HPDLC) grating as an external light feedback layer, specifically, by adopting an acrylate-based monomer with low functionality and a rubbed polyimide (PI) alignment layer. In such configuration, the phase separated LCs were aligned along the preferred direction, which gave an increased refractive index difference between the LC and the polymer, so it can provide better light feedback in the HPDLC layer. The pump efficiency for the laser, such as lasing output threshold and conversion efficiency, can be enhanced. The light loss, diffraction efficiency and driving voltage were also investigated for the HPDLC structures to identify the effects of the rubbing layer and monomer functionality.

Improvement of the switching frequency of a liquid-crystal spatial light modulator with optimal cell gap

In the application of a nematic liquid-crystal (LC) spatial light modulator, we derived the formula of retardation dynamic response of the device by solving the Erickson-Leslie equation. Then, the response time of the 2π phase change can be expressed as a function of the LC cell gap. The theoretical and experimental results all indicate that the response time of 2π first decreases and then increases with the LC cell gap increasing, and there is an optimal cell gap to obtain the shortest response time. Therefore, the method of optimizing the cell gap shows potential to improve the switching frequency for all type of nematic LC optical device with specific modulation quantity.

Accommodation-based liquid crystal adaptive optics system for large ocular aberration correction.

According to ocular aberration property and liquid crystal (LC) corrector characteristics, we calculated the minimum pixel demand of the LC corrector used for compensating large ocular aberrations. Then, an accommodation based optical configuration was introduced to reduce the demand. Based on this an adaptive optics (AO) retinal imaging system was built. Subjects with different defocus and astigmatism were tested to prove this. For myopia lower than 5D it performs well. When myopia is as large as 8D the accommodation error increased to nearly 3D, which requires the LC corrector to have 667 x 667 pixels to get a well-corrected image.