Xingyun Zhang

Advanced single-frame overdriving for liquid-crystal spatial light modulators

A single-frame overdriving scheme was employed to improve the temporal response of the active matrix addressing liquid-crystal spatial light modulator used in an open-loop adaptive optics system (OLAOS). Optimal time distribution giving minimum wavefront residual error for the OLAOS was demonstrated. As a result, the measured -3 decibels rejection frequency was increased from 26 to 35 Hz, and the image quality was significantly improved.

Improve the accuracy of interaction matrix measurement for liquid-crystal adaptive optics systems

We present a novel method to measure the interaction matrix of liquid-crystal adaptive optics systems, by applying least squares method to mitigate the impact of measurement noise. Experimental results showed a dramatic gain in the accuracy of interaction matrix, and a considerable improvement in image resolution with open loop adaptive optics correction.

A high precision phase reconstruction algorithm for multi-laser guide stars adaptive optics*

Adaptive optics (AO) systems are widespread and considered as an essential part of any large aperture telescope for obtaining a high resolution imaging at present. To enlarge the imaging field of view (FOV), multi-laser guide stars (LGSs) are currently being investigated and used for the large aperture optical telescopes. LGS measurement is necessary and pivotal to obtain the cumulative phase distortion along a target in the multi-LGSs AO system. We propose a high precision phase reconstruction algorithm to estimate the phase for a target with an uncertain turbulence profile based on the interpolation. By comparing with the conventional average method, the proposed method reduces the root mean square (RMS) error from 130 nm to 85 nm with a 30% reduction for narrow FOV. We confirm that such phase reconstruction algorithm is validated for both narrow field AO and wide field AO.

Measurement of gain characteristics of semiconductor lasers by amplified spontaneous emissions from dual facets

In this letter, we describe a novel gain measurement approach for semiconductor edge-emitting lasers, with which TE and TM gain spectra can be easily obtained by collecting the amplified spontaneous emissions at dual facets of the device. An unstrained and continuously-operated GaAs/AlGaAs single quantum well laser strip is used to illustrate this method. The measured gain spectra are compared with theoretical gain curves to analyze the gain polarization characteristics and the relevant subband structure in the valence band of the well using the measured gain spectra.

Visible light high-resolution imaging system for large aperture telescope by liquid crystal adaptive optics with phase diversity technique

There are more than eight large aperture telescopes (larger than eight meters) equipped with adaptive optics system in the world until now. Due to the limitations such as the difficulties of increasing actuator number of deformable mirror, most of them work in the infrared waveband. A novel two-step high-resolution optical imaging approach is proposed by applying phase diversity (PD) technique to the open-loop liquid crystal adaptive optics system (LC AOS) for visible light high-resolution adaptive imaging. Considering the traditional PD is not suitable for LC AOS, the novel PD strategy is proposed which can reduce the wavefront estimating error caused by non-modulated light generated by liquid crystal spatial light modulator (LC SLM) and make the residual distortions after open-loop correction to be smaller. Moreover, the LC SLM can introduce any aberration which realizes the free selection of phase diversity. The estimating errors are greatly reduced in both simulations and experiments. The resolution of the reconstructed image is greatly improved on both subjective visual effect and the highest discernible space resolution. Such technique can be widely used in large aperture telescopes for astronomical observations such as terrestrial planets, quasars and also can be used in other applications related to wavefront correction.

High precision system modeling of liquid crystal adaptive optics systems

In this paper, we present a heuristic method to simplify the liquid crystal adaptive optics system (LCAOS) into a single-input-single-output (SISO) system, then build the dynamic model of LCAOS based on subspace identification. Results show that the identified model could accurately describe the dynamical behavior of LCAOS (97% match), with extremely low complexity. The wonderful features of low complexity and high precision, make the identified model highly beneficial for model based controller design, system analysis and dynamical behavior simulation of liquid crystal adaptive optics systems.

DM/LCWFC based adaptive optics system for large aperture telescopes imaging from visible to infrared waveband

Almost all the deformable mirror (DM) based adaptive optics systems (AOSs) used on large aperture telescopes work at the infrared waveband due to the limitation of the number of actuators. To extend the imaging waveband to the visible, we propose a DM and Liquid crystal wavefront corrector (DM/LCWFC) combination AOS. The LCWFC is used to correct the high frequency aberration corresponding to the visible waveband and the aberrations of the infrared are corrected by the DM. The calculated results show that, to a 10 m telescope, DM/LCWFC AOS which contains a 1538 actuators DM and a 404 × 404 pixels LCWFC is equivalent to a DM based AOS with 4057 actuators. It indicates that the DM/LCWFC AOS is possible to work from visible to infrared for larger aperture telescopes. The simulations and laboratory experiment are performed for a 2 m telescope. The experimental results show that, after correction, near diffraction limited resolution USAF target images are obtained at the wavebands of 0.7-0.9 μm, 0.9-1.5 μm and 1.5-1.7 μm respectively. Therefore, the DM/LCWFC AOS may be used to extend imaging waveband of larger aperture telescope to the visible. It is very appropriate for the observation of spatial objects and the scientific research in astronomy.

Configuration optimization of laser guide stars and wavefront correctors for multi-conjugation adaptive optics*

Multi-conjugation adaptive optics (MCAOs) have been investigated and used in the large aperture optical telescopes for high-resolution imaging with large field of view (FOV). The atmospheric tomographic phase reconstruction and projection of three-dimensional turbulence volume onto wavefront correctors, such as deformable mirrors (DMs) or liquid crystal wavefront correctors (LCWCs), is a very important step in the data processing of an MCAOs controller. In this paper, a method according to the wavefront reconstruction performance of MCAO is presented to evaluate the optimized configuration of multi laser guide stars (LGSs) and the reasonable conjugation heights of LCWCs. Analytical formulations are derived for the different configurations and are used to generate optimized parameters for MCAO. Several examples are given to demonstrate our LGSs configuration optimization method. Compared with traditional methods, our method has minimum wavefront tomographic error, which will be helpful to get higher imaging resolution at large FOV in MCAO.

The openloop liquid crystal adaptive optics for astronomical application

Atmospheric turbulence deteriorates the image quality of astronomical objects and limits the spatial resolution of large aperture optical telescopes. Adaptive optics (AO) corrects the aberrations induced by atmospheric turbulence in real time, which concentrates the incident light and enables the telescope to achieve the diffraction-limited resolution on-axis or over an extended field-of-view (FoV). In this paper, we present the transfer function of our open-loop liquid crystal adaptive optics system. Based on the transfer function, we calculated the bandwidth of our nematic LC AO system: -3dB bandwidths are about 57.0 Hz and 82.4Hz for high order aberration and tilt-tip compensation control, respectively. Through further improvements on LC material, electronic interface port, shortened data processing time and optimized control, we will get a better LC AO system. 2013 IFAC.