Hao Xian

Performance evaluation of adaptive optics for atmospheric coherent laser communications

With extremely high sensitivity, the coherent laser communications has a large potential to be used in the long-range and high data-rate free space communication links. However, for the atmospheric turbulent links, the most significant factor that limits the performance of the coherent laser communications is the effect of atmospheric turbulence. In this paper, we try to integrate the adaptive optics (AO) to the coherent laser communications and analyze the performances. It is shown that, when the atmospheric turbulence condition D/r0 is not larger than 1, can the coherent laser communication system works well without the correction of an AO system. When it is in the gentle turbulent condition (around D/r0 = 2), only the tip and tilt correction can improve the mixing efficiency and the bit-error rate (BER) significantly. In the moderate (around D/r0 = 10) or relatively strong (around D/r0 = 17) turbulent condition, the AO system has to correct about 9 or 35 turbulent modes or more respectively to achieve a favorable performance. In conclusion, we have demonstrated that the AO technique has great potential to improve the performances of the atmospheric coherent laser communications.

Laboratory demonstrations on a pyramid wavefront sensor without modulation for closed-loop adaptive optics system

The feasibility and performance of the pyramid wavefront sensor without modulation used in closed-loop adaptive optics system is investigated in this paper. The theory concepts and some simulation results are given to describe the detection trend and the linearity range of such a sensor with the aim to better understand its properties, and then a laboratory setup of the adaptive optics system based on this sensor and the liquid-crystal spatial light modulator is built. The correction results for the individual Zernike aberrations and the Kolmogorov phase screens are presented to demonstrate that the pyramid wavefront sensor without modulation can work as expected for closed-loop adaptive optics system.

First light on the 127-element adaptive optical system for 1.8-m telescope

A 127-element adaptive optical system has been developed and integrated into a 1.8-m astronomical telescope in September 2009. In addition, the first light on a high-resolution imaging for stars has been achieved (September 23, 2009). In this letter, a 127-element adaptive optical system for 1.8-m telescope is described briefly. Moreover, star observation results in the first run are reported. Results show that the angular resolution of the system after adaptive optics correction can attain 0.1 arcsec, which approaches the diffraction limit of 1.8-m telescope at 700-900 nm band.

Progress on the 127-element adaptive optical system for 1.8m telescope

The 127-element adaptive optical system for the 1.8m astronomical telescope is being developed. In this system, the wavefront correction loop consists of a 127-element deformable mirror, a Hartmann-Shack (H-S) wavefront sensor, and a high-speed digital wavefront processor. The tracking system consists of a tip-tilt mirror, a tracking sensor and a tracking processor. The wavelength for the H-S wavefront sensor ranges from 400-700nm. The imaging observation wavelengths range from 700-1000nm and 1000-1700nm respectively. In this paper, the optical configuration of 1.8m telescope will be briefly introduced. The 127-element adaptive optical system is described in detailed. Furthermore, the preliminary performances and test results on the 127-element adaptive optical system is reported.

Measuring statistical error of Shack–Hartmann wavefront sensor with discrete detector arrays

A formula is derived that can be used to evaluate the angular position error caused by random noise; when a Shack–Hartmann wavefront sensor is used to measure the wavefront information of the signal, its standard deviation is given by σ = ωSNR − 1λ/D + ηVs − 1/2λ/D, where ω is the position-error constant, and it is fully defined in the main text, SNR is the ratio of the signal intensity to the noise intensity, η is a positive constant, λ is the measuring wavelength, D is the diameter of the aperture, and V s is the count of signal photoelectrons. The angular position error is inversely proportional to the signal-to-noise of the signal, and it is concerned with the scale of the discrete detector arrays. The experimental results are in agreement with the formula and prove that the formula can describe the angular position error precisely under the different scales of the discrete detector arrays and different kinds of noise.

Algorithm and experiment of whole-aperture wavefront reconstruction from annular subaperture Hartmann–Shack gradient data

A new method is proposed for testing a rotationally symmetric aspheric surface with several annular subapertures based on a Hartmann-Shack sensor. In consideration of the limited sampling of Hartmann-Shack subapertures in the matching annular subaperture, a new algorithm for whole-aperture wavefront reconstruction from annular subaperture Hartmann-Shack gradient data is established. The algorithm separates the tip, tilt, and defocus misalignments for each annular subaperture by introducing annular Zernike polynomials. The performance of the algorithm is evaluated for different annular subaperture configurations, and the sensitivity of the algorithm to the detector error of the wavefront gradient is analyzed. The algorithm is verified by the experimental results.

Field-of-view shifted Shack-Hartmann wavefront sensor for daytime adaptive optics system

For the daytime adaptive optics system, a field-of-view shifted Shack-Hartmann wavefront sensor (FSWFS), which is used to measure the aberrant wavefront under daytime conditions, is proposed. Because the field angle of the object signal in adaptive optics systems is much less than that of the sky background, the effective object signal is separated from the strong sky background. Experimental results indicate that FSWFS with a single focal-plane array can precisely and stably measure the aberrant wavefront information with a strong sky background under daytime conditions.

Adaptive optical system based on deformable secondary mirror on 1.8-meter telescope

In 2009, A 127-element adaptive system had been manufactured and installed at the Coude room of the 1.8-meter telescope at the Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. A set of new adaptive optical system based on a 73-element deformable secondary mirror is being developed and will be integrated into the 1.8-meter telescope. The 73-element deformable secondary mirror with convex reflecting surface is designed to be compatible with the Cassegrain focus of the 1.8-meter telescope. Comparing with the AO system of Coude focus, the AO system on the deformable secondary mirror adopts much less reflections and consequently restrains the thermal noise and increases the energy transmitting to the system. The design and simulation results of this system will be described in this paper. Furthermore, the preliminary test result of the deformable secondary mirror in the lab is also presented.

First observations on the 127-element adaptive optical system for 1.8m telescope

The 127-element adaptive optical system, which consists of a tracking loop with a tip-tilt mirror, a tracking system and a tracking processor, and a wavefront correction loop with a 127-element deformable mirror, a Hartmann-Shack wavefront sensor, and a wavefront processor, had been developed and integrated into the 1.8m astronomical telescope in September 2009. The First observations on the high resolution imaging for the stars had been done from September 23 2009 in the first light to March 2010. In this paper, the 127-element adaptive optical system for 1.8m telescope is described briefly and the star observation results in the first run are reported. The results show the angular resolution of the system can attain or approach the diffraction limit of 1.8m telescope at I band (700nm-1000nm) and J band (1000nm-1700nm).

Progress on the 1.8m solar telescope: the CLST

In order to study some special solar activities, such as the emergence, evolution and disappearance progress of the sunspot and magnetic flux, and the key role of magnetic field, a new 1.8-meter size high-resolution solar telescope —the CLST will be built in the Institute of Optics and Electronics(IOE), Chinese Academy of Science(CAS), which locates in Chengdu, China. The CLST has a classic Gregorian configuration, alt-azimuth mount, retractable dome. Besides that, a large mechanical de-rotator will be used to cancel the image rotation, and finally it will cooperate with another kind of mechanical de-rotator to cancel both of the pupil rotation and image rotation. Φ3 arc-minute field of view will help the CLST to observe the whole solar activity region, and if necessary the FOV can be enlarged to Φ 6 arc-minute. A 1.8m primary mirror with honeycomb sandwiches structure made by using ULE material will reduce about 70% of weight. Thermal controlling system will also be equipped for the CLST, which including Heat-Stop, primary mirror, tube truss, mount and the other optics elements. An experimental system for validating thermal controlling of primary mirror and Heat-Stop has been built, and the temperature tracking results will be illustrated in this paper. Currently, we have finished the detailed design of the CLST, and some important components also have been manufactured and finished. In this paper, we describe some important progresses and the latest status of the CLST project during these two years.