Search

To reduce the effect of servo lag error in the close-loop adaptive optics system (AOS), a prediction control algorithm (PCA) is proposed in this paper. We describe a linear predictive controller that uses the historical voltages of deformable mirror to predict their future voltages in advance. The parameters of the predictor are optimized in real time by a modified Recursive Least-Square (RLS) algorithm. Numerical simulations are carried out to show the significant improvements brought by the predictor. The close-loop Strehl Ratio of the residual wave-front error and the power spectrum density of the residual voltages of deformable mirror are calculated as the evaluation criterions. The comparison results between the PCA and the classical proportional integral (PI) control algorithm show that the residual errors caused by servo lag in the system could be more effectively reduced by using the PCA than using the classical PI control algorithm. In addition, the control bandwidth of AOS is improved by using the PCA.

Restricted by the observational condition and the hardware, adaptive optics can only make a partial correction of the optical images blurred by atmospheric turbulence. A postprocessing method based on frame selection and multi-frame blind deconvolution is proposed for the restoration of high-resolution adaptive optics images. By frame selection we mean we first make a selection of the degraded (blurred) images for participation in the iterative blind deconvolution calculation, with no need of any a priori knowledge, and with only a positivity constraint. This method has been applied to the restoration of some stellar images observed by the 61-element adaptive optics system installed on the Yunnan Observatory 1.2m telescope. The experimental results indicate that this method can effectively compensate for the residual errors of the adaptive optics system on the image, and the restored image can reach the diffraction-limited quality.

One of the important characteristic of adaptive mirrors is the thermal stability of surface flatness. In this paper, the thermal stability from 13 degrees C to 25 degrees C of a 20-actuator bimorph deformable mirror is tested by a Shack-Hartmann wavefront sensor. Experimental results show that, the surface P-V of bimorph increases nearly linearly with ambient temperature. The ratio is 0.11 mu m/degrees C and the major component of surface displacement is defocused, compared with which, astigmatism, coma and spherical aberration contribute very small. Besides, a finite element model is built up to analyse the influence of thickness, thermal expansion coefficient and Young's modulus of materials on thermal stability. Calculated results show that bimorph has the best thermal stability when the materials have the same thermal expansion coefficient. And when the thickness ratio of glass to PZT is 3 and Young's modulus ratio is approximately 0.4, the surface instability behaviour of the bimorph manifests itself most severely.

Adaptive optics can be used to compensate for the wave aberration of the human eyes to achieve high-resolution imaging in real time. However the correction is partial due to the limitation of hardware. We propose a kind of hybrid image post-processing method, which uses the blind deconvolution combined with the residual data in wavefront sensor to restore the partially adaptive optics corrected retinal image. This method is applied in the image restoration of the vivid human retinal images. The results show that it is effective to improve the retinal image quality.

An accurate pointing system is required in free-space optical (FSO) communication links. Low energy-transmission efficiency caused by pointing errors would decline the communication system’s performance. The statistics of the detected signal or return signal values could be used to estimate the pointing parameters, whereas atmospheric turbulence brings in serious challenges. A modified moment-matching estimation method is presented in this paper. The irradiance fluctuation caused by the atmospheric turbulence is considered, and the probability density function (PDF) in a weak turbulence condition is assumed to be lognormal. This modified approach is evaluated with wave-propagation simulation data and shows significant improvement over the conventional approach. The estimation accuracy and the properties of this new approach are also discussed. Although our method is based on lognormal irradiance PDF under a weak turbulence condition, the irradiance PDF would tend to be lognormal with aperture averaging effect under moderate to strong turbulence, and the ideas can be extended with appropriate PDF models to satisfy different conditions.

Laser beam steering or pointing, which stabilizes the beam direction, is critical in many areas, such as optical communication systems, astronomy and directed-energy systems etc. However, the disturbances including atmospheric turbulence and mechanical jitter on platform may degrade the pointing accuracy. A proportional-integral (PI) feedback control commonly has been used in the track loop with a fast steering mirror. To compensate dynamic disturbance effectively, the laser beam steering control system must have a larger bandwidth than the disturbance bandwidth. But the control bandwidth is limited by the noise of the sensor, computing latency, and the light energy. So, a simple proportional-integral (PI) feedback controller of a piezoelectric fast steering mirror (PFSM) can only compensate the broadband disturbance of the atmospheric turbulence, but cannot effectively compensate a larger amplitude narrowband jitter because of the low control bandwidth. Moreover, when the control bandwidth is tuned to high, the mechanical resonance of the PFSM can cause the instability of the system. An improved dual two-order filter assisted high-bandwidth control algorithm to improve the pointing accuracy and error attenuation capability is proposed. This method can control a PFSM for suppression of laser beam jitter. The influence of filter parameters on frequency characteristics is analyzed, and then, a practical design method is proposed. The dual two-order filter can combine the characteristics of traditional notch filter and two-order low-pass filter, and can also obtain any desired amplitude in the interesting frequency ragion with little influence on the others. The principle of the proposed filter for suppressing the mechanical resonance of the PFSM and the narrowband disturbance is elaborated. And then, the different dual two-order filters are designed according to the frequency content of the PFSM and the narrowband disturbance. Finally, the proposed dual two-order filter assisted PI control algorithm and classic PI control algorithm are compared with each other. Experimental results show that, in the same conditions, the pointing accuracy of the proposed two-order filter assisted PI control algorithm is nearly 5 times better than that of the classic PI control algorithm, and the error attenuation bandwidth is one time higher. It also indicates that the proposed algorithm does not need an additional sensor; it is simple and effective for the suppression of the mechanical resonance of a PFSM and that of the narrowband disturbance, hence it improves the system error attenuation bandwidth and the beam pointing accuracy.

The interaction matrix of an adaptive optical system is directly computed. In this method, based on the correlation between the measured and theoretical slope influence vectors of the selected actuators, the misalignment between the wavefront sensor and wavefront corrector is first estimated, and then an interaction matrix more close to the real one is obtained. Experimental results show that with considering the misalignment, the interaction matrix acquired by direct computation becomes more effective in closed-loop correction, and the slope residual error is reduced.

Based on the most widely used proportional-integrator controller of the tip-tilt correction loop in an adaptive optical system, the relationship between the optimal closed-loop bandwidth and the atmosphere turbulence as well as the sensor noise is analyzed. Numerical simulation is also performed by using the time serial signals that follow the power spectrum density of the global tilt of Kolmogorov turbulence. Results show that with different atmosphere turbulences and sensor noises, the correction capability of the tip-tilt correction loop can be brought into full play by choosing appropriate closed-loop bandwidth and the residual variance can be reduced.

Three-dimensional (3D) wavefront sensing of atmosphere turbulence is a key step to realize multi-conjugate adaptive optics technology. In this article, model tomography, which is one of the most important algorithms for 3D wavefront sensing, is analyzed in theory, the principle limitation of model tomography is proposed. Based on this view the reason for tomography error is discussed, and the simulation results for different errors are produced finally. The analysis shows that part Zernike model basis is used as a new basis of polynomial decomposition in model tomography, which leads to modal coupling error and aliasing error. The un-correlation of part Zernike model basis is the prerequisite to avoid modal aliasing error, but modal coupling error cannot be removed and we can only restrain its influence. Combined with simulation result a method with large field of view (FOV) sensing and small FOV reconstruction is proposed and gives a good performance to control modal coupling error.

Based on the time serial signal generated by power spectrum density inversion, the closed-loop compensation residual variances of tracking loop and high-order error correction loop with finite error rejection -3 dB bandwidth and time delay are simulated. The data fitting results of relationships between residual variances and corresponding error rejection -3 dB bandwidths are obtained. The system effective bandwidths are modified so as to make the estimate of residual variances more precise.