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The unstable ring resonator is a promising architecture for high power solid-state oscillators with intra-cavity adaptive optics. In this paper, we present adaptive compensation of intra-cavity tilt in an Nd: YAG unstable ring laser using a probe beam and a far-field camera to detect the tilt. A numerical model of the ring resonator is established based on the Fox–Li method, through which the influence of intra-cavity tilt is studied, and the optimal strategy of tilt correction is proposed and evaluated. An experiment with an unstable Nd: YAG slab ring laser is developed. With 40 μ rad tilt on the zigzag direction and 130 μ rad tilt on the non-zigzag direction compensated during lasing, the output power increased from 77.2W to 87.0W. The above results lay the foundation for adaptive optics techniques to compensate for the intra-cavity aberrations in high power solid-state unstable ring lasers.

A breakthrough result of a kW-level Nd: YAG slab laser with a unidirectional unstable ring resonator is presented. The reverse wave is efficiently suppressed by a diaphragm placed at the focal point of the negative-branch unstable ring resonator. With specifically developed multi-function sensor and precision placement of the diaphragm, we have achieved the maximum output power of 1350 W with reverse suppression ratio of 54:1 corresponding to an optical-optical efficiency of 21.6% and a slope efficiency of 33.7%. The output power is currently limited by the available pump power. To the best of our knowledge, this is the highest power demonstrated from an Nd: YAG slab laser based on an unstable ring resonator with efficient reverse wave suppression, which is a paramount step to the further development of high power solid-state unidirectional ring lasers.

The edge effects of high-power slab lasers cause severe wavefront distortions with large amplitude and gradients, which is a great challenge for the adaptive optics systems for improving the beam qualities. The standard least-squares wavefront reconstruction method would inherently assign higher priorities to correcting the edge aberrations which contribute little to the laser power in the near diffraction-limit areas in the far-field, while the other aberrations are not well corrected. To solve the problems, we propose a least-squares reconstruction algorithm with performance-based filtering, in which the correction capability of the deformable mirror is analyzed in the spatial frequency domain, and then the input phase distortions are filtered accordingly. The uncorrectable components of input distortions are ignored so that the residuals of the correctable components could be further minimized. Experiments with a 20 kW Nd:YAG slab laser indicate that the proposed method is effective to improve the beam quality of high-power slab laser with edge effect.

We present the results that the performances of the atmospheric coherent optical communications improved by a real 127-element adaptive optics system operating in 1550 nm wavelength on a 1.8 m telescope. The mixing efficiency and the BER of the homodyne free-space optical coherent communications have been analyzed with the correction of the adaptive optics system. It is shown that, the AO can expand the scope of the atmospheric turbulence condition D/r(0)(1550) from 1 to around 6.5 in which the coherent optical communications can function well. In order to acquire favorable results, the mixing efficiency should be larger than 0.4 and the residual wavefront error after the AO correction should be smaller than lambda/6. We also find that the mixing efficiency of the coherent homodyne receiver is approximate to the Strehl ratio of the far field of the signal light. In conclusion, we have shown that the adaptive optics is a very promising technique that can largely improve the performances of the free space coherent optical communications degraded by the atmospheric turbulence. (C) 2015 Elsevier B.V. All rights reserved.

We present a hybrid adaptive optics system for a kW-class solid-state slab master oscillator power amplifier laser that consists of both a low-order aberration corrector and a 59-actuator deformable mirror. In this system large defocus and astigmatism of the beam are first corrected by the low-order aberration corrector and then the remaining components are compensated by the deformable mirror. With this sequential procedure it is practical to correct the phase distortions of the beam (peak to valley up to 100 μm) and the beam quality β is successfully improved to 1.9 at full power.

We present adaptive compensation of intracavity tilts based on detecting the direction of the output beams. We use the initial direction of the output beam when the laser cavity is well collimated for reference. Then the difference between the actual direction of the output beam and the reference is used as feedback to control the intracavity tip/tilt mirror. The relation between the direction of the output beam and the intracavity tilt is investigated with both the Fox–Li method and measurements. A series of experiments demonstrate that intracavity tilts can be well compensated by the proposed method. We have also proved that it is possible to substitute the proposed method for conventional extracavity beam stabilization.

In this Letter, we present an adaptive aberration correction system to simultaneously compensate for aberrations and reshaping the beams. A low-order aberration corrector is adapted. In this corrector, four lenses are mounted on a motorized rail, whose positions can be obtained using a ray tracing method based on the beam parameters detected by a wavefront sensor. After automatic correction, the PV value of the wavefront is reduced from 26.47 to 1.91 μm, and the beam quality β is improved from 18.42 to 2.86 times that of the diffraction limit. After further correction with a deformable mirror, the PV value of the wavefront is less than 0.45 μm, and the beam quality is 1.64 times that of the diffraction limit. To the best of our knowledge, this is the highest performance from such a high-power, high-pulse repetition rate Nd:YAG solid-state laser ever built.

Direct liquid-cooling is a promising way of power scaling and heat management for solid-state lasers. A side-pumped direct liquid-cooled solid-state pulsed zigzag MOPA system is established based on this advanced concept. However, its beam quality is degraded by the thermal distortions in the non-zigzag direction and the flowing coolant. We develop an adaptive optics system to improve its beam quality, which primarily includes a low-order aberration compensator and a 59-actuator deformable mirror. The beam is first corrected by the low-order aberration compensator to remove large defocus and astigmatism, and its size is reshaped simultaneously to fulfill the demands of applications. Then the beam is further corrected by the deformable mirror. With collaborative operation of the low-order aberration compensator and the deformable mirror, we have achieved average beam quality of β=2.8.

Adaptive optics is an important technology for improving beam quality in solid-state slab lasers. However, there are uncorrectable aberrations in partial areas of the beam. In the criterion of the conventional least-squares reconstruction method, it makes the zones with small aberrations nonsensitive and hinders this zone from being further corrected. In this paper, a weighted least-squares reconstruction method is proposed to improve the relative sensitivity of zones with small aberrations and to further improve beam quality. Relatively small weights are applied to the zones with large residual aberrations. Comparisons of results show that peak intensity in the far field improved from 1242 analog digital units (ADU) to 2248 ADU, and beam quality β improved from 2.5 to 2.0. This indicates the weighted least-squares method has better performance than the least-squares reconstruction method when there are large zonal uncorrectable aberrations in the slab laser system.

In adaptive optics,Hartmann-shack wavefront sensor and image acquired from camera has been widely used to detect incident wavefront and the application of laser wavefront correction. A numerical simulation method of generating high resolution virtual camera image, which is based on Fourier optics and realized by FFT algorithm, is mentioned in this paper. In the condition of incident laser beam wavefront with only tilt phase aberration, calculating deviation of centroid offset between the result from virtual camera image and theoretical value, is compared to verify the accuracy of this numerical simulation method and virtual Hartmann-shack image generation model.

We present the results that the mixing efficiency and the bit-error-rate of the atmospheric coherent optical communications improved by a real 127-actuators adaptive optics system operating on 1550nm wavelength on a 1.8m telescope.

The search-based intra-cavity beam cleanup systems are typical feedback loop systems which have been used in both gas and solid-state laser cavity to enhance laser performance. The studies on search-based intra-cavity beam cleanup systems mostly rely on experiments. However there are few reports about numerical simulation. In this paper, we attempt to use numerical method to analysis the search-based intra-cavity beam cleanup system. These systems contain three main elements: an adaptive laser cavity, a fitness sensor of laser performance and a computer-based search algorithm. According to the compositions of beam cleanup systems, three modules have been set up in our numerical simulation framework. We detail the theory, structure and numerical method of these modules in this paper. To demonstrate the feasibility of the numerical method, we calculated the results based on stochastic parallel gradient descent algorithm.

The performance of Adaptive Optics (AO) real-time controller based on Central Processing Unit (CPU) has significantly progressed due to the introduction of the high speed frame-grabber and a 4-cores CPU, which make it possible to process at frequency over 2000 Hz for 4-meter-class telescope and to integrate the real-time task and the user interface program in this compact device. The detailed architecture of this computation system is demonstrated in this paper, and the performance and suitability of this architecture is also discussed by measuring the latency of the controller processing via an adaptive optics emulator system with 16 times 16 and 32 times 32 sub-aperture, and the overall typical processing time is 61 us and 322 us respectively. Test result turns out that it is well suited for the next generation 4-meter-class adaptive optics system and it is possible to process at frequency over 2000 Hz for a 3000-element AO system in 10- meter-class telescope with one board of art-of-the-state computer and a frame-grabber. Comparison with GPU and FPGA based architecture is also discussed in this paper.

A 3mm narrow interval deformable mirror (DM) with tip-tilt stage has been developed for astronomical instruments. Benefiting from its compact design, the adaptive optics system can be built with simple structure and smaller optical elements. First, a 37-elements prototype mirror has been developed for our 1.8-meter telescope, which interval space is 3mm, maximum tilt is ±10’, and maximum deformation is ±2μm. Based on this mirror, a simple adaptive optics system has been set up and its performance was tested in the laboratory especially the closed-loop correction ability. This adaptive optics subsystem is scheduled to be mounted at one folded Cassegrain focus of the 1.8-meter telescope this year, and comparison test for star compensation observation using this compact system and conventional adaptive optics system will also be carried out at the same time.

A first generation sodium Laser Guide Star Adaptive Optics System (LGS-AOS) was developed and integrated into the Lijiang 1.8m telescope in 2013. The LGS-AOS has three sub-systems: (1) a 20 W long pulsed sodium laser, (2) a 300-millimeter-diameter laser launch telescope, and (3) a 37-element compact adaptive optics system. On 2014 January 25, we obtained high resolution images of an mV 8.18 star, HIP 43963, during the first light of the LGS-AOS. In this paper, the sodium laser, the laser launch telescope, the compact adaptive optics system and the first light results will be presented.

We are developing a sodium guide star adaptive optics system for the 1.8 meter telescope, which consists of three main parts: (i) 20W microsecond pulsed laser system, (ii) Φ200mm laser launch telescope and (iii) 37-elements adaptive optics system. All of these three parts are mounted on the 1.8 meter telescope which is located in Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. The pulsed laser system and the launch telescope are rotated with the azimuthal base of the telescope. A miniaturized 37-elements low-order adaptive optics system including a 37-elelment deformable mirror and a 6x6 array Hartmann-Shack wavefront sensor is mounted at the Cassegrain focus taking account of the pulsed laser mode. A separate tip-tilt correction loop is also integrated into the system. This paper describes the details of this system, the simulation result and the test result in the lab. After the indoor test, the whole system will be shipped to 1.8 meter telescope. The latest commissioning status and results is presented also in this paper.

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 seco ndary mirror with convex reflecting surface is designed to be compatible with the Cassegrain focus of the 1.8-meter te lescope. 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. Key words: adaptive optics, telescope, deformable secondary mirror, fitting error

A low-order solar adaptive optics (AO) system, which consists of a fine tracking loop with a tip/tilt mirror and a correlation tracker, and a high-order correction loop with a 37-element deformable mirror, a correlating Shack-Hartmann wavefront sensor and a high-order wavefront correction controller, had been successfully developed and installed at 1-m New Vacuum Solar Telescope of Full-shine Lake (also called Fuxian Lake) Solar Observatory. This system is an update of the 37-element solar AO system designed for the 26-cm Solar Fine Structure Telescope at Yunnan Astronomical Observatory in 2009. The arrangement of subapertures of the Shack-Hartmann wavefront sensor was changed from square to hexagon to achieve better compensation performance. Moreover, the imaging channel of the updated system was designed to observe the Sun at 710nm and 1555nm simultaneously. The AO system was integrated into the solar telescope in 2011, and AO-corrected high resolution sunspots and granulation images were obtained. The observational results show that the contrast and resolution of the solar images are improved evidently after the correction by the AO system.

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).

A high speed real-time wavefront processing system for a solid-state laser beam cleanup system has been built. This system consists of a core2 Industrial PC (IPC) using Linux and real-time Linux (RT-Linux) operation system (OS), a PCI image grabber, a D/A card. More often than not, the phase aberrations of the output beam from solid-state lasers vary fast with intracavity thermal effects and environmental influence. To compensate the phase aberrations of solid-state lasers successfully, a high speed real-time wavefront processing system is presented. Compared to former systems, this system can improve the speed efficiently. In the new system, the acquisition of image data, the output of control voltage data and the implementation of reconstructor control algorithm are treated as real-time tasks in kernel-space, the display of wavefront information and man-machine conversation are treated as non real-time tasks in user-space. The parallel processing of real-time tasks in Symmetric Multi Processors (SMP) mode is the main strategy of improving the speed. In this paper, the performance and efficiency of this wavefront processing system are analyzed. The opened-loop experimental results show that the sampling frequency of this system is up to 3300Hz, and this system can well deal with phase aberrations from solid-state lasers.

Intracavity adaptive optics has been successfully used in solid state lasers to improve output laser beam quality. However, In order to utilize this technology to improve the output laser beam quality successfully, the distribution characteristics of phase aberration in the laser resonator should firstly be known. Thus, a set of Hartmann- Shack wave front sensor (HSWFS) to measure the time and space characteristics of phase aberration in a diode-side-pumped Nd:YAG laser was set up. In this paper, the HSWFS is briefly introduced. The experimental results for measurement of the phase aberration in a diode side-pumped Nd:YAG laser are presented. The experimental results show that, the main phase aberration in the resonator is generated by the Nd:YAG rod. The phase aberration induced by thermal deformation in the cavity mirrors is minor. The temporal behavior of phase aberration in the resonator and from cavity mirrors under light heating was also obtained.

A 37-element solar adaptive optics system, which consists of a fine tracking loop with a tip/tilt mirror and a correlation tracker, and a high-order correction loop with a 37-element deformable mirror, a correlating Shack-Hartmann wavefront sensor, and a real-time controller, was built and installed at the 26 cm solar fine structure telescope of the Yunnan Astronomical Observatory in 2009. In this system, the absolute difference algorithm is used. A new architecture with field-programmable gate array (FPGA) and digital signal processor (DSP) for the real-time controller based on systolic array and pipeline was developed. The computational latencies of the fine tracking loop and high-order correction loop are about 35 and 100 mu s, respectively. The tracking residual root-mean-square error is less than 0.1 arcsec, and the wavefront residual root-mean-square error is about 0.05 wavelengths (lambda = 550 nm) after correction. The observational results show that the contrast and resolution of the solar images are improved after the correction by this adaptive optics system. (C) 2010 Optical Society of America

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.