ming Wu

Beam shaping for uplink transmission of a relay mirror system

We establish the model of a two-Cassegrain-telescope system that is commonly used in a relay mirror system. With this model, uplink transmission of the relay mirror system is theoretically analyzed. We determined that uplink transmission with turbulence completely corrected is not an optimal mode. We improve power coupling efficiency of the two-Cassegrain-telescope system by optimizing the optical phase at the launching telescope by use of the stochastic parallel gradient descent algorithm. For a 10 km vertical uplink transmission, power coupling efficiencies of the system are 63.10%, 87.82%, and 97.80% corresponding to an open-loop mode, a closed-loop mode, and a closed-loop with optimization mode, respectively. For a 30 km vertical uplink transmission, power coupling efficiencies of the system are 22.35%, 82.66%, and 91.91%, corresponding to an open-loop mode, a closed-loop mode, and a closed-loop with optimization mode, respectively. The results show that power coupling efficiency of the two-Cassegrain-telescope system is significantly improved.

Realization of large energy proportion in the central lobe by coherent beam combination based on conformal projection system

In this paper, we experimentally validate a tiled-aperture conformal projection system with the largest array filling factor and element beam truncation factor to the best of our knowledge. The conformal projection system, which is made up of a hexagonal adaptive fiber-optics collimator (AFOC) array with the proximate ideal intensity distributions, is fabricated and the performance of output beam is tested and evaluated properly and carefully. Both of the active phase-locking control and precise tip-tilt control are implemented successfully in the CBC of the hexagonal seven-beam-array. Experimental results show a large energy proportion (47%, which increases by over 10% comparing with the previously demonstrated largest value) in the central lobe is achieved and the residual phase error is lower than λ/27. When the AFOC array performs, the precise tilt control makes the combining beams overlap well and the average normalized metric value is improved from 0.336 without control to 0.947 with both of active piston and tip-tilt phase controls while the fringe contrast increases from 19% to more than 91% correspondingly. This work presents a promising structure for the achievement of large energy proportion in the central lobe in high power fiber laser CBC systems.

Modeling on Bessel beam guide star beacon for wavefront sensing

Bessel beam has the advantages of reducing scattering artefacts and increasing the quality of the image and penetration. This paper proposed to generate a guide star by Bessel beam with vortex phase, and to use the beacon with special spot structure to measure the atmosphere turbulence aberrations. With the matching algorithm of measured characteristic spot in each subaperture, the detection accuracy of Hartmann wavefront sensor can be improved. Based on wave optics theory, the modeling of Bessel beam guide star and wavefront sensing system was built. The laser guide star beacon generated by Bessel beam with vortex phase and beacon echo wave measured by Hartmann sensor were both simulated. Compared with the results measured by echo wave from Gauss beam generated guide star beacon, this novel method can reduce the error of wavefront detection and increase the detection accuracy of Hartmann sensor.

Spatial light modulators based laser guide star simulator

Atmospheric turbulence induces laser guide star (LGS) spot wandering in the sodium layer, which introduces trouble to adaptive optics systems. Experimental study of LGS spot wandering usually needs on-sky test. However, the on-sky test of LGS spot wandering is expensive and complicated. Since spatial light modulators (SLMs) are able to simulate atmospheric turbulence, we have designed and set up a SLMs based LGS simulator to study LGS spot wandering. This LGS simulator is prominent to build a bridge between theoretical study and on-sky test. Its performance is tested for vortex beam generated annular LGS which was proposed to reduce the LGS spot wandering in our former paper.

Thermal distortion real-time detection and correction of a high-power laser beam-splitter mirror based on double Shack-Hartmann wavefront sensors

In a high-power laser system, a beam splitter refers to the mirror which locates at the cross point of the path of highpower beam and the weak light section. Because of the thermo-optic effect and elasto-optic effect, a beam splitter deforms under intense laser radiation. This deformation adds extra phase on the incident waves and deliveries inaccurate information to the wavefront sensor. Consequently, the output laser focuses at finite distance and gets divergent when arrives at the target. To settle the above problem, this paper presents a new method for real-time correction of the thermal distortion of beam splitter, based on algorithm of the data fusion of two Shack-Hartmann wavefront sensors (SH-WFS). Different from the traditional AO system, which contains a wavefront sensor, a corrector and a servo controller, two extra Shack-Hartmann wavefront detectors are adopted in our AO system, to detect the transmitted and reflected aberrations of beam splitter mirror. And these aberrations are real-timely delivered to the wavefront sensor. Based on coordinate conversion and data fusion algorithm, it makes the wavefront sensor of AO can “see” the aberrations of splitter mirror by itself. Thus, the servo system controls the corrector to compensate these aberrations correctly. In this paper, the theoretical model of data fusion algorithm is carried out. A closed-loop AO system, which consists of a typical AO system and two extra Shack-Hartmann wavefront detectors, is set up to validate the data fusion algorithm. Experimental results show that, the distortion of a CaF2 beam splitter can be real-time corrected when the AO closedloop control is on. The beam quality factor of output laser decreases from 4 to 1.7 times of diffraction limit.

Experiment research on uniformity of multi-beam illumination

Adaptive optics system can work with beacon produced by illumination the dim target with laser beam. But atmospheric turbulence distributed over the entire propagation path produces intensity scintillation of the illumination laser beam. The multi-beam illumination approach can improve the intensity uniformity by adding different intensity distributions produced by separate illumination beams passing through independent atmospheric paths. A 4 km horizontal propagation path was established to examine the illumination effect with multi-beam on the target. Individual laser beams were launched from a separate region of the transmitter telescope. The far-field irradiance profile was received by target-plane and recorded with a CCD camera operating in 1k Hz placed in front of the target-plane. The uniformity of target area was determined by the number of the illuminating beams and the turbulence strength. Data were recorded with 1, 3 or 6 incoherence lasers illumination and for a range of R0 parameter. We calculated the irradiance of a 8x8 cm2 area in the target-plane to characterize uniformity of illumination. Results show a good reduction in intensity variance with number of illumination beams increasing and that the uniformity of illumination on target turns to worse when the atmosphere turbulence became worse. The back backscatter from the target-plane was received by another telescope nearby the transmitter side and imaged by a CCD camera. The images of the rough reflector showed that the image resolution and the illumination effects improved with the number of illuminating beams increasing.

Simulation of the solid state laser relay mirror system

The laser relay mirror technique has been under extensive research in recent years. The concept and the working process of the relay mirror system are analyzed in this paper. The model of the relay mirror system is established, the effect on 1km-altitude target with 500 m/s flying speed of the relay mirror system and the conventional ground-based laser system are calculated respectively. Obtains that the maximum coverage range of the ground-based laser system is 2.0 km and the maximum coverage range of the relay mirror system is 23.2 km. The relay mirror can largely open up the coverage range of the laser system and strengthen the attacking ability of the ground-based laser.