In order to find out the wavefront distribution characteristics of supercontinuum light source, adding wavefront distortion description into the evaluation of supercontinuum light source beam quality, a comprehensive and accurate measurement about the wavefront distribution characteristics of a 400-1700nm supercontinuum light source developed by National University of Defense Technology (NUDT) is carried out in this paper. According to the experimental results, wavefront distribution characteristics in different wavelength bands of this 400nm-1700nm supercontinuum source are basically the same, mainly composing of defocus and astigmatism, however, values of distortions are different in specific wavelength bands.
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.
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.
The performance of the achieve laser beam propagation through atmospheric turbulence with adaptive optics is degraded by the fact that the wavefront aberrations difference. This error is only include the angular anisoplanatism when there is separation between the achieve laser beam and beacon. In the paper we derive an analytic expression for the effective angular anisoplanatism as a function of displacement angular when the turbulence profile is uniform. It shows that the effective angular anisoplanatism becomes weaker as the diameter of laser beams and the Fried transverse coherence length increase. We report results from field experiments that the effective angular anisoplanatism of a focused beam over horizontal path. It is found that measured and theoretical results are consistent. The effective wavefront variance increases with the strength of atmospheric turbulence and the angular displacement. And the constant phase of angular anisoplanatism has no effect on the Strehl ratio of the beam.
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.
Proc. SPIE. 9142, Selected Papers from Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics: Optical Imaging, Remote Sensing, and Laser-Matter Interaction 2013
For a beam from ground to space, the main optical turbulence effects are scintillation and beam wander. Multiple incoherent beams can reduce the scintillation. The scintillation is determined by the number of the beams, the beam separation and the size of the beam wander variance. A wave optics simulation was applied to study the scintillation index of 1-, 3, 6 collimated uplink Gaussian beams, where a hexagonal close-pack spacing is used. Based on the results of simulations, we propose an approximation to average spatial correlation in terms of the beam separation in the tracked and untracked cases. The relation between scintillation index and beam separation is different in the weak and moderately-strong fluctuation regimes when the number of beams is the same. And the average spatial correlation is determined by the beam waist radius, beam separation and beam wander variance.
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 cm<sup>2</sup> 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.
An 4km optical link was established to examine the effect of beam wander on the scintillation characterization for
multiple focus beams. Multiple beams that sample different portions of the atmosphere reduce the scintillation if the
atmospheric turbulence each beam traverses is independent. How much the beam wander affects scintillation is
determined by the number of the beams and the size of the beam wander variance. The number of mutually incoherent
beams was 1, 3 or 6. Individual laser was launched from a separate region of the transmitter and focused on the flat
screen with the horizontal distance of 4km by 350mm telescope. We recorded the illuminated flat screen with a CCD
camera operating at 1kHz. We calculated the scintillation index of the illuminated area with the size of 8x8cm on the
condition of untracked and tracked beams. A tracked beam generally refers to the situation in which the beam is tilt-corrected.
However, There was no tilt-corrected in the experiment. Instead, the rms beam centroid of the area was
aligned with the optical axis at each frame. Experimental results show that the scintillation index in tracked beam was
larger than in the untracked beam case when the size of the Rytov variance σ<sup>2</sup><sub>R</sub> is the same for one beam case, but for
three and six beams, the scintillation values are almost. It is the reason that the root mean square variance of wander
angle decreases as the number of beams increases. So the effect of beam wander is not evident for three and six beams.
And increasing the number of beams on the condition of untracked and tracked cases does reduce the scintillation index