Laser Beacon Research Articles

Calculation and analysis of the number of return photons from sodium laser beacon excited by the long pulse laser with circular polarization

The number of return photons from sodium laser beacon (SLB) greatly suffers down-pumping, recoil, and geomagnetic field when the long pulse laser with circular polarization interacts with sodium atoms in the mesosphere. Considering recoil and down-pumping effects on the number of return photons from SLB, the spontaneous radiation rates are obtained by numerical computations and fittings. Furthermore, combining with the geomagnetic field effects, a new expression is achieved for calculating the number of return photons. By using this expression and considering the stochastic distribution of laser intensity in the mesosphere under different turbulence models for atmosphere, the number of return photons excited by the narrow-band single mode laser and that by the narrow-band three-mode laser are respectively calculated. The results show that the narrow-band three-mode laser with a specific spectrum structure has a higher spontaneous radiation rate and more return photons than a narrow-band single mode laser. Of note, the effect of the atmospheric turbulence on the number of return photons is remarkable. Calculation results indicate that the number of return photons under the HV5/7 model for atmospheric turbulence is much higher than that under the Greenwood and ModHV models.

宏微脉冲激光激发钠信标回波光子数的数值模拟

宏微脉冲激光激发钠信标回波光子数的数值模拟

FPGA Implementation for CCD Driving and Laser Spot Position Detection

CCD sensor can be used as the imaging device in the initial aiming processing for the beacon laser, and can also be used as the detecting device for the communication lasers optical axis alignment error of the targeting captured trace (APT) unit in space laser communication system. In this paper, based on a double-FPGA structure, we proposed technical solutions to realize the imaging CCD sensor driving and the laser spot position detection algorithm for the CX285AL image sensor by using the overall design of the system block diagram. The spot position detection wave door track centroid algorithm for CCD sensor is also studied by using the analysis of laser transmission characteristics of the atmospheric turbulence. The simulation analysis and field tests have been verified the feasibility of this approach.

激光钠信标荧光回波光子数的影响因素及其数值模拟

激光钠信标荧光回波光子数的影响因素及其数值模拟

Control analysis of acquirement and locking in inter-satellite laser communications

Process of acquirement and locking-up of compound axis system in inter-satellite laser communication has been studied. The effect of different condition parameters on the process of acquirement and locking-up have also been researched and simulated. Simulation results show that when the system with appropriate bandwidth has been adopted, both fine pointing system and coarse pointing system can satisfy three requirements of step response, stability criterion and dwell time and then finish the acquirement and locking-up of beacon laser beam. The simulation results provide the suited condition parameters for both fine pointing system and coarse pointing system, which is very helpful to the subsequent point and tracking processes.

The Neutrino Telescope of the KM3NeT Deep-Sea Research Infrastructure

Abstract KM3NeT is a future deep-sea research infrastructure in the Mediterranean Sea that will hold a multi-cubic-kilometre neutrino telescope. Located in the northern hemisphere, KM3NeT will be able to observe point-like sources of cosmic neutrinos in a region of the sky that includes the Galactic Centre. The KM3NeT neutrino telescope will employ a number of innovative technologies that are the main subject of this article. It is planned to install optical modules of 17 inch diameter that will contain 31 photomultiplier tubes each. Triggered data will be digitised o_-shore. The optical modules will be installed on horizontal bar structures of about 6m length. Twenty of these structures will be stacked to form a vertical tower, interconnected by ropes at 40m distances. Fundamental to achieving the required pointing position for neutrino astronomy is the knowledge of the relative positions of the photomultiplier within 20 cm. Accordingly, a time synchronisation on the nanosecond level is required to reconstruct the Cherenkov cones of charged particles produced in neutrino interactions. To calibrate the complete timing in situ, LED beacons, integrated into the optical modules, and laser beacons installed on the sea floor will be used. To calibrate the positions of the optical modules on the towers, a system of acoustic transceivers at fixed positions on the sea floor and receivers along the towers will be used. Data transmission is done over a fibre optic network employing multiplexing of up to 80 wavelength over individual fibres.

Design and Control of Paired Mobile Robots Working Across a Thin Plate With Application to Aircraft Manufacturing

A pair of mobile robots acting on opposite sides of a thin plate is developed for a class of tasks where robots have to work together, carrying a pair of end-effectors and traversing across a plate surface. Using powerful magnets, the paired robots attract each other, support themselves against gravity, and generate traction force to move across the panel. First, the design concept of paired mobile robots is presented, followed by dynamic modeling and magnetic analysis. Conditions for preventing the robot from falling as well as from slipping on the plate surface are examined. Time-optimal control of the paired robots subject to the no-fall, no-slip conditions is formulated and solved numerically. Precision positioning control using a laser beacon is designed and tested. A prototype of the paired robots using Halbach array permanent magnets and Lorentz force actuators is developed, and the control methods are implemented and tested on the prototype.

太阳对成像型天基激光告警系统的影响

In view of the problem of solar radiation in the imaging space-based laser warning system,the effects of sunlight on the infrared detector of warning system were studied by theoretical analysis and numerical calculation.On the basis of analysis of the different spatial relationship of sun and detector,the influences of direct sunlight and sunlight reflected by earth to the signal to noise ratio(SNR) of infrared image,laser threat detection,the detector safety were investigated respectively.The results show that the sunlight is the mainly background radiation to space-based laser warning system.The sunlight reflected by earth is the main imaging background light of system′s near infrared detector,and the maximum energy density of reflected sunlight exposuring the CCD is 0.97 mJ/cm2.In the reflected sunlight background,the SNR of attack laser image exceeds 103,for the beacon laser is always below 1.While the reflected sunlight has no effect on attack laser detection,but it is strong background interference to the beacon laser detection.The sunlight direct exposure detector is unavoidable to the warning system.As the maximum energy density of the sun imaging spot is about 20 W/cm2,it will lead to the pixel of CCD saturation or even damaged.Since the imaging spot of sun is similar to the laser,direct sunlight will cause serious false alarm.

30 W级第二代钠信标激光器研究

30 W级第二代钠信标激光器研究

Wide-Angle Lens Array Spatial Acquisition With Ultrawide Field of View in Optical Links Under Atmospheric Scintillation and Background Radiation

We propose a wide-angle lens array scheme, which can provide spatial acquisition with ultrawide field of view (FOV) in free-space optical (FSO) links. This acquisition array is composed of sub-arrays, with each subarray including receiving elements. The elements in a subarray are arranged in the same direction to receive the laser beacon together, while the subarrays are arranged in different directions to cover an ultrawide FOV (a hemisphere with a solid angle of ). The elements in a subarray can increase the total aperture size to restrict the laser power fluctuation induced by atmospheric scintillation. The subarrays can disperse the ultrawide FOV to reduce the received background power on each element. Therefore, the elements in a subarray and the subarrays can both contribute to the improvement of the SNR, which can improve the spatial acquisition performance of the wide-angle array. The missed detection probability (MDP), as a parameter to describe the spatial acquisition performance, is investigated for the wide-angle lens array spatial acquisition scheme under atmospheric scintillation and background radiation. Results show that the proposed array scheme can improve the spatial acquisition performance degraded by atmospheric scintillation and background radiation. The acquisition performance can be further enhanced with the increase of element number in a subarray and subarray number , where the elements in a subarray and the subarrays are designed to suppress atmospheric scintillation and background radiation, respectively.

ON-SKY WIDE-FIELD ADAPTIVE OPTICS CORRECTION USING MULTIPLE LASER GUIDE STARS AT THE MMT

We describe results from the first astronomical adaptive optics system to use multiple laser guide stars, located at the 6.5-m MMT telescope in Arizona. Its initial operational mode, ground-layer adaptive optics (GLAO), provides uniform stellar wavefront correction within the 2 arc minute diameter laser beacon constellation, reducing the stellar image widths by as much as 53%, from 0.70 to 0.33 arc seconds at lambda = 2.14 microns. GLAO is achieved by applying a correction to the telescope's adaptive secondary mirror that is an average of wavefront measurements from five laser beacons supplemented with image motion from a faint stellar source. Optimization of the adaptive optics system in subsequent commissioning runs will further improve correction performance where it is predicted to deliver 0.1 to 0.2 arc second resolution in the near-infrared during a majority of seeing conditions.

Multiconjugate adaptive optics results from the laboratory for adaptive optics MCAO/MOAO testbed

We report on the development of wavefront reconstruction and control algorithms for multiconjugate adaptive optics (MCAO) and the results of testing them in the laboratory under conditions that simulate an 8 meter class telescope. The University of California Observatories (UCO) Lick Observatory Laboratory for Adaptive Optics multiconjugate testbed allows us to test wide-field-of-view adaptive optics systems as they might be instantiated in the near future on giant telescopes. In particular, we have been investigating the performance of MCAO using five laser beacons for wavefront sensing and a minimum-variance algorithm for control of two conjugate deformable mirrors. We have demonstrated improved Strehl ratio and enlarged field-of-view performance when compared to conventional AO techniques. We have demonstrated improved MCAO performance with the implementation of a routine that minimizes the generalized isoplanatism when turbulent layers do not correspond to deformable mirror conjugate altitudes. Finally, we have demonstrated suitability of the system for closed loop operation when configured to feed back conditional mean estimates of wavefront residuals rather than the directly measured residuals. This technique has recently been referred to as the "pseudo-open-loop" control law in the literature.

Constrained matched filtering for extended dynamic range and improved noise rejection for Shack-Hartmann wavefront sensing

We recently introduced matched filtering in the context of astronomical Shack-Hartmann wavefront sensing with elongated sodium laser beacons [Appl. Opt. 45, 6568 (2006)]. Detailed wave optics Monte Carlo simulations implementing this technique for the Thirty Meter Telescope dual conjugate adaptive optics system have, however, revealed frequent bursts of degraded closed loop residual wavefront error [Proc. SPIE 6272, 627236 (2006)]. The origin of this problem is shown to be related to laser guide star jitter on the sky that kicks the filter out of its linear dynamic range, which leads to bursts of nonlinearities that are reconstructed into higher-order wavefront aberrations, particularly coma and trifoil for radially elongated subaperture spots. An elegant reformulation of the algorithm is proposed to extend its dynamic range using a set of linear constraints while preserving its improved noise rejection and Monte Carlo performance results are reported that confirm the benefits of the method.

A Hybrid Stabilization Approach for Deep-Space Optical Communications Terminals

Near-Earth laser communication system designs typically use the near-symmetric power levels in duplex links as bright sources upon which to base active beam stabilization. In deep space, it may be difficult to provide a constant, high-power laser beacon as a stabilization reference. We describe here several means, aimed at different frequency ranges of control, for generating a combined pointing reference and for actively controlling beam position. Such a blended approach gives a highly flexible system for performing beam stabilization in deep space, where distances, conditions, and power levels can vary widely.

Shack-Hartmann wavefront sensing with elongated sodium laser beacons: centroiding versus matched filtering

We describe modeling and simulation results for the Thirty Meter Telescope on the degradation of sodium laser guide star Shack-Hartmann wavefront sensor measurement accuracy that will occur due to the spatial structure and temporal variations of the mesospheric sodium layer. By using a contiguous set of lidar measurements of the sodium profile, the performance of a standard centroid and of a more refined noise-optimal matched filter spot position estimation algorithm is analyzed and compared for a nominal mean signal level equal to 1000 photodetected electrons per subaperture per integration time, as a function of subaperture to laser launch telescope distance and CCD pixel readout noise. Both algorithms are compared in terms of their rms spot position estimation error due to noise, their associated wavefront error when implemented on the Thirty Meter Telescope facility adaptive optics system, their linear dynamic range, and their bias when detuned from the current sodium profile.

Pseudo-infinite guide stars for multi-conjugated adaptive optics on extremely large telescopes

We introduce a novel concept to sense the wavefront for adaptive optics purposes in astronomy using a conventional laser beacon. The concept we describe involves treating the light scattered in the mesospheric sodium layer as if it comes from multiple rings located at infinity. Such a concept resembles an inverse Bessel beam and is particularly suitable for multi-conjugated adaptive optics on extremely large telescopes. In fact, as the sensing process uses light apparently coming from infinity, some problems linked to the finite distance and vertical extent of the guide source are solved. Since such a technique is able to sense a wavefront solely in the radial direction, we propose furthermore a novel wavefront sensor by combining the inverse Bessel beam approach with the recently introduced z-invariant technique for a pseudo-infinite guide star sensor.

Optical guidance method for robots capable of vision and communication

The optical guidance of robots spans the research topics of robotics, computer vision, communication and real-time control. The proposed method aims to improve the accuracy of guidance along a desired route in an environment that is unknown to the robot. The key idea is to indicate the numerical coordinates of target positions by means of projecting a laser light onto the ground. In contrast with other guidance methods, which communicate the target position numerically, using optical commands avoids the need to maintain the coordinate transformation between the robot’s system and that of the environmental model (“world” reference coordinates). The image processing and communication ensure that the robot accurately follows the route indicated by laser beacons, and self-localization becomes less relevant for guidance. The experimental results have proved the effectiveness of this method.

First Tests of Wavefront Sensing with a Constellation of Laser Guide Beacons

Adaptive optics to correct current telescopes over wide fields, or even to correct future very large telescopes over narrow fields, will require real-time wavefront measurements made with a constellation of laser beacons. Here we report the first such measurements, made at the 6.5 m MMT with five Rayleigh beacons in a 2' pentagon. Each beacon is made with a pulsed beam at 532 nm of 4 W at the exit pupil of the projector. The return is range-gated from 20 to 29 km and recorded at 53 Hz by a 36-element Shack-Hartmann sensor. Wavefronts derived from the beacons are compared with simultaneous wavefronts obtained for individual natural stars within or near the constellation. Observations were made in seeing averaging 10 with two-thirds of the aberration measured to be from a ground layer of mean height 380 m. Under these conditions, subtraction of the simple instantaneous average of the five beacon wavefronts from the stellar wavefronts yielded a 40% rms reduction in the measured modes of the distortion over a 2' field. We discuss the use of multiple Rayleigh beacons as an alternative to single sodium beacons on 8 m telescopes and the impact of the new work on the design of a multi-sodium beacon system for the 25 m Giant Magellan Telescope.

The 4.2-m William Herschel Telescope as ELT testbed facility

The technical developments required to build the future Extremely Large Telescopes will be very demanding. Some of these developments, for instance in the field of Adaptive Optics, will rely on experimental work to test new techniques and concepts. The 4.2-m William Herschel Telescope located at a representative high-quality observing site, and with its stable Nasmyth optical bench for general access and its common-user Rayleigh laser beacon, is well placed to play a role as a testbed facility for such activities.

Characterizing the Adaptive Optics Off‐Axis Point‐Spread Function. II. Methods for Use in Laser Guide Star Observations11Based on observations obtained at the Lick Observatory, which is operated by the University of California.

ABSTRACTMost current astronomical adaptive optics (AO) systems rely on the availability of a bright star to measure the distortion of the incoming wave front. Replacing the guide star with an artificial laser beacon alleviates this dependency on bright stars and therefore increases sky coverage, but it does not eliminate another serious problem for AO observations. This is the issue of PSF variation with time and field position near the guide star. In fact, because a natural guide star is still necessary for correction of the low‐order phase error, characterization of laser guide star (LGS) AO PSF spatial variation is more complicated than for a natural guide star alone. We discuss six methods for characterizing LGS AO PSF variation that can potentially improve the determination of the PSF away from the laser spot; that is, off‐axis. Calibration images of dense star fields are used to determine the change in PSF variation with field position. This is augmented by AO system telemetry and simple computer simulations to determine a more accurate off‐axis PSF. We report on tests of the methods using the laser AO system on the Lick Observatory Shane Telescope. We observed with offsets typical of separations between dim science targets and the nearest suitably bright tip‐tilt guide star, up to 20″. If the tip‐tilt guide star is used as the PSF reference, the predicted Strehl ratio within an 8″ radius of the LGS can be more than a factor of 2 too high, as it does not take into account the field‐dependent anisoplanatism. A better result may be obtained by an improved empirical approach: repositioning the tip‐tilt guide star to compensate for the additional 8″ offset. This would result in a 27% relative error in Strehl ratio. A simple Gaussian model of PSF variation can reduce the error in the prediction to ∼20%. Some further improvement may be obtained semianalytically by using telemetry from the AO system (14% error) plus a more sophisticated theoretical model of PSF variation (13% error). It should be noted, however, that all but the first method achieve comparable accuracy in predicting PSF FWHM. This is important because the last method, unlike the others, is not directly applicable to 10 m apertures without further computational complexity.