Laser Beacon Research Articles

Orbiting Laser Beacons for Adaptive Optics Observations of Mars and Other Planets

ABSTRACTThe use of adaptive optics to correct the effects of seeing is rapidly becoming a standard technique in astronomical observing and is fundamental to current plans for extremely large telescopes. Adaptive optics has proved effective for studies of small solar system objects that can be used as their own reference sources. However, it is much harder to apply adaptive optics techniques to bright planets such as Mars and Venus, because of the difficulty of finding a suitable reference star that is not drowned out by the intense scattered light from the planet itself. A possible solution to the problem might be provided by current plans for laser communications systems. For example, the Mars Telecommunications Orbiter, planned for launch in 2009, will carry a 5 W laser to beam data back to Earth. This laser system in orbit around Mars will provide a very bright guide star, with a magnitude ranging from 1.8 to 5.8. Such a guide star is more than bright enough for existing adaptive optics systems and is in the range needed to support “extreme AO” systems, producing very high Strehl ratios. Used in conjunction with large ground‐based telescopes, this could allow studies of Mars with spatial resolutions down to a few kilometers and allow the ground‐based study of Mars to extend around much of its orbit, rather than be limited to the time around opposition.

Reduction of laser spot elongation in adaptive optics.

Adaptive optics systems measure the wave front to be corrected by use of a reference source, a star, or a laser beacon. Such laser guide stars are a few kilometers long, and when observed near the edges of large telescopes they appear elongated. This limits their utility significantly. However, with more sophisticated launch optics their shape and length can be controlled. We propose to string around the rim of a telescope a number of small telescopes that will add laser beams in the scattering medium to create a compact spot. The method could also be adapted for ocular adaptive optics.

Chemical imaging sensor and laser beacon.

Design and functional aspects of PANSPEC, a panoramic-imaging chemical vapor sensor (PANSPEC is an abbreviation for infrared panoramic-viewing spectroradiometer), were advanced and its optical system reoptimized accordingly. The PANSPEC model unites camera and fused solid-state interferometer and photopolarimeter subsystems. The camera is an eye of the open atmosphere that collects, collimates, and images ambient infrared radiance from a panoramic field of view (FOV). The passive interferometer rapidly measures an infrared-absorbing (or infrared-emitting) chemical cloud traversing the FOV by means of molecular vibrational spectroscopy. The active photopolarimeter system provides a laser beam beacon. This beam carries identification (feature spectra measured by the interferometer) and heading (detector pixels disclosing these feature spectra) information on the hazardous cloud through a binary encryption of Mueller matrix elements. Interferometer and photopolarimeter share a common configuration of photoelastic modulation optics. PANSPEC was optimized for minimum aberrations and maximum resolution of image. The optimized design was evaluated for tolerances in the shaping and mounting of the optical system, stray light, and ghost images at the focal plane given a modulation transfer function metric.

Extending the field of view of an adaptively corrected telescope by longitudinal pupil displacement

A simple scheme is proposed to increase the corrected field of view of a telescope equipped with adaptive optics. The optical system's entrance pupil is projected forward into the atmosphere and is slightly reduced in size. The field in any given direction is then imaged through just a portion of the main aperture, while the full aperture is used to collect light from the reference beacons, which may be either natural stars or laser beacons. The enlargement of the field of view is proportional to the reduction in pupil size and therefore comes at some cost in flux and resolution. A numerical simulation shows that the trade-off is modest and may in many cases be advantageous.

USING LASER BEACONS FOR DAYTIME ADAPTIVE OPTICS

This article examines the use of Laser Beacons for daytime astronomical observations. There are two potential applications: the diffraction limited observation of (1) the structure in the solar corona at all wavelengths, and (2) non-solar astronomical objects in the thermal infrared part of the spectrum. We examine the brightness of the Laser Beacon required as well as the limitations imposed by the daytime sky brightness and sky/telescope thermal emission on the observable magnitude limits. For both applications the use of Laser Beacon adaptive optics in daytime results in important research opportunities.

Tilt angular anisoplanatism and a full-aperture tilt-measurement technique with a laser guide star

A method is presented for sensing atmospheric wave-front tilt from a laser guide star (LGS) by observing a laser beacon with auxiliary telescopes. The analysis is performed with a LGS scatter model and Zernike polynomial expansion of wave-front distortions. It is shown that integration of the LGS image over its angular extent and the position of the auxiliary telescope in an array reduce the tilt sensing error associated with the contribution from the downward path. This allows us to single out only the wave-front tilt of the transmitted beam on the uplink path that corresponds to the tilt for the scientific object. The tilt angular correlation is analyzed in the atmosphere with a finite turbulence outer scale. The tilt correlation angle depends on the angular size of the telescope and the outer scale of turbulence. The tilt sensing error increases with the auxiliary telescope diameter, suggesting that an auxiliary telescope must be small. The Strehl ratio associated with the contribution from the downward path is in the range from 0.1 to 0.9 when the relative telescope diameter D/r(0) varies from 4 to 93 and the turbulence outer scale is in the 10-150-m range. Tilt correction increases the Strehl ratio compared with the uncorrected image for all the system parameters and seeing conditions considered. The method discussed gives a higher performance than the conventional technique, which uses an off-axis natural guide star. A scheme for measuring tilt with a beam projected from a small aperture is described. This scheme allows us to avoid phosphorescence of the main optical train for a sodium LGS.

A laser beacon for monitoring the mesospheric sodium layer at La Palma

We report the results of the first laser beacon experiment at the astronomical site of La Palma (Canary Islands). A continuous wave low power laser (a few hundreds of mW) system has been set up. The laser, tuned on the sodium D2 line at 589 nm, is launched close to zenith angle. The emission of the mesospheric sodium layer is observed from a telescope located 160 meters away from the laser. The layer is therefore resolved in altitude and the different features of its dynamics are investigated.

A laser beacon for monitoring the mesospheric sodium layer at La Palma

We report the results of the first laser beacon experiment at the astronomical site of La Palma (Canary Islands). A continuous wave low-power laser (a few hundred mW) system has been set up. The laser, tuned on the sodium D2 line at 589 nm, is launched close to the zenith angle. The emission of the mesospheric sodium layer is observed from a telescope located 160 m away from the laser. The layer is therefore resolved in altitude and the different features of its dynamics are investigated.

Shipborne Laser Beam Weapon System for Defence against Cruise Missiles

Sea-skim~ing cruise missiles pose the greatest threat to a surface ship in the present-day war scenario. The convenitional close-in-weapon-systems (CIWSs) are becoming less reliable against these new challenges requiring extremely fast reaction time. Naval Forces see a high energy laser as a feasible andjeffective directed energy weapon against sea-skimming antiship cruise missiles becauseof its .ability to deliver destructive energy at the speed of light on to a distant target. The paper comparesthe technology and capability of deuterium fluoride (DF) and chemical-oxygen-iodine laser (COIL) in effectively performing the role of a shipborne CIWS altainst sea-skimming missiles. Out of these twolasers, it is argued that DF laser wo.uld be more effective a,s a shipborne weapon for defence against sea-skimmin,g cruise missiles. Besides the high energy laser as the primary (killing) laser, othersub-systems required in the complete weapon system would be: A beacon laser to sense phase distor'ions in the primary laser, adaptive optics to compensate the atmospheric distortions, beam-directing optics, illuminating lasers, IRST sensors, surveillance and tracking radars, interfacing system, etc.

Verification of a System to Prevent Aircraft Illumination by Adaptive Optics Laser Beacons

ABSTRACTLaser beams directed into the sky from astronomical observatories to generate guide beacons for adaptive optics image‐sharpening systems are a potential hazard to aircraft. Detection systems are needed to sense aircraft and interrupt the laser beam to prevent accidental illumination. We describe here a system designed for this task. A computer examines CCD images of the sky over a wide field of view and uses a motion detection algorithm to find tracks made by aircraft lights. The results of tests, conducted with simultaneous radar coverage from Tucson International Airport, are reported. A test with an actual laser beacon at the Multiple Mirror Telescope in which Mars was used as a simulated aircraft is also described.

Experimental validation of a technique to measure tilt from a laser guide star

We have experimentally demonstrated for what is believed to be the first time a method for sensing wave-front tilt with a laser guide star (LGS). The tilt components of wave fronts were measured synchronously from the LGS by use of a telescope with a 0.75-m effective aperture and from the star Polaris by use of a 1.5-m telescope. The Rayleigh guide star was formed at an altitude of 6 km and at a corresponding range of 10.5 km by projection of a focused beam at Polaris from the full aperture at the 1.5-m telescope. Both telescope mounts were unpowered and bolted in place, allowing us to reduce substantially the telescope vibration. The maximum value of the measured cross-correlation coefficient between the tilt for Polaris and the LGS is 0.71. The variations of the measured cross-correlation coefficient in the range from 0.22 to 0.71 are caused by turbulence at altitudes above 6 km, which was not sampled by the laser beacon but affected tilt for Polaris, the cone effect for turbulence below 6 km, residual mount jitter of the telescopes, and variations of the signal/noise ratio. The results support our concept of sensing atmospheric tilt by observing a LGS with an auxiliary telescope and indicate that this method is a possible solution for the tip-tilt problem.

Two-deformable-mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere

A two-deformable-mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere is presented. This system uses a deformable mirror and a Fourier-transforming mirror to adjust the amplitude of the wave front in the telescope pupil, similar to kinoforms used in laser beam shaping. A second deformable mirror is used to correct the phase of the wave front before it leaves the aperture. The phase applied to the deformable mirror used for controlling the beam amplitude is obtained with a technique based on the Fienup phase-retrieval algorithm. Simulations of propagation through a single turbulent layer sufficiently distant from the beacon observation and laser beam transmission aperture to cause scintillation shows that, for an ideal deformable-mirror system, this field-conjugation approach improves the on-axis field amplitude by a factor of approximately 1.4 to 1.5 compared with a conventional phase-only correction system.

Full Adaptive Optics Images of Asteroids Ceres and Vesta; Rotational Poles and Triaxial Ellipsoid Dimensions

Adaptive optics (AO) images of asteroids Ceres and Vesta were obtained on September 18–20, 1993, with the 1.5-m telescope at the Starfire Optical Range of the USAF Phillips Laboratory located near Albuquerque, NM. The light source for higher-order wavefront correction was a Rayleigh laser beacon focused at a range of 10.5 km generated by a copper vapor laser. On April 27 and May 11, 1996, Vesta was again imaged, this time at its perihelic opposition using Vesta itself as the beacon for the AO. Images obtained at an effective wavelength of 0.85 μm were analyzed with a new reconstruction technique, called parametric blind deconvolution. The technique allows fits for relevant parameters in the frequency domain, where the convolution of the asteroid ellipse with the variable Lorentz-shaped point spread function produced by the AO system during these observations can be separated into a multiplication of analytic functions.The triaxial ellipsoid dimensions and rotational pole (with a two-fold ambiguity) of Ceres were obtained from 17 images and found to be in excellent agreement with its 1983 stellar occultation outline (975882) and the 1991 AO image produced by the COME-ON system (975882). A similar analysis of eight images of Vesta in 1993, 19 images in April 1996, and 28 images in May 1996, and combined with results from four previous speckle interferometry (SI) sets, yield dimensions and a rotational pole in reasonable agreement with the Hubble Space Telescope's results (975882). Differences may arise from a non-alignment between principal axes of inertia and the spin axis. The AO data from May 11, 1996, is of sufficient quality that Vesta's lightcurve can be decomposed into its cross-sectional area and surface brightness components, proving analytically that Vesta's lightcurve minimum is caused by a dark hemisphere. Combining the AO and SI observations with previous lightcurves yields a sidereal period of 0.22258874 days with an uncertainty of 4 in the last decimal place (3.5 ms) and shows that lightcurve minimum occurs 6° before a maximum in cross-section area.

Optimization of adaptive-optics systems closed-loop bandwidth settings to maximize imaging-system performance

We present the results of research aimed at optimizing adaptive-optics closed-loop bandwidth settings to maximize imaging-system performance. The optimum closed-loop bandwidth settings are determined as a function of target-object light levels and atmospheric seeing conditions. Our work shows that, for bright objects, the optimum closed-loop bandwidth is near the Greenwood frequency. However, for dim objects without the use of a laser beacon the preferred closed-loop bandwidth settings are a small fraction of the Greenwood frequency. In addition, under low light levels selection of the proper closed-loop bandwidth is more critical for achieving maximum performance than it is under high light levels. We also present a strategy for selecting the closed-loop bandwidth to provide robust system performance for different target-object light levels.

Highly Accurate Dynamic Position Measurement for Wheeled Mobile Robot Using Laser Beacon Measurement System.

The present research concerns a compensation method for dynamic measurement error(DME)using the laser beacon measurement system(LBMS), which is necessary for highly accurate positioning control and trajectory tracking control of a wheeled mobile robot(WMR). The LBMS uses particular laser beacons, each of which emits two symmetrical laser beams traveling in opposite directions around the beacon and is placed at two reference positions. Receiving beams from these beacons, a photo sensor incorporated in the sensor unit of a WMR yields four time intervals that are necessary to calculate the position of the WMR. From four time intervals, the position of a standing WMR can be measured with high accuracy by LBMS. However, using this method, the position of a moving WMR can't be measured as accurately, because the above-mentioned four time intervals change slightly owing to motion of the WMR, and DME arises consequently. The proposed method compensates the position of a moving WMR measured by LBMS, making use of the DME calculated from four different time intervals. In this paper, we report on this compensation method which has been found to yield satisfactory results in terms of accuracy and repeatability after various simulations and experiments.

Compact, holographic correction of aberrated telescopes

We demonstrate a compact reflector telescope design that incorporates the holographic correction of a large, low-quality primary spherical mirror by using a laser beacon located at the center of curvature. The simple design makes use of conventional optics and is easily scalable to much larger apertures. Experimental results indicate diffraction-limited performance from a heavily aberrated 0.5-m-diameter spherical mirror.

Highly Accurate Dynamic Position Measurement for Wheeled Mobile Robot Using Laser Beacon Measuring System.

The present research concerns a compensation method for dynamic measurement error using the laser beacon measuring system (LBMS), which is necessary for highly accurate positioning control and trajectory tracking control of a wheeled mobile robot (WMR). The LBMS uses particular laser beacons, each of which emits two symmetrical laser beams traveling in opposite directions around the beacon. The beacons are placed at two reference positions. Receiving beams from these beacons, a photosensor carried on a sensor unit of WMR yields four time intervals which are necessary to calculate the position of WMR. The position of a standing WMR can be measured with high accuracy by LBMS. However, the position of a moving WMR can not be measured as accurately, because the above-mentioned four time intervals change slightly owing to motion of WMR and consequently, dynamic measurement error arises. The proposed method is the one which compensates the position of a moving WMR measured by LBMS, making use of the dynamic measurement error calculated from four different time intervals. In this paper, we report on this compensation method which has been found to yield satisfactory results concerning its accuracy and repeatability after various simulations and experiments.

Calculation of the energy of a synthetic laser beacon

A statistical model of a synthetic laser beacon, formed by resonant scattering from the mesospheric sodium layer, is proposed and the average energy of the radiation emitted by this beacon is calculated. The beacon radiation can be used as the reference signal in compensation for the atmospheric turbulence and other sources of distortion in adaptive systems. A calculation formula is derived for the energy of a signal received from a synthetic beacon. Account is taken of the broadening of the resonant scattering line of sodium atoms because of their thermal motion, of the pulsed nature and partial coherence of the laser radiation, of the finite time taken by these atoms to cross the laser beam, and of the influence of the atmospheric turbulence.

Focus anisoplanatism: a limit to the determination of tip-tilt with laser guide stars

One limitation to using laser beacons to measure tilt is focus anisoplanatism. By use of a spectral representation for the atmospheric turbulence, the wave-front error is expressible as an integral over the distribution of turbulence with height. Results are calculated for turbulence profiles typical of astronomical sites. The decrease in Strehl ratio when a single laser guide star is used to measure tip-tilt is shown to be especially significant at visible wavelengths, precisely where tip-tilt is most difficult to obtain from natural stars.

Focus anisoplanatism effects on tip–tilt compensation for adaptive optics with use of a sodium laser beacon as a tracking reference

A Zernike decomposition of the pupil phase variance that is due to focus anisoplanatism for a sodium laser guide star is calculated, including the angular error variance associated with overall tilt. This quantity, or tilt focus anisoplanatism σFA,tt2, is necessary in evaluating the usefulness of new techniques developed to measure the wave-front tilt from a sodium laser guide star. The analytical expression of σFA,tt2 in the case of refractive-index fluctuation with a Kolmogorov power spectral density is given together with numerical results for two different Cn′2(z) profiles. Numerical calculations show that tilt focus anisoplanatism may be a limitation when a sodium laser guide star is used to sense the overall wave-front tilt at visible wavelengths, especially for 8-m-class telescopes.