A one meter diameter prototype lightweight composite mirror is being developed. The mandrel on which the composite mirror will be laid up is an ultra low expansion quartz glass, TSG, whose thermal expansion coefficient, 10-7/°C or less, is similar to that for the composite material. A precise layup procedure for the composite material is required to obtain this coefficient of expansion. The mandrel surface will be superpolished to 6-8 Å rms or better, resulting in ten times less scattered light in the visible region than displayed by a typical astronomical mirror. It has been shown experimentally that mandrel microroughnesses of this order can be successfully transferred to a replicated composite mirror faceplate. The composite faceplate is one to three millimeters thick, very tough, and unlike thin glass adaptive optic faceplates does not fracture easily. Actuators designed for atmospheric correction of the thinner adaptive optic faceplates have a response time of 1/2 msec, which is fast enough to correct for all atmospheric distortion. The thicker faceplates are used for active mirrors and are mounted differently. Controlled by stepper motors they can be used for computer controlled remote focusing over long distances or fine tuning of mirror tip-tilt as well as for gravitational sag of other mirrors in the optical train. All mirror actuators operate in the 30-70 v range.
New, very large telescopes with apertures of 30, 50, and 100 meters are being proposed by the astronomical community. Superpolished or ultrapolished mirrors with low scattered light levels and the use of adaptive optics for near-diffraction-limited performance would make such large telescopes a turning point in astronomy. The secondary mirror for the Euro50 will be a four meter adaptive optic made of a low expansion graphite-filled cyanate ester resin composite produced using a replica transfer technique. We have made three 1/3rd meter diameter prototype composite adaptive optic mirrors of this cyanate ester composite material. Because of the embedded graphite fibers, the composite material has a measured expansion coefficient in the 10-8 range, as has Zerodur or ULE glass. It is very much lighter, more rugged and more economical than Zerodur or ULE, and can be fabricated in weeks, not months. The Zerodur mandrels upon which these replica transfer mirrors are made are superpolished using centrifugal elutriation, so the replica surface has an rms roughness of 0.6 to 0.8 nm. It thus scatters about an order of magnitude less light than typical conventionally polished astronomical mirrors. In adaptive optic mirrors with sub-mm thick faceplates the number of plies used is insufficient to produce an isotropic surface. For mirrors 2 mm thick, with more plies, the surfaces are isotropic, and the slight astigmatism sometimes resulting from the mesh in the ply can be corrected by actuators to make them attractive mirrors. They must be supported to maintain a good optical figure over a meter diameter mirror. The support requirement may be met by using a new type of mechanical/piezoelectric actuator adjustable to a fraction of a wavelength. The mechanical actuators have a coarse adjust of over an mm and a fine adjust of less than a wavelength of light. They can be used in series with a novel type of piezoelectric actuator for final static adjustment. The low voltage, up to 2.5 kHz frequency piezoelectric actuators have a displacement of approximately one μm per volt, 82 times greater than conventional piezoelectric actuators, and a throw of ±30 μm or more. Compliant faceplates can be adaptive as well as active. Calculations indicate that for actuator spacings of about 4 cm the effective mirror stiffness equals that of a solid Zerodur mirror with a conventional 6:1 diameter to thickness ratio. The effect of gravitational sag for composite mirrors is calculated to be negligible. They are thus a good choice for the secondary mirror for the Euro50 as well as for the primary or secondary mirrors for other giant telescopes.
New very large telescopes with apertures as large as 100 meters are being proposed. They will be made up of mirror segments only a meter or two in diameter and phased together. The diffraction-limited resolution of a mirror is directly proportional to its diameter, and the light-gathering-power is proportional to the square of the diameter. Near-diffraction-limited performance using adaptive optics would make such large mirrors very exciting. We have built two small prototype composite adaptive optic mirrors of graphite fiber impregnated cyanate ester resin driven by actuators spaced 4 cm apart and with a faceplate influence function of 5 cm. The second mirror assembly also makes possible a 2 cm actuator spacing. The overall figure is not yet as good as desired, but we believe that much of this problem can be corrected by mechanical adjustment of the actuator rest positions and use of low expansion mandrels. This mirror concept, when realized in primary mirror segments a meter or more in diameter, should make correction possible for atmospheric turbulence under almost any observatory seeing conditions. The composite optical faceplate in the most recent prototype had a roughness of 0.6 to 0.8 nm. Two centrifugal elutriation super-polishers, each over 1.2 meters in diameter, are in place to produce superpolished mandrels on which to form superpolished faceplates over a meter in diameter. Scattered light from such a mirror surface will be reduced by as much as a factor of ten, as compared to conventional fresh feed polishing. The name "transfer mirrors" rather than the widely recognized but poorer quality "replica mirrors" is given to such faceplates. They have an expansion coefficient comparable to ULE quartz or Zerodur, and are lightweight with 10-20, an aerial density of 17 kg/m2 for the mirror with a 4 cm actuator spacing or 34 kg/m2 for the mirror with 2 cm actuator spacing. In both cases the effect of gravitational sag is minimized. A 60 volt potential results in actuator displacement of 5 mm as measured inter
The space elevator, a cable with one end attached to Earth and the other 100,000 km up in space that can be ascended by mechanical climbers, is a revolutionary system for carrying satellites into space. A 20.000 kg-capacity space elevator appears feasible with developing technologies at a cost of $40B. The basic design and implementation of the elevator have been worked out and many of the components are in use in other programs. The ComPower laser beaming design (free- electron laser and adaptive optics) has the performance required for the critical delivery of power to the climbers. With system designs in hand the next step is to perform a feasibility test of the overall system. One possible test utilizes a high-altitude balloon, carbon nanotube composite tether, protytpe climber, and laser power beaming system. A free-electron laser beam is directed to the climber using a lightweight composite mirror system with adjustable beam diameter. The climbers will ascend the tether to a 600 m altitude demonstrating the performance of the climber, power delivery and laser beaming system along with interactions of all the components. Combining this test with several others being conducted should demonstrate the viability of the space elevator.
Beaming of laser energy through the atmosphere is a means of supplying electrical power to orbiting satellites. By using a new ground based free electron laser developed by Lawrence Berkeley National Laboratory many times the amount of electrical power can be generated by the satellite from the same area of array now used for solar power. There is currently a shortage of power in space, and the demand is rising exponentially. Satellites which appear from earth to remain fixed at one point in the sky are said to be in geosynchronous orbit (GEO). They are used to relay long distance telephone communications, television, e-mail, credit card checks from the local gas pump and a myriad of other applications. In addition to the shortage in power there is also a shortage of available bandwidth. In response to that problem a higher frequency band, the Ka band, is being opened for satellites and fifty companies in the United States are planning to launch satellites which use it. Unfortunately rainfade is a serious problem at this frequency and ten times the usual power is sometimes needed to overcome the effects of rain. Laser power beaming is an answer to this problem. Key elements are a powerful 200 kW free electron laser and a fully compensated 11-meter diameter adaptive optics projection telescope. Remarkable progress has been made on both these ambitious objectives in the last two years.
Over the past fifteen years, the mid-infrared advanced chemical laser (MIRACL) and SeaLite beam director (SLBD) have been completed, moved to the High Energy Laser System Test Facility (HELSTF) at White Sands Missile Range, New Mexico, and integrated into the largest and most powerful high energy laser system in the U.S. The high energy laser system was modified, incorporating a shared aperture (pointing and tracking) capability to improve pointing performance. High power propagation, tracking, and beam control experiments over near vertical beam paths have been conducted. The system modifications, instrumentation, and test results are described.
A ground based laser beam transmitted to space can be used as an electric utility for satellites. It can significantly increase the electric power available to operate a satellite or to transport it from low earth orbit (LEO) to mid earth or geosynchronous orbits. The increase in electrical power compared to that obtainable from the sun is as much as 1000% for the same size solar panels. An increase in satellite electric power is needed to meet the increasing demands for power caused by the advent of 'direct to home TV,' for increased telecommunications, or for other demands made by the burgeoning 'space highway.' Monetary savings as compared to putting up multiple satellites in the same 'slot' can be over half a billion dollars. To obtain propulsion, the laser power can be beamed through the atmosphere to an 'orbit transfer vehicle' (OTV) satellite which travels back and forth between LEO and higher earth orbits. The OTV will transport the satellite into orbit as does a rocket but does not require the heavy fuel load needed if rocket propulsion is used. Monetary savings of 300% or more in launch costs are predicted. Key elements in the proposed concept are a 100 to 200 kW free- electron laser operating at 0.84 m in the photographic infrared region of the spectrum and a novel adaptive optic telescope.
First steps are now being taken to install a prototype laser power-beaming facility in the mountains near China Lake, California. A matching grant has been received from the State of California to establish the infrastructure necessary to fund the construction of a prototype laser power-beaming facility by private capital. The U.S. Navy and several major national laboratories have joined with the city of Ridgecrest, California and its citizens to contribute matching in-kind funding and to participate in the preparations needed for construction of such a facility. An environmental study is required before site approval can be obtained from the Navy at the highest levels. That process has begun and the requirements needed to create this new type of space utility industry in California are being addressed.
In order to effectively beam laser power into space to power satellites or to remove space debris in mid or high earth orbit very large mirrors (perhaps 12 m in diameter or more) and an adaptive optic systems to penetrate the atmosphere are required. Mirrors with adaptive optic segment sizes less than the equivalent. Fried coefficient for atmospheric turbulence (typically 3 - 5 cm at zenith in the visible region of the spectrum) are optimum for atmospheric penetration. These new mirrors may have hundreds of thousands of segments. The behavior of such mirrors under high powered laser irradiation is not clear, although for a large mirror both average and peak irradiation levels will be very low. Attention must be paid to penetration of laser energy into the gaps between segments and to the cumulative effect of edge diffraction. These problems do not appear to be severe and this new class of optics appears to offer us new possibilities for use in space. It may change the way in which we look at telescopes for space applications.
The site for the proposed National Advanced Optic Mission Initiative (NAOMI) facility will be in the mountains near China Lake, California. This location has 260 clear days per year (more than any other feasible site in the U.S.). In 1993 there were 5 completely overcast days all year. The area near the proposed site is unpopulated. The solar insolation in this general area is the greatest of any area in the United States. The NAOMI system will be installed at an altitude of 5600 feet. Astronomical seeing there is excellent. Even at a less favored site than that planned for NAOMI the average Fried seeing coefficient ro is 12 cm in the visible region and 20 cm values of ro (comparable to the best observatories) are commonly observed. The area is centrally located in and entirely surrounded by one of the largest restricted airspace/military operating airspace complexes in the United States, 12% of the entire airspace in California. Electrical power is available from either the nearly Coso Geothermal plant, second largest in the United States, or from the even closer cogeneration plant at Trona, California. Cooling water can be obtained from the nearby area or from the lake itself. Although a dry playa, the lake has a high brackish groundwater level. Most of the commercial satellites over the U.S. could be reached by a laser/telescope system located on government land at the Naval Air Weapons Station (NAWS) military reservation at China Lake. This telescope/laser system will be a prototype for five other systems planned for around the world. The complex will provide laser power beaming to all satellites and put the United States into the position of world leader in satellite technology and power beaming to space.
This paper addresses the potential augmentation of a quasi-stationary Unmanned Aerial Vehicle with a highly agile beam steering optical system. In addition to the primary application of relaying laser power from a ground station to low earth orbit satellites, applications include (1) precision tracking and ranging at distances of a few hundred kilometers, (2) covert communications to distances of 80 km utilizing only a modulable corner cube at the receiving end and (3) pollution detection and control and (4) continuous meteorological analysis of high altitude wind, CO2 content, liquid water content, ice particle effective radius, effective drop size, optical depth and density, turbulence structure and emissivity profile.
The National Advanced Optics Mission Initiative (NAOMI) consists of two proposed programs, the SpacE Laser ENErgy (SELENE) which includes the site, and the Advanced Telescope Technology Integrated Large Array (ATTILA). The infrastructure of the SELENE facility requires a systems engineering approach. There are several large scale projects for the water, power, access, and communications all of which are interactive elements. These projects need to be designed and constructed concurrently while taking environmental concerns into account before the SELENE facility becomes operational.
Satellites are of vital importance to the Department of Defense and the Navy as well as to the civilian economy. For example, about 90% of the communications to the fleet are by satellite. Economical means for putting satellites into orbit and maintaining and extending their lifetimes in orbit are just as important for the military as for civilian industries. There is also a significant economic impact to the ability to repair rather than replace satellites that are malfunctioning or have been inserted into the wrong orbits. Laser power beaming can not only accomplish these tasks but also promises to move satellites in orbit quickly and inexpensively, provide boost power for degraded satellites or those which suffer intentional jamming from adversaries, remove space junk even in geosynchronous orbit and provide very high resolution pictures of objects in space by eliminating atmospheric disturbances.
The Birchum Mesa SELENE (Space Laser Energy) facility will be dual use facility as it provides for progressive development of high power Free Electron Lasers (FEL) and commercial laser beam power transfer to space-borne vehicles. The facility will be comprised of SELENE mainsite containing two laser system bays and supporting facilities with transport tunnels coupling to the Beam Transfer Optical System (BTOS) which is the active optical array space beam director with its supporting facility. The first generation commercial grade laser will operate at 100 kW of quasi-CW laser power with a planned growth to 10 MW of output power. The BTOS beam director will direct a focus compensated laser power beam to provide power service to space vehicles within a +/- 50 degree (half angle from zenith) tracking cone service field. An underground hardened site is proposed for this facility to mitigate any potentially hazardous effects from operation of a very high energy CW electron beam laser, to protect the facility from inadvertent weapons splashdown during range Test and Evaluation operations, and to create minimum environmental impact upon historical and ecological elements of the range.
Laser power beaming of energy through the atmosphere to a satellite can extend its lifetime by maintaining the satellite batteries in operating condition. An alternate propulsion system utilizing power beaming will also significantly reduce the initial insertion cost of these satellites, which now are as high as $DLR72,000/lb for geosynchronous orbit. Elements of the power beaming system are a high-power laser, a large diameter telescope to reduce diffractive losses, an adaptive optic beam conditioning system and possibly a balloon or aerostat carrying a large mirror to redirect the laser beam to low earth orbit satellites after it has traversed most of the earth's atmosphere vertically. China Lake, California has excellent seeing, averages 260 cloud-free days/year, has the second largest geothermal plant in the United States nearby for power, groundwater from the lake for cooling water, and is at the center of one of the largest restricted airspaces in the United States. It is an ideal site for such a laser power beaming system. Technological challenges in building such a system and installing it at China Lake will be discussed.
Laser power beaming of energy through the atmosphere to a satellite can extend its lifetime by maintaining the satellite batteries in operating condition. An alternate propulsion system utilizing power beaming will also significantly reduce the initial insertion cost of these satellites, which now are as high as $72,000/lb for geosynchronous orbit. Elements of the power beaming system are a high-power laser, a large diameter telescope to reduce diffractive losses, an adaptive optic beam conditioning system and possibly a balloon or aerostat carrying a large mirror to redirect the laser beam to low earth orbit satellites after it has traversed most of the earth's atmosphere vertically. China Lake, California has excellent seeing, averages 260 cloud-free days/year, has the second largest geothermal plant in the United States nearby for power, groundwater from the lake for cooling water, and is at the center of one of the largest restricted airspaces in the United States. It is an ideal site for such a laser power beaming system. Technological challenges in building such a system and installing it at China Lake are discussed.
There are obvious differences but some surprising similarities between high-power laser mirrors designed for use in the visible and infrared wavelengths and synchrotron mirrors designed for use at x-ray wavelengths. The use of synchrotron mirrors at grazing incidence results in a relaxation of figure, surface microroughness, and thermal heating tolerances relative to the wavelength of nearly two orders of magnitude. As a result, the tolerances become roughly comparable to those desired for high-power laser mirrors for the visible region of the spectrum. Experience gained over the years by laser mirror designers in substrate design, the influence of metallic coatings on thermal and optical performance and limiting values for surface microroughness, may be helpful to designers of synchrotron systems. Exploitation of the promise of the new synchrotron systems still represents a challenging problem for researchers. The specifications and operational approaches to meeting them for the new Advanced Photon Source at Argonne National Laboratory are given.
Testing the optical figure and focal length of laser mirrors with radii of curvature in the 10 to 100 m range is difficult. If the mirror is concave, a source can be placed at the center of curvature. Air turbulence over these long path lengths makes interferometry difficult, however, and greatly reduces measurement accuracy. Convex mirrors are even more difficult to measure. A solution is to produce a slightly converging or diverging beam from a virtual source. The actual optical path in which turbulence may develop can then be made very short. A three-element test system consisting of a parabola, a transmission sphere, and a folding flat is described. It is capable of measuring both optical flats and convex or concave mirrors up to 40 cm in diameter with radii of curvature from 10 m to infinity. System accuracy is 1/20th wave rms in optical figure and 0.2% in radius of curvature. A discussion is given of the systematic errors introduced when the parabola is used in other than parallel light.
Linearly polarized light remains linearly polarized after reflection from a transparent material at oblique incidence. The reflected polarization angle is determined from the extinction position of the analyzer. If the incident polarization angle is 45 deg, the reflected polarization angle gives the ratio of the reflected p-wave to s-wave. This value can be used to determine the index of refraction from Fresnel equations. With our instrument, the uncertainty in the deduced refractive index is +/- 0.0004. This method is fast, convenient, and versatile enough to provide accurate results on small laboratory samples. In addition to measuring the refractive index, the method is sufficiently accurate to characterize the homogeneity of transparent materials.
The reflectance and the ellipsometric parameters for two black samples were measured at
5-j.tm wavelength and at multiple incident angles using an ellipsometer. Different models
were used to reduce the ellipsometric data and to calculate the reference specular
reflectance. With the correct model, the measured reflectance and the near-angle
scattering with respect to the reference specular reflectance can agree with Beckmann's
scattering theory. The roughness reduced from reflectometry is independent of
ellipsometric models and is used to select the correct set of solutions. A three-phase model
in which the complex dielectric constant is computed from Bruggeman's effective medium
theory can provide consistent solutions between roughness from reflectometry and
effective thickness from ellipsometry. A combination of ellipsometry, reflectometry, and
scatterometry can predict accurately the complex index of refraction, roughness, and other
optical properties of black samples.
A novel sapphire (99.99% Al2O single crystal) dome growth
technique is described whereby a dome of a nominal radius of 38
to 40 mm is produced. A modified Edge-Defined Film-Fed Growth
(EFGR ) technique is used to directly grow a dome blank with a
wall thickness of 2.5 to 3 mm which requires a minimum of
mechanical finishing and polishing. Total integrated scatter
(TIS) results for the polished dome are reported for .6328 um and
3.39 um wavelengths. An evaluation of striae and bulk
inhomogeneities is also given.