There is a certain amount of disconnect between the perception and reality of Free Space Optics (FSO), both in the marketplace and in the technical community. In the marketplace, the requirement for FSO technology has not grown to even a fraction of the levels predicted a few years ago. In the technical community, proposed solutions for the limitations of FSO continue to miss the mark. The main commercial limitation for FSO is that light does not propagate very far in dense fog, which occurs a non-negligible amount of the time. There is no known solution for this problem (other than using microwave or other modality backup systems), and therefore FSO equipment has to be priced very competitively to sell in a marketplace dominated by copper wire, fiber optic cabling and increasingly lower cost and higher bandwidth wireless microwave equipment. Expensive technologies such as adaptive optics, which could potentially increase equipment range in clear weather, do not justify the added cost when expected bad weather conditions are taken into account. In this paper we present a simple equation to fit average data for probability of exceeding different atmospheric attenuation values. This average attenuation equation is then used to compare the expected availability performance as a function of link distance for representative FSO systems of different cost.
Free-Space Optics (FSO) is a proven, reliable technology for last mile telecommunications applications, used worldwide for both enterprise network building-to-building connections and for wireless access to more traditional land line communications networks. In most mid-latitude coastal cities, link availability at distances above a few hundred meters is primarily affected by fog and low clouds. At longer distances, heavy rain and snow can also affect the link. The most mature technology used in FSO equipment relies on low cost semiconductor lasers or LED’s operating in the near infrared at wavelengths of 785 nm or 850 nm. In the past few years, systems operating at 1550 nm have also been developed. At first the vendors of these systems claimed that the 1550 nm wavelength had better propagation characteristics in severe weather than the 785 nm wavelength. With further analysis and research, those claims were withdrawn. Now there are claims that even longer wavelengths near 10 microns will solve the FSO link availability issues associated with severe weather. Hype about such magic wavelengths for FSO is both a disservice to the investors who will lose the money they are investing based on exaggerated claims, and to the rest of the FSO industry which should be creating realistic expectations for the capability of its equipment. In the weather conditions which normally cause the highest attenuation for FSO systems, namely coastal fog and low clouds, 10 microns offers no propagation advantage over shorter wavelengths.
Free Space Optics (FSO) has become a viable, high-bandwidth wireless alternative to fiber optic cabling. The primary advantages of FSO over fiber are its rapid deployment time and significant cost savings. The disadvantage of FSO over fiber is that laser power attenuation through the atmosphere is variable and difficult to predict, since it is weather airports, the link availability as a function of distance can be predicted for any FSO system. These availability curves provide a good indication of the reasonable link distances for FSO systems in a particular geographical area. FSO link distances can vary greatly from desert areas like Las Vegas to heavy-fog cities like St. Johns NF. Another factor in determining FSO distance limitations is the link availability expectation of the application. For enterprise applications, link availability requirements are generally greater than 99%. This allows for longer FSO link ranges, based on the availability curves. The enterprise market is where the majority of FSO systems have been deployed. The carriers and ISPs are another potential large user of FSO systems, especially for last-mile metro access applications. If FSO systems are to be used in telecommunication applications, they will need to meet much higher availability requirements. Carrier-class availability is generally considered to be 99.999% (5 nines). An analysis of link budgets and visibility-limiting weather conditions indicates that to meet carrier-class availability, FSO links should normally be less than 140m (there are cities like Phoenix and Las Vegas where this 99.999% distance limitation increases significantly). This calculation is based on a 53 dB link budget. This concept is extended to the best possible FSO system, which would have a 10 W transmitter and a photocounting detector with a sensitivity of 1 nW. This FSO system would have a 100 dB link margin, which would only increase the 99.999% link distance to 286 m. A more practical solution to extending the high availability range would be to back up the FSO link with a lower data rate radio frequency (RF) link. This hybrid FSO/RF system would extend the 99.999% link range to longer distances and open up a much larger metro/access market to the carriers. It is important to realize that as the link range increases, there will be a slight decrease in overall bandwidth. To show the geographical dependence of FSO performance, the first map of FSO availabilities contoured over North America is presented. This map is the first step to developing an attenuation map for predicting FSO performance, which could be used in similar fashion to the International Telecommunication Union (ITU)/Crane maps for predicting microwave performance.
A low Earth orbit (LEO) satellite laser communication (lasercom) terminal, built under funding by the Ballistic Missile Defense Organization (BMDO), was flown as part of the Space Technology Research Vehicle 2 (STRV-2) experiment module. The STRV-2 experiment module was housed on the Air Force Tri-Service Experiments 5 (TSX-5) spacecraft, which was launched on June 7, 2000. The lasercom flight hardware weighs 31.5 pounds, with a volume less than a cubic foot, and is capable of communication at data rates up to 1 Gbps. The LEO satellite-to-ground lasercom experiment is designed for slant ranges up to 2000 km and elevation angles above 15 degrees over the horizon. This experiment's goal was to demonstrate a satellite lasercom link that would validate the capability and readiness of lasercom for inter-satellite crosslinks, and data downlinks from LEO Earth sensing satellites. The mechanical and electrical systems of the satellite terminal survived launch and were functioning correctly. Unfortunately, no lasercom link from space was achieved because the satellite terminal had troubles acquiring and tracking the ground terminal beacons. The acquisition sequence was designed based on a spacecraft interface specification. The primary reason for the lack of acquisition was that the ephemeris and spacecraft attitude control was out of specification. The acquisition sequence required that the ground terminal beacons illuminate the satellite. The divergence of the ground terminal beacons was designed to the ephemeris specification, which stated that ephemeris be within +/- 100 m cross-track and +/- 500 m in-track error would be provided prior to a satellite pass.
12 There is currently a misconception among designers and users of free space laser communication (lasercom) equipment that 1550 nm light suffers from less atmospheric attenuation than 785 or 850 nm light in all weather conditions. This misconception is based upon a published equation for atmospheric attenuation as a function of wavelength, which is used frequently in the free-space lasercom literature. In hazy weather (visibility > 2 km), the prediction of less atmospheric attenuation at 1550 nm is most likely true. However, in foggy weather (visibility < 500 m), it appears that the attenuation of laser light is independent of wavelength, ie. 785 nm, 850 nm, and 1550 nm are all attenuated equally by fog. This same wavelength independence is also observed in snow and rain. This observation is based on an extensive literature search, and from full Mie scattering calculations. A modification to the published equation describing the atmospheric attenuation of laser power, which more accurately describes the effects of fog, is offered. This observation of wavelength-independent attenuation in fog is important, because fog, heavy snow, and extreme rain are the only types of weather that are likely to disrupt short (< 500 m) lasercom links. Short lasercom links will be necessary to meet the high availability requirements of the telecommunications industry.
KEYWORDS: Space operations, Satellites, Mid-IR, Sensors, Control systems, Defense and security, Imaging systems, Data communications, Composites, Environmental sensing
The STRV-2 program is the second in a series of three collaborative flight test programs between the U.S. Ballistic Missile Defense Organization (BMDO) and the United Kingdom (UK) Minstry of Defence (MoD). The STRV-2 Experiment Module contains five major experiments to provide proof-of-concept data on system design, data on the mid-earth orbit (MEO) space environment, and data on durability of materials and components operating in the MEO environment. The UK Defence Evaluation and Research Agency (DERA) has provided a mid- wavelength infrared (MWIF) imager to evaluate passive detection of aircraft from space. BMDO, in conjunction with the US Air Force Research Laboratory (AFRL) and the National Aeronautics and Space Administration (NASA), have provided experiments to evaluate use of adaptive structures for vibration suppression, to investigate the use of high bandwidth laser communications to transmit data from space to ground or airborne receivers, to study the durability of materials and components in the MEO space environment, and to measure radiation and micrometeoroid/debris fluence. These experiments are mounted on all- composite structure. This structure provides a significant reduction in weight and cost over comparable aluminum designs while maintaining the high stiffness required by optical payloads. In 1994, STRV-2 was manifested for launch by the DOD Space Test Program. STRV-2, the primary payload on the Tri-Service eXperiment (TSX)-5 spacecraft, was successfully launched on 7 June 2000 on a Pegasus XL from Vandenbery AFB, CA. The STRV-2 program, like the companion STRV-1 program, validates the viability of multi-national, multi-agency collaborations to provide cost effective acquisition of space test data. The experimental data to be obtained will reduce future satellite risk and provide guidelines for further system development.
Isaac Kim, Harel Hakakha, Brian Riley, Nicholas Wong, Mary Mitchell, Russell Howe, Carter Moursund, Wesley Brown, Prasanna Adhikari, Arunabh Lath, Eric Korevaar
A low earth orbit (LEO) satellite laser communication (lasercom) terminal built under funding by the Ballistic Missile Defense Organization will be flown as part of the Space Technology Research Vehicle 2 (STRV-2) experiment module. The STRV-2 experiment module will be housed on the Air Force Tri-Service Experiments 5 spacecraft, which is scheduled to launch on April 15, 2000. The lasercom flight hardware weighs 31.5 pounds, with a volume less than a cubic foot, and is capable of communication at data rates up to 1 Gbps. The LEO satellite-to-ground lasercom experiment is designed for slant ranges up to 2000 km and elevation angles above 15 degrees over the horizon. This experiment will demonstrate a successful satellite lasercom link that will validate the capability and readiness of lasercom for inter- satellite crosslinks, and data downlinks from LEO earth sensing satellites. Two transportable ground terminals have been built and are presently being characterized. This paper describes the results from open loop pointing tests of an STRV-2 ground terminal to currently orbiting LEO satellites illuminated by the sun, such as the Hubble Space Telescope.
An experiment comparing atmospheric scintillation for 785 nm and 1550 nm laser beam transmission is presented. Fluctuations in received optical power were recorded for both wavelengths at terrestrial ranges of 1.2 km and 2.2 km. The number of transmit apertures was also varied. The results indicated that scintillation fades are more of a problem at 1550 nm compared to 785 nm. This will require more scintillation fade margin built into the design of free-space laser communication systems operating at 1550 nm. As well, any advantage in decreased atmospheric attenuation margin at 1550 nm could be lost because of the need for greater fade margin. The overall reduction in scintillation with an increased number of transmit apertures was verified. A possible physical explanation will explain why more scintillation was observed at 1550 nm as compared to 785 nm.
Isaac Kim, Eric Korevaar, Harel Hakakha, Ron Stieger, Brian Riley, Mary Mitchell, Nicholas Wong, Arunabh Lath, Carter Moursund, Micah Barclay, John Schuster
The results from a 13.8 km horizontal laser communication (lasercom) link between the Space Technology Research Vehicle 2 (STRV-2) ground terminals are presented. These terrestrial tests are a precursor to the STRV-2 satellite-to-ground lasercom experiment scheduled to commence shortly after launch in June of 1999. Testing ground terminal performance through long terrestrial horizontal ranges (greater than 10 km) mimics the atmospheric effects of an uplink slant path to the low earth orbit (LEO) satellite. Bit error rates averaging 10-6 to 10-7 were achieved with only one of the three transmit telescopes operating. The terrestrial path also passed very close to a ridge, which most likely added additional ground scintillation. We expect much improved link performance once all three transmit telescopes are operational. These long-range horizontal tests will assist in the development of a long-range commercial terrestrial lasercom product, which will be baselined on the ground terminal design.
We describe a terrestrial free-space optical data link operating at 2.5 Gb/s using currently available 1.5 micrometers telecommunications electro-optic transmission components. The 2.4 km free-space optical data-link is characterized by bit-error-rate system performance. The optical link utilizes a 1.5 micrometers DFB laser device which is directly modulated and operating within the erbium amplification band.
Isaac Kim, Ron Stieger, Joseph Koontz, Carter Moursund, Micah Barclay, Prasanna Adhikari, John Schuster, Eric Korevaar, Richard Ruigrok, Casimer DeCusatis
KEYWORDS: Visibility, Signal attenuation, Laser communications, Scintillation, Visibility through fog, Telecommunications, Transmittance, Laser systems engineering, Local area networks, Data communications
A laser communications terminal built under funding from the Ballistic Missile Defense Organization (BMDO) will be flown as part of the Space Technology Research Vehicle 2 (STRV-2) experiment to be launched as part of Air Force mission TSX-5 in late 1998. The flight hardware, which weighs 31.5 pounds and is capable of communications data rates above 1 Gb/s, was delivered to JPL for integration in July 1997. The planned satellite-to-ground lasercom experiment is designed for slant ranges up to 2000 km and elevation angles above the horizon down to 15 degrees. The ground terminal, described in this paper, is being built now and will be used to measure various link parameters such as burst error rates due to scintillation in addition to demonstrating high speed communications.
The STRV-2 lasercom terminal (LCT) was designed and developed by AstroTerra Corporation of San Diego, California, under funding from the Ballistic Missile Defense Organization (BMDO). Scheduled for launch in late 1998 it will be used to demonstrate, for the first time, high data rate bi-directional satellite-to-ground optical communications. Concurrently with the development of the STRV-2 lasercom NASA/JPL was assembling the lasercom test and evaluation station (LTES), a high quality test platform for pre-flight characterization of optical communications terminals. The respective development schedules allowed evaluation of the STRV-2 LCT using LTES, for a month, prior to integration of the LCT with the spacecraft palette. Final co-alignment of the transmitter lasers to within plus or minus 20 (mu) rads with respect to the receive axis was achieved. This in turn allowed the specified 76 (mu) rad transmit beam divergence to be realized. However, subjecting the LCT to expected on-orbit temperatures revealed that the co-alignment deteriorated causing beam spreading, a finding which prompted the recommendation to operate the lasers warmed up during ground encounters. The 'bent-pipe' operation bit-error rates (BER) at 155, 194 and 325 Mbps were less than or equal to 1E - 10 over an approximately 20 dB range of irradiance measured at the receive telescope aperture. At 500 Mbps BER's of 1E-6 were achieved over an approximately 6 dB irradiance range, suggesting greater vulnerability to atmosphere induced fades. A pointing offset between the acquisition receivers and transmitter lasers of 1 mrad was measured. The impact of this offset will be to limit acquisition camera framing rates to 87 and 251 Hz, thus limiting the tracking loop bandwidth. Tracking performance test of the lasercom terminal, though planned could not be carried out because the software was not ready at the time of testing with LTES. The test results obtained for STRV-2 lasercom terminal will be used for designing the ground receiver.
Isaac Kim, Joseph Koontz, Harel Hakakha, Prasanna Adhikari, Ron Stieger, Carter Moursund, Micah Barclay, Alyssa Stanford, Richard Ruigrok, John Schuster, Eric Korevaar
AstroTerra's TerraLinkTM 8-155 laser communications equipment is designed for a clear weather range of 8 km and a data rate of 230 Mb/s, and TerraLink 4-155 is designed for a 2 km range. The TerraLink equipment achieves a reduction in scintillation-induced intensity fluctuations by using large receive apertures and multiple transmit apertures. We present measurements of received intensity fluctuations at different ranges through 4 inch and 8 inch receive apertures. We also present link margin data, with its implications for use of lasercom equipment in various weather conditions. Scintillation measurements were made while a communications link was operating by placing a second receive telescope with a PIN photodiode next to one of the lasercom transceivers. By plotting the probability of intensity vs. intensity, the necessary link margin to achieve a desired burst error rate can be calculated. At the longest ranges, the TerraLink equipment requires a scintillation fade margin of about 10 dB to achieve a 10-9 bit error rate. The equipment is designed with an additional margin of 4 - 5 dB for atmospheric attenuation.
Uplink scintillation is a serious issue for laser communication between a satellite and the ground. Fluctuations on the uplinked beacon and communications laser can be minimized by transmitting multiple independent lasers from separate apertures which then sum incoherently at the satellite. The objective of the experiment described here was to determine the number and spacing required for separate transmitters to reduce fluctuations in the received power due to atmospheric scintillation to acceptable levels. Received power vs. time data were collected for horizontal laser links established between a laser transmitting platform and a receive telescope assembly separated by distances of 1.2 and 10.4 km to mimic the expected atmospheric effects of an uplink slant path to a satellite. Reduction in signal fluctuations was observed as the number of laser transmitters was increased from 1 to 16. A ground terminal design with 16 lasers on an 18 inch diameter circle with a 7 dB fade margin should be adequate to compensate form sot scintillation fades, while the remaining deep fades may be corrected by using forward error correction techniques.
Laser communications hardware being built under funding from the Ballistic Missile Defense Organization (BMDO) will be flown on the Space Technology Research Vehicle 2 to be launched as part of Air Force mission TSX-5 in 1998. The flight hardware, which weighs 31.5 pounds and is designed for satellite-to-ground laser communications at data rates up to 1.24 Gb/s and ranges up to 2000 km, will be delivered to JPL for integration in March 1997. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low earth orbit satellite downlinks to the ground. This paper describes the operational scenario for satellite-to-ground lasercom experiments. Experiments at other facilities, including those in Europe and Japan, are encouraged.
Laser communication hardware being built under funding from the Ballistic Missile Defense Organization will be flown on the Space Technology Research Vehicle 2 to be launched as part of Air Force mission TSX-5 in 1998. The flight hardware, which weighs 31.5 pounds and is designed for satellite-to-ground laser communications at data rates up to 1.24 Gb/s and ranges up to 2000 km, will be delivered to JPL for integration in March, 1997. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low Earth Orbit satellite downlinks to the ground. This paper describes the hardware with photographs.
The Laser Communications Demonstration System (LCDS) Phase A/B was initiated by NASA-Headquarters through JPL to show that improvements in both technology and multi- discipline system engineering expertise since the initiation of previous programs enable the practical demonstration of space laser communication terminals that exhibit the anticipated benefits relative to rf communications. This paper presents an overview of the Laser Communication Demonstration System as developed by the Ball team.
For extremely high data rate communications between satellites, aircraft, and ground sites in situations where atmospheric interference is minimal, laser communications offers significant advantages over current radio frequency technologies in the areas of achievable data rate, size, weight, and power. Over the last two years, the Ballistic Missile Defense Organization (BMDO), contracting through the U.S. Army Space and Strategic Defense Command, has been funding the development of a laser communications terminal to be flown on the second Space Technology Research Vehicle (STRV-2) which is projected to launch in 1998. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low Earth orbit (LEO) satellite downlinks to the ground. The design of the terminal is based on direct modulation of semiconductor lasers, direct detection using avalanche photodiodes, separate acquisition/tracking and communications wavelengths, atomic line filter (ALF) technology for background light rejection, separate transmit and receiver apertures, and a hemispherical field-of-regard gimbal based upon a novel design. This paper discuses details of the optomechanical design of the terminal as presented at the program's critical design review.
KEYWORDS: Rubidium, 3D displays, Semiconductor lasers, 3D scanning, Mirrors, Chemical species, Laser scanners, 3D image processing, Absorption, Luminescence
The successful demonstration of a novel 3D volumetric display based on the intersection of two low power diode laser beams in an atomic vapor is presented. A 780 nm laser and a 630 nm laser are directed via mirrors and x-y scanners towards an enclosure containing rubidium vapor, where they intersect at 90 degrees. Rubidium atoms within the small intersection volume undergo 5s1/2 to 5p3/2 excitation from the 780 nm laser, and then 5p3/2 to 6d5/2 excitation from the 630 nm laser, resulting in red omnidirectional fluorescence from the intersection point. Tuning of the lasers to the exact excitation wavelengths resulted in an extended red spot with maximum brightness. By tuning the lasers slightly off the transition wavelengths, a very localized red spot with slightly less brightness was produced. A series of intersection points were scanned in a time less than the eye's 15 Hz refresh rate to create true 3D volumetric images such as a floating cube and rotating globe, which were viewable from many angles. The maximum speed of the mechanical scanners limited the complexity of the 3D images. By incorporating higher power lasers and faster acousto-optical scanners, this technique could allow the 3D viewing of real time air traffic control, medical images, or theater battlefield management.
Laser communications offers the potential for transferring information at data rates higher than 1 Gigabit per second (Gb/s) between satellites, aircraft, and ground sites using terminals with weights between 15 and 30 pounds, and volumes less than 1 cubic foot. The Ballistic Missile Defense Organization (BMDO), contracting through the U.S. Army Space and Strategic Command, has been funding the development of laser communications technology and demonstration hardware. Applications of near term interest include inter-satellite crosslinks, Low Earth Orbit satellite downlinks to the ground, and Aircraft-to-Aircraft crosslinks. To demonstrate the capability and readiness of laser communications for these applications, we are building a space capable laser communications terminal to be flown on BMDO's STRV-2 experiment with a projected launch date in 1997. The design of that terminal is based on direct modulation of semiconductor lasers, direct detection using avalanche photodiodes, separate acquisition/tracking and communications wavelengths, separate transmit and receive apertures, and a simple optical transceiver package mounted on a hemispherical field of regard gimbal.
Laser communications between satellites, high flying aircraft (such as JSTARS), and the ground offer the potential to transfer extremely high amounts of information faster and with a much smaller package than is possible using current radio frequency and microwave technologies. This can be especially important in downlinking time sensitive satellite reconnaissance information because the satellite stays within range of a ground station or aircraft for only a few minutes. A capability to downlink from a satellite to an aircraft can provide all weather performance, and multiple data transfers for every satellite orbit. Over the last few years, SDIO (now BMDO) has funded a number of technology efforts through the US Army Space and Strategic Defense Command reducing the risks associated with laser communications. This paper describes one of these efforts which is now being carried forward to an Advanced Technology Demonstration at ThermoTrex Corporation. The program will lead to the demonstration of high data rate communications of 270 MBPS (Mega Bits Per Second) to 1.08 GBPS (Giga Bits Per Second) between high altitude aircraft and possibly between a satellite and the ground. The Laser Communications Terminals incorporate Atomic Line Filter technology for background light rejection during acquisition, reactionless Roto-Lok offset cable drive gimbals for fast slewing and high accuracy pointing, and direct digital modulation of semiconductor diode lasers detected with low noise avalanche photodiodes. We present designs and preliminary performance results for both a simplified terminal appropriate for a near term satellite-to-ground data transfer experiment, and a full capability terminal appropriate for ground, aircraft, or satellite operations.
Laser satellite networking is a key element of effective communications operations to support both strategic and tactical missions. Lasercom offers a number of important advantages over conventional RF satellite communications. The shorter wavelength available using lasers provides higher data rates at less power and smaller apertures, both resulting in lower weight requirements. On the other hand, lasercom entails more difficulty in acquisition and tracking because of the narrow beams used. Technology problems to be overcome before intersatellite laser communications can reliably outperform RF communications include acquisition in the presence of significant background light from the earth, tracking to resolutions of a few microradians, high speed modulation of semiconductor lasers with close to one watt of power in a diffraction limited beam, high bandwidth low noise detector response, and demonstrated long term performance. We have developed critical technologies to solve some of these problems, and demonstrated them in a laboratory testbed which also supports development and testing of network protocols and algorithms. Our hardware provides new capability in background light rejection by using innovative atomic line filter technology, improved tracking accuracy by using innovative zero backlash Roto-Lok drive gimbal telescopes, and increased communications bandwidth by incorporating multi-link networking protocols.
Several electron beam activated diamond switches have been constructed and operated. In an initial set of experiments the electron source consisted of a LaB6 photocathode illuminated by approximately 15 nanosecond pulses of 248 nm light from a KrF laser. The photocathode could be biased at voltage of 10 - 80 kV. The type IIa diamond wafer was 12 microns thick with top and bottom electrodes consisting of Ti/Pt/Au sputtered metallizations (unannealed). Limited by surface flashover across a 12 micron broken edge of the diamond wafer, pulses with a peak power at the kilowatt level into 50 ohms were generated. The output pulse duration was set by the electron beam duration or the round trip time in the charged transmission line, whichever was shorter. Measurement of the output pulse rise time was limited by the diagnostic oscilloscope resolution but was less than one nanosecond. It was observed that the output pulse amplitude reached the expected value only when the bombarding electron beam voltage was sufficiently large that carrier pairs were generated throughout the thickness of the diamond sample.
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