Lasers in Space

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Lasers in Space

The word "laser" is an acronym for "light amplification by the stimulated emission of radiation." The laser is a unique device that produces a very pure color of light that is concentrated into a pencil-thin beam that stays concentrated, or focused, as it travels.

Lasers are commonly seen in several ordinary commercial applications, such as bar code scanners, laser pointers, CD players, CD-ROMs, videodiscs, laser surgery, and laser-light shows. However, lasers have many other applications as well. For instance, lasers enable us to communicate and transfer massive amounts of information, monitor our environments, provide protection from aggressive military attacks, and probe the deepest reaches of space and understand the origins of the universe. Lasers will have a myriad of applications in space, the following of which will be highlighted: (1) laser communications, (2) lasers for environmental and remote sensing, (3) space-based laser defense systems, and (4) lasers for astronomical applications in gravity wave detectors.

Laser Communications

The use of lasers as a tool to transmit information, such as telephone conversations, television programs, and data, is well known. As the information age continues to advance, the use of lasers in space as a communication tool will become critical. During the first decade of the twenty-first century, most of the lasers used in communication applications will be associated with optical fiber connections. The growth of the Internet, however, will eventually clog up today's fiber-optic cables . This will occur because many people will use computers that send information back and forth to each other through fiber-optic phone lines (or fiber-optic cables). This clogging up of the phone lines and cables by computer usage is similar to the clogging up of the phone lines on major holidays, such as Mother's Day.

One way to avoid this problem is to place lasers on satellites in space. In this way, data can be collected from multiple locations that are geographically close to one another and beamed up to a satellite by either a laser or microwave link . The satellite can then collect the data and retransmit the information from one satellite on an ultrahigh-capacity optical data link using lasers.

The importance of using lasers in space for communications is that since light is traveling in space (a vacuum ) the light signals are not corrupted as much as they would be traveling through optical fiber. In addition, instead of using one color of a laser to transmit information, a satellite could have many different lasers, each transmitting information on a different color. This method of using different colors to increase the amount of information to be transmitted is called "wavelength division multiplexing" and is similar to how conventional radio signals are broadcast on different radio frequencies. With this type of laser technology, optical communication links in space could easily handle many tens of trillions of bits of information being sent every second.

Environmental and Remote Sensing

One of the most common uses of lasers in space is for environmental and remote sensing. In this application, a laser stationed on a satellite can orbit Earth (or other heavenly bodies such as the Moon or Mars) and direct a sequence of short optical pulses onto the surface. These pulses are then reflected from the surface, and the reflected pulses are detected by the satellite that contains the laser. Since the speed of light is known accurately, the time it takes for the light pulses to leave the laser/satellite, travel to the surface, and return can be measured, as can the distance from the satellite to the surface. By repeatedly sending sequences of pulses from the satellite to Earth's surface, a three-dimensional topological map can be generated.

The truly amazing feature of using lasers for this type of geographical mapping is that a distance resolution of a few millimeters can easily be achieved. More importantly, different types of lasers emit different colors of light, and these different colors reflect in particular ways, depending on the type of surface the laser light reflects from. In this way, one can use different types of lasers that not only will map out the geographical terrain but also will be able to measure the composition of clouds and perhaps detect water,minerals , and other natural resources underneath the surface.

Laser Defense Systems in Space

The prospect of using lasers in space, as part of an overall strategic defense plan of the United States, was gaining significant support in the early twenty-first century. In this scenario, lasers would not be a source of directed energy in an offensive attack, but the lasers would primarily be used in a defensive mode to target, track, and identify potentially hazardous threats that may come in the form of intercontinental ballistic nuclear missiles. The types of lasers used would vary widely, depending on the functions to be performed by the laser. For example, small low-powered lasers would be used to realize optical radar functions and to determine the location and velocity of moving targets in space. More powerful solid-state or chemical lasers could then be used as a source of directed energy to disable rogue missile attacks. Several plans have been proposed to incorporate lasers in space as part of a unified missile defense plan, including ground-based lasers and orbiting reflectors to assist in tracking and directing the laser radiation. Owing to the harsh environment of space, novel engineering approaches would need to be employed to make these laser systems robust and reliable. In addition, the need for generating power to operate the lasers may easily be accomplished by a combination of solar cells or direct solar-pumped lasers.

Gravity Wave Detection in Astronomy

Lasers in space are also used in astronomy. Researchers use ground-based lasers and optical interferometry to detect gravity waves . Optical interferometry is a technique that splits a laser beam into two beams by using a partially silvered mirror. Each beam travels in a different direction (or arm of the interferometer) and is then reflected back to the silvered mirror. The two beams are recombined and the resulting combined beam can provide information about the differences between the two paths that each beam traversed.

This method is being used on Earth to detect the presence of gravity waves that could have been produced from exploding stars or colliding galaxies. Currently, the limitation in the ground-based approaches is that the sensitivity provided is not sufficient for detecting gravity waves. It should be noted that the lengths of the arms of the interferometer on ground-based gravity wave detectors are on the order of 1 kilometer (0.6 miles). By placing the laser and interferometer in space, the sensitivity can be improved by increasing the lengths of the arms of the interferometer to thousands of kilometers and by removing any disruptions caused by Earth-related effects. The detection of gravity waves would be an incredibly important finding in science, because it would serve as another verification of German-born American physicist Albert Einstein's theory of general relativity .

Outlook Towards the Future

This brief description of the potential applications of using lasers in space shows that these light sources are truly unique and can provide unprecedented performance in specific applications. Scientists and engineers worldwide are researching these and other applications of lasers in space, not only to consider and test the feasibility of specific uses but also to continue to develop state-of-the-art laser systems so that these applications will flourish. What will the newest applications of lasers in space bring? How will these applications change the way humans live their lives? No one can be completely sure, but the new uses that will be discovered will be limited only by the human imagination.

see also Communications, Future Needs in (volume 4); Laser Propulsion (volume 4); Military Space Uses of Space (volume 4); Mining (volume 4); Power, Methods of Generating (volume4); Scientific Research (volume 4); Space Industries (volume 4).

Peter J. Delfyett

Bibliography

Bass, Michael, et al., eds. Handbook on Optics, 2nd ed. New York: McGraw-Hill, 1995.

Chan, V. W. S. "Optical Space Communications."IEEE Journal of Selected Topics in Quantum Electronics 6 (2000):959-975.

Coyne, D. C. "The Laser Interferometer Gravitational Wave Observatory (LIGO)Project."IEEE Aerospace Applications Conference Proceedings 4 (1996):31-61.