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The Tendency to Say Optical Maser is Growing

The following are some interesting words of wisdom from May 1961.[1]

“The optical maser is a source of coherent, monochromatic light. By modulating and amplifying this coherent light emission, it will be possible to use light as radio waves are used today. Until now, there were coherent sources of electromagnetic waves only for frequencies less than 105 Mc (1011 cycles). The optical maser has raised this limit almost 10,000 times. And not only has the frequency been increased but the sharpness or fractional bandwidth has also been improved.”

“The ruby optical maser as reported by Bell Telephone Laboratories and similar to the Hughes Research Group’s development consists of a Linde synthetic pink ruby rod 0.20 inch in diameter and 2 inches long. Both ends of the rod are optically polished to make them flat within 2 X 10-6 inches and parallel to within 10 seconds of arc. The end surfaces are made partially reflective, allowing only about 5% of the light striking them to pass through. A source of input power, a xenon-filled flash lamp, surrounds the ruby rod. It is pulsed by discharging a bank of capacitors through it. Charged to 4,000 volts, the bank delivers 3,000 joules to the lamp in about 1 millisecond. Fig. 1 shows both the mechanical and electrical details of the construction of a pulsed ruby optical maser. In the bottom section is a 0-4,000 volt power supply, used to charge the bank of four 100 μf 4,000 volt capacitors. The series resistor limits the charging current. Also shown is the supply for the 15kv transient that triggers the FT524 flash tube. A funnel-shaped cone holds the ruby rod axially within the helical coil of the flash tube. Surrounding the entire assembly of holder and lamp is a reflector. It can be either highly polished aluminum sheet or powdered magnesium oxide. It contains the flash lamp output so a large fraction of it will be absorbed by the ruby. The output of the flash lamp is white light but only the green and violet portions of it can be absorbed by the ruby. Besides the essential parts ruby, flash lamp and reflector a coolant for the ruby and flash tube, and opaque shields surrounding the exterior parts complete the maser.”

“With the development of these powerful, coherent and monochromatic light sources, many new uses of light will become possible. In the past, no light combined all three of these properties to the same extent and at the same time. One immediate application would be optical radar. Since the wavelength is so much shorter than that of present radars, the resolution could be much higher. Radar systems operating at 1 cm with antennas several meters in height have a beam width, and hence definition, of about 0.5°. This might be compared with an optical maser 1/2 cm in width, with a beam width of 0.05°, which through the addition of a simple optical lens system an inch or so in diameter could easily be reduced to 0.005°. With such a narrow beam width, radar systems could easily identify the actual shapes of aircraft.”

“Because of the vast new frequency space that will be opened in the future, the communications industry is vitally concerned with maser developments. At present, communications links operate up to about 104 Mc. The optical maser will extend that range to 1014 cycles. This advance is particularly striking when one remembers that all of the frequency span now available to us is contained in 10% of the new region. To appreciate the significance of this, it is only necessary to realize that a color TV channel requires a bandwidth of nearly 10 megacycles. If the existing channels were used in a 10% modulation system, about 100 channels would be available. This is certainly adequate for TV but there is no space left for data transmission or telephony. On the other hand, an optical maser at 108 Mc with a 10% bandwidth could handle 106 TV channels and still have a bandwidth of 107 Mc left for other uses.”

“The directional property of optical masers will be useful in earth-to-satellite, satellite-to-satellite and earth-to-moon links. For example, if the output of an optical maser were sent through a simple lens system 4 inches in diameter, the radiation would cover only 2 miles at the surface of the moon. This indicates the possibilities of private communications.”

“In addition to these applications, the optical maser might be used as a tool to effect chemical reactions. Maser emission is coherent; it can therefore be focused into an area of dimensions comparable to wavelengths of light. Under these conditions all the maser energy could be concentrated within single living cells and selective destruction of tissue (surgery) be performed.”

“The optical maser can extend greatly the range over which interferometric measurements can be made. Present optical interference measurements are limited to about 100 cm due to inherent line width and low power of monochromatic sources, but significant increases in length are now possible.”

The technology developed quicker than most thought and the history of the optical maser informs today’s R&D. It’s time to reconstruct some of these early experiments.

[1] R. J. Collins and D. F. Nelson, “Communications at 450,000,000 MC, How the revolutionary new optical maser works” p. 57-60 Radio Electronics, May 1961.

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Maser-to-Laser Man

Charles Townes sketched out the basic design for the maser in 1951. It was not until 1953 that Townes, James Gordon and Herbert Zeigler arrived at the first maser; it operated at 24 GHz using ammonia. It found initial use as a UHF oscillator and precision clock. He immediately wanted to go to much shorter wavelengths.

This image has an empty alt attribute; its file name is Townes-Left-Gordon-Right.jpg
Townes (left) and Gordon (right)

Townes teamed up with his brother-in-law, Arthur Schawlow, and they generated the framework for infrared and optical masers.[1]

“The extension of maser techniques to the infrared and optical region is considered. It is shown that by using a resonant cavity of centimeter dimensions, having many resonant modes, maser oscillation at these wavelengths can be achieved by pumping with reasonable amounts of incoherent light. For wavelengths much shorter than those of the ultraviolet region, maser-type amplification appears to be quite impractical. Although use of a multimode cavity is suggested, a single mode may be selected by making only the end walls highly reflecting, and defining a suitably small angular aperture. Then extremely monochromatic and coherent light is produced. The design principles are illustrated by reference to a system using potassium vapor.”

Reflecting on the state of the art back in the late 1950’s is instructive. For example, “For a wavelength equal to 104 A, it was seen above that spontaneous emission produced a few milliwatts of power in a maser system of dimensions near one centimeter, assuming refectivities which seem attainable at this wavelength. Thus in the ultraviolet region at
wavelength equal to 1000A, one may expect spontaneous emissions of intensities near ten watts. This is so large that supply of this much power by excitation in some other spectral line becomes very difficult. Another decrease of a factor of 10 in wavelength would bring the spontaneous emission to the clearly prohibitive value of 100 kilowatts. These figures show that maser systems can be expected to operate successfully in the infrared, optical, and perhaps in the ultraviolet regions, but that, unless some radically new approach is found, they cannot be pushed to wavelengths much shorter than those in the ultraviolet region”

Townes in 2015, at 89 years, during an interview by Bonnie Azab Powell, “[The maser] was a new idea, a sudden visualization I had of what might be done to produce electromagnetic waves, so it’s somewhat parallel to what we normally call revelation in religion. Whether the inspiration for the maser and the laser was God’s gift to me is something one can argue about. The real question should be, where do brand-new human ideas come from anyway? To what extent does God help us? I think he’s been helping me all along. I think he helps all of us – that there’s a direction in our universe and it has been determined and is being determined. How? We don’t know these things. There are many questions in both science and religion and we have to make our best judgment. But I think spirituality has a continuous effect on me and on other people.”

A very nice and detailed obituary on Townes appeared in IEEE Spectrum on 28 January 2015, and it’s linked here.

References

1 A. L. Schawlow and C. H. Townes, “Infrared and Optical Masers,” Phys. Rev. 112, p.1940, 15 December 1958.

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A History of Military Mapping Camera Development – 1964

“For many years the Army’s Corps of Engineers has held the prime responsibility for the military development of map plotting equipment and for topographic mapping techniques in general. With the advent of the airplane, responsibility for the development of mapping cameras and other airborne mapping and surveying equipment was placed with the Army Air Corps. As a result, in 1920, the Army organized, at what is now Wright-Patterson Air Force Base, the Wright Field Military Detachment. This group was assigned as a branch of the Engineer Board at Fort Belvoir, Virginia, in the early part of World War II. The Engineer Board was reorganized several years later to form the U. S. Army Engineer Research and Development Laboratories. The present aerial mapping liaison group at Wright-Patterson Air Force Base is now a part of the U. S. Army Engineer Geodesy, Intelligence and Mapping Research and Development Agency, Fort Belvoir, Virginia, which operates directly under the Chief of Engineers.

The past forty years has seen the development in the Air Force of two lines of aerial cameras: the reconnaissance series and the mapping series. This division in development was influenced by the differing needs of the Army and the Air Force. In its prime responsibility for targeting, charting and reconnaissance interpretation, the Air Force has insisted that the major consideration in the development of reconnaissance cameras be the enhancement of photographic resolution. Hence, work in this area has included the development of lenses with high resolving power, films capable of producing high resolution, film magazines with image-motion compensation capabilities, and gyroscopically stabilized aerial camera mounts which reduce vehicle vibrations and acceleration effects.

The prime requisite of the Corps of Engineers for a mapping camera is a high degree of dimensional stability in the camera-lens-film
combination to produce photography capable of direct application to map compilation. Therefore, mapping camera development has been characterized by the production of frame-type cameras having low-distortion lenses, lens cones fabricated from alloys providing high structural stability and with fiducial markers placed on the lens cone
rather than on the magazine, also between-the-lens shutters, and appropriate data recordings which appear on the film negative
between frames. The cameras are stocked with low differential distortion, topographic base films, and are installed in stabilized aerial camera mounts. Development of mapping cameras is accomplished by the Air Force upon imposition of Corps of Engineers requirements. This work has been supported actively through the years by the previously mentioned Corps of Engineers personnel who have been assigned to duty at Wright-Patterson Air Force Base.

As early as 1955, Dr. James Baker was commissioned by the Air Force to design a low distortion mapping lens which would have a higher amount of photo resolution than the current Metrogon and Planigon lenses; these average about 22 and 24 lines/mm, respectively, in AWAR. The resulting Type T-ll lens, later called the Geocon I, proved to have lens distortions of less than 10 microns; its resolution measured 52 lines/mm on-axis and 35 lines/mm AWAR. The lens was mounted in a T-ll camera body for flight testing. The full F/5.6 speed of the lens, however, could not be realized in the prototype camera, which produced a maximum aperture of only F/8. This camera subsequently was used in high-speed, high-altitude flight tests.”

Reference: ROBERT G. LIVINGSTON, “A History of Military Mapping Camera Development,” Photogrammetric Engineering, p. 97-110, January 1964.