Here Comes the Laser
January 1963 Radio-Electronics

January 1963 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

In our present day where lasers of many wavelengths and powers in the megawatts are considered routine, this 1963 article from Radio-Electronics magazine shows how far the technology has come in the more than six intervening decades. Optical masers (i.e., lasers) in the field of radio and electronics technology are explored. Lasers generate coherent light beams and have a wide range of applications, from handling millions of telephone calls and TV channels to powering satellites and spacecraft. The article discusses crystal and gaseous lasers, their components, and how they function. It also highlights the research and development efforts by various organizations to improve laser technology. The article emphasizes the significance of lasers in communications, radar systems, and high-intensity beams, while acknowledging their limitations at the time, and the ongoing work to overcome them.

Here Comes the Laser

By Jordan McQuay

What's new in the field of optical masers?

A major advance in physical science is the generation of coherent light beams by a new electronic device: the optical maser, or laser. These newly developed electronic-optical devices promise to advance and revolutionize the extremely high-frequency fields of radio and electronics technology.

Potential uses of lasers are tremendous and breath-taking, with many bordering on science-fiction levels - but all are well within the realm of possibility in the next 5 years:

A light beam capable of handling 100 million telephone calls, or a million TV channels. A light beam that can power satellites and other spacecraft, or map areas of the moon. A light beam focused to an intensity brighter than the sun. A pulsed beam that enables radar to track objects billions of miles away in outer space. A concentrated light beam sufficiently intense to bore holes through metal-and even diamonds - in a few hundred microseconds.

These and other advanced techniques are possible with the laser - an acronym for light amplification by stimulated emission of radiation. Also called an optical maser, it is an extension of electronic techniques into the infrared and visible-light portions of the frequency spectrum (Fig. 1).  

Both masers and lasers are essentially energy converters, operating on similar principles. The maser operates in the microwave region; the laser in the infrared and visible-light regions.

Both utilize the natural resonant frequency of electrons within an atom as a frequency standard. This means that they have very high stability insensitive to environment.

In both masers and lasers, random energy is converted by atomic action into a stimulated, coherent, highly directional beam. But of these two, lasers show greater promise for future applications - exploring for the first time the use of optical frequencies for communications and other electronic techniques.

The optical portion of the spectrum is a new and almost-unfamiliar region to those accustomed to thinking of frequency in terms of megacycles. Wavelengths are so short that the angstrom (1 x 10^-10 meter) is used to define these waves.

In this region, a typical optical maser operates at about 6,330 angstroms, or 470 teracycles (10¹² cycles). At longer wavelengths in the infrared region, typical lasers operate between 11,114 angstroms (268.3 teracycles) and about 21,189 angstroms (137.1 teracycles), depending on the type of element used in each device. At least one recent optical maser worked in the visible (red) light region.

Coherent Waves

Although resonance in crystals and other materials is often encountered the most unusual characteristic of the laser is that oscillations are controlled and stimulated, by the device itself, so the output of light waves is coherent. This means the output is monochromatic, unipolarized and uni-phased.

Light is produced whenever the atoms of certain substances are raised sufficiently in temperature. Such light - whether from incandescent lamps, fluorescent tubes or the sun itself - is not coherent. It is emitted in a random pattern, haphazardly in all directions. The many light waves interfere with each other and are diffused. As a result, the potential energy is dissipated to a large degree. But coherent light behaves differently.

Coherent light waves have traveling surfaces of constant phase moving in an extremely narrow beam. The amplitude and phase of coherent waves can be specified closely. The frequency depends upon the active laser element.

These coherent waves proceed in strict formation, closely obeying the ideal laws of optics.

Although laser outputs have an extremely narrow beam width, they can be focused to even tighter beams. Under such controlled conditions, a laser beam spreads less than 2 feet in 1 mile. In comparison, sunlight would disperse as much as 100 yards.

This permits wide-band communications where - at least in theory - a 1 % bandwidth can handle over 100 million TV channels simultaneously. And with sufficiently high power, lasers will permit communication over phenomenal distances through outer space.

Much laser research is experimental. Only a few models are available for commercial use, and they are expensive. But some idea of the potential magnitude of lasers is indicated by the fact that over 500 Government-sponsored development contracts are now in existence in as many laboratories across the nation.

Despite their many advantages, these coherent light waves have some important limitations.

They cannot penetrate clouds, adverse weather or atmospheric conditions. They follow line-of-sight transmission paths - unless moved through optical type waveguides.

Atomic Energy

Coherent optical waves are generated and produced by harnessing and controlling the illimitable power and inherent frequency of atomic energy.

Certain elements, such as uranium, lend themselves to the production of nuclear energy. And we know what that produces!

In more peaceful pursuits, any of a relatively small group of solid-state crystals or certain gases are also suitable for producing a more beneficial and useful type of energy: optical waves.

The most successful crystalline substances have been the synthetic ruby - doped with any of several chemicals. Some of the gases are helium, neon, argon, krypton and xenon.

This is the way it works:

Every atom of such an active element has certain characteristic energy levels. The lowest of these is the normal - or "at rest" energy level, at which the atom remains relatively undisturbed (Fig. 2).

When stimulated by an external source of rf or light energy - called a radiation pump - the atoms of the element absorb some of the energy and jump quickly to a higher energy level, where they remain in an "excited" state.

This state is unstable and the atoms tend to return to their normal, "at rest" level. As they do so, they emit the absorbed energy in the form of light waves at a frequency depending upon the basic substance of the excited element, In a gas, these atoms collide with other atoms, causing further emission of light waves.

So far this light emission is random and not coherent.

But when this atomic process takes place in a resonant chamber or tube of critical dimensions and under carefully controlled conditions, the laser becomes a regenerative oscillator. Optical waves are stimulated and produce a magnified narrow-beam output.

Basic Operation

In a typical arrangement (Fig. 3), the active element - whether crystal or gas-is contained within a tube.

A mirror over each end of the tube makes it a resonant cavity. One of these mirrors, however, has a small transparent "window," or may be very thin, so that a small portion of the light escapes through it.

The laser is pumped by either optical or electrical means. Then, as excited atoms return from higher energy levels to their normal or "at rest" level, light waves are produced.

Reflected by the end mirrors, the waves move back and forth within the cavity, each time colliding with other excited atoms, and stimulating or increasing the production of light waves. All these light waves are in phase, which results in a major intensification of these waves along the resonant tube. Output is through the mirror at one end of the cavity. [For a more detailed description of optical maser action, see "Communications on 450,000,000 Mc," Radio-Electronics, May 1961, page 57.] The frequency of these oscillations depends on the atomic structure of the active element.

There are two broad types of lasers: the crystal and the gaseous.

Crystal Lasers

The active element of solid-state optical masers is a small crystal rod with silvered ends. It is pumped with white or green light from a spiral photoflash lamp surrounding the element.

The most common type of crystal laser (Fig. 4) uses a synthetic ruby crystal of aluminum oxide doped with chromium. Applying a green pumping light of the right frequency causes the chromium atoms to jump to a higher level of energy. Returning to their normal or "at rest" state, these atoms emit infrared light, which is trapped between the silvered ends of the ruby rod. By stimulating emission in the resonant cavity, intense radiation (Fig. 5) is developed along the length of the rod.

Work is continuing along several broad fronts by many research organizations. Notable among these are Hughes Aircraft Corp., Bell Telephone Laboratories, Raytheon Co., American Optical Co., Sylvania and many others.

The search for new active elements has led to exhaustive experiments with new crystal compounds. Prominent among the crystals successfully tested are calcium fluoride doped with dysprosium, samarium, thulium, neodymium, and even uranium. Also of potential merit are calcium tungstate doped with erbium, neodymium, holmium, thulium and other paramagnetic ions.

New methods of exciting the active element of a crystal laser have been developed. And Raytheon has offered one of the first pulsed crystal lasers on the commercial market.

An advanced type of optical maser, capable of continuous operation, has been developed by Bell Telephone Laboratories (Fig. 6). The active element of this laser consists of two substances grown together as one synthetic crystal. Shaped like a trumpet, the bell of the device is pure aluminum oxide (sometimes called sapphire) which receives the pumping radiation and then acts as a "condenser". The shank of the trumpet is synthetic ruby - aluminum oxide doped with chromium. Laser action takes place in the shank, producing an intense beam of infrared light. Output is about 4 mw.

Other advances in solid-state optical masers include increases in output power for crystal lasers plus new and diversified means of modulation.

One new technique which is reported to multiply output power by about 1,000 times, has been announced by General Dynamics Corp. The laser is surrounded by a strong non-uniform magnetic field, which forces the device to store up energy. When the field is removed, the laser releases the energy in a concentrate many more times powerful than for a non-modulated discharge.

Another specialized application of the crystal laser is a sun-powered device, proposed by the American Optical Co., which would permit direct use of sunlight to power a system for communicating with satellites and other space vehicles (Fig. 7).

Gaseous Lasers

Active element of most gaseous optical masers is a combination of inert gases-such as helium or neon - contained in a glass tube. This tube is pumped by a high-frequency signal of about 30 mc to change energy levels.  

When the atoms of one gas are excited, they collide with atoms of the second gas. Energy is transferred in the form of infrared or visible light. In a resonant tube, the energy is stimulated to form a continuous high-energy beam of coherent light (Fig. 8).

Output frequency depends upon the active elements - the inert gases. Helium-neon combinations produce visible light of about 6,328 angstroms. Combinations of neon-oxygen as well as argon-oxygen produce light waves in the near infrared region of 8,450 angstroms.

Optical masers using a pure (noble) gas have also been developed by the Bell Telephone Laboratories. These produce a beam of coherent infrared radiation at more than a dozen wavelengths between 16,900 and 21,890 angstroms. Cesium has been used to generate beams of 71,180 angstroms.

An important characteristic of this type of optical maser is that the output beam is several hundred times narrower than the output of crystal lasers.

The two types - crystal and gaseous - tend to complement each other, since the gaseous laser is essentially a low-power device. The crystal laser, a high-power device, is particularly useful in pulsed applications - for radar and pulsed-code modulation (pcm) communication.

Communications

Using either pulse-code or continuous-modulated CW, optical masers are particularly adapted to long-range communication - particularly in outer space. But for interstellar and inter-planetary communication, means and methods of modulation, which are now under development in research laboratories, are needed. General Telephone Laboratories at Bayside, N. Y. (formerly Sylvania Labs), announces, for example, a modulation method based on frequency change. This FM approach "allows the lasers to be 'tuned' rapidly like a home radio set" (Fig. 9).

For pulse-code modulation transmission or reception, a "traveling-wave" crystal laser that can amplify or intensify light waves directly has been. developed by Bell Telephone Laboratories.

An intensified image of a suitably illuminated object at the input is reproduced in the output. The laser type device consists essentially of two amplifying sections with an isolator of lead-oxide glass (flint glass) between them. This material is transparent to light waves over most of the optical range. The "isolator" tends to absorb backward traveling waves and to transmit forward traveling ones. This is done by a disc of polaroid type material. This apparatus provides a gain of about 13 db. But the amplifier has a bandwidth of about 100 kmc - as large as the entire spectrum of presently usable radio and microwave frequencies.

Not all optical masers need be large or ponderous. At least one light-weight though low-power crystal laser has been developed - by the Bell Telephone Laboratories. Weighing only a few pounds, it is used for short-range communications and demonstration (Fig. 10).

Radar Applications

In radar-for long-range surveillance, acquisition and tracking, and for guidance and control of space vehicles, the optical maser is destined to shine the brightest.

Both crystal and gas lasers are extremely adaptable to most radar operations, because they can be pulsed to achieve extreme peak power. Outputs greater than 3 megawatts have been generated by the Army Signal Corps and other research laboratories.

One of the first laser radars was the Colidar (for Coherent Light Detecting and Ranging) developed by Hughes Aircraft Co. (Fig. 11). A crystal (ruby) laser provides light pulses, which are collimated by a lens and transmitted toward a distant target. Reflected light signals (echoes) are collected by a large telescope, amplified and processed to provide distance, elevation and other data.

On a larger scale, MIT scientists at Cambridge, Mass., during 1962, used a Raytheon pulsed crystal laser and precise timing equipment to bounce signals off the moon - an old stunt for microwave radar, but a scientific "first" for the optical maser. Using a 12-inch telescope, transmitted laser pulses illuminated a circle on the moon about 2 miles in diameter, and then returned to earth. Each round trip took 2 seconds and was received weakly - but positively.

Optical masers fit perfectly into the special requirements of space exploration and discovery - once the light waves penetrate the atmosphere of the earth. Supplementing conventional radar applications, lasers could well bridge the gap between a manned spacecraft and a target vehicle as they approach a rendezvous point in space.

Although such an application may be a few years away, a suitable radar exists today - developed by the Martin Co. Called the "suitcase" laser, it transmits and receives radar pulses at ranges up to 10 miles. The equipment weighs about 30 pounds.

A military version of the same radar is the "laser ranger," also developed by the Martin Co. Designed for rugged field use, the operator simply aims the portable device toward a moving ground target, presses a button and reads the distance to the target on an indicator panel.

High-Intensity Beams

The ability of optical masers to pro-duce coherent light beams of extremely high intensity is utilized in a number of ways.

Using laboratory lasers, the Air Force has welded pieces of titanium - with no other source of power. And two pieces of hard-to-weld molybdenum and titanium have been fused by the American Optical Co. with a laser they developed.

Raytheon has demonstrated the tremendous power of their laboratory laser by burning holes through a stainless steel sheet 1/32 inch thick - in 2 milliseconds. The same coherent light beam can ignite paper at a distance of 15 feet. And a General Electric device can blast through diamonds with a temperature of about 10,000°F.

Already the laser has been adapted to the ophthalmologic area of medicine. Utilizing an intense laser beam, a new instrument called the laser retina coagulator is now successfully in use at the Presbyterian Hospital in New York. The beam is used to cauterize small areas and even kill individual cells of the retina without the dangerous and penetrating effects of X-rays and gamma rays (Fig. 12).

Other applications of the coherent light beams of optical masers are proceeding in many laboratories. Under the cover of military secrecy, some of these experiments are concerned with developing a "death ray" - to divert enemy satellites in flight, or even to knock down intercontinental ballistic missiles.

Such applications - and the optical maser, itself - are in their infancy. And much can be expected in the future from the coherent light beams produced by lasers.