Unpublished Manuscript
for Internal Circulation
1
DFO - LibTliorque
P
10028583
Bedford Institute of Oceanography
Dartmouth, N.S.
Institute Note 64-7
LASER AS A POTENTIAL YARDSTICK FOR SURVEYORS
by
F. L. DeGrasse
and
M. Duval
March 1964
Department of Mines and Technical Surveys
Marine Sciences Branch
Canada
TABLE OF CONTENTS
Page
Summary
1
Introduction & History
2
Description
3
Requirements
4
The Laser at Work
5
Conclusion
6
References
6
LASER AS A POTENTIAL YARDSTICK FOR SURVEYORS
SUMMARY
By definition, the LASER is a device that amplifies light
by means of stimulated emission of radiation.
The first laser was constructed in 1960 at the Hughes
Research Laboratories by an American named Maiman. Since then,
scientists working in the optical field, all over the world, have
explored the general physical conditions necessary for the operation
of different types of lasers and have analysed the feasibility of
several concrete systems.
Much literature has been published on the subdect since
1960, and new articles have appeared every month in scientific
magazines. "The growth of the entire field is so rapid that new
ideas often make the old ones obsolete before they can really be
assessed."
Many applications of the laser have already been found.
One of these is the use of the laser as a distance:measuring device,
or an optical yardstick for surveyors. In its present stage of
development, the ruby laser can measure distances up to 7 miles
with an accuracy of 15 feet. Dr. Malcolm L. Smith, of the Hughes
Laboratories claims that under ideal atmospheric conditions the
present model could measure distances up to 60 miles with the
same accuracy. This instrument has a digital readout and weighs
45 pounds including the back pack carrier. The operation and
handling of this instrument requires cautious attention, because
the laser light can damage delicate sensitive tissues such as
the human eye even at great distances.
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If the present rate of research and development continues
it is hoped that, in the near future, some laboratories will produce a light weight, accurate, and harmless laser that will compete
with other modern distance measuring devices.
DETAILED REPORT
Introduction
The word LASER stands for "Light Amplification by
Stimulated Emission of Radiation." A laser is essentially a
device that amplifies light by means of stimulated emission of
radiation, and is generally used as a source or generator of
radiation.
This unusual source of concentrated light - a billion
times brighter than the sun - excited the imagination of the
public, who were stimulated by an aggressive publicity campaign.
Commercial interest was soon aroused. Many private research
institutions and government agencies entered the field and adopted
some sort of a laser program.
The authors° intention is not to deal with the theory
of the laser but rather to give an idea of its development and
application to surveying.
History
It would seem unnecessary to mention the history of a
subject that is only three years old. However, the roots of the
subject go deeper than might be assumed.
Ever since the first stimulated micro-wave amplifiers
(Maser) were built by American and Soviet technicians in 1953-54,
there was speculation concerning the possibility of extending the
same principle to generation and amplification in the optical
(light) region. Considerable research preceded the successful
construction of the first Laser in 1960 by an American named
Maiman at the Hughes Research Laboratories. World scientists
carried out extensive research to explore the conditions necessary
for the operation of a laser in either the gaseous or the solid
state. The Soviets have been experimenting with a mercury discharge in which a transfer of excitation between different atoms
was employed to produce negative absorption, necessary to generate
amplification of light. However, Maiman has been experimenting
exclusively with ruby as have other scientists in the United
Kingdom and America. Henceforth, the ruby crystal laser only,
and its application to surveying, will be dealt with in this report.
Description
The relatively simple ruby crystal laser is a device consisting of a pair of parallel mirrors between which a cylindrical
shaped ruby is brought into a condition of negative absorption for
some frequencies. This device is represented in fig. 1, where
the reflectors are shown detached from the amplifying material
(ruby). However, this separation is not necessary in order to
obtain a power output from the laser. The end faces of the ruby
crystal can be precisely machined so as to have a completely
reflecting surface at one end and a partially transmitting one
at the other. Ability to transmit (t), reflectivity (r) and
loss (g) are connected with the equation t+r+g=1.
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When good reflectors are used, g may be neglected and we
write t=1-r. When the flash tube (L) is triggered to give
stimulated emission, light is generated within the laser. The
light that does not pass out through the sides is reflected back
and forth between the mirrors. Starting at one point, the
radiation will suffer two reflections before it passes the same
point in the original direction. In each passage through the
ruby the intensity gains by an enormous factor and the output would
become infinite if the loss factor was not present. On every complete two-way passage of the light through the laser, the portion
(t) leaves the laser as a coherent light ray of immense power
having a very narrow degree of divergency. Collimation can be
controlled by a relatively small optical system.
Requirements
The contemplated laser applications provide a motivation
for an intense industrial effort directed at further development
in the laser art. Each application requires different conditions
and special refinements. Applications involving heating, shaping
or destruction of matter requires large amount of energy per
radiated pulse while coherence of the beam may be a secondary
consideration.
In communications, the prime emphasis is not on power
but on constancy of frequency and amplitude. Most important is
the capability to modulate frequency and amplitude corresponding
to the intelligence to be transmitted.
In scientific experiments the requirements vary with
the conditions and no general statements can be made. For instance,
5
in the science of weights and measures, sources of high, longtime stability in frequency are needed.
Surveying applications require sharp, high-power pulses
at a fixed rate many times per second. Spacial coherence is a
consideration here because a sharp beam is required. Moderate
frequency excursions of the order of 1010 c/sec. can be tolerated.
The energy per radiated pulse need not be so large as for applications involving heat. A practical requirement here is a portable
and comparatively cheap instrument that can be used by the layman
as an accurate measuring device. There has been a great deal of
speculation about the possibilities of using the laser as an
optical yardstick for surveyors.
The laser at work
Ranging-measuring the distance to some remote object was, in fact, the first reported application for the laser. Four
scientists from Hughes Research Laboratories, California, birthplace of the laser in 1960, reported successful experiments in
1961. As a result of these -experiments, a low accuracy range
finding instrument has been marketed under the trade name,
"Mark II Colidar Rangefinder." This instrument in its unrefined
state resembles a carbine rifle and has measured distances up to
7 miles in daylight within an accuracy of 15 feet, using various
buildings and landmarks as reflective targets. Under ideal
atmospheric conditions, the present model could range 60 miles
against similar targets.
Distance is read out directly in meters and the device
is powered by self-contained batteries. This instrument, the fore-
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runner of the ranging laser applications was developed primarily
for the military as a homing device.
Conclusion
The potential applications of the laser are too numerous
to list. Much research is being done at the present in the field
of communication and ranging, but many problems are yet to be
solved. The laser will probably never displace radar, but with
its narrow beam, relatively free from interference, it will likely
become the base of a ranging system. When proper methods of
modulation and demodulation of the laser light are found, an
extremely potent information carrier will be achieved.
It is hoped that in the near future a lightweight
accurate and harmless laser will replace the surveyor's tape, and
the more expensive distance measuring devices. Accuracy of the
Laser is limited to the timing mechanism employed in the electronic
circuitry and not by the laser pulse which is 186,000 miles/sec.
It is the writer's belief that with the high energy,
accurately timed pulses and better discrimination against
interference by filtering, the ranging capability can be greatly
extended and the instrument will play an important role in the
requirement of the surveyor.
References
1. Bela A. Lengyel, Lasers, John Wiley and Sons, Inc.,
New York, 1963 (April)
2. New Scientist, Volume 20, No. 368, Dec. 1963, page
589.
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3. New Scientist, Volume 20, No. 362, Oct. 1963, page
201.
4. New Scientist, Volume 20, No. 365, Nov. 1963, page
392.
5. Science, Volume 142, No. 3597, Dec. 1963.
6. Nature, Volume 200, No. 4909, Nov. 1963, page 818.
7. C.R. Smith, R.L. Quandt, E.J. Woodbury, D.S. Price,
and M.L. Stitch, Optical Radar Design Using Lasers
(U), Aerospace Group, Hughes Aircraft Co., Culver
City, Calif.