Laser frequencies may be reduced to single frequency or selected frequencies. For single frequency performance, spacing of frequencies falls in natural line width. For two or more frequencies, thicker spacing is used. Configuration adapts to operation with Kerr or Pockel cell for Q switching

Heflinger, L. O.

English

NASA Technical Reports Server

December 1971	 Brief 71-10527 
S
NASA TECH BRIEF 
Ames Research Center 	 W 
______	
TTIII	
-.iIITT 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield,' Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Improving Laser Beam Coherence: A Concept 
Light emitted by a laser is not strictly monochro-
matic, for it actually consists of a narrow band of 
closely-spaced wavelengths. For certain applications, 
such as the production of holographic images with 
greater scene depth, it is desirable that the band of 
frequencies represented by the closely-spaced wave-
lengths be reduced even further to a single frequency 
or, in some special cases, to a few selected frequencies.
(Fabry-Perot interferometer) by using the arrange-
ment depicted in the diagram. Light from the laser 
rod propagating to the left is linearly polarized and 
passes into region A; then, passing through the first 
quarter-wave plate, it is circularly polarized in region 
B. Light reflected from the resonant reflector is re-
turned, traveling to the right in region B with the 
opposite sense of circular polarization. After passing 
.
D	 C	 B A 
Resonant Polarizer Laser Rod 
100%	 Reflector	 I Partially 
Reflector	 2nd	 St Reflecting 
Output 
Reflector 
Quarter-Wave Plates
In some techniques, the extent of narrowing of the 
frequency band depends on the thickness of a dielec-
tric parallel plate (resonant reflector) which is used 
as the output reflector. Reflectivity is sufficient to 
permit lasing only at frequencies such that odd mul-
tiples of quarter wavelengths are equal to the optical 
thickness of the reflector; at even multiples of quarter 
wavelengths, reflectivity is zero and lasing does not 
occur. Thus, the thickness of the reflector determines 
the comb of permitted frequencies, but the teeth of 
the comb are not narrow enough for the desired holo-
graphic images. 
It is proposed that the desired comb could be ob-
tained with a double-surfaced resonant reflector
through the quarter-wave plate, it is made into linearly 
polarized light in region A with a direction perpendic-
ular to the original polarization and thereby rejected 
by the polarizer. To this point, the system behaves 
like the one-way polarizers used on readout devices 
to reject reflected incident light. 
Light at a frequency which can pass through the 
resonant reflector into region C is still circularly polar-
ized, and then is made linearly polarized in region D 
by the second quarter-wave plate. After reflection 
and a second pass through the second quarter-wave 
plate, it is rendered circular of the same sense as 
before, passes through the double-surface resonant 
reflector and the first quarter-wave plate, and emerges 
Ah	 (continued overleaf) 
qW
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Aeronautics and Space Administration. Neither the United States
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into region A as linearly polarized light in the pass 
direction of the polarizer. The polarization elements 
thus permit reflection for only the narrow frequency 
intervals which have high transmission through the 
resonant reflector. 
The surfaces of the double-resonant reflector are 
coated with a dielectric material for high reflectivity, 
thus creating very narrow pass bands as is the case 
with a Fabry-Perot interferometer. The thickness of 
the reflector, i.e., distance between reflecting surfaces, 
determines the spacing of the comb frequencies. If 
single-frequency performance from the laser is de-
sired, then the spacing should be such that only one 
tooth of the comb will fall within the natural line 
width. When two or more simultaneous frequencies 
are needed, a thicker spacing is used so that two or 
more teeth of the comb fall within the natural line 
width. 
The configuration described is adaptable to opera- 
tion with a Kerr or Pockel cell for Q-switching. For 
example, the quarter-wave plate nearest the 100% 
reflector is replaced by the Kerr cell; application of 
voltage to the cell makes it into a quarter-wave plate, 
and absence of voltage makes the Kerr cell operate 
without birefringence so that all reflection is stopped.
Notes: 
1. The following documentation may be obtained 
from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: 
NASA CR-114274 (N71-17271), 
Holographic Instrumentation Studies. 
2. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B71-10527 
Patent status: 
No patent action is contemplated by NASA. 
Source: Lee 0. Heflinger of

TRW Systems Group, TRW, Inc.

under contract to
Ames Research Center 
(ARC-10417)
. 
. 
Brief 71-10527	 Category 03


Improving laser beam coherence - A concept

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