OPTIMIZATION OF LASERS

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• Collection: NGA Records (Formerly NIMA)
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• Document Creation Date: December 28, 2016
• Document Release Date: December 9, 2002
• Sequence Number: 2
• Publication Date: October 12, 1965
• Content Type: REPORT

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PAR 217
12 Oct 65
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SUBJECT: Optimization of Lasers
TASK/PROBLEM
1. Explore the production of 0.53 micron (blue-green) laser radiation
by harmonic doubling in KDP and ADP crystals.
DISCUSSION
2. The purpose of this program has been the production of a report
discussing the investigations and tests accomplished. At the start of
the program, the expected emphasis was upon reporting:
a. The knowledge gained regarding the combination of a laser with
a harmonic doubling cyrstal element as a source of coherent visible light
radiation.
b. Data regarding use of the laser with a variety of photographic
sensitized materials.
c. Recommendations regarding the breadboarding and building of
prototype equipment to support the photo exploitation community.
3. The production of blue-green laser radiation by harmonic doubling
had already been demonstrated at
There are many factors to encourage
the use of radiation in the 5000A and 6000A region in photographic systems.
These are:
a. Availability of a wide range of existing sensitized products
for which considerable performance data is already available.
b. Many existing optical system designs are corrected for this
wavelength range.
c. The possibility of using sensitized materials which may be
handled under safelights.
d. The possibility of visual observation of the image as an aid
to operation of the system.
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There was much activity in progress in the contractor's laboratory on "doped"
borate glass lasers which provide high-energy output pulses at 1.06 micron
wavelength. Reports in the technical literature, just prior to the time
of the project beginning, suggested the possibility of operating a glass laser
and a harmonic doubling element at a high-repetition rate using plasma-pinch
techniques. Repeated flashing at rates about 20 to 30 cps should provide the
visual effect of a continuously operating system for visual observation and
equipment adjustment.
THEORETICAL BASIS FOR HARMONIC DOUBLING
4, When a monochromatic beam of light is propagated through a medium,
it has associated with it a single or fundamental frequency, W . For this
to remain a "pure" frequency, a linear relationship would have to exist be-
tween the induced polarization, P, and the incident electric and magnetic
fields E and H. A linear relationship, however, does not exist and P is
more accurately expressed by a power series in E and H. In such an expan-
sion, one term is proportional to the square of the electric field. This
term leads to the frequency components of P at both 2 CO and zero, the 2 W
frequency being the second harmonic.
5. To understand the mechanism involved in efficient production of
second harmonic radiation, consider an isotropic medium having the same re-
fractive index for both (A1 and 2 W. Under this condition, both the funda-
mental and the generated harmonic would be propagated with the same velocity
and in the same direction. They would, therefore, continually be in phase
and the second harmonic would be propagated without interference. In real
media, refractive indices are not independent of frequency, and.W and 2tA)
would not be propagated with the same velocity or in the same direction. As
a result, the second harmonic generated at one point along the beam is
slightly out of phase with that generated just ahead or behind it. This con-
tinually varying phase results in a periodically destructive interference
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PAR 217
12 Oct 65
of the second harmonic with itself. Its intensity, rather than continuing
to increase with path in the nonlinear medium will oscillate about some
small value. To achieve maximum efficiency in such a situation, the medium
must be cut into a thin wafer to select the first peak in harmonic intensity.
To overcome this difficulty in producing second harmonic, a birefringent
material such as KDP or ADP can be used as a generating medium. In these
materials, the refractive indices are dependent on polarization and direction
of propagation, in addition to frequency. As a result, a propagation direc-
tion exists in some birefringent materials where one polarization of the fun-
damental frequency has the same refractive index as the other polarization of
the second harmonic. This is referred to as index matching and provides
one technique for producing second harmonic. This technique was used through-
out this program.
EXPERIMENTS IN PRODUCING SECOND HARMONIC RADIATION
6. The goal of the first part of the experimental program was to pro-
duce second harmonic radiation from a pulsed neodymium-doped borate glass
laser rod. To do this, an experimental breadboard was made using a 12-inch
long, 1/2-inch diameter glass laser to produce the fundamental 1.06 micron
radiation. The harmonic generating medium was a 1-inch square 5mm thick
KDP crystal, and the detector was a white cardboard located about two feet
beyond the crystal. When the crystal was correctly oriented, a bright green
spot of 0.53 micron radiation (second harmonic) was produced on the screen.
The durat..on of the pulse was approximately one millisecond, the same as that
of the fundamental. In addition to the green spot, several bright spots,
the order of 0.050 inches, were also observed on the screen. Experimenta-
tion showed this to be incandescence, a result of the intense 1.06 micron
beam interacting with the cardboard. This was verified by burn: marks which
became observable after 10 or 15 pulses. These spots were eliminated by
putting a filter glass (Pittsburgh 2043 Heat Absorbing) between the crystal
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PAR 217
12 Oct 65
and the screen. Transmission of the filter was 2 x 10-4% at 1.06 microns,
80% at 0.53 microns.
7. The next step in the program was to optimize the second harmonic
output with the crystal. This was done using a 929 Phototube as a detector
and recording the pulsed harmonic output as a function of crystal rotation.
A plot of these results is shown in Figure 1. To obtain quantitative measure-
ments of this output, the experimental equipment had to be modified. The
laser cavity was enclosed in a "lighttight" wooden box, having only an open-
ing at the output end. This opening was covered with a 2540 filter
having an average density of 6 throughout the visible, but transmitting
67%o at 1.06 microns. Also, a 925 Phototube was added to the system to mon-
itor the intensity of the fundamental beam incident on the crystal. This was
done by placing a beam splitter ahead of the crystal and splitting off about
10% of the energy. A schematic view of this setup is shown in Figure 2.
8. The goal of the effort described in paragraph 7 was to produce second
harmonic conversion efficiencies comparable to those obtained by R. W. Terhunel
using a ruby laser. The initial measurements toward this goal showed that
an input of 36 joules of 1.06 micron radiation produced 10-4 joules of 0.53
micron radiation -- a conversion efficiency of 3 x 10 4%o. By increasing the
ire;tdent 1.06 micron radiation to 120 joules, 9 x 10-4 joules of the har-
monic was produced -- an efficiency of 10-3%o. This increase in conversion
efficiency with increased input results from the second harmonic intensity
being a second order effect of the electric field. In other words, the po-rer
of the second harmonic varies as the square of the fundamental power. In
the data just quoted, for example, the input power was increased by a fac-
tor of 3. This increased the harmonic power by a factor of 9 and the con-
version efficiency by a factor of 3.
R. W. Terhune, "Non Linear Optics," SOLID STATE DESIGN, Vol. 4, No. 11, p. 38,
1963.
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100
80
a60
I-
0
20
10
0 0
'9~
0
0
0
O
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15 20 25 30
ANGLE IN MINUTES
35
PAR 217
12 OCT 65
40
45
50
RELATIVE OUTPUT OF 0.53 MICRON
RADIATION FUNCTION OF CRYSTAL
ALIGNMENT TO 1.06 MICRON INPUT BEAM
FIGURE I
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w
m
C'7
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12" NEODYMIUM
LASER
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2540 FILTER
(TO BLOCK VISIBLE
PUMP LIGHT)
KDP
CRYSTAL
2043 HEAT ABSORBING
GLASS (TO BLOCK 1.06
MICRON RADIATION)
2043 HEAT ABSORBING
GLASS (TO ATTENUATE
1.06 MICRON RADIATION)
925 PHOTOTUBE
(TO MEASURE 1.06
MICRON OUTPUT)
F I G U F34~prc2ed-For I I:Q&ikG/28G&AIEA-A"gd4t9AO 81%ffiT1 I C
929 PHOTOTUBE
( TO MEASURE 0.5:
MICRON OUTPUT)
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PAR 217
12 Oct 65
9. Efficiencies of the order of 10-3 and 10-4%o for the system, however,
were below Terhune's results and a reevaluation of the EDP crystal showed
this to be partially due to an error in orientation. The correct orientation
is shown in Figure 3. The z axis or optical axis is inclined to the surface
normal at the index matching angle l\f , and the x and y axes are symmetrically
positioned with respect to the normal. The index matching angle was deter-
mined by the equation2
(V2?)2
1/2
(V2E)2 (V2?)2
where Vl? is the velocity of the ordinary fundamental ray through the crystal,
V2? the velocity of the ordinary harmonic ray, and V2E the velocity of the
extraordinary harmonic ray, The solution was obtained by using index values3
Ni?
N2?
1.494.
1x5125
104706
E
N
2
The calculated angle was 41?1', and a 1-inch long 1-cm. square crystal having
this configuration was ordered.
2J. A. Giordmaine, "Mixing of Light Beams in Crystals," $.ell Telephone Technical
Memorandum MM 61-115-67, December 27, 1961.
3Fritz Zernike, Jr., "Refractive Indices of ADP and KDP Between 2,000 Angstroms
and 105 Microns," JOURNAL OF THE OPTICAL SOCIETY OF AMERICA, Vol. 53, No~ 10,
1964.
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NORMAL
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END VIEW
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PAR 217
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10. Using this new crystal, most of the experimental work, which had
been done with the 5mm long crystal, was repeated. This was done, however,
with an effective two-meter long laser cavity instead of the 12-inch cavity.
The purpose of increasing the length was to increase the harmonic efficiency
by reducing the fundamental beam divergence. The result of this was a reduc-
tion in the harmonic beam width from 25 minutes of arc (see Figure 1) to
30 seconds of arc, and an increase in efficiency of approximately 50 percent,
The end result of these measurements was that 4 x 10-3 joules of second har-
monic was produced with 140 joules of 1.06 micron radiation -- a conversion
efficiency of 2.8 x 10-3%, a percentage nearly that achieved by Terhune with
ruby.
11. Having developed an extremely bright spot of green light, a prelim-
inary qualitative study was begun on the beam uniformity. To do this, a
short focal length lens was used to diverge the harmonic beam and the result-
ing enlargement was displayed on a white matte screen. A photograph of
the resulting pattern is shown in Figure 4. The appearance of the lattice-
like structure in the beam suggested a diffraction pattern formed by the 1-
cm. square KDP crystal. This was quite probable considering the crystal was
the limiting aperture of the system and the only element in the system hav-
ing a straight line periphery. Before this could be verified, experimental
effort On the program was stopped, During some of the final observations,
however, it appeared visually that structure other than the lattice work
pattern was present in the beam.
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:L ure 4. Expanded Beam of 0.53 Micron Rad_,ttion
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PAR 217
12 Oct 65
REEXAMINATION OF PROJECT GOALS
12. The activity described above took place between the project author-
ization in March 196+ and December 1964. The findings on this project and
on PAR 216, "Exposure of Photographic Material with Lasers,"4 together with
developments of new visible light lasers in other laboratories made it evident
that the goals of this project should be reexamined.
13. In the PAR 216, Final Report, it was condlued that "No experimental
evidence or theoretical prediction was found that a photographic emulsion
(acting as a detector) reacts any differently to coherent than to non-coherent
radiation, provided they are of the same approximate wavelength and energy
level. Photographic materials or the detection of laser-generated radiation
may be chosen by the same criteria as for the detector of other radiation of
the same wavelength and energy level." No further effort toward providing
data specifically on the use of the harmonic doubling laser with photographic
sensitized materials was required.
l4. Technical reports on the "theta-pinch" source for laser activation
after the time of the preparation of the Project Authorization Request in-
dicated it was an inefficient technique. An early minor effort on this pro-
ject indicated that the equipment available in our laboratory was not adequate
for high-repetitive-rate operation of the theta-pinch source and several
thousand dollars worth of additional equipment would be required to make it
adequate. In view of the discouraging reports in the literature, tests on
this manner of operation were abandoned.
15. During 1964, there was information appearing in the technical liter-
ature on the development of new visible-light lasers. It appeared desirable
to make an orderly study of this information to attempt theoretical compari-
sons of those lasers with the harmonic doubling green light system.
Final Report, PAR 216, 15 February 1965
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PAR 217
12 Oct 65
16. A "Restatement of Project Goals" was prepared and submitted to
the customer as an attachment to the Monthly Report of 22 January 1965,
Approval of the indicated redirection of effort was given. The new goals
were summarized as:
a. Attempt to learn the causes of the nonuniformity in the beam
and to discover means to make the beam from the harmonic-doubling laser
source uniform in brightness.
bo Conduct a literature and vendor search from June 1963 to the
present on visible light lasers and attempt to make a theoretical comparison
of their performance with that achieved by the harmonic-doubling system.
BEAM UNIFORMITY STUDY
17. During the period of reexamination of the project goals, the con-
tractor-owned glass laser and power pack were transferred to another project
and were not available for the beam uniformity study. This condition con-
tinued to exist as late as May 1965, and we were requested by the customer
to terminate this portion of the project. No further work on beam uniformity
was performed beyond that reported in paragraph 11.
LITERATURE SEARCH ON VISIBLE LIGHT LASERS
18. The initial phase of the literature search was concerned only with
the beam uniformity of visible light lasers. The search was begun by the
contractor's "Technical Information Service" group (library). The material
collected by this group covered most of the laser articles in the technical
journals from June 1963 to early 1965. The sources and journals surveyed
are shown in Appendix I. Although a large number of articles and letters
have been published on visible lasers, none contained any specific informa-
tion on beam structure.
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PAR 217
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19. In May 1965, the approach toward the literature search was reviewed,
and it was decided that before any additional searching was done a list of
existing laser materials should be compiled. This was done with the list be-
ing divided into two sections: gas lasers and solid material lasers, both
including operational wavelengths. These lists, shown in Figure 5 and 6,
do not contain all the operational and experimental laser materials as of
May 1965, They do, however, contain a representative majority.
20. The power output of various laser types can vary with a variety of
design parameters, therefore, a meaningful comparison of the output of various
visible light lasers must be obtained from manufacturer's data. However,
at the customer's request, the project activity was terminated without mak-
ing a vendor search for this type of information.
CONCLUSIONS
21. A high power pulsed source of 0.53 micron wavelength (blue-green)
coherent radiation has been achieved. The beam produced has a character-
istic lattice-like pattern (nonuniform brightness) which makes it unsatis-
factory as an exposing source in many potential photographic applications.
22. With a one-inch long KDP crystal having optimum orientation of
the crystal axes 4 x 10-3 joules of second harmonic (0.53 micron wavelength)
radiation was produced from 140 joules of 1.06 micron wavelength primary
radiation input. The conversion efficiency was 2.8 x 10-3%0, nearly equal
to that achieved by Terhune with a ruby laser primary source.. The pulse
time in these experiments was about 1 millisecond, therefore, the average
power output of 0.53 micron radiation was about 4 watts.
23. As a basis for comparison, consider the performance of several
He-Ne continuous operating gas lasers offered for sale by several manu-
facturers. Their power output at 0.6328 micron wavelength ranges from about
3 milliwatts to 50 milliwatts in various models. This output level is
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IN MICRONS
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PAR 217
12 OCT 65
Ruby
PULSED LASERS
(2) Nd3+ JY3AI502
(1) Dy2+
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-- Ca F2
C.W. LASERS
SOLID MATERIAL LASERS
FIGURE 5
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PAR 217
12 oct 65
less than the peak from the second harmonic. system, but in only one second
of operation the smallest units can deliver as much energy as the second
harmonic pulse achieved in our experiments.
24. The efficiency of the second harmonic technique for the generation
of visible coherent radiation increases with the power level of the input
radiation. Its use appears practical only with a high energy pulsed laser
as the input source.
RECOMMENDATIONS
25. It appears likely that convenient, moderate-priced laboratory laser
units for continuous operation with blue-green and green light output will
soon be available as commercial units comparable to the present He-Ne units.
The possible availability of such equipment should be explored with potential
suppliers,
26. Photographic materials for the detection of visible coherent radi-
ation should be selected by the same criteria as for detection of noncoherent
radiation of the same. wavelength and power level.
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PAR 217
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APPENDIX I
LITERATURE SEARCH ON VISIBLE LIGHT LASERS
SOURCES CONSULTED
ENGINEERING INDEX
INSTRUMENTATION ABSTRACTS
PHYSICS ABSTRACTS
IEEE INDEX
NASA BIBLIOGRAPHIES
STA REPORTS (Scientific and Technical Aerospace Reports)
JOURNALS
BRITISH JOURNAL OF APPLIED PHYSICS
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA
INDUSTRIAL RESEARCH MAGAZINE
ELECTRONICS
ELECTRONIC INDUSTRIES
APPLIED OPTICS
JAPANESE JOURNAL OF APPLIED PHYSICS
I1 NUOVA CINIENTO
BRITISH COMMUNICATIONS AND ELECTRONICS
MISSILES AND ROCKETS
SOLID STATE COMMUNICATIONS
PHYSICS LETTERS
NATURE
PHYSICAL REVIEW
PROCEEDINGS OF THE IEEE
MICROWAVES
LASER - Abstracts
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