Note: Descriptions are shown in the official language in which they were submitted.
2/1598~ `J PCT/US92/0l~X
La~R SY$TEMS
This invention w~s made with Government support
under Contract DAAH01-88-C-0481 awarded by the United
State~ Army Missile Com~and. The Government has certain
rights in thi6 invention.
This application i5 ~ continuation-in-part of
United States patent npplication sQrial number 07/463,577,
filed January ll, 1990, of Harold M. Ep~tein, Allan H.
Clauer, Boyd A. Mueller, Jeffrey L. Dulaney, Bernerd E.
Campbell, and Craig T. Walters, for Improving Material
Properties, now abandoned.
This application is also a continuation-in-part
of United States patent application serial number
07/626,587, filed December 7, 1990, of Harold M. Epstein
and Jeffrey L. Dulaney, for Unpolarized Laser Oscillators,
now United 5tates Patent 5,075,893, issued December 24,
1991 .
FIELD OF INVENTION
This invention relates to laser systems for
providing coherent radiation pulses at high power. It has
to do typically with high-power laser apparatus that is
useful in systems such as those employed for improving
properties of solid materials by providing shock waves
therein. Such laser systems are especially useful for
enhancing or creating desired physical properties such as
hardness, strength, fatigue strenqth, corrosion
resistance, etc, in metallic materials, and the
improvement of welds between metal surfaces, etc.
The invention comprises significant improvements
30 in the apparatus of United States Patent 3,850,698,
November 26, 1974, of Philip J. Mallozzi and Barry P.
Fairand, for Altering Material Properties. Another
related patent is United States Patent 4,401,477, August
30, 1983, of Allan ~. Clauer, Barry P. Fairand, Stephen C~
Ford, and Craig T. Walters, for Laser Shock Processing.
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W~92/1598~ 2~ 9~67~ -2- PCT/US92/~1548 ~
BACKGROUND OF ~ INVE~QN
As the Mallozzi and Fairand patent points out,
old methods for the shock processing of solid ~aterial~
typically involve the use of high explosive materials in
contact with the solid, or high explo~ive materials are
used to accelerate a plate that strikes the solid to
produce shock waves therein. Such methods have several
disadvantages. For example: (a) it i6 difficult and
costly to shock process non-planar surfaces and
lo complicated geometries, (b) storage and handling of the
high explosive materials pose a hazard, (c) the processes
are difficult to automate and thus fail to meet some
industrial needs, and (d) the high explosive materials
cannot be used in extreme environments such as high
temperatures and high vacuum.
Shot peening is another widely known and accepted
process for improving the fatigue, hardness, and corrosion
resistance properties of materials by impact treatment of
their surfaces. In shot peening, many small shot or beads
are thrown at high speed against the surface of a
material. The shot or beads sometime6 escape from the
treatment equipment and scatter in the surrounding area.
Since particles might get into surrounding machinery and
cause damage, shot peening usually cannot be used in a
manufacturing line. Ordinarily it cannot be used on
machined surfaces without damaging them.
Laser shock processing equipment, however, can
fit right into manufacturing lines without danger to
surrounding equipment. It is also readily adaptable to
automatic control, making it further attractive for
production line applications. It can be used on machined
surfaces of harder metals and alloys with no da~age to the
surfaces.
The interaction of a pulsed laser beam with the
surface of a material gives rise to a pressure pulse
(shock wave) that propagates into the material and changes
its properties. In the case of metals, for example, the
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WO92/15985 PCT/U~92/01~8
changes in properties are caused by the introduction o~
cold work that increases the hardnes~ and stren,gth of the
material. By appropriate tailoring of the p~ak pressure
and width of the shock wave, it is possible to enhance
selected material properties, such as fatigue strength,
and at the same time not adver~ely affect other
properties, such as corrosion resiQtance. It is po6sible
also to shock process a finished piece of material without
disturbing its surface, where a thin sacrificial layer of
overlay material has been attached intimately onto the
surface of the workpiece.
Shock processing with coherent radiation has
several advantage6 over what has been done before. For
example: (a) The ~ource of the radiation is highly
controllable and reproducible. (b) The radiation i8
easily focused on preselected surface areas and the
operating mode i~ easily changed. This allows flexibility
in the desired shocking pressure and careful control over
the workpiece area to be 6hocked. (c) Workpieces immersed
in hostile environments 6uch as high temperature and high
vacuum can be shock processed. (d) It is easy to shock
the workpiece repetitively. This is desirable where it is
possible to enhance material properties in a ~tepwise
fashion. Shocking the workpiece several times at low
pressures can avoid gross deformation and 6pallation of
the workpiece. (e) The prQcess is readily amenable to
automation. (f) Nonplanar workpieces can be shock
processed without the need of elaborate and costly shock
focusing schemes.
Many publications have dealt with the use of
lasers to provide stress waves in solids. Several such
publications are cited and discussed in the first parent
application, identified above, of which this is a
continuation-in-part.
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WO9~/15985 2 ~ 0~',7 :~ -4- PCT/U592/01548
~ISCLOSURE OF THÆ ~vENT~Q~
In typical apparatus according to the pre~ent
invention for improving properties of a solid material by
providing shock waves therein, a laser oscillator lOa
provide~ a plurality of pulseg 112 of coherent radiation.
The leading edge of each pulse is sharpened either by a
metal foil 18 (Figure 1) or by phase conjugation
reflection means 18a (Figure 6) or 18e (Figure 11
including a stimulated Brillouin scattering (SBS) cell
o 18d,18e and optionally a Faraday isolator 18b.
Each pulse is directed from a preamplifier 20a
via a mirror 129, a retarder plate 130, lenses 131,132, a
mirror 133, and an iris 134, as a beam 112 having a
predetermined diameter, onto an amplifier 123 compri~ing
first and second laser amplifier rods 23a,23b in series.
At least a major portion of the radiation 112 amplified by
the first amplifier rod 23a i8 directed to the ~econd
amplifier rod 23b, where it is amplified and then directed
to a surface of the solid material 11 (Figure 1).
Substantially uniform ~patial amplitude is
achieved in the radiation 112 in at least one of these
ways: A pair of flashlamp6 70,71 tFigure 7) are included
with each laser amplifier rod 23a,23b for pumping the rod,
the axis of each flashlamp 70,71 and the axis of the fir~t
rod 23a are substantially parallel to each other and
substantially in the same horizontal plane; and the axis
of the second rod 23b and the parallel axes of its
associated flashlamps 70,71, continuing in approximately
the same direction as the axis of the first amplifier rod
23a, define a vertical plane; and/or the oscillator loc
(Figures 12-14~ provides a beam of coherent radiation 12T
that is not polarized and in which each succeeding pulse
is, in the spatial amplitude pattern of the beam,
substantially a mirror image of the pattern in the pulse
that preceded it (Figure 12-13), and/or is rotated about
its axis by a predetermined smaller angle from the pattern
in the pulse that preceded it tFigures 14-15).
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wos~/lsg8~ PCT/US92/015~X
f.` - ;
ERIEE DESCRI~ON ~F ?HE_~R~Ih9~
Figure 1 is a schematic view illustrating typical
embodiments of the pre~ent invention.
Figure 2 i~ a schematic plan view of typical
apparatus for providing rapid movement of a metallic film
as required in ~ome embodl~ent~ of the present invention.
Figure 3 is a schematic sectional view taken in
t~e plane 3-3 of Figure 2.
Figure 4 i8 a schematic end view of typical
apparatus as in Figures 2 and 3.
Figure 5 i~ an oscilloscope trace showing the
average relative intensity (ordinate Y) again~t time, nsec
(absci~sa X) of a typical pulse of radiation for providing
chock waves according to the invention.
Fiqure 6 is a schematic plan view of typical
apparatus according to the invention that includes some
useful features in addition to those disclosed in Figure 1
and the description thereof.
Figure 7 is an isometric view, with a middle
portion cut away to show a cross-section, of a typical
preamplifier in apparatus a6 in Figure 6.
Figure 8 is a graph of spectral irradiance
(~Wtcm2-nm at 50 cm) (ordinate Y) against wavelength (nm) ~ `
(abscissa X) for typical krypton flashlamps suitable ~or
use in apparatus as in Figure 6. Figure 8 indicate6 a
typical krypton lamp spectrum at 0.6 mm bore, 50.89 cm
arc, and 1.3 kW input power.
Figure 9 is a graph of spectral irradiance
(xlO 7 J/cm2-nm at 50 cm) (ordinate Y) against wavelength
(nm) ~abscissa X) for typical xenon flashlamps suitable
for use in apparatus as in Figure 6. Figure 9 indicates
flashlamps at low power density (E:TA Ratio = 2900) with
the detector 50 cm from lamp, viewing normal to lamp axis,
and Xenon, 390 Torr.
Figure 10 is a graph of relative energy density
(ordinate Y) against normalized radial position (r/R)
(abscissa X) for typical useful conditions in apparatus as
.
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W092/1598~ 21 !3 67 3 -6- PC~/U592/01548
in Figure 6. Figure lO indicates the radial energy
storage distributio~ at 15~m radius and 2 wt~ Nd.
Figure ll is a schematic plan view of alternative
laser oscillator circuitry, including pulRe 6harpening
means, that may replace part of the apparatus in Figure 6.
Figure 12 iB ~ schematic plan view illustrating
typical unpolarized laser o~cillators according to the
present invention.
Figure 13 is a similar view of part of the
oscillators as in Figure 12, illustrating an advantageous
charac~eristic thereof.
Figure 14 is a schematic plan view illustrating
typical embodi~ents of oscillators that are generally
similar to those of Figure 12 and include additional
devices to provide another ~dvantageous characteristic.
Figure 15 i6 a schematic view of an alternative
device for one of the devices in Figure 14.
CARRYING OU$ THE INVE~TION
Referring now to Figure l, typical apparatus lO
is shown, suitable for practicing the present invention
for improving properties of a metallic material in a
target l~ by-providing shock waves therein, and wherein
there are directed to the surface of the material ll a
plurality of pulses of coherent radiation 12 having
average energy fluence of at least about lO Joules per
square centimeter and rise time of not longer than about 5
nanoseconds within a fluorescence envelope lasting about
0.5 to 5 milliseconds, at a rate of about l radiation
pulse per lOO to 200 microseconds.
In apparatus lO for practicing the present
invention, the co~ponents lS'-l9' shown inside the dashed
polygon lO' are optional. Embodiments that do not include
the~e components will be described first.
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WOs~/l598~ 7 23 ) ,~ j PCT/U592/~lS48
Mul~iple Pulsest Fast Fil~_~ove~en~
The coherent radiation 12 is generated by ~n
oscillator 13-17 comprising a rear mirror 1~, a laser pump
cavity 14, a polarizer 15, a pockels cell 16, and an
output coupler 17. The laser pump cavity 14 comprises a
gain medium, such a6 a neodymium-glass la~er rod, pumped
by flashlamps that are driven at regular interval~ of
about 0.1 to 10 seconds by a pulse forming network (PFN).
One such laser pump cavity 14 that has been used
conveniently in the apparatus 10 comprises the following
components manufactured by Xigre, Inc. of Hilton ~ead,
South Carolina:
FC-500/2 Laser Cavity, 8" arc length
3/8 by 7.5" Q-98 Laser rod, 3% doping level, ends
1 degree opposed, antireflection coated at
both ends
Two Fluid-cooled Fla6hlamps
Model 883 Controller with integral 330 watt Power
Supply
20 Closed Cycle Cooling System
The oscillator 13-17 provides an approximately
rectangular fluorescence envelope lasting about 0.1 to 5
milliseconds. The ~oherent radiation 12 from the laser
pump cavity 14 is linearly polarized. The polarizer 15
breaks the radiation 12 down into two linearly polarized
orthogonal components; one of which (component B) it
reflects away as indicated at 12B; and the other
(component A) it transmits on, as indicated at 12A, to the
pockels cell 16.
With a proper potential present across it (about
3,300 volts for a cell of transverse deuterated pota~sium
dihydrogen phosphate), the pockels cell 16 retards the
coherent radiation 12A one-fourth wavelength (90 degrees)
while transmitting it on to the output coupler 17, which
reflects about one-half of it back toward the polari7er
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W092/lS98~ 7 ~ -8- PCT/US92/01548
15. The reflected energy proceeds back through the
pockel6 cell 16 with a further retardation of one-fourth
wavelength (90 degrees). So the b~ck radiation i8 one-
half wavelength (180 degrees) out of phase with the
forward radiation of component A, thus having the oppo~ite
polarization (B), and it is reflected away by the
polarizer 15, as indicated at 12C, so as not to return to
the laser pump cavity 14. Thu~, laser energy bu~ld6 up
and i8 stored in the laser rod of the pump cavity 14,
because oscillations cannot occur.
After at l~ast about 100 microseconds, the
potential across the pockels cell 16 is reduced to zero,
typically by shorting it to ground, for about 1 to 5
microseconds; and the shorting of the pockels cell 16 is
repeated at intervals of about 100 to 200 micro6econds
thereafter. While it i8 shorted, the pockel~ cell 16 does
not retard the radiation 12, and the oscillator 13-17
produces about 2 to 50 laser pulse6 12 in each
fluorescence envelope, with sufficient time between pulse6
for stored energy to build up in the laser rod, while
keeping fluorescent losses to a minimum.
The output coupler 17 comprises a partially
reflective mirror that transmits about half of the energy
in each pulse 12 on to the pulse Rharpener 18 comprising a
coating of aluminum about 150 to 5,000 angstroms thick on
a supporting film that is substantially transparent and
thin enough to be non-distorting to the radiation
wavefront. The supporting film typically comprises a
strong polyester material such as oriented, at least
partially crystalline, polyethylene terephthalate, about 1
to 40 micrometers thick. One such material that we have
used is Mylar, a product of E.I. du Pont de Nemours &
Company. Mylar is birefringent, and its optical axis
should be oriented to correspond with the polarization of
the polarizers 15 and 15'.
The radiation pulse 12 strikes the aluminum film
18, typically vaporizing an area of about 0.1 to 0.2
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W O 92/1598~ PC~r/US92/01548
9 _ .
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square millimeters of the film in about 0.1 to 3
nanoseconds, after which the area of vaporization
typically expand6 to about 1 to 1,000 square millimeter~
in about 2 to 10 nanoseconds. This sharpen~ the leading
edge of the radiation pul~e 12 passing through the hole
where the film 18 has been vaporized away, and the
modified pulse 12 is directed to a preamplifier 20. Where
necessary or convenient, planar mirrors l9 may be included
in the path of the radiation 12 to change or adjust the
direction of the beam of radiation 12.
The preamplifier 20, which may be (and typically
is) similar to the laser pump cavity 14, amplifies the
radiation pulse 12, typically by about 3 to 10 decibel6,
and the amplified radiation 12 proceeds by way of a
telescope, typically comprising a negative leni 21 and a
positive lens 22, to an amplifier 23, which typically
further amplifie6 the radiation pulse 12 by about 5 to 15
decibels. One amplifier 23 that has been used
conveniently in the apparatus 10 comprises the following
components manufactured by Xigre, Inc. of Hilton Head,
South Carolina:
.
Power Amplifier AYsembly, FA-1000/2
27 mm dia. x 810 mm long Q-89 Laser Rod
(described hereinafter)
; 25 Two Fluid-cooled Lampis, 63 cm arc length
Model 886-2 Power Supply compatible with 883
Controller
Dual PFN assemblies
Coolant-to-water Cooling System
The amplified radiation pulse 12 is focussed by a
positive lens 24 onto a desired area of the surface 25 of
the target 11, to provide an average energy fluence
therein typically of at least about 10 (and preferably
about 10 to 500) Joules per square centimeter, and an
average power flux on the target of at least about 107
.
W092~sg8~ ?~ 10- PCT/US92/0154~
(and preferably about lO9 to 10ll) watts per Rquare
centimeter, with pulRe lengths typically of about 10 to
1,000 nanoseconds. The maximum power flux will be li~ited
by the formation of ~ reflecting plasma at the target
surface. This maximum power flux will increa~e as the
laser wavelength decreases. For example for a laser
wavelength of 0.53 micrometer the maximum power flux will
be approximately four times that for a wavelength of 1.06
micrometers.
A portion of the output 12 of the amplifier 23,
typically about 10 percent, may be directed by a beam
splitter 37 and mirror~ 39 to a second similar a~plifier
23' to provide a second amplified radiation pulse 12',
focused by a positive lens 24' onto a desired area of the
surface 25 of the target ll simultaneously with the pulse
12 from the amplifier 23. In the same way, a portion of
the output 12' from the ~mplifier 23' may be directed by a
beam splitter 37' etc, and any convenient number of
amplifiers may be employed similarly to provide additional
pulses to the target 11.
Typically pulses from the different amplifiers
are directed to the same area on the surface 25 of the
target 11, to overlapping areas on the surface of the
target 11, and/or to areaR th~t are on opposite surfaces
of the target 11. When beams are directed onto an
overlapping area on the same surface, the optical pulses
should arrive on the target ~ubstantially simultaneously
so that the effective rise time of the overlapped pulses
is equivalent to the rise time of the individual pulse6.
This requires equivalent optical paths for all beam lines,
which may be accomplished by providing additional optical
path length for each beam that would otherwise arrive at
the target ahead of another beam.
The aluminum foil 18 blocks the path of the
radiation beam 12 briefly, but is rapidly vaporized by the
radiation beam 12, f~rst in a minute region, then rapidly
spreading outward from this region until the foil is
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W092/15985 2~ j7;) PCI/U592/01548
vaporized over the entire area in the path of the beam.
This action provides an extremely ~harp leading edge in
each radiation pul~e 12. Thus, the ~witching by the foil
18 is sp~tially transverse. An individual area of the
foil 18 switches from reflectinq to transmitting in the
time required for the foil vapor to expand to about three
times its original volume. At the boiling point of
aluminum, 2057 Celsius, the vapor expand~ at a velocity of
about 1200 meters per second. For a typical commercially
available film comprising a layer of aluminum
approximately 300 angstroms thick on a Mylar supporting
film about 10 micrometer~ thick, the irradiated aluminum
triples its volume in about 48 picoseconds. Thus, the
switching time at any given point within the irradiated
area is about 100 times faster than the time of ~witchi~g
averaged over the entire area.
In Figure 5, which shows an oscilloscope trace of
average relative intensity for a typical radiation pulse
12 applied to the target 11, the ri6e time of the pulse 12
(about 20 nanoseconds) is the time taken by the vaporized
area of the aluminum film 18 to expand to the full area of
the laser beam 12. The actual time for a small area of
the foil to switch from reflecting to transmitting is
about a factor of 100 shorter (about 0.2 nano econd).
Relevant phenomena are described in a conference
paper of the Society of Photo-optical Instr~mentation
Engineers of August 1976:
M. A. Duguay, M. A. Palmer, and R. E. Palmer,
Laser Driven Subnanosecond Blast Shutter, Proc.
SPIE, Vol. 94, High Speed Optical Techniques, pp.
2-6.
The abstract states: The opening time was
measured of the so called "blast shutter", presently used
as an isolator in high power laser systems. The shutter
consists of a 275 angstrom thick aluminum film deposited
.
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WO92/l598~ PCT/US92/0154~ ~
~ 7 0 -12- ~ ,
on a tran~parent plastic film 12 mi~rometer~ thick. The
aluminum film is suddenly removed by exposing lt to high
power pulse6 from a neodymiu~:glas~ laser (A ~ 1.06
micrometers). Pulse~ of 50 picoseconds in duration were
used that delivered betw~en 0.5 and 3 joules per square
centimeter of energy onto the alu~inum film. The absorbed
energy superheats the film and turns it into a rapidly
expanding cloud of aluminum vapor. A blue laser beam from
a continuous wave argon laser passe6 through the shutter
and is detected by an ultrafast streak camera, set for 50
picosecond resolution. With this arrangement 10 to 90
percent shutter opening times varying from 0.8 to 4
nanoseconds have been measured, the former occurring at
laser pulse energy densities of 2 joules per square
centimeter.
The isolator referred to in the abstract quot~d
above i8 described in a United States Patent of 1977:
Mallozzi et al, United States Patent 4,002,403,
January 11, 1977; Suppressing Superradiance.
Typically, the foil used therein comprises essentially
aluminum, gold, silver, platinum, copper, or lead, about
100 to 1000 angstroms thick; typically comprising a
coating on a transparent support that comprises
essentially glass, quartz, polyethylene terephthalate, or
other transparent plastic, and that preferably is less
than about 10 wavelengths thick. ~hese materials, and
other materials appropriately adapted, may be used in the
pulse sharpeners 18, 18' of the present invention.
Any metal film that allows essentially no laser
light tranemission through a few hundred Angstrom film
thickness is usable as a pulse sharpening foil. ~he main
advantage of using aluminum is that thin plastic film
coated with a few hundred Angstroms of aluminum is readily
available and is inexpensive. It is used extensively in
greeting cards. In addition the surface of the aluminum
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wos2/l59~ -13- PCT/US92/0154X
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is not adversely affected if oxidation occurs. The
surface of copper and silver films are deleteriously
affected by oxidation. Th~ fil~ of aluminu~ on plastic
used in reference M, abovQ, i~ the sa~e as the film u~ed
in reference N, above, and in the examples of the present
invention.
of cour~e the film 18 must be moved rapidly to
provide a different region of the film, not already
vaporized, in the path of the radiation 12 before the next
lo pulse is generated. A typical beam 12 from the oscillator
13-17 has a diameter of about 1 centimeter, 60 the
aluminum film 18 must be moved across the path of the beam
12 at a speed of about 50 to 100 meters per second to move
the vaporized area out of the path of the beam in about
100 to 200 microsecond~. However, the beam can
conveniently be focussed to a diameter of about 0.3
centimeter before it impinges on the aluminum film 18,
reducing the required speed to a~out 15 to 30 meters per
second. Further concentration of the beam 12 might not be
desirable, because the power flux density might then
exceed the threshold above which an aluminum plasma is
formed that would itself block the laser beam 12. This
can be avoided by reducing the output power from the
oscillator 13-17 and increasing the gain of the
preamplifier 20 enough to provide the required power input
level to the final amplifier 23.
Figures 2, 3, and 4 illustrate schematically a
suitable device for moving the film 18 at the required
speed.
A cylindrical drum 41 rotates about its axis 42
at a speed high enough to provide the required rate of
movement in the film 18 mounted just inside the periphery
of the drum 41, between a supply spool 43 and a drive
spool 44. During part of each revolution of the drum 41,
as indicated by the arrow 45, the slots 46, 47 at opposite
sides of the drum 41 cross the path of the radiation
pulses 12; so that several pulses 12 in succession proceed
W092/15985 2.'~ 14- PCI/US')2/~2s48
in through the entry slot 46, strike the aluminum fil~ 18,
vaporizing it, and continue out from the exit ~lot 47 and
on to the mirrors 19, eto, as ~hown in Figure 1.
At the appropri~te time, just be~ore the slots
46, 47 begin to cross the path of the la6er radiation 12,
any suitable actuating device (not ~hown) i6 triggered by
an actuating component 48 mounted on the drum 41. For
example, the triggering component 48 may comprise a mirror
that reflects a beam of light to a photodetector to
provide a timing pulse that fires the flashlamps in the
laser pump cavity 14 when the slots 46, 47 begin to cross
the path of the laser beam 12. Then 6everal laser pulses
12 in succession ~trike the aluminum film 18, each in a
different area of the fil~ 18, and proceed as described
above in connection with Figure 1.
The process is repeated periodically, typically
about every 0.1 to 10 seconds, with the actuating device
automatically turned off in the interim. Meanwhile the
film 18 is advanced by the drive spool 44 to move a new
region of the film 18 in llne with the entry slot 46.
Alternatively, the film 18 can be advanced continuously at
a slower rate by the drive spool 44 to provide a different
portion of the film'6 surface for each serie6 of pulse6
12. Typically a roll of aluminized Mylar film
approximately 6 inches wide and 1000 feet long can provide
the pulse sharpening for about lOo,OOO to 1,000,000 laser
pulses 12.
Two Pulses. Slow FiAlm Movement
Other typical embodi~ents of the present
invention, which do not require special fast means for
moving the film, also are illustrated in Figure 1. In
such embodiments the components enclosed within the dashed
polygon 10' are included in the apparatus 10.
The portion of the apparatus 10 already described
above provides one properly sharpened pulse in the same
manner as is described above. However the pockels cell 16
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W092/1~8~ -15- PCT~US92/01548
2 ~ , . i J ,~ ~
is fihorted only once, typically ab~ut 200 ~icro~econd~
after the firing of the fl~h lamps in the laser pump
cavity 14; so only one pulse 12 of coherent radiation of
component A i8 provided to the target 11 within each
fluorescence envelope. Also the pulse 12 proceeds from
the pulse harpener 18 through a second polarizer 15'
~efore being directed to the preamplifier 20 by the
mirrors 19.
The laser pump cavity 14, the polarizer 15, the
lo pockels cell 16', and the output coupler 17~ form a second
oscillator 14-17' ~y means of which the other radiation
component 12B provides a single sharpened pulse 12' by way
of the pulse sharpener 18', the mirror 19' and the
polarizer 15', which reflects the component B radiation to
the first mirror 19; and from there the path of the
radiation 12 is the same as that of the component 12A.
The operation of the second oscillator 13, 14,
15, 16', 17' is similar to that of the first oscillator
13-17. The second oscillator 13-17' also provides an
approximately rectangular fluorescence envelope lasting
about 0.1 to 5 milliseconds, and the coherent radiation 12
from the laser pump cavity 14 is linearly polarized. The
polarizer 15 breaks the radiation 12 down into two
linearly polarized orthogonal components; one of which
(component B) it reflects, as indicated at 12B, to the
pockels cell 16'. The other (component A) it transmits
on, as indicated at 12A, to the pockels cell 16.
With a proper potential present across it (about
3,300 volts for a cell of transverse deuterated potassium
dihydrogen phosphate), the pockels cell 16' retards the
coherent radiation 12B one-fourth wavelength (90 degrees)
while transmitting it on to the output coupler 17', which
reflects about one-half of it back toward the polarizer
15. The reflected energy proceeds back through the
pockels cell 16' with a further retardation of one-fourth
wavelength (90 degrees). So the back radiation is one-
half wavelength (180 de~rees) out of phase with the
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w092/~598~ ~ , 16 PCI/US92/Ot548
2 3 3 6 7 ~
forward radiation of component B, thus having the opposite
polarization (A), and it i8 transmitted away through the
polarizer 15, as indicated at 12C, BO as not to return to
the laser pump cavity 14. Thus, there can be no
06cillation and laser energy builds up and i8 stored in
the laser rod of the pump cavity 14.
At the desired predetermined instant, the
potential across the pockels cell 16' is reduced to zero,
typically by shorting it to ground, for about 1 to S
microseconds. While it is shorted, the pockels cell 16'
does not retard the radiation 12B, and the oscillator 13-
17' produces a second laser pulse 12' in each ~luore6cence
envelope.
; Typically the pockels cell 16' is shorted about
lSO microseconds later than is the pockels cell 16, so
that the second radiation pulse 12 will strike the target
11 about 150 microseconds after the first pulse 12. The
polarizer lS', the pockels cell 16', the output coupler
17', the pulse sharpener 18', and the mirror 19' typically
are substantially identical to the correspondingly
numbered items lS-19, respectively.
Thus the apparatus 10 including the components
enclosed within the dashed polygon 10' provides two
radiation pulses 12 to the target 11 for each fluorescent
envelope.
The process is repeated periodically, typically
about every 0.1 to 10 seconds. To provide a different
portion of their surfaces acros~ the path of each pulse of
radiation 12, the films 18, 18' need to move only about 1
centimeter per second. Movement at such a speed can be
provided easily by any suitable conventional or special
means. The movement may be either continuous or
intermittent.
As described above, there is no oscillation (and
3S laser energy builds up and is stored in the laser rod of
the pump cavity 14) throughout each fluorescence envelope,
except while one or the other pockels cell 16 or 16' is
- - , . ~ ~ - ,
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.
.
. . ..
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WQ~2/ 1 59B~ --17 - PCT/ US92/0 1548
f;fl 3
shorted; because all of the back radiation is directed
away from the pump cavity 14 by the polarizer 15, in the
direction 12C. The back radiation through the pockels
cell 16 is reflected by the polarizer 15 along the path
12C, and the back radiation through the pockels cell 16'
is transmitted through the polariz~r lS along the path
12C.
Except during the pulees of oscillation, the
radiation that passe6 through the output coupler6 17,17'
to their respective pulse sharpeners 18,18~ is f~r tos
weak to vaporize the metal foil, and it remains intact.
Hiqh Efficie~cy
In shock treatment according to the present
invention, each individual pulse of radiation must have
enough energy to exceed a particular threshold so ac to
produce the shock wave~, and the rate at which the solid
material can be treated is approximately proportional to
the average power of the radiation pulses. For high
efficiency and optimum results, the laser and associated
apparatus that provide the coherent radiation to the solid
material should produce a high ratio of output energy to
heat in the laser ~edium. This ratio depends on the
extraction efficiency of the stored energy And on the
fraction of energy stored at the time of Q-switching by
the pockels cell. The extraction efficiency, in the
region of interest, is a monotonically increasing function
of the ratio of the output fluence to the saturation
parameter, S. S = hL where hv is the laser photon energy
and OL iS the cross section of the stimulated emission-
Since the output fluence in a single pulse is limited bythe damage threshold of the laser glass, the energy
extraction in a single pulse is limited. For 8 Joules per
square centimeter output fluence, the efficiency has been
calculated to be about 40 percent for a typical phosphate
glass. However, this limitation on extraction efficiency
.. . .. ~ . ; ,, , ~ . .. .. -
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.
. ~: ' ' " ~ ' ' ' . : .
.- . . . .
W 0 92/lS98` 2~ 18- P(-r/US9~/01548
can be circumv~nted by providing two or more laser pul3es
within a single fluorescence pulse envelope.
The efficiency can be improved by making the
pumping pulse width short compared to the fluorescence
lifetime. However, this require~ ~uch a high power
loading in the flashlamp~ that the resulting spectrum i~
too far into the ultraviolet region to pump the laser
efficiently, and the lamp lifetime is ~reatly shortened.
It is preferable to circumvent the efficiency 108E3 due to
incomplete storage of energy at the time of Q-switching by
extracting more than one Q-switched pulse fro~ a ~ingle
fluorescence envelope. Typically about half of the total
energy pumped into the upper laser level is 6tored at the
time of Q-switching. The remainder of the energy i~
either pumped into the upper laser level after Q-switching
or i8 lost by fluorescent decay before Q-switching. ~hus,
multiple pulses under a single fluorescence envelope,
where the time between pulses is about 100 to 200
microseconds, can produce a substantial improvement in
efficiency. Typically the efficiency of the system can be
increased from about 2 percent with a single pulse to
about 6 percent with a plurality o~ pulses.
~plifier Materials
.
As mentioned above, the rate at which metallic
material can be treated according to the present invention
is approximately proportional to the average power of the
radiation pulses that produce the shock waves, provided
each individual pulse has enough energy to exceed a
particular threshold. We have found that the rate of
production according to the invention can be increased
approximately five fold by using a recently developed type
of strengthened phosphate laser glass in the amplifier 23
that delivers the radiation pulses to the target 11.
At first blush, it may seem obvious to use
strengthened glass in this manner. However, strengthened
laser glasses have been available for more than a decade
.
.. . . .
.
- ~ , . . .
-' - :. - :,
. ~ , : , . :
'' - ' .
WO.~Z/l5985 19 PCI/U59~/UI~ ~
2 ~ ~ 3 ~ 7~
without finding widesprQad use in laser system~ requiring
high average power. In most l~ser application~ the
brightness and coherence of the laser beam are of prime
importance, and both of them degrade at high average power
to the extent that the laser beam i8 uBeless for such
applications even before the fracture stre~s i8 reached
for ordinary unstrengthened glas~. Moreover, the silicate
glasses that have been available in strengthened form for
many years have such high temperature coeffiaients of
index of refraction and such high stress coefficients of
index of refraction that the strengthened glasses produce
no more average power than do the unstrengthened gla6ses.
Also the coefficient of gain in silicate glas i6 only
about two-thirds that of phosphate glass, making it
; 15 unsuitable for applications requiring high power.
Heretofore, the main reason for using
strengthened glass has been that it is less likely to
-; break during shipping or other handling. ~he greatly
increa~ed rates of production obtainable by using a
strengthened phosphate glass amplifier in shock proce~sing
` according to the present invention were unexpected and far
~` from obvious.
: Ion-exchange-strengthened phosphate glas6 became
available around 1985. It has an average power capability
about five times as great as that of the older phosphate
; glass. The temperature coefficient of index of refraction
tends to cancel the density coefficient and the resultant
change in index of refraction with temperature is much
lower than for silicate glasses, so it is possible to
obtain higher output power, approximately proportional to
the greater strength of the glass. Because of the special
glass composition that is required for the strengthening,
however, the efficiency of the strengthened glass is only
about 75 percent of the efficiency of the equivalent
unstrçngthened glass. It is only because the present
hock treatment does not require a radiation beam of high
briqhtness, and because the ~ixed polarization re6ulting
.. . ., ~ .. . ..... .. .. .. . .. . . . .
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. .
, . . .
WO92~159X~ PCT/US92/01548
2~n3~ 20- ~ ,
from stress birefringence does not have any adver~e
affect, that the strength~ned phosphate gla66 can provide
significant improvements in average power such as to yield
much higher rates of production in the shock proces6 of
the present invention.
A laser glass that has been used to advantage in
practicing the prssent invention i8 Xigre Q-89
strengthened phosphate la~er glas6. Q-89 is
~ trengthened phosphate la~er glas6 that combines the high
gain and high damage-threshold properties of the pho~phate
glasses with the high strength and durability
characteristics of the silicate glasse~. Extensive
research efforts of Kigre, Inc. in conjunction with the
University of Rochester have resulted in the development
lS of a laser glass composition combined with an ion-exchange
proces6 that results in an increase in rupture strength by
factors of 5 and 6 over conventional pho~phate glAsses.
The end result i6 a new laser material which promi~es to
provide a breakthrough in the capability of laser glas6 to
provide high average power.
The manufacturer, Kigre, Inc. of Hilton Head,
South Carolina, lists the following properties of Q-89:
8pectroscopio Prop-rt$-~
Peak Wavelength (nm) . . . . . . . 1054
Cross Section (xlO~~cm2) . . . . . . 3.8
Fluorescent Lifetime (usec) . . . . 350
Radiative Lifetime (usec) . . . . . 308
Linewidth (nm) FWH . . . . . . . . 21.2
Loss Q Lasing Wavelength (% ~ cm~1) 0.08
optical Proporti-s
Index of Refraction (Nd) . . . . . l.559
Abbe. No. . . . . . . . . . . . 63.6
, ~ - . . ........ , ... -
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..... . . :
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W~ ~)2/1598~ PCl~US92~01548 .
1 -21- :
:` :
~h-r~l Prop-rt~-~
Transformation Point C . . . . . . 495
T~ermal Expansion (lO 7/oC) (20-40C) 93
Thermal Conductivity (W/m X) . . 0.82
Physic~l Properti~o
Density (gm/cc) . . . . . . . . . 3.14
,
Solari~tion a~ t-n~e
. . . . . . . . . Excellent
Further information about Q-89 glass is brought
out in a paper published in 1988:
K. A. Cerqua, M. J. Shoup III, D. L. Smith, S. D.
Jacobs, and J. H. Kelly, Strenqthened phosphate
glass in a high rep rate active-mirror amplifier
geometry, Applied Optics, Vol. 27, No. 12, 15
June 1988, pp. 2567-2572.
The abstract states: Ion-exchange strengthened
phosphate glass in an active-mirror geometry remained
un~ractured at pump power levels 3 times the average pump
fracture limit of un trengthened pho~phate glass in the
same geometry. In addition, pretreatment and
posttreatment measurements of surface wavefront and
roughness were made on a set of rectangular substrates to
quantify any ion-exchange-induced surface modifications.
Experimental measurements of treatment-induced wavefront
deformation of strengthened blocks were shown to be less
than modeled values of distortion attributable to extended
treatment times.
The solid target 11 typically comprises at least
one metal, alloy, intermetallic compound, or other
metallic material. Some typical target materials for
which the present invention is especially useful are
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W092/l598~ 3 ~ 7 J -22- PCT/US92/01s4~ r
silver, copper, magne6ium, alumi~um, cadmium, zinc, iron,
nickel, and titanium.
Typically a layer 26 of ~olid or liquid overlay
material is attached to a surface 25 of the target 11, and
the radiation pulse 12 i~ directed to the layer 26 of
overlay material. The thickness of the target 11 plu8 any
overlay ~6 that is absorbent to the radiation 12
preferably i5 at least about two micrometers greater than
the mean free path of the radiation 12 therein. The
target 11 preferably i8 mounted against a substantially
larger solid support member 31 or is rigidly held by a
fixture, either of which is rigidly attached to a table or
other large fixed object.
Overlays may be applied over the surface of the
target 11 being treated. The~e overlay materials may be
of two types, one transparent to the laser radiation and
one opaque to the laser radiation. They may be used
either alone or in combination with each other; but it is
preferred that they be u~ed in combination, with the
overlay 26 directly on the surface 2S of the target 11
being opaque and the outer overlay 30 or 27 being
transparent.
The layer of overlay material 26 should be
attached securely over the surface 25 of the target 11 so
as to be in intimate surface contact throughout the area
to be radiated. Where some or all of the overlay material
comprises a liquid, a~ at 27, it may be held ~ithin an
enclosure 28, of which at least the front portion 29
preferably is transparent to the radiation 12, or it may
flow over the area to be treated without restriction by an
enclosure. Where a liquid transparent overlay 27 is used,
the solid transparent overlay 30 may be omitted, if
desired. Where only the solid transparent overlay 30 is
desired, the liguid 27 and the enclosure 28 may be
omitted.
Various typical and preferred kinds of overlay
and target materials and combinations thereof are
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W092t1598~ -23~ PCT/US92tO1~8
discussed in the first above-mentioned parent appllcation.
Also included in the specification are examples showing
some of the advantage6 of the present invention.
Embodiments of Fia~res 6 and 11
Referring now to Figure 6, typical iapp~r~tus
according to the present invention, ~or improving
properties of a ~olid material by providing shock waves
therein, includes means, such a6 a laser 06cillator lOa,
for providing a plurality of pulse6 112 of coherent
radiation; and means 18a for sharpening the leading edge
of each pulse 112.
The laser oscillator lOa comprises a rear mirror
13a, typically a 100 percent reflective spherical mirror;
a Q-switch 16a, typically a pockels cell; a polarizer l5a;
a laser pump cavity 14a; and an output coupler 17a.
The laser pump cavity 14a typically comprises a
gain medium, such as a neodymium-glass laser rod, pumped
by flashlamps that are driven at regular intervals of
about 0.1 to 10 seconds by a pulse forming network (PFN).
One such laser pump cavity 14a that has been used
conveniently in apparatus as in Figure 6 comprises the
following components manufactured by Kigre, Inc. of Hilton
Head, South Carolina:
FC-500/2 Laser Cavity, 8H arc length
25 3/8 by 7.5" Q-98 Laser rod, 3% doping level, endc
1 degree opposed, antireflection coated at
both ends.
Two Fluid-cooled Flashlamps
Model 883 Controller with integral 330 watt Power
Supply
Closed Cycle Cooling System
The oscillator lOa provide~ an approximately
rectangular fluorescence envelope lasting about o.l to S
milliseconds. The coherent radiation 112 from the laser
,
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WO92/1598~ PCT/US92/0154X
2~ ~357~ -24~
pump cavity 14a is linearly polarized. The polarizer l5a
breaks the radiation 112 down into two linearly polarized
orthogonal components; one of which it reflectR ~way; and
the other it transmits on betw~en the pump cavity 14a and
the pockels cell 16a.
With a proper potential pre6ent acros6 it (about
3,300 volts for a cell of transverse deuterated potassium
dihydrogen phosphate), the pockels cell 16a retard~ the
coherent radiation 112 one-fourth wavelength (9o degrees)
while transmitting it on via the pump cavity l~a to the
output coupler 17a, which reflects a fraction of it
(typically about 10 to 60 percent) back toward the
polarizer 15a. The reflected energy proceeds back through
the pockels cell 16a with a further retardation af one-
fourth wavelength (90 degrees). So the back radiation isone-half wavelength (180 degrees) out of phase with the
forward radiation of the transmitted component, thu6
having the opposite polarization, and it is reflected away
by the polarizer 15a, so as not to return to the laser
pump cavity 14a. Thus, laser ensrgy builds up and is
stored in the laser rod of the pump cavity 14a, because
oscillations cannot occur.
After at least about loO microseconds, the
potentiaI acro6s the pockels cell 16a is reduced to zero,
typically by shorting it to ground, for about 1 to 5
microseconds; and $he shorting of the pockels cell 16a is
repeated at intervals of about 100 to 200 microsecond~
thereafter. While it is shorted, the pockels cell 16a
does not retard the radiation 112, and the oscillator lOa
produces about 2 to 50 laser pulsec 112 in each
fluorescence envelope, with sufficient time between pulses
for stored energy to build up in the laser rod, while
keeping fluorescent losses to a minimum.
The output coupler 17a typically compriseC a
partially reflective mirror that transmits a fraction of
the energy in each pulse 112 on to the pulse sharpening
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'~
: . , . : :-
w092/1598~ 2 i ~ PCTJUS9~10t~8
-25-
means 18a, and the sharpened pulse 112 proceeds to a
preamplifier 20a.
The preamplifier 20a, which may be (and t~pically
is) ~imilar to the laser pump cavity 14a, amplifies the
sharpened radiation pulse 112, typically by about 3 to 10
decibel6, and the amplified radiation 112 proceeds to
means for directing each pul6e 112 as a beam having a
predetermined diameter onto amplifying means (amplifier)
123 comprising first and second laser amplifier rods
23a,23b in series.
The means for directing each pulse 112 from the
preamplifier 20a to the first laser amplifier rod 23a
comprises directing, equalizing, and beam expanding mean6
including a mirror 129, a retarder plate 130 (typically a
half-wave plate), a negative lens 131, a positive len~
132, and a mirror 133. The retarder plate 130 typically
is rotated to adjust the fractions of the energy delivered
by the beam splitter 137 to the second and third laser
amplifier rods 23b and 23'a so as to equalize the energy
delivered by the amplifier chains 123 and 123'. The beam
expander comprises the negative lens 131 followed by the
positive lens 132, to expand the diameter of the beam by a
factor of about 2.5, from about 1 centimeter (the diameter
of the preamplifier rod 20a) to about 2,5 centimeters (the
diameter of the amplifier rod 23a).
The amplifier 123 includes means, typically
including a beam splitter 137, for directing at least a
major portion of the radiation 112 amplified by the first
amplifier rod 23a to the second amplifier rod 23b.
The apparatus further include6 means for
directing the radiation 112 amplified by the second
amplifier rod 23b to a surface of the solid material; and
means for providing substantially uniform spatial
amplitude in the radiation 112 directed to the surface of
the solid material.
The first laser amplifier rod 23a typically
further amplifies the radiation pulse 112 from the
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WO 92/159Y5 ~ 1 n 3 ~ r~ 16- PC-r/US92/0154~
preamplifier 20a by about 5 to 15 decibels. One amplifier
23a that has been used conveniently in the apparatus
comprises the following co~ponQnts mAnufactured by Kigre,
Inc. of Hilton Head, South C~rolina: I
5 Power Amplifier Assembly, FA-1000/2
27 mm dia. x 810 mm long Q-89 Laser Rod
(described hereinafter)
Two Fluid-cooled Lamps, 63 cm arc length
Model 886-2 Power Supply compatible with 883
Controller
Dual PFN assemblies
Coolant-to-water Cooling System
Referring now to Figure 1 for typical means for
directing the radiation 112 to a 6urface of the solid
material; the amplified radiation pul~e 112 typically may
be focussed by a positive lens 24 onto a desired area of
the surface 25 of the tArget 11, to provide an average
energy fluence therein typically of at least about 10 (and
preferably about 10 to 500) Joules per square centimeter,
and an average power flux on the target of at least about
107 (and preferably about 109 to 1011) watts per square
centimeter, with pulse lengths typically of about 10 to
1,000 nanoseconds. The maximum power flux will be limited
~y the formation of a reflecting plasma at the target
surface. This maximum power flux will increase as the
laser wavelenqth decreases. For example for a la6er
wavelength of 0.53 micrometer the ~aximum power flux will
be approximately four times that for a wavelength of 1.06
micrometers.
A portion of the output 112 of the first
amplifier rod 23a, typically about lo percent, ~ay be
directed by the beam splitter 137 and mirrors 139 to a
second similar amplifier 123' to provide a second
amplified radiation pulse 112', focused by a positive lens
onto a desired area of the surface 25 of the target 11
i rj~ ,~ 3
WO92/159~ PCT/US92/0154X
-2~-
~ ~ .
simultaneously with the pulse 112 ~rom the amplifier 123.
In the same way, a portion of the output 112~ ~rom the
amplifier 23'a may be directed by a beam splitter 37~ etc,
and any convenient number of amplifiers may be employed
similarly to provide additional pulse6 to the target 11.
Typically pul es from the different amplifiers
are directed to the same area on the surface 25 of the
target 11, to overlapping areas on the surface of the
target 11, and/or to areas that are on opposite surface6
of the target 11. When beams are directed onto an
overlapping area on the same surface, the optical pulse~
should arrive on the target substantially 6imultaneously
so that the effective ri3e time of the overlapped pulse6
is equivalent to the rise time of the individual pulses.
This requires equivalent optical paths for all beam lines,
which may be accomplished by providing additional optical
path length for each beam that would otherwise arrive at
the target ahead of another beam.
Uniform Spatial Amplitude
In typical preferred embodiments of the invent$on
the means for providing substantially uniform spatial
amplitude in the radiation 112 comprises the amplifying
means 123. In such embodiments, as illustrated in Figure
7, the amplifying means 123 typically includes with each
laser amplifier rod 23a,23b a pair of flashlamps 70,71 for
pumping the rod 23a, the axis of each flashlamp 70,71 and
the axis of the rod 23a being substantially parallel to
each other and substantially in the same plane; and the
plane containing the axis of the second laser amplifier
rod 23b and the axes of its associated flashlamps 70,71 is
substantially perpendicular to the plane containing the
axis of the first laser amplifier rod 23a and the axes of
its associated flashlamps 70,71.
For example, as shown in Figure 7, the axes of
the first amplifier rod 23a and the flashlamps 70,71
typically may lie in the same horizontal plane. Then the
:
,
: : . . . ... . ..
, . . .
-
.. . . . . .. , . ~ . ,
. . - - ~ . ~
., ~ .; . .:
,. . . : , , . , . .. ~ -
.
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WO92/1598~ 2 ~ ~ 3 & 7 3 28 PCT/US92/0l54X
axis of the ~econd amplifier rod 23b typically extend6 in
nearly (but not quite) the same direction ~at least
optically) and in the same horizontal plane as the axis of
the first amplifier rod 23a. The supporting structure 72,
however, i6 rotated by substantially so degree ~bout the
axis of the amplifier rod 23b ~o that it and the axes of
the flashlamps 70,71 (although still pointing in the same
horizontal direction) define ~ substantially vertical
plane. Of cour6e the arrangements may be reversed; or
other equivalent arrangements may be substituted.
The apparatus typically comprises also means 137,
143-148 for directing a minor portion of the radiation 112
amplified by the first amplifier rod 23a to second
amplifying means 123' (which m~y be, and typically i8,
similar to the amplifying means 123) compri6ing third and
fourth laser amplifier rods 23~a,23'b in series. The beam
splitter 137 directs a selected fraction of the beam 112
from the first amplifier rod 23a via a positive lens 143,
mirrors 144,145, a negative lens 146, and a mirxor 147,
through an iris 148 to the third amplifier rod 23'a.
The apparatus further typically includes means
for directing at least a major portion of the radiation
112' amplified by the third amplifier rod 23'a, through an
iris 149, to the fourth amplifier rod 23'b; and means
(typically comprising components such as those described
above and shown in Figure l.) for directing the radiation
112' amplified by the fourth amplifier rod 23'b to a
surface of the solid material 11; and so on similarly for
any desired number of additional similar amplifying means.
Typically each means for directing radiation
112,112' to a surface of the solid material 11 provides
the radiation to the surface at an intensity of about 10
to 500 Joules per square centimeter. Also each means for
directing radiation to a surface of the solid material
directs its portion of the radiation substantially
simultaneously to an area on the same surface that
overlaps at least a portion of the area to which another
. . . ~ . . - , . . ~ . .
.
,.," ~ : :
- . .: , ~
W092~1598~ -29 ~ ~f;~ 7~ PCT/US92/0154X
I
said means directs its portion of the radiation.
Typically the optical path length of each beam is ~elected
so as to provide the 6ubstantially simultaneous arrival at
the surface by each portion of the radiation. optimally
s each means for directing radiation to the sa~e ~urface of
the solid ~aterial directs it~ portion of the radiation to
an area on the surface ~uch that all of sa$d mean6
- together direct the total radi~tion approximately e~enly
over a continuous area at least about one-third as large
-~ 10 as the area that the radiation would cover if directed
entirely to areas of the surface that do not overlap. In
other words, the overlapping of each beam with a second
beam is at least substantially complete over the area, and
with as little further overlapping as is conveniently
- 15 possible with a third beam.
Alternatively at least one of the means for
directing radiation to a surface of the solid material may
direct its portion of the radiation substantially
simultaneously to an area on a different 6urface of the
solid material from the surface to which at least one
- other said means directs its portion of the radiation; and
said means typically direct their respective portions of
the radiation to area6 that are substantially opposite to
each other and on opposite sides of the solid material.
In typical embodiments of the invention, the
means for providing substantially uniform spatial
amplitude in the radiation comprises the amplifying means
123,123';
each amplifying means includes with each laser
amplifier rod 23a,23b; 23'a,23'b a pair of flashlamps
70,71 for pumping the rod, the axis of each flashlamp and
the axis of the rod being substantially parallel to each
other and substantially in the same plane (in Figure 7,
the horizontal plane);
the plane containing the axis of the second laser
amplifier rod 23b and the axes of its associated
flashlamps 70,71 is Eubstantially perpendicular to the
., : ~ - ...... , ~. . - . .
. ~ .
~ .
W092/1598~ j _ PCT/US92tOl548
7 ~ 30- ~
plane containing the axis of the first laser ampllfier rod
23a and the axes of its associated flashlamp~ 70,71;
the plane containing the axis of the fourth laser
amplifier rod 23'b and the axes of its associ~ted
flashlamps 70,71 is substantially perpendicular to the
plane containing the axis of the third la~er amplifier rod
23'a and the axes of its associ~ted flashlamps 70, 71;
and 80 on similarly for any additional similar
: amplifying ~ean6.
Typically the pulse providing means comprise6 a
laser oscillator lOa,lO"a including a rod and a pair of
flashlamps for pumping it, the axis of each flashlamp and
the axis of the rod lOa,lO"a being substantially parallel
to each other and sub6tantially in the same plane; and a
lS preamplifier 20a including a rod and a pair of flashlamps
for pumping it, the axis of each flashlamp and the axis of
the rod being substantially parallel to each other and
substantially in the same plane;
the means for providing substantially uniform
spatial amplitude in the radiation comprises the laser
oscillator lO,lO"a and the preamplifier 20a; and
the plane containing the axis of the preamplifier
rod and the axes of its associated flashlamps is
substantially perpendicular to the plane containing the
axis of the oscillator rod and the axes of its associated
; flashlamps.
In some typical embodiments of the invention, the
means for providing substantially uniform spatial
amplitude in the radiation comprises the pulse providing
means lOc, and the pulse providing means compri~es means
for providing a beam of coherent radiation 12" that is not
polarized and in which each succeeding pulse is, in the
spatial amplitude pattern of the beam, substantially a
mirror image of the pattern in the pulse that preceded it.
3s In such embodiments, the means for providing a
beam of coherent radiation typically comprise:
,
.
: . 7 -
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- : :. ,: , . ... .. .: . ' ~
Wr~92/lS985 -31-2'133r~ ) PCI/US92/~)lS48
:- a. laser means 14c for providing a beam of
unpolarizad coherent radiation 12 n in a
predetermined ~irst direction (upward in Figure
12);
b. polarizing means 15c located in the path of
the beam 12", for breaking down the radiation 12"
~: into tWQ linearly pola~ized orthogonal component~
12A',12B', transmitting one component 12A'
through it in the first (upward) direction, and
. lo reflecting the other component 12B' from it in a
predetermined second direction; (upper rights in
Figure 12);
c. means 17,18,19 for reflecting the component
12A' that waC transmitted through the polarizing
means 15c, along a first closed path
15c,17c,18c,19c,15c (clockwise in Figure 12) that
ends in directing it back to the polarizing m~ans
15c in a predetermined third direction (lower
.. : left in Figure 12), opposite to the second
. 20 direction, and for reflecting the component 12B'
. that was reflected from the polarizing means 15c,
along a second closed path 15c,19c,18c,17c,15c
(counterclockwise in Figure 12) that is opposite
to the first closed path 15c,17c,18c,19c,15c, and
ends in directing it ~ack to the polarizing means
15c in a predetermined fourth direction (downward
in Figure 12), opposite to the first direction;
d. Q-switch (pockels cell) means 16c located
in the closed paths 15c,19c,18c,17c,lSc;
15c,17c,18c,19c,15c for retarding each component
12A',12B' of the radiation by a predetermined
fraction of a wavelength (typically about 0.1 to
0.25 wavelength), so that a substantial fraction
(typically about 10 to 50 percent) of the
transmitted component 12A' is transformed into
the opposite component 12B' before reaching the
polarizing means 15c, and so that a sub tantial
.:: ~ . . - , . . ., -
,, , . . - - ~ -, -
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.. . . . . . ..
- - ,
W~92/1598~ 3~ PCT/US92/01~4
2 ~ Q ~3 ~
fraction (typic~lly th¢ saDIe range a~ ~bove) of
the reflect~d component ~ 2B~ i~ transformed into
the opposite component 12A' before again reaching
the polarizing means 15c;
e. so that the transformed transmitted
component (now 12B') iB refl~cted from the
polarizing ~sans 15c in the fourth (downward~
direction to the laser means 14c to maintain
oscillation therein, and the transformed
reflected component (now 12A') is transmitted
throush the polarizing means 15c in the fourth
(downward) direction to the laser means 14c to
maintain oscillation therein; and
f. so that the fraction of the transmitted
component 12A' that remains not transformed is
transmitted through the polarizing mean6 15c in
the third direction (lower left), and the
fraetion of the reflected component 12B' that
remains not transformed is reflected from the
polarizing means 15c in the third direction
(lower left); and thus
g. so that the two last mentioned components
12A',12B' combine to form a- beam 12T of
unpolarized useful output radiation Eout that i6
directed in the third direction (lower left).
In other, somewhat similar, typical embodiments,
wherein the means for providing substantially uniform
spatial amplitude in the radiation comprises the pulse
providing means, the pulse providing means typically
comprises means for providing a beam of coherent radiation
that is not polarized and in which each succeeding pulse
is, in the spatial amplitude pattern of the beam, rotated
about its axis by a predetermined angle from the pattern
in the pulse that preceded it.
Such embodiments typically comprise, in addition
to the means a-g described above,
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- : . : . :
- . . . . . . .
: . -
: .
~'`92/1598:~ _33_ f~ ;r1~3 Pcr/us92/ols4~
h. mean~ (typically either a dove prism 30c
(as in Figure 14) or an optionally equivalent
arrangement 30'c of ~irror6 34c,35c~36c) as in
Figure 15) located in th~ closed p~th6
15c,17c,18c,19c,15c; 15c,19c,18c,17c,15c for
rotating the beam of radiation 12" about it~ ~xis
(typically at least about 5 degrees); and
i. ~ean6 31c (typically a (preferably qu~rter
wave) retardation pl~te) in the laser ~ean~ 14c
for retarding the radiation beam 12N from each
pass through the closed paths to the next pa~s
therethrough by a predetermined fraction of a
wavelength (typically by about 1/8 to 3/8, and
preferably about 1/4, wavelensth) per one-way
pass through the retarding means in each
direction; ie about 1/4 to 3/4 (and preferably
about 1/2) wavelength total retardation per two-
way pass through the retarding means); and thus
j . 50 that the radiation baam 12~ i8 rotated
from each pass through the closed paths to the
next pass therethrough by a predetermined angle
(typically at least about 5 degrees).
Pulse Sharpe~in~
In typical apparatus according to the invention,
the means for sharpening the leading edge of each pulse
co~prises means for providing phase conjugation reflection
of the radiation to li~it the rise time of the pulse to
not longer than about 5 nanoseconds. Such means typically
comprises a stimulated Brillouin scattering (SBS) cell
wherein the reflecting material comprises a liquid or a
gas. Typically the reflecting material comprises carbon
tetrachloride, sulfur hexafluoride, methane, acetone,
benzene, carbon disulfide, or ethylene glycol.
The fsllowing articles describe the use of
stimulated Brillouin scattering for Q-switching of lasers,
including conditions for stable operation. The first two
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W092/1598~ PCT/US92/0154~ ~
2 ~ 7 ~ ~34~ ~` !
article6 include profile~ of pulse6 (somewhat ~imil~r to
Figure 5 herein~ showing sharpening that prov~des rise
ti~es short enough to be u~erul in carrying sut the
present invention.
5 H. Meng, V. Aboites, H. J. Eichler, SBS Q-
switched Nd: YAG laser, Revista Mexicana
de Fisica 38 No. 3 (1990) pp. 335-339.
S. B. Kormer, G. G. Roche~asov, S. M. Kulikov,
Val. D. Nikolaev, Vik. D. Nikolaev, and S.
A. Sukharev, Use of stimulated Mandelstam-
Brillouin ~cattering for peaking pulses and
interstage decoupling in laser fu~ion
experiments, Sov. Phys. Tech. Phys. 25(6),
June 1980 pp. 757-758
15 V. I. Bezrodnyi, F. I. Ibragimov, V. I. Ki~lenko,
R. A. Petrenko, V. L. Strizhevskii, and E.
A. Tikhonov, Mechanism of laser Q switching
by intracavity stimulated ~cattering, Sov.
J. Quantum Electron 10(3), March 1980, pp.
382-383.
The stimulated Brillouin 6cattering cell 18e may
be located in the oscillator cavity lOa", as in Figure 11;
or it may be located external to the oscillator cavity
lOa, as in Figure 6. (Figure 11 comprises an alternative
embodiment for the ¢ombination of the o~cillator lOa, the
pulse sharpener 18a, the preamplifier 20a, and the
directing mirror 129 in Figure 6 (everything in the left
portion of Figure 6 from the back mirror 13a to the point
129a).)
The means for providing phase conjugation
reflection by photoacoustic scattering ~ay comprise also
means for providing Faraday rotation of the radiation.
Such means typically comprises a Faraday isolator 18b and
a stimulated Brillouin scattering cell 18d.
. .. ....... . . . . .
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WO 92/1598~ --3S-- ~ i f; J ~ Pcr~us92/0ls4~
Brillouin Bcattering t 6 the nonlinear optical
phenomenon of the spontaneous scattering of light in a
medium by it~ interaction with sound wave6 passing through
the medium. The scattering ta~es place on an atomic
level. Faraday rotation is the effect, di~covered by
Faraday in 1845, wher~by optically inactive materials or
substances become capable of rotating the polarization
plane of polarized radiation (light~ pas~ed through them
when they are placed into a 6trong magnetic field that has
a component in the direction of rotation. A familiar
optical instrument utilizing this effect is the Faraday
rotator, and a well-known present day application is in
the protective devices used to prevent the destruction of
high power laser systems by back reflections from the
target or other "downstream'~ system points.
In typical e~bodiments of the invention, the
diameter of each laser amplifier rod 23a,23b; 23'a,23'b is
about 2 to 3 centimeters. The output fluence level in
each amplifier rod 23a,23b; 23'a,23'b typically i6 about 6
to 20 Joules per square centimeter.
The components in the amplifier means 123,123'
affecting flashlamp efficiency, cavity transfer
efficiency, quantum defect, quantum efficiency, and
extraction efficiency preferably are selected to provide
substantially maximum overall efficiency therein.
Typically cerium is provided in the walls of the
flashlamps 70,71 to minimize the number of photons
emitting with wavelengths substantially shorter than 0.85
micrometer, and krypton is provided in the flashlamp~
70,71 to minimize the number of photons emitting with
wavelengths longer than about 0.85 micrometer, to confine
the radiation from the flashlamps 70,71 predominantly to a
narrow range of wavelengths just less than about 0.85
micrometer.
Typically the amplifier rods 23a,23b; 23'a,23'b
comprise a material having an absorption cross-section of
less than about 10 3 per centimeter and a stimulated
.
: , :. : : : ~: ~ : : :
WO92tl598~ 7~ -36- PCT/US92/0154X
emiss~on cro6s-section of gr~ater than about 4xlO 20
square centimeters. Such rod6 typically comprise
neodymium-doped glass, wherein the concentration of
neodymium i~ ~bout l.5 to 2.5 percent by weight.
A pulse with a fast ri~e time is es~ential for
shock process~ng. Peak pressure~ are approximately
proportional to PMr where P~ is the peak power and ~ i8
the rise time. While a short pulse with high peak power
would meet the peak pressure requirements, the total
energy in the pulse mu~t be high to achieve the necessary
depth of shock wave penetration in the material. Thi8
leads to the need for an asymmetric pulse for efficient
shock processing. The leading edge mu~t be sharpened.
This can be done in a number of ways. Thin films, Pockels
cells, fast nonlinear dyes, and stimulated Brillouin
mirrors have been considered. Aluminum film6 and
;i stimulated Brillouin scattering cells have been the most
effective for us so far. New developments in technology
~` may lead to a preference later on for one of the other
sharpening means.
'',;
Ampli~fier E~iciency
In the apparatus of Figure 6, the 06cillator and
preamplifier are fairly standard, but the amplifiers have
been designed to optimize performance in this application.
Shock processing places greatest importance on high-
average power and pulse energy capabilities of the
amplifiers. Beam brightness or divergence is important in
most other laser ap~lications; but for shock processing,
divergence affects only the details of the relay optic~.
A pulse energy of at least about 50 Joules is
considered necessary to meet minimum target spot size and
fluence requirements. The amplifiers are designed largely
around these requirements. Safe operation of the best
platinum-free laser glasses limits the fluence to about lO
Joules per square centimeter, requiring at least about 5
square centimeters of beam area. The optimum rod diameter
:. ~ . . : .;; . : ~ : ' : . : . '
: : : - ' : . . : - ' . '::` . .
W092/159~ -37- ~J~ `? ~ ,j PcT/us92/o1~48
for this application is about 2 to 3 centimeters. ~ecause
maximum pump rate depends only on pump length, larger
diameter 8tage6 do not produce higher average power.
Amplifier efficiency i8 ~ product of five
separate efficiency term~: flashlamp efficiency, cavity
transfer efficiency, quantum defect, quantum efficiency,
and extraction efficiency.
Fla hlamp efficiency i6 the fraction of
electrical energy radiated by the lamps. Since the energy
loss is in tbe lamp walls and electrode~ only, this
efficiency doe6 not affect the maximum average power of
the rod. So lamp effici~ncy may be sacrificed to improve
the quantum defect.
Cavity transfer efficiency relates to the
transfer of energy from lamps to rod, and has no direct
effect on average power limits. However, there are major
indirect effects. Unle~s the ab60rption is kept low in
the reflector in the pump cavity, the reflector' 6
contribution to rod pumping is not high enough to ~aintain
uniform pumping, and the average laser fluence falls far
below the 10 Joules per square centimeter needed for
spatial uniformity. So the amount of energy per pulse is
less than the desired 50 Joules. Also, the flashlamps
would have to be pumped beyond their best wall loading.
This would shift the emitted spectrum toward the
ultraviolet and thus would reduce the quantum defect (the
efficiency for converting photon energy absorbed in the
gain medium to available stored energy).
An absorbed photon with a wavelength of 0.85
micrometer has a quantum defect of O.78. Only the energy
lost in the transition from the lower laser level to the
ground state is lost. Absorbed photons with wavelengths
longer than 0.85 micrometer do not contribute to stored
energy, as they have a quantum defect of zero. Absorbed
photons with wavelength A shorter than 0.85 micrometer
have a quantum defect egual to 0.78A/0.85~ where ~ is in
micrometers. To minimize heating in the rod, it is
'- , :
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: . . - : . . .
-: - :
WO 9~/15985 2 ~ ~ 3 G '7 0 -38- PCI/US9~/01548 f
., . C
i~portant to ~inimize the number of ultraviolet photon~
and photons with wavelengths longer than 0.85 micrometer.
Ultraviolet radiation i8 ~ilter~d out of the flashlamp
spectrum by adding cerium to the lamp wall~. Longer
wavelength emis6ions are mini~ized by substituti~g krypton
filling gas for the more efficient xenon (Figures B and
Quantum efficiency is the fraction of transition~
from the upper laser level going to the ground state via
lo the lower laser level. The quantum efficiency i6 affected
by composition and impurities in the laser glas~. (This
is not generally known.) ~attelle set up a special
interferometer to measure heat simultaneously with
available stored energy. Xigre Q-88 and Schott LG760 were
the best of those tested with %=1.55, where X is the ratio
of heat to available stored energy.
Finally, extraction efficiency increases with
output fluenca, although the efficiency nearly levels off
for output fluence level6 qreater than 10 Joules per
square centimeter. Thi~ W~8 one of the reasons for
choosing 10 Joules per square centimeter as the design
output. Low absorption cross-section, ~, at the laser
wavelength, 1.05 micrometers, and high stimulated emission
cross-section, o, are also needed for high extraction
efficiency. The gain, G, is related to these cross-
sections by the equation
G = exp (aNl-~) L
where N~ is the number of atoms in the upper laser level,
and L is the length of the gain medium. For this
application a laser glass was chosen with ~<10-3 per
centi~eter and ~>4x10-2 square centimeters.
Efficient use of the energy deposited in the gain
medium is only one of the two requirements for high
average power. The other is the choice of laser glass
with optimum physical properties, high strength, high
thermal conductivity, and low expansion coefficient.
Glasses strengthened by ion exchange on the surface have
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:
WO92/159N~ PCT/us92/0l548
39-
been shown to inc~ease the fracture power threshold by as
much as a factor of 6.
Becau8e the total energy per pulse i8 li~ited by
the peak energy density on the output face of the
amplifier rod, spatial uniformity iB essential. Energy i8
pumped into the rod through the cylindrical surf~ce and
attenuates as it penetr2te~ toward the center. Without
any focusing of the pump radiation, the absorbed energy
would be at a maximum at the cylindrical surface and a
minimum at the center. However, the rod acts as a
cylindrical lens increasing the intensity at the center.
By choosing the optimum neodymium concentration, these two
effects can be nearly balanced, producing a fairly flat
distribution. The result~ of our calculation6, shown
graphically in Figure 10, indicate that 2 percent
neodymium, by weight, provides good radial flattening.
There is also an appreciable circumferential
nonuniformity, due to the placement of the two fla~hlamps.
This distortion has been mini~ized by rotating the second
amplifier in the chain by 90 degree~. Rotating one
amplifier with respect to the other i6 better than putting
four lamps in each head; because it costs less, gives
greater spacing between lamps (which protects against
electrical breakdown), and requires less reflector area
(producing less reflector absorption loss).
An alternative approach to solving the
circumferential lamp pumping is to use helical flashlamp6,
which surround the rod equally around the circumference of
the rod. However, this approach is somewhat more
expensive in both flashlamp and power supply cost.
Unpolarized Laser Oscillators
Useful alternatives to the oscillator 13-17 or
10a in apparatus as in Figure 1 or Figure 6 are shown in
Figures 12-15. These oscillators provide coherent
radiation that is not polarized.
.. : .
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:: ' . : , . :- , ~ .. -
: ~ . ' ' ' ' :
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W09.!/~598~ 21~)3670 40- PCI/U59Z/01548
The Q-switch (pockels cell) time sequence
operation of the oscillator shown in Figure 12 i~ the
following:
At time to the *la~hlampe are fired. ~here i8 no
voltage on the pockels cell at to. The tran6mitted
polarization travels around the ring in the clockwise
direction and is trans~itted out when it ~trikes the
polarizer again. The reflected polarization travel6
around the ring in the counterclockwi~e direction and is
lo reflected out when it strikes the polarizer again. The
cavity is open with no voltage on the pockels cell, and
energy is ~tored in the rod. At time tl, voltage i~
applied to the pockels cell. When quarter wave voltage i5
applied, half of the beam is trans~itted and half i6
reflected. Thi6 i6 equivalent to a fifty percent output
coupler. Lower voltage lower6 the effective reflectivity
and higher voltage increases it.
Since the beam is unpolarized going through the
rod, there is no stress birefringence loss to cause
nonuniformity in the beam. Likewise the cavity can have
no unwanted 106ses due to reflections from the polarizer.
This configuration has several advantages:
A. ~he pockels cell can be operated at the
lowest voltage consistent with the desired
output, minimizing the recirculating energy in
the cavity, and improving the lifetimes of the
polarizer and the pockels cell.
B. In addition to the improvement in
uniformity arising from the elimination of the
Maltese cross, the beam pattern flips top to
bottom, or side to side, with each pas~, as shown
in Figure 13.
C. No output couplers are needed. A
conventional system requires a large array of
output couplers.
D . This configuration can operate in the
cavity dump mode without modification if the
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- :,
, . .: .: . .
3 ~ -
~092/l59~s -41- PCT/Us9~/0l548
pockels cell i8 operated at half wave voltag~ ~or
a buildup time of typically ~ever~l photon
transit times.
The sequence of operation in more detail i5 the
following:
At time to~ the flashlamps (not shown) in the
laser pump cavity 14c are fired with no voltage on the
pockel~ cell 16c. The transmitted polarization 12A'
travels around the ring of mirrors in the clockwise
lo direction 17c,18c,19c, and i8 transmitted out at the
polarizer 15c, as indicated at ~2T. So the cavity is open
for the transmitted polarization 12A', and cannot lasQ.
In a similar manner, the reflected polarization 12B'
travels around the ring of mirrors in the counterclockwi~e
direction l9c,18c,17c, and it i~ reflected out at the
polarizer 15c, as indicated at 12T. So the cavity iB also
open for this polarization 12B', and cannot lase.
At time t1, after the flashlamps have been on for
typically several hundred (eg about 100 to 500)
microseconds, the pockels cell 16c receive~ a voltage of
typically about 1 to 4 kilovolts, causing about a
tenthwave to a quarterwave retardation in the path of each
polarization of radiation described above. So the 12A'
polarized radiation passing upward through the pockels
cell 16c is transformed into the 12B' polarization, and i
reflected by the mirrors 17c,18c,19c in the clockwise
direction to the polarizer l5c from which it i6 reflected
back to the rear mirror 13c. Meanwhile the 12B' polarized
radiation passing downward through the pockels cell 16c is
transformed into the 12A' polarization and is transmitted
through the polarizer 15c back to the rear mirror 13c.
This closes the cavity for both polarizations when the
stored energy in the gain medium (the rod 20c) hac built
up to the desired level. The rear mirror of the cavity is
the concave mirror 13c, and the polarizer 15c is the
effective output coupler.
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W0 92/1s9~c ~ 42- PCI/US9~/01548
7 ~
The fraction of energy coupled out at each pa6s
is determined by the voltage on the pockels cell 16c. For
a ~D*P (deuterated potassium dihydrogen phosphate) cell,
half of the energy i6 coupled out at about 3.3 kilovolts,
and none of the energy couples out at about 6.6 kilovolts
(Conventionally Q switched o c~llators operate with about
20 to 50 percent of tha energy r~moved by the output
coupler.) The voltage r~ain6 on the pockelG cell
typically for a few (eg about 0.1 to lo) microsecond6.
The contrast ratio of a typical dielectric
polarizer is much better for the transmitted beam than for
the reflected beam. ~he poor contrast ratio of the
reflected bea~ allows energy to leak out of the oscillator
during the charging period. This problem can be corrected
by using two dielectric polarizers, with their polarizer
surfaces adjacent, typically ~bout 0.1 to 1 millimeter
apart. The other surfaces must be antireflection coated,
because the usual Brewster angle geometry to prevent 1066
of the desired polarization i8 not effective in a cavity
wit~ both polarizations.
The coherent radiation 12" is generated by an
oscillator lOc (all of Figure 12) compriging a rear mirror
13c, a laser pump cavity 14c, a polarizer 15c, a pockel~
cell 16c, and three mirrors 17c,18c,19c. The laser pump
cavity 14c comprises a gain medium, such as at least one
neodymium-glass laser rod 20c pumped by flashlamps (not
shown~, that are driven by a pulse for~ing network (PFN)
(not shown). One such laser pump cavity 14c that has been
used conveniently in the oscillator lOc comprises the
following components manufactured by Compagnie Generale
Electrique, of France:
CGE-640 laser cavity, 67 centimeters long,
helical flashlamps
64 millimeters x 670 millimeters laser
rod with 1 percent Nd doping.
Antireflection coated at both
ends.
., .. . ., . ~ . .. .. . .
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1, . `!, ~ 1 i/
WOg2/1598~ PCT/US92/~1~48
~;. 43
8 Melical fla~hlamps
For simplicity, Figure~ 12 and 13 show only one
laser rod 20c. We typically use two of the rods ds~cribQd
above, and any convenient number may be employed. The
rear mirror 13c typically may be either planar or
~pherical. A spheric~l mirror with ~00 percent
reflectivity and a 20 meter radius of curvature is
convenient in combination with the above equipment.
The oscillator lOc provides a fluorescence
lo envelope. The coherent radiation 12" from the laser pump
cavity 14c is unpolarized. The polarizer 15c break6 the
radiation 12" down into two linearly polarized orthogonal
components 12A~,12B'; one of which (component A') it
transmits in the clockwise direction, as indicated at
12A'; and the other (component B'~ it reflects to the
right in the counterclockwise direction, as indicated at
12B'; to the pockels cell 16c.
With a proper potential present acro6s it (about
1 to 4 kilovolts for a cell of tran6verse deuterated
potassium dihydrogen pho~phate), the pockels cell 16c
retards the coherent radiation component 12A' while
transmitting it on in the clockwise direction to the
mirrors 17c,18c,19c, and thence to the polarizer 15c,
which reflects a fraction of it back through the rod 20c,
to the rear mirror 13c and transmits the remainder 12T to
the target, as indicated at Eout~ The reflected radiation
component 12B' proceeds back in the counterclockwise
direction by way of the mirrors l9c,18c,17c and thence
through the pockels cell 16c, with a retardation, to the
polarizer l5c, which transmits a fraction of it back
through the rod 20c to the rear mirror 13c and reflects
the remainder 12T to the target, as indicated at Eout~
When the flashlamps are originally fired, n~
voltage is on the pockels cell 16c. Neither the clocXwise
12A' nor the counterclockwise 12B' radiation component can
return to the rear mirror 13c, because all of the energy
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W~92/15~8~ 44 PCT/VS92/01~8 ~ (
2 ~ ~ ~ 7 !~ ,
exit6 the cavity; being eith~r tran~mitted (12A') or
reflect~d (12B'), by the polariz~r 15c, away from the rod
20c, as indicated at 12T ~nd Eout~ During this time,
energy builds up on the rod 20c. After about 100 to 500
microseconds, voltage i~ applied to the pockels cell 16c
to initiate the laser pulse. Voltage i8 removed from the
pockel6 cell 16c after about 0.1 to 10 microsecond~.
The uniformity of the output beam from an
oscillator as in Figure 12, which we have dubbed the "P-
Oscillator" because of its P-shaped arrangement apparent
from Figures 12 and 14, is achieved largely becau~e of the
fact that the output 12T comprises the combination of both
components 12A',12B' of the radiation 12" from the laser
pump cavity 14c, as explained above. Moreover, the
uniformity is enhanced by virtue of a feature inherent in
the invention that is illustrated in Figure 13; namely the
pattern of the beam in each pass is flip-flopped left and
right ~as viewed in Figures 12 and 13) from the pattern of
the preceding pass, as is explained below.
In the first pass of the beam 12", the component
12B' at the left side of the beam 12", shown as a solid
line following the counterclockwi~e inner path abcdef, is
transformed between e and f (by the pockels cell 16c) to
the component 12A', and i8 transmitted down through the
polarizer 15c from f to g.
This 12A' component (on the right side of the
beam 12n) is reflected back up from the rear mirror 13c
(This is the second pass.) from g to f wher~ it is
transmitted up through the polarizer 15c. Then it is
transformed by the pockels cell 16c into the component
12B', which proceeds along the clockwise inner path edcb
(solid line) and is reflected down by the polarizer 15c
from b to a and thence to the rear mirror 13c.
In the next (third) pass, this 12B' component
(now again on the left side) is re~lected back along the
path abcdefg in the same manner as described above for the
. .
2 ~
W~92/1598~ 45 PCT/US92/01548
component 12B' at the left side of the beam 12" in the
first pass.
Similarly, the description for the fourth pass i~
the same aæ the description above for the second pas~, the
description for the fifth pass i8 the same as that for the
third pass, and ~o on for the subsequent passe6.
In the fir~t pas6 of the beam 12", the component
12B' at the right ~ide of the beam 12 n ~ shown as a dashed
line following the counterclockwise outer path gfhijb, i5
transformed between j and b (by the pockels cell 16c) to
the component 12A', and is transmitted down through the
polarizer 15c from b to aO
This 12A' component (on the left side of the beam
12") is reflected back from the rear mirror 13c (This is
the second pass.) from a to b where it is transmitted up
through the polarizer 15c. Then it is transformed by the
pockels cell 16c into the component 12B', which proceeds
along the clockwise outer path jihf (dashed line) and i5
reflected down by the polarizer 15c from f to g and thence
to the rear mirror 13.
;~: In the next (third) pass, thi~ 12B~ component
. (now again on the right side) is reflected back along the
path gfhijb in the same manner as described above for the
component 12B' at the right side of the beam 12 n in the
first pass.
Similarly, the description for the fourth pass is
the same as the description above for the second pass, the
description for the fifth pass is the same as that for the
: third pass, and so on for the subsequent passes.
The descriptions for the 12A' component on each
side of the beam 12" in the first pass are the sane as
those for the second pass above, the descriptions for the
second pass are the same as those for the third pass
above, and so on in the same manner.
To summarize, each component starting on each
side of the beam in each pass returns to the rear mirror
13c as the opposite component at the opposite side of the
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W092/15~8~ -46- Pcr/US92/01548 ~
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beam, and start~ as such from there in the next pa86.
Thus both components flip-flop from side to side with each
successive pass, and any side to side nonuniformity i8
substantially eliminated.
A standard oscillator exhibits output beam
angular nonuniformity due to pumping nonunifor~ities.
Typical pump cavity configur~tion~ include a single
ellipse cavity with the flashlamp at one focal axi~ of the
cylindrical ellipse and the la~er rod at the other. This
geometry i~ efficient because rays from one focal axis
intercept the other focal axis. However, most of the pump
radiation strikes the side of the rod adjacent to the
lamps. This nonuniformity can be partially smoothed by
forming the pump cavity from multiple partial ellipses,
each with a lamp, but there remains a scallop effect, and
complexity of the cavity increases. Another typical
configuration comprise a close coupled cavity without
elliptical focusing. The highest pump intensity still
occur~ in the areas of the rod adjacent to the lamps.
The apparatus shown in Figure 14 provides
coherent radiation that is free from any angular
nonuniformities except possibly a negligible amount. It
contains all of the equipment shown in Figure 12 plu6 a
dove prism 30c in the closed paths 15c,17c,18c,19c,15c;
15c,19c,18c,17c,15c and a retarder plate 31c, preferably a
quarterwave plate, between the rear mirror 13c and the
laser rod 20c. The operation is the same as that
described above for Figure 12 except that the pattern of
the laser beam in each pass instead of flip-flopping from
the pattern of the preceding paths, rotates about its axis
by at least a few degrees in the same direction from each
pass to the next, and thus provides even greater
uniformity of output than does the apparatus of Figure 12.
The closed paths l5c,17c,18c,19c,15c;
15c,19c,18c,17c,15c lie in a plane (the plane of the paper
in Figures 12 and 14) and the dove prism 30c is located
with its axis along a portion of the closed paths and with
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W~9~/l598~ -47 PCT/US92/0154X
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the normal to its ba~e at an angle away from the plane of
the closed path~, as indicated at 32c. Thi~ cau8e5 each
component of radiation 12A',12B' to proceed out of the
plane of the closed path~ and then back into it a6 it
passes through the dove prism 30c, and thus cauRes the
beam to rotate by an a~gle that i6 approximately twice the
angle between the plane of the clo~ed paths and the normal
to the base of the dove prism. The fraction of the
rotated beam that i6 directed by the polarizer 15c back
throuyh the rod 20c and the quarter wave plate 31c i8
reflected by the rear mirror 13c so that the component
12A',12B' that was directed down is reflected back up by
the rear mirror 13c and thus passe6 through the quarter
wave retarder plate 31c and the rod 20c a second time, so
that the next radiation beam 12" i8 retarded by a half
wavelength and thus each component 12A',12B' return~
upward to the polarizer 15c as the same component as in
the preceding pass around the closed paths. So with each
pass around the closed paths the beam is rotated by the
same amount and in the same direction, so that after
several passes it has made a complete rotation and
continues in the same manner, providing an extremely high
degree of angular uniformity in the output beam.
The apparatus of Figure 15 operates in the same
25 manner with the combination 30'c of mirrors 34c,35c,36c in
the closed paths instead of the dove prism 30c. The unit
30'c is positioned similarly to the dove pri~m 30c as
described above. The unit 30'c is positioned so that the
normal to each mirror 34c,35c,36c is at an angle,
typically at least about 2.5 degrees, to the plane of the
closed paths, and rotates each component 12A',12B' of the
beam by twice that angle, as explained before for the dove
prism 30c.
Maior Com~onents
An appendix filed with the application from which
this patent issued, and present in the file of the
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Wo92/1598~ 21 ~ 3 6 7 D -4~- pc~/us92/nls4R ~ ¦
application, lists the ~a~or component~ (and most of the
other components) in an ~mbodi~ent of the invention that
includes mo6t of the ~eatures i~ Figures 1 and 6 and ha6
provided excellent results. The appendix also identi~ies
the supplier and model number for each purchased
component.
: While the forms of the invention herein di~closed
constitute presently preferred embodiment~, many others
are possible. It is not intend~d herein to mention all of
the possible equivalent forms or ramifications of the
invention. It is to be understood that the terms used
herein are merely descriptive, rather than limiting, and
that various changes may be made without departing from
the spirit or ooope of the invention.
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