Note: Descriptions are shown in the official language in which they were submitted.
~4~i~
CONTINUOUS WAVE, FREQUENCY-DOUBLED SOLID STATE
LASER SYSTEMS WIT~I STABILIZED OUTPUT
Field of the Inventlon
This invention relates in general to solid
state laser systems and more particularly to a con-
tinuous wave solid state laser system having an output
frequency which is twice that of the laserls fundamental
frequency.
Background of the Invention
Laser systems utilizing frequency-doubling
solid-state crystals to generate an output beam at a
frequency twice the fundamental frequency of t~e laser
~ rod employed are well known in the art. Typically such
systems embody a two-mirror cavity configuration in
which reflectors (or mirrors) at either end of the laser
rod are coated to reflect substantially all of the opti-
: 20 cal energy at the fundamental frequency while passing
therethough a substantial portion of the optical beam at
twice that frequency~ In such an arrangement a suit-
able frequency-doubling crystal, such as KTP (potassium
-t.itanyl phosphate), manufactured by Airtron, a division
~ 25 of Litton Industries, Inc., of Morris Plai.ns, N.J., is
~;
placed between one end of the laser and one of the t"o
mirrors. Accordingly, the optical beam at double the
fundamental frequency is emitted from the mirrors at
either end of the laser. In such an arrangement
substantially half of the energy at the doubled fre-
quency is lost since it is propagated in opposite direc-
tions. Additionally such prior art lasers have been
capable of operating only in the pulse mode because of
the intensity level required at the doubled frequency
1~ for the laser to be efficient.
One approach that has been employed in the
past for producing a pulsed, frequency-doubled solid
state laser output is a three-mirror L-shaped optical
configuration in which at one end the laser cavity has a
lS mirror for reflecting all of the optical energy at the
fundamental frequency which is incident upon it. At the
opposite end a folding mirror ~i.e., a mirror which is
coated for reflection of the fundamental frequency only
and positioned at a 45 angle relative to the incident
beam) ls placed centered on the axis of the laser rod to
reflect the beam from the laser rod in a direction nor-
mal thereto. A third reElecting mirror~ coated to
reflect substantially all of the optical energy incident
upon it at both the fundamental frequency and at double
that frequency, is positioned to intercept the light
reflected from the folding mirror and direct it back
along that axis onto the folding mirror. Positioned
between the folding mirror and the third reflecting
mirror is a frequency-doubling crystal. The folding
- 2
mirror is characterized by providing substantially total
reflection of optical energy incident thereon which is
at the fundamental frequency emitted by the laser rod
while bein0 substantially transparent to incident light
at twice that frequency. Accordingly, in such a three-
mirror arrangement the Erequency-doubled light output is
emitted from the laser only at one place9 i.e., along
the axis through the folding mirror. In some such
three-mirror systems it is conventional to employ some
focusing, both at the third reflecting mirror at the far
side of the solid-state Erequency doubling crystal and
also on the laser beam as it is emitted from the end of
the rod onto the folding reElector. With the frequency-
doubling crystals currently available this focusing is
needed in the operation of such system in a continuous
wave, as opposed to pulsed, mode in order to achieve
sufficient intensity inside the frequency-doubling
crystal to permit efficient conversion to the doubled
frequency.
In attempting to operate the above-described
configuration in the CW (continuous wave~ mode, high
efficiency is very difficult to achieve because it is
desirable to utilize substantially the entire diameter
of the laser rod for generating the laser beam, in other
words, to operate the laser in the so~called transverse
TEMoo mode, which corresponds to a uniform gaussian
distribution of the lightenergy over the cross~section
of the laser rod. However, the pump radiation from the
excitation lamp would ordinarily cause the temperature
- 3 -
of the laser rod to rise correspondingly. Cooling
techniques, such as the insertion of an optical filter
between the excitation lamp and the laser rod to reduce
radiation incident on the laser rod which is not useful
for producing excitation at the fundamental frequency,
have been utilized, and fluid cooling as well as coating
of both the laser rod and the excitation lamp have also
been employed. ~owever, in such prior art arrange-
ments a stabili2ed frequency-doubled CW output in the
TEMoo mode had not heretofore been achieved--in part
because of non-uniform cooling of the laser rod.
sy way of explanation, cooling of the laser
rod is accomplished by convective transfer of heat from
the laser rod into the cooling fluid flowing over the
surface of the rod. This causes uneven cooling of the
rod surface in both time and space. The resulting tem~
perature variations create optical distor-tions because
of the dependency of the index of refraction of the
laser rod on temperature. The effect of the optical
distortions on the output power from the laser depends
in turn on the laser resonator mirror geometry. This
effect is most severe when the laser is operating in the
fundamental mode. A time-dependent cavity loss is
generated in this case from these optical distortions
and results in a significant variation in output power.
This output power variation becomes very severe when the
fundamental mode is designed to fill most of the rod
volume, as in the TEMoo mode, because space is occupied
out to the periphery of the laser rod where the index of
refraction variations are the greatest.
- 4
~p~
Summary of the I vention
Broadly speaking, in the present invention the
configuration oE the three-mirror L-shaped optical
system described previously is employed with an ellip-
S tical laser pump cavity including a Nd: Y~G laser rod, asuitable excitation lamp and a frequency-doubling
cryskal. The system optics are arranged to provide for
employment of the full diameter of the laser rod, that
is, in the TEMoo transverse mode at the fundamental
frequency, and at sufficient intensity inside the
frequency-doubling crystal, in CW operation, accom-
plished, for example, by focusing the light emitted by
the laser rod within a small cross-sectional area inside
the frequency-doubling crystal, so that efEicient output
power at the doubled frequency can be achieved. It has
been found that by employing the techniques described
below the temperature of the laser rod and hence its
lasing stability can, in contrast to prior art systems
of this type, be maintained substantially constant
(within a fraction of a percent) during operation in the
CW TEMoo mode. The preEerred way of accomplishing
this is to provide for a fluid-cooled pump cavity
together with a cooling jacke-t of specific size and
materials (such as Spinel or quartz) cladding the laser
; 25 rod. Also, by the use of a speciEically-sized orifice
for the pump cavity -the fluid flow across the surface of
the laser rod can be maintained laminar which provides
more constant cooling as a Eunction oE time. Either
the cladding or the laminar fluid Elow can provide
-- 5
sufficient cooling so as to maintain uniform temperature
along the length of the laser rod and thereby a stable
output power level.
The foregoing and other objectives, features,
and advantages of the invention will be more readily
understood upon consideration of the following detailed
description of the invention, taken in conjunction with
the accompanying drawings.
Brief Descr_ption of the Drawin~s
FIG. 1 is an illustration in generally block
diagrammatic form of a frequency-doubled solid state
laser constructed in accordance with the principles of
this invention.
FIG. 2 iS a generally cross-sectional,
diagrammatic view of an embodiment of an optical laser
cavity suitable for employment in the system of FIG. 1.
FIG. 3 is a cross-sectional, generally
diagrammatic view, taken along the line 3-3 of FIG. 2,
of the em-bodiment of the optical laser cavity shown in
FIGo 2 ~
FIG. 4 is a cross-sectional, generally
~; diagrammatic view oE a modified embodiment of the
optical laser cavity suitable for employment in the
system of FIG. 1.
FIG. 5 is a cross-sectional, generally
daigrammatic view, taken along the line 5 5 of FIG. 4,
of the modified embodiment of the optical laser cavity
shown in FIG. 4
'~
-6-
Descri~tion of _referred Embodiment
In the exemplary system illustrated in FIG. 1
the pump cavity 1l encloses a conventional parallel
excitation lamp 15 which is optically coupled to an
elongated solidstate laser rod 13, typically a 4 mm
diameter Nd: YAG rod, which produces along the axis oE
the rod an output laser beam at typically a wavelength
of 1.064 um. A re~lector mirror 17, highly reflective
("H.R.") at 1.064 um, is positioned at one end of the
laser cavity to reElect the laser beam back along the
axis. This reflector 17 is formed with an appropriate
coating to reflect substantially all of the incident
light which is at a wavelength of 1.064 um. At the
opposite end of the axis there is positioned a folding
mirror 19 coated to reflect substantially all of the
light incident upon it at a wavelength of 1.064 um.
This light is reflected normal to the axis of the fun-
damental beam from the laser rod and is then directed to
a concave reflecting mirror 21 coated to reflect sub-
stantially all light incident upon it at wavelengths ofboth 1.064 um and 0.532 um. Positioned between the
folding reflector 19 and the concave reflector 21 is a
frequency-doubling crystal 23.
The frequency doubler crystal may be formed of
KTP which has the characteristic of converting a portion
of the light energy incident upon it at a wavelength of
1.064 um to a wavelength of 0.532 um~ The efficiency oE
; this conversion depends significantly upon the intensity
of the incoming light beam. Accordingly, a focusing
element 25, which typica]ly~ is a lens, i5 positioned to
focus the longer wave length light incident upon the
frequency-doubling crystal 23 -to a small area to enhance
this conversion efficiency. (While the focusing element
25 is shown as a separate lens, the same effect can be
achieved by grinding a curvature in the end oE the laser
rod 13~) Similarly, the concave reflector 21 also
focuses the reflected light back onto the crystal 23.
The folding mirror l9 is arranged to be substantially
transparent to light at the 0.532 um wavelength and
hence serves as an exit window from the system for a
laser beam of this wavelength. Since the frequency-
doubling is efficient only in selected polarization
planes, a wave plate 18 may be positioned, if desired,
between the end of the laser rod 13 and reflector 17 to
rotate the polarization of the beam and align it as
necessary.
In order to operate the above-described system
in the CW mode with sufficient output power, it is
important to achieve both a very highly efficient beam
generation as well as high stability in the power out-
put. In the present invention this is achieved by
utilizing the full diameter of the Nd: ~AG laser rod to
generate the beam. In order to operate at these levels
of power stability, several extraordinary measures for
maintaining the entire volume of laser rod at highly
uniform temperature is required. Thus, as illustrated
in FIG. ~, the laser pump cavity itself is typically
j formed of a pyrex glass cylinder 31 of elliptical cross
--8--
section with the excitation~lamp 15 located along one
axis and the Nd: YAG laser rod 13 located along the
other axis. The excitation lamp can be a conventional
krypton lamp having a quartz envelope and tungsten
electrode such as that available from ILC Company, of
Sunnyvale, California, under the designation ILC No.
L3243. The interior surface of the sidewall of the
pyrex cavity 31 is coated with a gold deposit, or other
highly-reflective coating, to provide for reflection of
the lamp energy onto the laser rod. A colored glass
filter 34 is positioned between the lamp 15 and the
laser rod 13 to absorb that portion of radiation from
the lamp which does not serve to excite the lasing
energy states of the rod, but yet would heat the rod.
The entire elliptical cavity 31 is cooled by a
fluid such as water and includes a cooling flow tube 35
around the lamp 15 and a second cooling flow tube 37
positioned around the laser rod 13. The flow tubes 35
and 37 may be formed of any suitable material such as
uranium-doped quartz providing that it is substantially
transparent to light from lamp 15 which is of wavelength
effective to excite the laser energy states in rod 13.
The inner diameter of the flow -tube 37 around the 4mm
laser rod 13 may suitably be 9.5 mm. The nature of the
fluid Elow through this tube is effected by the sizing
of the orifice~ This orifice is sized such that the
flow through the tube along the surface of the rod is
substantially laminar, thereby introducing a minimum of
variation in the cooling action and maintaining the
, d~
surface oE the rod at a sta~ble temperature distribution.
(The surface temperature along the length of the rod is
not necessarily the same but the temperature distribu-
tion pat tern is maintained constant.) This temperature
constancy is absolutely critical for providing the
stable output laser power with variations, for example,
of less than one per cent.
In FIG. 3 there is illustrated a typical flow
configuration in which the cooling fluid is passed in
parallel through the pump cavity 31 and laser rod cool-
ing tube 37 and is then returned through the excitation
lamp flow tube 35.
In FIGs. 4 and 5 there is illustrated a
modiied embodiment of the optical laser cavity in which
cladding of the laser rod is employed, rather than limi-
nar fluid flow, to maintain the desired constant tem-
perature distribution along the rod4 (In this embodi-
ment the same reference numerals refer to the same or
similar elements in the first-described embodiment.)
In the modified embodimen-t the entire ellip-
tical cavity 31 is cooled by a Eluid such as water and
includes a cooling flow tube 35 around the lamp 15 and a
sleeve-like cladding 38 around the laser rod 13 with a
thin layer of an optically-transparent, thermally-
conductive material, such as a static layer of water or
a silicone gel, in-terposed between the laser rod and the
cladding. The cladding 38 may be constructed of any
suitable material such as crystalline quartz or Spinel
providing that it is substantially transparent to light
--10--
from the lamp 15 at the wav~elength efEective to excite
the laser energy states in rod 13, and has appropriate
thermal and mechanical properties similar to quartz and
Spinel to maintain a stable temperature distribution
along the laser rod. Exemplarily, the cladding sleeve
for the laser rod may have a 4 mm inside diameter and a
14 mm outside diameter and be constructed from the
material Spinel (MgO:A12O3). As depicted, fluid flows
through the elliptical pump cavity 31 around the
cladding 38 and is returned through the lamp 15 via flow
tube 35.
In certain applications it may be desirable to
combine both the laminar fluid flow with the cladding
sleeve in order to attain a desired cooling effect on
the laser rod and thereby maintain the constant tem-
perature distribu-tion pattern required for a stabilized
output in a continuous wave, frequency-doubled laser
; system.
In a typical example a system of the type
embodying the principles of the present invention would
have the following characteristics:
TABLE 1
. .
Laser Rod ~ Nd: YAG, 4 mm diameter
Frequency-Doubling Crystal - KTP
25 Fundamental Wavelength - 1.064 um
System Output - 2 wat-ts CW at 0.532 um
Transverse mode - TEMoo
Beam Diameter - 3mm
Electrical Input - 20~/240 single phase,
10 amps.
Cooling Requirement - 1500 watts.
--11--
~æ~
The terms and exp~re.ssions which have been
employed in the :Eoregoing specification are used therein
as terms of description and not of limitation, and there
is no intention in t.he use of such terms and expressions
of excluding equivalents of the features shown and
described or portions thereof, it being recognlzed that
the scope of the invention is deEined and limited only
by the claims which follow.
-12-
