High brightness cold burst solid state laser operation

A solid state laser having an elliptical chamber 22, a solid state laser rod 10, a flashtube 12 for optically pumping the rod 10, external mirrors 24,26 providing an optical cavity, the flashlamp 12 being energized by a pulse generator 14 which generates electrical pulses across the flashlamp 12, is provided with cold burst pumping of the flashlamp 12 by a burst 120 of a predetermined number of pulses, e.g., 40 pulses, each having a predetermined pulse width, e.g., 650 msec, and the burst lasting a predetermined length of time, e.g., 200 msec, not to exceed 1/16 to 1/12 of the thermal time constant of the solid state laser rod 10. Cold burst pumping provides a substantially constant thermal rod profile which reduces divergence of the output beam 36 and increases beam brightness, thereby improving the productivity and precision of solid state lasers.

TECHNICAL FIELD 
This invention relates to solid state lasers and more particularly to a 
technique for optically pumping a solid state high power laser. 
BACKGROUND ART 
It is known in the art of solid state high power lasers such as a 
doped-insulator neodymium doped yttrium aluminum garnet (Nd:YAG) laser, 
that pumping is normally achieved by using an intense flash of white light 
from a flashtube (or flashlamp). The flashtube is typically excited 
electrically by a strong surge of electrical voltage across opposite ends 
of the flashtube with the tube containing an appropriate light-emitting 
gas. 
A typical configuration of an Nd:YAG laser comprises a lasing medium 
(Nd:YAG) in the form of a cylindrical rod and a linear flashtube placed 
inside a highly reflecting elliptical chamber. The flashtube is located 
along one focal axis of the ellipse and the laser rod along the other 
focal axis of the ellipse. In this configuration, the properties of the 
ellipse insure that most of the radiation from the flashtube passes 
through the laser rod, thereby providing efficient pumping. 
The optical cavity of the laser is typically formed by providing mirrors 
external to the elliptical chamber, one mirror being totally reflecting 
while the other being less then totally reflecting (e.g., transmitting 
about 10%) to provide an output light. 
Because a large amount of heat is dissipated by the flashtube during 
pumping, the laser rod quickly becomes very hot. To avoid damaging the 
laser rod from the extreme heat, cooling is typically provided, e.g., by 
pumping cooling water through a jacket which surrounds the laser rod or by 
forcing cool air into the elliptical chamber. 
Conventional operation of high power Nd:YAG lasers typically employs 
continuous repetitive pulsing of the pump light which provides an output 
beam which is also pulsed. Also a shutter is typically used to control 
which optical output pulses are exposed to the workpiece. 
However, such continuous repetitive pump pulsing causes a thermal gradient 
to be introduced into the laser rod due to the heating of the entire rod 
by the flashtube and the cooling of the outer perimeter of the rod by the 
cooling process. In particular, a radially parabolic (or quasi-parabolic) 
temperature distribution exists in the rod along the cross-section of the 
rod, with the center of the rod being at a peak heated temperature and the 
outer radius being at the cooling temperature. Such a parabolic 
temperature distribution causes a corresponding variation in refractive 
index of the rod. This refractive index variation causes the optical path 
length for regions of the oscillating beams within the laser to have 
different optical path lengths, thereby causing portions of the internal 
beams (related to transverse lasing modes) to focus at different points 
along the rod and laser cavity. This is called a thermal lens effect (or 
thermal lensing) and introduces divergence into the output beam, thereby 
reducing the beam brightness or beam quality at the focal spot. 
Consequently, this parabolic thermal gradient limits the usefulness of the 
laser for the processing of materials. In particular, it precludes 
drilling small holes or cutting fine lines in material and requires loose 
tolerances on the larger operations. 
One way to minimize the effect of the parabolic temperature distribution, 
is to restrict the pumping rate to low repetition rates to allow 
sufficient time for the rod to completely cool between each pumping pulse, 
i.e., the pump time equals the thermal time constant. For typical Nd:YAG 
systems, the thermal time constant is approximately 2-4 seconds. Thus, 
when used at such low repetitions rates, the productivity and efficiency 
of the laser are severely limited by greatly extending the length of time 
required to operate on a workpiece (e.g., drill a hole or weld a joint). 
One technique for improving the brightness of solid state lasers includes 
lasers having complex optical configurations, e.g., apertures and/or 
lenses, within the laser cavity to reduce the effects of beam spreading. 
However, this technique is very costly, is difficult to maintain in a 
production environment, and requires wasting much of the optical energy. 
Another technique is to use face-pumped rectangular "slab" crystal lasers. 
However, such laser crystals are very expensive (e.g., $20,000) and are 
hard to manufacture because the laser crystals require precise dimensional 
control in manufacturing, precise mounting and adequate cooling of the 
slab to prevent thermal gradients from distorting the slab. Still another 
technique used in the art to reduce thermal lensing is to drill a hole 
through the center of the rod and pump coolant through the hole, as well 
as the around the outer diameter, thereby reducing the thermal time 
constant and increasing the allowable repetition rate. However, this 
technique reduces the overall volume of the gain medium, thereby reducing 
the available gain and output energy of the laser. 
Thus, it would be desirable to devise a scheme of operation which minimizes 
the quasi-parabolic temperature distribution and the resultant thermal 
lensing effects while not adding significant cost or complexity to the 
laser nor reducing the available gain or output power. 
DISCLOSURE OF INVENTION 
Objects of the invention include provision of a high power solid state 
laser system which does not exhibit a quasi-parabolic temperature 
distribution in the laser rod and does not exhibit a thermal lensing 
effect. 
According to the present invention a solid state laser includes pump light 
means for providing optical pulsed pump light in response to a pulsed 
electrical pump signal; pump pulse means for providing the pulsed 
electrical pump signal; solid state lasing means, disposed in the path of 
the pump light, for becoming excited by the pumping light, for becoming 
heated by the pumping means, and for emitting lasing light in response to 
the pump light; an optical cavity, containing the solid state lasing 
means, providing a predetermined amount of internal reflection of the 
lasing light, and allowing a predetermined amount of the lasing light to 
exit as output laser light; cooling means, thermally interacting with a 
portion of the lasing means, for cooling the lasing means; and the pulsed 
electrical pump signal being a burst of a predetermined number of pulses 
having a predetermined pulse width, the burst lasting for a predetermined 
burst time, so as to cause the temperature distribution profile along a 
cross-section of the lasing means during the burst time to be 
substantially flat over a predetermined region, thereby reducing 
divergence of the laser output beam. 
According further to the present invention the burst time does not exceed 
about 1/16 to 1/12 of the thermal time constant of the solid state lasing 
means. According further to the present invention, the pump pulse means 
comprises a pulse generator. According still further to the present 
invention, the solid state lasing means comprises a laser rod. 
The invention represents a significant improvement over the prior art by 
providing a substantially flat temperature gradient along a large portion 
of the cross-section of a solid state laser rod by pumping the laser with 
a burst of high frequency pulses and then terminating the pulsing before 
the effect of cooling can establish a thermal gradient across the core of 
the rod. The invention permits the operation of conventional solid state 
rod lasers in a high brightness mode without changing the 
mechanical-optical design, without the addition of complex intra-cavity 
optical components, and without changing the cooling design. Furthermore, 
the invention provides a laser that can drill and cut material more 
quickly and to finer tolerances than prior art lasers. Thus, the invention 
improves the productivity and precision of conventional solid-state laser 
systems currently employed in production use. Applications for the laser 
of the present invention include, but are not limited to, welding, hole 
drilling, cutting, and slicing (perforating). 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent in light of the following detailed 
description of exemplary embodiments thereof as illustrated in the 
accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, a solid state laser, e.g., a Nd:YAG laser, comprises a 
laser rod 10 made of yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12) 
doped with the rare earth metal ion neodymium (Nd.sup.3+). The laser also 
has a linear flashtube 12 (or flashlamp), which contains a gas, e.g., 
xenon, capable of emitting white light when excited with an electrical 
excitation across the gas. Other gases may be used if desired. A pulse 
generator 14, provides electrical pulses along the lines 16,18 which are 
connected to opposite ends of the flashtube 12. When the pulse generator 
14 generates in a pulse of electrical power, the gas within the flashtube 
12 becomes excited and emits an intense flash of white light 20 which 
optically pumps the laser rod 10. 
Both the laser rod 10 and the flashtube 12 are located inside a highly 
reflecting elliptical chamber 22 and are each located at one of the focal 
axes of the elliptical chamber 22. The location of the flashtube 12 and 
laser rod 10, together with the shape of the ellipse, insures that most of 
the radiation 20 emitted from the flashtube 12 passes through the laser 
rod 10, as discussed hereinbefore. 
The optical cavity of the laser is formed by providing mirrors 24,26 
external to the chamber 22. The mirror 24 is totally reflecting (i.e., 
100% reflector) and the mirror 26, reflects slightly less than 100%, e.g., 
is 10% transmitting. The light 20 from the flashtube 12 excites the laser 
rod (gain medium) to a level which allows population inversion to occur, 
thereby allowing lasing to occur. The associated emission of photons due 
to lasing are indicated by the arrows 30. The light 30 exits the rod 10 
and is reflected by the mirrors 24,26, as indicated by the lines 32,34, 
respectively. Because the mirror 26 is not totally reflecting, it provides 
an output light 36, which is the output of the laser. The output light 
beam 36 is ideally collimated; however, when the prior art pumping scheme 
is used, the beam 36 becomes divergent, as discussed hereinafter. 
Instead of the external mirrors 24,26, the ends of the rod 10 may be 
polished and coated with a reflective coating to provide the required 
internal reflection. 
To allow cooling of the laser rod 10, a jacket or sheath 40 is provided 
around the laser rod 10 and cooling water 42 is injected into one end of 
the tube 40 and exits from the other end. Other coolants may be used if 
desired, as is known. 
An optional shutter 44, is provided and is controlled by a shutter control 
signal on a line 46 from known shutter control logic 48. The shutter 
control logic 48 comprises known electronic signal processing hardware 
capable of performing the functions described herein. When the shutter 44 
is open, the light 36 passes through the shutter 44 unattenuated and 
unaffected as a collimated light beam 50. However, when the shutter 44 is 
closed, none of the light 36 passes through the shutter, thus the output 
beam 50 is nonexistent. The light 50 from the laser is incident on a 
focussing lens 52 which converts the collimated beam 50 to a focussed beam 
54 which is focussed on a workpiece 56. 
Referring now to FIG. 2, a prior art pumping scheme for the laser of FIG. 1 
uses the pulse generator 14 to provide continuous stream of pump pulses. 
When the beam is processing the workpiece, the shutter 44 is open, and at 
the other times, the shutter 44 is closed, as indicated in FIG. 2. The 
pulses 100 are typically separated in time by about 100 milliseconds (or 
10 Hz). However, the pulse rate may be as high as 100 Hz, depending on the 
desired beam quality. The higher the pulse rate, the lower the beam 
quality. The time width of the pulses 100 are approximately 1-2 
milliseconds and the laser beam output light pulses accordingly. 
Referring now to FIG. 3, for the prior art pulsing profile shown in FIG. 2, 
the temperature distribution across the cross-section of the laser rod 10 
is a parabolic (or quasi-parabolic) temperature profile 104. The profile 
104 has a peak temperature 106 at the center 108 of the rod 10 and a 
minimum temperature at the points 110 at the outer radius R of the rod 10. 
Because the temperature profile 104 is non-linear and parabolic in nature, 
a corresponding gradient in refractive index exists across the 
cross-sectional profile of the laser rod 10, as discussed hereinbefore. As 
a result, the lasing light 30 within the rod, exhibits unequal path 
lengths for various regions of the light within the rod, also discussed 
hereinbefore. Thus, beam spreading occurs at the output beam 36, thereby 
causing the diameter of the output beam 36 to be wider than it would 
otherwise be. 
Also, such beam spreading causes a degradation of the beam brightness at 
the focal spot on the workpiece. In particular, the laser output beam 
brightness is defined by a known factor defined as: 
EQU M.sup.2 =D.sub.B .times..theta. [Eq. 1] 
where D.sub.B is the beam diameter prior to focussing, and .theta. is the 
divergence (or beam spread) of the laser output beam prior to focussing. 
The M.sup.2 factor determines how small a given lens can focus the beam. 
In general, a "high brightness" beam has a small M.sup.2, with the best 
quality having an M.sup.2 value of 1. For the present inventions M.sup.2 
values in the range of about 5 to 10 were achieved; however, other values 
may be used if desired. 
Referring now to FIG. 4, instead of pulsing the laser as indicated in FIG. 
2, the present invention pulses the flashtube 12 (FIG. 1) with a burst 120 
of pulses, the burst lasting about 1/12 to 1/16 of the thermal time 
constant, as discussed hereinafter. 
When the pulse burst 120 is complete, the pulse generator 14 (FIG. 1) 
pauses for a time T.sub.2 equal to approximately the time it takes for the 
rod to return to the pre-pumped temperature, i.e., the thermal time 
constant, which is about 4 times the thermal relaxation time. The thermal 
time constant, as is known, is a function of the rod diameter and thermal 
diffusivity constant (k) for the rod material, as is known. During the 
pause time (T.sub.2), the laser is moved to the next location to be 
processed on the workpiece. 
After the pause time T.sub.2, a second burst 122 is provided for the burst 
time T.sub.1 having approximately the same number of pulses and the same 
pulse spacing as the burst 120. The laser is then moved to the next 
location on the workpiece, and the sequence is repeated for another burst 
124 which occurs T.sub.2 seconds after the burst 122. I have called this 
technique "cold burst" laser pumping or laser operation. 
As a specific example, a burst of 40 pulses at about 200 Hz (or about 5 
msec apart) lasting for a burst time T.sub.1 of about 200 msec (0.2 
seconds) with individual pulse energy of about 1.25 Joules/pulse and a 
pulse-width of about 650 microseconds (0.65 msec) was used to trepan (or 
outline) cut 1.25 mm diameter holes in 1.38 mm Inconel X-750 (brand name; 
a nickel-based alloy) during the 0.2 second burst (Inconel X-750 
composition being Cr(15.5%) C(0.08%) Fe(7%) Ti(2.5%) Al(0.7%) Ni(74.22%)). 
The focussed laser beam spot size on the workpiece was 0.25 mm. This 
represents a productivity improvement of a factor of about twenty over 
conventional trepan cutting, done at a pulse rate of about 6 Hz, which 
would require about 4 seconds for the same cut. Also, I have run tests 
where the pulse widths varied from about 125 to 1000 microseconds (0.125 
to 1 msec) and the optical pulse energy varied from about 0.37 to 1.7 
Joules/pulse, and have obtained similarly favorable results. However, 
other pulse widths, energies, and spot sizes may be used if desired. Also, 
the invention will work equally well on other metals. However, because a 
nickel-based alloy is such a hard metal, these energy levels will work on 
softer metals. For harder metals higher energy levels may be needed. 
For a diameter of the rod is about 5/16" the thermal time constant is about 
3.4 seconds. However, other radius diameters may be used if desired, which 
will change the time constant T.sub.2 accordingly. 
Referring now to FIG. 5, using the cold burst pumping technique of FIG. 4, 
the resultant temperature profile along the rod cross-section is indicated 
by a curve 150. In this case, however, a maximum temperature 152, which 
exists at the center of the rod 108, also exists across a large region 154 
(approximately 80%) of the cross-section and is substantially flat across 
this region. Thus, the cold burst pumping technique of the invention 
essentially eliminates the thermal gradient across a majority of the 
cross-section of the rod 10, thereby allowing the refractive index of the 
laser rod 10 to be maintained at a substantially constant value. 
Consequently, thermal lensing and beam spreading will not occur in this 
region, thereby minimizing beam spread and allowing maximum output 
brightness of the beam. 
Referring to FIGS. 4 and 5, I have found that the longer the burst time 
T.sub.1, the narrower the region 154 of the temperature profile becomes. 
Accordingly, I have found that the burst time T.sub.1 should be no longer 
than about 1/16 to 1/12 of the thermal time constant (or about 1/4 to 1/3 
of the thermal relaxation time). Thus, for a thermal time constant of 
about 3.4 seconds, the burst time T.sub.1 should be no longer than about 
0.213 to 0.283 seconds. 
I have also found that the number of pulses in the burst is not critical to 
the temperature profile, provided the maximum burst time length is not 
exceeded. Thus, one can use 70 pulses at 100 Hz, or 150 pulses at 400 Hz, 
without significantly altering the profile. 
The number of pulses to use in a pulse burst is determined by the type of 
processing and the type of material used. For example, if it takes 10 
pulses to drill a hole through a given material, this can be accomplished 
by using a 50 msec burst with the pulses 5 msec apart (or a rate of 200 
Hz) or a 25 msec burst with the pulses 2.5 msec apart (or a rate of 400 
Hz). Other combinations of burst length and pulse rate may be used 
provided the total burst time does not exceed the criteria discussed 
hereinbefore. 
One way to explain the outstanding results of the invention is as follows. 
With the continuous pumping technique of the prior art shown in FIG. 2, 
the thermal profile of the rod and the cooling water dictate the shape of 
the temperature profile. In particular, after a pulse is generated, the 
system is allowed to cool enough such that the temperature profile takes 
the shape of the parabolic distribution shown in FIG. 3. Then, when the 
next pulse is issued, while it initially provides a constant temperature 
increase, it does not compensate for the fact that portions of the rod 
have already begun to cool. Thus, the thermal profile has already been 
established and is merely elevated in temperature with each pulse. 
However, when using the pumping technique of the present invention, each 
pulse in a given burst follows quickly after the previous pulse, thereby 
not allowing the rod to cool and create a parabolic temperature 
distribution along the rod cross-section. Therefore, each time a pulse 
occurs it adds a constant amount of energy (and heat) to a prior 
temperature profile, thereby merely increasing the temperature in 
step-wise increments. Consequently, the resultant overall temperature 
distribution profile is maintained substantially flat, thereby obviating 
the problems of the prior art techniques. 
Although the invention has been described as being using with an Nd:YAG 
laser, it should be understood by those skilled in the art that any 
flashtube pumped solid state high power laser having a four level solid 
state material for a gain medium, where the lower lasing level is not the 
ground state of the medium, may be used if desired. 
Also, other means for optically pumping the laser may be used, such as a 
plurality of flashlamps pulsed together, one or more helical flashlamps, 
or one or more laser diodes. Also, shapes other than an ellipse may be 
used for the chamber 22 (FIG. 1). Further, the laser chamber 22 may 
contain a plurality of laser rods configured in series or parallel or in 
other configurations. Also, a single chamber may comprise a plurality of 
laser rods for a plurality of different lasers having separate output 
beams, may be used if desired. 
Although the invention has been described and illustrated with respect to 
the exemplary embodiments thereof, it should be understood by those 
skilled in the art that the foregoing and various other changes, omissions 
and additions may be made without departing from the spirit and scope of 
the invention.