Annular chamber flashlamp including a surrounding, packed powder reflective material

A coaxial flashlamp for optical pumping of a tunable dye laser. The flashlamp includes electrodes preferably made from a tungsten-based alloy for reduced metallic vapor deposition within the flashlamp. Positioned around the outer tube is a packed powder material having a diffuse reflectivity of the order of at least about 98%. The outer surface of the outer tube is placed under an inwardly-directed pressure provided by a pressurized cooling liquid that serves both to cool the flashlamp and simultaneously to offset internally generated pressure within the flashlamp caused by the pressure wave resulting from the movement of the ionization front through the gas within the annular gas chamber defined between the inner and outer tubes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a flashlamp for use in an optically-pumped 
dye laser, and more particularly to a coaxial flashlamp structure having 
an inner dye cell and an annular gas chamber surrounding the inner dye 
cell, wherein the flashlamp provides improved operating efficiency, longer 
operating life, and improved reliability. 
2. Description of the Related Art 
Optically-pumped lasers utilize photons from a source of light to excite 
molecules of an organic dye carried in a liquid medium, to cause the dye 
to emit coherent light having a desired wavelength. The light source can 
be a flashlamp, which provides pulses of light energy to the dye material. 
One form of flashlamp that has been found suitable for use in such an 
optically-pumped laser is a coaxial flashlamp wherein an inner tubular dye 
cavity or cell is provided to receive the laser dye material. Surrounding 
the outer surface of the tubular dye cavity or cell is a flashtube defined 
by a coaxially positioned quartz or silica glass tube that has a greater 
diameter than that of the tubular dye cavity to define therewith a closed, 
annular flashtube of predetermined length. A suitable gas, such as xenon 
gas, is provided within the annular flashtube, and electrodes are 
positioned at each end of the annular flashtube so that an electrical 
current passes through the xenon gas to cause it to ionize and produce a 
large quantity of white light. A coaxial tube structure for one such type 
of flashlamp is disclosed in the present inventor's previously-issued U.S. 
Pat. No. 4,250,427, which issued on Feb. 10, 1981, and is entitled, "Dye 
Laser Flashlamp and Method of Making Same". 
Although flashlamps having the structure disclosed in the present 
inventor's earlier patent are quite adequate for use in dye lasers, and, 
in fact, constitute a significant improvement in reliability and service 
life over previous such structures by virtue of the improved electrode 
arrangement and seal therein disclosed, further improvements in operating 
efficiency, in effective operating life, and in reliability are desirable. 
It is an object of the present invention to provide an improved flashlamp 
structure for use in liquid tunable dye lasers, to provide improved 
operating efficiency, longer useful life, and improved reliability, to 
thereby enable the flashlamp to be more effectively utilized in medically 
directed laser devices. 
SUMMARY OF THE INVENTION 
Briefly stated, in accordance with one aspect of the present invention, a 
flashlamp is provided as a source of light excitation in a tunable dye 
laser. The flashlamp includes a pair of coaxially disposed inner and outer 
tubular members each having different diameters to define therebetween an 
annular gas chamber. The tubular members are fused together at a pair of 
longitudinally spaced positions to provide at each position a 
substantially gas-tight seal therebetween. A ring electrode is positioned 
at each axial end of the gas chamber, and a connector extends from each of 
the electrodes for connection of the electrodes with a source of 
electrical energy. A reflective material surrounds and is in contact with 
the outer tubular member and has a reflectivity of at least about 0.98. 
In accordance with another aspect of the present invention the electrodes 
are made from a high density material to avoid sputtering of electrode 
material within the gas chamber. 
In accordance with a further aspect of the present invention a cooling 
jacket is provided around the reflective material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, there is shown a flashlamp 10 that includes an 
inner quartz tube 12 and an outer quartz tube 14 that has a larger 
diameter than that of inner tube 12 and that surrounds and is coaxial with 
inner quartz tube 12. The inner diameter of outer tube 14 is greater than 
the outer diameter of inner tube 12 to define therebetween an annular gas 
chamber 16. Outer tube 14 has a shorter axial length than that of inner 
tube 12 and has opposite end portions 18 that each include a reduced 
diameter, annular, inwardly extending sealing ridge 20 that contacts and 
is sealed to the outer surface of inner tube 12 to define the ends of 
annular gas chamber 16. 
Positioned interiorly of chamber 16 adjacent each end thereof is an inner 
electrode 22 in the form of an electrically conductive metallic ring to 
encircle inner tube 12. Positioned exteriorly of chamber 16 at each end 
thereof is an outer electrode 24, also in the form of an electrically 
conductive metallic ring that also encircles inner tube 12. Extending 
between each of inner electrodes 22 and its respective outer electrode 24 
are a plurality of electrically conductive connector strips 26 that extend 
axially along the outer surface of inner tube 12. As taught in the present 
inventor's earlier U.S. Pat. No. 4,250,427, the disclosure of which is 
hereby incorporated herein by reference to the same extent as if fully 
rewritten, a number of connector strips 26 are equidistantly 
circumferentially spaced about the outer surface of inner tube 12, and 
they can advantageously be made from molybdenum. Preferably, sealing ridge 
20 is heat fused to the outer surface of inner tube 12 to surround and 
hold those portions of each of connector strips 26 that pass through the 
seal area at sealing ridge 20 between inner tube 12 and outer tube 14, and 
simultaneously to close and seal the ends of chamber 16 to preclude the 
entry of outside air into chamber 16. Fusing the inner and outer tubes 
together also avoids the entry into chamber 16 of compounds given off by 
solvents that were contained in the adhesives formerly employed to seal 
the ends of the outer tube to the inner tube. 
In the past, because of the degree of opacity of the gas within chamber 16 
during the generation of short light pulses (i.e., those having a pulse 
duration of from about 0.5 to about 1 microsecond) within such an annular 
coaxial flashlamp, special reflectors were frequently not provided around 
the outer surface of the outer tube because part of the generated light 
that radiated outwardly from chamber 16 was blocked by the opacity of the 
plasma generated in the gas contained within chamber 16. When they were 
provided, such reflectors often were in the form of aluminum foil that was 
wrapped around the outside of the outer tube, and resulted in about a 10% 
addition to the laser pulse length. However, as the length of the desired 
laser pulse is further increased, the peak currents within chamber 16 are 
reduced, and it was found that the aluminum foil reflectors employed with 
short pulse flashlamps resulted in an inefficient flashlamp. For example, 
a standard aluminum foil wrap, which has a reflectance of the order of 
about 0.85 to about 0.88 resulted in a lamp that had an efficiency of 
production of laser light of less than about 0.05%. A lamp of such low 
efficiency is very sensitive to laser dye contamination and deterioration, 
each of the latter of which can cause a significant reduction in laser 
light output. 
It has been found that providing a very highly reflective material around 
the entire outer surface of outer tube 14 can result in a substantial 
improvement in flashlamp output to the lasing dye material, and thereby 
increases the laser light output. Materials having reflectance values of 
the order of about 0.94 or more have been found particularly effective in 
improving laser light production efficiency to as much as about 0.25%, a 
five-fold improvement, for a pulse length of 450 microseconds. Such 
reflectance values, and the resulting improvement in lamp operating 
efficiencies, have been found to be obtainable by tightly packing a highly 
reflective material 28, such as barium sulfate powder, around and in 
direct and continuous contact with the outermost surface 30 of outer tube 
14, to completely cover the outer tube. A packing pressure in excess of 
100 psi., to provide a tightly packed barium sulfate outer layer having a 
density of about 1 to 4 gm/cc was found to give very good results. In 
addition to barium sulfate, other suitable materials that are expected to 
have high reflectance when tightly packed about the outer tube include 
titanium dioxide, alumina, manganese oxide, and the like. 
To obtain longer laser pulse lengths requires that longer duration light 
pulses be generated in the flashlamp. Longer duration light pulses cause 
the flashlamp electrodes to become hotter, sometimes to the extent of 
overheating the electrode material to the point that the material from 
which inner electrode ring 22 is made begins to sputter away from the 
electrode surface and to condense on the clear fused silica glass defining 
gas chamber 16. As the outer surface 32 of inner tube 12 becomes coated 
with deposited electrode material, the transparency of inner tube 12 
diminishes, thereby restricting the quantity of flashlamp light that can 
reach the lasing dye material within inner tube 12. And after a number of 
shots of the flashlamp the extent of deposition of electrode material on 
the flashlamp inner surfaces can reach a point that lasing efficiency is 
so reduced that replacement of the flashlamp is required. Consequently, 
instead of the usual nickel electrodes utilized in previous coaxial 
flashlamps, it has been found that electrodes made from a high density 
alloy dramatically improve the flashlamp life. Tungsten alloys are 
suitable because such materials have a relatively high density, of the 
order of at least about 16 gm/cc, and electrodes formed from such high 
density materials are less prone to sputtering at the electrode 
temperatures common when providing laser output pulses of the order of 
about 400 microseconds. The electrodes can preferably be formed from a 
high density refractory metal alloy consisting essentially by weight of 
from about 80% to about 98% tungsten, from about 2% to about 8% nickel, 
and from about 1% to about 4% copper. Alternatively, the copper can be 
replaced with about 1% to about 4% iron, and up to about 5% molybdenum can 
be added. 
One tungsten-based alloy that has been found to be effective as an 
electrode material for such applications is alloy CMW 1000, which is 
available from CMW Inc., of Indianapolis, Ind., and which has a density of 
16.96 gm/cc and which consists, by weight, of 90% tungsten, 6% nickel, and 
4% copper. Other high density refractory metal alloys that are believed to 
be suitable include CMW 2000, CMW 3950, and Anviloy 1150, each of which is 
also available from CMW Inc., of Indianapolis, Ind. 
In the operation of a flashlamp in accordance with the lamp is typically 
energized by discharging a charged capacitor connected with the 
electrodes, to provide the electrical energy to cause ionization of the 
gas within chamber 16. Generally that gas is xenon. During the initial 
discharge of electrical energy, the gas around one of the electrodes is 
ionized, and an ionization wave front is formed that progresses 
longitudinally through the gas within chamber 16 toward the opposite 
electrode. Movement of the ionization wave front produces a pressure wave 
that also travels longitudinally within chamber 16, and because chamber 16 
is a closed cavity, the pressure wave causes an inwardly-directed pressure 
to be imposed on inner tube 12 and an outwardly-directed pressure to be 
imposed on outer tube 14, thereby generating a hoop stress within the wall 
of outer tube 14. 
Fused silica glass tubing, which is the commonly employed material from 
which the inner and outer tubes are made, can withstand a significant 
compressive stress, which is the type of loading imposed on inner tube 12 
by the pressure wave. However, such tubing cannot withstand a large 
tensile stress, which is the type of loading imposed on outer tube 14 by 
the pressure wave. And for laser pulse lengths of the order of about 450 
microseconds, which is the preferred laser pulse length for several types 
of medical treatments using laser light, failure of the flashlamp can be 
expected to occur because of the high pressure forces imposed on outer 
tube 14. Accordingly, positioned around the outer surface of the packed 
barium sulfate powder is an outer metallic casing 34 that defines an 
annular enclosing pressure cell 36 for receiving a pressurized liquid, 
such as water or the like, to maintain an inwardly-directed pressure on 
outer tube 14, to, in effect, preload outer tube 14 with a compressive 
stress, thereby to offset the effect on outer tube 14 of the pressure wave 
that is generated during illumination of the flashlamp. Preferably, the 
pressure under which the liquid is maintained is of the order of about 40 
psi. or greater, to maintain a positive inward pressure on outer tube 14 
during the entire operating cycle of the flashlamp. 
In addition to providing a compensating inward pressure on outer tube 14, 
the provision of a cooler, circulating, pressurized liquid within outer 
casing 34, through suitable inlet and outlet connections, also serves to 
cool the flashlamp and extend its effective operating life. Because of the 
buildup of heat that results from operation of the flashlamp, some amount 
of cooling is desirable to maintain the fused silica glass at a 
temperature below which devitrification or other breakdown of the glass 
structure can occur. Additionally, it is also desirable to maintain the 
ends of outer tube 14, adjacent the electrodes and the metal to glass 
seals, at a temperature sufficiently low to minimize electrode temperature 
and stresses at the joints that could cause leakage of the flashlamp gas 
or that could permit the entry of outside air into the interior of the 
flashlamp. 
Incorporation of each of the above-described improvements in the structure 
of a coaxial flashlamp for a tunable dye laser can be expected both to 
significantly lengthen the effective operating life and also to improve 
the operating efficiency of the flashlamp. 
Although particular embodiments of the present invention have been 
illustrated and described, it will be apparent to those skilled in the art 
that various changes and modifications can be made without departing from 
the spirit of the present invention. Accordingly, it is intended to 
encompass within the appended claims all such changes and modifications 
that fall within the scope of the present invention.