Ultraviolet range laser and method for manufacturing the same

An ultraviolet range laser comprising a vacuum vessel having an open end portion hermetically sealed with an output mirror glass substrate, which is featured in that the output mirror glass substrate is formed of a glass material which is capable of allowing a laser beam of 337 nm in wavelength to pass therethrough at a transmittance of 80 to 96% as measured when the thickness of the glass substrate is set to 5 mm and whose thermal expansion coefficient falls in the range of 44.times.10.sup.-7 /.degree.C. to 55.times.10.sup.-7 /.degree.C. at a temperature of 0.degree. C. to 300.degree. C., and in that a metallic sealing ring is hermetically attached to the open end portion of vacuum vessel and the hermetical sealing of the open end portion of vacuum vessel with the output mirror glass substrate is achieved by hermetically adhering the output mirror glass substrate onto the metallic sealing ring via a low melting point solder glass interposed therebetween.

CROSS-REFERENCE TO THE RELATED APPLICATIONS 
This is a continuation application of Application No. PCT/JP96/00934, filed 
Apr. 5, 1996, now abandoned. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an ultraviolet range laser such as a nitrogen gas 
laser or an excimer laser, and also to a method of manufacturing the 
ultraviolet range laser. 
2. Description of the Related Art 
An ultraviolet range laser-oscillating apparatus such as a nitrogen gas 
laser, an excimer laser, etc. is widely utilized in various kinds of 
chemical experiments and chemical analysis, in the field of biochemistry, 
or as an excitation light source for a dye laser. For example, the 
nitrogen gas laser capable of generating a laser beam of ultraviolet range 
of mainly 337.1 nm in wavelength is featured in that it can be operated as 
a pulse oscillator and is capable of achieving a high peak output in spite 
of its relatively simple structure. On the other hand, the excimer laser 
is well known as an oscillating source of a high output laser beam of 308 
nm in wavelength or of other ultraviolet range or vacuum ultraviolet 
range. 
The nitrogen gas laser of ordinary transverse excitation type for example 
is generally constructed as shown in FIG. 1. Namely, it comprises a 
nitrogen gas laser tube device 11 and a power source device 12 connected 
electrically to the nitrogen gas laser tube device 11 for supplying an 
operating voltage to the nitrogen gas laser tube device 11. The power 
source device 12 is provided with a high voltage-generator 13 to which a 
switching device 14 such as a spark gap or a thyratron is connected. The 
power source device 12 is connected to the nitrogen gas laser tube device 
11 via a storage capacitor 15. 
The interior of the vacuum vessel of the nitrogen gas laser tube device 11 
is filled with nitrogen gas as a laser mediuma at a predetermined 
pressure, and provided with a pair of discharge electrodes 17 and 18 
positioned to face to each other. At the occasion of discharging, one of 
the discharge electrodes, i.e. the electrode 18 functions as an anode, 
while the other electrode 17 functions as a cathode, and hence the 
electrode 18 is defined as being an anode 18, while the electrode 17 is 
defined as being a cathode 17. Further, between these anode 18 and cathode 
17 are connected in parallel a peaking capacitor 22 and a charge resistor 
23. 
In the operation of the nitrogen gas laser tube device 11 constructed 
above, a charging from the high voltage-generator 13 to the storage 
capacitor 15 is performed under a condition where the switching element 14 
is kept open. When the switching element 14 is closed after the charging 
is finished, the electric charge stored in the storage capacitor 15 is 
transferred to the peaking capacitor 22 via the switching element 14. When 
the electric charge is transferred in this manner, a pulse of 
predetermined high voltage is impressed between the anode and the cathode, 
thus generating a glow discharge. As a result, an optical resonance is 
caused to generate between a high reflection mirror (not shown) and an 
output mirror (not shown), thus allowing a laser beam to be emitted from 
the output mirror. 
By the way, the mirror portion constituting the optical resonator of the 
conventional nitrogen gas laser tube device is constructed as shown in 
FIG. 2. Namely, a mirror glass substrate 26 having a mirror film 25 
deposited thereon is disposed to hermetically seal the open end of the 
cylindrical vacuum vessel 24 formed of glass or ceramics. On the other 
hand, the mirror portion (not shown) on the high reflection side is 
provided with a mirror substrate having a high reflection mirror film 
adhered thereon, the high reflection mirror film being formed of aluminum 
or a multi-layered dielectric film so as to reflect 99.5% or more of the 
laser beam of the oscillating wavelength of nitrogen laser. Since the 
mirror substrate on the high reflection side is not required to have a 
capability to allow a laser beam to pass therethrough, the material for 
the mirror substrate is not so restricted. 
On the other hand, the mirror glass substrate 26 on the laser output side 
is required to be formed of a material which is capable of exhibiting a 
sufficiently high transmittance of a wavelength of 337.1 nm so as to allow 
a laser beam of the oscillation wavelength of nitrogen laser, i.e. a 
wavelength of 337.1 nm to pass through the mirror glass substrate 26 with 
the minimum possible loss of the laser beam. In view of this requirement, 
a quartz glass such as a synthetic quartz has been generally employed up 
to date as a mirror glass substrate on the laser output side. The quartz 
glass exhibits a transmittance of as high as 85% or more of light having a 
wavelength of up to nearly 270 nm in the ultraviolet range, and is readily 
available. 
However, the thermal expansion coefficient of the mirror substrate made of 
quartz glass is about 5.5.times.10.sup.-7 /.degree.C., whereas the thermal 
expansion coefficient of alumina ceramics which is generally employed as a 
vacuum vessel 24 is about 70.times.10.sup.-7 /.degree.C., i.e. higher than 
that of the quartz glass by more than ten times. Therefore, it has been 
difficult to attain a strong and stable hermetical bonding of both mirror 
substrate and the vacuum vessel in the form of hard seal structure. 
Under the circumstances, there has been extensively adopted a structure as 
shown in FIG. 2 where the bonding between the mirror glass substrate 26 on 
the output side which is made of quartz glass and the open end portion of 
the vacuum vessel 24 which is made of alumina ceramics is achieved by 
making use of an epoxy-based adhesive 27 which is relatively excellent in 
elasticity. This hermetical bonding is generally called a soft seal. 
However, the conventional nitrogen laser of the aforementioned soft seal 
structure where the hermetical sealing of the mirror glass substrate is 
effected by making use of an epoxy-based adhesive as explained above is 
accompanied with a drawback that the hermetical sealing strength is 
deteriorated with time due to the effect of ambient moisture, resulting in 
the invasion of the outer air into the vacuum vessel, thus deteriorating 
the laser output within a relatively short period of time. In particular, 
the epoxy-based adhesive is generally vulnerable to moisture so that the 
deterioration in operational reliability of the laser under a humid 
climate or circumstance would become more conspicuous. 
In order to overcome such a problem and to provide a laser which is capable 
of maintaining the initial condition of filled gas for a long period of 
time so as to maintain a high reliability of the laser for a long period 
of time, it is desired to fabricate the hermetic sealing portion of the 
laser into a structure which is free from any influence of ambient 
atmosphere, i.e. a hard seal structure. One example of such a structure is 
disclosed in Japanese Patent Unexamined Publication Hei/4-348091. 
According to this Publication, Covar (trade name) and Covar glass are 
hermetically adhered onto a vacuum vessel and quartz glass is further 
adhered thereover with a glass having an intermediate thermal expansion 
coefficient interposed therebetween, the mirror glass substrate formed of 
quartz glass of the same kind as the aforementioned quartz glass being 
fuse-bonded onto the vacuum vessel via the quartz glass. 
However, the construction proposed in this Publication is very complicated, 
thus making the manufacture thereof very troublesome, hence badly 
affecting the reliability of the device and at the same time making the 
device larger in length and diameter. 
As for a laser other than the ultraviolet range laser, there is proposed to 
hermetically bond a metallic part constituting a portion of a vacuum 
vessel to a mirror glass substrate through a low melting point solder 
glass as disclosed for example in Japanese Patent Unexamined Publication 
Shou/63-10578 (corresponding to U.S. Pat. No. 4,727,638) or Japanese 
Utility Model Publication Hei/7-11477. According to the former 
Publication, a borosilicate glass such as BK-7 (trade name) which is 
generally useful for an optical device is used for the laser mirror 
substrate. However, since such a glass substrate for optical use is poor 
in transmittance as explained below, it is not suited for use as an output 
side mirror substrate for the ultraviolet range laser for emitting a 
relatively short wavelength such a nitrogen gas laser. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an 
ultraviolet range laser which is capable of overcoming the aforementioned 
problems in the prior art, compact in structure and capable of maintaining 
a highly reliable operation performance thereof for a long period of time. 
Another object of the present invention is to provide a method of 
manufacturing such an ultraviolet range laser. 
Namely, according to the present invention, there is provided an 
ultraviolet range laser comprising a vacuum vessel having an open end 
portion hermetically sealed with an output mirror glass substrate, which 
is featured in that the output mirror glass substrate is formed of a glass 
material which is capable of allowing a laser beam of 337 nm in wavelength 
to pass therethrough at a transmittance of 80 to 96% as measured when the 
thickness of the glass substrate is set to 5 mm and whose thermal 
expansion coefficient falls in the range of 44.times.10.sup.-7 /.degree.C. 
to 55.times.10.sup.-7 /.degree.C. at a temperature of 0.degree. C. to 
300.degree.C., and in that a metallic sealing ring is hermetically 
attached to the open end portion of vacuum vessel and the hermetical 
sealing between the open end portion of vacuum vessel and the output 
mirror glass substrate is achieved by hermetically adhering the output 
mirror glass substrate onto the metallic sealing ring with a low melting 
point solder glass interposed therebetween. 
According to the present invention, there is further provided a method of 
manufacturing an ultraviolet range laser comprising the steps of: 
positioning discharge electrodes in an interior of a cylindrical ceramic 
vacuum vessel and at the same time hermetically bonding a metallic sealing 
ring onto an open end portion of the vacuum vessel; press-contacting part 
of inner surface of a mirror glass substrate having if required a mirror 
film adhered thereon to the open end face of the vacuum vessel while 
feeding a low melting point solder glass at an interface between the 
metallic sealing ring and the mirror glass substrate; baking the low 
melting point solder glass so as to hermetically adhere the mirror glass 
substrate onto the metallic sealing ring; and filling the interior of the 
vacuum vessel with gas constituting a laser medium. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will be explained further with reference to drawings 
illustrating one embodiment of this invention. Throughout these drawings, 
the same parts are indicated by the same reference numerals. FIG. 3 shows 
a nitrogen gas laser which has been accomplished according to this 
invention. Referring to FIG. 3, the nitrogen gas laser according to this 
embodiment is provided with an output mirror glass substrate 32 which is 
hermetically bonded in the form of a hard seal structure to one of the 
open ends of a cylindrical vacuum vessel 31 formed of alumina ceramics and 
also with a high reflection mirror glass substrate 33 which is 
hermetically bonded also in the form of a hard seal structure to the other 
of the open ends of the cylindrical vacuum vessel 31. 
At the middle of the interior of the cylindrical vacuum vessel 31, a pair 
of main discharge electrodes, i.e. a cathode 17 and an anode 18 both 
formed of a nickel or brass rod are disposed parallel with the axial 
direction of the vacuum vessel 31 and in a manner to face to each other 
with a laser beam path being interposed therebetween. These cathode 17 and 
an anode 18 are fixed and supported by a pair of supporting rods 34 and 
35, and another pair of supporting rods 36 and 37 respectively, these 
supporting rods 34, 35, 36 and 37 being mounted on the vacuum vessel 31 in 
such a manner that they are hermetically pierced into the interior of the 
vacuum vessel 31 through the wall thereof. 
An output mirror film 38 is adhered onto the central portion of the inner 
surface 32a of the output mirror glass substrate 32. The peripheral 
portion of the output mirror glass substrate 32 is press-contacted with 
the open end face 31a of the vacuum vessel 31. A substrate-sealing ring 39 
made of Covar or an iron/nickel alloy is hermetically soldered by making 
use of a metallized layer M and a silver solder layer 40 onto the outer 
peripheral wall of the open end portion of the vacuum vessel 31 so as to 
maintain the vacuum of the vacuum vessel 31. 
The substrate-sealing ring 39 is provided with a top portion slightly 
extended along the axis thereof from the open end face of the vacuum 
vessel where a curved portion 39a for alleviating stress is formed at the 
intermediate portion thereof and a flange portion 39b is formed at the 
distal end thereof expanding externally from the curved portion 39a. The 
flange portion 39b is disposed in close to an intermediate portion in 
thicknesswise of the outer circumferential wall 32b of the output mirror 
glass substrate 32. The inner diameter of the curved portion 39a of the 
substrate sealing ring is made equal to or slightly larger than the outer 
diameter of the glass substrate 32. Therefore, the mirror film can be 
easily and precisely aligned with the axis of the laser beam path at the 
occasion of inserting the glass substrate 32 into the substrate sealing 
ring so as to press-contact it with the open end face 31a of the vacuum 
vessel, and at the same time it is possible to prevent a solder glass 
material from unwillingly flowing into the interface between the inner 
surface 32a of the glass substrate and open end face 31a of the vacuum 
vessel at the baking step of a low melting point solder glass to be 
explained below. 
The outer circumferential wall 32b of the output mirror glass substrate 32 
is hermetically adhered onto the flange portion 39b of the 
substrate-sealing ring by making use of a low melting point solder glass 
41 deposited covering both circumferential wall 32b and the flange portion 
39b thereby vacuum-sealing the vacuum vessel. Likewise, the mirror glass 
substrate 33 disposed on the high reflection side and having a high 
reflection mirror film 42 attached thereto is adhered onto the vacuum 
vessel 31 by making use of a metallic sealing ring 44 which is 
hermetically bonded to the outer circumference of the open end portion of 
the vacuum vessel via a metallized layer and a silver solder layer 43 as 
well as by making use of a low melting point solder glass 45 so as to 
maintain the vacuum of the vacuum vessel. The interior of the vacuum 
vessel is filled with nitrogen gas of predetermined pressure, the nitrogen 
gas constituting a laser medium. The vacuum vessel is connected with a 
power source device 12 for supplying a pulse voltage to the cathode and 
anode. These components are accommodated in a case (not shown) thereby 
accomplishing a laser oscillation apparatus. 
Next, preferred procedures of manufacturing this laser tube apparatus will 
be explained. 
First of all, as shown in FIG. 4, the metallized layer M is deposited at a 
soldering area of the outer peripheral wall of the open end portion of the 
vacuum vessel 31 made of alumina ceramics. The deposition of the 
metallized layer M can be carried out by coating a slurry mainly 
comprising molybdenum powder and manganese powder dissolved in advance in 
a solvent and, after being baked, the baked layer is covered by a nickel 
plating layer. 
Subsequently, the open end face 31a of the vacuum vessel is 
surface-finished by polishing the open end face 31a in a direction 
perpendicular to the central axis, i.e. the axis C of the laser beam path. 
With this surface finishing, any portion of the metallized layer that 
might be protruded up to the open end face 31a may be erased away. 
Then, as shown in FIG. 5, the substrate sealing ring 39 and the silver 
solder are placed in the region of the metallized layer on the outer 
peripheral wall of the open end portion of the vacuum vessel 31, and 
thereafter hermetically soldered by heating them at a temperature of about 
780.degree. C. Concurrently with this soldering step, each of the 
electrode-supporting rods bearing in advance the cathode 17 or anode 18, 
and an exhaust tube (not shown) are hermetically soldered onto 
predetermined portions of the vacuum vessel. Since this soldering step of 
each metallic component onto the vacuum vessel is performed by heating a 
solder up to such a high temperature which may cause the mirror glass 
substrate and the mirror film deposited on the surface of the mirror glass 
substrate to become denatured, the soldering step is performed before 
mounting the mirror glass substrate on the vacuum vessel, i.e. in a 
condition where only the vacuum vessel can be heated. 
Meanwhile, the output mirror film 38 formed of a dielectric multi-layered 
film is vapor-deposited in advance on one main surface of the output 
mirror glass substrate 32 which is made of a material to be explained 
hereinafter and shaped into a disk form. Thereafter, as shown in FIG. 6, 
the vacuum vessel 31 is placed with its central axis directed 
perpendicularly and then the mirror glass substrate 32 is inserted into 
the flange 39b of the substrate-sealing ring. In this step, the glass 
substrate 32 is just fitted in the inside of the curved portion 39b of the 
sealing ring so that the glass substrate 32 can be precisely positioned in 
place. As a result, the peripheral portion surrounding the mirror film on 
the inner surface of the glass substrate 32 is press-contacted with the 
inner circumference portion of the open end face 31a of the vacuum vessel, 
and then a heat resistant weight 46 is placed on the glass substrate so as 
to keep this pressed condition. It is of course possible to make use of a 
suitable fixing tool and an elastic member in place of employing the 
aforementioned weight for keeping this press contact condition. 
As for the high reflection mirror side not shown in figures, the mirror 
substrate is brought into contact with the vacuum vessel and placed in 
position. With respect to the material for the mirror substrate on the 
high reflection mirror side, any material such as a metallic material can 
be used. It is however preferred to employ the same material as that of 
the output mirror glass substrate 32 in view of accomplishing the 
hermetical sealing of the vacuum vessel with both mirror substrates in one 
sealing step as well as in view of achieving a sealing of high 
reliability. 
Subsequently, a slurry of a low melting point solder glass material 41a 
which is prepared by dissolving the low melting point solder glass 
material in a suitable solvent is applied in a suitable amount to the 
corner portion where the outer peripheral wall of the mirror glass 
substrate 32 is closely contacted with the flange portion 39b of the 
substrate-sealing ring by making use of a nozzle 47 of a coating 
apparatus. The slurry thus coated is allowed to be temporarily solidified 
by drying it. Likewise, the low melting point solder glass is applied to 
the high reflection mirror side. 
Subsequently, the vacuum vessel under this condition is introduced into a 
heating furnace for sealing, and the temperature of the furnace is 
gradually raised up to for example 460.degree. C. which is the baking 
temperature of the low melting point solder glass and then this 
temperature is maintained for a predetermined period of time after which 
the temperature of the furnace is gradually cooled down. As a result, the 
low melting point solder glass is baked, i.e. solidified after being 
fused, thus achieving a hermetical and strong sealing of the interface 
between the outer peripheral wall 32b of the mirror glass substrate and 
the flange portion 39b of the substrate-sealing ring. By the way, the 
heating temperature in this sealing step is controlled such that the 
mirror glass substrate and the mirror film consisting of a dielectric 
multi-layered film would not be or not substantially be denatured. 
After being fabricated in this manner, the interior of the vacuum vessel is 
exhausted through an exhaust tube (not shown), or the vacuum vessel and 
electrodes inside the vacuum vessel are heated to a suitable temperature 
so as to release any gas remained therein thereby exhausting the vacuum 
vessel. Subsequently, nitrogen gas is introduced into the vacuum vessel 
through the exhaust tube so as to fill the vacuum vessel with a 
predetermined pressure of nitrogen gas, and then the exhaust tube is cut 
and sealed. Thereafter, the vacuum vessel is accommodated in a case 
together with a power source if required, thereby accomplishing the 
nitrogen laser as shown in FIG. 3. 
The output mirror glass substrate 32 is formed of a glass material which is 
capable of allowing a laser beam of 337 nm in wavelength to pass 
therethrough at a transmittance of 80 to 96% as measured when the 
thickness of the glass substrate is set to 5 mm, and whose thermal 
expansion coefficient is in the range of 44.times.10.sup.-7 /.degree.C. to 
55.times.10.sup.-7 /.degree.C. at a temperature of 0.degree. C. to 
300.degree. C. More preferably, the output mirror glass substrate 32 
should be formed of a glass material which is capable of allowing a laser 
beam of 337 nm in wavelength to pass therethrough at a transmittance of 85 
to 94% as measured when the thickness of the glass substrate is set to 5 
mm. Suitable examples of such a glass substrate include borosilicate 
glass, which may be available for example from Toshiba Glass Co. as a 
Material Code 084 Glass (trade name: FP-3) or from Corning Co. as a 
Material Code CGW-7056. 
The Material Code 084 Glass has a thermal expansion coefficient of about 
52.times.10.sup.-7 /.degree.C., and the spectral transmittance at the 
ultraviolet range thereof was found to be as shown by the solid line A in 
FIG. 7. The sample measured in this case had a thickness of 5 mm and was 
not subjected to any kind of thermal chemical treatment or coating 
treatment. This sample allowed a laser beam of 337.1 nm which is an output 
wavelength (La) of the nitrogen laser to pass therethrough at a 
transmittance of 92%, and a laser beam of 308 nm which is a wavelength 
(Lb) of the Xe.sub.2 Cl excimer laser to pass therethrough at a 
transmittance of 85%. By contrast, the borosilicate glass BK-7 which is 
known as being suited for use in soldering or fusing it with Covar was 
found to be as shown by the dot and dash line B in FIG. 7. This 
borosilicate glass BK-7 allowed a laser beam of 337.1 nm to pass 
therethrough at a transmittance of 73%, and a laser beam of 308 nm to pass 
therethrough at a transmittance of as small as 22%, so that it would be 
impossible to employ such a borosilicate glass in an ultraviolet range 
laser. 
The output mirror glass substrate which is suited for use in an ultraviolet 
range excimer laser is formed of a glass material which is capable of 
allowing a laser beam of 308 nm in wavelength to pass therethrough at a 
transmittance of 75 to 92% as measured when the thickness of the glass 
substrate is set to 5 mm, and whose thermal expansion coefficient is in 
the range of 44.times.10.sup.-7 /.degree.C. to 55.times.10.sup.7 
/.degree.C. at a temperature of 0.degree. C. to 300.degree. C. More 
preferably, the output mirror glass substrate should be formed of a glass 
material which is capable of allowing a laser beam of 308 nm in wavelength 
to pass therethrough at a transmittance of 80 to 90% as measured when the 
thickness of the glass substrate is set to 5 mm. 
It has been found as a result of extensive studies by the present inventors 
that a borosilicate glass comprising 65.0 to 70.0% of SiO.sub.2, 2.0 to 
8.0% of Al.sub.2 O.sub.3, 6.0 to 10.0% of K.sub.2 O, 0.5 to 1.5% of 
Li.sub.2 O, 15.0 to 22.0% of B.sub.2 O, and not more than 2.0% in total of 
other elements in a relative mass ratio is suited for a practical use as a 
material for the output mirror glass substrate. 
As for the output mirror film 38, a dielectric multi-layered film 
comprising silicon oxide and hafnium oxide, and exhibiting 10 to 50%, 
preferably 20 to 40% in reflectance to the laser beam of 337 nm in 
wavelength is desirable. In the case of a pulse laser of such an 
ultraviolet range, the output mirror film may be omitted provided that the 
reflectance at both surfaces of the glass substrate itself is about 10% or 
more. 
As for the material for the low melting point solder glass and the 
substrate-sealing ring to be soldered to the vacuum vessel, those having a 
thermal expansion coefficient falling in the range of 44.times.10.sup.-7 
/.degree.C. to 55.times.10.sup.-7 /.degree.C. at a temperature of 
0.degree. C. to 300.degree. C. are suited. Examples of such a low melting 
point solder glass include a product from Nihon Denki Glass Co., Material 
Code LS-0111, which is a powdered glass for electronic parts. This 
powdered glass may be employed by transforming it into a slurry by 
dissolving it in a solvent comprising mainly acetic acid, the slurry being 
subsequently coated and heated to a baking temperature thereby achieving 
the fuse-bonding thereof. The heating of the slurry should preferably be 
performed for obtaining an excellent hermetical bonding under the 
conditions that the slurry is first heated up to the baking temperature of 
460.degree. C. at an increment of about 10.degree. C. per minute, and then 
gradually cooled down. The thermal expansion coefficient of the 
aforementioned Material Code LS-0111 which is a low melting point solder 
glass is 50.times.10.sup.-7 /.degree.C. at a temperature of 0.degree. C. 
to 300.degree. C. The material for the substrate-sealing metal ring should 
be selected from materials which exhibit an excellent soldering property 
in relative to Covar or ceramics such as alumina, and, if required, may be 
provided with a strain-absorptive portion such as a thin wall portion or a 
curved portion. By the way, the Covar has a thermal expansion coefficient 
ranging from 45.times.10.sup.-7 /.degree.C. to 52.times.10.sup.-7 
/.degree.C., for example 50.times.10.sup.-7 /.degree.C. at a temperature 
of 0.degree. C. to 300.degree. C. 
When the glass substrate, the low melting point solder glass and the 
substrate-sealing ring, each formed of the aforementioned materials, are 
employed, it is possible to achieve an excellent hermetical sealing which 
is free from strain and highly reliable, because the thermal expansion 
coefficient as well as the shrinkage factor of these materials at each 
temperatureraising or lowering process during the sealing step using a low 
melting point solder glass are very close to each other. 
In the above example, the surface of the output mirror glass substrate 
where the mirror film is adhered is in direct contact with the open end 
face of the vacuum vessel. However, the surface of the output mirror glass 
substrate may be in indirect contact with the open end face of the vacuum 
vessel with a spacer ring interposed therebetween. The vacuum vessel and 
the substrate-sealing ring may be hermetically bonded together by making 
use of the aforementioned low melting point solder glass in stead of using 
a solder. 
According to the above example, since the sealing portion of the vacuum 
vessel constituting the laser is hermetically sealed by making use of an 
inorganic solder glass, there is substantially no possibility that the 
vacuum sealing of the vacuum vessel would be deteriorated under the 
ordinary working environment. Furthermore, since the hermetical sealing is 
achieved with the use of materials which are very close in thermal 
expansion coefficient to each other, it is possible to carry out the 
exhaustion of the laser, i.e. the exhaustion of the laser while baking the 
laser at a relatively high temperature before a laser medium is introduced 
into the vacuum vessel. As a result, any impurity gases or other 
impurities adhered on or included in the inner wall or electrodes inside 
the laser can be sufficiently removed by this exhaustion under a high 
temperature baking. Therefore, it is possible, even if the temperature of 
the inner wall or inner electrodes of the laser are raised during the 
discharging operation, to inhibit the deterioration of laser output than 
might be caused by the release of impurity gases or other impurities. 
Moreover, since the bonding structure is relatively simple, the number of 
parts to be used can be minimized and at the same time any parts which 
might be a cause of increasing the length or diameter of the vacuum vessel 
can be dispensed with, thereby making it possible to make a laser into a 
compact structure. It is also possible according to this invention to 
maintain the initial state or condition of the laser medium filled in the 
vacuum vessel for a long period of time. Moreover, according to this 
invention, since the transmittance of the mirror substrate to the output 
laser beam is sufficiently high and the denaturing of the mirror film 
would be hardly brought about, it is possible to obtain an ultraviolet 
range laser which is free from any deterioration of laser output for a 
long period of time and excellent in reliability. 
In the above example, a pulse laser of lateral discharge excitation type is 
explained. However, the discharge type of the laser is not limited to that 
of the above example, but this invention can be also applied for example 
to an ultraviolet range laser of horizontal discharge excitation type 
where the laser oscillation is effected coaxial with the discharge 
direction. 
As explained above, it is possible according to this invention to obtain an 
ultraviolet range laser which is compact in structure and capable of 
maintaining highly reliable operational properties for a long period of 
time. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, representative embodiments shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.