Laser medium for use in a composite slab type laser

A laser medium for use in a composite slab type laser, wherein laser active layers are divided in the longitudinal direction of the laser medium by removing at least a part of a region, which is deviated from a zigzag path and a laser beam to be extracted therefrom does not pass through. Thereby amplified spontaneous emission can be weaken and parasitic oscillation can be effectively suppressed, and further laser oscillation and light amplification can be performed for a long period of time.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
This invention generally relates to a laser medium and more particularly to 
a laser medium for use in a composite slab type laser (hereunder referred 
to simply as a composite type slab laser medium) which can weaken 
amplified spontaneous emission (hereinafter abbreviated as ASE) and 
suppress parasitic oscillation to thereby increase an oscillation 
efficiency or an amplification efficiency. 
2. Description of The Related Art 
As a conventional solid state laser medium, is publicly known a slab laser 
medium which has a slab structure provided with two parallel planes facing 
each other as reflecting inner surfaces (hereunder referred to simply as 
reflecting surfaces) as disclosed in, for example, Japanese Patent 
Application Publication No. 48-15599 Official Gazette. This conventional 
slab laser medium is used to perform laser oscillation or optical 
amplification by extracting a laser beam therefrom. Further, in this 
conventional slab laser medium, the laser beam follows a zigzag path 
undergoing internal reflection at the alternate reflecting surfaces. 
Therefore, even if the distance between the reflecting surfaces is short, 
the optical path followed by the laser beam can be sufficiently long. In 
other words, even if the laser medium is made thin, a desired path length 
can be obtained. Thereby, the laser medium can be efficiently cooled. 
Thus, large pump energy can be supplied to the laser medium. This realizes 
laser oscillation providing a large laser output. 
Further, in general, where a thermal gradient is presented within a laser 
medium, thermal lensing and thermal birefringence occurring due to 
thermally induced distortion and stress cause phase differences among 
laser beams to be extracted. This results in degradation of beam quality. 
However, in case of this conventional slab laser medium, the laser beam 
goes along the zigzag path between the reflecting surfaces as described 
above. Thus, the laser beam equally and repeatedly travels obliquely to a 
transverse direction, in which the thermal gradient is presented, 
perpendicular to the two reflecting surfaces. Consequently, the phase 
difference due to unevenness of refractive index in the laser medium, 
which is caused by the thermal lensing and the thermal birefringence, is 
substantially cancelled, and further a laser beam with relatively good 
beam quality can be obtained. 
Further, in order to obtain a larger laser output and good beam quality, it 
is favourable for such a slab laser to have the thinnest possible laser 
medium. On the other hand, such a slab laser has a minimum thickness 
required to maintain prescribed mechanical strength and accuracy of finish 
of the reflecting surfaces to be formed in such a fashion to be in 
parallel with and face each other. Thus, there is a lower limit to 
thickness of the conventional slab laser which can be realized by using 
ordinary methods. 
As a conventional laser medium obtained by making better use of the 
characteristic of this slab laser medium to improve beam quality and lower 
the lower limit of thickness, is publicly known what is called a composite 
slab type laser medium proposed by J. L. Emmett et al (see The Potential 
of High-Average-Power Solid State Lasers UCRL-53571, Lawrence Livermore 
National Laboratory, California, 1984). This composite slab type laser 
medium is devised to make a thermal gradient therein very small by 
including a laser activating material only in a specific region between 
the reflecting surfaces and moreover making the layer including the region 
containing the activating material very thin. Generally, in a slab laser 
medium, temperature is high in a central portion in the transverse 
direction between the two reflecting surfaces. Further, the closer to end 
portions (i.e., to the reflecting surfaces) a portion, the lower 
temperature. Thus, by removing the laser activating material from the 
central portion, generation of heat therein is prevented. Moreover, by 
making laser pumping regions of the end portions extremely thin, the 
thermal gradient in the transverse direction is made to be very small. 
FIG. 2 is a perspective view showing the construction of a conventional 
composite slab type laser medium. In this figure, reference numeral 100 
designates the conventional composite slab type laser medium; 100a an 
incident surface; 100b an exit surface; 100c and 100d reflecting surfaces 
facing each other; 101 a substrate portion forming an inactive layer; and 
102 and 103 laser glass plate portions forming an active layer. The 
incident and exit surfaces 100a and 100b are formed in such a manner to be 
inclined at a predetermined angle, which meets Brewster's condition, away 
from the reflecting surfaces 100c and 100d when a laser beam is incident 
on the surfaces 100c and 100d in the direction parallel to the reflecting 
surfaces 100a and 100b. These laser glass plate portions 102 and 103 
contain a laser activating material. In contrast, the substrate portion 
101 does not contain any laser activating material. A mirror (not shown) 
to be used for causing optical resonance is placed at both ends of the 
laser medium in the longitudinal direction. Thereafter, when the laser 
medium 100 is pumped by an external pump source (not shown), is generated 
a laser beam which resonates in the laser medium (hereunder sometimes 
referred to as laser resonance light) which follows a zigzag path 
undergoing internal reflection at the alternate reflecting surfaces. Thus, 
laser oscillation is performed. In this case, a laser pumping is effected 
only in the laser glass plate portions 102 and 103 and namely is not 
performed in the substrate portion 101. As a result, in the laser medium 
100, the rise of temperature is suppressed and a temperature distribution 
becomes uniform in the transverse direction. 
Further, results of performance tests of a composite slab type laser, of 
which the laser medium is manufactured for trial by inventors of the 
instant invention, reveal that when the pump energy applied to the slab 
laser is less than a predetermined value, it is favourable to employ the 
thinnest possible glass plates, which contain the most possible laser 
activating material, as the laser glass plate portions 102 and 103 and 
that when the pump energy is increased and becomes equal to or larger than 
a certain value, a gain of the laser is driven into saturation. As a 
result of further study, it is found that the latter phenomenon is caused 
by the ASE and the parasitic oscillation effected in the inside of the 
laser medium. Incidentally, the ASE is light emitted, which is stimulated 
and amplified by fluorescence in a laser medium and attenuates energy 
stored prior to normal laser oscillation and optical amplification. 
Further, the parasitic oscillation is a phenomenon that in a laser medium, 
the ASE goes along an optical path other than a normal optical path to be 
followed by a laser beam which resonates in the laser medium (hereunder 
sometimes referred to as a resonant optical path) but perform a harmful 
oscillation by forming a closed resonant optical path. Further, when the 
ASE and the parasitic oscillation occur, the stored energy is spent for 
the ASE and the parasitic oscillation, so that energy of the laser beam 
following the normal optical path cannot be increased and consequently, a 
larger laser output cannot be obtained. 
It has been known that the ASE and the parasitic oscillation occur in a 
conventional ordinary disk type laser medium and the conventional slab 
laser medium. Further, with respect to the ordinary slab laser medium, has 
been proposed a method for weakening the ASE and suppressing the parasitic 
oscillation. 
Namely, a known method for weakening the ASE and suppressing the parasitic 
oscillation is what is called a segmented spacer method (see "New Slab and 
Solid-State Laser Technology and Application", SPIE., Vol. 736, p. 38, 
1987). 
FIG. 3 is a sectional view of an example of application of this segmented 
spacer to an ordinary slab laser medium 110. As illustrated in this 
figure, according to this method, gasket members 116, . . . , 116 made of 
rubber and so on are put into contact with outer surfaces of parts, at 
which a laser beam is not reflected, of the parallel reflecting planes 
110c and 110d in order to prevent conditions of total internal reflection 
from holding. As described above, in the slab laser medium, a laser beam 
l.sub.1 to be extracted therefrom (hereunder sometimes referred to simply 
as an extraction beam) goes along a zigzag path undergoing total 
reflection at the alternate reflecting surfaces. As a consequence, each 
reflecting surface is scattered with parts of a region 115 (hereinafter 
referred to as a non-path region), through which the extraction beam 
l.sub.1 does not pass. Therefore, the efficiency of oscillation is not 
decreased in case where the conditions of total reflection of the laser 
beam l.sub.1 are made not to hold for parts of the non-path region 115. 
Moreover, by preventing the conditions of total reflection from holding 
for parts of the non-path region, reflection of light l.sub.2 generated by 
the ASE or the parasitic oscillation having reached the parts of the 
non-path region 115 can be prevented. 
Further, as another method for weakening the ASE and suppressing the 
parasitic oscillation, is known a method disclosed in Japanese Unexamined 
Patent Application Publication No. 63-211779 Official Gazette. According 
to this method, wrapping processing is performed on portions corresponding 
to the parts, with which the gasket members are put into contact, of the 
parallel reflecting planes 110c and 110d used for effecting the segmented 
spacer method to form diffused reflection surfaces thereof. Thereby, 
parasitic oscillation can be effectively suppressed by suppressing 
reflection of light emitted due to parasitic oscillation (hereinafter 
referred to as parasitic oscillation light), which comes from the inside 
of the laser medium to the portions corresponding to the parts, with which 
the gasket members are put into contact, of the parallel reflecting planes 
110c and 110d, without using gasket members and so on. 
Moreover, as still another method for weakening the ASE and suppressing the 
parasitic oscillation, is known a method disclosed in Japanese Unexamined 
Patent Application Publication No. 61-287287 Official Gazette. According 
to this method, sandblasting processing is performed on portions 
corresponding to the parts, with which the gasket members are put into 
contact, of the parallel reflecting planes 110c and 110d used for 
effecting the segmented spacer method to form sandblasted surfaces 
thereof. Alternately, etching processing is performed on such portions to 
form diffused reflection surfaces thereof. Otherwise, V-shaped grooves are 
formed in portions corresponding to the non-path regions 115. Thereby, 
parasitic oscillation can be effectively suppressed by suppressing 
reflection of parasitic oscillation light which comes from the inside of 
the laser medium to the portions corresponding to the parts, with which 
the gasket members are put into contact, of the parallel reflecting 
surfaces 110c and 110d. 
However, when the inventors of the instant invention applied the segmented 
spacer method to a composite slab type laser medium, expected effects were 
not obtained. According to the inventors' study of the cause, the 
conclusion was as follows. 
The segmented spacer has been developed on the basis of an idea that 
reflection of a laser beam at parts of the non-path region is restrained 
by making the conditions of total reflection from holding for the parts of 
the non-path region. Thus, the gasket member 116 is used as a member for 
making the conditions of total reflection from holding. In other words, 
the ASE and the parasitic oscillation light impinge on the reflecting 
surface at a certain angle can be effectively made extinct by using the 
segmented spacer, while the segmented spacer method is ineffective against 
the ASE and the parasitic oscillation light go on in parallel with the 
reflecting surfaces. Generally, in an ordinary slab laser medium, a 
relatively large part of the ASE and the parasitic oscillation light 
impinges on the reflecting surface at a certain angle, so that the 
segmented spacer method is effective to a certain extent. 
However, in case of the composite slab type laser medium, most part of the 
parasitic oscillation light advances in the laser glass plate portions 102 
and 103 in parallel with the reflecting surfaces 100c and 100d. 
Consequently, if the segmented spacer method is applied to the composite 
slab type laser medium without change, expected effects cannot be 
obtained. 
Further, the results of the experiments made by inventors of the present 
invention reveals that the gasket member 116 is very easily deteriorated 
by iteration of the laser oscillation and optical amplification. From an 
investigation, it is found that the cause of this is a phenomenon that the 
gasket member 116 is not also heated by heat conducted from the laser 
medium but also absorbs pumping light and light emitted due to parasitic 
oscillation light and generates heat and thus temperature of the gasket 
member 116 is liable to rise to a permissible temperature and higher. 
Especially, this phenomenon is conspicuously presented in case that an 
air-cooling method with low cooling efficiency is employed for cooling the 
laser medium. 
Moreover, in case of the method disclosed in Japanese Unexamined Patent 
Application Publication No. 63-211779 Official Gazette, differently from 
the segmented spacer method, it is unnecessary to use gasket members 
having low heat resistance, and degradation of gasket members owing to 
heat does not occur. This method, however, has a problem of suppressing 
parasitic oscillation light advancing in parallel with the reflecting 
surfaces similarly as the segmented spacer method does. Thus, similarly as 
in case of the segmented spacer method, if the method disclosed in 
Japanese Unexamined Patent Application Publication No. 63-211779 Official 
Gazette is applied to the composite slab type laser medium without change, 
expected parasitic oscillation suppressing effects cannot be obtained. In 
addition, as described above, according to this method, the wrapping 
processing is performed on the surfaces of the laser glass plate portions. 
Thus, the surfaces of the laser glass portions are as good as scratched in 
a sense. It is well known that scratches on the surface of glass 
considerably decrease mechanical strength of the glass and a maximum 
stress, which is a cause of thermal destruction occurring when pump light 
is absorbed, is generated on the surface of the laser medium. Thus, this 
method has a drawback in that the laser glass plate portions are liable to 
cause thermal destruction and consequently, there is a limit to an average 
input of the pump light applicable to the laser medium. 
Furthermore, the method disclosed in Japanese Unexamined Patent Application 
Publication No. 61-287287 Official Gazette is similar to the method 
disclosed in Japanese Unexamined Patent Application Publication No. 
63-211779 Official Gazette in respect of scratching the surface of glass 
and vicinities thereof, and accordingly has the similar defect as the 
method disclosed in Japanese Unexamined Patent Application Publication No. 
63-211779 Official Gazette. 
The present invention is intended to obviate the above described drawbacks 
of the prior art. 
It is therefore an object of the present invention to provide a composite 
slab type laser which can effectively weaken ASE and suppress parasitic 
oscillation and stably perform laser oscillation and light amplification 
for a long period of time. 
SUMMARY OF THE INVENTION 
To achieve the foregoing object and in accordance with a first aspect of 
the present invention, there is provided a composite slab type laser 
medium having a layer of a first type and two layers of a second type 
holding the layer of the first type therebetween and containing a laser 
activating material, wherein the layer of the first type is made up of a 
member that contains a laser activating material of which the quantity is 
less than that of the laser activating material contained in the layers of 
the second type or contains substantially no laser activating material, 
and the sides of the layers of the second type opposed to boundary 
surfaces between the layer of the first type and the layers of the second 
type are the two parallel surfaces facing each other, and being used to 
perform laser oscillation or optical amplification by extracting a laser 
beam, which follows a zigzag path undergoing internal reflection at the 
alternate reflecting surfaces therein, therefrom, and wherein the second 
layers are divided in the longitudinal direction of the laser medium by 
removing at least a part of a region, which is deviated from the zigzag 
path and the laser beam to be extracted therefrom does not pass through. 
Thereby, the optical path to be followed by the parasitic oscillation 
light, which mainly advances in parallel with the reflecting surfaces in 
the laser medium, can be divided without dividing normal resonant optical 
paths. Moreover, the layers of the second type, which are active layers, 
are divided in the longitudinal direction of the laser medium. Thus, there 
is no necessity of scratching the surface of the second layer and the 
vicinities thereof. Consequently, the composite slab type laser medium can 
obstinately resist thermal destruction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, preferred embodiments of the present invention will be 
described in detail by referring to the accompanying drawings. 
First, by referring to FIGS. 1, 4 and 5, a first embodiment of the present 
invention will be described in detail hereinbelow. FIG. 1 is a sectional 
view of a first embodiment (i.e., a first composite slab type laser 
medium) of the present invention. Further, FIG. 4 is an enlarged sectional 
view of a part A indicated by a dashed circle in FIG. 1. FIG. 5 is a 
perspective view of the first embodiment. 
In these figures, reference numeral 10 designates a laser medium; 11 a 
substrate portion composing an inactive layer; 121, 122, 123, 131, 132 and 
133 glass substrate portions made of glass (hereunder referred to as laser 
glass plate portions) composing active layers; and 14 a non-path region 
portion. 
The laser medium 10 is a plate-like portion and is approximately 8 
millimeters (mm) in thickness, 25 mm in width and 85 mm in length. 
Further, a top and back surfaces (hereunder sometimes referred to as 
surfaces in the transverse direction) of this laser medium 10, as viewed 
in FIG. 2, are reflecting surfaces 10c and 10d, respectively. Further, an 
incident and exit end surfaces 10a and 10b facing each other in the 
longitudinal direction are formed to be inclined at a predetermined angle 
.alpha. away from the longitudinal direction and are abraded like mirrors. 
Incidentally, the angle is set such that a laser beam l.sub.1, which 
enters and exits from the laser medium the in the longitudinal direction, 
meets Brewster's condition. (In this embodiment, the angle .alpha. is set 
to be 33.1.degree.). Thereby, total reflection of only polarized light can 
be effected at the alternate reflecting surfaces 10c and 10d. 
Further, as illustrated in FIGS. 1 and 2, this laser medium 10 is 
constructed by welding two groups of the laser glass plate portions 
121-123 and 131-133 (respectively corresponding to the above described two 
layers of the second type and hereinafter sometimes referred to as the 
layers of the second type) to the top and back surfaces of the substrate 
portion 11 (corresponding to the above described layer of the first type 
and hereinafter sometimes referred to as the layer of the first type), 
respectively. 
The substrate portion 11 is a plate-like portion made of transparent 
phosphate glass, which contains substantially no laser activating 
material, and is approximately 6 millimeters (mm) in thickness, 25 mm in 
width and 80 mm in length. This substrate portion 11 is an inactive layer 
which does not effect laser oscillation and optical amplification, 
substantially. Further, a refractive index n.sub.d and a thermal expansion 
coefficient .epsilon. of the glass composing the substrate portion 11 are 
1.55 and 98.times.10.sup.-7 /.degree.C., respectively. 
In passing, the substrate portion 11 may contain laser activating material 
a little. In such a case, it is necesary that a quantity of the laser 
activating material contained in the substrate portion 11 should be less 
than a quantity of the laser activating material contained in the laser 
glass plate portions 121-123 and 131-133. Namely, it is necessary that 
quantity of heat generated in the substrate portion 11 due to laser 
pumping should be less than a quantity of heat generated in the laser 
glass plate portions. 
Further, as illustrated in FIG. 1, the laser glass plate portions 121-123 
and 131-133 are welded to the surfaces 11a and 11b facing each other in 
the longitudinal direction in such a manner that a group of the laser 
glass plate portions 121-123 are separated from another group of the laser 
glass plate portions 131-133 by a predetermined distance (1.35 mm in this 
embodiment). Namely, parts of the layers of the second type, which are 
active layers, are removed and thus the layers of the second type are 
divided into sections in the longitudinal direction. In this case, 
separation portions 15 are made to be positioned in the non-path regions 
14, through which the laser beam l .sub.1 going along a zigzag path in the 
laser medium 10 at the time of effecting laser oscillation and optical 
amplification does not pass. Further, as viewed in FIG. 4, a top end of 
the left end surface of the laser glass plate portion 131 and a top end of 
the right end portion of the laser glass plate portion 132 are positioned 
on lines of intersection of a boundary surface between the laser beams l 
.sub.1 and the non-path regions 14 and the surface 11b of the substrate 
portion 11. 
Further, the laser glass plate portions 12, . . . , 12 and 13, . . . , 13 
are plate-like portions each made of phosphate glass containing Nd.sup.3+ 
ions of 1.times.10.sup.21 /c.c. as laser activating material, and is 
nearly 1 mm in thickness and 25 mm in width. Incidentally, the length and 
arrangement of the laser glass plate portions are geometrically determined 
from the geometry of the zigzag path. Namely, the length and arrangement 
of the laser glass plate portions are necessarily determined by 
positioning the separation portions 15 in the non-path regions 14 in the 
above described manner. This embodiment is constructed such that the laser 
resonance light (or light to be amplified), which is incident on the 
incident and exit end surfaces 10a and 10b in the direction parallel to 
the reflecting surfaces 10c and 10d and meets Brewster's condition (e.g., 
in this embodiment, the laser beam is incident on the reflecting surface 
10d at an angle i equal to 23.8 degrees), is reflected four times by the 
reflecting surfaces 10c and 10d (i.e., is reflected two times by each of 
the surfaces 10c and 10d) and thereafter is extracted to the outside of 
the laser medium. Thus, the central positions of the non-path regions 14 
in the longitudinal direction of the laser medium 10 are respectively 
positioned 15.21 mm (on the reflecting surface 10c), 33.35 mm (on the 
reflecting surface 10d), 51.49 mm (on the reflecting surface 10c) and 
69.63 mm (on the reflecting surface 10d) away from the left end of the 
laser medium 10 of FIG. 1, as viewed in this figure. Therefore, the length 
of each of the laser glass plate portions 121-123 and 131-132 are as 
follows: 
The length of each of the portions 121 and 131 is about 35 mm; 
The length of each of the portions 122 and 132 is about 33 mm; and 
The length of each of the portions 123 and 133 is about 4 mm. 
Furthermore, a refractive index n.sub.d and a thermal expansion coefficient 
.epsilon. of the glass composing the laser glass plate portions 121-123 
and 131-133 are 1.549 and 100.times.10.sup.-7 /.degree.C., respectively. 
When irradiated with predetermined pump light L, these laser glass plate 
portions 121-123 and 131-133 performs stimulated emissions of light of the 
wavelength is 1.06 micrometer (.mu.m). Further, when the laser glass plate 
portions are positioned in a predetermined resonant optical path, laser 
oscillation occurs at wavelength of 1.06 .mu.m. Moreover, when a laser 
beam passes through the laser glass plate portion, light amplification is 
effected. 
Moreover, surfaces of the laser glass plate portions 121-123 and 131-133 
are abraded like mirrors (flatness: .lambda. (632 nanometers (nm)). 
Further, the abraded surfaces of the laser glass plate portions 121-123 
and 131-133 are alternately arranged and are pushed and welded to the 
surfaces 11a and 11b of the substrate portion 11 as illustrated in FIG. 1 
(welding temperature: 450.degree.-550.degree. C.). Further, the other 
surface of each of the laser glass plate portions 121-123 and 131-133 is a 
boundary surface between the portions 121-123 and 131-133 and the outside 
thereof and composes the reflecting surfaces facing each other. In 
addition, the refractive index of the laser glass plate portions 121-123 
and 131-133 is different from that of the substrate portion 11 by a 
quantity equal to or less than 0.03 and the thermal expansion coefficient 
of the laser glass plate portions 121-123 and 131-133 is different from 
that of the substrate portion 11 by a quantity having an absolute value 
equal to or less than 5.times.10.sup.-7 /.degree.C. in such a manner to 
prevent occurrence of Fresnel reflection and thermal distortion as far as 
possible. In passing, end surfaces of the portions 121-123 and 131-133 in 
the direction of their optical axis are not abraded (flatness: 10 micron 
(.mu.) or so). 
Hereunder, will be considered effects obtained in case where the laser 
having the above described arrangement is Q-switched. When mirrors for 
effecting laser resonance are placed at the both ends of the laser medium 
10 in the longitudinal direction and further the laser medium 10 is 
irradiated with pump light L from a pump source (not shown), laser 
resonance light l .sub.1 is generated between the mirror and the laser 
medium. The laser resonance light l .sub.1 follows a zigzag path 
undergoing total reflection at the alternate reflecting surfaces 10c and 
10d facing each other in the transverse direction. In this case, a region 
portion 14 deviated from the zigzag path (i.e., a non-path region portion) 
is formed in the laser medium 10. Namely, the non-path region portion 14 
is a portion through which the laser resonance light l .sub.1 does not 
pass. As above described, portions contained in the non-path regions 14 of 
the active layers in case of the conventional composite slab type laser 
medium, are removed and thus the separation portions 15 are formed. Thus, 
ASE l .sub.2 or parasitic oscillation light generated in the laser glass 
plate portion 131 and traveling from left to right, and vice versa, as 
viewed in FIG. 4, can be emitted from a left end surface 131a to the 
separation portion 15. At that time, the ASE is attenuated due to Fresnel 
loss when passing through the left end surface 131a. For example, in case 
the ASE is generated in the laser glass plate portion 133 and then passes 
through one of the separations 15, the portion 131 in this order and 
another one of the separation portions 15 and finally reaches the laser 
glass plate portion 122, a loss at a boundary surface of each of these 
composing elements owing to Fresnel reflection is about 4% and thus, total 
Fresnel loss becomes approximately 17.5% Moreover, the ASE is attenuated 
also due to scattering thereof caused on the end surface 131a which is not 
abraded. Furthermore, the ASE emitted to the separation portions 15 is not 
pumped therein and therefore is attenuated therein. Thereby, the ASE 
advancing in the longitudinal direction of the laser glass potions can be 
effectively weaken and parasitic oscillation can be suppressed. 
As a matter of course, ASE is generated not only in the longitudinal 
direction but in the traverse direction. However, the projective component 
of an optical path onto the traverse direction is extremely short. 
Therefore, in the traverse direction, attenuation of the stored energy is 
very little in comparison with the attenuation of the stored energy in the 
longitudinal direction. Thereby, in case of this embodiment, the 
attenuation of the stored energy due to the ASE and the parasitic 
oscillation can be effectively suppressed, and oscillation and 
amplification with good efficiency can be achieved. 
Additionally, in this embodiment, there is no necessity of using gasket 
members made of rubber and so forth. Moreover, it is unnecessary to 
scratch the laser glass plate portions 121-123 and 131-133. Thus, this 
embodiment can endure iterated oscillation attended with a large laser 
output for a long period of time. 
Referring next to FIG. 6, there is illustrated a graph showing results of 
measurement of a single pass gain (=optical path length.times.a gain) of 
the laser medium 10 of this embodiment and of a single pass gain of a 
prior art composite slab type laser medium which has the same structure as 
the laser medium of this embodiment does except being provided with no 
separation portions. In FIG. 6, the vertical axis represents single pass 
gains expressed by relative values; the horizontal axis electrical input 
energy (i.e., input pump energy) expressed in kilojoule (kJ). 
Next, a second embodiment of the present invention will be described in 
detail hereinbelow. FIG. 7 is a sectional view of the second embodiment of 
the present invention. Further, FIG. 8 is an enlarged sectional view of a 
part B of FIG. 7. 
In this embodiment, the laser medium 20 has the same construction as the 
first embodiment does, except that predetermined end surfaces in the 
longitudinal direction of laser glass plate portions 221-223 and 231-233 
corresponding to the laser glass plate portions 121-123 and 131-133 of the 
first embodiment are inclined. Therefore, composing elements of the second 
embodiment, which are the same as the corresponding elements of the first 
embodiment, are designated by the same reference numerals as used to the 
corresponding elements of the first embodiment. Further, detailed 
descriptions of the composing elements of the second embodiment, which are 
the same as the corresponding elements of the first embodiment are omitted 
herein for brevity of description. Hereunder, the differences between the 
first and second embodiments will be mainly described. 
In this embodiment, are inclined end surfaces 221a, 221b, 222a, 223a, 231a, 
231b and 232`a in the longitudinal direction of the laser glass plate 
portions 221-223 and 231-233 which are in contact with surfaces of the 
separation portions 25, . . . , 25 corresponding to the separation 
portions 15, . . . , 15. For instance, as illustrated in FIG. 8, the left 
end surface 231a is formed in such a manner to be inclined at an angle 
.beta.(.beta.=23.8.degree. in this embodiment) away from a boundary 
surface 11a between the substrate portion 11 and the laser glass plate 
portion 231. By setting the inclination angle to be 23.8 degrees as 
described above, the left end surface 231a is fromed along the boundary 
surface between the region (hereunder sometimes referred to as the path 
region) including the optical paths and the non-path region of FIG. 4. In 
passing, it is not necessary to form the inclined left end surface 231a 
along the bundary surface between the path region and the non-path region. 
In short, the end surfaces 221a, 221b, 222a, 223a, 231a, 231b and 232a may 
be inclined such that the optical path to be followed by the ASE recedes 
away from the laser medium without reducing the non-path regions. 
Moreover, each of the end surfaces 221b, 222a, 223a, 231a, 231b and 232a 
is formed in a manner similar to the end surface 231a. Incidentally, 
reference character 20a designates an incident end surface of the laser 
medium 20; 20b an exit end surface thereof; and 20c and 20d reflecting 
surfaces thereof. Further, in the second embodiment, the dimensions and 
the constitution of each of these end surfaces and the laser glass plate 
portions 221-223 and 231-233 are the same as those of each of 
corresponding end surfaces and the laser glass plate portions 121-123 and 
131-133 of the first embodiment. 
In case of this embodiment, as illustrated in FIG. 8, most of the ASE l 
.sub.2, which goes from right to left in the laser glass plate portion 
231, as viewed in this figure, and is emitted from the end surface 231a, 
goes away from the laser medium and is never incident on the laser glass 
plate portion 232. Thereby, in comparison with the first embodiment, ASE 
can be more effectively weaken and parasitic oscillation can be more 
effectively suppressed. Moreover, can be obtained technical advantage 
which is similar to the technical advantage of the first embodiment. 
Hereinafter, a third embodiment of the present invention will be described 
in detail with reference to FIGS. 9 and 10. FIG. 9 is a sectional view of 
the third embodiment of the present invention, and FIG. 10 is an enlarged 
sectional view of a part C of FIG. 9. 
This embodiment has the same construction as the first embodiment does, 
except that a member 35 made of transparent glass and having a high 
refractive index is fitted into the separation protions 25, . . . , 25 of 
the second embodiment and is welded to glass members in the neighborhood 
in such a manner to be integral with them. Therefore, composing elements 
of the second embodiment, which are the same as the corresponding elements 
of the first embodiment, are designated by the same reference numerals as 
used to the corresponding elements of the second embodiment. Further, 
detailed descriptions of the composing elements of the third embodiment, 
which are the same as the corresponding elements of the second embodiment 
are omitted herein for simplicity of description. Hereunder, the member 35 
having a high refractive index will be mainly described. 
As the member 35 having a high refractive index, may be used any 
transparent glass material having a refractive index larger than the 
refractive index of glass material composing the laser glass plate 
portions 221-223 and 231-233. An example of such a transparent glass 
material is a glass material of which the primary components are P.sub.2 
O.sub.5, K.sub.2 O, PbO, Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5. 
Incidentally, this glass material contains a blend of PbO, Nb.sub.2 
O.sub.5 and Ta.sub.2 O.sub.5 of 45% by weight. 
The refractive index n.sub.d and the thermal expansion coefficient 
.epsilon. of the member 35 are 1.63 and 104.times.10.sup.-7 /.degree.C., 
respectively. 
In case of the third embodiment, as illustrated in FIG. 10, most of the ASE 
l .sub.2, which goes from right to left in the laser glass plate portion 
231, as viewed in this figure, and is incident from the portion 231 on the 
member 35, goes in the direction indicated by a solid arrow in this 
figure. Namely, in this embodiment, the ASE l .sub.2 is turned away from 
the laser medium by the member 35. This can prevent the ASE l .sub.2 from 
being once again incident on the laser glass plate portion 232. Thereby, 
similarly as in case of the second embodiment, ASE can be more effectively 
weaken and parasitic oscillation can be more effectively suppressed. 
Further, in case of the third embodiment, the boundary surfaces between the 
laser medium 30 and the outside thereof in the transverse direction is a 
uniform plane. As a result, a flow of a refrigerant, which is in contact 
with the boundary surface, becomes uniform. Thereby, efficiency of cooling 
the laser medium can be increased, and temperature distribution in the 
laser medium can be made more uniform. 
While preferred embodiments of the present invention have been described 
above, it is to be understood that the present invention is not limited 
thereto. For example, the above described embodiments can be used for 
effecting laser oscillation or optical amplification of laser light having 
wavelength other than 1.06 .mu.m. In addition, the composing elements of 
the laser medium may be made of a known crystalline material such as YAG 
instead of a glass material. 
Further, it is to be understood that other modifications will be apparent 
to those skilled in the art without departing from the spirit of the 
invention. 
The scope of the present invention, therefore, is to be determined solely 
by the appended claims.