Solid state lasers

A solid state laser has an elongate slab of lasing material having a rectangular cross section with the lower face of the slab contacting a slab mount which is of a high thermal conductivity material. Energy to drive the lasing medium is provided by a flash lamp. Upper and lower faces of the slab are polished to an optically smooth finish so that light is able to propagate in a generally axial direction through the slab. Side faces of the slab are polished and then re-roughened to provide a finish with a surface damage zone comparable in depth to the wavelength of the lasing emission. For a lasing wavelength of one micrometer, the depth of surface damage is in the region of one micrometer.

The present invention relates to solid state lasers and in particular, 
though not necessarily, to solid state lasers employing a slab-type lasing 
medium. 
Many conventional solid state lasers employ a cylindrical lasing rod, for 
example of Nd:YAG, with mirrors placed at opposed ends of the rod. Lasing 
light propagates axially backwards and forwards along the rod causing 
amplified stimulated emission to occur. A problem with this arrangement is 
that the optical pump light applied to the rod generates heat which in 
turn gives rise to temperature gradients across the rod, i.e. transverse 
to the direction in which light propagates. The temperature gradients 
cause non-uniformities in the optical properties of the rod to arise, 
causing distortion and power loss in the light output of the laser. Whilst 
it is possible to alleviate the steady state problem by various means 
including use of liquid coolants, the problem of dynamic changes in 
temperature remains significant with respect to lasing performance. 
More recently, rod-type lasing media have been replaced with elongate slabs 
having a rectangular or square cross-section, in an attempt to further 
reduce the problems caused by temperature gradients within the lasing 
media. In a slab-type medium, light propagates lengthwise along the medium 
in a zig-zag manner, reflecting alternately off the two opposed longer 
side faces of the slab which are polished smooth to maximise internal 
reflections. This is illustrated in FIG. 1. 
The zig-zagging of the light path effectively averages out the effect of 
temperature gradients .DELTA.T.sub.b between the two opposed faces 1, 2 
from which the light reflects, reducing distortion of the light beam and 
therefore improving collimation of the laser output beam. To remove heat 
from the slab it is usually cooled via one or both of the internally 
reflecting faces 1, 2, e.g. using a liquid coolant. 
With slab-type lasing media however, there still remains the problem of 
temperature gradients .DELTA.T.sub.66 arising across the width of the 
media, i.e. between the side faces 3, 4 from which the light beam is not 
reflected. 
It is an object of the present invention to overcome or at least mitigate 
disadvantages of known solid state lasers. 
It is a further object of the present invention to reduce heat generation 
within solid state laser media and to reduce temperature gradients arising 
therein. 
According to a first aspect of the present invention there is provided a 
method of reducing temperature gradients within an elongate solid state 
lasing medium, the medium comprising one or more substantially 
non-reflecting faces for emitting or scattering radiation generated by 
amplified spontaneous emission (ASE), the method comprising treating said 
non-reflecting face or faces to reduce the amount of heat generated by 
radiation passing therethrough. 
Where the medium is provided with one or more preroughened faces, the or 
each face may first be polished visually smooth and then reroughened, such 
that the depth of surface damage at the roughened face is less than that 
of the original face. Thus, the scatter path at the roughened face is 
reduced and heat generation, due primarily to pump light, is also 
consequently reduced. "Roughening" may be taken to include producing a 
surface finish which scatters incident light. The finish may comprise 
periodic or random patterning. 
Alternatively, faces of the lasing medium through which it is required to 
emit or scatter parasitic ASE light can be polished visually smooth and 
coated with a material whose thermal and optical properties are the same 
or similar to those of the lasing medium, but in which heat dissipation is 
less than that in the lasing medium. The outer surface of the coating is 
then roughened to provide a substantially non-reflecting scattering 
finish. Given the relatively low heat dissipation which occurs within the 
coating material, even a relatively large scatter path at the outer 
surface of the coating material will result in a relatively small amount 
of heat generation compared to that which would occur at a roughened 
surface of the lasing medium. Where the lasing medium is in the form of a 
slab, the surface coating may take the form of thin sections of undoped 
lasing material or dielectric bonded to the polished short faces of the 
slab. Alternatively the coatings may be deposited using thin film 
deposition techniques. 
The reduced level of heat generation at the periphery of the lasing medium 
reduces the need for heat sinking or thermal impedance matching to the 
side faces. In the case of a slab-type medium, this makes it possible to 
thermally isolate the slab on three side faces with a gas filled or 
vacuous gap (thereby reducing the affect of external temperature 
variations) and to provide a heat sink on only one of the beam reflecting 
faces to permit heat removal. 
In contrast to conventional approaches to reducing the effects of 
temperature gradients within a solid state lasing medium, which generally 
involve increasing the conductivity of heat within or around the lasing 
medium, the present invention relies upon reducing the levels of heat 
generation within the lasing medium itself. 
This reduction has been achieved as a result of realising the significant 
role which the rough surface finish and resulting depth of surface damage 
(i.e. crystal discontinuity) given to faces of lasing media play in the 
generation of heat within the media. In order to allow parasitic light 
generated by amplified spontaneous emission (ASE) to be removed from a 
lasing medium, side faces of the medium are often ground so as to 
significantly reduce internal reflection of this light. In the case of 
slab-type media, the two shorter side faces from which amplified 
stimulated light is not reflected are provided with this ground finish. 
Grinding generally results in a rough surface finish and a significant 
depth of surface damage to the lasing crystal. Whilst the surface 
roughness may have an rms peak to peak amplitude of around 1 .mu.m, 
surface damage may extend into the crystal by up to 20 .mu.m (many times 
the wavelength of the lasing light) causing light, particularly pump 
light, exiting and entering the ground faces to be scattered by multiple 
bounce reflections. As the light gives up a given amount of energy per 
unit length of its travel path, a relatively large amount of heat is 
generated at the ground faces (the pump light contributing the majority of 
energy given up as heat). The present invention seeks in particular to 
reduce heat generation by light transmission at rough faces. 
According to a second aspect of the present invention there is provided a 
laser comprising an elongate solid state lasing medium having a plurality 
of polished faces arranged to support lasing within the medium at a laser 
wavelength and at least one non-reflecting face arranged to provide for 
egress of ASE radiation from the lasing medium, wherein said at least one 
non-reflecting face is provided with a surface finish arranged to minimise 
heat generation in the vicinity of the non-reflecting face. 
In one embodiment of the invention the or each non-reflecting face has a 
rough surface finish, wherein the depth of surface damage produced is less 
than 5 .mu.m but greater than 100 nm and more preferably greater than 0.5 
.mu.m. 
In an alternative embodiment of the present invention, the or each 
non-reflecting face is provided by a polished face of the lasing medium 
and a layer of relatively low heat dissipating material covering at least 
a portion of said polished face, the cuter surface of the covering layer 
having a rough surface finish. The coating layer may comprise undoped 
lasing material or alternatively may be a dielectric whose optical 
properties are matched to those of the lasing material. The coating layer 
may be a thin slice of material bonded, e.g. by diffusion bonding, to the 
corresponding surface of the lasing medium. 
In a first embodiment of the present invention, the lasing medium is a 
slab-type lasing medium having upper and lower polished faces for 
supporting lasing and non-reflecting side faces to provide for egress of 
ASE radiation, the laser comprising an air gap surrounding the lasing 
medium on the two non-reflecting side faces and on one of the reflecting 
faces, and a relatively high thermal conductivity material contacting the 
lasing medium over the other of the reflecting faces. Preferably, the 
laser comprises a source of pump light adjacent to the reflecting face of 
the slab which is not in contact with said high conductivity material and 
means opposed to the non-reflecting side faces for reflecting pump light 
into the lasing medium whilst absorbing ASE light. Preferably, the source 
of pump light is surrounded on three sides by reflecting means for 
directing pump light from the source to the lasing medium. More 
preferably, said reflecting means and the means for reflecting and 
absorbing opposed to the non-reflecting sides, comprise beryllia or 
alumina. 
In an alternative embodiment of the invention, the laser may be 
double-pumped, i.e. with pump sources arranged on two sides of an elongate 
slab-type lasing medium. A liquid coolant may be contained between the 
pump sources and the medium.

There will now be described with reference to FIGS. 2 and 3 a solid state 
laser embodying the present invention. The laser comprises a slab of 
lasing material 5 which may for example be Nd:YAG. However, it will be 
appreciated that any other suitable type of material may be used. The slab 
is elongate having a rectangular crosssection, with the lower face 5a of 
the slab contacting a slab mount 6 which is of a high thermal conductivity 
material. A preferred material for the mount 6 is beryllia. The slab mount 
6 is in turn mounted on an aluminium base plate 7. Due to different rates 
of thermal expansion of the slab mount and the base plate materials, the 
two are kinematically mounted together in a known manner such that they 
are rigidly fixed to one another whilst still allowing for relative 
displacement due to thermal expansion. 
Energy to drive the lasing medium 5 is provided by a cerium doped quartz 
flash lamp 8 which is clamped between appropriately contoured sapphire 
blocks 9. The sapphire blocks 9 conduct heat away from the flash lamp 8 
whilst being substantially transparent to the light generated by the flash 
lamp. The sapphire blocks are bounded on their outwardly facing surfaces 
by respective blocks of high conductivity material 10, again preferably 
beryllia which combines high thermal conductivity, electrical isolation, 
and good diffuse light reflection. The beryllia blocks 10 in turn contact 
lamp heat sinks 11 which are thermally isolated from the base plate 7. 
The sides 7a of the base plate 7 extend upwardly towards the non-reflecting 
side faces 5c, 5d of the slab 5 and mounted to the sides of the base 
plate, opposed to the non-reflecting side faces of the slab, are reflector 
bars 12. Preferably, the reflector bars are coated on their outwardly 
facing surfaces 12a with a dielectric material providing a wavelength 
specific reflectivity. Whilst the reflector bars preferentially absorb ASE 
light which is transmitted by the coating, pump light is reflected back 
into the lasing medium by the reflective coating. 
The upper and lower faces 5a, 5b of the slab are polished to an optically 
smooth finish such that light is able to propagate in a generally axial 
direction through the slab by a reflecting alternately off the top and 
bottom faces (as illustrated in FIG. 1). In contrast, the side faces 5c, 
5d of the slab (FIG. 3) have a finely ground finish such that light is 
generally not reflected by these faces and is able to exit the slab. By 
allowing ASE light to leak out through the side faces, the parasitic 
effect of this light is significantly reduced and energy is allowed to 
build up in an axial lasing mode. ASE light exiting the side faces 5c, 5d 
is substantially absorbed by the reflector bars 12 whilst pump light 
exiting the side faces is reflected back into the slab 5. Mirrors 13 are 
arranged at respective ends of the slab 5 and redirect light exiting the 
slab end faces back into the slab. 
It is generally the case that slab-type lasing media are provided with a 
rough surface finish, often resulting from the cutting of the bulk 
material into slabs. This finish typically results in an rms peak to peak 
amplitude of 1 .mu.m or more. However, the roughening process produces 
surface damage, e.g. crystal fractures, down to a depth of 20 .mu.m. As 
discussed above, this depth of surface damage (sometimes termed 
the"surface damage zone") results in an unacceptably high level of 
scattering for light exiting and entering the slab which in turn results 
in significant amounts of heat being generated at the roughened faces. In 
order to reduce this problem, pre-roughened faces are first polished to a 
relatively smooth finish (e.g. &lt;100 nm) and are then reroughened or 
patterned to provide a finish with a surface damage zone of depth 
comparable to the wavelength of the lasing emission. Thus, for a lasing 
wavelength of 1 .mu.m, the depth of the surface damage zone will be in the 
region of 1 .mu.m. This level of finish is sufficient to substantially 
prevent internal reflection of light at the roughened or patterned faces 
5c, 5d whilst also significantly reducing the level of scattering of light 
exiting and entering the roughened faces. 
As can be seen from FIG. 3, the slab 5 is surrounded on three sides by an 
air gap 14 and only makes contact with the slab mount 6 over the surface 
of the lower face 5a. The air gap is 3 mm or less in width and provides 
thermal isolation for the slab 5 from the reflector bars 12 and the flash 
lamp 8, whilst minimising the flow of convection currents. The temperature 
distribution within the slab 5 is therefore substantially determined by 
the heat generation processes within the medium and by the slab mount 6. 
This arrangement achieves a reduction in temperature gradients between the 
non-reflecting side faces of the slab as compared to conventional 
arrangements where a multiplicity of factors affect the temperature 
gradients within the slab. 
As an alternative to reroughening the side faces 5c, 5d of the slab, it is 
possible to employ a slab of the type shown in FIG. 4. Following polishing 
of the side faces 5c, 5d, thin sections 15 of undoped lasing material are 
diffusion bonded to the side faces of the slab 5. The outer surfaces 14 of 
the thin sections 15 are provided with a ground finish. As the optical 
properties of the doped and undoped lasing media are substantially the 
same, the boundary between the two media has little effect upon light 
crossing it. However, because the undoped lasing medium possesses a 
relatively low light absorbtion coefficient, little heat is generated at 
the outer faces 16 of the thin sections and even a relatively large level 
of scattering at the outer faces will result in only relatively low levels 
of heat generation. Again, this facilitates effective removal of ASE light 
whilst giving rise to little heat generation due to the passage of pump 
light. 
As an alternative to using undoped lasing material to provide a finish for 
the side faces of the slab, it is possible to use any suitably matched 
dielectric material. This may be in the form of a thin section bonded to 
the side faces or may be provided by a thin film deposition process. 
It will be appreciated by the skilled person that various modifications may 
be made to the above described embodiment without departing from the scope 
of the present invention. It will also be appreciated that the invention 
is applicable in general to solid state lasers and is not limited to 
lasers employing slab-type lasing media. For example, the invention can be 
applied to reduce heat generation in cylindrical rod lasing media.