Solid state lasers

Details Solid state lasers Solid state lasers

2000-01-27

2001-02-13

Arroyo, Teresa M. (Department: 2881)

Coherent light generators

Particular operating compensation means

C372S066000

Reexamination Certificate

active

06188706

ABSTRACT:

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 &Dgr;T
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 &Dgr;T
a
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 pre-roughened 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 &mgr;m, surface damage may extend into the crystal by up to 20 &mgr;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 &mgr;m but greater than 100 nm and more preferably greater than 0.5 &mgr;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 outer 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

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