FR2635231A2 - Mirror for power laser - Google Patents

Abstract

The invention relates to a mirror for a power laser. This mirror is formed of three superimposed metal blades 4, 5 6 fixed together and made of different materials. A fluid circulates successively inside an end plate 4 comprising the reflection face 2 of the mirror, then in the other end plate 6. Application to high power carbon dioxide and nitrogen laser generators.

Description

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  The present invention relates to a mirror for a power laser having

  is the subject of a main patent filed under No. 74 22 428.

  According to claim I of the main patent, this power laser mirror, comprising a thick plate whose front face is polished so as to reflect a large proportion of the incident light and the conduits in thermal contact with said plate to allow the circulation of light. a cooling fluid, is characterized in that said ducts are disposed in a fraction of the thickness of said plate in the vicinity of

said front face.

  In one embodiment of the mirror according to the main patent, the plate is formed on almost all of its thickness of the same

  good thermal conductor metal such as copper.

  Such mirrors are used in particular for producing the resonant optical cavity of carbon dioxide and nitrogen type lasers which are capable of emitting a beam of infrared radiation of high Dense. However, it appears that when the power of the beam is very high, the efficiency of lasers equipped with mirrors according to the main patent decreases rapidly during operation. The present invention aims to improve the laser mirrors according to the main patent in order to allow high-power lasers using mirrors of this type to maintain a good performance during the

f unction.

  The present invention relates to a power laser mirror according to claim 1 of the main patent, comprising: - a thick plate whose front face is polished so as to reflect a high proportion of the incident laser light, - conduits disposed in a fraction of the thickness of said plate in the vicinity of said front face, and means for circulating a fluid in said ducts in order to cool said plate

  characterized by the fact that said plate is composed of three superficial blades

  posed - a first blade comprising said front face and consisting essentially of a first metallic material, - a second blade, one face of which is fixed on the other face of said first blade, this second blade being constituted by a second metallic material, said ducts being disposed substantially along the contact surface between said first and second blades, - a third blade, one face of which is fixed on the other side of said second blade, this third blade consisting of a third metallic material,

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  channels being disposed in the thickness of this third blade, the expansion coefficients of said first and third materials being greater than that of said second material, the thermal conductivities of said first and third materials being greater than those of said second material, and said means to circulate a fluid in said ducts also pass in said channels the fluid exiting said

  conduits for heating said third blade.

  The invention is described below, by way of illustration but in no way limiting, with reference to the appended drawing in which FIG. 1 represents an embodiment of the mirror according to the invention and FIG.

  schematic along plane XX of the mirror illustrated in FIG.

  In FIG. 1, a thick cylindrical plate I has a polished flat front face 2 for reflecting a large proportion of the power of a 10.6 micron laser beam 3 from a nitrogen laser generator and carbon dioxide. For this purpose, the face 2 of the plate 1 has been previously

  coated with a layer of gold a few microns.

  The plate I is composed of three metal blades 4, 5 and 6 superimposed

  and assembled together by brazing for example.

  The blade 4, made of molybdenum, comprises the front face 2 which was discussed above. The face of the blade 4 opposite to the face 2 is fixed by soldering to a face of the blade 5 made invar. Many ducts such as 7 and 8 allowing the circulation of a cooling fluid are arranged substantially along the contact surface XX between the blades 4 and 5. The other face of the blade 5 is brazed to one face of the blade 6 made of copper. of the

  channels such as 9 and 10 are arranged in the thickness of the blade 6.

  Of course the metallic materials of which the blades 4, 5 and 6 are made may be different from those mentioned above, but these materials must be such that the expansion coefficients of the materials constituting the blades 4 and 6 are greater than the coefficient of expansion. of the material constituting the blade 5. In addition, the thermal conductivities of the materials constituting the blades 4 and 6 must be greater than those of the material constituting the blade 5.

  Means are further provided for circulating a cooling fluid.

  such as water, in the ducts and channels of the mirror. The water flows first in the ducts 7, 8, then is directed into the channels 9 and the blade 6. For this purpose a network of pipes 11 and 12 connects the ducts to the channels; the circulating water enters the mirror through inputs 13 and 14 and leaves it through outlets 15 and 16. These circulation means

  further comprise a pump system not shown.

  Figure 2 illustrates an embodiment of the conduits. Ducts 7,

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  8, 17, 18, 19 are arranged in concentric circles around a point

disposed on the X-X surface.

  The cooling fluid enters in particular at a point 21 of any duct 7. The flow of this fluid is then divided into two parts 22 and 23 that propagate in opposite directions along each other along this duct. and other point 21, this flow out of said conduit at a point 24 opposite the

  point 21 on the circular duct 7.

  In this case, it is preferably provided that the cooling fluid irrigating the conduit 8 adjacent the conduit 7 enters the conduit 8 at a point 25 of this conduit, located near the point 24. The corresponding flow is divided into two parts 27 and 28 propagating in opposite directions along the conduit 8 and out of this conduit at a point 29 located near the point 21. The circulation of the fluid in the various conduits is then such that the senses of fluid flow in any two adjacent conduits

  are opposed to each other.

  The density of the duct network in the blade 4 can be variable from one point to the other of this blade as a function of the distance to the center 20. Thus it is shown in Figure 2 the density of the ducts in the central area near point 20 may be greater than that of the area

peripheral.

  Likewise, the section of the different ducts and the flow of the fluid in these

  ducts can be variable from one duct 3 to the other.

  To easily achieve these ducts, grooves of substantially rectangular section can be hollowed in the blade 4 or in the blade 5 along the X-X contact surface between these two blades. In the device shown in FIGS. 1 and 2, these grooves are hollowed out in the blade 4 and are covered by

  portions of the face of the blade 5 in contact with the blade 4.

  The shape of the channels disposed in the blade 6 may be similar to that of the ducts of the blade 4 described above. But more simply, as shown in FIG. 1, these channels can be rectilinear, parallel to the face I and

  distributed homogeneously in the mass of the blade 6.

  Thanks to the thin layer of gold deposited on the front face 2, the reflection coefficient of the mirror is very high and close to 99%. The face 2 absorbs and transforms into heat about 1% of the incident light. Thanks to the good thermal conductivity of the material constituting the blade 4, this heat is rapidly transmitted to the ducts and largely evacuated by the cooling fluid circulating in these ducts. However, the non-evacuated heat tends to heat the blade 4 and to expand it. Since the coefficient of expansion of the blade 5 is less than that of the blade 4, the expansion of the blade 4 generates a mechanical stress acting on the blade 5 which would tend, if the blade 6 did not exist, to cause a curvature of the front face 2 so as to make the mirror convex. But we know that it is enough for the mirror to take

  a low convexity so that the laser efficiency drops considerably.

  The cooling fluid which has heated up by passing through the ducts of the blade 4 then comes to heat the blade 6 by circulation in the channels of this blade. In general the temperature rise of the blade

  6 is smaller than that of the blade 4. Since the expansion coefficient

  the material of the blade 6 is greater than that of the blade 4, the result is

  for the blade 6 a dilatation approximately equivalent to that of the blade 4.

  The expansion of the blade 6 generates a mechanical stress exerted on

  the blade 5, substantially equal and opposite to that exerted by the blade 4.

  The two mechanical stresses acting on the blade 5 thus compensate for each other.

  siblement. The resulting curvature of the front face 2 of the mirror is then much lower than that of a mirror according to the main patent made of a single material. This curvature reduction of the front face makes it practically possible to use the improved mirrors according to the invention for laser powers that are significantly higher than the maximum power.

  authorized by mirrors made of a single material.

  Those skilled in the art can predetermine, by a calculation of strength of the materials, the optimum values to be given to the thickness of the blades 4, 5 and 6 and the characteristics of the cooling system according to the materials chosen for the blades and the power density of the laser beam. As an indication a mirror 10 centirmtres of diameter consisting of a blade 4 molybdenum thickness 0.2 cmri, a blade 5 invar thickness

  1.8 cry and a 6 copper blade thickness of 2.3 cm provides a function-

  a satisfactory laser level for an average power density of the laser beam of 10 KW / cm 2 if a cooling system capable of providing an average density of water flow in the ducts of the blade 4 equal to 10 grams per second per square centimeter of useful area of the

  front section, the section of these ducts being defined by a width of 0, 1 centime-

  meter and a depth of 0.15 centimeters, the distance between the axes of two

  consecutive ducts being of the order of 0.25 cm.

  The shape and arrangement of the ducts described above and illustrated in FIG. 2 makes it possible to compensate for an inhomogeneous spatial distribution of energy.

  laser that hits the blade 4 and decreases the efficiency of the cooling

  as the spread of the liquid in each

the blade 4.

  It is known that a laser beam rarely has a homogeneous transverse distribution of its power. Most often this distribution is of Gaussian type .. that is to say that the axial density of power is much

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  stronger than the peripheral density. In this case, the blade 4 which receives the laser energy on the face 2 tends to become deformed so as to produce a convexity of the face 2, this deformation coming to increase that caused by

  global warming of the lamina 4.

  It is further known that heat exchange at the wall of a conduit in which a cooling fluid flows is degraded as the fluid progresses in the conduit and its temperature increases. The circular shape of the ducts illustrated in FIG. 2 is used

  when the distribution of the energy of the beam received on the mirror is sensi-

  rotation around the axis of the beam, the center of the circles

  concentric being placed on this axis.

  The density variation of the duct network in the blade 4 is intended to compensate for the thermal effect caused by the variation of energy density in a cross section of the beam. Thus, when the energy distribution is of the Gaussian type, the density of the duct network is higher in the axial zone than in the peripheral zone (see Figure 2). Of course this compensation of the thermal effect caused by the variation of

  energy density of the beam can also be obtained by a corresponding variation

  the section of these ducts or the flow rate of the coolant

in these ducts.

  The alternation of the direction of circulation in the successive ducts is intended to compensate for the decrease in cooling efficiency as the liquid propagates in the ducts, and to render

  thus more homogeneous the cooling of the blade 4.

  The mirrors according to the present invention can be applied to the production of resonant optical cavities of large laser generators.

  power, including generators of carbon dioxide and nitrogen type.

  Of course, the invention is not limited to the embodiments described and shown which have been given by way of example. In particular, without departing from the scope of the invention, it is possible to change certain provisions and

  replace certain means by equivalent means.

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Claims (10)

  1 / mirror for a power laser, according to claim 1 of the main patent, comprising: - a thick plate whose front face is polished so as to reflect a large proportion of the incident laser light, - ducts arranged in a fraction of the thickness of said plate in the vicinity of said front face, and means for circulating a fluid in said ducts for cooling said plate characterized in that said plate (1) is composed of three blades (4, and 6 ) superimposed   a first blade (4) comprising said front face (2) and consisting essentially of   firstly a first metal material, - a second blade (5) whose one face is fixed on the other side of said first blade (4), this second blade (5) being made of a second metal material, said conduits (7, 8) being disposed substantially along the contact surface (XX) between said first and second blades (4, 5), - a third lane (6), one face of which is fixed on the other face of said second blade ( 5), this third blade (6) consisting of a third metallic material, channels (9, 10) being arranged in the thickness of this third blade (6), the expansion coefficients of said first and third materials being greater than to that of said second material, the thermal conductivities of said first and third materials being greater than those of said second material, and that said means for circulating a fluid in said ducts (7, 8) additionally pass in said naux (9, 10) the fluid   exiting said ducts (7, 8) to heat said third blade (6).   2 / Mirror according to claim 1, characterized in that said conduits   (7, 8) are arranged in concentric circles.   3 / mirror according to claim 1, characterized in that said first   material is molybdenum, said second material is invar and said third sine material of copper.   4 / Mirror according to claim 1, characterized in that said   blades (4, 5, 6) are fixed to each other by brazing.   5 / mirror according to claim 2, characterized in that said means for circulating a fluid in said ducts give the fluid opposite flow directions (22, 28) in any two adjacent ducts (7, 8).   6 / mirror according to claim 5, characterized in that said means for circulating a fluid in said conduits are such that the flow of said fluid enters each conduit (7) at a point (21) of the corresponding circle and then divides into two parts (22, 23) so as to traverse this conduit in the opposite direction on either side of this point (21), the flow of said fluid exiting said conduit (7) at a point (24) opposite this circle by report at the point of entry (21).   7 / mirror according to claim 2, characterized in that the density of said ducts (7, 8, 17, 18, 19) on the surface of said plane varies according to   the distance of said conduits to the center (20) of said concentric circles.   8 / mirror according to claim 2, characterized in that the density of said conduits on the surface of said plane is stronger in the central zone   of said concentric circles only in the periphery.   9 / Mirror according to claim 2, characterized in that the section   different ducts is variable from one conduit to another.   Mirror according to Claim 2, characterized in that the flow rate   said fluid in the different ducts is variable from one conduit to another.   11 / mirror according to claim 1, characterized in that said ducts are constituted by the grooves (7, 8) cut in one of said first and second blades (4, 5) along the contact surface (X-X) between these blades.