FR2933817A1 - LASER COMPRISING A PURIFIER, INERTIAL CENTRAL AND METHOD OF MANUFACTURING THE SAME - Google Patents

Abstract

The invention relates to a laser (2) having - a cavity (20) having an inner surface (25) and containing a gas (23), and - a device (27) for exciting the gas (23) for the emission a laser radiation in the cavity (20), said laser (2) being characterized in that it further comprises: - a purifier (10) placed in the cavity and capable of adsorbing impurities (11) in the gas , and having a developed surface (101) such as the ratio between, on the one hand, the area of said developed surface (101) and, on the other hand, the area of the inner surface (25) of the cavity (20) is greater than 100. The invention also relates to an inertial unit comprising such a laser and a method for manufacturing such a laser.

Description

General technical field

The invention relates to a laser having a cavity, having an internal developed surface and containing a gas, and a device for exciting the gas for emitting laser radiation into the cavity.

It also relates to a laser gyro comprising such a laser, as well as a method of manufacturing such a laser.

State of the prior art

Vehicle navigators are known which include inertial units using gyrolasers.

A laser gyro makes it possible to determine the speed of rotation of the vehicle with respect to an axis, by comparing the travel distances of two laser beams circulating in the laser gyro.

Figures 1a and 1b show schematically a known embodiment of a laser gyro 50.

Two laser beams 54 and 55 are emitted by a laser 2 in two opposite directions. They are guided by mirrors 53 to traverse the gyro laser in a ring-shaped path, for example in the form of a triangle (Figure la) or square (Figure lb).

The annular path of the beams 54 and 55 is in a plane whose normal is the axis of rotation around which the gyrolaser 50 performs the speed measurement. If the vehicle rotates in the forward direction of Figures 1a and 1b, for example, the beam 54 then has a longer travel distance than in the case where the vehicle rotates in the indirect direction 52. 1 The speed of the beams being constant, the variation in the travel distance of the two beams 54 and 55 is determined in a detector 58, which makes it possible, according to a known method, to measure the speed of rotation in the directions 52 or 51.

Figure 2 shows schematically an example of a known laser 2, which can be included in a laser gyro. The laser 2 is composed of a resonant cavity having semi-reflecting blades 21 and 22 and enclosing an amplifying medium composed of a gas 23. The laser 2 is further provided with an excitation device 27 exciting the gas 23 to form a plasma, which emits a laser radiation within the cavity 20. The laser radiation can be transmitted for example in two opposite directions through the blades 21 and 22. The power Pi of radiation within the cavity 20 is given by the following relation: ## EQU1 ## where G is the gain of the cavity; and P is the loss of the cavity.

It will be understood that the higher the gain G, and the lower the loss P, the greater the power, which guarantees, for example, a good quality of measurement of the speed of rotation in the case of a laser gyro.

When the power is no longer sufficient, the laser is no longer usable. The known lasers have disadvantages. Indeed, over time there is a decrease in the power Pi affecting the long-term lifetime of the lasers.

Indeed, when the cavity 20 comprises impurities 11, even in gaseous form, the latter constitute a pollution because they can - absorb a portion of the excitation energy initially intended for the plasma, which decreases G; and - be fixed by the blades 21 and 22, thus creating solid structures that absorb the laser radiation, which increases P.

It is found that the effect of impurities is very quickly harmful and greatly deteriorates the life of lasers.

In addition, the lasers 2 used in the gyrolasers generally have a cavity 20 of small volume (of the order of a few cm3), and the gas 23 is at low pressure (of the order of a few tones or mbar). The impact of the pollution is therefore all the more important as the number of molecules making up the gas 23 is low, since the lesser impurity 11 then represents a relatively large pollution.

Lasers 2 thus tolerate only a few tens of parts per million (ppm) of impurities in the gaseous state within the cavity.

However, the cavity, even hermetically closed, is never completely devoid of openings (sealing defects) allowing the entry of impurities into the cavity.

In addition, there is a degassing of impurities in the cavity, that is to say a release in gaseous form of impurities initially contained in the elements forming the cavity 20.

In order to maintain a low level of impurities in the cavity, FIG. 2 shows that a known solution consists in placing in the cavity 20 an impurity pumper (also known as a getter by those skilled in the art) absorbing the impurities. impurities but do not trap the molecules of the gas 23 constituting the amplifying medium. The pomp 30 is generally in the form of a metal alloy.

The known pumper fixes the impurities 11 by absorption (also called chemisorption).

As shown in FIG. 3, the absorption is carried out by creating bonds 32 between the impurities 311 and the surface 301 of the pumpor 30, in particular by chemical transformation: the absorbed impurities 313 react chemically, for example by reactions 32 oxidation or molecular dissociation.

However the use of these getters 30 is limited by major drawbacks of the getters.

As shown in Figure 3, the getters fail to absorb certain chemical compounds 312, such as hydrocarbons or other organic compounds. But in the case of use of a laser 2 in a gyrolaser 50 embedded within a vehicle, these chemical compounds are present in the environment of the laser 2.

In addition, the getters have a low pumping capacity.

Finally, the temperatures which make it possible to increase their capacity to absorb the impurities are too high compared with the usual operating temperatures of the lasers.

The use of such getters 30 is then not sufficient to meet the lifetime requirements of a laser device 2, namely a few years.

PRESENTATION OF THE INVENTION The present invention aims to overcome at least one of these disadvantages.

According to the invention there is provided a laser comprising a cavity having an inner surface and containing a gas, a device for exciting the gas for emitting laser radiation into the cavity, said laser further comprising a purifier placed in the cavity capable of adsorbing impurities in the gas, and having a developed surface such as the ratio between, on the one hand, the area of this developed surface and, on the other hand, the area of the internal surface of the cavity is greater than 100.

The invention is advantageously completed by the following characteristics, taken alone or in any of their technically possible combination: the ratio between, on the one hand, the area of the developed surface and, on the other hand, the area the internal surface of the cavity is greater than 1000, preferably greater than 5000; the purifier is composed of: a porous material whose specific surface, namely the developed surface relative to its mass, is of the order of 100 m 2 / g; for example a zeolite; the purifier comprises a pumper capable of absorbing impurities in the gas, the pumper possibly being in the form of inclusions in the purifier, preferably in the form of metal inclusions; the laser comprises a heating of the purifier, the heating being able to heat the purifier to a temperature of the order of 300 ° C .; the gas is a mixture of rare gases, preferably Helium and Neon; and / or the gas has a low pressure preferably less than 20 mbar; and / or the cavity has a small internal volume preferably less than 50 cm 3.

In addition, according to the invention, a laser gyro comprising a laser mentioned above is proposed.

Finally, a method of manufacturing a laser mentioned above is proposed, the steps of the manufacturing process being: the provision of a purifier in a metal cup, the purifier being composed of a material capable of adsorbing impurities in the gas and having a developed surface whose area is greater than the area of the inner surface of the cavity; - The tabletting by mechanical pressure of said material in the metal cup; the insertion of this cup into the cavity; - Vacuum holding of the cavity; induction heating of the cup which, by conduction, causes degassing of said material at a temperature of the order of 300 ° C., during the vacuum holding of the cavity; filling the cavity with the low-pressure gas; and sealing the cavity. The invention has many advantages. It thus allows an increase in the life of the device.

In particular, it makes it possible to purify the amplifying medium of the laser cavity from all the molecules that constitute impurities, including molecules that do not react chemically with the surface of conventional getters.

Presentation of figures

Other features and advantages of the invention will be apparent from the detailed description which describes exemplary embodiments. This description will be made with reference to the accompanying drawings in which: FIGS. 1a and 1b, already discussed, are schematic representations of two known embodiments of gyro lasers; FIG. 2, already discussed, is a schematic representation of an embodiment of a laser according to the prior art; FIG. 3, already discussed, represents an operating diagram of the chemisorption mechanism implemented by a known getter; FIG. 4 is an operating diagram of the adsorption mechanism implemented by a purifier according to the invention; ; FIG. 5 is an operating diagram of the adsorption mechanism on a developed surface larger than that of FIG. 4; FIG. 6 is a diagram illustrating the difference between an apparent surface and a developed surface; FIG. 7 is a graph comparing the evolution over time of the partial pressure of the impurities of a gas for different developed surfaces of purifiers; FIG. 8 is a diagram of a first embodiment of the invention; FIG. 9 is the curve representing the efficiency of the adsorption as a function of the ratio between the area of the developed surface of the purifier and the area of the internal surface of the cavity; FIG. 10 is a representation of an embodiment of the purifier according to the invention comprising inclusions of absorbent materials; - Figure 11 is a diagram showing the degassing phenomenon of the purifier; and FIG. 12 is a diagram of one embodiment of the laser purifier heating means.

detailed description

FIG. 8 shows a possible embodiment of a laser according to the invention.

The laser 2 comprises a cavity 20 containing a gas 23, and a device 27 for exciting the gas 23 for emitting laser radiation into the cavity 20.

The gas 23 constituting the amplifying medium is preferably a chemically inert gas of the rare gas type, such as for example a mixture of helium-neon noble gases.

The cavity 20 has an internal developed surface, which is hereinafter referred to as the internal surface 25.

Figure 6 shows schematically the difference between two developed surfaces 90 and 91 (also called specific surfaces by those skilled in the art when related to a mass or volume) of two faces. The two developed surfaces 90 and 91 correspond to the same apparent surface S.

In the case where the face of which the developed area is measured is perfectly flat, the apparent surface S and the developed surface 90 are identical and have the same area.

On the other hand, in the case where the face has irregularities 92, the apparent surface S is always the same, while the developed surface 91 represents the total dimension of the surface of the face, including the internal surface of the irregularities 92. Figure 6 is a non-limiting sectional representation of the development of a surface. The developed surfaces may have more complex configurations, which the simplified representation of Figure 6 does not rule out, for example in the case of three-dimensional porous materials. However, it is understood from FIG. 6 that the developed surface may be significantly greater than the apparent surface.

The inner surface 25 of the cavity 20 takes up all the elements forming the cavity (as for example the walls and semi-reflective plates), but is of a size relatively close to the apparent surface of the cavity.

The laser 2 further comprises a purifier 10 placed in the cavity 20. The purifier 10 has a large developed surface 101. Indeed, according to the invention, the ratio between, on the one hand, the area of the developed surface 101 and, on the other hand, the area of the inner surface 25 of the cavity 20 is greater than 100.

The purifier 10 is thus able to adsorb impurities 11 in the gas.

Adsorption, also called physisorption, is the fixation of gas molecules by any solid surface. Adsorption maintains the chemical integrity of molecules, unlike absorption. The reverse process of adsorption is desorption.

As shown in FIGS. 4 and 5, the adsorption allows a fixation 111, on sites 102 of adsorption of the developed surface 101, of a large number of different molecules 311 and 312, which does not allow the phenomenon of absorption, as we have seen, some molecules can not be absorbed. Adsorption is indeed capable of fixing all the molecules 311 and 312 having an electric dipole moment, which is the case of a large number of molecules. Of course, the gas 23 is inert and is not adsorbed by the purifier 10.

As long as the surface 101 has free adsorption sites 102, the impurities 11 of the gas 23 tend to fill its adsorption sites 102 by physisorption 111. The adsorbed impurities are then no longer present in the gas 23 and no longer represent of pollution.

Some of the molecules already adsorbed by the surface 101 are however released by desorption 112 in the gas 23.

The quantity of molecules adsorbed on this surface 101 is thus in dynamic equilibrium with the amount of impurities in the gas 23. This reversible adsorption / desorption phenomenon is known in thermodynamics by those skilled in the art, thanks to theories of isotherms of Langmuir and BET. These theories show that the equilibrium depends in particular on the temperature.

However, the temperature of use of the lasers can not be modified to increase the amount of impurities adsorbed by the purifier 10, for technical reasons.

The inventors have found that a large developed surface 101 comprising a large number of adsorption sites, ie having a low recovery rate (the recovery ratio is the ratio between, on the one hand, the number of molecules adsorbed and secondly the number of adsorption sites 102 of the surface 101), can act as a gas purification system.

In other words, the number of adsorption sites 102 influences the dynamic balance of adsorption / desorption.

As can be understood from a comparison between FIGS. 4 and 5, if the developed surface 101 of the purifier is increased, the number of adsorption sites 102 is increased. The balance of the adsorption / desorption is thus displaced. in the sense of greater adsorption of impurities.

The influence of the increase of the developed surface 101 on the presence of impurities 11 in the cavity over time has been studied, using a simulation based on the Langmuir theory.

FIG. 7 shows that with a purifier 10 having a developed area of 100 m 2, the partial pressure of impurities 11 in the cavity (the partial pressure of impurities is directly related to the number of impurities 11 present in the cavity 20) is always 100 times lower than the partial pressure of impurities when no purifier is placed in the cavity.

Each surface inside the laser cavity participates in the degassing and the adsorption / desorption of impurities 11. The efficiency of the purification of the gas 23 then depends on the ratio between, on the one hand, the surfaces participating mainly in the adsorption of impurities 11, that is to say the developed surface 101 of the purifier, and secondly the surfaces participating mainly in the degassing of impurities 11, that is to say the inner surface 25.

FIG. 9 shows that, in the case of a ratio of the areas of the order of 100, the gain on the partial pressure of the impurities 101 is perceptible. The number of impurities 11 in the cavity 20 therefore decreases.

The operation of the purifier 10 does not attach to any particular material. The purifier 10 can be in any material, from the moment it has a large developed surface.

The purifier is however preferably composed of a porous material, so that the developed surface 101 of the purifier 10 has an area greater than 100 times the area of the inner surface of the cavity 20.

According to FIG. 9, it is preferable to use a purifier whose area of the developed surface is at least 1000 times greater than the area of the internal surface of the cavity (a partial pressure gain is obtained. impurities of about 5, i.e. the partial pressure of impurities is 5 times lower), and even at least 5000 times higher (gain of about 10).

Thus, the purifier 10 may be composed of a material whose specific surface area (i.e., the developed area of the purifier based on its mass) is greater than 100 m 2 / g, for example 600 m 2 / g.

The zeolite type materials, which have significant trapping surfaces relative to their volume and mass, seem particularly suitable for this type of stress. However, depending on the materials that make up the purifier, the energies of the physical bonds between the impurities 11 and the surface 101 of the purifier 10 will be modified. This property will have a direct impact on the previously described adsorption / desorption equilibrium. It is clear that some materials, and some molecules, will be more effective in trapping or trapping.

Absorption mechanisms can also be used in combination with the adsorption phenomenon, to further reduce the number of impurities in the cavity. The combination of these two sorption phenomena allows purification of the still larger cavity.

The absorption is carried out by specific materials, chemically reactive, such as titanium, molybdenum, tungsten, palladium, platinum, vanadium, zirconium or an alloy of several of these metal elements.

According to a variant of the invention shown in FIG. 10, the purifier 10 may thus be composed of a material whose developed surface is large and further comprising inclusions 31 of materials that are capable of being absorbed, for example inclusions of metal type.

The impurities reacting with the chemisorption 311 can thus be absorbed by the metal inclusions, in addition to being adsorbed by the surface 101.

The laser 2 described by the invention has many advantages in the case of use as a laser gyro 50 for an inertial unit.

Indeed, the inertial units are embedded in a vehicle with relatively long lifetimes. The laser 2 described by the invention thus makes it possible to extend the life of the laser 2 and thus limit the use of the shutdown 30 of the vehicle for the replacement of the inertial units. The use of a laser 2 described by the invention allows the adsorption of impurities 11 present in the environment of the gyrolasers, such as organic compounds and particularly hydrocarbons.

In the case of a laser of a gyrolaser, the pressure of the gas 23 is less than 20 mbar, while the volume of the cavity is preferably less than 50 cm3

As shown in FIG. 11, the purifier 10 contains, in its initial state, impurities 11 which it is capable of degassing when it is placed inside the cavity, thus contributing to additional pollution of the gas 23 the cavity 20.

The degassing of the impurities 11, initially contained in the purifier 10, can be caused before the purifier 10 is disposed in the sealed cavity 20. The prior degassing of the purifier makes it possible, of course, to avoid degassing, but also to reduce the rate of recovery of the surface 101, by freeing up adsorption spaces in particular.

For this purpose, the laser advantageously comprises a heating 40 of the purifier 10. The heated purifier 10, at a temperature of the order of 300 ° C, degasses the impurities 11 that it initially contained.

Heating degassing can be done directly in cavity 20 before it is sealed.

The invention thus also relates to a method of manufacturing a laser 2.

According to a preferred method of manufacture shown in FIG. 12, 80 mg of zeolite material is available for example in a metal cup 41 belonging to the heater 40. A zeolite material is chosen to constitute a purifier 1015 whose area of the developed surface 101 is much greater (for example, greater than 5000 times) in the area of the inner surface of the cavity.

The purifier 10 is mechanically pressed into the cup 41 to form a pellet.

The metal cup 41 containing the emptied purifier 10 is then inserted into the laser cavity. The cup 41 is held on the wall of the cavity 20 using, for example, a spring. Vacuum is then made in the laser cavity.

During the evacuation, the purifier 10 is degassed by heating 40. For this purpose, the metal cup 41 is heated by induction means 41 also belonging to the heater 40.

The purifier 10 being in contact with the cup 41, the latter, heated by induction, heats the purifier 10 by radiation and by conduction at a temperature above 300 ° C. This temperature allows sufficient degassing of the purifier 10, in particular freeing up a maximum of adsorption sites 102.

The cavity 20 being actively maintained under vacuum during the degassing time, the released impurities 11 are evacuated by the evacuation means (not shown). Once the degassing is complete, the heating is stopped and the cavity 20 can be filled with the gas 23 constituting the amplifying medium.

The cavity 20 is then hermetically sealed. During the life of the laser 2, impurities 11 which will be degassed by the different surfaces of the laser 2 or entered through the openings (leaks) of the cavity 20 will be picked up by the purifier 10.

The purifier then makes it possible to maintain the power of the laser 2 by adsorbing the impurities 11 present within the cavity 20.

Claims (8)

REVENDICATIONS1. Laser (2) having - a cavity (20) having an inner surface (25) and containing a gas (23), and - a device (27) for exciting the gas (23) for emitting laser radiation in the cavity (20), said laser (2) being characterized in that it further comprises: - a purifier (10) placed in the cavity and capable of adsorbing impurities (11) in the gas, and presenting a surface developed (101) such that the ratio between, on the one hand, the area of said developed surface (101) and, on the other hand, the area of the inner surface (25) of the cavity (20) is greater than 100. 2. Laser (2) according to claim 1, wherein the ratio between, on the one hand, the area of the developed surface (101) and, on the other hand, the area of the inner surface (25) of the cavity (20) is greater than 1000, preferably greater than 5000. 3. Laser (2) according to one of claims 1 or 2, wherein the purifier (10) is composed of a porous material whose specific surface, namely the developed surface relative to its mass, is of the order of 100 m2 / g. 4. Laser (2) according to one of claims 1 to 3, wherein a material component of the purifier (23) is a zeolite. 5. Laser (2) according to one of claims 1 to 4, wherein the purifier (10) comprises a pump (30) adapted to absorb impurities (11) in the gas (23). 6. Laser (2) according to claim 5, wherein the pompeur (30) is in the form of inclusions (31) in the purifier (10), preferably in the form of metal inclusions, said metal inclusions consisting of titanium , molybdenum, tungsten, palladium, platinum, vanadium, zirconium or an alloy of several of these metal elements. 7. Laser (2) according to one of claims 1 to 6, comprising a heating 5 (40) of the purifier (10), the heating (40) being adapted to heat the purifier at a temperature of the order of 300 ° vs. 8. Laser (2) according to one of claims 1 to 7, wherein: - the gas (23) is a mixture of rare gases preferably Helium and Neon; and / or the gas (23) has a low pressure preferably less than 20 mbar; and / or - the cavity (20) has a small internal volume preferentially less than 50 cm3. 12. Inertial unit comprising a laser gyro (50) characterized in that the gyro laser (50) comprises a laser (2) according to one of claims 1 to 8. 13. A method of manufacturing a laser (2) according to the one of claims 1 to 8, the laser having a cavity (20) having an inner surface (25) and containing a gas (23) at low pressure, the method being characterized in that it comprises the steps of: - provision a purifier (10) in a metal cup (41), the purifier (10) being composed of a material adapted to adsorb impurities (11) into the gas (23), and having a developed surface (101) whose area is greater than the area of the inner surface (25) of the cavity (20), - tabletting by pressure of said material in the metal cup (41), - insertion of this cup into the cavity (20), 30 - maintaining under vacuum the cavity (20), 15- inductively heating the cup (41) which, by conduction, causes the degassing said material at a temperature of the order of 300 ° C, while maintaining the cavity (20) in a vacuum, - stopping the heating once the degassing has been completed, - filling the cavity (20) with the gas (23) ) at low pressure, and - hermetic closure of the cavity (20).