FR2466118A1 - Gas laser waveguide structure - has chamber defined by spaced conductive blocks sandwiched between insulating blocks with surrounding metal tube - Google Patents

Abstract

The laser waveguide structure has an elongate chamber which has a cross-section allowing propagation of laser light and in which an alternating electrical field is set up for stimulation of a gas laser within the chamber. The chamber is defined by a pair of spaced conductive blocks, sandwiched between a pair of insulating blocks (132,134). A resilient metal tube (150) surround the blocks (132,134) over the full length of the chamber and press them together. Pref. the interior of the tube (150) has two diametrically opposed ridges (154) extending radially inwards, each with a wider, lipped end face fitting over one of the respective insulating blocks (132,134).

Description

 The present invention relates to lasers and more precisely to a gas laser comprising a waveguide.

 U.S. Patent No. 4,159,251 discloses a waveguide laser excited by high frequency transverse discharge between about 30 MHz and 3 GHz with an active gas in an elongated chamber and a discharge. being established in the gas by an alternating electric field applied to the chamber in a direction transverse to its length. The excellent characteristics of this waveguide laser excited transversely at high frequencies as well as its small size and low complexity compared to those of existing lasers as described for example in United States Patent 3,772 611 and 3,815,047, are described in detail in the aforementioned patent and are not rewritten.

 The invention also relates to a waveguide laser whose excitation is provided at high frequency but which presents significant improvements over the aforementioned patent. Accordingly, the invention relates to high frequency transverse excitation waveguide lasers, having the many advantages described in the aforementioned patent, but not having the difficulties and restrictions of these lasers so that the laser formed according to the invention is much more efficient and flexible to use. One of the improvements is the use of a longitudinal excitation which allows the selection, in the form of independent parameters, of the size of the waveguide chamber and the separation of the electrodes so that the high frequency can be optimized regardless of the size of the chamber.

 Another improvement according to the invention, with respect to the known high-frequency excitation laser, relates to a device for eliminating a serious and very restrictive disadvantage of the device described in the aforementioned patent, namely the formation of defects in uniformity or "hot spots" apparently resulting from the bistable characteristics of the impedance of the discharge. Such hot spots tend to reduce the gain, output, and output power of the laser. The invention re-mediates this disadvantage of the known device by using a combination of structural and circuit enhancements, including the use of a series-connected capacitive load that improves the waveguide structure. is constituted by an original control circuit used for detecting the occurrence of low impedance hot spots and for adjusting the input power accordingly so that the non-uniform negative impedance characteristics otherwise are made minimum.

 Another improvement relates to the use of a totally homogeneous structure which, while promoting the resolution of the hot spots problem indicated above, allows the formation of a waveguide laser which is more compact, more stable in this respect. which relates to the composition of the gas and more reliable than a laser formed simply according to the teachings of the aforementioned patent, relating to a high frequency transverse excitation waveguide lazer.

 Thus, the invention has all the advantages of the known device, described in the aforementioned patent, namely the high frequency transverse excitation waveguide laser, but in addition, the invention eliminates the difficulties and restrictions presented by the high frequency transverse excitation technique. The main advantage of the high frequency longitudinal excitation on the high frequency transverse excitation is the resulting independence of the selection of the size of the active part of the laser chamber and the distance separating the electrodes in the device excitation.

The advantage also lies in the possibility of optimizing the high driving frequency irrespective of the size of the chamber.

 The aforementioned patent indicates that high frequency excitation efficient lasers require a sufficiently high driving frequency that the electrons drift only a negligible distance from the gap separating the electrodes during a half cycle from the alternating electric field. In the opposite case, space-charge regions are established in the vicinity of the electrodes and cause the formation of a higher electric field in the space separating the electrodes so that the electronic temperatures are increased.

It is known that, in the physics of molecular gas lasers, the optimum electronic temperature that maximizes the efficiency of a laser head tends to be considerably less than the electronic temperature in self-sustaining discharges. In addition, the high electronic temperatures tend to increase the rates of CO 2 dissociation so that the duration of the tube is reduced. Accordingly, in self-sustaining landfills, it is desirable that the electronic temperature be minimal and therefore the intensity of the electronic field required for the maintenance of the landfill is also minimal. Thus, for high frequency discharges, there is a high minimum driving frequency which maximizes the efficiency of the laser head for a given electrode separation distance.

 In a high frequency transverse excitation waveguide laser of known type, the size of the laser chamber and the distance separating the electrodes are intrinsically the same. Thus, the dimension of the chamber imposes the minimum frequency of high frequency control and, when this high frequency of control is increased, the coupling efficiency between the high frequency energy source and the discharge decreases and the formation of sources of High frequency driving which are effective becomes more and more difficult. Thus, the overall efficiency of the laser, corresponding to the high frequency transverse discharge of the known device, is reduced because of the low efficiency of the laser head when the high driving frequency is lower than the desirable minimum. On the other hand, if the high frequency of control is too high, the efficiency decreases due to the reduction of the coupling efficiency between the high frequency source and the discharge.

 The aforementioned restrictions of the high frequency up and down pose particularly delicate problems when the chamber size is small and therefore requires a relatively high frequency of control, for the performance of the laser head. Unfortunately, such a high driving frequency gives a reduced coupling and a poor feed efficiency, this situation being paradoxical.However, when using the longitudinal excitation at high frequency according to the invention and thanks to the indepen As a result of the separation distance of the electrodes and the size of the chamber, it is possible to choose the high driving frequency giving a high coupling and a high efficiency of the supply, and then to choose separately the separation distance of the electrodes. so that the efficiency of the laser head is optimal.

 Thus, the invention by implementing a high frequency longitudinal excitation, allows the formation of a waveguide laser having a higher overall efficiency than the waveguide lasers described in the aforementioned patent, i.e. having a high frequency transverse excitation. In addition, the degree of increase in the efficiency of the replacement of the transverse excitation by the longitudinal excitation increases and thus gives an important additional advantage for the higher high frequency driving that would be required for smaller chamber devices. using the transverse excitation configuration.

 The invention, which uses longitudinal excitation, has a further improvement in the overall efficiency with respect to the transverse excitation, either because the separation of the electrodes is increased or because the surface of the electrodes is reduced with respect to the corresponding electrodes of the electrodes. a device with transverse excitation. As a result, the capacity of the resulting waveguide structure is reduced so that the load overvoltage factor of the waveguide structure is reduced. The overvoltage factor under load is the overvoltage factor observed during the discharge ignition. A lower value of the load overvoltage corresponds to a better coupling efficiency given the lower circulating power and the reduction in energy dissipation. in other circuit elements. In addition, the possibility of using a lower high control frequency for a given chamber size further reduces the circulating power while maintaining the same efficiency at the laser head so that overall efficiency is further increased.

 An example of a known transverse discharge waveguide laser construction includes a cross-sectional discharge section coupled to a 50 ohm transmission line circuit and adapted by a w-resonant circuit. The described structure uses a transformer with a shunt choke placed at the edge of the discharge section so that the reactance is eliminated. The discharge section is used in balanced mode. The structure comprising the transformer and the shunt choke allows tight coupling of the high frequency driving with the discharge and independent adjustment of the imbalance adaptation with respect to the tuning of the reactance. The balanced structure described in FIG. The invention minimizes the retained charge, the voltage at each electrode, relative to ground, being equal to half of the total voltage in the spaces separating the electrodes.

 In various embodiments of the invention, certain improvements eliminate or reduce the sharpness of the problems posed by the hot spots which could otherwise reduce the characteristics of the waveguide laser. In one embodiment, the basic construction of the laser is modified. High frequency lasers with transverse excitation are for example manufactured using a conventional sandwich construction comprising two metal strips arranged between two blocks of alumina. The waveguide chamber formed between the metal strips and the alumina blocks, for example, is in the form of a square having a side of 1 to 3 mm.

This construction makes it possible to obtain 0.2 W / cm of laser length in a CO2 waveguide laser.

 As the length of the laser increases, it arrives at a time when, for a length of about 20 cm and a power of about 2 to 4 W / cm, the discharge of the laser becomes bistable and localized discharge hot spots develop. . It has been experimentally determined that bare aluminum electrodes are the most desirable material as it is more resistant to spraying than most common materials. However, a problem with the use of a bare aluminum electrode is that the oxidation of the electrode is very important and affects the long-term stability of the gas composition.

The problem of oxidation is solved according to the invention by a hard anodizing coating of the aluminum electrodes.

This aluminum oxide coating forms an insulating layer on the metal. The problem posed by the hot spots, as indicated above and in particular for a high power, is solved by implementing a dielectric charge forming device for the longitudinally excited high frequency waveguide laser. This structure comprises two strips of alumina 1 to 3 mm thick arranged between two sheets of alumina. The waveguide chamber thus formed has a rectangular section of 1 to 3 mm and the alumina sheets have a thickness of between 0.5 and 2 mm. The electrodes are placed outside the guide structure. The resulting structure is formed entirely of homogeneous aluminum oxide and can therefore be sealed by solder, the gas being confined in the chamber only. This characteristic differs from that of transversely excited lasers, placed entirely in a cavity filled with gas. Another advantage of the invention is that the electrodes are totally insulated from the discharge and consequently they do not degrade as fast as the electrodes of the known devices over time.

 In another embodiment of the invention, the tendency of known lasers to form localized hot spots is further reduced by an original control circuit. When a discharge laser section excited in a waveguide is properly excited, the characteristic impedance is positive and the discharge is uniform over the entire length of the device. In this mode, when the reactance has been eliminated, the impedance of the discharge appears in resistive form and is adapted to 50 ohms by a greenhouse-coupled ferrite transformer.

When the discharge forms hot spots, the measurements showed that the total voltage and the discharge impedance decreased by about 20 to 40%. In one embodiment of the invention, a driver circuit is provided to instantly reduce the input power when a reduction in impedance is detected. As a result, the discharge section can not operate in unstable mode or in the presence of hot spots and the result is a proper continuous excitation of the discharge.

 In one embodiment, such a control circuit comprises a transmission line of the quarter-wave impedance type greater than that of the impedance adapted to the 50-ohm laser head. For example, a quarter-wave section of a 75 ohm cable may be chosen as a suitable driver circuit since a quarter-wave section has the characteristic of transforming the impedance changes around its characteristic value.

When a load of 75 ohms is applied to this section, a real impedance of 75 ohms is seen at the input of the cable. When a 50 ohm load is applied at one end, a real impedance of 112.5 ohms exists at the other end. On the contrary, if the load drops to 25 ohms, as in the case of a hot spot, for example, the input impedance is 225 ohms. When such a quarter-wave section is used between a 50-ohm laser head and a source adapted to 50 ohms so that its power is maximum, the power delivered decreases almost instantaneously and causes the extinction of the operating mode in the presence of hot spots.

 In another embodiment of the invention, the structure of the laser head is such that the chamber in which the laser effect takes place is formed by aluminum electrodes placed opposite, arranged between two blocks of alumina. This assembly is maintained aligned and compressed by an elastic tube which acts as a spring-loaded casing which can be evacuated and a radiator which positions the waveguide and optics with proper alignment. .

 Thus, the invention relates to a high frequency excited waveguide laser which substantially reduces or eliminates some of the disadvantages of known high frequency waveguide lasers, yet with the many advantages of these lasers over more conventional excitation waveguide lasers.

 The invention also relates to a waveguide laser operating by high frequency longitudinal excitation for selecting the size of the waveguide chamber independently of the selection of the high frequency of the excitation signal so that the Parameters can be chosen so that the efficiency of the laser is increased.

 It also relates to a high frequency excitation waveguide laser in which a charging technique enables the significant reduction or elimination of problems caused by hot spots due to an unstable discharge caused by operation in high frequency conditions. bistable impedance regions.

 It also relates to a high frequency excited waveguide laser structure which has a long life, small size, high reliability and efficient operation, relative to the corresponding properties of high excitation waveguide lasers. frequency of known type.

 It also relates to a control circuit intended to ensure the adaptation of a high frequency source to a laser head so that the high frequency input power is automatically reduced after detection of the occurrence of a low impedance due to instability when operating in the presence of hot spots, which could occur for higher constant powers.

 The invention also relates to a waveguide assembly package which enables inexpensive, mass-produced waveguide laser production and allows alignment and compression of the waveguide assembly by an elastic tube which acts as a vacuum-springable housing and a radiator, providing positioning of the waveguide and optical elements, with proper alignment.

Other features and advantages of the invention will become more apparent from the description which follows, given with reference to the accompanying drawings in which:
FIG. 1 is a partly diagrammatic sectional view of a known type of transverse-discharge waveguide gas laser, the double arrow indicating the direction of the electric field in the waveguide chamber;
FIG. 2 is a perspective of a longitudinal discharge excitation arrangement embodiment for waveguide gas lasers
FIG. 3 is a plan section of the configuration of the electrodes of the waveguide gas laser, in the plane indicated by the line 3-3 in FIG. 2.
FIG. 4 is an elevation of a homogeneous embodiment of a longitudinal discharge structure according to the invention;
FIG. 5 is a graph showing the characteristic bistable impedance which poses certain problems in transverse and longitudinal excitation waveguide gas lasers but which are solved or reduced in large part by the practice of the invention. , the ordinates representing the effective voltage and the abscissa the input power per unit length, the upper curve corresponding to a stable mode and the lower curve to a mode of operation in the presence of hot spots
FIG. 6 is an end view of a transverse discharge waveguide gas laser in which a capacitive load according to the invention is used for solving the problem posed by the bistable impedance.
Fig. 7 is a diagram of the equivalent circuit of the device of Fig. 6;
FIG. 8 is an end view of a longitudinal discharge excited waveguide gas laser having a capacitive load as well.
FIG. 9 is a sectional view of a longitudinal waveguide gas laser implementing a capacitive load, but in lateral elevation better representing the structure
FIG. 10 is a circuit diagram of the longitudinal excitation and capacitive load gas waveguide laser circuit of FIGS. 8 and 9;
FIG. 11 is a diagram of a circuit used for the description of the control circuit according to one embodiment of the invention.
FIG. 12 is a graph showing, on the ordinate, the variation of the excitation power as a function of time as abscissa, in one embodiment of the invention, allowing a further reduction in the probability of the harmful effects of the Bistable characteristic impediment shown in Figure 5;
FIG. 13 is a perspective of an advantageous embodiment of longitudinal excitation waveguide gas laser according to the invention.
Figs. 14 and 15 are respectively a partial perspective and an end view of two other laser head configuration embodiments including the waveguide of Fig. 13
Fig. 16 is an electrical diagram showing the connection of the waveguide of Fig. 13 to a suitable driver circuit and a high frequency source for excitation of the laser; and
Figs. 17, 18 and 19 show another embodiment of laser head configuration comprising a resilient metal tube for holding the waveguide laser in compressed form and aligned with respect to its container.

 In the detailed description of various embodiments of the invention, it appears that this relates to improvements in waveguide gas lasers, by implementing a new form of high frequency excitation called excitation. longitudinal. A number of additional improvements, both for transverse and longitudinal excitation gas lasers, are also described. These additional improvements are intended to further improve the performance and to eliminate the problems posed by the characteristic bistable impedance, common to lasers having both forms of excitation, as described in more detail below.

 For a better understanding of the waveguide gas laser technology, it is preferable to quickly consider the prior art represented by the transverse discharge waveguide laser described in the aforementioned US patent. . In particular, FIG. 1 shows the basic configuration of a transverse discharge waveguide gas laser corresponding to the aforementioned patent, in which the waveguide laser 10 comprises two elongated conductive electrodes 12 and 14 placed in position. look, between two elongated dielectric members 16 and 18 placed opposite, so that an elongate chamber 20 is delimited, for the creation of a laser excitation excitation discharge. The chamber 20, for example, may have a length of about 20 cm and a square section, with sides of 2 mm. The electrodes 12 and 14 are formed of a metal such as aluminum while the dielectric members 16 and 18 may be formed of a material such as beryllium oxide, alumina or glass. The structure of the waveguide may be based on a block 22 formed of a material having a high thermal conductivity, for example copper, so that the heat is removed from the waveguide during operation. The discharge chamber 20 is filled with a desired laser active gas such as a conventional gas mixture for a CO2 laser.

 A high frequency energy source Vs is coupled between the electrodes 12 and 14 and provides the excitation suitable for establishing an electric discharge in the active gas, in a manner sufficient to invert the population of the energy levels of the desired laser transition. An ir coupling circuit comprising adjustable capacitors C v and a choke L forms an impedance matching circuit for canceling the reactive impedance of the structure defining the discharge chamber and for adapting the actual value of the input impedance which is for example about 50 ohms.

 An important difference between an embodiment of the invention and the known device shown in FIG. 1 is that the electric field created in the discharge chamber 20 is vertical between the electrodes 12 and 14 so that it is transverse to the longitudinal axis of the chamber 20. It is clear that the separation distance of the electrodes is equal to the vertical dimension of the chamber. Thus, the size of the waveguide chamber sets the high minimum driving frequency, so that the aforementioned restrictions to the overall efficiency of the laser appear.

 The invention relates to a device for applying the excitation by an electric field, parallel to the longitudinal axis of the waveguide chamber. An embodiment of longitudinal excitation is shown in Figures 2 and 3, and Figure 4 shows an advantageous embodiment of longitudinal structure.

 As shown in FIG. 2, the configuration of the laser according to the invention is such that two ceramic blocks 30 and 32 are substantially parallel to one another and are separated from one another by two dielectric blocks 34 and 36 which are themselves separated from each other so that they delimit a waveguide chamber 38 which forms the discharge region of the waveguide laser.

 The electrodes of the waveguide laser, in the embodiment of FIG. 2, are formed on the respective inner surfaces of the ceramic blocks 30 and 32, i.e. the surfaces which are contiguous with the dielectric blocks 34 and 36 which delimit the chamber 38. The configuration of the electrodes is better represented in FIG. 3 which shows the internal face of the ceramic block 32.

As shown in FIG. 3, there is an electrode 40 of a first polarity and an electrode 42 of a second polarity. The electrodes 40 and 42 are placed along the edges of a ceramic block 32, in a direction substantially parallel to the chamber 38. Each electrode has a series of substantially perpendicular portions 41 and 43 respectively disposed transversely to the chamber 38 and overlapping.

It should be noted that two other electrodes of the same configuration are formed on the internecorresponding surface of the upper ceramic block 30. The electrodes 40 and 42 each have a weld bead 39 which allows electrical interconnection as will be described hereinafter. More specifically, the electrode 40 is electrically connected to the corresponding electrode which is just above and parallel to it along the surface of the upper ceramic block 30. Similarly, the electrode 42 is electrically connected to the corresponding electrode which is just above and parallel to it, also on the upper ceramic block 30. The electrodes 40 and 42 and the corresponding electrodes of the upper ceramic block 30 to which they are connected are electrically coupled to the opposite phase connections d a high voltage source such as V
s shown diagrammatically in FIG. 1, through a suitable control circuit described hereinafter. Given this electrical connection and the geometric configuration of the electrodes according to the invention, a high frequency electric field is induced in a direction parallel to the longitudinal axis of the chamber 38, the internal walls being placed relative to the electrodes as represented by the broken lines in Figure 3, the double arrow thereof representing the direction of the electric field in the waveguide chamber.

 The spacing A of the central axes of the perpendicular portions 41 and 43 of the overlapping electrodes 40 and 42 may vary with the high frequency applied by the source and, as previously indicated, it varies independently of the dimensions of the guide chamber 38. wave, unlike the known configuration shown in Figure 1. The apparatus of the invention can be used for example with a dimension A between about 5 and 12.5 mm, with a corresponding voltage difference of between 500 and 1000 V. It should be noted that the electrodes can be fixed to the surface of the ceramic blocks by a metallization operation, for example by spraying. However, various methods of depositing a thin film of high conductivity metal on a ceramic substrate may be suitable for the implementation of the invention. The dimension B corresponding to the width of the parts 41 and 43 is about 0, 5 mm.

 FIG. 4 represents an advantageous embodiment of longitudinal excitation waveguide gas laser according to the invention. This advantageous configuration of FIG. 4 has a certain number of advantages over the known type of waveguide laser, as described in more detail below. It is shown experimentally that a bare aluminum electrode represents one of the best electrode materials since aluminum seems to withstand very well the effects of laser discharge. However, when the length of the laser reaches a value which is about 20 cm, for a charging power of the order of 2 to 4 W / cm, the discharge of the laser has a characteristic bistable impedance that causes the problem mentioned above In addition, the aluminum electrodes oxidize rapidly and significant oxidation occurring after only a few hours of operation is sufficiently large that the conductivity of the walls is reduced and that the characteristics of the laser are also reduced. Significant oxidation is probably caused by acid vapors formed during the discharge. The configuration according to the invention, shown in FIG. 4, solves the problem posed by the oxidation of the electrodes, tends to reduce the problem presented by the hot spots and furthermore constitutes very advantageous improvements in the structure if Although the laser form is compact and highly reliable.

 More specifically, as indicated in the configuration of FIG. 4, the longitudinal waveguide gas laser comprising two sheets 50 and 52 of alumina, between which two strips of alumina 54 and 56 which delimit a chamber 55 of waveguide. In addition, a thin metallization layer is applied to the alumina substrates to form the excitation electrodes or the outer surfaces 58 and 60 of the sheets 50 and 52, respectively. It is clear that the surfaces 58 and 60 are insulated from the discharge chamber 55 of the waveguide. The electrodes of the opposite sides of the waveguide structure are connected by electrical conductors 62 and 64 which also provide electrical access to a control circuit. For example, the alumina strips 50 and 52 have a thickness between 0.5 and 2 mm, as indicated by the dimension C, and the strips of alumina 54 and 56 have for example a thickness of 1 to 3 mm as indicated by the dimension D. The chamber 55 has for example a square section having sides of 1 to 3 mm.

 The structure of the embodiment of FIG. 4 is completely homogeneous since all the interfaces as well as all the surfaces forming the walls of the chamber 55 are formed of alumina, that is to say Al 2 O 3 formed by hard aluminum anodizing. This structure can be sealed tightly by welding so that only the chamber 55 must be filled with the active gas of the laser.

This characteristic is distinguished from the configuration of the known laser of FIG. 1 which, given the difficulty of the sealed welding of different materials which form the sides of the chamber 20 of the known transversely excited laser of FIG. assembly of the structure in a gas-filled cavity or in a housing so that the dimension of the overall structure is increased. Accordingly, according to the invention, the structure is bulky and, since the electrodes are isolated from the gas discharge, the laser has increased reliability and durability than the known lasers.

 The aforementioned problem presented by the bistable impedance tends to manifest itself and to reduce the characteristics of the gas waveguide lasers, that their configuration ensures a longitudinal or transverse excitation. Accordingly, further improvements described hereinafter with respect to the elimination or substantial reduction of the characteristic and efficiency disturbances caused by the problem of the characteristic bistable impedance, apply to the guide lasers. It can be seen that the problems presented by the bistable impedance can be solved by means of a capacitive load, a new control circuit and an original operation. controlled power-up, these characteristics being considered individually or in combination.

 The cause of the aforementioned hotspot problem is represented by the graph of FIG. 5, which indicates, on the abscissa, the limited range of input powers for which it is found that the high-frequency excited discharge is stable and spatially uniform (upper curve). ). For sufficiently low input powers, it is found that the discharge ceases. In the case of a known type transverse waveguide laser, during unstable operation, the discharge is only triggered along limited portions of the length of the waveguide and the length of the discharge. and its location appear random.It is noted that a similar effect occurs in the longitudinal waveguide laser according to the invention in which all the discharge segments do not congregate to form a stable and continuous discharge, and the various Segments of discharge appear to turn on and off randomly. For greater input power, the same type of uniformity defects are observed in transverse and longitudinal devices.

 It is considered that the instabilities appearing at both ends of the power range are due to a characteristic bistable impedance of the discharge. The effects of such a bistable impedance can be compensated to some extent by using an increased high frequency drive or a larger waveguide chamber dimension or an impedance load.

The stabilization of the discharge by an impedance load is considered the most desirable technique because it has no effect on the flexibility of application of the waveguide as to its high frequency or the size of the bedroom. The most advantageous solution is therefore the impedance load, ie the addition of a series impedance which tends to minimize the effect of the impedance change in the discharge of the guide. waves, depending on the input power. The use of a load series capacitor is best when compared to the use of a series load resistor, for example, because no energy is dissipated in the charging element. a series capacitor is easily used in waveguide structures as shown hereinafter with reference to FIGS. 6 to 10.

 In the case of the known type of transverse discharge configuration, the capacitive load is obtained as shown in FIG. 6, in which the chamber 70 of the waveguide is offset from the center of the ceramic block 72 separating the two electrodes 74 and 76. In the example shown in FIG. 6, a waveguide chamber of height a is separated from the two electrodes by ceramic blocks having a dielectric constant e of a distance d.

The equivalent circuit is shown in FIG. 7 on which it is noted that the equivalent impedance of the discharge, represented by the parallel arrangement of the capacitor
C2 discharge and discharge conductance G2, is in series with the charge capacitor C1. As x1 WE C2 WE indicates the equations at 7x = d and Tx = a, the variation with the length X of the waveguide of the corresponding capacitance can be considered equal to the width W of the section of the structure of waveguide multiplied by the dielectric constant and divided by the distance d.The variation of the discharge capacitance C2 with respect to the length of the waveguide may be similarly determined to be equal to the width W multiplied by the dielectric constant and divided by the height of the chamber a. It is clear that, when varying the dimension d with respect to the dimension a, a large and stable capacitance can be introduced in series with the discharge capacitor so that the characteristic impedance can be better stabilized and cancel or reduce substantially the probability of the formation of unstable hot spots in the discharge, for a waveguide laser of any length with transverse excitation.

FIGS. 8 and 9 show a similar form of capacitive load of a longitudinally excited waveguide laser in an embodiment of which at least one set of electrodes is separated from the waveguide chamber by a thickness The overlapping electrode portions corresponding to the portions 41 and 43 of FIG. 3 are offset relative to one another by a distance d.sub.r as a dielectric material, for example a ceramic or alumina. indicated in FIG. 9 and are offset vertically with respect to the chamber 82 by the thickness of the dielectric 80
Fig. 10 is an equivalent circuit showing the various capacitances in the waveguide configurations of Figs. 8 and 9 for which it is assumed that the lower dielectric block 84 is connected to ground for example by ground plane arrangement. The distance separating the electrodes from the wall of the chamber, corresponding to the dielectric 80, over a thickness t, increases the load capacity cob between the discharge capacitor Cd and each electrode. In addition, there is a capacitance between each side of the discharge and the mass, through the lower dielectric block 84. As indicated by the formula
Cb = due to the value of the capacitance C b existing between each electrode and the conductance Gd of the discharge is approximately equal to the distance separating the electrodes ds multiplied by the dielectric constant C of the dielectric block 80 and divided by four times the thickness t of the dielectric block 80. It should be noted that the original position of the electrodes, that is to say, the upper face of the waveguiding structure, separated from the guide chamber by a dielectric material, is doubly advantageous because, in addition to isolating the electrodes against the harmful effects of direct contact with the discharge in the waveguiding chamber, it forms a charging capacitance which minimizes the effects of the characteristic bistable impedance of discharge.

 Reference is now made to FIGS. 11 and 12 for the description of another device for substantially eliminating or reducing the deleterious effects of the characteristic bistable impedance indicated above.

In FIG. 11, the discharge capacitor Cd is connected to a voltage generator Vs by a coaxial line 90 having a predetermined length 1, expressed in number of electrode lengths, and having a characteristic impedance ZOZ and by a transformer T having a winding ratio equal to N. In addition, a Lp chord coil is mounted
p in parallel with the discharge capacity and it is set to ensure resonance of the capacitance for the excitation frequency.

dans lesquelles 10 = 1d N et VO = Vd/N et Id et Vd sont l'intensité et la tension de la décharge (dans l'hypothèse de la résonance).When the coaxial line 90 is a quarter-wave line, the power and the input current are given by the formulas:

where 10 = 1d N and VO = Vd / N and Id and Vd are the intensity and voltage of the discharge (assuming resonance).

We can connect Ven to I by the relation
in
(2) Ven = Vs - R5Ien
If we take the values of Vd and Id, we get

 This equation is that of a load line.

The only meaning of j is that the current and the discharge voltage are 900 out of phase with the source. The effective impedance of the source is therefore

(étant donné que l'impédance résultante est positive).Stability is obtained when

(since the resulting impedance is positive).

 As a result, the use of a characteristic impedance much higher than the impedance of the source allows a considerable increase in the effective impedance of the source. The condition of equation (5) can then be satisfied.

 Another way to consider this behavior is to represent the quarter-wave line as an impedance inverter. If the discharge begins to form an arc, the impedance of the discharge decreases. At the input of the quarter-wave line, the impedance begins to increase. In general, less energy is then transmitted to the load which interrupts the arc.

When the coaxial line 90 is a line demionde, the equation of the load line is given in this case by the relation
(6) Vs = R5 Id N + Vd / N
The effective impedance of the source is therefore

The stability of the discharge is guaranteed by the relation

 Unlike the case of the quarter-wave line, the effective impedance of the source is just that of the source and tends to be much lower. For a source impedance of 25 ohms and a 75 ohm cable, the effective impedance of the source (divided by N2) is 225 ohms for a quarter wave section and only 25 ohms for a half wave section. As shown in FIG. 11, the configuration according to the invention, used for the excitation of high frequency lasers comprises a high frequency generator Vs connected to a coaxial cable of length a and of characteristic impedance Z0, connected to The secondary of the transformer is connected to the discharge section in which the capacitance Cd is resonated by a coil Lp connected in parallel.

dans laquelle Z représente l'impédance présentée au transformateur et kl est égal à 2X/Am (Am étant la longueur d'onde).As also shown in Figure 11, the impedance presented at the line is given by the relation:

where Z represents the impedance presented to the transformer and k1 is equal to 2X / Am (Am being the wavelength).

dans laquelle V et V représentent l'onde progressive se déplaçant vers l'avant et vers l'arrière ou, si l'on tire
V+, on

The voltage Ven is given by the relation

where V and V represent the traveling wave moving forward and backward or, if
V +, we

<tb> gets <SEP>: <SEP> z
<tb><SEP> in
<tb><SEP> V <SEP> R <SEP> + <SEP> Zen
<tb><SEP> (11) <SEP> V + <SEP> = <SEP> s <SEP> s <SEP> in
<tb><SEP> Z <SEP> + <SEP> Z1 <SEP> Z <SEP> e <SEP> -Z
<tb><SEP> 1 <SEP> o <SEP> -jkl
<tb><SEP> e <SEP> zl <SEP> F <SEP> ZO <SEP> e
<Tb>
The voltage at the input of the transformer is given by the relation

dans laquelle Qu représente le facteur de surtension sans charge et N le rapport d'enroulement.Dans un exemple dans lequel C = 40 pF, f = 40 MHz et N = 4, on a Qu sensiblement égal à 100.

For the evaluation of Z1, it is convenient to show that Z1 is given by the relation

where Qu is the load-free overvoltage factor and N is the winding ratio. In an example where C = 40 pF, f = 40 MHz and N = 4, Qu is substantially equal to 100.

Assuming Z1 Z0, we have
(15) VO ~ 2 V +
If we consider two particular cases, those of the quarter-wave and half-wave lines, we have, for the quarter-wave line (that is to say 1 = / 4)

The voltage created across the discharge section is therefore equal to

In the case of a half-wave line (i.e., 1 = A / 2), we have
(20) Zen = Z

The voltage across the discharge section is therefore equal to

 Assuming a 50 ohm source impedance and a typical 50 ohm impedance, both cases (X / 4 or A / 2 for line length) are equivalent.

However, in the half-wave case, the voltage is independent (to a first-order approximation) of the impedance of the source and the impedance of the cable whereas, in the case of a quarter wave, an increase in the Voltage can be achieved by using a cable impedance greater than the source impedance. For example, a 75 ohm line and a 50 ohm source impedance give a 50% increase in voltage.

 Thus, it is noted that the use of the original control circuit comprising a coaxial line or another suitable transmission route, depending on the operating frequency, having a line length equal to a quarter of the wavelength and a Characteristic impedance equal to or greater than 1.5 Rs, the stability of the discharge is ensured and there is a significant increase of the discharge voltage with respect to the voltage of the source.

 Figure 12 corresponds to another device allowing significant reduction or elimination of the problem posed by hot spots, caused by the characteristic impedance bistable laser discharge, transversely or longitudinally excited. This additional device comprises the controlled application of energy to the excitation electrodes as indicated graphically in FIG. 12, that is to say the instantaneous application of a sufficient quantity of energy so that the breakdown is triggered, then the more gradual increase of energy transmitted for a time greater than or equal to a minimum value of 1 millisecond, until the maximum excitation power is reached. For example, for the parameters indicated previously with reference to FIG. 11, the power P b is equal to 20 W and the power P max is equal to 80 W. It is considered that the excellent stability obtained by progressive application of the energy As shown in FIG. 12, the excitation frequency is due to the tendency of the discharge to follow the most stable impedance curve of the two positive impedance slope portions which form the above-mentioned bistable characteristic. The realization of an automatic device for progressive application of the excitation energy can be easily performed using a suitable capacitive circuit placed at the level of the high frequency generator, this circuit being well known to those skilled in the art. and not described in detail in this memo.

 FIG. 13 is a perspective of an advantageous embodiment of longitudinal excitation waveguide 100 according to the invention in which the chamber 102 of the waveguide is formed by two side blocks 104 and 106 of alumina which are they themselves arranged between two ceramic plates 108 and 110. Each of these ceramic plates is coated on its outer face, as shown in Figure 13 for the upper plate 108, by elongate electrodes as described above and which have a configuration ensuring the forming a longitudinal electric field in the chamber 102. More precisely, the electrode 112 and its perpendicular parts such as the part 114, at any time, have a polarity opposite to that of the corresponding electrode 116 having perpendicular portions such as part 118.

 In the advantageous embodiment of FIG. 13, the dimension F is for example equal to 0.5 mm, the dimension G is for example equal to 1 mm and the dimension H to 6.35 mm. In addition, the chamber 102 has a square section with walls of 2 mm which delimit the chamber.

 FIGS. 14 and 15 show two different laser head configurations comprising the waveguide 100 of FIG. 13. As shown in FIG. 14, this configuration comprises the waveguide 100 placed in a rectangular section metal box 120. providing high frequency shielding. The waveguide is centered inside on a base 122. This has a chamber 124 of rectangular section which constitutes a gas reservoir for the waveguide. The chambers 102 of the guide 100 and 124 of the base 122 are both at a suitable pressure below atmospheric pressure, suitable for the operation of the laser. The remaining internal volume of the box 120 is at ambient pressure and forms a closed channel for cooling the guide 100 by forced circulation of air.

 In the variant of FIG. 15, the waveguide 100 is placed inside a metal box 130 intended to form a high frequency shielding and to be at a suitable depression. The waveguide structure is mounted between the ceramic blocks 132 and 134 which prevent discharge to the outside of the chamber 102.

 The remaining volume delimited between the walls of the box 130 and the structure of the waveguide is also filled with an active gas which serves as a reserve, the gas being at the same pressure lower than the atmospheric pressure that the laser gas placed in room 102.

 Fig. 16 schematically illustrates the electrical interface of an advantageous embodiment of the invention in which different devices for reducing or eliminating problems presented by hot spots, as previously described, are used.

For example, the waveguide 100 has electrodes as shown in FIG. 13, i.e. in the form of a thin metallization layer applied to the upper and lower faces respectively of ceramic plates 108. and 110. Keeping the electrodes away from the chamber by the plates 108 and 110 introduces the aforementioned charging capacity. A resonance choke Lr is mounted between the electrodes and the upper and lower pairs of electrodes of the same polarity are connected by the conductors 140 and 142 which also interconnect the transformer 144. This is connected to a suitable source of Vs voltage through the control circuit according to the invention comprising a quarter-wave line 146 having a characteristic impedance greater than or equal to 1.5 times the impedance of the source; in FIG. 16, it is assumed that the impedance of the source is 50 ohms and that the characteristic impedance of line 146 is equal to or greater than 75 ohms, according to the improvement described above. Furthermore, the delayed characteristic of the excitation energy, as previously described with reference to FIG. 12, is represented by incorporation in the circuit of FIG. 16 of a suitable device 148 for controlling the generator, the details of which are known to those skilled in the art.

 Figures 17, 18 and 19 show another embodiment of laser head configuration implementing an improved construction technique that allows the use of mass production techniques and low cost according to the invention. In the configurations described, the active chamber is formed by spaced-apart and placed aluminum electrodes arranged between two blocks of alumina. The assembly is kept aligned and compressed by an elastic tube 150 shown in section in FIG. 17. The elastic tube forms a casing of the spring type and intended to be evacuated and a radiator, ensuring the positioning of the waveguide and optical elements with proper alignment.

In the embodiment shown, an elastic cylinder 150 is in the form of a cylindrical tube of circular section having an outer casing 152 and two parallel and remote trays 158 disposed longitudinally and parallel to the axis of the cylinder 150. The trays 158 are arranged along the length of the tube 150 from locations which are at the inner periphery of the casing 152, by corresponding necks 154. Each plate 158 has two parallel walls 156 which are perpendicular to it so that grooves of rectangular section allowing the housing and the alignment of blocks 132 and 134 of alumina of the waveguiding structure are delimited. In one embodiment of the invention having an elastic metal tube 150 that can be easily manufactured by extruding materials such as aluminum 6063-T5 and the like, the dimensions of the various parameters shown in FIGS. 17 and 18 can be the following
Dimensions References
Figures 17 and 18 (mm)
K 12.07 + 0.25
L 9.91 + 0.25
M 3.18 + 0.25
N, 9.53 + 0.25
S 0.64 + 0.13
P 31.75 + 0.25
Q 0.89 + 0.13
R 3.18 + 0.25
For example, the distance between the opposite external faces of the blocks 132 and 134 of alumina is slightly greater than the distance L before the setting up. Thus, during the introduction of the waveguiding structure into the tube 150, a compression force is applied in the planes 151 and 153 as shown in Figure 18, so that the distance L between the trays 158 increases and reaches a value allowing the housing of the waveguiding structure. Then, the overpressure of the force applied in the planes 151 and 153 causes the application of a compressive force between the plates, this force being applied via the waveguiding structure and corresponding to a pressure included between 1.4.106 and 7.106 Pa. As a result, it is noted that the plates 158 ensure the alignment of the alumina blocks 132 and 134, in compression, and the alignment of the entire waveguiding structure of the fact the elasticity of the tube 150. This not only ensures the rigidity of the entire laser head but also ensures a better thermal contact for a more effective effect of heat removal.

 As shown in Figure 19, the ends of the tube 150 are chamfered at an angle O advantageously equal to about 450. This feature has the important advantage of providing automatic alignment of the mirror mounting devices 160 and those skilled in the art can appreciate that this is an important advantage of the configuration according to the invention in which Reflective surfaces of extremities are sometimes desirable. When the mirror mounting device 160 is also chamfered as shown in Fig. 19, it can be easily sealingly connected to the end of the tube 150 by a continuous weld bead 162 which can be applied over the entire circumference of the interface. of the mounting device 160 and the tube 150.

 It should be remembered that the invention relates to a waveguide gas laser having excellent characteristics as well as excellent performance, reliability and durability properties. One of the improvements relates to the longitudinal excitation of the laser discharge in a suitable waveguide chamber containing an active gas. The longitudinal excitation makes the spacing of the electrodes variable independently of the geometrical configuration of the waveguide chamber so that the efficiency is improved. In addition, it should be remembered that a number of improvements are each intended to reduce or eliminate the problems posed by hot spots commonly present in transverse or longitudinal excitation waveguide gas lasers. significantly reduce the characteristics of lasers.These improvements concern for example the use of a capacitive load, by implementation of original configurations comprising the isolation of the electrodes relative to the waveguide chamber, by a dielectric material suitable spacer such as a ceramic or alumina, on an original control circuit of predetermined length and characteristic impedance, and on the controlled application of the excitation energy for a minimum period.

 The invention also relates to an additional improvement due to the elastic metal tube for accommodating the waveguiding structures and comprising parallel and spaced plates formed in an extruded metal tube and intended to cooperate by compression with the wave guiding structure. and align it. Such a tubular casing, constituting a vacuum elastic container and a radiator, ensures the positioning of the waveguide and optical elements with a proper alignment.

 It is understood that the invention has been described and shown only as a preferred example and that we can bring any technical equivalence in its constituent elements without departing from its scope.

Claims (4)

1. Laser waveguide, characterized in that it comprises  a device defining an elongated chamber whose section has dimensions that are suitable for guiding the laser light,  an active gas capable of having a laser effect and placed in the chamber  a device for forming an alternating electric field in the chamber so that a laser effect excitation discharge is established in the gas, and  an elongated elastic metal tube (150) containing the device defining the elongate chamber and cooperating with this device, over its entire length, by compressing it. 2. Laser according to claim 1, characterized in that the device delimiting an elongated chamber comprises two remote blocks conducting electricity, arranged between two blocks (132, 134) non-conducting electricity, and the metal tube (150 ) further comprises two elongated and parallel spaced apart trays (158) protruding from opposite points of the inner periphery of the tube (150) over at least a portion of the length of the tube, these two trays being intended to house the device defining the elongated chamber by cooperating with the non-conductive blocks of the electricity (132, 134) by compressing them. 3. Laser according to claim 2, characterized in that each of the parallel plates (158) has walls (156) substantially perpendicular to the plates and intended to define a rectangular groove whose width is substantially equal to that of the non-conductive blocks of the electricity (132, 134) so that they are automatically aligned. 4. Laser according to claim 3, characterized in that the elastic metal tube (150) is intended to undergo a compressive force along a plane (151, 153) which is substantially parallel to the plates (158) so that the distance separating the parallel plates (158) is temporarily increased without changing their parallelism and without deformation of the rectangular grooves.