IL32190A - Generator for the production of laser pulses of short duration - Google Patents

Description

Generator for the production of laser pulses of short duration I COMMISSARIAT A L'ENBRGIB ATOMIQUE Oi-30391 .

This invention relates to a generator for producing laser pulees which are of very short duration and/or which ^n& e a very Bteep leading edge as obtained by external chopping of the emission of a triggered- latser.

In the present state of the 'art; triggered lasers usually produce pulses which are scarcely less than about twenty nanoseconds in duration and which have a rise time of 10 nanoseconds under the best conditions. However, it -is well known that some fields of application of lasers entail the need to utilize light pulses which are of minimum duration and/or which have an extremely fast rise time.

The generator in accordance with the present Invention provides a highly satisfactory solution to this problem since a pulse of a few tenths of a nanosecond may be obtained, for example, from am initial pulse of 30 nanoseconds duration.

It is al30 jnpwn that, when the laser pulse is generated by an assembly consisting of a control laser and at least one amplifying chain1, two major disadvantages interfere with the operation of said assembly, namely super-radiance (that is to say amplification during pumping of the fluorescence radiation which corresponds to the spontaneous depopulation, of the excited levels) which has the effect of reducing the efficiency of said amplifying chain and the phenomenon of return of light produced by a targe having a mirror effect, this phenomenon being liable to result In damage to the control laser and to the first stages 1 1 of the amplifying chain.

The arrangemen which is pro osed :in the present invention is intended to overcome both of these disadvantages at the same time* ] ..

According to the present invention, there is provided a 52190/2 assembly arranged to be triggered by appropriate means, ( an electro-optical shutter placed on the path of the laser pulse emitted by said assembly and outside a resonator of said assembly, said shutter being coupled to the triggering means via a delay line.

'.Further properties and advantages of the invention will become apparent from the following description in which a number of embodiments of said generator are given by way of explanation but not in any limiting sense, reference being made to the accompanying drawings, in which: Pig. 1 represents a first embodiment of a generator In accordance with the invention j Fig* 2a, 2b and 2c are explanator graphs relating to the description of eaid generator ; Figs# 3a, 3b, 3c represent the shape of the pulses obtained ; Pig. 4 represents a second embodiment of the invention ; Pig. 5 illustrates an alternative embodiment for increasing the steepness of the leading edge of the pulses ; and finally, Fig. 6 represents the arrangement which is adopted in the case of an amplifying chain.

As is apparent from Fig. 1, a laser rod 1 surrounded by a coaxial flash tube 2 is placed within a resonant cavity limited by two mirrors 3 and 4, the mirror 4 being semi-transparent. An electro-optical cell 5 and a polarizer 6 are also disposed within the cavity on each side of the rod 1.

The cell 5 consists of a crystal of potassium monophosphate (abbreviated to KDP) or ammonium monophosphate (abbreviated to ADP), said crystal being placed between two annular electrodes 7 and 8 so that, its crystallographic axis Oz should be parallel to the lines of force of the electric field produced by said electrodes. The cell itself is oriented in such a manner that the axis Oz should be parallel to the direction of propagation of the light emitted by the rod 1. It is known that the application of an electric field to a crystal of this type induces birefringence in said crystal by virtue of the Pockels effect. If the axes Ox and Oy (Fig. 2a) constitute with the axis Oz a direct right-angled trihedron, the Pockels effect results in the appearance of two axes Ox' and Oy' which are located at 4-5° from the axes Ox and Oy in the plane xOy ; the velocities of propagation of the polarized waves along Ox' and Oy' are a function of the intensity of the electric field which is applied along Oz.

The polarizer 6 is disposed in such a manner as to ensure that the axes Ox and Oy are respectively parallel and perpendicular to the plane of polarization of the wave which it is capable of transmitting.

Since any light vibration can be split up into two vibrations which are polarized in a direction parallel to the axes Ox and Oy, a triggered laser is thus made available.' In fact, if there is applied between the electrodes 7 and 8, during optical pumping of the laser rod 1, a voltage such that the optical path difference caused by passage through . the crystal between two vibrations having wavelengths A which are polarized along Ox' and Oy* is equal to A/ , the vibration which is polarized in a direction parallel to Ox is polarized in a direction parallel to Oy after passing twice through the cell and is therefore not transmitted by the polarizer 6, On the other hand, the vibration which is polarized in a' direction parallel to Oy is immediately stopped by said polarizer. The cavity is therefore open and laser emission cannot take place. However, when the voltage applied to the cell is restored to a zero value, said cell no longer has induced axes and therefore has noveffec i on_ the light. The cavity is thus closed and a laser pulse is emitted and polarised in the direction parallel to the axis Ox by the polarizer 6.· This pulse is then received by a three-plate electro-optical cell 9 which comprises two single crystals 10 and 11 of KDP (or ADP) which are placed side by side, separated by an annular central electrode 12 and surrounded by two lateral electrodes 13 and 14 which are also annular and connected electrically. The principal crystallographic axes Oz^ and Ozg (Fig. 2b) of said crystals are directed parallel to the lines of force of the electric field E which can be produced by said electrodes. The cell itself is oriented so as to ensure that the- crystals should be traversed successively by the laser pulse with a direction of propagation which is parallel to the axes Oz^^ and 0ζ£. Under these conditions, the application of an electric field to each crystal induces by Pockels effect a birefringence which results in the appearance of two axes O ^ and O ^ in the ■ crystal 10, 0χ'≥ and 0y'2 in the crystal 11, said axes being disposed as shown in Fig. 2c.

The central electrode 12 is interposed in the central conductor of a coaxial line 15 having a value of 10 ohms, for example, whereas the two lateral electrodes 13 and 1 are interposed in its outer conductor.

A polarizer 16 whose plane of polarization is crossed with respect to that of the polarizer 6 receives the light transmitted by the cell · The polarizer 16 and the cell 9 constitute with the polarizer 6 an electro-optical shutter which serves to transmit only a part of the pulse provided that its instant of opening is perfectly synchronized with the passage of said pulse. In fact, when said pulse which is polarized parallel to the axis Ox arrives at the cell 9» no voltage is applied. The crystals 10 and 11 therefore do not have any induced axes and the light passes through them without any change in its polarization. However, the pulse is not transmitted by the polarizer 16 since its plane of polarization is perpendicular to that of the light which impinges thereon. However, when the light pulse attains its maximum intensity, a voltage Vg is applied to the crystals very rapidly (0.5 nsec . ) and is such that the optical-path difference which is produced as a result of passage through the cell between two waves polarized along Ox' and Oy1 is equal to λ/2, that is to say to A / "by passing' through each crystal. As a result of passage through the cell, there thus takes place a rotation by ΤΓ/2 of the plane of polarization of the wave which was initially polarized to the axis Ox. The wave is thus polarized parallel to the axis Oy and can therefore be transmitted by the polarizer 16.

There is shown in Pig. the shape of the laser pulse which passes through the cell 9. The pulse' which is chopped by said cell and transmitted "by the polarizer 16 is represented diagrammatically in Fig, Jb, It is seen that this pulse has a very steep leading edge whereas its trailing edge remains the same as that of the incident pulse. If it is desired to obtain a very short pulse, it is only necessary to ensure that the voltage £ which is applied to the cell 9 should be brought back to a zero value as rapidly as possible. The pulse then has the shape which is shown in Pig. j5c .

Since the cell 5 has a single crystal and the cell 9 bas two crystals, the voltage Y^ which is applied to the first and the voltage which is applied to the second must have the same value Y. By inserting the two cells in the same coaxial line 15, this therefore makes it possible to employ the same electric pulse to cause the opening of the first cell and then the opening of the second. Such an arrangement serves to define with a high degree of accuracy the time interval . t which elapses between the instant t^ at which the voltage which is applied to the cell 5 s restored to a zero value and the instant tg at which the voltage Y^ is applied to the cell 9. This time interval must in fact be equal to the time of formation of the laser pulse, that is to say to the time which elapses between the instant at which the resonant cavity is closed and the instant at which the intensity emitted is of maximum value.

In accordance with the invention, there is initially applied between the electrodes of the cell 5 a direct-current voltage having a value Y. At the instant ^, there is applied between these electrodes a voltage pulse having an amplitude Y and a sign which is contrary to that of the direct-current voltage as obtained by the discharge of a ca acitor 1 in the coaxial line 1 Said volta e ulse nullifies the electric field on the cell. The capacitor 17 is charged through a resistor 18 to the desired voltage and is mounted in series with a coaxial spark-gap 19 which controls its discharge. In order not to affect the shape of the leading edge of the pulse, the electrode 7 s connected to the central conductor of one of the five 50-ohm cables which are placed in parallel and accordingly constitute a portion of the 10-ohm coaxial line. After having passed through a given length of cable which defines the time interval Δ. t, the electric pulse is again applied at the instant t2 "bo the second cell 9 . A second coaxial spark-gap 20 of the self-ignition type having an interelectrode distance which is much smaller than the first is disposed just before said cell 5 said second spark-gap serves to shorten the pulse rise time although this is not an essential requirement .

If it is desired to obtain a chopped laser pulse having a profile as shown in Fig. 3 , the coaxial line must be terminated by a matched load 21 . If it is desired to obtain a chopped laser pulse having the profile which is shown in Pig. 3c , the coaxial line must be short-circuited, at the extremity. In the latter case, the pulse is returned with a change of sign and therefore nullifies the voltage of the cell. The width of the chopped pulse is accordingly determined by the distance between the short-circuit termination and the cell, Fig. illustrates a second advantageous embodiment of the invention in the case especially of a laser which is triggered by a rotating prism. The resonant cavity which is limited on one side by the rotating prism 22 and on the other side by a semi-transparent mirror then only comprises the laser rod 1 surrounded "by its flash tube 2. A part of the light pulse emitted is deflected by a semi-reflecting plate 23 towards a spark-gap 24 and causes the ignition of this latter by being brought to a focus on one of its two electrodes. The spark-gap is mounted in series with a capacitor 2^ which is charged to a desired voltage through a resistor 26. The capacitor 25 can be replaced by an open line. In order to ensure good reproducibility of measurements, it is an advantage in this form of construction to isolate the spark-gap 24 within a pressure vessel 27 in which is formed a window 28. The. other part of the laser pulse is then received as in the previous case by a first polarizer 6, then by the three-plate cell 9 which is inserted in the coaxial line 15 which is terminated either by a matched load 21 or by a short-circuit element prior to reaching a polarizer 16, the plane of polarization of which is crossed with respect to that of the polarizer 6. The cell 5 is advantageously preceded by a second coaxial self-ignition spark-gap 20.

In this form of construction, the electric discharge pulse of the capacitor 25 which is induced by the ignition of the spark-gap 24 travels towards the three-plate cell 9 and opens this latter at a given instant of the passage of the light pulse after having passed through a precisely-determined length of cable.

The description of this second mode of application of the invention has been given with reference to the case of a laser which is triggered by a rotating prism. However, it is evident that the utilization of the laser pulse itself in order to cause the opening of the three-plate electro-optical shutter can also be carried out when, as in the first embodiment herein described, the laser emission is triggered by means of an electro-optical cell.

Systems such as those which have just been described make it possible to obtain laser pulses having a steep leading edge with a high dynamic value, that is to say a ratio E = !„„„ / In of the order of lCr f wherein re-presents the maximum intensity and IQ represents the intensity immediately prior to the pulse. However, this ratio can be considerably improved by carrying out further chopping of the pulse obtained : the dynamic value is in fact equal to Rn if n "cell-polarizer" assemblies are employed, provided that perfect synchronization of the optical and electrical pulses is achieved.

Fig. illustrates the solution adopted for this purpose in the present invention. The chopped pulse transmitted by the polarizer 16 is deflected through 90 ° by a prism 29 towards a second prism 0 and directed by this latter towards a second three-plate cell 31 which is supplied by the same coaxial cable 18 as the first cell and followed by a polarizer 32 which is crossed with respect to the polarizer 16. The pulse is then deflected by the prisms 33 and 4» then passes through a third cell 35 which is identical with the two preceding cells and is finally transmitted by a polarizer 6 which is crossed with respect to the polarizer 32. Synchronization of the optical pulse and of the electric pulse as applied successively to the three cells is obtained in this device by making strictly equal the times of passage from one cell to the other of the optical pulse and of the electric pulse which are propagated within the central conductor of the coaxial cable 15 .

When it is desired to obtain a pulse of the type shown in Fig. 3c, it is necessary as in the previous instants to replace the matched load 21 of the coaxial line by a short-circuit termination.

Fig. 6 shows the arrangement adopted according to the invention when the laser pulse is generated by an assembly consisting of a control laser and an amplifying chain.

The light produced by the control laser which is constituted by the rod 9, the excitation flash-tube 2, the semi-transparent mirror 4 and the rotating prism 22 is injected into the amplifying chain constituted by the series · of rods 37» 38 and 39 and their respective flash tubes 40, 41 and 42. The diameters of these different rods increase in a geometrical progression in a ratio which is approximately equal to two. The afocal optical devices 3, 44 and 5 formed by the association of two lenses, namely a convergent lens and a divergent lens disposed between each stage of the chain, have the intended function of matching the dimensions of the beam which issues from one rod to the dimension of the following rod.

The means employed for obtaining a chopped pulse are strictly the same as in the case of the generator which is illustrated in Fig. 4 and are accordingly designated by the same reference numerals. In this case, however, the three-plate cell 9 is inserted between two stages of the chain, for example between the rods 8 and 39· If the length of the cable 13 is so adjusted as to achieve perfect synchronisation between the instant of opening of the shutter and the instant at which a pulse derived from the control laser then amplified in the two first stages appears at the level of the cell 9 with a maximum value of intensity, a chopped pulse having a steep leading edge is injected into the rod 39.

Since the shutter is opened only at the moment at which each pulse reaches the level of said shutter, it can readily be seen that the parasitic fluorescence radiation derived from the final stages of the chain cannot reach the first stages.. In fact, it is known that this radiation has its maximum activity at the level of the first stages since it has been concentrated on each pass through the afocal devices. Superradiance in the chain is therefore attenuated to a considerable degree. Moreover,, for the same reason, the light reflected from the target which is placed at the end of the chain is prevented from returning towards the control stage.

It is apparent that, in the event that the amplifying chain is disposed in a straight line as is the case in Fig. 6, the path along which the light travels between the control stage and the shutter is shorter than the path followed by the electric control pulse in the coaxial line. In this case, it is practically impossible to open the shutter at the moment of passage of the laser pulse. This problem can be solved in a very simple manner by looping the amplifying chain by means of a deflection system such as prisms which make it possible to reduce the distance between the spark-gap and the cell.

Although only a single shutter is employed in the exemplified embodiment which is illustrated in Fig. 6, there is no reason to prevent any increase in the number of shutters. Accordingly the same control voltage may be applied to a plurality of coaxial lines in order to supply a first shutter which is disposed between the control stage and the first stage of the amplifying chain followed by one or a number of shutters disposed between different stages of said chain. Thus, it also follows from the foregoing that a plurality of lines can supply a number of shutters disposed in different amplifying chains, said chains being preferably fed by pulses derived from a single control stage.

The foregoing description of the arrangement employed in conjunction with an amplifying chain has been given with reference to a control laser^triggered by a prism but it remains apparent that an electro-optical system can also be adopted as in the first embodiment hereinabove described. It is equally apparent that the arrangement described with reference to Fig. 5 can be employed in conjunction with an amplifying chain.

The present invention is clearly not limited to the modes of construction which have been illustrated in the drawings and described by way of explanation. The scope of this patent also extends to alternative forms of either all or part of the arrangements described which remain within the definition of equivalent means as well as to any application of said arrangements. 32190/2

Claims (8)

1. A generator for producing laser pulses which are of short duration and which have a steep leading edge comprising a laser assembly arranged to be triggered by appropriate means, an electro-optical shutter placed on the path of the laser pulse emitted by said assembly and outside a resonator of said assembly, said shutter being coupled to the triggering means via a delay line,

2. A generator according to Claim 1 wherein said triggering means are constituted by an electro-optic cell located internally with respect to said resonator, said cell and said shutter being successively fed across said line which is constituted by a coaxial line.

3. generator according to Claim 1 wherein said triggering means are constituted by a rotating prism, said delay line comprising a beam splitter, one of the component split beams being incident on a spark-gap located on a coaxial feed line of the external, electro-optic cell.

4. A generator in accordance with Claim 1, wherein the electro-optical shutter which is placed on the path of the laser pulse comprises an electro-optical cell constituted by two single crystals which exhibit the Pockels effect and are placed side by side, said orystals being separated by a central electrode interposed in the central conductor of a coaxial line and being surrounded by two lateral electrodes which are interposed In the outer conductor of said line, all said electrodes being intended to permit the passage of the laser beam, the principal crystallographic axes of said crystals being directed parallel to the lines of force of the 32190/2 eleotrlo field which can be produced by said electrodes and said crystals being traversed successively by the laser pulse, and two polarizers having crossed planes of polarization and placed on each side of said cell, said shutter being such that its opening is obtained by applying to the central electrode thereof a voltage having a value such that the optical path difference caused by the passage through the cell between two vibrations having a wavelength Tv. which are polarized in a direction parallel to the induced axes is equal to * /2.

5. · A generator in accordance with Claim lt wherein the electro-optical shutter which is placed on the path of the laser pulse comprises a plurality of unitary shutters as defined in Claim 7 and whose electro-optical cells are Inserted In the same coaxial line, and optical reflection systems whloh define the path of the lase pulse so that said pulse should pass successively through each unitary shutter and which are so arranged that the times of passage of the laser pulse and of the electrical control pulse from one cell to the other are made strictly equal to each other.

6. A generator in accordance with Claim 7 or Claim Θ, wherein the means for determining the opening time of the shutter comprise a short-circuit element provided in the coaxial line at a given distance from the shutter,, the electrical pulse which has initiated the opening of said shutter being reflected with a change of sign and intended to initiate the closure of said shutter after a time interval corresponding to one back-and-forth passage of said pulse between the shutter and the short-circuit element. 32190/2

7. A generator according to Claim 1 comprising a triggered laser assembly and amplification stages wherein the external electro-optical shutter is located between two amplifying stages.

8. A generator in accordance with Claim 1 for producing laser pulses of short duration, substantially as hereinbefore described with referenoe to and as illuetrated in the accompanying drawings* For "theeApplicants■ DR, JSEIHHOLD COHK HD FARTHERS IS:CB