GB2108752A - Laser system - Google Patents

Abstract

To produce multiple pulses a single laser resonator has two separate discharge regions and two sets of electrodes 12, 14 and 16, 18 and preionizers 21, 23. The optical resonator is folded by mirrors 32 and 34 and the beam passes through both discharge regions between end mirrors 30 and 36. Since the multiple discharges occur within the same resonator, multiple identical pulses are produced and since the discharge regions are separated, the shock wave and medium inhomogeneity produced by a discharge in one discharge region will not disturb the others. A discharge in the region between the electrodes 12 and 14 is triggered directly from a timer 50 by a pulse forming network 70. A subsequent discharge is triggered indirectly from the timer 50 by a pulse forming network 110 coupled through a delay 90 to the timer 50. The overall mirror separation and electrode spacing define a Fresnel number suited for single transverse mode operation. <IMAGE>

Description

SPECIFICATION Laser system This invention relates to laser systems capable of multiple pulse operation.

There is a need for a laser system which can produce multiple pulses having a controllable separation between pulses. For instance, such a system is useful in systems performing averaged cross-wind velocity measurements by using a correlation technique, and in incoherent designator systems.

One technique that may be used is to employ two separate lasers pulsing consecutively. The multiple pulses are then transmitted along the same optical axis. The problem is that the mode from the different lasers are not exactly the same, thus causing problems in applications requiring correlation between the pulses. Additionally, it is extremely difficult to maintain the required alignment stability between the two lasers.

Another technique which may be used consists of pulsing a standard laser at the required interval to produce the multiple pulses. A problem with this is that it requires a power supply with a high enough current capability to recharge the energy storage capacitors in less than the minimum interval required. Another problem is that when the laser is pulsed with a short interval between pulses, as required in certain applications, the shock waves and the medium inhomogeneities produced by a first discharge do not substantially dissipate, prior to subsequent discharge, thus creating problems by interfering with the subsequent discharge.

In a preferred embodiment of the present invention a laser system produces multiple pulses, without the problems encountered with presently known techniques, by using multiple discharge regions within a common resonator. The advantage of this system is that the overall mirror separation and the electrode spacing define a Fresnel number which is almost ideal for single transverse mode operation. By folding the resonator a more compact structure is achieved.

Additionally, since the separate discharge regions can have their own pulse forming network and trigger circuit, the multiple discharges can be fired simultaneously for maximum output, or they can be fired sequentially separated by a delay for producing multiple discharge output that is useful in the applications mentioned herein. Another advantage of this system is that since the discharge regions are inside the same resonator, the multiple pulses generate an identical output beam. Additionally, in the folded resonator design of the preferred embodiment, the multiple discharges are separate and isolated from each other, thus the discharge in a first region will not disturb the medium in a second region.

Thus the invention provides means for providing a plurality of discharge regions in a single optical resonator, a gain medium disposed in said discharge regions, and means for producing a discharge in each of said regions, said discharges being produced in a predetermined time relationship.

Additionally, the invention provides means for providing a plurality of separate but optically coupled discharge regions including a gain medium and forming a single optical resonator, and means for selectively producing a discharge in said discharge regions. In one application, said discharges may be produced simultaneously, while in another application, said discharges are produced successively and are separated by a predetermined time interval. Preferably, said discharges are separated by a time interval greater than the duration of turbulence generated in the resonator by a prior discharge.

A preferred embodiment of the present invention has an optical resonator, a plurality of discharge regions disposed within said resonator and having a gain medium therein, means for independently producing a discharge in each of said discharge regions, and means for controlling said discharge producing means to establish a predetermined timing between discharges from different ones of said discharge regions. Each of said discharge regions comprises a different predetermined portion of said resonator between opposite electrodes of an electrode pair. Said discharge producing means comprise means for switching a predetermined electric pulse across a predetermined one of said electrode pairs. The electrode spacing defines the beam diameter, and the relative dimensions of the resonator produce single mode beam operation.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawing in which the combined Figs. 1 and 2 are respectively a block diagram of an electronic control apparatus for an embodiment of the invention and Fig. 2 is a schematic perspective view of components of a laser apparatus embodying the invention.

Referring now to the drawing, there is shown a transverse electric field laser 10 comprising a folded optical resonator defined by mirrors 30, 32, 34 and 36. Mirrors 30 and 36 are the end mirrors that define the opposite extremes of the optical cavity, and mirrors 32 and 34 are used to fold or turn around a resonating beam in order to limit the length of the device. The mixture of a lasing gas is confined within the resonator structure by a folded laser envelope, not shown.

For example, an appropraite mixture comprising carbon dioxide, nitrogen and helium, as is known in the art, may be used.

One set of main electrodes is placed between each region defined by one of the end mirrors and a turn-around mirror. Thus, electrodes 12 and 14 are placed between reflector 30 and 32 symmetrically about a center line of the folded laser and electrodes 1 6 and 18 are similarly placed between mirrors 36 and 34 symmetrically about a corresponding center line. A preionizing flash board is also placed adjacent to each of the two regions thus defined. Each flashboard 21 and 23 comprises a support plate 20 and 22, respectively, and a plurality of auxiliary electrodes 24 disposed on a first surface of each support plate facing a respective main discharge region defined by the volume between a pair of main electrodes. The two preioinizing flash boards are placed back-to-back and thus can share a common conductive member 26.The center auxiliary electrode of each flashboard is electrically coupled to the conductive member 26 which is electrically grounded. The two end auxiliary electrodes of each flash board are electrically coupled to an output terminal of a respective pulse forming network. The operation of each flash board is as follows. Upon application of a high voltage pulse between the two end auxiliary electrode and ground, a discharge will be produced between the end electrodes and their adjacent neighbours, resulting in a transfer of charge to these neighbours since the auxilliary electrode in combination with the backing plate and the conductive member form a plurality of spaced capacitors. The charge is then transferred by consecutive discharges from the two end electrodes to the grounded center electrode.The preionized flash boards are located so that the chain of discharges between the auxiliary electrodes of each flash board 21 and 23 illuminates a respective main discharge region between a pair of two main electrodes. This is because the flash board converts a fraction of the stored energy into a pulse of ultraviolet radiation which in turn produces free electrons which when accelerated by an electric field promote uniform ionization of the gas in the interaction space between the main electrodes. This is necessary to preclude the occurrence of arcing, which would dump all the energy otherwise needed for pumping the gas mix.

The curvature and spacing of mirrors 30 and 36 and the size of the defining aperture determine the mode in which the optical resonator will oscillate. The fundamental or lowest order radially symmetric mode, (TEMoo mode), is desired since it has the least beam divergence and no nodes or gaps in the far field. Mirrors 30 and 36 have a concave surface of similar radius of curvature to define a resonator having a mode volume with a waist in the center line of the folded resonator between the two turn-around mirrors. When the two sets of electrodes are placed symmetrically about the center, the mode volume contained between one pair of electrodes is the same as the other. This is important to make sure that both pulses are the same.Although the emitted beam might be slightly divergent, due to the curvature of the mirrors, this divergence may be compensated for by using an output mirror 36 with a curved rear surface, not shown in the figure, to effectively act as a lens. The aperturing is effected by making the mirrors slightly larger than the region between the main electrodes. An aperture is then placed somewhere in the resonator, typically at the output mirror. The diameter of the aperture is approximately equal to the electrode spacing. Using the main electrode spacing for defining the size of beam aperture insures optimum use of the gain medium, since most of the gas excited by the discharge is used to contribute to the formation of a laser pulse.

Alternatively, aperturing may be effected by placing an aperture stop 33 between the two folding mirrors 32 and 34.

The operation of a laser system of the present invention is as follows. A timer 50 supplies a timing pulse at a predetermined repetition rate, for example 20 Hz. The timing pulses are used by trigger 60 to trigger pulse forming network 70 to supply a pulse of voltage to one of the flash boards, flash board 21 for example in order to preionize the laser medium, in this case in the region adjacent main electrodes 12 and 14, as described hereinabovs. Preferably, the duration of the preionizing pulse is a few nanoseconds to 1 50 nanoseconds and has an amplitude of a few thousand volts. Trigger 60 is preferably formed by a spark gap having a trigger electrode connected to timer 50. The spark gap is used to electrically connect suitable energy storage capacitors, which are part of pulse forming network 70, across the required points on the auxiliary electrodes.The energy storage capacitors in pulse forming network 70 are charged to their required level by high power voltage supply 80. A main laser discharge pulse is applied by pulse forming network 70 to main electrodes 12 and 14 approximately after the cessation of the preionizing discharge between the auxiliary electrodes of the flash board.

The same timing pulses applied to trigger 60 are also applied by timer 50 to delay 90. Delay 90 is used to provide a suitably delayed timing pulse to trigger 100, which in turn is used to trigger pulse forming network 110 to supply a voltage pulse to the second flash board, flash board 23 for example, to preionize the laser medium adjacent main electrodes 16 and 18. At the end of this preionizing pulse, pulse forming network 110 also applies a main discharge pulse across the second set of main electrodes 1 6 and 1 8. Thus, the system just described produces a first laser pulse at time to defined by each timing pulse from timer 50 and a second laser pulse at a subsequent time t, determined by the amount of delay selected in delay 90.

It should be understood that even though a specific implementation for a preionizating flash board has been described to produce a preionization pulse to uniformly ionize the gas mix, other methods of producing the required level of ionization for proper operation of the laser may be used.

It should be appreciated that since there are provided two separate discharge regions that can be fired off independently, the laser system can be used to generate a larger laser pulse by setting the delay in delay 90 to zero, thus causing simultaneous discharges in the two sets of electrodes. Alternatively, any amount of delay may be set by delay 90 to generate two discharges that are separated in time by any predetermined amount less than the timing pulse repetition rate.

In general, each time that a discharge takes place, a portion of the gas mixture in the discharge breaks down and decomposes into different compounds. In the case where CO2 is used as the active species, the breakdown produces CO and 2 Under normal conditions, the disassociated components will recombine, and catalysts may be used to aid the recombination process. However, a problem occurs in applications requiring multiple pulses separated by a time duration shorter than the recombination time. The disassociated gas causes a smaller amount of active molecules to be available in the discharge region, and thus it provides less gain, possibly less than the minimum required, for the generation of a laser pulse. In addition to producing a decomposition of the gain medium, each discharge produces a shock wave which heats the medium in the vicinity of the discharge.The rise in temperature reduces the gain of the medium further, thus precluding the successive generation of substantially identical laser pulses if each discharge takes place before the heat dissipates An important advantage of the laser system of the present invention is that these problems are eliminated by providing multiple discharge regions, each utilizing a substantially stable active having independent gas volumes, thus enabling the firing of successive discharges in different regions, ach utilizing a substantially stable active volume of the gain medium. It should also be appreciated that since the two discharges are separated and are isolated from each other the shock wave from one discharge should not significantly disturb the other discharge.

It has been found that if a delay time of approximately 100 microseconds is used, there is no measurable interference in the two laser pulses thus generated. This is because the shock wave has been sufficiently dissipated within that period of time. Smaller intervals may be achieved by placing around the discharge regions materials that absorb the shock wave and prevent it from bouncing off the walls of the envelope and returning to discharge the optical cavity.

An advantage of the present laser system is that since the two discharge regions are within the same optical resonator pulsing either one of them produces substantially identical output pulses. Additionally, the overall mirror separation in the electrode spacing defines a Fresnel number which is almost ideal for single transverse mode operation. The Fresnel number is proportional to the radius square of the beam divided by the operating wavelength times the length of the optical resonator. A Fresnel number approximately equal to 2 is found to easily satisfy the single transverse mode requirements. The folded design makes possible the building of such a resonator in a very compact structure. This may be achieved by having a main electrode separation of 0.85 centimeters and having a single discharge region length of 25 centimeter for an operating wavelength of 10.6 microns for a CO2 TEA laser.

It is understood that the above described embodiments of the invention are illustrative only and that modifications thereof may occur to those skilled in the art. Accordingly, it is desired that this invention is not to be limited to the embodiments disclosed herein but is to be limited only as defined by the appended claims.

Claims (20)

Claims

1. A laser system comprising: means for providing a plurality of discharge regions in a single laser resonator; a gain medium disposed in said discharge regions and means for producing a discharge in each of said regions, said discharges being produced in a predetermined time relationship.

2. A laser system according to claim 1, wherein said discharges are produced simultaneously.

3. A laser system according to claim 1, wherein said discharges are produced successively and are separated by a pre-set time interval.

4. A laser system according to claim 3, wherein said discharges are separated by a time interval greater than the duration of turbulence generated in the resonator by a prior discharge.

5. A laser system according to claim 3, wherein said discharges are separated by a time interval greater than 100 microseconds.

6. A laser system according to claim 1 wherein means are provided for controlling said discharge producing means to establish a predetermined timing between discharges from different one of said discharge regions.

7. A laser system according to claim 6, wherein each of said discharge regions comprises a different predetermined portion of said resonator.

8. A laser system according to any preceding claim, wherein each of said discharge regions has a pair of opposite electrodes disposed longitudinally about said discharge region.

9. A laser system according to claim 8, wherein said discharge producing means comprises means for switching a predetermined electric pulse across a predetermined one of said electrode pairs.

1 0. A laser system according to any preceding claim, wherein the relative dimensions of the optical beam produced by each of said discharges and length of the resonator define a Fresnel number approximately equal to 2.

11. A laser system according to claim 10, wherein the electrode spacing defines the beam diameter.

12. A laser system according to claim 11, wherein the relative dimensions of the resonator produce single mode beam operation.

1 3. A laser system according to any preceding claim, wherein said resonator is folded.

14. A laser system according to claim 1, wherein there are provided a plurality of pulse forming means for producing a discharge in each of said discharge regions; and means for selectively triggering said pulse forming means to produce a plurality of laser beam pulses in a predetermined time relationship.

1 5. A laser system according to claim 14, wherein said laser resonator includes means which provide a folded cavity to spatially isolate said discharge regions.

1 6. A laser system according to claim 14 or 1 5, wherein each of said discharge regions has a pair of elongated electrodes disposed longitudinally about the longitudinal axis of each discharge region and coupled to said pulse forming means.

17. A laser system according to claim 16, wherein means are provided for aperturing said laser beam pulses to a size approximately equal to the spacing between the electrodes of said electrode pairs.

1 8. A laser system according to any one of claims 14 to 17, wherein said laser resonator has an optical length suitable for single mode laser beam operation.

1 9. A laser system according to any preceding claims, wherein means are provided for preionizing said gain medium prior to each of said discharges.

20. A laser system substantially as described hereinbefore with reference to the accompanying drawings.