GB2258754A

GB2258754A - Excimer laser using negative ions to create a gas discharge.

Info

Publication number: GB2258754A
Application number: GB9214438A
Authority: GB
Inventors: John Michael Green; Michael Robert Osborne; Richard John Winfield; Arthur Frederick David Taylor
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1991-08-16
Filing date: 1992-07-07
Publication date: 1993-02-17
Anticipated expiration: 2012-07-07
Also published as: DE4226716A1; FR2683400A1; GB9117686D0; GB9214438D0; GB2258754B

Description

Excimer Laser The present invention relates to lasers which employ rare

gas halides as their lasing medium, that is to say to what are known conventionally as excimer lasers.

Excimer lasers are of the gas discharge type and require preionisation to enable the lasing discharge to take place. In existing excimer lasers this is done by means of short, intense, pulses of ultra violet light or Xrays. However, the liberated electrons rapidly are lost to the halide gas which provides the halogen for the rare gas halide. For example, if the rare gas halide is xenon chloride (Xe Cl) produced in a lasing medium consisting of xenon with 0.2% by volume of hydrogen chloride, then as the rate constant for the reaction HCl + e - H + Cl- is greater than about 10-10 CM3 sec-', for a gas mixture at a pressure of about 3 atmospheres the free electron density decay time is less than about 60 nanoseconds. As the free electron density required for effective preionisation is about 106 electrons CM-3 it can be seen that very high intensity preionisation energy sources are required.

The present invention uses a fundamentally different approach to the preionisation of the excimer lasing medium, greatly reducing the preionisation intensity required.

According to the present invention there is provided a method of operating an electrically excited gas discharge laser wherein the lasing medium includes a rare gas and a halogen donor molecule, including the operations of producing continuously or quasi- continuously within the lasing medium-a population of negative ions of the halogen donor molecule and subsequently removing electrons from the ionised halogen donor molecules to facilitate the establishment of an electric discharge in the lasing medium.

The ionisation of the halogen donor molecules may be achieved by irradiating the lasing medium with continuous or long-pulse X-rays produced by a suitable source or similar radiation from a laser which produces radiation in the ultra violet region of the electromagnetic spectrum. The removal of the electrons from the ionised halogen donor molecules may be achieved by means of an auxiliary laser which produces radiation in the ultra violet region of the electromagnetic spectrum, which may in practice be the same laser as is used to ionise the halogen donor molecule. The auxiliary laser may be a smaller rare gas halide laser or a nitrogen laser. The auxiliary laser may also be used to is produce a desired shape to the output pulses from the main rare gas halide laser.

Suitable lasing media include helium or neon as a carrier gas together with xenon, argon or krypton as an active.rare gas and hydrogen chloride, or fluorine as the halogen donor molecule. Typical proportions may be: He, Ne 1-10 bar: xenon, argon or krypton 10-200 mbar; HCl, F2 1-10 mbar.

A preferred lasing medium has the- constitution Ne 3 bar, Xe 40 mbar, F2 2 mbar.

Also according to the invention there is provided an electrically excited gas discharge laser including a lasing medium comprising an active rare gas and a halogen donor molecule, wherein there is included means for producing continuously or quasi- continuously a population of negative ions of the halogen donor molecule and means for subsequently removing electrons from the ionised halogen donor molecules.

1 - 3 The means for producing the ionised halogen donor molecules can be an X- ray source or a laser producing an output in the ultra violet region of the electromagnetic spectrum. Preferably, the laser has an output wavelength less than 360 mm. Suitable lasers are rare gas halide or nitrogen lasers.

A preferred gaseous lasing medium comprises a mixture of helium or neon as a carrier gas, together with xenon, argon or krypton as an active rare gas and hydrogen fluoride or fluorine as a halogen donor molecule.

The invention will now be described, by way of example, with reference to the accompanying drawings in is which Figure 1 is a schematic representation of an embodiment of the invention and Figure 2 is a schematic representation of another embodiment of the invention.

Referring to Figure 1 of the drawings, a transversely excited rare gas halide gas discharge laser includes a lasing region 1 formed in a clos ed loop of ducting, only a portion 2 of which is shown, through which a mixture of gases including a halogen donor can be circulated. The remainder of the loop of ducting and circulating fans are not shown as they can conform to well known practice and do not form part of the present invention as such. The lasing region 1 includes two output windows 3 and 4, respectively, and a pair of exciting electrodes 5, only one of which is shown as the other is directly beneath it. An optical cavity is formed by a full reflecting mirror 31 and a partially transparent output mirror 41. The lasing medium comprising a mixture of He; Xe F2 in the proportions 1500:20:1 is passed between the electrodes 5 in the direction shown by the arrow 6.

A continuous X-ray source 7 producing some 100 mW of KeV X-rays is so positioned as to irradiate the lasing medium through a window 8 just prior to its entering the lasing region 1. At a total operating pressure of 3 atmospheres, such an X-ray source will produce an ion density of about 109 ions Cm-3, with an ion lifetime to half density of about 1 mS. In practice, the ion lifetime is inversely proportional to the initial ion density and directly proportional to the gas pressure. Gas pressures in the range 1 to 3 atmospheres are suitable.

A pulsed laser 9 which produces radiation having a wavelength less than about 360 nm is arranged to illuminate the region between the electrodes 5 via a beam-folding mirror 10t a dichroic. mirror 11 and the exit window 4 from the lasing region 1. The laser 9 has a power output of about 100 kW cm-2 which is sufficient to liberate some 106 electrons cm-3 from the 109 ions cm-3. This is sufficient to enable a gas discharge to be maintained in the lasing medium. The laser 9 may be a subsidiary rare gas halide laser such as a Xecl laser producing radiation using a wave length of 308 nm, or a.nitrogen laser.

In the embodiment shown in Figure 2, the X-ray source 7 has been replaced by another ultra violet laser 21 of equivalent power output. Other components which correspond to those described with reference to Figure 1 have the same reference numerals. In practice, the lasers 9 and 21 may be one and the same with the electron- liberating pulses obtained by known beam-handling techniques.

An advantage of the use of an auxiliary laser to release the electrons is that the shape of the output pulse from the main laser can be varied by altering the shape of the output pulses from the auxiliary laser.

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Claims

Claims - 1. A method of operating an electrically excited gas discharge

laser wherein the lasing medium includes a rare gas and a halogen donor molecule, including the operations of producing continuously or quasicontinuously within the lasing medium a population of negative ions of the halogen donor molecule and subsequently removing electrons from the ionised halogen donor molecules to facilitate the establishment of an electric discharge in the lasing medium.

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2. A method according to Claim 1 wherein the ionisation of the halogen donor molecules is achieved by irradiating the lasing medium continuously with electromagnetic radiation the photon energy of which is capable of ionising the halogen donor molecules.
3.. A method according to Claim 1 wherein the ionisation of the halogen donor molecules is achieved by irradiating the halogen donor molecules with pulses of electromagnetic radiation the photon energy of which is capable of ionising the halogen donor molecules and the pulse length of which is longer than the lifetime of the ionised halogen donor molecules.
4. A method according to Claim 2 or Claim 3 wherein the electromagnetic radiation is X-radiation.
5. A method according to Claim 4 wherein the X-radiation has an energy of 50 KeV and a power level of 100 mW.
6. A method according to Claim 2 or Claim 3 wherein the frequency of the ionising radiation lies in the ultra violet region of the electromagnetic spectrum.

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7. A method according to any preceding claim wherein the removal of the electrons from the ionised halogen donor molecules is carried out by irradiating the ionised halogen donor molecules with radiation having a frequency in the ultra violet region of the electromagnetic spectrum.
8. A method according to Claim 7 wherein the ultra violet radiation used to remove electrons from the ionised halogen donor molecules is derived from the same source as that 10 used to ionise the halogen donor molecules.
9. A method according to Claim 7 or Claim 8 wherein the ionised halogen donor molecules are irradiated with ultra violet light having a wavelength less than 360 nm.

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10. A method according to any of Claims 7, 8 or 9 wherein the ultra violet radiation is at a power level of the order of 100 kW cm-2.
11. A method according to any of Claims 7 to 11 wherein the ultra violet radiation is derived from a rare gas halide laser.
12. A method according to Claim 11 wherein the rare gas halide laser is a xenon chloride laser.
13. A method according to any of Claims 7 to 11 wherein the ultra violet radiation is derived from a nitrogen laser.
14. An electrically excited gas discharge laser including a gaseous lasing medium comprising a rare gas and a halogen donor molecule, wherein there is included means for producing continuously or quasi-continuously a population of negative ions of the halogen donor molecule and means for subsequently removing electrons from the ionised i halogen donor molecules.
15. A gas discharge laser according to Claim 14 wherein the lasing medium comprises helium or neon as a carrier 5 gas. xenon, argon or krypton as the active rare gas and hydroqen chloride or fluorine as the halogen donor molecule.
16. A gas discharge laser according to Claim 15 wherein the lasing medium includes helium or neon at a pressure in the range 1-10 bar, xenon, argon or krypton at a pressure in the range 10-200 mbar and hydrogen chloride or fluorine at a pressure in the range 1-10 mbar.
17. A gas discharge laser according to Claim 16 wherein the lasing medium comprises neon at a pressure in the range 13 bar, xenon at a pressure of 40 mbar and fluorine at a pressure in the range 2-3 mbar.
18. A gas discharge laser according to Claim 17 wherein the lasing medium comprises a mixture of neon, xenon and fluorine at pressures of 3 bar, 40 mbar and 2 mbar respectively.
19. A gas discharge laser according to any of Claims 14 to 18 including a pair of electrodes by means of which an electric discharge may be maintained in the lasing medium, a source of ionising radiation arranged to irradiate the gaseous lasing medium so as to ionise the halogen donor molecules prior to their passage between the electrodes and a laser arranged to irradiate the ionised lasing medium as it passes between the electrodes so as to release electrons from the ionised halogen donor molecules thereby to facilitate the creation and maintenance of the electric discharge in the lasing medium.

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20. A gas discharge laser according to Claim 19 wherein the source of ionising radiation is an auxiliary laser the output from which has a wavelength less than 360 nm.
21. A gas discharge laser according to Claim 20 wherein the auxiliary laser is a rare gas halide laser.
22. A gas discharge laser according to Claim 21 wherein the auxiliary laser is a xenon chloride laser.
23. A gas discharge laser according to Claim 22 wherein the auxiliary laser is a nitrogen laser.
24. A gas discharge laser according to any of Claims 20 to is 23 wherein there is included means for directing a proportion of the output of the auxiliary laser through the ionised lasing medium as it passes between the electrodes so as to release electrons from the ionised halogen donor molecules thereby to facilitate the creation and maintenance of the electric discharge in the lasing medium.
25. A gas discharge laser according to any of Claims 20 to 24 wherein the auxiliary laser has a power output of at -2 least 100 kW cm
26. A gas discharge laser according to Claim 19 wherein the source of ionising radiation is an X-ray source.
27. A gas discharge - laser according to Claim 26 wherein the X-ray source is adapted to produce X-rays having an energy of 50 KeV and a power level of at least 100 mW.
28. A method of operating a gas discharge laser substantially as hereinbefore described and with reference to the accompanying drawings.
29. A gas discharge laser substantially as hereinbefore described and with reference to the accompanying drawings.

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