WO1984002233A1

WO1984002233A1 - Transversely excited recombination laser

Info

Publication number: WO1984002233A1
Application number: PCT/AU1983/000172
Authority: WO
Inventors: Milan Brandt
Original assignee: Commw Of Australia
Priority date: 1982-12-03
Filing date: 1983-11-25
Publication date: 1984-06-07
Also published as: EP0147398A4; IT1194495B; EP0147398A1; JPS60500035A; IT8323998A0

Abstract

A recombination laser, incorporating a single anode (15) and opposing cathode (14) arrangement to provide a transverse-discharge configuration and between which an atmosphere of metal-ion vapour, in the form of strontium-ion, is generated. The anode (15) and said cathode (14) are of semi-circular cross-section with their semi-circular surfaces facing each other.

Description

TRANSVERSELY EXCITED RECOMBINATION LASER Technical Field

This invention relates to lasers, particularly a transversely-excited metal-ion recombination laser, and more particularly but not exclusively to a strontium-ion

-. (Sr ) recombination laser.

Background Art

Observation of laser oscillation on the

2 2

430.5-nm (6 S, ,_y - 5 P* ₂) transition of singly ionized strontium was first reported in the afterglow of a pulsed He-Sr discharge by E.L. Latush and M.F. Sem in Sov. J. Quantum Electron., 3,216 (1973). It was shown that electron-ion recombination and electron de-excitation are both responsible for generating laser action on this and other strontium-ion transitions. To date, experiments with the strontium-ion recombination laser have been carried out mainly with longitudinally-excited devices. Such devices are relatively easy to construct and when operated in the self-heating mode (the energy required to heat the laser tube and vapourize the strontium comes from the discharge itself) have yielded average output powers of 2 watts and overall efficiencies of approximately 0.2% (see reports by L.M. Bukshpun, V.V. Zhukov, E.L. Latush and M.F. Sem in Sov. J. Quantum Electron., 11,804 (1981) and V.V. Zhukov, V.S. Kucherov, E.L. Latush and M.F. Sem in Sov. J. Quantum Electron., 7,708 (1977)). These devices, however, operate at a relatively-low optimum buffer-gas pressure approximately 8 kPa (60 Torr) which inhibits high rates of recombination and electron de-excitation with consequent reduced performance.

An alternative approach involves the use of a transverse-discharge configuration which should allow operation at high gas pressures and therefore result in improved laser performance. Laser action on the 430.5-nm line of Sr has been reported previously in a transversely-excited device with a pin-array anode and a common cathode (see report by A.M. Bogus, V.L. Dzhikiya and A. . Chernov, in Sov. J. Quantum Electron., 8,259 (1978)). Laser oscillation was only observed for buffer gas pressures in the range 4 to 13 kPa (30 to 100 Torr) and no details were reported of the laser output power obtained.

Disclosure of the Invention We have found if a transverse-discharge configuration as formed by a single anode and opposing cathode is employed, together with a metal-ion vapour such as a strontium-ion, significantly improved pulse energy and peak power are obtained. The invention therefore envisages a recombination laser, incorporating a single anode and opposing cathode arrangement to provide a transverse- discharge configuration and between which an atmosphere of metal-ion vapour is generated. Preferably the vapour is strontium-ion, and means are provided to achieve a strontium evaporation temperature of 725 degrees Celsius at a vapour pressure of 107 Pa (0.8 Torr) whilst helium gas pressure is maintained at 120 kPa (900 Torr) . With such a configuration and under such operating conditions high laser pulse energies have been achieved and no saturation in laser output was observed up to the maximum discharge peak currents employed.

Brief Description of the Drawings One preferred form of laser tube in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the transverse-discharge laser tube;

Figure 2 is a plot of laser pulse energy as a function of the tube temperature in accordance with a preliminary test conducted with the laser tube of Figure

1;

Figure 3 is a plot of laser pulse energy as a function of helium pressure obtained from the preliminary test; Figure 4 is a plot of laser pulse energy dependence on discharge peak current obtained from the preliminary test;

Figure 5 is a representation of the oscilloscope traces of the temporal dependence of: (a) current pulse; vertical axis 1 kA/div, horizontal axis 50 ns/div; and (b) laser pulse; vertical axis 2 kW/div, horizontal axis 50 ns/div, obtained in the preliminary test. Figure 6 is a plot of laser pulse energy dependence on discharge current obtained from a further test conducted; and

Figure 7 is a plot of average power as a function of repetition rate obtained from a still further test conducted.

O PI Best Mode for Carrying Out the Invention

Referring to Figure 1 of the drawings the preferred form of laser tube was constructed from stainless-steel tubing 10 of thickness 3 mm, internal diameter 80 mm and length 700 mm. The tube was terminated at the ends by stainless-steel flanges 11 which incorporated demountable metal end-sections 12. Brewster windows 13 were clamped to the end-sections 12 to allow operation at gas pressures above one atmosphere. Helium was chosen as the buffer gas to satisfy the requirement for rapid cooling of the recombining electrons. Industrial grade helium entered the tube at one end via an inlet needle valve and was exhausted from the other end by a rotary pump. During laser operation only static gas fills were employed.

The electrode structure is formed by two stainless-steel rods 14 and 15 of semi-circular cross- section and of 12.5 mm effection diameter. The gap between the rods is 25 mm with semi-circular surfaces facing each other. The upper electrode 14 is 200 mm long and its mounting is electrically isolated by a section of heavy-wall silica tubing 16 having an external connection 21. The lower electrode 15, at earth potential, is secured to the flange at one end while the other is free to expand. A section of shaped firebrick 17 is used to keep the upper electrode 14 in place. The possibility of the discharge firing to the tube wall is eliminated by employing a length of large-bore ceramic tubing as a liner 18. The ceramic tube extends 200 mm beyond the ends of the upper electrode 14. In accordance with this preferred form of the present invention, pieces of strontium metal (total weight 2 g and purity 99%) are distributed at intervals along the central region of the tube, and the tube is mounted in a firebrick oven 19. The oven extended 150 mm beyond the ends of the upper electrode 14. The temperature is monitored with a chrome1-alumel thermocouple positioned inside the tube. The ends of the tube are cooled by passing water through water cooling pipes 20 surrounding the ends of the tube and in order to prevent overheating of the O-ring connections shown between the flanges 11 and the end sections 12.

It was observed that, in the. absence of any preionization, the introduction of strontium vapour into the active region produced a stable, homogeneous discharge which filled the volume between the electrodes. This behaviour was noted for a wide range of operating conditions. At present this phenomenon is not fully understood. It is well known that metal oxide coatings, in particular those of alkali metals, considerably improve the electron emission current of the cathode in vacuum tube devices (see for example G. Herrmann, S. Wagener, "The oxide-coated cathode", volume one and two, Chapman and Hall (1951)). This improved performance is principally due to the much lower work function of the oxide coating as compared to that of the metal substrate. In addition the alkali metal-oxide coating is resistive in nature and at the optimum temperature appropriate to

2 the present experiments its resistance is about 3Ω/cm

(see D.A. Wright, Proc. Royal Soc. London (A) 190, 394 (1947)). Visual inspection of our electrodes revealed a uniform coating of white, powdery substance, believed to be that of the strontium oxide, across the full area of the electrodes. It is inferred then that the strontium oxide coating due to its resistive nature forms a resistively ballasted electrode which has the ability to stabilize the discharge, that is prevent arc formation. Resistively ballasted electrodes of a different type have been used previously in other transversely-excited lasers (see A.F. Gibson, K.R. Rickwood and A.C. Walker, Appl. Phys. Letters, vol. 31, 176-178, (1977); K. Fujii, A.J. . Kearsley, A.J. Andrews, K.H. Errey and C.E. Webb, Quantum Elect. Letters, vol. QE-17, 1315-1317, (1981)).

The discharge circuit employed in the test conducted was of the "capacitor discharge type" and has been described in a report by M. Brandt and J.A. Piper in IEEE J. Quantum Electron., QE-17, 1107 (1981). A low- inductance storage capacitor was charged from a high- voltage dc supply. When the high-voltage switch (an EEV CX 1174 thyratron) was closed the charge stored in the storage capacitor is transferred rapidly to a series of dump capacitors (Murata 30 kV, 1700 pF) in parallel with the discharge channel. The voltage across the laser tube rose as charge accumulated in the dump capacitors until breakdown occurred in the mixture, followed by a rapid discharge of the dump capacitors. Charging voltages up to 30 kV at repetition rates of 1 to 5 Hz were employed. Tests were performed for a range of storage and dump capacitor values in order to investigate the effects of current amplitude and duration on laser performance. The current pulses were detected by a Pearson 110A current monitor and the voltage pulses were detected by a Tektronix high voltage probe. The resultant signals were displayed on a Tektronix 7844 oscilloscope.

In this preferred embodiment the laser resonator is formed from a pair of spherical concave mirrors 25 mm in diameter and with radii of curvature of 3 m, separated by 1.5 m. One mirror is highly reflecting (R > 0.99) and the other is partially reflecting (R = 0.6). In the tests laser output pulses were detected by calibrated KORAD K-Dl detector (ITT FW114A photodiode, S 20 photoresponse) in combination with Oriel neutral- density filters. A facility existed on the detector for measuring laser pulse energy directly by monitoring the voltage change across the photodiode high-frequency capacitors. The signals were displayed on the Tektronix 7844 oscilloscope.

A plot of laser pulse energy as a function of tube temperature (Sr vapour pressure) for fixed helium gas pressure prepared from the test results is illustrated in Figure 2. The increase in laser pulse energy with increasing Sr vapour pressure is believed to be due to the increase in the concentration of doubly- ionized strontium. The decline in laser power for temperatures above 725 degrees Celsius is possibly due to itJ £ _ O PI slowing down of the recombination of doubly-ionized Sr as a result of plasma heating caused by the recombination of singly-ionized Sr. In addition, the probability of filling the lower laser level by electron impact from the strontium-ion ground state increases with increasing strontium vapour pressure.

Figure 3 illustrates laser pulse energy plotted as a function of He gas pressure prepared -from the test results. While maximum pulse energy was obtained for He pressure of approximately 120 kPa (900 Torr), satisfactory performance has been achieved for pressures from 80 to 200 kPa (600 to 1500 Torr). Operation over so wide a range of helium pressures coupled with static-fill conditions is very attractive from the point of view of sealed-off operation.

Figure 4 is a plot of laser pulse energy dependance on peak current for a combination of storage and dump capacitor values also prepared from the test results. It can be seen that the pulse energy increases with increasing current amplitude up to the maximum charging voltage permitted by the voltage rating of the capacitors. The maximum output energy obtained was 1 mJ per pulse at a peak current of approximately 6 kA. Under this condition the laser pulse had a duration of 120 ns (FWHM), and reached peak power of 8.5 kW. Laser energy conversion efficiency, calculated in terms of the energy stored in the dump capacitors, was estimated to be about 0.1 percent. The increase in laser pulse energy with peak current was observed for all combinations of storage and dump capacitor values employed. It was noted that for the same voltage and current settings, the laser pulse duration, increased with increased storage and dump capacitor values while the peak power remained the same. For example, doubling storage and dump capacitor values from 7.8 WF and 5.1 μF respectively to 15 μF and 10.2 μF increased laser pulse duration (FWHM) from 60 ns to 110 ns. In addition, there existed an optimum ratio of storage to dump capacitance (about 1.5) for which the laser pulse energy was maximum. Any large deviation from this ratio caused a dramatic decline in pulse energy through both the decrease in peak power and laser pulse duration.

A representation of the oscilloscope trace of a single laser pulse together with a current pulse for operating conditions appropriate to Figure 4 and peak current of 4.8 kA, is shown in Figure 5. The pulse width is 120 ns (FWHM) and the peak height about 6 kW. Since the delay between the termination of the current pulse and the beginning of the laser pulse is of the order of ten nanoseconds high-repetition-rate operation of the present device (say, between 5 and 10 kHZ) should result in a relatively efficient device with an average output power in the 5 to 10 W region.

In summary, the strontium-ion recombination transverse-discharge configured laser of this preferred embodiment of the invention, produced laser oscillation - li ¬

on 430.5-nm transition of strontium-ion over a range of discharge parameters, including He gas pressure up to 200

3 kPa (1500 Torr), with output energy densities of 25uJ/cm being obtained from an active volume of in the order of

3 approximately 40cm .

Further tests have been undertaken with the aim of establishing the optimum operating conditions for maximum output energy. The output power and energy of a single-segment transversely-excited tube, of active volume of approximately 60 cm , was monitored as a function of temperature (Sr vapor density), the nature and pressure of the buffer gas, the peak current, and the energy deposited in the discharge. Under optimum conditions for helium as the buffer gas, laser specific energies of 35 μJ/cm were obtained at a conversion efficiency, based on the energy deposited in the discharge, in the range 0.1 to 0.2%. Laser output pulse energies of 2.1 mJ per pulse and peak powers of 17kW have been obtained from that active volume, and Figure 6 of the drawings is a plot of laser pulse energy dependence on discharge current resulting from such further tests. This performance was achieved with stable, homogeneous discharges produced in the mixture in the absence of any discharge preionization. These results demonstrate that a transversely-

+ excited Sr recombination laser, in contrast to a longitudinally-excited device, can, at least for low repetition-rate operation, generate large pulse energies and peak powers. Still further tests in connection with the laser of the present invention, and aimed at.increasing the operating frequency of a transversely-excited Sr recombination laser, have been conducted. The tests were performed with a single-segment TE tube of active volume of approximately 40 cm in the absence of any discharge preionization. The excitation circuit was a modified version of the low repetition rate circuit, and was capable of supplying high voltage ( " 20kV) high current ( ^~-5kA) excitation pulses at frequencies of up to 5 kHz. The laser average output power and pulse energy were monitored as a function of temperature, the gas pressure, the peak current, and the different values of the storage and dump capacitances. The highest average output power recorded was

90mW at a repetition frequency of 200 Hz. The corresponding laser output pulse energy was 450 μJ. Further increase of the repetition frequency resulted in an unstable, striated discharge. The operation frequency was increased up to 500 Hz but at a reduced input power level. The plot of Figure 7 of the drawings shows the effect of repetition rate on average power observed in these tests. It will be apparent from the above that operation of a transversely-excited strontium-ion recombination laser at relatively high repetition-rates has been achieved with the laser of the present invention. The results suggest that stable, uniform discharges can be produced in the mixture in the absence of any discharge preionization up to a few hundred hertz,

^~ ΕE

OMPI although operation beyond this frequency results in large discharge instabilities with consequent decrease in laser performance.

Claims

CLAIMS : -

1. A recombination laser, incorporating a single anode and opposing cathode arrangement to provide a transverse-discharge configuration and between which a atmosphere of metal-ion vapour is generated.

2. A recombination laser as claimed in Claim 1, wherein metal-ion vapour in the form of strontium-ion is used to form said atmosphere.

3. A recombination laser as claimed in Claims 1 or 2, wherein said anode and said cathode are of semi¬ circular cross-section with their semi-circular surfaces facing each other.