GB2038080A - Gas laser - Google Patents

Abstract

In a longitudinal or transverse flow gas laser, there is provided a thermal gas circulation system in which a gas is evaporated from a heated liquid reservoir 16, supplied to an inlet port 13 of the laser tube 10, collected from an outlet port 15 of the laser tube, condensed at 22, and returned to the liquid reservoir through pressure equalising means, such as a liquid seal 24 maintaining a head of liquid, or a porous material which resists passage of the liquid. <IMAGE>

Description

SPECIFICATION Improved gas laser This invention relates to gas lasers, especially to lasers in which the gas is a fluorinated alkyl halide. In the prior art, typical halides used are iodides such as CM31, C3F51 and C3F71, and lasers operating with such gases are termed iodine lasers.

In this specification the word "gas" also includes a vapour.

An iodine laser operates by photodissociation of the iodide in the vapour state caused by ultraviolet light, typically in the spectral region 250 to 300 nanometers depending on the gas to be dissociated; excited states of the iodine atom are generated, and laser emission takes place from these states at 1.315 micrometres, i.e. in the near infra-red. An iodine laser has the typical advantages of a gas laser, that is, a low-cost amplifying medium, so that large installations and therefore large pulse energies or continuous wave powers are possible, and relatively easy removal of the heat generated in the pumping process by flowing the gas, so that a high pulse rate or continuous laser operation are feasible.

In an iodine laser, molecular iodine is generated during laser operation and is an efficient quenching agent of the excited iodine states so that subsequent operation yields a much lower laser energy. In addition, there is partial consumption of the laser gas, due to partial irreversibility of the photolysis. In one prior art arrangement it has been the practice to refill the iodine laser with iodide gas before each pulse, but this limits the pulse repetition rate. In an alternative arrangement, the gas is pumped continuously through the laser and through a cold trap in which the molecular iodine condenses and the partial pressure of the iodide is restored. Such a pumped recirculation system is described by W. Fuss and K.

Hohla in Optics Communications, Volume 18, Number 4, September 1976, page 427.

A disadvantage of the pumed recirculation system is that mechanical vibration of the pump may be coupled to the optical resonator, causing alignment stability problems when the iodine laser is used in the oscillator mode. There may be gas leakage at the rotary seal on the pump, and if atmospheric oxygen is admitted to the laser, it forms an extremely active laser quenching agent. A third disadvantage is that the metal of the pump, even if it is stainless steel, may react to form an iodide, particles of which may flake off and be carried to the laser windows in the gas flow, possibly causing damage to the laser windows under high power laser irradiation in addition to generating serious absorption and scattering losses.For the particular case of continuous wave operation of the laser, extremely high flow rates at low gas pressure are re quired which are difficult to obtain in a closed cycle with mechanical pumps.

According to the invention, in a gas laser there is provided a thermal gas recirculation system comprising in series connection a liq uid reservoir; a longitudinal or transverse flow laser tube; condenser means; and a connection between the condenser means and the liquid reservoir; there being also provided means to heat at least a part of the liquid reservoir, and pressure equalising means be tween said heated part and the condenser means.

In use, the laser tube will be placed in a cavity resonator or optical amplifier arrange ment adjacent laser pumping means, and the liquid reservoir will contain a suitable liquid.

The difference in the pressure of the gas or vapour above the heated part of the liquid reservoir and in the condenser acts to drive the gas through the laser tube, and the pres sure-equalising means balances the difference in gas pressure between the ends of the laser tube.

In one form the reservoir and the condenser are at a lower vertical level than the laser tube and there is a liquid seal in a tube connection between the condenser and the reservoir, the head of liquid in the tube connection consti tuting the pressure-equalising means.

In another form the reservoir comprises a porous material which contains substantially all of the liquid in the gas laser, the resistance of the porous material to passage of the liquid constituting the pressure equalising means.

Also according to the invention, a method of supplying gas to a gas laser comprises evaporating gas from a liquid reservoir; sup plying the gas to an inlet port of a longitudi nal or transverse flow laser tube; collecting gas from an outlet port of the laser tube; condensing the collected gas; and returning the condensed gas to the reservoir via pres sure equalising means.

Suitable liquids for use in the reservoir include the fluorinated alkyl halides; either one such compound or a mixture of two such compounds may be used; if an iodide is used the gas laser is known as an iodine laser. To provide a buffer gas for such a laser, the reservoir may also contain a fluorocarbon hav ing the general formula CnF2n+2 or CnF2n, where n is between 3 and 5; such fluorocar bons have boiling points in the range - 1 0 to + 30"C. Alternative buffer gases comprise other molecular gases which are condensible at about these temperatures to form solutions with the fluroinated alkyl halides, such as the gas SeF6.

The invention will be described with refer ence to Figs. 1 and 2 of the accompanying drawings, which illustrate schematically two forms of a gas laser according to the inven tion.

In Fig. 1 a silica laser tube 10 is arranged between flashlamps 12 (or continuously oper ating lamps) which constitute the laser pumping system. The laser tube 10 is horizontal; an inlet port 13.of the laser tube is connected by a supply tube 14 to a liquid reservoir 1 6 which is vertically below the laser tube; the reservoir is surrounded by an electricallyheated jacket 1 8. An outlet port 1 5 of the laser tube is connected by a collector tube 20 to a condenser 22 which is also vertically below the laser tube; the lower end of the condenser is connected by a connection tube 24, surrounded by thermal insulation 26, to the reservoir 16, to which the connection is made below the surface of the liquid in the reservoir.

In use, vapour boiling off the liquid in the reservoir passes through the supply tube 1 4 into the laser tube 10, and out through the collector tube 20 to the condenser where it is cooled to liquid form. The liquid runs down the connection tube and returns to the reservoir. The thermal cycle is continuous. The pressure difference required to drive the gas through the system is generated by the difference in vapour pressure of the gas in the evaporator and the condenser. This pressure difference causes the difference P in liquid levels between the connection tube 24 and the reservoir 16; the height of the apparatus must be sufficient to accommodate the difference which obviously depends on the density of the liquid and the flow rates required.

Typically the gas supplied to the laser is C3F71. When the pumping lamps 1 2 are operated to pump the laser, several chemical reactions take place, including the formation of molecular iodine. The gas flow rate through the laser tube, which is controlled by the temperature difference in the evaporator and condenser, and in the pulsed mode the repetition rate of the flashlamp operation are chosen so that all of the molecular iodine is swept from the laser tube between pulses. The iodine dissolves in the liquid alkyl halide, and is not recirculated. Eventually, the concentration of dissolved iodine will reach a level such that the alkyl halide must be replaced, and iodide losses due to chemical reactions will also reach an unacceptacle level but it is believed that the laser can be operated for a considerable period-weeks or months-before this is required.

The entire thermal recirculation system can be hermetically sealed, so that there is no risk of atmospheric contamination of the laser and can be constructed of glass or other nonmetallic, iodine-resistant material, so that there is no possibility of the formation of metallic iodides. No circulation pump is needed, so no vibration problems arise from that source.

In the arrangement illustrated in Fig. 1 the thermal circulation of the gas is assisted by gravity. It would also be possible to provide thermal circulation using an evaporator and condenser on the same vertical level as the laser tube; the pressure drop along the laser tube could then be matched by use of a filter; other alternatives are the provision of an osmotic system, or the use of valves; in no case will a mechanical pump be required. Any of these devices may be used in conjunction with the gravity-assisted arrangement, to minimise the depth of the apparatus below the laser tube.

One slight disadvantage of the illustrated arrangement is that the gas entering the laser tube 10 may be relatively cool, i.e. less than ambient temperature, and condensation of atmospheric humidity may occur on the outside of the laser window. To minimise this effect a heat exchanger 28, 30 (shown chain-dotted) may be provided between the collector and supply tubes; this has the disadvantage that there is an inevitable increase in resistance to gas flow, but in many cases, supply of a cold gas will be acceptable, and even advantageous since the heating of the gas during laser operation will not result in an unacceptably high gas temperature.

For most efficient operation, there should be zero, or only a small, temperature difference between the condenser and ambient temperature; this minimises heat conduction at the coldest part of the system.

An alternative form of gas laser is illustrated in Fig. 2, in which integers identical to Fig. 1 are given the same reference numerals. In Fig. 2 a wide-bore glass tube 34 between the supply and collector tubes 14, 20 contains a body of porous material which forms the reservoir of the laser system; a suitable porous material is a sintered "Pyrex" (Registered Trade Mark) glass; the capillaries of the material contain substantially all of the liquid in the laser system, and the material must have sufficient surface tension to retain the liquid during operation of the laser whatever the orientation of the tube 34. The porous material is arranged in three sections; a first cylindrical part 36 forming a thin layer on the inner wall of the tube 34 near the supply tube 14; a central part 38 which forms a plug of substantial thickness in the wide bore tube; and a second cylindrical part 40 forming a thin layer on the inner wall of the tube 34 near the collector tube 20.

The central plug 38 consists of two circular portions 38A, 38B which each completely fill the tube 34, and a connecting stem 38C of smaller diameter, which leaves an annular space 39 between the stem and the tube 34; this is a liquid expansion space.

The two cylindrical parts 36, 40 form respectively an evaporation zone and a condensation zone. Around the wide bore tube 34 in each zone, in contact with the outer surface of the tube, are the first ends of respective heat pipes 42, 44, the second ends of the pipes being in contact with opposite faces of a first thermoelectric module 46. A third heat pipe 48 is arranged between the first module 46 and a second thermoelectric module 50, which is also in contact with a heat sink 52.

The thermoelectric modules are known devices each including a plurality of junctions between dissimilar metals, and having provision for passing a current through the junctions in either direction. The devices operate by means of the Seebeck and Peltier effects, and are capable of operating as heaters or refrigerators or heat pipes. The integers 42, 44 and 48 are heat pipes of more conventional construction, comprising a wick material in a closed cavity.

In the Fig. 2 arrangement, heat is transferred by heat pipe 42 from the electrically operated thermoelectric module 46 to the evaporation zone, heating the first cylindrical part 36 of the porous material. Liquid within the capillaries is evaporated, passes through the laser tube 10, and is condensed in the second cylindrical part 40 of porous material in the condenser zone, which is cooled by heat pipe 44. The condensed liquid passes through the central plug and is recirculated.

Resistance to the liquid movement balances the pressure drop along the laser tube.

The arrangement of heat pipes 42 and 44 allows some of the latent heat of condensation to be recirculated by transfer of heat from the condensation zone to the evaporation zone; some heat is also supplied by the module 46; heat is transferred out of the laser system by the heat pipe 48 to the ambient temperature heat sink 52; this heat may be of thermoelectric origin, or may be photolysis heat generated by laser operation. Control of the laser gas pressure and flow rate is obtained by adjusting the magnitude and direction of the elctric currents circulating in the thermoelectric modules, and variations in ambient temperature can also be compensated, for example by supplying heat from the heat sink 50.

The large contact areas between the heat pipes 42, 44 and the tube 34 in the evaporation and condensation zones minimise the temperature difference between the zones; the thermal efficiency is dependent on a small difference. The main contribution to thermal resistance is the low thermal conductivity of the glass tube 34.

When the temperature control system is inactive, the liquid is no longer constrained by the porous material. To minimise start-up time, the material should be at the lowest point of the apparatus in its most probable orientation. However, it is a great advantage of the Fig. 2 arrangement that the gas circulation system is no longer gravity dependent; it is believed that such a form of laser according to the invention may be used in portable arrangements, and in vehicles and aircraft, since the orientation of the laser when it is operating is not critical.

The heat transfer system illustrated in Fig. 2 may in some circumstances be unnecessarily complex; the use of a porous-body reservoir is also possible with separate heat inlet and outlet connections, without the recirculation facility.

Considering now the gas used in a laser according to the invention; this will have a molecule containing a halide radical which is easily dissociable into an excited state, usually by photolysis, but alternatively by ion discharge or a proton beam. In the present invention there is the additional requirement that the material can be easily condensed and vaporised, preferably but not essentially at temperatures such that thermoelectric or mechanical refrigerators can be used; however, cryogenic agents such as liquid nitrogen can be used in the condenser if necessary.

Typically, materials are fluorinated alkyl halides of the general form (CnF2n+,)R, where R is a halide, such as CM31, C2F51, C3F71, and C4Fgl, which have boiling points at or near room temperature. Bromides and chlorides may also be suitable, as may fluoro-chlorinated alkyl halides such as CF2CII. Other alternatives are fluorinated halides including an arsenic atom such as (CF3)2Asl, or with phosphorous or antimony atoms instead of arsenic. Silicon-based compounds of the general form SiXFyl may also work. In general, the compound will not contain any hydrogen atoms, and will not be an aromatic compound.

Instead of a single halide, a mixture of two or more compounds may be used when, for example, a wider ultra violet absorption band is required to give a better spectral match to the pump light, thus improving the efficiency of the laser.

The partial pressure of the laser gas should be controlled to give the correct degree of absorption of the pump light.

When the gas used in C3F71, the laser can operate in a long-pulse, free-running mode of oscillation without Q-switching or in a continuos wave mode; no buffer gas is needed to control the gain. For a laser tube 10 having a diameter between 1 and 5 centimetres, and operating at ambient temperature, the temperature in the evaporator should be about - 20'C. For larger diameter tubes, which require a lower gas pressure, either the operating temperature can be reduced, or the less volatile C4Fgl can be substituted.

If Q-switched operation is required or if the device is required to act as an amplifier, then a buffer gas is essential to reduce the gain of the laser to a controllable level. The conventional buffer gas, argon, is unsuitable in a laser according to the invention because it is not easily condensable, and, if present in the necessary quantities, would effectively stop the thermal recirculation.

It has been found that materials suitable for use as buffer gases in a gas laser having a thermal recirculation system comprise the heavier fluorocarbons such as C3F8, C4F8, C4F10; C5F,2; these materials have boiling points at about room temperature, form solutions with fluoroalkyl halides and iodine and have satisfactory pressure broadening rates and beam determining parameters, that is, they have high specific heat, high molecular weight,- and low speed of sound.

.In a gas laser according to the invention, the thermal recirculation system may be initially charged with a mixture of a fluorinated alkyl halide and a heavier fluorocarbon, proportioned so that, at the temperature of the gas just after evaporation, the required partial pressure of each component is obtained. Since alkyl halides absorb ultra violet light and fluorocarbons do not, a convenient method of determining the partial pressure of the halide is the use of an ultra violet absorption spectrometer. Usually the partial pressure of the halide is about 10 to 100 torr, and that of the buffer gas is between 0.1 and 5 atmospheres.

It is possible to use a mixture of fluorocarbons to provide the exact degree of volatility required, or to discourage dimerisation of the fluorocarbon in the laser; for example, C6F,4 may be added when C3F7l is the main working gas.

Since fluorocarbons have considerable thermal mass, the use of the optional heat exchanger is particularly advantageous in reducing the power consumption. In large installations, operating with high buffer gas pressures, the thermal efficiency can be improved by using the waste heat from the refrigeration system to heat the liquid in the evaporation section, thereby recycling the latent heat released on condensation; the heat path is indicated by the dotted line 32 in Fig. 1, and is fully illustrated in Fig. 2.

In general, a gas laser according to the invention is expected to operate to give Qswitched pulses in the energy range 1 to 100 joules with good quality beams and a pulse repetition rate of between 1 and 10 Hz, or in continuous wave mode with powers of 1 to 100 watts.

Claims (12)

1. A gas laser in which there is provided a thermal gas circulation system comprising in series connection a liquid reservoir; a longitudinal or transverse flow laser tube; condenser means; and a connection between the condenser means and the liquid reservoir; there being also provided means to heat at least a part of the liquid resrvoir, and pressure equalising means between said heated part and the condenser means.

2. A gas laser according to Claim 1 in which the liquid reservoir and the condenser means are at a lower vertical level than the laser tube and there is a liquid seal in a tube connection between the condenser means and the liquid reservoir, the heat of liquid in the tube connection constituting the pressure equalising means.

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3. A gas laser according to Claim 1 in which the liquid reservoir comprises a porous material which contains substantially all of the liquid in the gas laser, the resistance of the porous material to passage of the liquid constituting the pressure equalising means.

4. A gas laser according to Claim 3 further comprising heat exchanging means connected between the heated part of the liquid reservoir and the condenser means.

5. A gas laser according to any preceding claim in which the liquid reservoir contains a fluorinated alkyl halide.

6. A gas laser according to Claim 5 in which the liquid reservoir contains a fluorinated alkyl iodide of the general form CnF2n+,1 where n is an integer less than 5.

7. A gas laser according to Claim 5 in which the liquid reservoir contains (CF3)2Asl.

8. A gas laser according to Claim 5 in which the liquid reservoir contains CF2Cll.

9. A gas laser according to any preceding claim in which the liquid reservoir contains a material of the general form SiXFyl.

10. A gas laser according to any preceding claim which further contains a fluorocarbon buffer gas.

11. A gas laser according to Claim 10 in which the buffer gas is C3F8 or C4F8 or C4Fro or C5F,2-

1 2. A method of supplying gas to a gas laser comprising evaporating gas from a liquid reservoir; supplying the gas to an inlet port of a longitudinal or transverse flow laser tube; collecting gas from an outlet port of the laser tube; condensing the collected gas; and returning the condensed gas to the reservoir via pressure equalising means.

1 3. A gas laser as hereinbefore described with reference either to Fig. 1 or to Fig. 2 of the accompanying drawings.