Abstract: A gas ion laser employs two or more independently controlled electrical gas discharges including a longitudinal plasma arc created by a thermionic cathode and an anode and a transverse glow discharge electron beam created within a photocathode reflector. These separately controlled gas discharges are arranged in a unique topology that produces highly excited atomic ions on the optical axis of a laser cavity, giving rise to a beam of laser optical radiation characteristic of the gas ion species being excited. The longitudinal plasma arc on the optical axis is heavily populated with ground state thermal ions that are not sufficiently excited to the energetic states required for laser action. The additional energy to excite these ground state thermal ions to the upper laser states is provided by a second more energetic photo-electron and glow discharge electron beam generated by a negatively biased coaxial photocathode reflector.

Abstract: A TE laser amplifier has a fully closed metal housing. This fact, its geometry, and that of its electrodes (5-7) and/or the reversal of the output polarity make possible a homogenous field distribution, a capability to influence the local amplification process in the laser medium, and a relatively long lifetime of the gas and the laser.

Abstract: The generator is excited by an electric discharge through the gas between two electrodes (6, 7), at least one of which (6) is made of silicon carbide.

Abstract: Disclosed is a cathode for a ring laser comprising a housing composed of substantially a nickel-iron composition having an inner cavity with a coating of a low sputtering electrically conductive material thereon.

Abstract: A gas discharge device includes a block having one or more gas filled cavities or passageways along which the laser beams may propagate. Coupled to one of these passageways is an exit port which is coupled to a cathode mounted on an outer surface of the block. The exit port is formed so that the cross-section of the exit port decreases with distance from the outer surface of the block.

Abstract: A cathode assembly for a gas laser housing has weldless sleeves for securing a cathode to the laser housing. A support flange, brazed or heliarc welded to a gas reservoir at one end of a laser tube, supports a cathode. Ceramic feedthroughs brazed or heliarc welded to holes in the flange bring wires through the flange while maintaining insulation therefrom. The helical cathode is joined to copper stubs from the feedthroughs by crimped conductive sleeves.

Abstract: A transverse gas flow RF pumped waveguide laser has been developed utilizing RF discharge waveguide technology in several infrared lasers. Two potential applications have been identified; the pulsed chemical laser and the CW CO.sub.2 laser. In the chemical laser, the flowing gas device provides rapid gas replenishment to maintain high electrical efficiency at high repetition rates. In the CW CO.sub.2 laser, the flowing gas provides efficient cooling so that high output power per unit gain length can be achieved.

Abstract: The invention is a switch to permit a laser beam to escape a laser cavity through the use of an externally applied electric field across a harmonic conversion crystal. Amplification takes place in the laser cavity, and then the laser beam is switched out by the laser light being harmonically converted with dichroic or polarization sensitive elements present to alter the optical path of the harmonically converted laser light. Modulation of the laser beam can also be accomplished by varying the external electric field.

Abstract: The present invention relates to a device in which a laser medium in a discharge space is excited by a discharge. The device includes a positive electrode and a plurality of negative electrodes in opposition, connected to a simmer power source and a main discharge power source. There is also a plurality of first stabilized resistances, which restrict the current of the simmer power source, each of which is respectively connected to one of the negative electrodes. There is also a plurality of first reverse current prevention elements, each of which is connected to the main discharge power source, and connected in parallel with one of the first stabilized resistances.

Abstract: A metal vapour laser is disclosed having an optical cavity with a sputtering cathode and anode located at one end of the cavity. The sputtering cathode and anode includes a gaseous jet inlet to introduce a gas stream of inert gas into the optical cavity for entraining metal vapour sputtered by the sputtering cathode and anode. An electrical discharge separate to the sputtering cathode and anode is provided in the optical cavity for exciting the metal vapour entrained in the inert gas stream.

Abstract: A highly efficient laser employs a glow discharge electron gun and includes a solid wall cathode that emits a beam of electrons resulting in a plasma that is a negative glow discharge having an electron distribution that has a larger number of high energy electrons than would be present in a Maxwellian distribution of the same electron density.

Abstract: An improved negative glow discharge hollow cathode device for generating laser radiation. The device comprises a hollow cathode assembly substantially surrounded by an anode envelope having a body section and a pair of end sections, each of the end sections terminating in a radiation transmission member. The metal laser medium is enclosed in the vicinity of the perforations in the cathode such that the metal vapor can be readily introduced into the cathode central region effectively and efficiently. The anode to cathode discharge paths is reduced by the use of an anode screen such that the glow discharge is directed into the cathode central region with a controlled discharge gap for optimum pressure and confined negative glow discharge within the cathode. The metal laser medium, in the form of a metal foil or sheet, is lined adjacent the cathode outer surface and perforated cathode holes in the lower half portion of the cathode.

Abstract: An apparatus for the generation of laser radiation according to the invention comprising a discharge chamber of essentially rectangular cross section made from insulating material, one pair of discharge electrodes between which a gas discharge initiated by UV preionization arising from a corona discharge can be produced in a direction transverse to an optical resonator, as well as a charging and discharging circuit and a gas circulation and cooling system. The two opposite walls of the discharge chamber on which the electrodes are arranged are provided with a metal coating, e.g. metal plates of sheets. The remaining lateral walls of the discharge chambers are provided with conducting structures, preferably wire grids which are insulated against the interior space and extend up to the level of the metal coating.

Abstract: Gas laser discharge tube with copper halide vapors. The tube has a vacuum tight quartz casing equipped with at least two electrodes which are disposed in extensions of the quartz casing. The electrodes are made of copper particles of irregular form which make contact with the vacuum tight terminals. The copper particles are piled in quartz tubes disposed coaxially in the quartz extensions, so that between the quartz tube and the extensions of each electrode there is formed a cavity. Each quartz tube is perforated along its entire surface and it is plugged by a perforated copper plate through which the vacuum tight terminal passes. The apertures in the quartz tube and in the copper plate have a smaller diameter than the diameter of the copper particles. This gas discharge tube may be used in scientific research, laser locating, and the like.

Abstract: A laser discharge tube comprises a discharge envelope and an anode and cathode both disposed within the envelope. The envelope includes a capillary tube having a hole. In this hole the tip portion of the anode is inserted. Outside the envelope there is provided a magnet generating a magnetic field which lies in the vicinity of the anode.

Abstract: The turn-on delay of a cold cathode gas tube, such as a gas laser, is significantly reduced by the inclusion of an electric field concentrator in a region of the tube between the tube anode and cathode. The concentrator can take the form of a projecting conductive member having a sharp corner, linear or curved knife edge, needle like points, or combinations of such geometries.

Abstract: A high frequency, high energy RF powered laser is shown and described. This laser utilizes a stripline wave guide electrode, a ground plane, and dielectric materials for the lasing tube to provide for resistance to spurious discharge and parasitic breakdown between the stripline and the ground plane. The use of a high voltage pulse to initiate lasing gas breakdown, the use of a dielectric section in the stripline wave guide to protect the RF source and a choke to protect the high voltage pulse source from RF energy are also shown and described.

Abstract: A thermal structure and cooling system for an argon-ion laser that is virtually free of thermal asymmetries includes a laser cathode having a housing constructed of a material of high thermal conductivity and relatively low thermal expansion in conjunction with a cooling structure configuration which readily and uniformly dissipates heat. In a specific embodiment the support structure is fabricated of copper material of at least a minimal thickness and the cooling fins are disposed in one and two stages of radial fins at specified regions along the support structure forming the discharge tube and enclosed by a tubular thermally conductive band.

Abstract: The present invention is a configuration of electrodes in a transversely excited gas laser which includes an elongated cylindrical chamber with a laser gas confined therein, a pair of reflectors which reflect light energy from a discharge of the laser gas within the chamber so that the light energy travels longitudinally the length thereof, an rf generator which applies a voltage of alternating polarity between the electrodes at a frequency ranging from 10 Mhz to about 3 GHz to establish the laser gas discharge, a coupling circuit which matches the steady state reactive impedance of the chamber to the impedance of the rf generator and couples the rf generator to the electrodes. The configuration of the electrodes, each of which is formed from an electrically conductive material and is in the shape of a continuous helix, includes a first electrode and a second electrode each of which is disposed opposite to the other on the outer surface of the chamber in a first plane.

Abstract: A gas laser having an outer conductive shell and two axially displaced spaced plastic cylinders mounted within the shell. The inner plastic cylinder supports the high voltage components to produce a transverse electrical discharge. The outer plastic cylinder is spaced from the outer shell by rubber feet and forms an acoustic baffle to decrease the noise radiated during operation. High speed fans circulate the laser gas around the annular path between the two cylinders and between the high voltage electrodes. The inner plastic cylinder is manually removable for repair or other reason and carries with it all of the electrical components within the outer shell. High voltage is fed to the high voltage electrodes by means of a circuit that does not include the outer shell. This arrangement minimizes operating noise, provides maximum safety for operating personnel, and minimizes electrical interference.

Abstract: In a laser using an electrical discharge produced between a pair of electrodes a catalyst is used for the recombination of gain medium components dissociated by the discharge. The catalyst material is confined by a catalyst container made of conductive material. The container has apertures smaller than the smallest catalyst particle and bigger than the molecule size of the gain medium. The conductive surface forms a cage which protects the catalyst, usually a powder, from the dispersing effects of the large electric fields present in the laser, and the aperture size allows the gain medium to contact the catalyst while preventing the catalyst powder from dispersing throughout the laser. The cage is placed adjacent the region in which the discharge takes place. Preferably, the cage is placed in contact with one of the electrodes in order to transfer heat from the electrode to the catalyst.

Abstract: A transversely excited gas laser, includes: an elongated gas-filled discharge chamber having an optical axis; a main cathode electrode and a main anode electrode extended along the optical axis in the discharge chamber defining a main discharge space between the main cathode and the main anode electrodes through which the optical axis extends, the main electrodes bordering two sides of the main discharge space leaving two other sides open; two pairs of auxiliary electrodes each being extended parallel to the optical axis in vicinity of a respective one of the open sides of the main discharge space and mutually spaced apart by a given distance, each of the auxiliary electrodes including a conductor and a dielectric shell surrounding the conductor, each of the auxiliary electrodes being spaced from all of the other electrodes by a set breakdown distance; and a driving unit connected to the electrodes, the driving unit supplying different potentials to the main cathode and main anode electrodes producing a main

Abstract: Gas laser of circular cross section perpendicular to the direction of beam propagation, having an optical resonator. The laser has, for continuous or modulated continuous beam operation, a tube as the inner electrode (2), which is connected to a high-frequency voltage source. The outer electrode (4) likewise comprises a tube which is disposed coaxially with the inner electrode (2). Between the two electrodes (2, 4) is the annular excitation chamber (3). The tube wall serving as outer electrode (4) is provided with slot-like cavities (1) which are open toward the excitation chamber (3) and serve for the cooling of the gas therein.

Abstract: A lateral excitation type gas laser device having a small size and high output. An anode and a cathode are arranged opposite one another with a flow of laser gas therebetween. A high DC voltage is applied between the anode and cathode to cause a glow discharge therebetween. A dielectric electrode is disposed in the flow of laser gas upstream of the anode and cathode, and a high frequency high voltage is applied between the dielectric electrode on one hand and the anode and cathode on the other for causing a silent discharge therebetween. The dielectric electrode has a diameter of 3 mm<D<d/2, where D is the diameter of the dielectric electrode and d is the length of the gap between the anode and cathode. The dielectric electrode is disposed in a region defined by 0.ltorsim.l.sub.1 .ltorsim.2d and 0.ltorsim.l.sub.2 .ltorsim.d, where l.sub.1 is the distance in the direction of the laser gas flow between the dielectric electrode and the cathode and l.sub.

Abstract: The efficiency of pulsed laser systems is enhanced substantially by injecting a minor amount of radiation energy into the system .DELTA.t seconds prior to the emergence of the laser pulse. The degree of power enhancement is a function of both radiation wavelength and time delay.

Abstract: The invention provides a novel means of stabilizing a glow discharge device against electrothermal instabilities and electrode induced instabilities comprising imposing a magnetic field on the discharge current to create a sheared flow system within the discharge volume. There is further provided an apparatus for producing a stable high power glow discharge comprising an anode and a cathode adapted to be connected to an electric power source and, when so connected, to establish an electric field and a glow discharge between said anode and cathode and an electromagnet adapted to be connected to an electric power source and, when so connected, to establish a magnetic field across said electric field. This application relates to high power glow discharge devices and to a means of stabilizing such devices against electro-thermal instabilities and electrode induced instabilities.

Abstract: An inductively stabilized, long pulse duration transverse discharge apparatus. The use of a segmented electrode where each segment is attached to an inductive element permits high energy, high efficiency, long-pulsed laser outputs to be obtained. The present apparatus has been demonstrated with rare-gas halide lasing media. Orders of magnitude increase in pulse repetition frequency are obtained in lasing devices that do not utilize gas flow.

Abstract: Exploding conducting film laser optical pumping apparatus. The 342-nm molecular iodine and the 1.315-.mu.m atomic iodine lasers have been optically pumped by intense light from exploding-metal-film discharges. Brightness temperatures for the exploding-film discharges were approximately 25,000 K. Although lower output energies were achieved for such discharges when compared to exploding-wire techniques, the larger surface area and smaller inductance inherent in the exploding-film should lead to improved efficiency for optically-pumped gas lasers.

Abstract: An orthogonal type gas laser oscillator is constructed in such a manner that a laser medium gas is caused to flow through an exciting region for the laser oscillation to perform the laser oscillation on the optical axis which runs in the direction orthogonal to the flowing direction of the laser medium gas; the exciting region is provided independently in even number and the flowing direction of the laser medium gas in one half of the exciting region and the flowing direction of the laser medium gas in the remaining half of the exciting region are mutually opposed.

Abstract: Method of fabricating an cold cathode for a laser such as ring laser gyro characterized by the use of vitreous carbon as the material of the cathode. For this purpose a predetermined amount of resin is pressed formed into the shape of a hollowed cathode body which is then heated in an oxygen free environment form a vitreous shaped cathode by thermal degradation of the resinous body. The vitreous carbon material reduces the difference between the coefficients of thermal expansion of the housing and electrode which can cause the indium seal to fail, e.g., in the case of prior aluminum cathode. The electrical connection may be made via either a silver epoxy or indium solder.

Abstract: A circular bore transversely excited gas discharge laser is disclosed which may be constructed and operated with no physical contact between the active discharge and the metal excitation electrode structure. The discharge is excited by RF in the frequency range 10 MHz to 1 GHz applied to a transverse metal electrode structure designed to maintain a relatively uniform electric field in the discharge bore. The disclosed laser may be configured as a waveguide laser or a large bore laser operating in a non waveguide mode.A means for inductively coupling RF energy from a suitable RF energy source to the electrode structure and a means for attaching mirrors to the discharge tube using no organic sealing material is disclosed. Without physical contact between the active discharge and the metal excitation electrode structure or organic sealants, long lifetime, superior laser performance, and capability for liquid cooling of the discharge tube is achieved.

Abstract: A laser device with a discharge tube forming a closed envelope with cylindrical end portions connected by an elongated constricted center portion and having external electrodes at the end portions, fed from a source of short pulses with a width measured in nanoseconds, capacity coupled to the discharge plasma. The discharge plasma may include metal vapor, a metal halide, including mercury halide, and internal electrodes may be included in the closed envelope but coupled to the external electrodes by capacity coupling so as to avoid any connections from electrodes passing through the glass or fused silica envelope.

Abstract: An X-ray generator 7 directly connected to the electrode 8 internally of a hollow cathode 3 emits electrons under a field effect to generate X-rays, the generator 7 supplying 50 to 100 Kv with a rise time of less than 10 nanoseconds. X-rays rapidly created, in turn, rapidly create electrons in the active medium 1 between anode 2 and cathode 3 to trigger a discharge therebetween, across which anode and cathode the voltage of a laser capacitor is pre-applied at a level below the breakdown voltage of the active medium.

Abstract: A gas laser device has an anode and a cathode arranged oppositely in a discharge tube. The cathode comprises a plurality of cathode members each having a predetermined discharge effective cathode surface and each being electrically isolated from others. A switching circuit selects the cathode member(s) used for a continuous wave oscillation and the cathode member(s) used for a pulsed oscillation. The total area of the discharge effective cathode surface(s) of the cathode member(s) used for the pulsed oscillation is selected to be smaller than, preferably 1/3 to 1/2 of that of the discharge effective cathode surface(s) of the cathode member(s) used for the continuous wave oscillation.

Abstract: The present invention is an improved gas laser which includes an elongated cylindrical chamber of cross-sectional dimension which has an internal cylindrical wall and which is suitable for confining a laser gas discharge. The internal cylindrical wall has a plurality of grooves extruded on its internal wall surface. A laser gas is disposed in the elongated cylindrical chamber. A pair of reflectors reflect light energy from the laser gas discharge within the elongated cylindrical chamber so that the light energy travels longitudinally the length of the elongated cylindrical chamber and is optically independent of the internal walls of the elongated cylindrical chamber. The improved gas laser also includes a pair of electrode plates which excite the laser gas and an rf generator which applies a voltage between the pair of electrode plates in order to establish a laser gas discharge in said laser gas. The plurality of grooves provide optical mode control.

Abstract: A cathode arrangement for a ring laser gyro. A metal cathode member is sunken into a bore disposed in the gyro body to provide a low profile cathode arrangement. An annular groove is formed at the bottom of the bore around the periphery thereof. The cathode member is suspended from a cover such that the lower edge of the cathode member is disposed in the annular groove, the cover being secured to the gyro body to provide a seal therebetween.

Abstract: Apparatus for establishing and maintaining a stable discharge across a stream of flowing gas, particularly useful for high-power lasers, comprises working electrodes disposed on opposite sides of the gas flow channel, and starting electrodes adjacent to the upstream working electrodes. Each working electrode includes a ballast impedance and is connected between a high-voltage source and a point of fixed reference potential. Each starting electrode includes a substantially larger ballast impedance and is connected to the point of fixed reference potential such that each starting electrode is effective to bring the point of fixed reference potential to the upstream working electrode to start the discharge, and to draw little current after the discharge has been started.

Abstract: An electrode assembly and a D.C. glow discharge system incorporating the electrode assembly. The system is stabilized by a lower pulse repetition rate (1 KHz-5 KHz) and power (<5% of sustainer power) than that required for pulse-sustained discharge. It operates with a self-sustained discharge at lower voltage than usual due to lowered work functions, increased secondary emission and effective ionization. The pulse-stabilized discharged provides an extended stable operation suitable for high power gas lasers. A water ballasted electrode unit used therein, having individually ballasted electrode elements with discrete water resistors so formed when water flows, eliminates temporally varying spatial current fluctuations and exhibits such unique characteristics as; providing capacitive coupling for a pulse, and resistive coupling for D.C., and self-optimizing current uniformity with the operating time.

Abstract: An excitation circuit for a TE high-energy laser system, includes at least one high-speed, high-voltage switch, activatable for energizing a pulse-generating network to generate high-voltage pulses at a pair of laser electrodes; the pulse generating network having first and second strip-line capacitors associated with the high-voltage switch and the laser chamber and with appertaining equivalent inductances of the excitation circuit formed from self-inductance of the high-voltage switch, the laser chamber, leads of the system and the stripline capacitors has at least a third stripline capacitor included in the pulse-generating network in addition to the first and second stripline capacitors, a series connection of the high-voltage switch and one of the equivalent inductances being connected in parallel with the first stripline capacitor and with a series-parallel circuit formed of the second stripline capacitor and a parallel connection made up of the third stripline capacitor in parallel with an impedance as

Abstract: A vascular anastomosis apparatus using a carbon dioxide laser is proposed. The vascular anastomosis apparatus satisfies three conditions: a long focal distance of not less than 10 cm which is suitable for surgical operations under microscopic observation; a beam spot diameter of 0.1 to 0.3 mm when a laser beam is converged; and a stable, small beam output of not more than 100 mW. A visible guide beam having the same focusing point as that of the carbon dioxide laser beam is used in the vascular anastomosis apparatus.

Abstract: A harmonic amplifier for a laser beam amplifies a harmonic frequency of the beam. The amplifier generates a series of electric fields across the beam, successive fields being directed oppositely. The spacing between the fields is related to the speed of propagation so as to amplify a particular harmonic. Typically this will be the second harmonic so that the amplifier acts as a frequency doubler. The fields can be provided by an array 6 of electrode pairs positioned on opposite sides of the beam 2 with a voltage generator 8 connected across the sets in alternately opposite sequence.

Abstract: A light beam modulating and deflecting system comprising a stationary electrode secured to a stationary body, and a rotatable electrode secured to a rotating body, on which a light source is mounted, and opposed in slightly spaced relation to the stationary electrode to form a capacitor together with the stationary electrode. A high frequency signal modulated with a light modulating signal is applied to the stationary electrode, thereby to feed the light modulating signal from the stationary electrode to the rotatable electrode. The system may comprise a plurality of light sources, stationary electrodes and rotatable electrodes, and high frequency signals modulated with various light modulating signals may be applied to the respective stationary electrodes, thereby to independently modulate the light sources via the rotatable electrodes.

Abstract: A high power, electrically excited laser is disclosed wherein a laser gas is caused to flow through an excitation region in which an electric discharge is established along a direction transverse to the direction of flow of the laser gas between an anode and a porous cathode disposed on opposite sides of the excitation region, and wherein an electron beam is introduced into the excitation region along a direction parallel to the direction of the discharge. A first quantity of an auxiliary gas (preferably helium) having a normal glow current density at least an order of magnitude less than that of the laser gas is introduced into the upstream end of the excitation region adjacent to the cathode and caused to flow across the cathode surface along a direction parallel to that of the laser gas. A second quantity of the auxiliary gas is "bled" through the porous cathode and gradually added to the auxiliary gas stream flowing across the cathode surface.

Abstract: Laser of the TE type, including a gas-tight housing defining a laser chamber having a gas space, at least two laser electrodes disposed parallel to the optical axis of the laser within the housing, the electrodes being disposed mutually opposite and at a given distance from each other, excitation of the laser being provided by an arc-free capacitor discharge being as homogeneous as possible in the gas space between the electrodes, a pre-ionization device disposed within the housing, current leads and a current return disposed in the housing, each of the electrodes being connected to one of the current leads and current return, a device disposed in the laser chamber for conducting laser gas therethrough, a pulse-forming network having a capacitor contact surface and being connected to the current leads and current returns, the laser electrodes being in the form of at least two subelectrodes facing toward the pulse-forming network and subelectrodes facing away from the pulse-forming network, an electrode bridge

Abstract: A gas laser having preionization means which comprises an elongated glass tube extending beside the glow discharge volume for the length of the laser cavity, there being a central conductor inside the tube and a conductive strip or spaced strips bonded to the outside of the tube. The inner wire and outer strip are connected respectively to the same high voltage pulse leads which provide the main laser discharge. Ultraviolet radiation from the conductive strips on the tube preionizes the laser gas and creates a stable uniform laser electric discharge.

Abstract: A sealed-off CO.sub.2 TEA laser employs a corona discharge preionization technique and improved mechanical construction to improve vibration resistance and shelf life and to decrease sensitivity to the form of the electrical input.

Abstract: A gas laser of circular cross section perpendicular to the direction of beam propagation and having an optical resonator is disclosed. The laser has, for continuous or modulated continuous beam operation, a metal tube as inside conductor, which is connected to a high-frequency voltage source. An outside conductor, which is coaxial with the inside conductor consists of a tube of metal of good electrical conductivity. The inside conductor is surrounded, leaving an intervening space, by a jacket tube of dielectric or metallic material. A dielectric cooling liquid of electrical insulating quality and low loss factor for high-frequency fields flows through the space between the inside conductor and the jacket tube.

Abstract: The generator comprises an envelope and two mirrors defining a resonant cavity, in which is maintained a monolithic wave-guide with a capillary. The capillary is formed in a thinned portion of the guide. Two electrodes extend over the thinned portion of the guide. The capillary is circular in cross section, this being obtained by drilling or extrusion of a cylindrical bar made of dielectric material. The circulation of the laser gas is excellent and the laser plasma uniform. The generator has multiple applications, for example in the medical field.

Abstract: Methods and apparatuses for operating a gas laser which utilize the electrical discharge between the electrodes to increase gas circulation. In a first embodiment, electrical discharges cause a control current (pulse) which activates an electromechanical device disposed within a closed gas flow loop. The electromechanical device acts as a pump and effects gas exchange and heat dissipation with a minimum of elements. In other embodiments, electrical discharges cause pressure fluctuations which increase gas circulation without the use of mechanically moved parts. To this end, connecting lines that determine the direction of the flow are placed between the chambers in order to facilitate equalization of gas pressure. A flow circulation loop is thereby created in a quasi-passive manner when suitably constructed.

Abstract: A transverse flow gas transport laser including a closed loop of ducting through which a gaseous lasing medium can be circulated so as to pass continuously through a lasing region of the ducting in which it can be excited into lasing activity, in which there is included means for extracting a portion of the gaseous lasing medium from a region of the ducting other than the lasing region, comprising the said portion of the lasing medium and injecting it back into the remainder of the lasing medium thereby to induce flow of the gaseous medium through the lasing region of the ducting.