RU2429554C1 - Method of pulse oscillation of emission of slot-type gas laser and device for its implementation - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: emission from slot-type discharge gap of laser is directed to one of resonator mirrors through additional passive optic forwarding section. At that, pulse generation of emission is performed by periodic interlocking of resonator emission amplification in this passive section. Device for implementation of the above method includes tight chamber filled with active gaseous medium and having an emission outlet opening and optic resonator with output mirror. Two opposite lying extended metal electrodes connected to HF generator and forming a slot-type discharge gap are installed in the chamber. Optic resonator is equipped with external passive section - optic retranslator with possibility of retranslation of distribution of intensity and phase of light beam in plane of edges of electrodes on plane of output resonator mirror installed outside the chamber, and with emission modulator installed on passive section.

EFFECT: providing the possibility of modulated change of losses in resonator of slot-type laser.

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Description

The group of inventions relates to the field of quantum electronics and can be used to create gas lasers with high-frequency excitation of an active medium and a high level of pulsed radiation generation, used for technological processing of materials with special properties, for example, non-ferrous metals and brittle materials.

Slit CO ₂ lasers are known (US5048048 (A), H01S 3/03, 1991-09-10; EP0275023 (A1), H01S 3 / 03.1988-07-20: US5131003, H01S 3/097, 1992-07-14 ), which include a pair of extended metal electrodes connected to the RF generator, forming a gap discharge gap with the active medium, and resonator mirrors installed near the ends of the discharge gap electrodes. Typical dimensions of the discharge gap: the height of the gap is 1.5-2 mm, and the width is tens of times larger. A transverse high-frequency (HF) discharge is excited in the slit discharge gap and the radiation propagates, as in a planar waveguide along the electrodes and freely in the lateral direction. The combination of waveguide and non-waveguide radiation propagation allows one to realize high pump power densities of the active medium and, accordingly, to obtain high levels of radiation generation power.

In known slotted CO ₂ lasers, two modes of excitation of a gaseous active medium and, accordingly, two modes of radiation generation are realized: continuous and pulse-periodic. In continuous mode, the radiation power of sealed off CO ₂ lasers, depending on the volume of the active medium, is from 100 to 500 watts. In the pulse-periodic mode when modulating the RF voltage at the discharge (see, for example, US 5131003, H01S 3/097, 1992-07-14; JP 7226556 (A), H01S 3/097, 1995-08-22) receive pulses with a duration of 30 μs or more with a peak radiation power exceeding the average power by 2-2.5 times.

The maximum pulse repetition rate at a 100% depth of light modulation in sealed-off C0 ₂ slit lasers is in the range up to 10 kHz. This is due to the kinetic processes of pulse development in the active medium at typical gas pressures of 50-80 Torr and typical pump levels of 20-60 W / cm ³ . At higher frequencies the pump pulse repetition modulation depth decreases due to long times (50-70 microseconds) rise and fall pulse generation (see., E.g., "Slab waveguide RF-excited _CO2 laser for material processing", AIDutov, NANovoselov, VNSokolov, AAKuleshov, Proc.SPIE, vol. 2713, pp. 51-57, 1995).

An increase in the pulsed lasing power in slotted CO ₂ lasers can be achieved by increasing the gas pressure to 150–200 Torr and by a multiple increase in the pump power density. For example, in the work “The gain of the active medium and the generation parameters of a slit CO ₂ laser with high pulsed power”, A.I. Dutov, A.A. Kuleshov, N.A. Novoselov, N.L. Orlov, A.A. Sokolov, S.A. Motovilov, Izv.AN, ser.phys., Vol. 66, No. 8, pp. 1179-1181, (2002), it is reported that a peak generation power of up to 4-5 kW with a pulse duration of 20-50 μs in a laser operating at a pulse repetition rate of up to 600 Hz. Similar generation parameters are given by F.Villarreal, PRMurray, HJBaker, GAJMarkillie, RJRamirez, Q. Cao, DRHall, "High peak power CO ₂ planar waveguide lasers for direct high resolution machining", Proc. SPIE, vol. 4184, pp. 258-261, (2001), where the maximum pulse repetition rate without decreasing the amplitude power was 1 kHz. In both studies, the pulse repetition rate was limited by the influence of acoustic density waves, which develop in the active medium with a pulse-periodic contribution with high specific pump densities (300-800 W / cm ³ ).

A known method of pulsed generation of radiation of a gas laser of a slit type, carried out in a known gas CO ₂ laser of a slit type (US 5131003; H01S 3/097; 1992-07-14), which, by the combination of essential features, is the closest analogue of the claimed laser.

The known gas laser contains a sealed chamber filled with active gas medium with a window for outputting radiation, installed in the chamber and connected to the RF generator two opposite extended metal electrodes forming a gap between them, and an optical resonator with an output mirror.

When implementing the known method, the excitation voltage is applied to the electrodes of a known laser from an RF generator in the form of a pulse with a given frequency, resulting in a pulse discharge. The flow of discharge pulses ensures the creation of a short-term inversion of the population of energy levels in the active gas medium located in the discharge gap. Forced periodic emission of particles by the gas medium within the optical cavity during their transition from the excited to the ground state modulates the laser radiation at the corresponding wavelength.

The known method implemented in known slotted CO ₂ lasers does not allow to realize simultaneously high pulsed light powers and high pulse repetition frequencies at 100% modulation depth. The maximum peak powers of these lasers are only a few kilowatts, while for technological applications (for example, for processing non-ferrous metals, as well as brittle materials), peak powers of tens of kilowatts and more are required.

The reasons that impede the achievement of the technical result indicated below when using the known method of pulsed radiation generation by a known device include violation of the homogeneity of the active medium in the regime of high-power pulsed pumping and instability of a gas discharge.

It is known that when shortening the duration of a laser pulse, the efficiency of the interaction of radiation with matter increases due to a decrease in the melting thresholds and evaporation of the irradiated material, which depend on its thermal conductivity, density, heat capacity, and other properties. Reducing the pulse duration from 1 millisecond to 100 does not allow to lower the threshold for metal evaporation by 100-300 times (see "Laser Technologies in Microelectronics". V.P. Veiko, S.M. Metev. Publishing House of the Bulgarian Academy of Sciences, Sofia, 1991) .

An effective method of shortening the pulse duration in lasers and increasing the peak power is the method of modulating the quality factor of the resonator. For any resonant system, the quality factor is defined as the ratio of the stored energy in the system to the energy lost in one oscillation cycle, i.e. the high quality factor of the laser cavity means that the resonant system has low losses (see O. Zvelto. "Principles of Lasers". M: Mir, 1990).

If radiation amplification is blocked in the optical path of the resonator with a shutter, then lasing cannot occur. In this case, the population inversion can reach a value that far exceeds the threshold that occurs in the absence of a shutter. The closed state of the shutter is equivalent to the appearance of very large losses in the laser cavity. If the shutter is now opened sharply, the gain in the laser will significantly exceed the losses and the stored energy will be released in the form of a short and intense radiation pulse.

The known method of pulsed generation of radiation of a gas CO ₂ laser (High power repetition rate Q-switch CO ₂ laser and its application to study the optical breakdown in supersonic air stream ", ANMalov, AMOrishich, VBShulatyev, Proc.SPIE, vol. 731, 7131 IP -1-7 (2009), which uses the modulation of laser radiation by periodically blocking the amplification of radiation in the cavity.

In the known method, the blocking of radiation amplification in the resonator is carried out using a modulator in the form of a rotating disk with slots.

The known method has only been implemented on the basis neschelevogo industrial gas CO ₂ laser with low pressure gas flow. At repetition frequencies of 30-40 kHz were obtained laser pulses with an amplitude in the range of 50-100 kW, which expands the technological possibilities of industrial CO ₂ laser, such as the ability to cut non-ferrous metals and glass.

Technological non-slot lasers of this type are very bulky, consume a lot of gas mixture and are ineffective, because excited either by direct current or by alternating current of low frequency.

Also known are waveguide CO ₂ lasers with Q-switched resonators using an electro-optical modulator (EOM) (See Precision Microfabrication with Q-Switched CO ₂ Laser ", C. Dunsky and H. Matsumoto, Proc.SPIE, Voi. 4830, pp. 85 -90, (2003), as well as "Q-switched cavity dumped CO ₂ laser for material processing", Kennedy JT et al., US Pat. 6,697,408; H01S 003/22; 2004-02-24). Waveguide CO ₂ lasers have a round or square cross-section of the channel with dimensions in the range from 2 to 4 mm, in which the active medium is pumped in a gas discharge.

The radiation propagates in the channel, as in a fiber, along both coordinates. One or both resonator mirrors are located outside and are separated from the active medium by Brewster windows. The EOM is located in the external optical circuit and, together with the phase plate, forms a modulator-gate for radiation.

The known method for modulating resonator losses using EOM cannot be used in a slit laser for two reasons: firstly, the efficiency of the crystal in the EOM is limited to 10-15 watts of radiation and, secondly, the light aperture or the cross section of the crystal in the EOM is too small (from 4 × 4 mm to 8 × 8 mm) for installation in the cavity of a slit laser.

It is not possible to use a mechanical modulator (shutter) as a shutter in the cavity of a slit laser, since the light aperture of a resonator in lasers of this type is too large to allow fast ("instantaneous") blocking of light.

The prior art methods are not known for modulating the Q factor of a resonator in slot-type gas lasers.

The problem to which the claimed group of inventions is directed is to simultaneously increase the power and pulse repetition frequency of CO ₂ laser pulses of slot-type lasers, in particular, to obtain a sequence of high-power laser pulses with a peak power of 5 kW or more, followed by a high repetition rate, mainly in the range of 10 kHz and more, with an average laser radiation power of 50 to 500 W in sealed mode. The values of these parameters depend on the specific embodiment of the device, in particular, on the volume of the active medium and on the design of the radiation modulator. The peak power is approximately 100 times higher than the average laser radiation power, and the pulse repetition rate depends on the number of slots on the disk and on the rotation speed of the modulator disk.

The technical result obtained by the implementation of the claimed group of inventions is to provide the possibility of modulated loss up to 100% loss in the cavity of a slot-type laser.

The specified technical result is achieved by the implementation of the claimed group of diverse inventions that form a single creative concept and constitute a method and device for its implementation.

The specified technical result in the implementation of the claimed group of inventions is achieved by the fact that in the inventive method of pulsed generation of gas radiation of a slotted gas laser, radiation from the slotted discharge gap of the laser is directed to one of the cavity mirrors through an additional passive optical relay section, and pulsed generation of radiation is carried out by periodically blocking the gain radiation in the resonator in this passive section.

The specified technical result in the implementation of the claimed group of inventions is also achieved by the fact that the periodic blocking of radiation amplification in the resonator is carried out on a passive section at the waist of the radiation beam.

The specified technical result in the implementation of the claimed invention is achieved by the fact that in the inventive slit laser containing a sealed chamber with an active gas medium with a window for outputting radiation, two opposing extended metal electrodes installed in the chamber and connected to the RF generator, forming a gap discharge gap between them , and an optical resonator with an output mirror, in contrast to the known laser, the optical resonator is additionally equipped with an external passive section - optical a repeater with the possibility of relaying the distribution of the intensity and phase of the radiation beam in the plane of the end face of the electrodes to the plane of the output mirror of the resonator mounted outside the camera, and a radiation modulator installed in the passive section.

The specified technical result in the implementation of the claimed group of inventions is also achieved by the fact that the radiation modulator is installed on the passive section at the waist of the radiation beam.

The specified technical result in the implementation of the claimed group of inventions is also achieved by the fact that the optical repeater is made in the form of two confocal lenses, in which the front plane of the relay is aligned with the ends of the discharge electrodes, and the rear plane is with the output mirror of the resonator.

Figure 1 shows the inventive laser with a cavity of an unstable type of negative branch, in which the beam path is shown in the plane of the waveguide coordinate; figure 2 shows the optical diagram of the resonator slit laser with the path of the rays in the plane of the free coordinate; figure 3 is a graph of the calculated dependences of the generation pulse and resonator losses obtained for a resonator with Q-switching at a modulation frequency of 1 kHz.

In the inventive laser (figure 1) in an airtight chamber 1 filled with active gas (containing CO ₂ ) with a window 2 for radiation output, two opposite extended metal electrodes 4 and 5 are connected to the RF generator, forming a gap discharge gap 6 between them The optical laser cavity forms an end spherical mirror 7 installed in the chamber 1 and an output spherical mirror 8 installed outside the chamber 1. An external optical lens is installed in the cavity between the plane of the ends of the electrodes 4 and 5 and the output mirror 8 Sky repeater 9, consisting of two confocal positive lenses 10 and 11 with focal lengths f ₁₀ and f ₁₁ .

The front and rear focal planes I and II of the relay 9 (figure 2) are aligned with the plane of the ends of the electrodes 4 and 5 and the output mirror 8, respectively, to ensure the relaying of the radiation field (i.e. for reproduction in amplitude and phase) from plane I to plane II and vice versa up to the passband of the repeater 9. Thus, the section between the planes I and II has a zero effective length and fully preserves the structure of the resonator mode, its loss and sensitivity to aberrations, that is, it is a passive section in opt cal scheme.

On the passive section (in the repeater 9) between the lenses 10 and 11, preferably in the place of the waist of the radiation beam, there is a mechanical shutter 12 in the form of a disk with slots around the periphery.

The inventive method of pulsed generation of radiation of a gas laser slit type is carried out in the claimed laser as follows.

The RF generator 3 continuously supplies voltage to the electrodes 4 and 5 and excites the active medium in the discharge gap 6. The flow of current between the extended metal electrodes 4 and 5 creates an inverse population of the energy levels of CO ₂ molecules in the gas active medium located in the gap 6 . In the initial state, generation cannot occur, since in the optical path of the resonator the disk of the shutter 12 overlaps the light aperture at the waist, i.e. blocks gain. In this case, the population inversion accumulates and can reach a high level, which far exceeds the threshold value. If the slot on the disk opens the radiation flux when the disk of the obturator 12 is rotated, another laser state is realized in which the gain in the active medium significantly exceeds the losses, and for a short time t ', determined by the rotation speed of the obturator 12 and the cross section of the waist of the radiation beam formed in the repeater 9 with focus lenses 10 and 11, the radiation field (mode) is set. The energy accumulated in the active medium is released in the form of a short and intense pulse. The time t 'is determined by the speed of rotation of the obturator 12 and the cross section of the waist of the radiation beam formed in the repeater 9 by confocal lenses 10 and 11. To minimize optical losses in the resonator and maintain the radiation mode structure, the plane of the output mirror II and the plane of the ends of the discharge electrodes I are conjugated using an optical repeater 9 formed by lenses 10 and 11.

A mechanical obturator 12 in the form of a disk with peripheral slots located in the region of the repeater 9 waist can practically have slots 0.5-1.0 mm wide, the distance between them is in the range of 5-20 mm and the disk rotation speed is 200-300 rpm. Thus, the cavity can be opened for a time of 1-10 μs, during which the pulsed mode of laser generation is carried out. In this case, the peak radiation power of tens of kilowatts is realized, the duration in the peak at half maximum is 40-500 ns, and the duration of the "tail" of the pulse is in the range from 2 to 10 μs.

The optical circuit of the repeater for an unstable type of laser resonator in combination with a radiation chopper - an optical loss modulator - was used in calculating the energy, duration and shape of the generation pulses by computer simulation. Figure 3 presents graphs of calculation results for an active medium of a slit laser with dimensions of 2 × 60 × 600 (mm), a gas pressure of 70 Torr, and an RF pump power density in the discharge of 40 W / cm ³ obtained for an unstable Q-switched resonator at modulation frequency 1 kHz.

For an unstable resonator with an increase in M = 1.2, a peak power of 20 kW was obtained, and the energy in the pulse was 9 mJ. The calculation results confirm the possibility of implementing the claimed invention, achieving the specified technical result and solving the problem.

Claims (5)

1. A method of pulsed generation of radiation of a slot-type gas laser, characterized in that the radiation from the slotted discharge gap of the laser is directed to one of the resonator mirrors through an additional passive optical relay section, and the pulsed generation of radiation is carried out by periodically blocking radiation amplification in the resonator in this passive section . 2. The method according to claim 1, characterized in that the periodic blocking of radiation amplification in the resonator is carried out on a passive section at the waist of the radiation beam. 3. A slit-type gas laser containing a sealed chamber filled with active gas medium with a window for radiation output, two opposite extended metal electrodes installed in the chamber and connected to the RF generator, forming a gap discharge gap between them, and an optical resonator with an output mirror, characterized the fact that the optical resonator is additionally equipped with an external passive section - an optical repeater with the ability to relay the distribution of the intensity and phase of the light beam in the square the bores of the ends of the electrodes on the plane of the output mirror of the resonator mounted outside the camera, and a radiation modulator installed on the passive section. 4. The gas laser according to claim 3, characterized in that the radiation modulator is mounted on a passive section in the area of the radiation beam waist. 5. The gas laser according to claim 3, characterized in that the optical repeater in it is made in the form of two confocal lenses, in which the front plane of the relay is aligned with the end of the discharge electrodes, and the rear plane is with the exit mirror of the resonator.

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