EP2810345A1 - Co2 laser with rapid power control - Google Patents

Abstract

The invention relates to a CO₂ laser which allows for rapid power modulation, particularly highly efficient Q-switching. The key concept is the sub-division of the resonator into a high-power branch, containing inter alia the active medium (1), and a low-power feedback branch (14), in which the power-sensitive beam-shaping elements, particularly the modulators, are arranged. This is made possible by a suitable arrangement of a polarisation beam splitter (5) and a λ/4-phase shifter (2). The free adjustability of an angle φ between said two components permits the extremely flexible realisation of various operating modes, particularly optimisation of the feedback degree during pulse generation.

Description

C0 ₂ laser with fast power control

For fine machining (precision machining)

different materials with lasers is pulsed in the vast majority of applications

Radiation used. This concerns all typical ones

Material processing laser alike. Use cases are e.g. the cutting, drilling and the defined

Material removal of metals, ceramics, plastics etc.

Modern solid-state laser systems (diode-pumped Nd: YAG lasers, disk lasers, fiber lasers, Ti: sapphire lasers, etc.) are characterized by wide-ranging variable pulsability (from 100 fs over ps and ns to the ts range), but are in view on the costs and above all the long-term experience in the industrial employment still far behind the C0 ₂ -Laser back. An essential principle disadvantage of all available so far

However, for commercial C0 ₂ lasers suitable for material processing, their limited fast power control and, associated with that, their limited pulsability. Limits are mainly set when it comes to most effectively implement in C0 ₂ -Hochleistungslasern with, for example cw output powers in the kW range in the latter pulsed radiation. There is still no commercial C0 ₂ laser that emits pulsed radiation at high average power with pulses that have quasi-Q-switched properties, ie

Power increases of at least a factor of 10 compared to the cw power at pulse lengths in ns and μβ- With additional requirements, the relatively good K number (at least 0.6) typical for most C0 ₂ lasers is largely retained and an effective conversion of the potentially available power (cw) into the mean Performance of the pulsed system is feasible.

The equipment of a material processing plant with such a C0 ₂ laser would be a great technological

Jump means several aspects: a) The applications that were previously realized with the C0 ₂ laser could be carried out even more efficiently. b) Numerous applications previously reserved for other laser types (eg precision drilling and cutting of copper and aluminum and other metals whose processing is linked to special pulse parameters - called titanium) or completely new applications could be achieved with such a C0 ₂ laser realize. c) The flexibility of the system would be extremely high, as it could handle a wide variety of tasks which, in the current state of the art, would be linked to different types of lasers. Here is the overall efficiency in the production, for example one

complicated component with fine holes, difficult cutting contours, etc. to call. Equally relevant is the possible rapid change of the type of material, eg from metal to ceramic. The prior art can be summarized summarized as follows.

Because of the very good storage characteristics of its active medium, the C0 ₂ laser is available for a wide variety of Q-switching types with power up to one

Factor 100 and more suitable. As a result, in the first two decades of its rapid evolution from active Q-switching, it has become simpler

Rotating mirrors on electro- and acousto-optical modulation up to passive Q-switching with SF ₆ and even mode locking in C0 ₂ TEA lasers (see W. Jitman, "The C0 ₂ Laser", Springer-Verlag 1987) investigated countless variants. A broad overview can be found eg in: SPIE Milestone Series Vol. MS 22, "Selected Papers on C0 ₂ Lasers", ed. By James D. Evans, SPIE 1990. Because of this fact, it appears at first

It is surprising that practically none of these methods has been widely used in C0 ₂ lasers for material processing. They remained an interesting subject of basic research up to huge facilities for

Investigations on laser-controlled nuclear fusion, but only played a role in industrial applications in niches.

In contrast, the simple one has, though

functionally reliable and cheap method of pulsing of C0 ₂ lasers prevailed over the gas discharge, which is used in virtually every material processing laser, although they have serious weaknesses such as low power overshoot of the generated pulses, relatively large pulse durations and has low pulse repetition frequencies. Consequently, the short-pulse range (με and below), which is important for countless applications, is almost exclusively occupied by the abovementioned solid-state laser systems. The reason for this is less in the reinforcing properties of the active

Medium, rather than in the wavelengths justified.

While it is for the lasers in the visible and in the near

Infrared by 1 μπι a variety of excellent suitable optical materials, e.g. Crystals or glasses, which are u.a. due to low absorption, high

 Radiation resistance, large electro- and elasto-optical constants and excellent possibilities for

Distinguish processing and coating, that is

Material spectrum at wavelengths by 10 μπι strong

restricted, especially when it comes to special

 Properties goes like the electro-optic effect, which is practically limited to CdTe, or good

acoustooptic properties that are only in the Ge

owns desired way. A general problem is the limited radiation load capacity, whereby not primarily the destruction of the component by too high

Intensities can be seen, but the already occurring well before the destruction threshold and in particular to the relatively high dn / dT (refractive index change per

Temperature change) of these materials bound optical effects that lead to deformation of the wavefront and especially in applications within the laser resonator, eg in the Q-switching, are unacceptable because they have a strong performance-dependent beam quality of the laser result. A promising approach to optimally convert the power that is potentially available in a C0 ₂ laser into intense radiation pulses was provided by the outcoupling modulation by means of an interference decoupling element (see Schindler, K., Staupendahl, G.: "A novel C0 ₂ -pulse laser for material processing ", Yearbook LASER (3rd Edition), ed. H. Kohler, Vulkan-Verlag 1993, pp. 9-14 and GDR Patent WP H 01 S / 286 072 5 (1986)" Arrangement for wavelength selection and internal

Power modulation of the radiation of high-performance C0 ₂ - lasers "). Again, however, failed

industrial implementation in the range of higher average power to the power sensitivity of the crucial component, the interference - decoupling element.

Because of the high practical relevance, the realization of optimally pulsed C0 ₂ lasers is still an important goal of laser development, so that patents on this problem have appeared again in the last decade. For example, US Pat. No. 6,826,204 describes a C0 ₂ pulsed laser for material processing with a CdTe electro-optical Q-switch. For the basic problem of the lowest possible radiation load of the Q-switch with the highest possible average laser output power, which is responsible for efficient

Material processing have special significance, there is no solution in the patent. The situation is similar with a subsequent publication by the same applicant, US Pat. No. 7,058,093. Here is the principle The electro-optical Q-switching using CdTe modulator with the principle of a special power extraction, the cavity dumping linked. The goal here is the achievement of pulse trains with the highest possible increase of the

Pulse peak power relative to the cw power of the laser with a very high pulse repetition rate. The problem of radiation exposure is not solved here either. Because of the significantly better optical properties of Ge compared to CdTe, the Q-switching of C0 ₂ lasers by means of acousto-optic modulators based on Ge is of interest. In DE 112008001338 T5 such a laser is described. Special arrangements in the

Resonator design for the realization of high medium

 Output power at the same time good beam quality does not show the patent.

The aim of the arrangement according to the invention is to C0 ₂ laser conventional design, especially lasers used in material processing such as slow or fast längsgeströmte systems, but also those with stationary gas filling to modify so that completely novel ways of fast

Power control, especially the generation of

Radiation pulses result, which are characterized by a very wide range of parameters, in particular on the one hand the timing down to the ns range and on the other hand, a power range, with peak pulse powers up to the order of 100 kW and in the average power up to the kW range.

This task is solved with the objects of the

Claims.

If in the claims of a straight-line or kinked course of the resonator is spoken, this refers to the geometric center line in the longitudinal extension of the laser. This is not the case

 A beam through the polarization beam splitter is not kinked only if its two major surfaces are exactly orthogonal to the beam. At the (passing) beam

angled polarization beam splitter is a twofold bend of the beam, wherein the beam path on the two sides (exit or entrance) is parallel to each other. In detail, there are various possibilities of execution and these are described below as non-limiting variants, with some or all technically useful mutually combinable features can be combined.

Thus, the invention is also solved with a C0 ₂ laser with active medium in the low or medium

Pressure range to a maximum of about 0.1 bar, so that cw operation is possible by appropriate pumping power, and with respect to conventional C0 ₂ laser resonators, which by a highly reflective end mirror on the one and a decoupling element at the other end of the active medium are modified

A resonator, characterized in that between the one end of the active medium and a first resonator end mirror of high reflectivity, which is preferably greater than 99%, a λ / 4 phase shifter and between the other end of the active medium and a second Resonatorendspiegel high reflectivity , which is also preferably greater than 99%, a polarization beam splitter is arranged and the

 Polarization beam splitter one from the direction of the active medium impinging on him beam with any

Polarization divides into a linearly polarized outcoupling beam of power P _A and a

back-coupled beam of power P _R with also linear, but perpendicular to the polarization of the.

the polarization beam to be coupled out, wherein the λ / 4 phase shifter or the polarization beam splitter are rotatably mounted around the resonator axis, so that by setting a freely selectable angle φ between a characteristic axis of the λ / 4 phase shifter, which is perpendicular to the resonator axis , and a characteristic axis of the polarization beam splitter which is also perpendicular to the resonator axis, any desired power ratio P _A / PR

can be adjusted and that between the

Polarization beam splitter and the second

Resonatorendspiegel, ie in the feedback branch of

Resonator, elements for beam shaping, in particular

Elements for fast power modulation and for Wavelength selection and special apertures can be arranged.

The active medium can only in the area between the first Resonatorendspiegel and the

 Be set polarization beam splitter. Then this area with gas tight walls is against others

Sections of the laser and sealed against the environment (with the exception of any Gaszuführ- and / or

Gas discharge lines).

The electrodes are typically electrical electrodes.

The polarization beam splitter may be a ZnSe-based thin film polarizer arranged at Brewster angle a _B to the resonator axis 11.

In the feedback branch of the resonator elements for (preferably fast) power modulation, preferably electro-optical or acousto-optic modulators,

 Interference laser radiation modulators, mechanical

Chopper or (preferably fast) tilting mirror

be arranged. In the feedback branch of the resonator can be a

Electro-optical modulator and between the latter and the polarization beam splitter a telescope, preferably of the Galilean type, be arranged to adjust the beam diameter D to the free opening d of the electro-optical modulator, wherein the ratio D / d is preferably between 1.2 and 5, and that an absorber (26) the returning and upon application of a λ / 4-wave voltage to the electro-optical modulator in its polarization rotated by 90 ° beam, which is deflected by the polarization beam splitter from the resonator beam path intercepts.

In the feedback branch of the resonator can be a

acousto-optic modulator and between the latter and the polarization beam splitter a telescope, preferably of Galilei type, be arranged for adjusting the beam diameter D to the free opening d of the acousto-optic modulator, wherein the ratio D / d is preferably between 1.2 and 5, and that two absorbers intercept the beam components, which are bent out of the resonator beam path when a switching voltage is applied to the acousto-optic modulator.

When applying a switching voltage to the

acousto-optic modulator diffracted beam can be reflected from the second Resonatorendspiegel and as

used back-coupling beam and destroyed the non-deflected beam portion of an absorber, between the telescope and the acousto-optic modulator either a special aperture to secure the optimum beam quality is attached.

In the feedback branch of the resonator can firstly a

Interference laser radiation modulator so be arranged at a small angle ε its optical axis to the direction of the rückzukoppelnden beam that reflected by him radiation components from the

Resonator beam path and deflected by absorbers Second, a wavelength-selective element the function of the laser on exactly one

Wavelength secures. In the feedback branch of the resonator can optionally

 Prisms, preferably Doppelbrewster prisms of ZnSe or NaCl, or interference filters are used as wavelength-selective elements. In the feedback branch of the resonator can be a Kepler-type telescope with intermediate focus and a

Chopper disc be arranged with drive element so that the rückzukoppelnde beam exactly in this

Intermediate focus is locked or released by the chopper disc.

The second resonator end mirror may be a preferably fast tilt mirror and, between the latter and the polarization beam splitter, optionally a telescope, preferably of the Galilean type, for adapting the

 Beam diameter D be arranged at the free opening d of the fast tilting mirror, wherein the ratio D / d is preferably between 1.2 and 10. The optional elements used to adjust the beam diameter D to the free openings d of

Power modulation elements can be either Galilei or Kepler telescopes in lens design or Galileibzw. Kepler telescopes in mirror design or

Combinations of a collective lens or a Be collecting mirror with a second Resonatorendspiegel be suitable curvature.

By means of the optionally usable wavelength-selective elements, the laser can be forced to work on a fixed, but freely selectable line of the rotational oscillation spectrum of the C0 ₂ laser in the range 9 μιτι <λ <11 μπι, the properties of the other optical elements of the laser , in particular the λ / 4 phase shifter and the polarization beam splitter, adapted to this selected line.

All listed optical elements can be accommodated in a common vacuum-tight enclosure and the beam to be coupled out through a window of transparent material, preferably of ZnSe leaves the laser.

In a material processing system according to the invention can in the beam path between the laser output and the

Workpiece an interference laser radiation modulator with the proviso be integrated, the transmitted beam runs as a power-regulated beam in the direction of the workpiece and the reflected beam optionally one

Absorber / detector for destruction or for on-line measurement is supplied. In the beam path between the laser output and the workpiece, an acousto-optic modulator can be integrated with the proviso that the deflected beam as a power-regulated beam in the direction of workpiece (33) runs, while the non-deflected beam either an absorber / detector for destruction or on- line measurement is supplied, optionally between the

Polarization beam splitter and the acousto-optic

Modulator Elements for beam shaping, e.g. a telescope and / or a special aperture, optionally arranged.

The basic idea of the solution according to the invention consists in modifying the usually used basic structure of the laser resonator with a 100% level at the one and the decoupling element at the other end of the system so that the resonator is subdivided into a high-power branch, which i.a. is formed by the active medium and a special decoupling element, and in a low-power feedback branch, u.a the

Contains elements for fast power control. The performance ratios between high and low

Low power branch can be varied within wide limits by the arrangement variants explained below, so that for the control and very high

Services only a small fraction of it, eg 10%, is required. Thus, all for C0 ₂ - laser technology, although existing, but relatively

power sensitive modulator systems, e.g.

acousto-optic, electro-optic or interference laser radiation modulators, for the fast

Power control, in particular an effective

Q. be used. The novel resonator arrangement according to the invention will now be described in detail (see also Fig. 1).

Central element for the division of the resonator into a high-power branch and a low-power

Feedback branch is a polarization beam splitter. In the case of the C0 ₂ laser, a thin film polarizer (TFP) based on ZnSe can be used for this purpose. The latter is characterized in that the TFP is brought into the beam path at the Brewster angle a _B and, as a result of the special coating, an incident beam of power P ₀ is split so that its portion of the power P _p polarized parallel to the plane of incidence of the TFP is fully transmitted and its polarized perpendicular to the plane of incidence

Proportion of the power P _{s is} fully reflected. ie it applies

Po ⁼ Pp + Ps where losses, eg by absorption in the TFP,

were neglected.

The TFP will be around in the place of the usual

Auskoppelspiegels positioned and also serves in the laser according to the invention as a decoupling element, ie either the TFP reflected or the transmitted beam is coupled out and leaves the resonator. The other sub-beam is used for the resonator feedback, which is achieved, for example, by an adjustable 100% mirror, which sends the beam back into itself can be. The beam path between this mirror and the TFP forms the said low-power feedback branch, in which any elements for the power control of the laser can be arranged.

A second central idea of the invention is devoted to the problem of how the power ratio P _p / P _s can be _set as flexibly as possible, so that the laser modified in each case according to the invention corresponds to the gain of its active medium in accordance with its basic properties, in particular its performance the respective desired goal of the novel to be achieved

Parameter, in particular special pulse parameters, is adjustable to the optimum. This is achieved by the targeted influencing of the polarization properties of the radiation generated in the laser by "at the other end" of the resonator, in front of the existing end mirror with about 100% reflectivity, a device with a phase shift of λ / 4 per passage is arranged. For high-performance C0 ₂ laser one will be doing in the

 Laser material processing proven λ / 4 phase retarder mirror (PRS) use. With an appropriate geometric arrangement, this component transforms linearly

polarized radiation after passage into circularly polarized radiation. If the latter is now reflected at the first end mirror Sl and passes through the λ / 4 phase shifter a second time, it becomes circular

polarized radiation transformed back into linearly polarized, but rotated 90 ° from the original direction. The described properties of the TFP and the λ / 4 phase shifter and their arrangement according to the invention in the resonator now allow a number of novel options of the laser function, which are discussed in detail below.

1. The quasi-axial mode free continuous laser

We start the examination at the TFP and assume that an arbitrarily polarized beam of rays out of the interior of the resonator, ie from the direction of the active

 Medium that TFP falls for. Here is the

 described splitting in transmitted and reflected beam, which are then both polarized linearly and perpendicularly to each other. In principle, each of these two beams can be considered a laser beam

 decoupled and each other as

 Feedback beam can be used. Et al Because of the strong wavelength dependence of the properties of commercially available TFP based on ZnSe, which will be discussed in more detail in the exemplary embodiments, it makes sense to decouple the reflected beam and feed it back into the transmitted one, so that the

 the following considerations underlie this option.

First, the laser is to be operated without additional elements for power modulation, i. the beam transmitted at the TFP falls directly on the second 100% end mirror S2, where it is exactly in itself

reflected back, happens a second time (practically lossless) the TFP and is then amplified in the active medium, whereby its predetermined by the position of the TFP direction of the linear polarization is maintained. After passing through the active medium of the beam reaches the combination of λ / 4 phase shifter and Sl and would, with a corresponding precise adjustment of the phase shifter, again linearly polarized, but rotated by 90 ° relative to the incoming beam, again in the active medium, now in opposite Direction, happen. At this point, a significant difference between conventional lasers and the laser according to the invention reveals itself: The waves traveling back and forth in the active medium are typically linearly polarized in the same direction in the same direction, that is, fully capable of interfering, which leads to

Forming the known axial mode structure leads. Although the two waves are also linear in the laser according to the invention, but polarized perpendicular to each other, so that no interference and thus no axial mode structure occurs.

In material processing lasers, the axial

In most cases, the fashion structure is only given subordinate attention, but this is not justified a priori. Since they are extremely sensitive (μπι area) with the

Resonator length coupled, rich in the relatively large resonator lengths of C0 ₂ - material processing lasers already

Temperature changes of the order lCf ² ° C in order to change the axial mode structure relevant. By

Averaging effects this usually goes unnoticed, but with the highest accuracy requirements one finds out that from it both performance and

Room direction variations of the beam

can result. Another problem caused by the axial modes, ie the standing waves in the resonator, is the so-called "spatial hole-burning", which reduces the output power of the laser, especially in solid-state lasers, due to the periodic intensity variation of the laser

standing wave between 0 and a maximum value with the period K / 2, which leads to an incomplete query of the population inversion via stimulated emission. In a laser without axial mode structure, these negative effects do not occur.

The path of the beam in the resonator should now be followed up. After the second passage through the active medium, it again encounters the TFP with the fatal effect that, under the previously accepted and described conditions, the beam is reflected to almost 100%, i. There is no feedback, the laser process stops. This very special situation, which is a specific feature of the laser according to the invention, will be discussed in more detail later in the third option, the so-called "self-oscillation"

In order for a "normal" laser function, both

continuous as well as pulsed, required

To achieve feedback, the laser according to the invention has a very simple and flexible at the same time

Possibility of a defined feedback adjust. The λ / 4 phase shifter is rotatable about its beam axis, which in this case is the axis of the beam incident thereon from the direction of the active medium. Depending on how strongly the phase shifter is rotated against its "ideal" position, no linear, but a more or less elliptically polarized one runs

Beam back in the direction of the TFP with the result that then a certain, precisely adjustable portion of the TFP is transmitted and is available as a feedback beam. On the one hand, this proportion is made as large as necessary in order to achieve a reliable laser function while optimally querying the population inversion of the active medium, but on the other hand kept as small as possible, so that the illustrated advantages of the arrangement according to the invention are not lost, namely one side the lowest possible radiation intensity in the

Feedback branch and on the other the quasi-axialmode-free operation of the laser.

At this point, depending on the desired mode and the power class of the laser must be compromised because of the dependency between the laser output power and feedback. If it is desired to operate the laser, in particular in continuous operation with optimum output power, higher feedback rates are required than, for example, in the pulsed described below

 (Q-switched) operation. At the here discussed

C0 ₂ lasers for material processing with a However, typical power range of several 100 to several 1000 W are already sufficient at relatively low losses of cw power feedback levels between 5 and 20%, so that the above requirement for the lowest possible intensity in the feedback branch can be well met in cw operation.

The quasi-axial mode Q-switched laser

The main application of the C0 ₂ laser according to the invention are applications requiring a fast

Power control, especially the generation

require defined radiation pulses by means of Q-switching. The required elements are in

Feedback branch, which is characterized by low intensities, arranged. In contrast to conventional C0 ₂ lasers, all typical modulation variants available for 10 μνη wavelength can be used, which are generally relatively sensitive to high intensities and, for example, more direct

Arrangement in high-performance resonators either the beam quality worsen crucial or even destroyed. Below, five variants of such power control are discussed:

 Electro-optical and acousto-optic modulators,

Interference laser radiation modulators the simple chopper disk and fast oscillating

Tilting mirror. a) Use of electro-optical modulators (EOM) The use of the linear electro-optic effect (Pockels effect) for the resonator-internal

Power control of lasers is characterized mainly by the extremely short

achievable switching times down to the sub-ns range, ie due to their extremely good suitability for the Q-switching of lasers and, moreover, due to their very high degree of flexibility with respect to the

Switching parameters such as rise times or

Pulse repetition frequency off. While in the visible and near infrared spectral range, many very suitable crystals for electro-optical

Switches exist, this option is in

Wavelength range of C0 ₂ laser practically limited exclusively to commercially available CdTe modulators. By their compared to ZnSe substantially less favorable optical properties, in particular their relatively high absorption, these modulators

However, only be used at relatively low intensities. The laser according to the invention offers here by its special

Feedback branch with its compared to the usual laser resonator by about one order of magnitude

reduced intensity (at the same

Laser output power!), An advantageous

Way out. Another exceptionally favorable feature of the novel arrangement is the fact that the polarization sensitive element (the analyzer) used for the Modulation effect of the EOM in conventional

Resonators must be additionally introduced, is already present in the resonator according to the invention in the form of TFP immanent. However, due to the relatively small cross-sectional area of CdTe-EOM, which is generally smaller than the typical beam cross-section of a high power C0 ₂ laser, in most cases an adaptation of the

Beam diameter, e.g. with help of a

Telescopes, required.

The switching or modulation function then runs as follows. The bundle coming from the TFP into the feedback branch, which is polarized linearly and parallel to the plane of incidence of the TFP, passes through the beamforming (telescope) and the stress-free EOM and is fed back from the 100% mirror, whereby with optimal setting of said elements that in the active Medium returning bundles the same

Propagation properties (divergence) and the same polarization as the incoming bundle, so that a quasi-ideal resonator function (transverse mode structure!) Is guaranteed, ie the laser is running at optimum performance. If one then applies to the EOM a λ / 4 voltage which makes the linearly a circularly polarized beam, the latter is linearly polarized again after reflection at the 100% mirror and the second passage through the EOM, but now perpendicular to the incoming beam , Does this beam on the TFP, he will be completely out of the

 Resonator beam path out and intercepted by an absorber, i. the feedback goes to zero. Radiation generation stops at the moment that it generates

Resonator losses are so great that the system is below the "laser threshold." It should be stressed again that in this way a laser power is switched, which is about one

Magnitude above the power in the feedback branch is! The achievable minimum switching times are determined by the properties of the EOM itself and its control as well as the resonator length and are typically in the

Order of magnitude ns.

Use of acousto-optic modulators (AOM)

Modulators based on the acousto-optic effect are usually made of crystals for C0 ₂ lasers. These are, as well as CdTe, in their allowable load capacity, which is given by the requirement that the beam path in the resonator must remain largely unaffected even with changing loads, for example, when varying the laser power, significantly limited. 100 W / cm ² should not be exceeded. Again, the principle of the laser according to the invention provides the way out. Since AOM is limited in its free opening analogous to the EOM, the basic structure is described in a) similar, ie a telescope is used and to the position of the EOM comes the AOM. The free laser function is typically given again for the stress-free AOM. The shutdown of the laser, so the reduction of the feedback below the threshold, can be achieved by

Activation of the AOM, so that when it passes through twice so much radiation is bent out of the feedback path and intercepted with absorbers that the laser function stops. In the embodiments, the second possibility is described in which the rejected beam is used for the feedback.

The achievable switching times are in

and below, i. also with AOM can

Modulation frequencies in the MHz range can be realized. Advantages of the AOM use are u.a. the higher robustness and optical homogeneity of Ge compared to CdTe, the lower

required switching voltages and the

lower costs.

Use of interference laser radiation modulators (ILM)

Modulators of this type are based on the principle of the Fabry-Perot interferometer (FPI) and are typically equipped with two ZnSe plates as optically active elements. Because of the very favorable features of ZnSe and its great Range of application in C0 ₂ laser technology offers ILM the advantage that on the one hand they can be easily adapted to the intracavity beam diameter, so that in general no additional telescopes are required, and on the other hand the radiation load capacity is significantly higher than for CdTe and Ge. As a result, with such modulators and multi-kW laser of

inventive design be switched.

ILMs operate as variable beam splitters, i. The incident laser power becomes practical

lossless divided into a transmitted and a reflected beam, wherein the

Divider ratio very flexible, but only in the kHz range, can be varied by a corresponding control. As an ILM in the

Transmission maximum reaches 1, it is in the beam path (similar to EOM and AOM) arranged so that this corresponds to the state of full laser function. The more you now tune it by means of a control current in the direction of increasing reflection, the rise

Resonatorverluste, since the reflected components are reflected by a small inclination of the ILM axis against the resonator axis out of the feedback beam path and destroyed by absorbers. If you fall back below the laser threshold due to the losses, the laser function stops. Typical achievable switching or pulse parameters of this arrangement variant are switching times and pulse durations in the s range as well as

 Pulse repetition frequencies up to the order of 10 Hz. Since ILM modulators can be loaded with up to several 100 W, average laser output powers of several kW can be achieved.

Use of mechanical switches

For the Qüteschaltung of the laser according to the

Invention can also be simple mechanical

Switch, in particular rotating hole or slit diaphragm or fast oscillating

Tilting mirror, be used advantageously. For example, a Kepler telescope with a sharp intermediate focus can be placed in the feedback branch, and at this point, this focus can be switched by means of a rapidly rotating perforated or slotted disk in short times in the μ range, depending on the number and arrangement of the free openings on the disk and Their rotational speed can be a very efficient implementation of the available average power of the laser in pulse with high power increase at

Pulse repetition frequencies up to several 10 kHz and typical pulse durations in the μΞ range can be achieved. Again, the low affects

Radiation intensity in the feedback branch favorable: When generating powerful pulses, the switching edges of the rotating Disc exposed to high intensities, which in conventional lasers can lead to Abtragsprozessen and thus a relatively rapid destruction of the sharp switching edges, while this is avoided in the laser according to the invention.

 In the embodiments is also a

 Variant with fast oscillating tilting mirror described.

The self-oscillation

As already indicated above, the laser according to the invention, due to its special resonator structure a very specific mode of operation -

Self-oscillation. This novel effect will be explained in more detail below. The basis for the occurrence of self-oscillation is the precise

Adjustment of the two characteristic of the described laser elements, the λ / 4 phase shifter at one end of the resonator and the TFP at the other, wherein if necessary, a wavelength-selective element must ensure that the laser on a precise

defined, the specifics of phase shifter and TFP corresponding wavelength works. "Precise

Setting "means that the incidence levels of the λ / 4 phase shifter (if one assumes that this is a common PRS) and TFP are exactly 45 ° twisted against each other The two 100% end mirrors of the resonator must also be in the be adjusted in the usual way. For the qualitative understanding of the expiring

Operations after switching on the laser, it is assumed that the population inversion has reached a quasi-equilibrium state in the active medium and is now being tracked, as a start radiation beam, which initially exclusively spontaneously

emitted photons randomly run exactly in the direction of the laser axis, on the way of further propagation in the resonator behaves. The effect becomes most apparent when one assumes that this is the case

Start radiation beam starts at the end of the active medium, which is at the TFP, and moves towards the interior of the active medium, ie in the direction of the λ / 4 phase shifter. On the way there, the bundle is amplified, its unpolarized state, which is typical of the spontaneously emitted start radiation beam, remains practically preserved. This also changes the path section phase shifter - 100% end mirror - phase shifter nothing, because here all radiation components are rotated equally by 90 °, so the bundle remains unpolarized. After further amplification during the second pass through the active medium, it now strikes the TFP and is essentially split into two equally strong sub-beams, which are polarized linearly but perpendicular to one another. One of them is decoupled, the other fed back. The latter now runs again in the direction of λ / 4 phase shifter through the active medium, but is significantly modified in its properties compared to the start radiation beam: First, it is linearly polarized and has secondly through induced emission already a much higher

Power. At the second round trip through the

Resonator it is further amplified and - which is crucial for the self-oscillation - at

Double pass through the λ / 4 phase shifter rotated in its polarization direction by 90 °, so that it is now fully decoupled when reaching the TFP, the feedback is 0. Thus, the further amplification collapses via induced emission, the output power of the laser is briefly practical to 0 before a new cycle of the described form starts. From the qualitative description of the

Process, it becomes clear that the laser output power becomes maximum when the bundle has two rounds, ie the distance 4L (L is the resonator length)

graduated. This results in a

Pulse repetition frequency f _imp of fimp = c / 4L, where c is the speed of light. For typical resonator lengths of several meters arise

Pulse repetition frequencies in the order of 10 MHz.

The prerequisite is that the population inversion in the active medium by the quadruple passage of the

Radiation bundle is not degraded so much that a certain "pumping time", which depends on the pumping rate of the respective laser, is required to restart the cycle.Of the latter, of course, falls the pulse repetition frequency. The effect of the self - oscillation, the novel and the special resonator configuration according to the

Invention is bound, thus leading

continuous pumping to corresponding periodic pulse trains without an additional

power modulating device in the

Resonator beam path must be integrated!

It is also noteworthy that the average power of the "self-oscillating radiation" practically corresponds to the cw value of the laser.

Radiation decoupling laser - workpiece

In addition to the above-described possibilities of fast power modulation, the C0 ₂ laser according to the invention offers a further attractive advantage in practical use in one

Material processing plant.

Often, highly reflective materials,

In particular, metals are processed, which reflect or scatter a significant portion of the incident radiation. Since this radiation is usually directed by the focusing element very well in parallel back towards the laser and through the

Decoupling element can penetrate into the resonator, the intracavity radiation generation is significantly disturbed, resulting in a deterioration of the

Beam quality and in power fluctuations, especially at the peak power of pulses noticeable. Therefore it is common state of the art Technology, for a radiation decoupling between laser and workpiece by means of a combination of ATFR mirror, so a polarization-dependent

Reflector / absorber, and a λ / phase retarder mirror to build a kind of "optical diode", the

Laser radiation passes in the direction of the workpiece, but absorbs returning portions.

If you work with a C0 ₂ laser according to the

Invention, the effect of the ATFR mirror is inherent in the laser in the form of the polarization beam splitter. As already stated, the beam leaves the laser linearly polarized. Passing twice on a λ / 4-phase retarder mirror on the way to and from the workpiece, its polarization plane is rotated by 90 °, so it is automatically on impact with the polarization beam splitter from the

Resonator beam path is deflected and can be intercepted by an absorber.

There are thus two advantages: First, it is possible to dispense with the component ATFR mirror and secondly, the beam components to be destroyed are not - as with the ATFR mirror - of

temperature-sensitive component itself absorbed, but deflected out of the beam path in the desired manner and fed to a suitable absorber.

External power modulation In many tasks of laser material processing must be during the machining process the

Laser power vary. Mostly this is done via an intervention in the laser process itself, i.a. via a variation of the pump energy supply. However, this will affect the beam quality, i. the K-number changes with the retrieved performance, resulting in a reduced processing quality. A solution is provided by external modulators, which, while maintaining the beam quality, are a variation of the

 Performance on the workpiece within wide limits

allow.

Also, the laser according to the invention has for a certain selected parameter set, e.g. Pulse duration,

sequence frequency and peak power, a defined optimal operating regime with regard to best

Beam quality. That's why it's good

required power variations via an external modulator that does not use the laser function itself

influenced to realize.

Two efficient possibilities are offered for this purpose, namely acousto-optic and interference laser radiation modulators, which can each be placed in the vicinity of the laser output and further beam shaping measures, if necessary, z, B. the above-discussed radiation decoupling Läse - workpiece, do not disturb. With the AOM, it is convenient to use the diffracted beam as a processing beam, since it can be regulated in its power from 0 to the maximum value. The undeflected portion can either be destroyed by an absorber or, for example, be supplied to a detector for on-line control of the laser power. Depending on the requirements, beam-shaping elements (telescope, aperture) are provided for optimally adapting the radiation field coming from the laser to the modulator.

 use.

The ILM can be integrated into the beam path without such additional elements, since the free diameter of the interferometer plates can easily be adapted to the laser radiation. The FPN plates made of ZnSe can be loaded with several hundred watts of radiant power, without a deterioration of the beam quality in the transmitted beam, which will typically be used as a processing beam occurs. The unused reflected portion can either be destroyed by an absorber or used for on-line control.

It should be mentioned that it is advantageous to live in the laser according to the invention as a whole so that all directly related to the laser components against external

Influences like dust, humidity and

Climatic fluctuations are generally protected. Typically this will be solved constructively so that the entire enclosure is in direct communication with the active medium, ie the components are surrounded by the laser gas. As a result, their life span is common for lasers

Standards are adapted.

The object of the invention is explained below with reference to exemplary embodiments, which are shown schematically in the drawings. In these show:

Figure 1: Schematic representation of the C0 ₂ laser according to the

 invention

FIG. 2: Basic arrangement of a λ / phase retarder mirror (PRS) as λ / phase shifter

FIG. 3 The Functioning of a ZnSe-based Thin-Film Polarizer (TFP)

Figure 4 arrangement variant with TFP and transmitted

 Beam as beam to be coupled out and reflected beam as beam to be fed back

Figure 5 arrangement variants of the C0 ₂ laser according to the

 invention

 a) Variant with TFP and elements for fast power modulation

 b) variant for the realization of

 Selbstoszillation - first Resonatorumlauf c) Variant for the realization of

Self-oscillation - second resonator circulation Figure 6 arrangement variant for fast

 Power modulation using EOM

Figure 7 Two arrangement variants for fast

 Power modulation using AOM

 a) Feedback by means of transmitted beam b) Feedback by means of diffracted beam

Figure 8: arrangement variant for fast

 Power modulation using ILM

FIG. 9: Arrangement variant for pulse generation by means of

 Chopper disk Figure 10: Arrangement variant for pulse generation by means of

 tilting mirror

FIG. 11: Radiation decoupling laser - workpiece,

Arrangement when using a C0 ₂ laser according to the invention

FIG. 12: For external power control of the

 laser radiation

 a) Variant using ILM

 b) Variant using AOM

FIG. 13: Vacuum-tight enclosure at the coupling-out end of the

FIG. 1 shows in a highly schematic manner the basic structure of the C0 ₂ laser according to the invention. It does not play at first Role, which concrete geometrical conditions, in particular with regard to the active medium 1,

available. The sketch shows that the resonator is terminated at both ends by a highly reflective mirror 3 and 4 respectively. Through the polarization beam splitter 5, the resonator is transformed into a high power branch, which i.a. contains the active medium 1, and the feedback branch 14, which is characterized by relatively low power divided. This desired division is made by the

Interaction of the polarization beam splitter 5 with the λ / 4 phase shifter 2 at the other end of the resonator achieved in the following manner. If, coming from the direction of the active medium 1, radiation 6 of initially arbitrary polarization impinges on the polarization beam splitter 5, it is divided into two perpendicular to one another linearly

polarized components, one of which is reflected and the other is transmitted. In FIG. 1, these are the beam 7 to be coupled out with horizontal polarization 10 and the beam 8 to be fed back with vertical

Polarization 9. The latter happens after reflection on

 End mirror 4 again the polarization beam splitter 5, is amplified in the active medium 1 and passes through the λ / 4 phase shifter 2. Depending on which angle φ now

between a characteristic axis 13 of the

Polarization beam splitter 5 and a characteristic axis 12 of the λ / 4 phase shifter 2 has been set, this can change the polarization state of the incident linearly vertically polarized wave. In the first special case it remains unchanged, in the second special case it will be circular, in the general case elliptical.

After reflection of the wave at the end mirror 3 and a second pass through the λ / phase shifter 2 is in the second special case again linearly polarized radiation, but now with horizontal polarization arise, in the general case, the elliptical polarization is maintained, but with a changed axis ratio. The relation between the vertical and the horizontal part of this polarization ellipse is now decisive for the desired power distribution on the

Polarization beam splitter 5, which finally after further amplification in the active medium 1 of the modified

Wave is reached again. As already stated, it is a principal object of the C0 ₂ laser according to the invention to keep the power of the vertical polarization beam 9 to be coupled as low as possible without the desired function of the laser

affect. The optimum can easily through

corresponding adjustment of the angle φ are found when the λ / -phase slider 2 (which is preferably realized) or the polarization beam splitter 5 or both are arranged rotatable about the resonator 11. Here is another significant advantage of the solution according to the invention over conventional lasers: While in the latter the optimization of the feedback must be done by complex replacement of coupling elements with different reflectivity, here is sufficient to simply change the angle cp to find the optimum of the laser function.

In the feedback branch 14 with optimized relative

low power can now different elements for beam shaping 15, in particular elements for fast Power modulation and / or wavelength selection and, for example, suitable spatial filter to ensure the high beam quality of the laser to be integrated. A particular advantage of the arrangement is that, for example

Elements with high functionality, but great

 Power sensitivity, which is therefore not advantageous in conventional lasers of this power class

can be used in C0 ₂ laser according to the invention

can be used easily.

A favorable practical solution for the λ / phase shifter 2 is illustrated in FIG. 2, namely the use of a λ / 4-phase retarder mirror (PRS) 16. These mirrors are also suitable for high powers in the kW range. The left picture shows in section its compact arrangement with the

adjustable end mirror 3, the right image the possibility of rotation of this unit about the resonator axis 11. As shown in the left image, where the plane of the plane of incidence corresponds to the radiation, the relative arrangement of the components must be chosen so that the angle ß both between the resonator axis 11 and the

Einfallslot 43 of the PRS 16 and between the latter and the entrance slot 44 of the end mirror 3 45 °. Now suppose that the incident on this unit

Radiation bundle in the plane of incidence, ie in the

 Drawing plane of the left image, linearly polarized, it will be on both mirrors without changing the

Polarization reflects, so runs virtually unchanged back into the active medium. However, as shown in the picture on the right, rotate the unit by an angle φ

opposite to this starting position, results from the Special case φ = 45 ° circularly polarized radiation after the first reflection at the PRS 16 and after the reflection at the end mirror 3 and the following second reflection at him again linear, but perpendicular to the original direction polarized radiation. For values in the interval 0 ° <φ <45 ° elliptically polarized radiation is obtained.

A decisive feature of the laser according to the invention is that, by means of the described unit, linearly polarized radiation emerging from the active medium (for example perpendicularly polarized as in FIG

Setting a suitable angle φ is modified so that the returning radiation is a desired

Power ratio between the vertical and the

has parallel polarized component.

As a practical solution for the polarization beam splitter 5, a thin film polarizer (TFP) 17 based on ZnSe is suitable for C0 ₂ lasers. Its operation is illustrated in FIG. 3. A specially coated ZnSe plate is brought into the beam path at the Brewster angle a _B and divides an incident beam of arbitrary polarization into a transmitted beam which is incident in the beam path

Incidence plane, and a reflected beam, the

perpendicular to it is linearly polarized. As the

shown dependency of the reflectivity of the TFP 17 of the wavelength for these two beam components, this distribution is almost perfect at the main wavelength of the C0 ₂ laser of 10.59 μπι. In conjunction with the λ / 4 phase shifter 2, the TFP 17 now allows the division according to the invention of a beam 6 coming from the direction of the active medium into a powerful beam 7 to be coupled out (power P _A ) and a relatively low-power backfeed

Beam 8 (power P _R ). In real high power C0 ₂ lasers for material processing, often only a few percent of the incident radiation suffices for effective feedback, so that power ratios P _A / P _R of 10 and more are applicable, ie, the radiation loading of the elements

 Beam forming, which can be integrated into the feedback branch 14, is extremely low. As already described, this ratio can easily be over the

Angle φ can be set and optimized.

The beam splitting at the TFP 17 can in principle be done in two ways. Either one disengages the reflected beam and uses the transmitted to the feedback or vice versa. Both variants have advantages and disadvantages, which result mainly from two properties of the TFP 17: First, the absorption for the p-component is much higher than for the s-component of the radiation and, secondly, as shown in FIG. 3, the reflectivity strongly dependent on the wavelength for the p-component.

If the reflected beam and the transmitted one are now coupled back, one has the two advantages of firstly reflecting the strong power component as the s-component at the front side of the TFP 17 and only minimal absorption losses, and secondly the λ-dependence of the transmitted one and for the Feedback competent p-component even for the laser has a function-stabilizing effect. A certain disadvantage is the double passage of the back-coupling beam as a p-component, ie at a relatively high level

Absorption, by the TFP 17 with the risk of

Distortion of the intracavity wavefront. This

Problem disappears when the reflected portion is fed back. However, there are two others for this, namely the risk of considerably influencing the divergence of the intense coupled-out beam 7 through a "thermal lens" in the TFP 17 and through the

Wavelength dependence of the p component used for

Avoidance of the oscillation unwanted laser lines makes an additional wavelength selection in the feedback path 14 is required. The latter variant is shown in FIG. 4 with a grating mirror 25 as a wavelength-selective element.

The more commonly used version will be that shown in Figure 5, where the reflected beam

is decoupled. FIG. 5 a) illustrates the most important case with the TFP 17 as a beam splitter and elements 15 for power modulation in the feedback branch 14 as well as the typical polarization ratios. The returning beam 43 with linearly perpendicular polarization 9 is at the λ / -phase slider 2 at the first pass in radiation with weakly elliptical polarization 46 and after reflection at the end mirror 3 at the second pass in radiation with highly elliptical polarization 47th

formed, whose main polarization component is horizontal, so that the reinforced medium in the active medium 1 6th is split at the TFP 17 in the strong beam to be coupled out and reflected as s component and the weak and 8 transmitted as a p-component beam 8. The latter happens twice the beam-forming,

in particular power-modulating elements 15,

then without further power losses the TFP 17 and runs again as a returning beam 43 with linearly perpendicular polarization 9 through the active medium. 1

FIGS. 5b) and c) illustrate the special case of self-oscillation. For a better clarification of the

Effect was the appearance in the first

Resonator circulation (5b)) and the second Resonatorumlauf (5c)), which together correspond to a period of the self-oscillation divided. In Figure 5b) starts

picked out radiation beam 45 at the point 44, which initially consists exclusively of those spontaneously emitted photons, which run exactly in the direction of the resonator 11. This unpolarized (48) bundle is amplified in the active medium 1, passes twice through the λ / 4 phase shifter 2 and, after further amplification, finally reaches the TFP 17 as a still unpolarized bundle 6. This splits it into two equal parts 7 and 8, which are each linearly polarized in the manner shown. The beam 8 is fed back and upon reaching the point 44 is the first revolution

completed . The already relatively strong beam 8 with the linear polarization 9 reaches the λ / 4 after further amplification. Phase shifter 2, which is set exactly (via the angle φ), that the bundle after the first round has exactly circular polarization 49 and therefore after

Reflection at the end mirror 3 and the second passage again linear, but now horizontal (10) is polarized. This bundle reaches after further reinforcement the TFP 17 and is now completely reflected, so decoupled. The feedback is 0, the process shown must restart again, i. the pulse repetition frequency of

In principle, self-oscillation is given by a double "round trip" through the resonator: The exact temporal power curve in decoupled

Radiation bundle 7 depends complex on the laser parameters and can be determined only by solving the balance equations or of course experimentally.

From the multitude of possible arrangement variants of the C0 ₂ laser according to the invention, FIGS. 6 to 10 show characteristic examples.

In FIG. 6, an EOM 18 is first inserted into the feedback branch 14 of the resonator. The use of such

Modulators are problematic for the large wavelengths of C0 ₂ laser, you have to relatively small and expensive

Switching crystals, e.g. from CdTe, which require high switching voltages and in their optical

Parameters (radiation load capacity and absorption) are not optimal. Positive, however, is their extremely high switching speed compared to their use

desirable. Compared to conventional C0 ₂ lasers, the laser according to the invention offers significant advantages, solve the mentioned problems. First, the

Power in the feedback branch 14, even at relatively high average power in the outgoing beam 7, are reduced so much that e.g. can be adapted to the free opening d of the small switching crystals 18 by using a Galilei-type telescope 22 of the diameter D of the back-coupling beam 8, without destruction of the crystal by the increased

Power density must be feared. Secondly, with the TFP 17 already the polarization-selective element, which is required in modulation by means of electro-optical crystals, already inherent in the resonator, so does not need to be additionally introduced. This is enough for completely switching off the feedback

Applying a quarter-wave voltage to the modulator to rotate the rückzukoppelnden beam 8 in its polarization by 90 °, so that it is completely reflected during the return of the TFP 17 as a beam 28 and destroyed by the absorber 26.

FIG. 7 shows a similar arrangement, but with AOM 19. Since the switching speed depends, inter alia, on the free diameter d (small d-high switching speed), these modulators are generally only available with d <10 mm, so that in most cases the integration of a telescope Galilei type 22 is required. Since germanium, which is used as an acousto-optic crystal in C0 ₂ lasers, also reacts relatively sensitively to high intensities, the low power is again in the

Feedback branch of the decisive advantage of the laser according to the invention. FIG. 7 shows two variants of the AOM insert. In FIG. 7a), the feedback, ie the state in which the laser operates, takes place via the directly from the AOM 19 without

Control signal in the direction of end mirror 4 transmitted beam. When applying a control signal, so the

Generation of a refractive index grating in the modulator, back and forth running beam to a greater or lesser extent depending on the control signal from the

Resonator beam bent out (rays 29). This makes it possible to feedback and thus the

To modulate laser output power. With sufficiently high diffraction losses absorbed by the absorbers 26, the laser can be brought below its threshold and thus completely switched off - cleaner

Pulse mode is possible.

In the second variant shown in FIG. 7b), the beam 29 diffracted by the modulator upon application of a control signal is used for the feedback. Here it is clear that with control signal = 0 also the feedback becomes 0, thus the laser is switched off. Likewise, lasers with a very high gain can be used, even for very small ones

Apply feedback, be well pulsed. Another positive aspect of this arrangement is its

 Wavelength selectivity inherent in the diffraction process. So may, if necessary, to others

Wavelength-selective elements are omitted in the resonator beam path. To ensure the high beam quality and to eliminate a possible influence on the transverse mode structure of the laser by unwanted Diffraction effects in the edge area of the AOM can be a

Special aperture 53 are set as a suitable spatial filter between the telescope 22 and the modulator 19. Figure 8 illustrates the use of ILM 20 for rapid power control of the C0 ₂ laser according to the invention. In the illustrated arrangement, the laser operates when the modulator is ideally set to transmission = 1 and the beam 8 to be retransmitted can pass through virtually without loss. If a corresponding control current is applied, the distance of the interferometer plates is changed and more or less intense reflected radiation components 30 occur, which are again destroyed by absorbers 26. Because of T = 1 - R, the transmitted component and thus the feedback decrease to an analog extent, the laser can be modulated in its output power or even switched off and thus pulse operation can be realized.

The scenario described only works flawlessly when the laser is forced to operate at exactly one wavelength. This requires a wavelength-selective element - in Figure 8, this is the diffraction grating 25, which replaces the end mirror 4 at the same time. ILMs are also relatively sensitive to high levels

Performances, since the two interferometer plates can significantly affect the transmitted wavefronts under high load. Consequently, here too the low radiation load in the feedback branch 14 is a relevant factor. Another variant, which is not as flexible as the above, but it is very easy and cheap, Figure 9. By means of a fast

rotating chopper disc 21, which is driven by a controllable motor 24 and can be well regulated at least in the speed, the rückzukoppelnde beam 8 is periodically switched on and off. So that especially the turn-on process takes place as fast as possible in the μβ-range and to a true Güteschaltungseffekt with strong

Overcosting beam 8 is not "chopped" at its original diameter, but in the intermediate focus of a Kepler-type telescope 23rd

Other elements are not required in principle. The advantage of the low performance in the

Feedback branch 14 is in this system is that despite the sharp focus in the telescope even when generating very powerful pulses at the switching edge no sparking and thus no material removal takes place, which would greatly reduce the life of the chopper wheel 21.

A variant created by the development of modern

High-performance scanner systems of interest,

FIG. 10 illustrates, in order to show how the lens-based telescopes can be replaced by mirror versions, a Galilean telescope consisting of a concave mirror 50 and a curving mirror 51 is used here. That through this telescope in his

Diameter reduced beam 8 hits the tilting mirror 52, which replaces the end mirror 4. By fast oscillation of this tilting mirror 52 of the Laser resonator quickly toggled between adjusted and unadjusted state and in this way

Radiation pulses are generated. The achievable

Pulse repetition frequencies are in the order of 10 ⁴ Hz. Since this pulse repetition frequency depends on the mass of the tilting mirror 52 and thus its diameter, the reduction of the bundle diameter makes sense. Again, the low power in the feedback path 14 is extremely advantageous because very small mirror diameter in the order mm and thus very high pulse repetition frequencies without risk

Mirror destruction can be used.

The advantages of the C0 ₂ laser according to the invention are not limited only to the radiation properties of the laser itself. Figure 11 shows a significant advantage that this laser when used in a

 Material processing plant brings. For such systems, it is typical that on the one hand non-linear, but circularly polarized radiation 36 is sent to the workpiece 33 and on the other hand measures for radiation decoupling between the laser and the workpiece are made, so that the e.g. radiation returning from highly reflective materials towards the laser does not cause instabilities in the process of radiation formation in the laser. In

conventional plants become two for this purpose

Devices, an ATFR mirror that is s-polarized

Radiation reflected and absorbed p-polarized, and a λ / 4-phase shifter 34 used in combination. If the C0 ₂ laser is used according to the invention, can be dispensed with the ATFR mirror, since its task is the

Polarization beam splitter, in Figure 11 so the TFP 17, can automatically fulfill. After passing through the external λ / 4-phase shifter 34 twice that is from the

Workpiece 33 coming radiation beam 38 linearly, but polarized perpendicular to the laser radiation 35 and is thus completely transmitted by the TFP 17, so eliminated from the resonator beam path. An absorber 26 destroys this radiation.

During real material processing processes a variation of the laser power is usually required. In order not to influence optimally set parameters of the laser function itself, in particular the quality of the laser radiation, is an external power modulation

advantageous. Figure 12 illustrates two possibilities that can be used in conjunction with the C0 ₂ laser according to the invention. FIG. 12a shows the use of an ILM 54 for external power modulation. The beam 35 coming from the laser is split by the ILM 54 into the power-regulated transmitted beam 59, which is fed to the workpiece 33, and into the reflected beam 58, with the remaining power. The latter is in the component 55, the optional one

Absorber or a radiation detector can be either destroyed or used for on-line monitoring. Of the

The advantage of using ILM lies in its relatively high radiation load capacity, but the modulation speed is limited to typical times in the range 10 to 100 μβ. The achievable maximum-minimum modulation range of the power depends on the interferometer plates used. Allow typical ILM models Attenuation of the laser beam 35 by factors between 10 and 100.

Very high modulation speeds down to the sub-s range allow the use of an AOM 57, the i.a. optical elements for beam shaping 56, e.g. a telescope for adjusting the beam diameter and a special aperture for securing the beam quality, are connected upstream. In this example, the diffracted beam is supplied to the workpiece 33 as a power-regulated beam 59. The residual beam 58 is selectively destroyed or measured again in an absorber / detector 55. Another advantage of this arrangement is the fact that the beam 59 can be attenuated as much as desired, in the minimum to 0 W. However, depending on the AOM model, the controllable power is limited.

FIG. 13 shows in a highly schematized form a factor which is important for the practical realization of the C0 ₂ laser according to the invention. To the long-term stability of

sensitive intracavity components

So, in particular, protect them from dust and

To protect climatic influences, the entire system should be housed in a vacuum-tight enclosure 31.

FIG. 12 shows this for the laser end with the

Thin-film polarizer 17 and the elements of the

Feedback branch 14. The outgoing beam 7 leaves the laser through the window 32 of transparent material, preferably of ZnSe. Analogously, the elements at the other end of the resonator, ie the λ / 4-phase retarder mirror 16 and the end mirror 3 are to be included in the enclosure. Conveniently, the entire vacuum-tight enclosure 31 can be connected to the volume of the active medium 1.

LIST OF REFERENCE NUMBERS

1 active medium

 2 λ / 4 phase shifter

3 end mirror 1

 4 end mirror 2

 5 polarization beam splitters

 6 On polarization beam splitter 5 or 17

 incident beam

7 beam to be decoupled

 8 back-coupling beam

 9 Vertical polarization direction

 10 Horizontal polarization direction

 11 resonator axis

12 Characteristic axis of the λ / 4 phase shifter 2

13 Characteristic axis of the

 Polarization beam splitter 5

 14 feedback branch of the resonator

 15 elements for beam shaping

16 λ / 4 Phase Retarder Mirror (PRS)

 17 thin film polarizer (TFP)

 18 Electro-Optical Modulator (EOM)

 19 Acousto-Optic Modulator (AOM)

 20 interference laser radiation modulator (ILM)

21 chopper disk

 22 Galilei-type telescope

 23 Telescope of the Kepler type

 24 drive element

 25 reflection grids

26 absorbers

27 Optical axis of the ILM 20 28 Beam rotated 90 ° by EOM 18 in its polarization direction

 29 Beam bent by the AOM 19

 30 Ray reflected by the ILM 20

31 Vacuum-tight enclosure

 32 windows made of transparent material

 33 workpiece

 34 external λ / phase shifters

 35 Linearly polarized radiation of the laser

36 Circularly polarized radiation in the direction of the workpiece

 37 Circularly polarized radiation from the workpiece towards the laser

 38 Coming from the workpiece and from the λ / phase shifter

 (34) linearly polarized radiation

 39 deflection mirror

 40 machining head

 41 Incidence slot of the PRS 16

 42 Incidence slot of the end mirror 3

43 Running in the direction of the λ / 4 phase shifter 2

 beam

 44 Starting point of spontaneous radiation

 45 Weak, spontaneously emitted beam in the direction of the resonator axis 11

46 Weak elliptical polarization

 47 Strong elliptical polarization

 48 Unpolarized Radiation

49 Circular polarization

50 concave mirrors

51 vault mirror

52 tilting mirror 53 special aperture

 54 externally arranged ILM

 55 Optional absorber or detector

 56 External elements for beam forming

57 External AOM

 58 Eliminated radiation fraction

 59 Power-regulated radiation c Speed of light

d reduced diameter of the laser beam

D diameter of the laser beam

fimp pulse repetition frequency

 L resonator length

P ₀ Radiation power before splitting

P _p power of the parallel polarized

 radiation component

P _s power of the vertically polarized

 radiation component

P _A Power of the beam to be coupled out P _R Power of the beam to be fed back

Si, S ₂ end mirror

 AOM acousto-optic modulator

 ATFR Absorbing Thin Film Reflector

cw continuous wave

EOM electro-optical modulator

 FPI Fabry-Perot interferometer

 ILM interference laser radiation modulator

PRS λ / 4-phase retarder level

 TFP thin film polarizer

a _B Brewsterwinkel ß angle between the resonator axis and incident solder of the

PRS

ε inclination of the ILM axis against resonator axis

λ wavelength

φ angle between characteristic axes of

 Polarization beam splitter and λ / phase shifter

Claims

claims

C0 ₂ laser with one at both ends with

 Resonator end mirrors (3, 4) closed resonator containing an active medium (1) and with electrodes for the pumped power supply, wherein the resonator in the direction of a Resonatorachse (11), extending orthogonal to the

 Resonatorendspiegel (3, 4), between the

 Resonator end mirrors (3, 4) in one

 High power branch and in a feedback branch (14) is divided, wherein the high-power branch and the feedback branch (14) by a polarization beam splitter (5) for decoupling a part of the laser beam generated in the resonator (7) are separated from each other, wherein in the high-power branch between a first Resonatorendspiegel (3) and the

 Polarization beam splitter (5) the active medium (1) and a lambda / 4 phase shifter (2) is arranged, wherein in the feedback branch (14) between a second Resonatorendspiegel (4) and the

 Polarization beam splitter (5) Elements for

 Beam shaping (15) are arranged, wherein the lambda / 4 phase shifter (2) and the

Polarization beam splitter (5) against each other Angle φ are rotatable, about a rotation axis with at least one rotational component parallel to

Resonator axis (11) or around the resonator axis (11), wherein the resonator axis (11) either straight or bent by the polarization beam splitter (5) and wherein the second Resonatorendspiegel (4) by a wavelength selective element as an element to

Beam shaping (15) may be replaced.

C0 ₂ laser according to claim 1, wherein the active medium in the resonator (14), in particular the high-performance branch, a pressure of less than 0.2 bar, in particular less than 0.1 bar, and / or wherein the λ / 4 Phase shifter (2) between the active

Medium (1) and the first Resonatorendspiegel (3) is arranged.

C0 ₂ laser according to claim 1 or 2, wherein both

Resonatorendspiegel (3, 4) have a reflectivity of more than 95%, in particular more than 99%. C0 ₂ laser according to one of claims 1 to 3, wherein the elements for beam shaping (15) are selected from the group consisting of elements for

Power modulation, for wavelength selection,

Special apertures and combinations of 2 or more such elements. C0 ₂ laser according to one of claims 1 to 4, wherein the λ / phase shifter (2) is a λ / phase retarder mirror (16), preferably for high powers.

C0 ₂ laser according to one of claims 1 to 6,

 wherein the angle φ is adjustable with the proviso that the beam (6) traveling through the active medium (1) in the direction of the polarization beam splitter (5) and the beam (43) traveling in the direction of the λ / 4 phase shifter (2) respectively are linear but polarized perpendicular to each other, or

 wherein the angle φ is adjustable with the proviso that the beam (43) traveling through the active medium (1 in the direction of the λ / 4 phase shifter (2) is linear and that through the active medium (1) is directed in the direction of the polarization beam splitter (5 ) running beam (6) is elliptically polarized, eccentricity and position of the ellipse being determined by φ,

 preferably of the

Polarization beam splitter (5) reflected beam to be coupled out as the beam (7) and the transmitted beam to be fed back as the beam (8) and wherein the resonator (11) preferably

straight through the polarization beam splitter (5).

C0 ₂ laser according to claim 1 to 6, wherein the of

Polarization beam splitter (5) transmitted beam to be coupled out as the beam (7) and the reflected beam to be fed back as the beam (8) and wherein the resonator (11) preferably kinked by the polarization beam splitter (5) extends.

C0 ₂ laser according to one of claims 1 to 7, wherein in the feedback branch (14) of the resonator as

wavelength-selective element a diffraction grating (25) instead of the second resonator end mirror (4)

is used.

Material processing plant with a

A workpiece holder for a workpiece and having a C0 ₂ - laser (35) according to any one of claims 1 to 8, wherein the workpiece is positionable by means of the workpiece holder relative to a laser output of the laser, wherein in the beam path between the laser output and the workpiece (33). , preferably directly at the laser output, a λ / 4-phase shifter (34) is provided with the proviso that the linearly polarized radiation of the laser (35) is transformed into circularly polarized radiation (36), one after reflection or scattering back on the workpiece in the direction of laser radiation component (37) after the second

Passing through the λ / 4 phase shifter (34) again linearly, but polarized perpendicular to the emitted laser radiation (35) and this radiation component (38) from penetrating into the active medium (1) in the resonator of the polarization beam splitter (5, 17) from the direction deflected by the laser beam and destroyed by an absorber (26).

10. Material processing plant with a

A workpiece holder for a workpiece and having a C0 ₂ - laser (35) according to any one of claims 1 to 8, wherein the workpiece is positionable by means of the workpiece holder relative to a laser output of the laser, wherein in the beam path between the laser output and the workpiece (33). Power modulation elements, preferably interference laser radiation modulators (54) or acousto-optic modulators (57), are integrated with the proviso that the direction

 Workpiece (33) running beam (59) is adjustable in its performance within wide limits, without

 Control parameters are changed in the laser ..