FR2702891A1 - Compact laser. - Google Patents

Abstract

The invention relates to a compact laser. In this laser device (1), which comprises an excited gain region (9) confined between two cooled electrodes (6, 7) in the form of thin sheets, with a three-plate in-line excitation configuration, a formed resonator is provided. a focusing mirror and a second reflector plane comprising different areas having different reflectivities so that a folded path with six or more "segments" is the only internal distribution of radiation maintained within of the laser cavity. Application in particular to machining, cutting or drilling machines in a computer-controlled positioning device.

Description

The present invention relates to a compact gas laser device and

  robust type of triplacue in-line excitation configuration and which can physically replace the spindle unit or the machining / cutting / drilling tool in a position device.

  computer controlled operation.

  In conventional engraving, cutting and piercing devices using a low power laser (less than 100 W), fixed lasers having a beam delivery system (mobile optical system) consisting of a series of mirrors are used. , lenses and / or fibers, which transmit the beam from the opening of the laser to the desired variable position on the workpiece These systems can be of a very complex nature and often require recalibration and adjustment and Generally, the initial investment and the cost of maintenance are significantly increased, while increasing the base area of the system (the other option is to move the whole workpiece, and the disadvantages of

  process are often even more important).

  The fixed nature of the laser in conventional systems is required by the large dimensions and the significant weight of the gas laser, as well as by their relatively fragile construction. On the other hand, small, robust lasers have powers that are too low for the treatment of materials. or tend to produce poor quality beams, which can not be focused to obtain maximum power flux densities (solid-state lasers in the near-infrared range or in the visible can eliminate these two disadvantages, but

  are currently prohibitively expensive.

  Extremely compact, conduction-cooled gas lasers can be designed using a triplet-in-line configuration, in which the excited gain region is confined between two cooled electrodes having the geometry of a thin sheet (see Figure 2 (a)). appended to this application) In such a configuration, the power per unit length is proportional to the lateral dimension of the sheet and is not independent of the cross section of the gain region (as in the case of gain regions). These lasers can therefore be arranged in such a way as to produce high powers from very compact devices. In the case of using an I-box optical resonator, having a total reflection focusing reflector and a planar output coupler, the in-line laser configuration produces a beam which takes the form of the cross section of the plane. the gain region, i.e. a thin band

  such beam configurations are undesirable

  in the perspective of applications, since the diffraction beam in the focal plane of a focusing system would have an asymmetrical shape and the focal depth would be weak A method for extracting a narrow beam from regions of bar-shaped geometry gain consists in using a hybrid resonator with unstable wavelengths, that is to say that the optical cavity consists of an unstable resonator in the lateral (longitudinal) dimension, whereas the configuration in stable waveguide is maintained in the transverse direction (across the gap) The two types of positive and negative branches of unstable resonators (Figure 4) have already been described (US Patent

  N 04 719 639 positive branch, and US Patent No. 05 048 048 -

negative branch).

  For low-power compact lasers, the unstable resonator poses two problems First, the beam exiting a resonator of this type has axial mode spacing in the frequency range of c / 2 L, where c is the velocity of light and L the length of the resonator For very short lasers, this spacing becomes important and it is often difficult to adapt the

  maximum of one axial mode to the maximum of the gain curve.

  In addition, especially for short-length lasers, in order to obtain round-trip gain greater than unity, total losses (including coupling losses) must be kept to a minimum value. reduce the coupling losses of the resonator

  unstable, it is necessary to reduce the opening of the outgoing beam.

  This leads to a reduction in the quality of the beam of

  output under the effect of diffraction.

  Another method for beam extraction

  focussed from a system with three-dimensional configuration.

  plate was developed by Synrad Inc., under the

  shape of their C 02 micro-miniature laser networks "Poly-

  Lase "series 22, models D 22-1 and M 22-1 These lasers are

  really compact, provide relative powers

  high and consist of a triplate line configuration, the plate being subdivided lengthwise into three regions, by ceramic spacers The outgoing radiation is in the form of three distinct beams, without any coherence between the beams. focus of such a system is complex and one can not obtain flux densities

  maximum with a large focal depth.

  Synrad describes appropriate applications for this laser,

  as applications in which the ultimate factor

  me in MY {mode quality or spot diameter) is not required. "In previous articles (Optics Letters Vol 14, N 023, (1989), pp 1309-1311 and SPIE, Vol 1276, CO 2 Lasers and Applications II (1990), pp 98-105), the original inventors of this patent have described a preliminary solution to the problem of extracting a Gaussian beam from the wide gain zone of the linear configuration. Triplate A folded trajectory has been obtained for the mode M by placing a stop within a semi-confocal cavity of the resonator,

  near the focusing mirror, which realizes a

  total bending, to suppress the formation of other modes There are three disadvantages to the optical cavity system in the M mode: 1 The path in this system has only four segments and therefore is not suitable for excitation regions having a significant lateral extent since it does not effectively sample the excited region This prevents the use of the M mode for very high power

high per unit length.

  2 The insertion of a stop between the electrodes and the mirrors (which increases the distance between these elements) leads to a significant reduction of the power and the efficiency, because of the significant losses by diffraction at the edge of the electrodes This effect is even more pronounced when the spacing between the electrodes is low (as is necessary for

high powers).

  3 The abutment region is located at the level of the total reflection spherical mirror, which requires that the plane mirror and the spherical mirror are optical compounds made to measure This increases the cost of

elements of the cavity.

  The present invention relates to a compact and robust gas laser device in which the excited gain region is confined between two cooled electrodes having a thin sheet-shaped geometry and producing high powers from a very compact device, said laser gas emitting a Gaussian symmetric focused beam, while effectively sampling the extended gain region having a

  configuration of excitation in line triplaque and in the-

  which said gas laser is characterized in that it comprises a resonator constituted by a focusing mirror and a second reflector plane comprising different zones having different reflectivities so that a folded trajectory comprising six or a greater number of "segments "is the only internal radiation distribution maintained

  inside the laser cavity.

  Said device can be used as a spindle unit or a machining / cutting / drilling tool in a machine.

  computer-controlled positioning device.

  The invention also relates to a resonator of the gas laser device Said resonator is constituted by a focusing mirror and a second reflective plane comprising different zones having different reflectivities so that a folded trajectory comprising six or more segments is the only internal radiation distribution maintained

  inside the laser cavity.

  In addition, the present invention also relates to a beam emerging from such a resonator parallel to the optical axis, with an axial mode spacing six times greater than or greater than that of a beam delivered by a beam.

usual resonator.

  The object of the present invention is to provide a

  robust gas laser capable of withstanding the accelerating forces

  during a trip, while maintaining its performance (ie the power output, the

quality of the mode, the output).

  In addition, the laser described is compact and lightweight, which facilitates its movement by a small positioning device controlled by

  computer (that is, the arm of a robot, a pantom-

phe, etc.).

  In addition, the laser is both formally and dimensionally compatible with existing computer-controlled cutting, piercing or engraving machines and is able to physically replace the existing machining tool. or the existing spindle assembly, which combines the current mechanical capabilities with those of a laser and allows a dynamic control of the area of the dimension uu point of impact of the beam on the piece to

  machining, by means of the z-axis of the positioning device.

  Another object of the present invention is to provide a laser having a beam of excellent quality (i.e. a symmetric, Gaussian, low center width beam), which can be focused to obtain densities of flows and a maximum M 2 factor, while still sampling the region efficiently

wide gain of the excited medium.

  In addition, the coupling losses of the optical resonator according to the present invention can be reduced more and more, without this affecting in any way

  damaging the quality of the outgoing beam.

  In addition, the optical network configuration of the laser is particularly suitable for its short length, with corresponding axial mode spacing.

  to a laser cavity six or more times greater than the

  This allows a higher density of axial modes in a given range of frequencies and facilitates the matching of maxima.

  axial modes with the maximum of the gain curve of the

excited place.

  In addition, the compact and solid nature of the present invention as well as its relatively high power (per unit length) and its symmetrical focused beam make it possible to insert the system according to the invention into existing positioning devices, in place of the tool for machining, drilling,

  engraving or cutting or set of pins.

  Other features and benefits of the

  present invention will emerge from the description given

  Hereinafter with reference to the accompanying drawings, in which: Fig. 1 (a) is a schematic of the cross-section of the laser head, which is the first embodiment of the present invention; Figure 1 (b) shows the internal configuration of the electrodes and spacers; Fig. 2 (b), already mentioned, shows a schematic side elevational view of the optical resonator of the present invention as well as a representation of the internal distribution of the radiation and the outgoing beam; Fig. 3 shows the embodiment of the optical resonator according to the present invention; Figure 4 (a) shows an unstable resonator with positive branch; and Figure 4 (b) shows a resonator

unstable to negative branch.

  Figure 1 (a) shows a diagram of the cross-section of the laser head. Two parallel electrodes (6, 7) are held in position by

  ceramic spacers (2, 3, 4) and a clamping screw (5).

  An electrode (6) is electrically insulated from the other electrode (7) and with respect to the surrounding foil (1). RF voltage is applied between the two electrodes and a discharge is formed in the gas in the region. (9) present between these electrodes The electrodes are cooled by means of a circulation of water in channels (8) extending inside the electrodes The metal sheet 1 forms a wall for sealing the gas in the laser and also acts as a Faraday cage to provide protection against HF radiation and reduce HF noise leakage. Figure 1 (b) shows the electrodes and spacers without the wraparound metal housing. The gain region in this laser is a leaf

  thin confined between the two electrodes (Figure 2 (a)).

  Since the configuration of the tri-plate cavity is long and thin and it can be assumed that the Maxwell equations for the radiation distribution in the laser cavity can be separated for the direction (y) parallel to the electrodes. and the direction (x) perpendicular to the electrodes, the cavity can be considered essentially in a two-dimensional system taking into account the radiation distribution in the y direction. The current two-dimensional distribution is a hybrid of the waveguide mode in the x-direction , and the free space mode, which will now be discussed, in the y direction Since the number of Fresnel (n = 2 / λL) is high, an optical resonator with an I-case (mirrors having a uniform U-reactivity) on the height a of the gain region), the optical radiation pattern is a superposition of a large number of Hermitic-Gaussian modes including HG modes of order which take essentially the shape of the section of the cross-section of the gain region

  (a rectangle of height a and width b).

  The length of the cavity, the radius of curvature of the mirrors and the distribution of losses inside the cavity can be chosen in such a way that certain HG modes are favored. This can also be intuitively understood in the case FIG. 2 (b) shows a resonator comprising a plane mirror (1) and a focusing mirror (2) having a focal length equal to twice the distance L between the mirrors A beam, which starts from a point P1 situated on the plane mirror (1) and moves parallel to the optical axis, is reflected by the focusing mirror (2), meets the plane mirror at the point P 2, and then reaches the center of the spherical mirror Its path continues symmetrically with respect to the axis and passes through point P 3 and point P 4, at which point it returns following the same path This path is closed, that is, to say that the beam returns in the same position and in the same direction after a finite

  passages inside the cavity (12 in this case).

  This discussion is compatible with the analysis of the ABCD radiation matrix of an optical cavity (which is valid for Gaussian beams as well as for geometrical rays), in which one mirror is spherical and the other is the matrix. Starting unit cell at the plane mirror (Laser Electronics, J T. Verdeyen, Prentice-Hall, 1981, pp 35-41) is provided by 1 2 Ll 1 O È 1-2 L / f 2 L

T = (1)

  In the configuration described here with f = 2 L, -OZ 2 T = 1 (2) The position of the radius after S traverses of the unit cell is provided by the relation rs = rmaxsin (s O + a) (3) In the plane S = 0, a beam moving parallel to the axis and at a distance rmax of this axis is triggered In this case, we obtain a immediately as equal to n / 2 O is provided by the relation A + D cos O = = 1/2 (4)

O = (5)

  Thus, the position after a round trip is provided by rs = rmax Sin (sn / 3 + n / 2) (6) Obviously, this function is repeated after six round trips or twelve reflections, as was

indicated previously.

  An abutment (3) is inserted into the cavity to suppress all other radiation distributions or closed trajectories which otherwise could be maintained.

in the cavity.

  Since the radiation intersects the plane mirror at distinct points Pl P 4, the plane mirror can be designed to completely reflect the beam at three points P 2 P 4, while transmitting partially

  the beam at the fourth point P 1.

  FIG. 3 shows the side elevational view of the configuration of the optical resonator of the present invention, in accordance with the preceding discussion. The total reflection spherical reflector 1 has a focal length equal to 2 L, where L is the distance between the mirrors The mirror plane is aligned in a single plane through the use of a single substrate having different coatings on its face turned towards the inside of the cavity The zones (3) and (5) are

  covered in such a way as to have a reflective

  maximum, and the region (2) is partially transmissive.

  The region (4) of the plane mirror is non-reflective and

  corresponds to the stop in FIGS. 2 (b) (3).

  The outgoing radiation (6) has the characteristic characteristic of a Gaussian beam moving parallel to the optical axis. Moreover, the radiation is identical to that of a conventional laser whose cavity length is equal to six times that of the present configuration (6 L), (i.e. beam divergence, axial mode spacing, etc.) Single lens (7) mounted directly on the laser housing

  focuses the beam on a defined point (8).

  In the case of the optical resonator described in an earlier article (FIG. 4), the unstable resonator, both the positive branch and the negative branch, to reduce the overall losses of coupling, it is necessary to reduce the ratio of the opening to the reflecting surface. (a 1 / a 2)> For a given lateral dimension of the discharge zone (a), when the opening (a 1) is decreased, the quality of the outgoing beam is

  altered more strongly by diffraction.

  However, in accordance with the present invention, coupling losses can be reduced by causing them to

  a desired low value by increasing the reflec-

  In this case, for cavities of short length, in which the overall gain is small and the coupling losses must therefore have a low thief to obtain the same effect as in the output coupling area (FIG. 3 (2)). of a return gain greater than unity, the present embodiment of the optical resonator is more desirable than that of the

unstable resonator.

  These two advantages (the correspondence to the characteristics of a longer laser, and the possibility of reducing the coupling losses) make the present invention particularly suitable for very short cavities. The laser described here has the combination of a very compact robust device, with high output power per unit length and a beam of excellent quality. This allows the laser to be inserted into a computer-controlled positioning device, such as example a pantograph, a milling unit, drilling or CNC cutting or the arm of a robot The laser being arranged to adapt to the precise dimensions of the machining tool or the machining pin currently mounted on the positioning device, this tool can be physically replaced by the present invention. In this way, the manufacturer can have the capabilities of a laser without the need to acquire affected laser positioning devices.

  cost and space and reduces the time and experience of

  necessary to train the operator again for

  that he is competent on two separate systems.

  Another advantage of the present invention is its ability to modify the size of the spot by means of a modification of the distance between the laser and the workpiece. The width of an etching channel, for example, can be controlled dynamically at from a minimum value of 0.1 mm to 1 mm or more by changing the z-axis position of the laser. Z-axis movement is a possibility currently available in numerically controlled systems and in pantograph systems used to effectively maintain the interaction between the lathe tool and the workpiece material In laser systems, this capability is provided by the electronic system of the excitation circuit (energy source) and the z axis is therefore free to control the size of the focused region. An example of a particular embodiment of the present invention is applicable to etching in a plastic material. Laser etching provides an improvement over mechanical systems for a number of points: elm of chip or cut

remains on the workpiece.

  No resistance (for fragile parts or

difficult to maintain).

  Fine line resolution (up to 0.1 mm).

  Dynamic stroke width control (by changing the distance from the workpiece to

machining).

  A computer-controlled mechanical engraving system, which is usually available, is the so-called Gravograph VX 2 system. The cutting elements are held by a support pin of the cutting elements (part NO 63435-000) having dimensions 18 cm x P 3.5 cm and weighing 2 kg The dimensions of the laser head according to the present invention can be precisely adapted to that of the spindle The spindle unit can be replaced by the laser head, which provides the possibilities of laser engraving with the use of the existing mechanical engraving system, and this for a

  fraction of the cost of a specialized laser engraving system.

Claims (4)

  A compact and robust gas laser device in which the excited gain region is confined between two cooled electrodes (6, 7) having a thin sheet-shaped geometry and producing high powers from a very compact device, said a gas laser emitting a symmetrical Gaussian focused beam, while effectively sampling the extended gain region (9) having a triplate line excitation pattern and wherein said gas laser device is characterized in that it comprises a resonator consisting of a focusing mirror (2) and a second reflector plane comprising different areas having different reflectivities so that a folded trajectory with six or more "segments" is the only internal distribution of radiation maintained at   inside the laser cavity.   A compact gas laser device according to claim 1 for use as a spindle unit or a machining / cutting / drilling tool in a   a computer controlled positioning device.   Compact gas laser device according to claim 2, characterized in that it does not require any other optical system for manipulating the beam to perform etching, cutting, drilling or other work in the target material, and a dynamic control of the diameter of the point of impact on the workpiece is provided by means of the z-axis of the positioning device. 4 Resonator usable in the device   gas laser according to one of claims 1 to   3, characterized in that it is constituted by a focusing mirror (2) and a second reflector plane (1) comprising different zones (2 ', 3, 5) having different reflectivities so that a folded trajectory with six or a larger number of segments is the only internal radiation distribution maintained at   inside the laser cavity.   Device according to claim 4, characterized in that the beam (6) issuing from such a resonator is parallel to the optical axis, with an axial mode spacing six times greater than or greater than   that of a beam delivered by a usual resonator.