Coherent light generators – Particular resonant cavity – Unstable resonator
Reexamination Certificate
1998-10-26
2001-09-04
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular resonant cavity
Unstable resonator
C372S099000
Reexamination Certificate
active
06285703
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention refers to a laser resonator with first and second end mirrors and with coaxial electrodes between which laser radiation is reflected back and forth in the direction of an axis of symmetry of the laser resonator and passes azimuthally segment by segment through a resonator volume having an annular cross section.
Resonators of this type with a coaxial discharge structure are conventionally used for high-performance lasers, i.e. for lasers with up to several kW of laser power, and in particular for gas lasers such as CO
2
lasers.
A laser resonator with an annular cross section on which the preamble of claim
1
is based is described in German Patent No. 41 23 024 C2. This laser resonator has coaxial, regular-cylindrical electrodes, with mirrors facing one another provided at their ends, between which the laser radiation travels azimuthally around the axis of symmetry of the resonator in the manner described above.
In this case, one of the end mirrors in particular can be directly connected to the inner electrode or be an integral part thereof, such that merely the outer electrode and the other end mirror have to be adjusted.
Aside from this, other resonators with an annular cross section are known, in which two or more mirrors are adjustable relative to the electrodes and to one another, German Patent No. 41 29 530 C2 or U.S. Pat. No. 5,353,299 are cited here as examples.
The radiation also travels azimuthally in the lasers known in these publications. The differences between the lasers and/or laser resonators known from the aforementioned publications lie primarily in the design of the end mirrors and in the manner in which the laser beam exits.
Another type of coaxial laser, which produces a laser beam with a cross section in the shape of a ring sector, is known from German Patent No. 44 21 600 A1. In this known laser additional mirrors for shaping the laser beam are required in addition to the two end mirrors. The amount of effort necessary to adjust the laser is correspondingly large.
A coaxial laser with a stable resonator, in which the laser radiation is guided over a very complex configuration of mirrors, such that an at least approximately homogenous laser beam emerges centrally, is known from German Patent No. 44 24 729 C1.
A disadvantage of all the resonators described above is that the course of the beam in the resonator volume depends very heavily on the precise adjustment of all components which enclose the resonator volume. Even in the event that only one single mirror requires adjustment, any angular deviation—no matter how slight would result in a clearly out-of-center beam path and, therefore, inferior beam quality. The above adjustment problems, as well as the use of additional folding mirrors, frequently make it more difficult if not impossible to ensure adequate beam quality.
SUMMARY OF THE INVENTION
The object of the invention is to further develop a state-of-the-art laser resonator such that it is substantially robust with regard to adjustments and therefore can be folded with no hazard to or loss of beam quality in spite of the use of additional folding mirrors, if applicable.
SUMMARY OF THE INVENTION
This object is carried out in accordance with the invention in that the second end mirror is a substantially conical mirror that deflects the laser radiation striking it to respective azimuthally opposite regions of the resonator volume.
Between the reflections by the first and the second end mirrors, the laser radiation passes through an annular cross-sectional surface oriented perpendicular to the optical axis, and in time travels azimuthally along this ring around the axis of symmetry lying in the center of the ring. It then strikes a corresponding annular segment of the conical second end mirror according to the invention. This conical mirror first of all reflects each respective impacting laser beam at an approximate right angle towards the optical axis and thereby at an offset of approximately 180° into the azimuthally opposite region of the annular impact surface, where it is deflected back towards the first end mirror by a second reflection. The important thing here is for the doubly reflected beam if applicable, excepting the azimuthal beam inclination required for the travel of the beam to return at the same angle at which it struck the conical mirror beforehand.
In conventional ring resonators, the result of any maladjustment of a mirror is that the laser beam first of all strikes the other mirror at a certain tilt and therefore with a radial offset, and then, after being reflected back, strikes the first mirror with double the radial offset. The same tilting angle in the laser resonator according to the invention results at first in a corresponding radial offset of the points of impact on the conical mirror. Through the double reflection at the conical mirror, with an aperture angle of approximately 45° to the optical axis (the exact aperture angle will be discussed further below), the reflected laser beam strikes the first mirror again at the same angle, i.e., with zero radial offset, namely in the azimuthally opposite region. This inherent compensation of possible adjustment inaccuracies ensure high beam quality, even in resonators with single or multiple folds. The first mirror, as well as the second mirror, which is conical according to the invention, can be circular or conical, or particularly for the purpose of cooling the inner electrode it can be annular in shape. A plurality of corresponding ring segment-shaped mirror sections could likewise be envisaged without limiting the general inventive concept.
In a first embodiment of the invention the first end mirror is radially curved to form a torus and has a beam exit window in a small azimuthal area. The toroidal curvature guarantees that the laser beam keeps to the optimal radius during its progression always offset by 180° according to the invention around the circle.
The exit of the laser beam following the complete round is accomplished in a structurally simple manner by a window which takes up only a small portion of the ring circumference of one of the mirrors.
In a second embodiment the first end mirror has a helical slope by regions, preferably over two azimuthally opposite regions, for the selection of a specific azimuthal beam inclination. The purpose of the helical slope, by which the first end mirror assumes the approximate shape of a spiral strip segment, is not only to produce, but above all to favor a certain azimuthal inclination of the laser radiation. Since in accordance with the invention the radiation is reflected alternately back and forth between two halves of the ring gap, a special feature of the invention is that the laser resonator has two such helical mirror sections azimuthally opposite one another.
In a third embodiment the first end mirror has a helical slope over two semicircular ring segments, to increase the azimuthal beam inclination. A vertex radius associated with the torus curvature of the second end mirror diminishes there as the azimuthal angle grows. In this embodiment, according to which the first mirror has an azimuthal slope over its entire circumference, the azimuthal inclination of the radiation increases continuously during half of the travel cycle, up to its exit from the beam exit window. In order for the radiation to strike the conical mirror within the prescribed radius in spite of this increase, the vertex radius, which determines the torus curvature of this first mirror, diminishes as the azimuthal angle grows and thus counteracts the radially outwardly directed deflection caused by the increasing azimuthal inclination of the radiation.
In a fourth embodiment, for the selection of a specific azimuthal beam inclination, the conical mirror has an aperture angle tuned to a vertex radius associated with the toroidally curved first end mirror. The advantage of this is that instead of an entirely or partly helically climbing first end mirror which is therefore difficult to manufacture, a merely toroidal
Arroyo Teresa M.
Inzirillo Gioacchino
Pepe & Hazard LLP
Trumpf Lasertechnik GmbH
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