Stripline laser

A stripline laser includes a discharge chamber between two flat electrodes. A flat multipass resonator which is stable in relation to the width of the discharge chamber has a folding mirror configuration inside the resonator and is associated with end surfaces directed perpendicularly to the longitudinal direction of the discharge chamber.

BACKGROUND OF THE INVENTION
 Field of the Invention:
 The invention relates to a stripline laser.
 Stripline or slab lasers are disclosed, for example, in U.S. Pat. No.
 4,719,639 and U.S. Pat. No. 5,048,048. In the case of those lasers, an
 elongate narrow parallelepipedal discharge chamber for a gas, in
 particular CO.sub.2, is formed between planar electrodes which are
 parallel to one another. The gas is electrically excited by a
 radiofrequency voltage applied to the electrodes. In order to achieve a
 laser effect, resonator mirrors are disposed opposite rectangular end
 surfaces of the discharge chamber. Those resonator mirrors form a
 resonator only in a plane which is parallel to the electrodes, that is to
 say in the direction of the width of the discharge chamber. Transversely
 thereto, that is to say in the direction of the distance between the
 electrodes or the height of the discharge chamber, the two electrodes
 behave as waveguides.
 U.S. Pat. No. 4,719,639 explains in further detail that both stable and
 unstable resonators are suitable. In particular, an unstable confocal
 resonator of the negative branch is proposed in U.S. Pat. No. 5,048,048.
 Unstable resonators have a number of advantages which are important for
 high-power lasers, in particular. Thus, with unstable resonators it is
 possible, for example, to achieve high mode volumes and better utilization
 of the total volume region of the discharge chamber, that is to say of the
 entire excited gas, even in relatively short resonators. Compact
 high-power lasers can thus be built with the aid of unstable resonators.
 However, unstable resonators have the property that the intensity
 distribution in the cross-section of the laser beam differs in the
 near-field and far-field range and that the far-field distribution
 moreover has, as a rule, secondary maxima which have to be filtered out by
 suitable devices.
 SUMMARY OF THE INVENTION
 It is accordingly an object of the invention to provide a stripline laser,
 which overcomes the hereinafore-mentioned disadvantages of the
 heretofore-known devices of this general type and which is distinguished
 by a high beam quality and a high laser power in conjunction with a
 compact overall structure of the laser.
 With the foregoing and other objects in view there is provided, in
 accordance with the invention, a stripline laser, comprising two planar
 electrodes; a discharge chamber or space disposed between the two planar
 electrodes and having a longitudinal direction, a width and end surfaces
 oriented perpendicularly to the longitudinal direction; and a planar
 multipass resonator being associated with the end surfaces and being
 stable referring or relative to the width of the discharge chamber, the
 resonator having a folding mirror configuration within the resonator.
 The use of a stable multipass resonator configuration having a
 resonator-internal folding mirror configuration ensures that, on one hand,
 the entire discharge chamber is utilized and that, on the other hand, the
 advantageous properties of the laser beam that are associated with the use
 of a stable resonator, are obtained. Those advantageous properties are, in
 particular, an intensity distribution over the beam cross-section that is
 virtually independent of the distance from the beam exit window, as well
 as the absence of interfering secondary maxima.
 In accordance with another feature of the invention, the folding mirror
 configuration is a telescopic folding mirror configuration.
 In accordance with a further feature of the invention, the telescopic
 folding mirror configuration has an optical axis running outside the
 discharge chamber.
 In accordance with a concomitant feature of the invention, the telescopic
 folding mirror configuration includes a mirror associated with one of the
 end surfaces and having a reflecting surface curved convexly toward the
 discharge chamber; and a mirror facing the other of the end surfaces and
 having a reflecting surface curved concavely toward the discharge chamber.
 Other features which are considered as characteristic for the invention are
 set forth in the appended claims.
 Although the invention is illustrated and described herein as embodied in a
 stripline laser, it is nevertheless not intended to be limited to the
 details shown, since various modifications and structural changes may be
 made therein without departing from the spirit of the invention and within
 the scope and range of equivalents of the claims.
 The construction and method of operation of the invention, however,
 together with additional objects and advantages thereof will be best
 understood from the following description of specific embodiments when
 read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring now to the figures of the drawings in detail and first,
 particularly, to FIGS. 1 and 2 thereof, there is seen a stripline laser
 that contains two planar electrodes 2 and 3 in plate form which are
 disposed parallel to one another and separated by a gap s. A laser gas, in
 particular a CO.sub.2 -containing gas mixture which is situated between
 these two electrodes 2 and 3, is excited (pumped) by a radiofrequency
 voltage applied to the electrodes 2 and 3. The electrodes 2 and 3 define a
 parallelepipedal discharge chamber 4 having a length 1, for example 500 cm
 to 1000 cm, a width and a height s corresponding to the distance between
 the electrodes. The distance s is in the region of a few millimeters and
 is distinctly less than a width b, which may be several centimeters, for
 example 20 cm.
 The discharge chamber 4 has end surfaces 42 and 44 which are oriented
 perpendicularly to its longitudinal direction and have a rectangular
 cross-section with the width b and the height s. Mirrors 8, 16 and 12, 20
 which are respectively disposed opposite these end surfaces 42 and 44 form
 a stable planar multipass resonator parallel to planes spanned by the
 electrodes 2 and 3. In the exemplary embodiment, the multipass resonator
 includes an end mirror 8 and an end mirror 12 as well as a folding mirror
 configuration disposed within the resonator. In the exemplary embodiment,
 the folding mirror configuration includes two folding mirrors 16 and 20,
 which are inclined with respect to the end surfaces 42 and 44 and have
 respective planar reflecting surfaces 18 and 22. Each end surface 42 and
 44 is associated with a respective folding mirror 16 and 20. The end
 mirror 8 has a high degree of reflection (near 100%), serves as a back
 mirror and is likewise a planar mirror having a planar reflecting surface
 10. The end mirror 12 is coated in such a way that it is semi-transparent,
 serves as an output-coupling mirror, is a cylindrical mirror which is
 curved concavely in the direction of the discharge chamber 4 and is
 spherical or disposed with its cylinder axis perpendicular to the
 electrodes 2 and 3. Its degree of reflection R is chosen in such a way
 that a maximum efficiency is achieved, which is between 60% and 80%, for
 example. Instead of a planar end mirror 8 for back-mirroring and a concave
 end mirror 12 for output coupling, it is also possible to use a concavely
 curved back mirror and a planar output-coupling mirror or two concave end
 mirrors.
 The mirror curvatures of the end mirrors 8 and 10 are chosen in this case
 in such a way that a stable resonator is produced.
 Propagation conditions parallel to the width b of the end surfaces 42 and
 44 are determined by the physical laws underlying free beam propagation. A
 stable multipass resonator is present only in the plane spanned by this
 direction and the plane spanned by the longitudinal direction 1 of the
 electrodes 2 and 3. Perpendicularly to the electrodes 2 and 3, the beam
 propagation within the stripline laser is essentially determined by the
 waveguide properties of the narrow discharge chamber 4 formed by the
 electrodes 2 and 3.
 The folding mirrors 16 and 20 lying opposite one another compel a beam 50
 which is propagating within the resonator to effect a plurality of
 resonator-internal passes that in each case are laterally offset with
 respect to one another. Consequently, the effect of this folding mirror
 configuration is, on one hand, that the total volume of the discharge
 chamber is utilized for the optical amplification. On the other hand, the
 folding mirror configuration also effects an increase in the effective
 length of the resonator, which has an advantageous effect on the frequency
 purity and the frequency stability of the output beam. Instead of the
 simple embodiment having two folding mirrors 16 and 20 which is
 illustrated in the figure, embodiments containing more than two folding
 mirrors are also conceivable.
 In accordance with FIG. 3, a telescopic folding mirror configuration is
 provided which includes a folding mirror 30 having a reflecting surface 32
 that is curved concavely toward the discharge chamber 4, and a folding
 mirror 34 having a reflecting surface 36 that is curved convexly toward
 the discharge chamber 4. The two folding mirrors 30 and 34 form an
 astigmatic telescopic imaging system with an optical axis 38 that is
 disposed outside the discharge chamber parallel to the longitudinal
 direction of the electrodes. The advantage of such telescopic folding is
 that a small beam cross-section is expanded in accordance with a low
 Fresnel number with a small number of passes and correspondingly low
 losses, and consequently a relatively large beam cross-section is
 available, which is associated with low loading on the transmission
 optics.
 The telescopic folding mirror configuration 30, 34 corresponds to an
 unstable confocal off-axis resonator of the positive branch. The entire
 mirror configuration forms a stable resonator with a resonator-internal
 telescope. A typical exemplary embodiment has a width a1.apprxeq.1.7 cm
 for the end mirror 8 and a width a2.apprxeq.4.4 cm for the end mirror 12,
 a distance 1'.apprxeq.100 cm between the mirrors 30 and 34 on the optical
 axis 38, a distance x=10 cm from the optical axis 38, a width b=20 cm of
 the discharge chamber 4, a focal length of the folding mirror 30 of
 f1.apprxeq.688 cm and a focal point of the folding mirror 34 of
 f2.apprxeq.588 cm (confocal configuration, focal point F outside the
 resonator to the side of the folding mirror 34). The result is 6 cyclic
 passes of the beam 50 and a Fresnel number Nf=1.2 in the direction of the
 width b, that is to say a "single-mode-operation" in the direction of this
 width b and in a direction perpendicular to the electrode surfaces, as
 well as an axial mode separation of about 25 MHz. Such a stripline laser
 generates a stable-frequency laser beam having a Gaussian-like intensity
 distribution in both directions.