Coherent light generators – Particular active media – Gas
Reexamination Certificate
2002-10-21
2004-09-28
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S055000, C372S007000, C372S046012
Reexamination Certificate
active
06798816
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to radio frequency (RF) excited, diffusion-cooled, sealed-off CO
2
lasers. The invention relates in particular to a waveguide CO
2
laser including a tapered-waveguide gain-region.
DISCUSSION OF BACKGROUND ART
There are three types of RF-exited, diffusion-cooled CO
2
lasers in common use. These are the slab laser, the folded waveguide laser, and the folded, free-space-resonator laser. In a folded free-space-resonator laser, lasing modes are determined primarily by the configuration of mirrors forming the laser resonator. A slab laser includes a laser resonator in which the lasing mode or modes are constrained in one of two mutually perpendicular directions, transverse to the resonator axis, by slab-like electrodes used to excite an RF discharge in the lasing (CO
2
) gas. The mode shape in the other transverse direction is determined by the configuration of mirrors forming the resonator. In a folded waveguide laser, lasing modes are constrained in mutually perpendicular directions in zigzag arrangement of waveguide-channels in a dielectric slab, typically a slab of a ceramic material. The ceramic slab is bounded by electrodes for exciting an RF discharge in a lasing gas in the waveguide-channels. It is generally accepted that the power output of slab lasers scales with the discharge area for a given electrode spacing while the power out put of prior-art waveguide lasers scales with length.
Slab CO
2
lasers have the highest power output capability. Slab lasers having a power output of 1000 Watts (W) are commercially available. It is generally accepted, however, that waveguide CO
2
lasers have superior mode-quality to that of slab lasers and have higher efficiency. One factor contributing to the higher efficiency is diffusion cooling in both the height and width of the waveguide dimensions. One factor contributing to this superior mode-quality is the use of waveguide dimensions that constrain lasing into a single oscillation mode. The higher efficiency and superior mode-quality are presently obtained at lower output power than is available in commercial slab lasers. Waveguide CO
2
lasers are commercially available with power outputs in a range between 25 W and 140 W, although waveguide lasers with power outputs up to 300 W have been custom produced for specialized applications.
FIGS. 1 and 2
schematically illustrate a prior art waveguide-block
30
of a type used in a prior-art waveguide CO
2
laser. Other features of the laser such as gas containment arrangement, resonator mirrors, arrangements for sustaining an RF discharge, and cooling arrangements are omitted from
FIG. 1
for convenience of illustration. Such features are well known to those skilled in the art to which the present invention pertains. A detailed description of a prior art laser including such a waveguide-block is given in U.S. Pat. No. 6,192,061 the complete disclosure of which is hereby incorporated by reference.
Waveguide-block
30
is typically formed from a ceramic material such as high density Aluminum Oxide (Al
2
O
3
) and includes two or more waveguide-channels, with 3 to 7 channels being preferred. Three waveguide-channels (waveguides)
32
,
34
and
36
are depicted in
FIGS. 1 and 2
. Each waveguide has a height or depth H and a width W, each of which is assumed, here, to be constant. There is little freedom in varying the cross sectional dimensions H and W of a waveguide if single mode operation is desired. By way of example, dimensions of a single-mode waveguide-channel for a CO
2
laser are about 0.28 centimeters (cm) high, and between about 0.28 and 0.47 cm wide.
A longitudinal resonator axis
38
, folded into a Z-shape by mirrors (not shown in
FIG. 1
) extends through the waveguides. Waveguides
32
,
34
, and
36
are arranged at an angle &thgr; from each other to accommodate the folded resonator axis. Angle &thgr; is exaggerated in
FIG. 1
for convenience of illustration. In practice, angle &thgr; is relatively small, for example less than about fifteen degrees (15°) with about 6° or less being preferred. End
32
B of waveguide
32
overlaps (is juxtaposed with) end
34
A of waveguide
34
. End
34
B of waveguide
34
overlaps end
36
A of waveguide
36
. The degree of overlap depends on angle &thgr; and the distance at which mirrors (not shown) used to fold the resonator axis
38
are located from the ends of the waveguides. Those skilled in the art will be aware that this distance and the angle &thgr; are usually kept as small as practically possible to minimize the length and the width of the laser. The selection of the angle &thgr; is a design compromise between keeping the width of the laser small, and minimizing the waveguide overlap area. Reducing &thgr; reduces laser width, while increasing &thgr; decreases the overlap area. Reducing &thgr; also reduces the positioning sensitivity of the folding mirror for ease of resonator alignment.
Given that height H is constant, total laser power output capability provided in the uniform-width waveguides is proportional to the total area (width times length) of the waveguides. The overlapping or juxtaposition of the waveguides gives rise to common areas (A
C
) of the waveguides that can be considered to provide gain in only one of the waveguides or the other. Common areas A
C
are small compared with the total waveguide area for an angle &thgr; less than 6°. Similarly, the length of waveguide
34
can be considered to be approximately equal to the length of waveguides
32
and
36
. Accordingly, the total area of the waveguides can be considered as approximately equal to the product of the number of waveguides (here, 3), the waveguide width W, and the length of any one of the waveguides. In other words, the power output of single-mode, waveguide CO
2
lasers scales with the total length of the waveguides for a given width and height of the waveguide. By way of example, a total waveguide length of about 2.3 meters (m) may be required for an output power of about 150 W. A waveguide-block
30
having five folded channels providing a total waveguide length of 2.3 m may be about 47.5 centimeters (cm) long and about 7.6 cm wide.
One potential limit to the prior-art folded-resonator or folded waveguide approach to increasing total waveguide length is that, for a fixed physical length of a single waveguide, the folded waveguide-block can become as wide as it is long if the number of waveguides is increased. In addition, increasing the number of waveguides increases the number of mirrors required to fold the resonator axis to the point where alignment of the mirrors becomes very difficult. Further, as dimensions of a folded-resonator laser-package and output power increase, it becomes increasingly difficult to design uniform cooling arrangements for the laser-package that minimize temperature gradients.
Temperature gradients resulting from non-uniform cooling can cause flexing of a laser housing, resulting in beam pointing errors, among other problems. Difficulty in obtaining ceramic blocks greater than one meter in length also limits the length and thus the power scaling of CO
2
waveguide lasers.
Increasing the number of waveguides increases the total area of the laser, which, in turn, increases the area of electrodes needed to maintain the RF discharge in the waveguides. As the electrode area increases, the capacitance seen by an RF power supply energizing the electrodes increases causing a decrease in impedance. The lower the impedance the more difficult it is to couple RF energy into the discharge.
Still another problem encountered in power scaling waveguide-lasers is damage to intra-resonator optical components, particularly optically coated components. In prior art CO
2
waveguide lasers operated in a cavity-dumped, Q-switched, pulsed mode, for example, it is possible that intra resonator power density (power per unit area) can reach the damage threshold of intra-resonator optical components such as electro-optic switches (EO-switches) and reflective phase retarders
DeMaria Anthony J.
Laughman Lanny
Seguin Vernon A.
Coherent Inc.
Nguyen Dung
Stallman & Pollock LLP
Wong Don
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