Resonator array for solid-state lasers

Details Resonator array for solid-state lasers Resonator array for solid-state lasers

1999-12-16

2004-09-07

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Active media with particular shape

C372S040000, C372S041000

Reexamination Certificate

active

06788723

ABSTRACT:

TECHNICAL FIELD AND STATE-OF-THE-ART
During the past years, Nd:YAG welding lasers have found increasing applications in the areas of jewelry and dentistry. These so-called handheld welding lasers are suitable to perform precise point and seam welding in the sub-millimeter region. They have not only the advantage of providing solder-free joints, but also protect the workpiece as compared to the conventional flame welding technique. A typical construction of devices presently on the market is illustrated in FIG.
1
. The conventional “classic” resonators are mostly constructed in the manner illustrated in
FIG. 2
a
and have a planar output mirror
2
and a concave reflecting mirror
3
. The Nd:YAG laser rod
1
is located approximately in the center between the mirrors
2
and
3
.
The exiting laser beam
4
is focused onto the work plane
8
by a beam expander
5
with an adjustable divergence, a turning mirror
6
(for example, 1064 nm—HR, visible range—AR) and a focusing lens
7
.
All devices encounter problems associated with “thermal lensing” of the Nd:YAG rod and the “initial pulse characteristics” associated therewith. Pumping with a flash lamp and water cooling produces a radial temperature profile in the Nd:YAG rod, which is transformed by the characteristic material constant dn/dT into a refractive index profile and thereby into a lensing effect. Depending on the injected pump energy and the cooling provided by the cooling water, respectively, this lensing effect depends on the pump power.
FIG. 3
illustrates the initial pulse characteristics for a state-of-the-art “classic resonator”, wherein the spot size at the focal point is simulated without beam expansion as a function of the pump power, i.e. with varying thermal lensing effect and for different radii of the reflecting mirror. The increase of the focal diameter with increasing pump power is clearly seen. Smaller radii of curvature of the reflecting mirror lead to a smaller relative change, but to overall larger values of the focal diameter. For single pulses or for initial pulses (low pump power), the lensing effect is still small. For continuous pulses (high pump power) and a predetermined frequency the lens increases to a value which depends on the average pump power in continuous operation. This lensing effect affects the beam quality and thereby also the spot size in the work plane as well as (to a lesser degree) the pulse energy. The user who is mainly interested in the energy density, i.e. the pulse energy divided by the spot size, will recognize this phenomenon as a strongly variable welding outcome which depends on the welding history.
One possibility to avoid this problem is to transmit the laser beam through a sufficiently long glass fiber. Since the glass fiber does not preserve the diameter of the beam, the beam which is coupled out typically has a constant diameter and an approximately constant divergence. However, this approach degrades the beam quality, so that the focusing unit has to be adapted accordingly. Moreover, the so-called “benign behavior” of the welding process suffers, since the depth of focus in the work plane is reduced. Another possibility to avoid this problem is to use a stronger beam expansion before the beam splitter and to also work outside the focusing range, where the image of the rod surface remains approximately constant. This approach also reduces the initial pulse characteristics. However, the “benign behavior” is again adversely affected (depth of focus of the laser focusing system in the work plane).
The publication by MAGNI, V. et al.: “Recent Developments In Laser Resonator Design” in Optical and Quantum Electronics 23, 1991,pp. 1105-1134, in particular page 1106,second paragraph, describes additional measures to counteract or even compensate the effect of thermal lensing. These conventional measures, however, are only effective at a specified value of the pump power.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a stable resonator which is stable over an extended range of the pump power against the effects caused by thermal lensing, rather than only at a specific value of the pump power. The resonator according to the invention should also reduce the initial pulse characteristics below the detection limit of the user, while at the same time maintaining the “benign behavior”, i.e. the depth of focus, of the laser.
The solution of this object is provided by resonators as described in the commensurate claims
1
,
3
,
4
and
5
. The applicant has realized that, unlike in state-of-the-art devices, the beam quality as a function of the pump power has a comparably flat maximum for relatively short resonator lengths due to the extremely asymmetric construction of the resonators according to the invention. Accordingly, the applicant achieves a comparably constant beam quality over a larger pump power range. As a result, the thermal lensing effect has no effect or only an insignificant effect on the welding result; the characteristics features of the initial pulse are negligibly small. While the laser rod in the embodiments recited in the commensurate claims
1
and
4
is completely displaced towards the output side, in the other advantageous embodiments recited in claims
3
and
5
the laser rod may be located at a very short distance from the output mirror.


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Publication by Magni, V. et al.: “Recent Developments in Laser Resonator Design” in Optical and Quantum Electronics 23, 1991, pp. 1105-1134.
Weber, H: Laserresonatoren und Strahlqualität—Resonators and Beam Quality. In: Laser and Optoelektronik No. Feb. 1988, p. 60-66.
Magni, V., et al.: Recent developments in laser resonator design. In: Optical and Quantum Electronics 23, 1991, p. 1105-1134.
Pavel, N., et al.: Positive-branch unstable resonators with thermal lens compensation. In: Optics & Laser Technology, vol. 28, No. 6, 1996, p. 451-455.
Metcalf, David, et al.: Laser resonators containing self-focusing elements. In: Applied Optics, vol. 26, No. 21, Nov. 1987, p. 4508-4518.

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