Solid-state laser oscillator and machining apparatus using...

Details Solid-state laser oscillator and machining apparatus using... Solid-state laser oscillator and machining apparatus using...

1999-05-18

2001-09-04

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular resonant cavity

Specified cavity component

C372S092000, C372S105000

Reexamination Certificate

active

06285705

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a solid-state laser oscillator which can stably generate a laser beam with a high power output and exhibiting a high quality factor (Q), and a machining apparatus using the same.
FIG. 5
is a schematic view showing the configuration of an oscillator section of a conventional solid state laser device which has been used to oscillate a laser beam with high quality (Q). In
FIG. 5
, reference numeral
1
denotes a rod-like solid-state element (host), e.g., a yttrium aluminum garnet (YAG) crystal doped with neodymium (Nd), i.e., Nd: YAG laser. Reference numeral
2
denotes an excitation light source, e.g., Krypton arc lamp, Xenon flash lamp, etc. Reference numeral
4
denotes a condenser formed so as to enclose the solid-state element
1
and excitation light source
2
. Numeral
31
denotes a partial reflecting mirror and numeral
32
denotes a total reflecting mirror.
FIG. 6
is a sectional view of a laser oscillator which is directed to the prior art for stabilizing a laser oscillator with high quality Q as disclosed in Solid-State Laser Engineering, 2nd Edition, Springer-Verlag, pp. 192 to 193. Reference numerals
11
and
12
denote first and second rod-like solid-state elements, respectively, and reference numerals
21
and
22
denote first and second excitation light sources, respectively. Reference numeral
31
denotes a partial reflecting mirror;
32
a total reflecting mirror;
60
a 90° crystal optical rotator; and
61
a Brewster window.
Referring to
FIG. 5
, a conventional laser oscillator is described as follows. It is well known that the quality of a laser beam improves as the ratio of a beam in a solid-state element to that of a Gaussian beam calculated theoretically in the solid-state element decreases. To increase the laser output, the length of the resonator can be increased, or an aperture can be made in the resonator to restrict oscillation to the lowest order of transverse mode (TM
00
) in the solid-state laser cavity, and thereby obtain a high Q. An “offset” laser resonator can be employed to boost output power and energy by using a reflecting mirror with a small radius of curvature, e.g., lm or less, typically 0.1-0.5 m, where the laser beam is converged to a small spot at the front surface of the reflecting mirror. A convex partial reflecting mirror can also be arranged in the vicinity of the solid-state element to form a reflecting mirror, increasing the effective length of the laser cavity by substantially several meters, taken together with a convex thermal lens effect produced by the solid-state element.
In an experiment by the inventors of the present invention, where the resonator was structured as above, such that the diameter of the Gaussian beam theoretically calculated in the vicinity of the solid-state element was increased to, e.g., about ⅕ the diameter of the solid-state element, a laser beam with high quality (Q) could be obtained that was about {fraction (1/20)} the refraction limit, i.e., a transverse mode order of about 20 that is about {fraction (1/10)} that of a normal laser oscillator.
However, this resonator structure has the problem in that it gives rise to a reduction of oscillation efficiency and fluctuation in the laser output. This is notable in the case where the resonator is operated with a high output of 100 W or larger, in which distortion of the solid-state element is increased. This tendency to distortion is noticeable as the quality of the beam is increased.
FIG. 7
graphically shows one example of the oscillation characteristic acquired in an experiment on the conventional solid-state laser oscillator. In the graph, line A illustrates the oscillation characteristic of a laser beam with poor beam quality, e.g., about {fraction (1/200)} times a theoretical limit, i.e., having the transverse mode order of 200, and line B illustrates that of a laser beam with high beam quality having the transverse mode order of 20. The line B does not exhibit a linear oscillation characteristic but a curved characteristic including several peaks. It can be seen that the fluctuation of the output is notable at the sections where there are valleys in the oscillation curve, i.e., under the condition where the output is relatively low.
On the other hand, in the prior art shown in
FIG. 6
, it is known that with a first and a second rod-like solid-state element
11
and
12
arranged in tandem, and a 90° crystal rotator
60
located at the center between these solid-state elements, if the incidence of birefringence generated by the first solid-state element is canceled out by the second solid-state element, a laser beam with a stabilized output and with high efficiency can be obtained. Specifically, birefringence refers to the effect of causing two polarization components orthogonal to each other to discern different refractive indices owing to thermal stress generated in the solid-state element. Thus, the laser beam incident on the birefringent solid-state element will discern either one of two kinds of thermal lenses according to its polarization direction.
The 90° crystal rotator
60
rotates the polarized light of the laser beam which has permeated through the first solid-state element
11
, and causes it to be incident on the second solid-state element
12
. Thus, the laser beam incident on the first solid-state element
11
equally discerns two kinds of thermal lenses when it has passed through both solid-state elements. Accordingly, with the laser beam polarized in either polarization direction, and with birefringence being exhibited by both the solid-state elements, the laser beam discerns both thermal lenses in the combination of the two solid-state elements, and two polarized beams oscillate under substantially the same conditions to provide an effect as if the separation of polarized light by birefringence of the solid-state elements has been canceled out.
The prior art shown in
FIG. 6
was designed to drive a linear polarized light efficiently and stably. In addition, according to the experiment carried out by the inventors of the present invention, it was also confirmed that in a resonator in which linear polarized light is not the objective and a Brewster window is not present, careful insertion of a 90° crystal rotator
60
between two solid-state elements
11
and
12
to cancel the influence of birefringence can improve the efficiency of the laser oscillation. The oscillation characteristic illustrated by curve C as shown in
FIG. 7
, provides an oscillation waveform with no substantial fluctuation.
A theoretical explanation follows regarding the difference between the prior art shown in
FIGS. 5 and 6
. First, in the prior art shown in
FIG. 5
, the solid-state element is excited by the environment and becomes thermally deformed. For example, the solid-state element with a circular section provides a difference in extension of the crystal and change in the refractive index between a diameter direction and a radial direction of the section. The directions of the two extensions are orthogonal to each other, to provide two lens functions in the respective directions. Therefore, the laser beams having two basic polarization modes; i.e., polarization components shown in
FIGS. 8A and 8B
, are subjected to different dimensions of lens function when they pass through the solid-state element. For this reason, the diameters of the Gaussian beam theoretically calculated within the solid-state element, for the output of an excitation light source, can be plotted as two curves of B
1
and B
2
, as shown in
FIG. 9B
, for the laser beams having the basic polarization modes shown in
FIGS. 8A and 8B
. The section represented by the two curves with diameters B
1
and B
2
of the Gaussian beam, is an area where oscillation can occur, which can be calculated for the respective basic polarization modes. In the other areas, oscillation does not occur owing to great losses in the resonator.
A comparison between the oscillation characteristic represented by curve B in FIG.
7
an

Affiliated with

Also associated with

Rating

Solid-state laser oscillator and machining apparatus using... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Solid-state laser oscillator and machining apparatus using..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solid-state laser oscillator and machining apparatus using... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2538928