Coherent light generators – Optical fiber laser
Reexamination Certificate
1998-06-30
2001-08-07
Schuberg, Darren (Department: 2872)
Coherent light generators
Optical fiber laser
C385S089000, C385S115000, C372S071000
Reexamination Certificate
active
06272155
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a fiber bundle composed of a plurality of optical fibers and a method of manufacturing the fiber bundle.
This invention relates also to a laser apparatus and a laser machining apparatus, each apparatus using the fiber bundle.
In the fields of optical communication and laser machining, it is desired to develop a laser apparatus which is greater in output power and is less expensive. An optical fiber laser apparatus is known for its greater potential to meet the above-mentioned demand.
In the optical fiber laser apparatus, a single transverse mode of laser oscillation can relatively easily be achieved by appropriately selecting a core diameter and a difference in refractive index between a core and a clad. In addition, by confining a light beam at a high density, it is possible to enhance an interaction between a laser active material and the light beam. Furthermore, by increasing the length of an optical fiber, the interaction can be extended so as to produce a high-quality laser beam with a high efficiency. Because of the above-mentioned superior characteristics, it is possible by the use of the optical fiber laser apparatus to obtain at a relatively low cost a laser beam excellent in quality and having a transverse mode free from an influence of the intensity of laser output, heat, and vibration.
In order to realize further increase in output power and efficiency of the optical fiber laser apparatus, it is necessary to efficiently introduce an excitation beam into the optical fiber in a region (typically, a core) where laser active ions, pigments, or other luminescent centers (hereinafter referred to as the “laser active material”) are doped. Generally, when the core diameter is determined so as to satisfy a single-mode waveguide condition, the core diameter is restricted to a value not greater than ten and several micrometers. Therefore, it is generally difficult to efficiently introduce the excitation beam within the core diameter. To overcome the difficulty, proposal is made of a so-called double-clad fiber laser (for example, disclosed in H. Zellmer, U. Willamowski, A. Tunnermann, and H. Welling, Optics Letters, Vol. 20, No. 6, pp. 578-580, March, 1995).
The double-clad fiber laser comprises a core, a first clad surrounding the core and having a first refractive index lower than that of the core, and a second clad arranged outside of the first clad and having a second refractive index lower than the first refractive index. With this structure, an excitation beam introduced into the first clad is kept confined within the first clad during propagation because total internal reflection occurs due to the difference in refractive index between the first and the second clads. During the propagation, the excitation beam repeatedly passes through the core to excite the laser active material contained in the core. In the double-clad fiber laser, the excitation beam is introduced into the first clad. The first clad has a sectional area corresponding to several hundreds to one thousand times that of the core. Therefore, a greater quantity of the excitation beam can be introduced so as to increase the output power.
Thus, the double-clad fiber laser is advantageous in that the oscillation efficiency is high and that the transverse mode of oscillation is a single mode and stable. Therefore, the double-clad fiber laser has a high ability as a machining laser for fine cutting or fine welding.
However, the double-clad fiber laser is disadvantageous in that its laser output is restricted due to the increase of loss resulting from the nonlinear effect such as Brillouin scattering and Raman scattering at the core and to the damage by intense light at the core. With a core material currently available, the double-clad fiber laser is restricted in output power to a range between several tens watts (W) to hundreds and several tens watts (W).
In order to overcome the above-mentioned disadvantage, it is a straightforward idea to increase the core diameter. However, the increase in core diameter of the fiber laser inevitably results in multiple modes of laser oscillation. In case of the multiple modes, the stability of the transverse mode as the advantage of the fiber laser will lost. In this event, the transverse mode of the laser output readily varies due to the intensity of the output, slight vibration of the fiber, and the change in shape. For example in laser machining, this results in unstable distributions of light intensity at a condensing point.
As another approach to compensate the disadvantage of the double-clad fiber laser, it is proposed to use a fiber bundle (a bundle of fibers). This is because a bundle of a plurality of fiber lasers of a single transverse mode provides the increase in output power corresponding to the number of the fibers.
However, if a plurality of fiber lasers of a single transverse mode are simply bundled, each core is surrounded by a clad far greater (about 100 times in diameter) than the core. Therefore, even if the fiber bundle is used in a laser apparatus, the cores as emission points are dotted in a wide spread so that the luminance is decreased.
SUMMARY OF THE INVENTION
In view of the above, it is an object of this invention to provide a fiber bundle in which optical fibers are bundled so that a plurality of laser beams of a single transverse mode are condensed in a small area.
It is another object of this invention to provide a method of manufacturing a fiber bundle, which is capable of bundling a plurality of optical fibers with their cores arranged at a small interval.
It is still another object of this invention to provide a laser apparatus which is capable of producing a laser beam high in luminance and output power with a stable single transverse mode maintained.
it is yet another object of this invention to provide a laser machining apparatus which is capable of carrying out laser machining by the use of a laser beam high in luminance with a stable single transverse mode maintained.
Fiber bundles according to this invention are as follows:
(1) A fiber bundle in which one ends of a plurality of optical fibers are bundled as a bundled portion, the topical fibers including cores, respectively, a laser beam being propagated through each of the cores, the fiber bundle comprising in the bundled portion a clad having a refractive index less than that of each of the cores and surrounding the cores so that the cores are positioned near to each other with a minimum distance left between two adjacent ones of the cores, the minimum distance being decided so that transverse modes of the laser beams propagated through the two adjacent ones of the cores do not affect each other.
(2) A fiber bundle as mentioned in Paragraph (1), wherein different ends of the optical fibers are kept separated as a separated portion without being bundled, the fiber bundle comprising in the separated portion: first clads, each of which has a first refractive index less than that of each of the cores and surrounds each of the cores; and second cores, each of which has a second refractive index less than that of each of the first clads and surrounds each of the first clads.
(3) A fiber bundle as mentioned in Paragraph (2), wherein each of the cores includes a laser active material which produces, when excited by an excitation beam incident to the different end of each of the optical fibers, the laser beam propagated through each of the cores from the different end of each of the optical fibers to the one end of each of the optical fibers.
(4) A fiber bundle as mentioned in Paragraph (1), the laser beam inevitably leaking from each of the cores to the clad as an evanescent wave, wherein the minimum distance is decided so that the evanescent wave leaking from one of the two adjacent ones of the cores do not affect the laser beam propagated through a different one of the two adjacent ones of the cores.
(5) A fiber bundle as mentioned in Paragraph (4), wherein the minimum distance is longer than a first distance and shorter than a secon
Hoya Corporation
Schuberg Darren
Sughrue Mion Zinn Macpeak & Seas, PLLC
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