Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-10-03
2002-06-04
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S099000
Reexamination Certificate
active
06400745
ABSTRACT:
This application is based on application No. H11-305130 filed in Japan on Oct. 27, 1999, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser radiating optical system that shapes a laser beam having an uneven intensity distribution emitted from a light source in such a way as to obtain an even intensity distribution on an irradiation target surface, and in particular to a laser radiating optical system that shapes a laser beam that diverges from a light source in an anisotropic fashion.
2. Description of the Prior Art
Laser beams are characterized by offering high intensity with small beam widths, and are widely used in minuscule machining operations on the surfaces of materials and in data transfer through optical fibers. As light sources that emit laser light, various types are known such as gas lasers as exemplified by carbon dioxide lasers, solid lasers as exemplified by YAG lasers, and semiconductor lasers as exemplified by laser diodes. Any laser light source emits a laser beam that has an uneven intensity distribution, specifically a Gaussian intensity distribution in the single mode.
In machining operations of materials, an object is machined into the shape corresponding to the intensity distribution of the laser beam used. Therefore, the laser beam needs to be controlled so as to exhibit a desired intensity distribution on the object according to the shape into which the object is to be machined. For example, to form a hole having a square cross section and having a uniform depth in the surface of an object, it is necessary to use a laser beam that has a square outline on the sectional plane perpendicular to the optical path and that has an even intensity distribution on that sectional plane. Moreover, the minimum machinable area depends on the beam width of the laser beam used, and therefore it is necessary to give the laser beam a beam width according to the machining precision required. In general, the beam width required in high-precision machining is about a few tens of microns.
When a laser beam is used in data transfer through an optical fiber, which typically has a diameter of a few microns, it is necessary to minimize the loss of laser light by making the beam width still smaller. In addition, an optical fiber has an individual light propagation mode, and a laser beam propagated through an optical fiber has a Gaussian intensity distribution in the single mode. For this reason, if a laser beam emitted from a light source that exhibits a Gaussian intensity distribution is made to have a minuscule beam width simply by being made to converge, it occurs that very little of the laser beam is propagated through the optical fiber when the intensity distribution of the laser beam and the intensity distribution that can be propagated through the optical fiber are out of phase. Moreover, as temperature varies, the degree to which they are out of phase varies, and accordingly the data transferred also varies. This makes it impossible to achieve the desired function.
To prevent this inconvenience, it is necessary to reduce the error in the position at which a laser beam is shone into an optical fiber to about one-tenth of the fiber diameter or less. This requires that the optical fiber and the optical system that directs the laser beam at the optical fiber be positioned precisely relative to each other and that their positions be fixed so as not to vary with temperature. Thus, their alignment requires much time and also a high-precision fixture.
In machining of materials, a laser radiating optical system is used, which is provided with a shaping optical system that makes a laser beam emitted from a light source converge and that converts the intensity of the laser beam in such a way that an even intensity distribution is obtained at the position at which the laser beam converges. Also in data transfer through optical fibers, such a laser radiating optical system can be used so that a laser beam having an even intensity distribution and having a beam width somewhat greater than the fiber diameter is directed to an optical fiber. This makes it possible to keep constant the intensity of the laser beam propagated through the optical fiber even when a small error occurs in the position, relative to the optical fiber, at which the laser beam converges, and is thus expected to make their alignment and fixing easier.
When the aperture through which a light source emits a laser beam is not much different from the wavelength of the laser beam, diffraction occurs in the laser beam traveling out of the aperture and forms the laser beam into a divergent beam that fans out in a conical shape. With gas or solid lasers, the aperture is circular or square, and therefore the laser beam diverges in an isotropic fashion so as to have a substantially circular cross section. Thus, in a laser radiating optical system that employs a gas or solid laser as a light source, it is possible to obtain an even intensity distribution by the use of a shaping element that has an isotropic shaping characteristic.
Usually, to facilitate intensity conversion achieved by a shaping element, a divergent beam emitted from a light source is formed into a parallel beam beforehand by the use of a collimator lens. The shaping element makes the parallel beam converge to make its beam width smaller, and simultaneously converts the intensity distribution of the laser beam so that an even intensity distribution is obtained at the position at which the laser beam converges.
On the other hand, a laser diode has a structure in which a thin active layer is laid between cladding layers so that a laser beam is emitted from a side face of the active layer, and thus has a rectangular aperture. As a result, the angle of diffraction of the laser beam traveling out of the aperture is large in the direction of the shorter sides of the aperture (i.e. in the direction in which the semiconductor layers are laid over one another) and small in the direction of the longer sides thereof. Thus, the laser beam diverges in an anisotropic fashion so as to have an oval cross section. For example, a typical laser diode emits a laser beam whose vertical angle is about 25° in the direction of the shorter sides of the aperture and about 10° in the direction of the longer sides thereof. Quite naturally, with a laser beam that diverges in an anisotropic fashion, whenever it exhibits a Gaussian intensity distribution, its intensity also is distributed in an anisotropic fashion.
However, in a conventional laser radiating optical system, even when a laser diode is used as a light source, no consideration is given to the fact that the laser beam diverges in an anisotropic fashion. That is, even then, the laser beam is formed into a parallel beam by the use of an isotropic collimator lens, and then its intensity distribution is converted by the use of an isotropic shaping element. As a result, some anisotropy remains even at the position at which the laser beam converges, and thus the range in which an even intensity distribution is obtained differs greatly between two mutually perpendicular directions. Accordingly, in applications where a laser beam having an even intensity distribution is shone onto a region extending equally in two mutually perpendicular directions, only part of the area in which an even intensity distribution is obtained is used, and the remaining part is discarded. Thus, the laser light is utilized with quite low efficiency.
This inconvenience is alleviated by using an anisotropic shaping element instead of an isotropic shaping element. However, the shaping performance of a shaping element that serves to make the intensity distribution of a laser beam even depends heavily on the numerical aperture at the exit side of the shaping element. Therefore, even if an anisotropic shaping element is used, it is extremely difficult to make the intensity distribution satisfactorily even in both of two mutually perpendicular directions. T
Jr. Leon Scott
Minolta Co. , Ltd.
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