Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
1999-02-24
2001-08-21
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S640000, C359S678000
Reexamination Certificate
active
06278557
ABSTRACT:
CROSS REFERENCES TO CO-PENDING APPLICATIONS
None.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to beam shaping, and, more particularly, to beam stacking devices and methods for stacking beams which change the shape of a beam from a highly asymmetric non-diffraction-limited laser source, such as a high power laser diode bar, to a more symmetric, and thus more desirable and useful, profile.
2. Description of the Prior Art
While semiconductor lasers are widely used in both scientific and consumer devices, the relatively low power output of such devices has limited their use. The output power of semiconductor lasers has been increased by several different techniques, one being fabricating a multiple diode device in the form of an array of low power laser diodes on a single chip. While it is possible to fabricate such a multiple diode device having, for example, 10 laser diodes, the output of such a device is a beam having a highly elliptical shape or profile. That is, the output beam is much wider in the dimension corresponding to the necessary spacing between the laser diodes than in the dimension corresponding to the width of a single laser diode. In other words, the output beam has dramatically unequal M
2
dimensions in two orthogonal directions, thus resulting in a beam which is highly asymmetrical.
High power laser diode bars are attractive as optical sources for applications in pumping solid state lasers, in materials processing, and in the medical field. Unfortunately, the inconvenient shape, i.e., highly elliptical nature, of the output beam of laser diode bars makes them, by themselves, inappropriate for many of the uses which require a higher power output than is attainable from a single laser diode source. However, this has not been a fatal shortcoming because there exist techniques for reconfiguring the shape or profile of the output beam to make it less elliptical in nature; that is, to create an output beam whose dimensions in two orthogonal directions are more nearly equal, thus giving rise to an output beam which is substantially symmetrical. One such technique is the beam stacking device disclosed in European patent number EP0731932 and in corresponding International Application (PCT) number WO 95/15510, and it is that sort of technique which the present invention serves to improve.
The beam stacking device of European patent number EP0731932 and International Application number WO 95/15510 consists of two high reflectivity planar mirrors aligned approximately parallel and separated by a small distance. The two mirrors are offset transversely from each other in two directions so that small sections of each mirror are not obscured by the other. These unobscured sections form input and output apertures of the beam stacking device. The action of the beam stacking device is to chop the incident beam into a specific number of chopped parts and then to redirect and reposition these chopped parts so that they emerge from the beam stacking device one on top of another. Although this beam stacking device does reconfigure the output beam to a more favorable profile, it nevertheless has several disadvantages, the principal disadvantage being that, because the incident beam is chopped many times and many reflections are needed between the mirrors, losses due to reflection are great and, therefore, the transmission is low.
SUMMARY OF THE INVENTION
The general purpose of the present invention is to provide new beam stacking devices and methods of beam stacking which overcome the disadvantages of the prior art. This is achieved through the use of a prism assembly including entrance and exit apertures and total internal reflection surfaces.
More particularly, the beam stacking devices of this invention utilize a prism assembly into which an elliptical incident beam to be shaped, i.e., an incident beam effectively having greatly different M
2
parameters in two directions, is introduced through an entrance aperture and then sequentially reflected by four total internal reflection surfaces of the prism assembly such that on each round trip a chopped part of the beam emerges from an exit aperture and is stacked on top of the preceding chopped part. The lengths of the four total internal reflection surfaces provide for inward displacement of adjacent parts of the input beam incident on the entrance aperture, thereby compressing the input beam in one orthogonal direction. The width of the exit aperture provides the dimension of the emergent beam along the other orthogonal direction and is selected so that the input beam is expanded in the other orthogonal direction. Consequently, in comparison with the input beam at the entrance aperture, the output or emergent beam at the exit aperture has a reduced orthogonal dimension in one orthogonal direction and an increased orthogonal dimension in the other orthogonal direction. Stated in another manner, the effect of the beam stacking device is to decrease the size of the highly elliptical input beam along one orthogonal direction and to increase the size of the highly elliptical input beam along the other orthogonal direction, thereby reconfiguring the highly asymmetrical input beam into an output beam which is substantially symmetrical. Since the first round trip of the incident beam involves four total internal reflections, and each succeeding round trip involves another four additional reflections, the losses due to reflection are minimized. For example, in accordance with each of two embodiments of the invention, an elliptical incident beam comprising the output of a five laser diode bar can be combined to produce a single, more favorably shaped output beam with a total of only 55 internal reflections and, in another embodiment of the invention, with a total of only 60 internal reflections.
In one embodiment, a beam stacking device is formed of a prism assembly consisting of a right angle prism which provides the entrance aperture, and a rectangular parallelepiped prism with one edge cut away which provides the four total internal reflection surfaces and the exit aperture.
In a second embodiment, a beam stacking device is formed that includes two right angle prisms. One right angle prism provides the entrance aperture and two of the four total internal reflection surfaces. The other right angle prism, which has a portion cut away for the path of the entrance, provides the exit aperture and the other two of the four total internal reflection surfaces.
In a third embodiment, a beam stacking device consists of two prisms. One prism provides the entrance aperture and two of the four total internal reflection surfaces. The other prism provides the exit aperture, the other two of the four total internal reflection surfaces, and the path of the entrance.
In all of the embodiments the beam stacking devices permit focusing beams to small diameters, i.e., more nearly equal orthogonal dimensions, without significantly decreasing the brightness. The total internal reflection surfaces improve the quality of the transmission. The number of stacked beams is a function of the incident beam width and the size of the exit aperture. The shift of stacking beams in one direction is implemented by the size difference of the total internal reflection surfaces.
The entrance and exit apertures of the various embodiments can be switched, and can be implemented by cut away surfaces or by additional components such as right angle prisms.
REFERENCES:
patent: 5339186 (1994-08-01), Weverka
patent: PCT/GB94/02614 (1994-11-01), None
Epps Georgia
Jaeger Hugh D.
Semiconductor Laser International Corporation
Seyrafi Saud
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