1. Field of the Invention
The present invention relates to semiconductor diode lasers and more particularly to brightness enhancement of semiconductor laser diodes utilizing stimulated Raman scattering.
2. Description of the Prior Art
Semiconductor laser diodes are known in the art. Such semiconductor laser diodes are used in various applications including optical pumping of solid state lasers. Examples of such semiconductor diode lasers used in such applications are disclosed in commonly owned U.S. Pat. Nos. 5,555,254; 5,646,773; 6,094,297; and 6,178,040. Such semiconductor laser diodes are used to optically excite the Nd:YAG slabs to a relatively high energy meta-stable state.
Techniques are known for increasing the output power and brightness or intensity of such semiconductor laser diodes. For example, U.S. Pat. No. 5,764,675 discloses a semiconductor diode laser array that optimizes the laser output power by optimizing the heat removal from the diode laser array. In particular, the '675 patent discloses a diode laser array which includes a diode laser bar that is thermally connected to a heat exchanger. With such configuration, waste heat removal is optimized, thereby allowing the brightness of the semiconductor diodes to be increased.
In order to further increase the brightness of such semiconductor laser diodes, the coupling efficiency of the output light beam from the semiconductor diode has been improved. For example, U.S. Pat. No. 5,299,222 discloses a system for optically coupling the output light beam from a semiconductor laser array to a plurality of optical fibers; each optical fiber being coupled to an individual semiconductor diode laser within the array. A plurality of mirrors and lenses is used to optically couple the output light beams from each of the semiconductor diodes forming the array into a respective optical fiber.
The brightness or intensity level of such semiconductor diode lasers has been further increased by combining the individual output light beams from a semiconductor laser diode array into a single compound beam having a higher brightness than the brightness of an output light beam of any of the individual semiconductor diode lasers. In particular, U.S. Pat. Nos. 5,668,903; 5,793,783; and 6,005,717 disclose various techniques for combining the output light beams of a plurality of semiconductor diode lasers into a compound light beam having a relatively high brightness. In particular, the '903 patent discloses a semiconductor diode array which includes a plurality of lenses for focusing the output of each semiconductor diode laser into an optical fiber with a circular cross section. The plurality of optical fibers, in turn, is coupled to an optical combiner, formed from a optical waveguide having a cross section sufficient to encompass the individual cross sections of all of the optical fibers optically coupled thereto. The output of the optional fiber is a single output light beam having a brightness much greater than any of the individual output light beams of the semiconductor laser diode. The '903 patent also discloses specially formed optical waveguides to further improve the coupling efficiency of the output of the optical combiner to a downstream device.
U.S. Pat. No. 5,793,783 discloses yet another method to provide enhanced brightness of a semiconductor laser diode. In particular, the '783 patent discloses a system which includes a semiconductor laser diode array in an optical system for focusing the individual output light beams from each of the semiconductor laser diodes in the semiconductor diode laser array into a single light spot. The optical subsystem includes a cylindrical lens configured with a length equal to the length of the semiconductor laser diode array. The cylindrical lens is used to reduce the transverse divergence and thus collimate the light from the individual semiconductor laser diodes in a particular direction. A lens array is provided downstream of the cylindrical array lens. The lens array includes a single lenslet for each semiconductor diode in the array. The lens array further reduces the divergence of the light beam. A focusing lens 19 is used to converge the output light beams from each of the individual lenslets in the lens array to a single light spot having a relatively increased brightness.
U.S. Pat. No. 6,005,717 also discloses a technique for increasing the brightness of an output beam of a semiconductor diode by combining the output beams of a array of semiconductor diodes providing a combined beam with increased brightness and power density. The '717 patent is similar to the '783 patent and utilizes a cylindrical lens and a lens array for focusing the output light beams from the array of semiconductor diodes to a single light spot by way of a focusing lens.
Although the systems discussed above provide a semiconductor diode array having increased brightness, these systems require fairly accurate optical alignment in order to produce an output beam having increased brightness. In addition, such prior art systems require a number of lenses which increase the cost of the system. Thus there is a need for a method for increasing the brightness of a semiconductor diode which is less expensive and does not require relatively precise alignment.