Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-05-07
2001-04-24
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S108000, C359S569000, C359S711000, C359S719000
Reexamination Certificate
active
06222864
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to optical systems forming a coherent light source of high power and brightness, and in particular to such sources that include a high brightness, semiconductor laser diode, such as a single-spatial-mode broad area laser diode, a flared-resonator-type (unstable resonator) laser diode or a MOPA device, in combination with astigmatism correcting optics for that source.
BACKGROUND OF THE INVENTION
In U.S. Pat. No. 5,321,718, Waarts et al. describe a coherent light source having an astigmatism-correcting lens system positioned in the path of a high power, but astigmatic, coherent light beam from a semiconductor optical source such as a flared-amplifier-type MOPA device or a flared-resonator-type laser diode. A number of lens configurations are described, which include combinations of cylindrical and spherical lens surfaces. While most of the embodiments use from two to four lenses, one embodiment employs a single lens with two crossed positive cylinder lens surfaces. Another embodiment uses a lens having a positive toric surface and a planar surface. All of the lens systems are adapted to provide a modified astigmatism-free light beam from the astigmatic light received from the semiconductor optical source. The astigmatism-free light is useful for many laser applications, including frequency conversion, of which a number of configurations are disclosed.
In U.S. Pat. No. 5,369,661, Yamaguchi et al. disclose an optical system for coupling light from a semiconductor laser array into a solid-state laser medium or into an optical fiber. The optics include a gradient index (GRIN) lens array to condense the individual light beams emitted with a large divergence angle from the semiconductor laser array to form parallel collimated light beams. A separate aspherical lens then converges the light beams into a single beam spot. Stacks of two or more laser arrays with corresponding stacks of two or more GRIN lens arrays are also disclosed, which form a 2-D array of parallel light beams. An aspherical lens then condenses the array of light beams to a beam spot for coupling to a fiber. Plural sets of stacked arrays may be combined by arranging their respective optical fibers to form a fiber bundle.
U.S. Pat. No. 5,229,883 to Jackson et al., U.S. Pat. No. 5,081,639 to Snyder et al., and U.S. Pat. No. 5,293,269 to Burkhart et al. disclose lens optics for collimating the diverging light output from diode lasers and diode laser arrays. Jackson et al. use a first cylindrical lens for collimating the light in the fast axis (or transverse direction) and a second binary diffractive optical element or array of such elements, simulating one or more aspheric lens surfaces, for collimating the light in the slow axis (or lateral direction). Snyder et al. use a cylindrical lens having an elliptical or hyperbolic cross-section, while Burkhart et al. use a lens with a circular-cylindrical back surface and an acircular-cylindrical front surface. Both of these cylindrical lenses are formed by means of a fiber lens drawing process from a master or preform having the desired cross-section.
In U.S. Pat. No. 5,216,687, Fujino et al. employ a spherical first lens or a GRIN lens array for collimating the light from a semiconductor laser or laser array in the fast axis, and a bicylindrical second lens with crossed (orthogonally oriented) cylindrical surfaces for focusing the light in both its fast and slow axes to a spot.
In providing lens optics for semiconductor laser sources that emit highly astigmatic light beams, such as flared-resonator-type laser diodes or flared-amplifier-type MOPAs, it is desirable that the optics not only correct for the astigmatism in the light, but also be compact, have a minimum number of refracting surfaces within the constraints of manufacturability, be easily positioned in front of the laser source at the proper locations within the design tolerances, and preferably be inexpensive to make. A minimum loss of brightness is preferred, so that numerical aperture is an important design parameter. Likewise, when arrays of such astigmatic laser sources are used, the corresponding lens arrays need to provide a precise center-to-center spacing between lenslets and be designed, if possible, for maximum beam filling of the emitted array of light beams. Unfortunately, many of these requirements conflict so that trade-offs must be made. A theoretical design calculated from purely optical considerations may include lens surfaces which are difficult and very expensive to manufacture. If the design is limited to easily manufactured lenses with circular-cylindrical and spherical lens surfaces, multiple lenses are required, which must be precisely positioned, and which generally limit the numerical aperture and beam filling factor that are achievable, thus reducing brightness.
An object of the invention is to provide a coherent light source in which the astigmatism-correcting lens optics for high power semiconductor laser sources that emit astigmatic light beams preserve the brightness of the emitted light, while being compact, inexpensively manufacturable and easily positioned for astigmatism-correction and beam collimation. Another object of the invention is to provide an astigmatism-correcting lens array for a diode laser array that is inexpensive to manufacture with maximum beam filling and brightness conservation of the array of emitted beams.
SUMMARY OF THE INVENTION
The objects of the invention are met with a coherent light source comprising a semiconductor optical source generating and emitting a high power coherent light beam that is astigmatic, and a single astigmatism-correcting lens positioned in the path of the light beam, where this single lens has a first acircular-cylindrical or toroidal lens surface and a second aspheric or binary diffractive lens surface. The objects are also met with a coherent light source comprising a semiconductor optical source generating and emitting an array of high power coherent light beams, each of which is astigmatic, and a single astigmatism-correcting lens array positioned in the paths of the light beams, such that each lenslet is aligned with a corresponding laser light emitter of the source, and where a first surface of the lens array is either an acircular cylinder extending across the width of the array or an array of toroidal lens surfaces aligned with the light beams. The second lens array surface is an array of either aspheric or binary lens elements.
The toroidal surfaces of these lenses or lens arrays can be made easily and inexpensively with a mold, in which either the mold itself or a master for the mold is machined using a technique that involves cutting with a diamond-tipped cutting tool into the circumferential surface of a cylindrical blank mounted on a rotating spindle. The depth of the cut varies axially to create a toroidal surface or an array of toroidal surfaces whose axial cross-section can be acircular. The section cut perpendicular to the master's rotation axis is necessarily circular. When the mold itself is machined in this way, the toroidal surface formed by the diamond turning technique is a negative of the resulting toroidal lens surface. When the machined surface is used as a master to create the mold, the original toroidal surface of the master can be a positive of the final lens surface created by the mold.
The semiconductor optical source that is combined with the single astigmatism-correcting lens may be included in an optical cavity having a gain region with a lateral dimension along its length in the cavity that is greater than a lateral dimension of the light path along other portions of the optical cavity. For example, it can be a flared-amplifier-type MOPA device, a flared-resonator-type (unstable resonator) laser diode, or some other laser diode with a light diverging region therein. Alternatively, it may be a wide-area laser diode with an angled DFB grating or any other spatially coherent source with strong astigmatism.
The coherent light sources with
Endriz John
Lang Robert J.
Osinski Julian S.
Waarts Robert G.
Wolak Edmund L.
Arroyo Teresa M.
Carothers, Jr. W. Douglas
Leung Quyen Phan
SDL Inc.
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