Patent Application
20160313516
The present invention relates in general to high-power continuous wave (CW) fiber-lasers. The invention relates in particular to directing the outputs of a plurality of such lasers into a single transport fiber.
CW fiber-lasers are rapidly replacing solid-state lasers for laser machining operations, such as metal cutting, where high CW power, for example several kilowatts (kW), is required. CW fiber lasers are commercially available with power output up to about 5 kW.
Such high-power fiber-laser are very complex and require sophisticated arrangements for providing pump-radiation and coupling the pump radiation into a gain-fiber. Gain-fibers must be specially formulated to resist damage through photo-darkening, and resonant-cavity mirrors must be arranged to avoid instability of the laser output through nonlinear effects. A detailed description of such arrangements is provided in U.S. Pre-grant Publication No. 20130028276, the complete disclosure of which is hereby incorporated herein by reference.
Many of the above discussed complexities and sophisticated arrangements can be avoided if the output power of a CW fiber-laser is limited to about 1 kW. Were a means available to combine the outputs of a plurality of such 1 kW lasers into single transport fiber, without significant power loss or degradation of beam quality, high-power cutting operations could be performed without the need for a multi-kW fiber-laser.
In one aspect, optical apparatus in accordance with the present invention comprises a plurality N of fiber lasers each thereof emitting a diverging beam having first numerical aperture, and a transport optical fiber. The apparatus includes a collimator assembly including N juxtaposed lens-segments each thereof having an optical-axis and arranged about a geometric axis of collimator assembly. The optical-axes of the lens segments and the geometric-axis of the collimator assembly are parallel to each other. The optical-axes of the lens-segments are arranged on a circle having a radius centered on the geometric-axis of the collimator assembly. Each of the optical fibers is aligned with the optical axis of a corresponding one of the lens segments such that the diverging beams of the fiber lasers are collimated by the collimator assembly. A focusing lens is arranged to focus the collimated beams combined into the transport optical fiber, with combined focused beams having a second numerical aperture.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.
Referring now to the drawings,
Apparatus 10 includes a composite lens 20, including three juxtaposed lens segments 22, 24, and 26, each having positive optical power, which are arranged to collimate the output beamlets of fibers 12, 14, and 16 respectively. The collimated beamlets are each focused by a positive (focusing) lens 32 into optical fiber 18. In the drawing of
In this embodiment, the three optical fibers carrying the laser radiation are arranged so that the axis of the output beams exiting the ends of the fibers are parallel. Further, the ends of the fibers are arranged on the three points of an imaginary equilateral triangle.
One method of constructing composite lens 20 is to fabricate three circular lenses which are then cut with straight sides (edges) at 120° to each other to form the segments. Juxtaposition of the segments is such that the straight edges of the segments meet on the geometric-axis of the composite lens. The segments can then be bonded together or held together in a mechanical frame, whichever is convenient. It is also possible to make the composite lens as a single element. Segmented lens arrays in a single element can be generated by Power Photonic Ltd, of Dalgety Bay UK.
The lens segments are spaced from the fiber-lasers by a distance f1, which is about the focal length of the lens segments. Accordingly the beams are collimated by the assembly centered on the optic-axes of the lens segments. The optic-axes are parallel to the geometric axis 30 of composite lens 20. Transport fiber 18 is spaced apart from focusing lens 32 by a distance f2 which is about focal length of lens 32. The composite and focusing lenses are preferably spaced apart from each other by a distance of about f1+f2. Also depicted are the beamlet input NA, i.e., the fiber output NA), the beamlet output NA, and the combiner output NA. The NA of a diverging or converging beam, as is known in the art, is the Sine of the convergence or divergence half-angle. Exemplary NA values specified herein are assumed to be measured at the 1/e2 points of a beam.
An initially discouraging aspect of the inventive combiner arrangement, evident from the illustration of
In order to quantify the consequences of this BPP increase, an extensive study was performed of the interdependence of the various parameters depicted in
In the graph, black dot X is located at a BPP-ratio of about 1.9, with combining losses of only about 95%. Here, the beamlet NA is about 0.045 with optical axes of the beamlets located on a circle of radius (δ) equal to about 0.05 mm about geometrical axis 30 of composite lens 20. The NA of the focused beams is about 0.09.
The graph indicates that a compromise is necessary between BPP-ratio and throughput. High throughput (low losses) comes at the expense of large increase of BPP. A small BPP increase comes at the cost of large losses. Black dot X represents a good compromise, where the losses are 5% and the increase in BPP (BPP ratio) is 1.9. High throughput is obtained by selecting a low value for the ratio of the output NA to displacement δ, so as to avoid clipping a beam into the “wrong” lens-segment, but a high value of δ increases the divergence of the combined output beam and the BPP-ratio.
While the present invention is described above in terms of combining the outputs of three fiber-lasers, in principle it is possible to combine the outputs of four fiber lasers using a four-segment lens assembly, five fiber lasers using a five segment assembly, and so on. In order to do this however, the radius δ of the circle on which the optic axes of the segments are arranged would need to be progressively increased with the number of fibers and segments. The above-discussed parametric analysis of
Here, the beamlet NA is again about 0.045 but with optical axes of the beamlets located on a circle of radius (δ) equal to about 0.067 mm about geometrical axis 30 of composite lens 20. The NA of the focused beams is about 0.105.
The BBP increase of 2.3 is near a margin of usefulness. A similar analysis for combining the outputs of 5 fiber-lasers with a five-segment lens requires a BPP increase of 2.6 to restrict losses to 5% with a loss of brightness (Power/BPP2) of 30%. This is probably too high a penalty to pay for the beam-combination.
In summary, the present invention is described above in terms of a preferred and other embodiments. The invention however is not limited by the embodiments described and depicted herein. Rather the invention is limited only by the claims appended hereto.
