# Patent

## Title
Fiber-based output couplers for wavelength beam combining laser systems

## Abstract

In various embodiments, wavelength beam combining laser systems incorporate optical fibers and partially reflective output couplers or partially reflective interfaces or surfaces utilized to establish external lasing cavities.

## Background
RELATED APPLICATION
This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/051,523, filed Sep. 17, 2014, the entire disclosure of which is hereby incorporated herein by reference.


TECHNICAL FIELD
In various embodiments, the present invention relates to laser systems, specifically wavelength beam combining laser systems having fiber-based output coupling schemes.
BACKGROUND
High-power laser systems are utilized for a host of different applications, such as welding, cutting, drilling, and materials processing. Such laser systems typically include a laser emitter, the laser light from which is coupled into an optical fiber (or simply a “fiber”), and an optical system that focuses the laser light from the fiber onto the workpiece to be processed. The optical system is typically engineered to produce the highest-quality laser beam, or, equivalently, the beam with the lowest beam parameter product (BPP). The BPP is the product of the laser beam's divergence angle (half-angle) and the radius of the beam at its narrowest point (i.e., the beam waist, the minimum spot size). The BPP quantifies the quality of the laser beam and how well it can be focused to a small spot, and is typically expressed in units of millimeter-milliradians (mm-mrad). A Gaussian beam has the lowest possible BPP, given by the wavelength of the laser light divided by pi. The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is denoted M2, or the “beam quality factor,” which is a wavelength-independent measure of beam quality, with the “best” quality corresponding to the “lowest” beam quality factor of 1.
Wavelength beam combining (WBC) is a technique for scaling the output power and brightness from laser diode bars, stacks of diode bars, or other lasers arranged in one- or two-dimensional array. WBC methods have been developed to combine beams along one or both dimensions of an array of emitters. Typical WBC systems include a plurality of emitters, such as one or more diode bars, that are combined using a dispersive element to form a multi-wavelength beam. Each emitter in the WBC system individually resonates, and is stabilized through wavelength-specific feedback from a common partially reflecting output coupler that is filtered by the dispersive element along a beam-combining dimension. Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed on Feb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat. No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107, filed on Mar. 7, 2011, the entire disclosure of each of which is incorporated by reference herein.
A variety of WBC techniques have been utilized to form high-power lasers for a host of different applications, and such techniques often involve the formation and in-coupling of the resulting multi-wavelength beams into optical fibers. However, such schemes are often quite complex and require several different optical components for producing the multi-wavelength output beam (via the external WBC cavity), coupling the beam into the fiber, and aligning resonator modes to the core of the fiber, when some of the resonator modes may be lasing in particular modes outside the preferred modes of the fiber and/or its core. Thus, there is a need for improved, simpler optical arrangements that facilitate the integration of WBC laser systems with output (or delivery) optical fibers.
SUMMARY
In accordance with embodiments of the present invention, wavelength beam combining (WBC) laser systems feature multiple emitters (or “beam emitters”), e.g., diode bars or the individual diode emitters of a diode bar, which are combined using a dispersive element to form a multi-wavelength beam. Each emitter in the system individually resonates and is stabilized through wavelength-specific feedback from a common partially reflecting (or “partially reflective”) output coupler that is filtered by the dispersive element (e.g., a diffraction grating, a dispersive prism, a grism (prism/grating), a transmission grating, or an Echelle grating) along the beam-combining dimension. In this manner, laser systems in accordance with embodiments of the present invention produce multi-wavelength output beams having high brightness and high power.
In accordance with various embodiments of the present invention, the output (or delivery) fiber, or a portion thereof, is utilized to replace the discrete output couplers of conventional WBC systems. Thus, a portion of or an interface with the fiber is itself partially reflective and utilized, in conjunction with a reflective surface of each beam emitter, to form and stabilize the external WBC laser cavity. Thus, component count and alignment complexity of fiber-coupled WBC laser systems are reduced, and efficiency of such systems is enhanced. The output coupler/feedback mechanism within the delivery fiber may include or consist essentially of, for example, a fiber Bragg grating within the core of the optical fiber. In other embodiments, a partially reflective coating at the input face of the fiber, or between the input face and an end cap attached thereto, or between two fiber segments, stabilizes the WBC laser via Fresnel reflection of the incoming beams.
Embodiments of the present invention couple the one or more input laser beams into an optical fiber. In various embodiments, the optical fiber has multiple cladding layers surrounding a single core, multiple discrete core regions (or “cores”) within a single cladding layer, or multiple cores surrounded by multiple cladding layers.
Herein, “optical elements” may refer to any of lenses, mirrors, prisms, gratings, and the like, which redirect, reflect, bend, or in any other manner optically manipulate electromagnetic radiation. Herein, beam emitters, emitters, or laser emitters, or lasers include any electromagnetic beam-generating device such as semiconductor elements, which generate an electromagnetic beam, but may or may not be self-resonating. These also include fiber lasers, disk lasers, non-solid state lasers, vertical cavity surface emitting lasers (VCSELs), etc. Generally, each emitter includes a back reflective surface, at least one optical gain medium, and a front reflective surface. The optical gain medium increases the gain of electromagnetic radiation that is not limited to any particular portion of the electromagnetic spectrum, but that may be visible, infrared, and/or ultraviolet light. An emitter may include or consist essentially of multiple beam emitters such as a diode bar configured to emit multiple beams. The input beams received in the embodiments herein may be single-wavelength or multi-wavelength beams combined using various techniques known in the art.
Laser diode arrays, bars and/or stacks, such as those described in the following general description may be used in association with embodiments of the innovations described herein. Laser diodes may be packaged individually or in groups, generally in one-dimensional rows/arrays (diode bars) or two dimensional arrays (diode-bar stacks). A diode array stack is generally a vertical stack of diode bars. Laser diode bars or arrays generally achieve substantially higher power, and cost effectiveness than an equivalent single broad area diode. High-power diode bars generally contain an array of broad-area emitters, generating tens of watts with relatively poor beam quality; despite the higher power, the brightness is often lower than that of a broad area laser diode. High-power diode bars may be stacked to produce high-power stacked diode bars for generation of extremely high powers of hundreds or thousands of watts. Laser diode arrays may be configured to emit a beam into free space or into a fiber. Fiber-coupled diode-laser arrays may be conveniently used as a pumping source for fiber lasers and fiber amplifiers.
A diode-laser bar is a type of semiconductor laser containing a one-dimensional array of broad-area emitters or alternatively containing sub arrays containing, e.g., 10-20 narrow stripe emitters. A broad-area diode bar typically contains, for example, 19-49 emitters, each having dimensions on the order of, e.g., 1 μm×100 μm. The beam quality along the 1 μm dimension or fast-axis is typically diffraction-limited. The beam quality along the 100 μm dimension or slow-axis or array dimension is typically many times diffraction-limited. Typically, a diode bar for commercial applications has a laser resonator length of the order of 1 to 4 mm, is about 10 mm wide and generates tens of watts of output power. Most diode bars operate in the wavelength region from 780 to 1070 nm, with the wavelengths of 808 nm (for pumping neodymium lasers) and 940 nm (for pumping Yb:YAG) being most prominent. The wavelength range of 915-976 nm is used for pumping erbium-doped or ytterbium-doped high-power fiber lasers and amplifiers.
A diode stack is simply an arrangement of multiple diode bars that can deliver very high output power. Also called diode laser stack, multi-bar module, or two-dimensional laser array, the most common diode stack arrangement is that of a vertical stack which is effectively a two-dimensional array of edge emitters. Such a stack may be fabricated by attaching diode bars to thin heat sinks and stacking these assemblies so as to obtain a periodic array of diode bars and heat sinks There are also horizontal diode stacks, and two-dimensional stacks. For the high beam quality, the diode bars generally should be as close to each other as possible. On the other hand, efficient cooling requires some minimum thickness of the heat sinks mounted between the bars. This tradeoff of diode bar spacing results in beam quality of a diode stack in the vertical direction (and subsequently its brightness) is much lower than that of a single diode bar. There are, however, several techniques for significantly mitigating this problem, e.g., by spatial interleaving of the outputs of different diode stacks, by polarization coupling, or by wavelength multiplexing. Various types of high-power beam shapers and related devices have been developed for such purposes. Diode stacks may provide extremely high output powers (e.g. hundreds or thousands of watts).
In an aspect, embodiments of the invention feature a laser system that includes or consists essentially of an array of beam emitters each emitting a beam, focusing optics for focusing the beams toward a dispersive element, a dispersive element for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam, an optical fiber for receiving the multi-wavelength beam, and a fiber Bragg grating disposed within the optical fiber. The fiber Bragg grating receives the multi-wavelength beam, reflects a first portion thereof back toward the dispersive element, and transmits a second portion thereof as an output beam composed of multiple wavelengths.
Embodiments of the invention may include one or more of the following in any of a variety of combinations. The dispersive element may include or consist essentially of a diffraction grating (e.g., a transmissive diffraction grating or a reflective diffraction grating). The focusing optics may include or consist essentially of one or more cylindrical lenses, one or more spherical lenses, one or more spherical mirrors, and/or one or more cylindrical mirrors. A portion of the optical fiber may be disposed within an external lasing cavity established between the array of beam emitters and the fiber Bragg grating. The optical fiber may include or consist essentially of one or more cores at least partially surrounded by one or more cladding layers. At least a portion of the fiber Bragg grating may be disposed within at least one core of the optical fiber. A first end cap may be attached to the optical fiber and disposed optically upstream of the fiber Bragg grating. A second end cap may be attached to the optical fiber and disposed optically downstream of the fiber Bragg grating.
In another aspect, embodiments of the invention feature a laser system that includes or consists essentially of an array of beam emitters each emitting a beam, focusing optics for focusing the beams toward a dispersive element, a dispersive element for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam, an optical fiber for receiving the multi-wavelength beam, and a partially reflective interface disposed within or on the optical fiber. The partially reflective interface receives the multi-wavelength beam, reflects a first portion thereof back toward the dispersive element, and transmits a second portion thereof as an output beam composed of multiple wavelengths.
Embodiments of the invention may include one or more of the following in any of a variety of combinations. The dispersive element may include or consist essentially of a diffraction grating (e.g., a transmissive diffraction grating or a reflective diffraction grating). The focusing optics may include or consist essentially of one or more cylindrical lenses, one or more spherical lenses, one or more spherical mirrors, and/or one or more cylindrical mirrors. An end cap may be attached to the optical fiber. The partially reflective interface may be disposed between the end cap and the optical fiber (e.g., where the end cap and the optical fiber contact each other). The end cap may include, consist essentially of, or consist of a first material. The optical fiber may include, consist essentially of, or consist of a second material. The first material may be different from the second material. The first material may include, consist essentially of, or consist of sapphire. The second material may include, consist essentially of, or consist of glass or fused silica. The partially reflective interface may include, consist essentially of, or consist of a coating applied to at least a portion of a surface of the end cap and/or at least a portion of a surface of the optical fiber. The partially reflective interface may be disposed between a portion of the optical fiber and a void (e.g., a bubble) disposed within the optical fiber. The void may include, consist essentially of, or consist of air or vacuum. The optical fiber may include or consist essentially of first and second optical fiber portions joined together. The partially reflective interface may be disposed between the first and second optical fiber portions. The first and second optical fiber portions may have different numerical apertures and/or different core diameters and/or different mode fields. A mode field adaptor or a fiber taper may be disposed optically downstream of the partially reflective interface. A mode field adaptor or a fiber taper may be disposed between the first and second optical fiber portions. The first optical fiber portion may include a core region optically upstream of the partially reflective interface. The second optical fiber portion may include a core region optically downstream of the partially reflective interface. A diameter (or lateral dimension) of the core region of the second optical fiber portion may be different from (e.g., larger than) a diameter (or lateral dimension) of the core region of the first optical fiber portion.
In yet another aspect, embodiments of the invention feature a laser system that includes or consists essentially of an array of beam emitters each emitting a beam, focusing optics for focusing the beams toward a dispersive element, a dispersive element for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam, a first optical fiber for receiving the multi-wavelength beam, a second optical fiber disposed optically downstream of the first optical fiber, and a partially reflective output coupler disposed between the first and second optical fibers. The partially reflective output coupler receives the multi-wavelength beam, reflects a first portion thereof back toward the dispersive element, and transmits a second portion thereof as an output beam composed of multiple wavelengths.
Embodiments of the invention may include one or more of the following in any of a variety of combinations. The dispersive element may include or consist essentially of a diffraction grating (e.g., a transmissive diffraction grating or a reflective diffraction grating). The focusing optics may include or consist essentially of one or more cylindrical lenses, one or more spherical lenses, one or more spherical mirrors, and/or one or more cylindrical mirrors. The first optical fiber may be disposed within an external lasing cavity formed between the array of beam emitters and the partially reflective output coupler. A first end cap and/or a first focusing lens may be disposed between the first optical fiber and the partially reflective output coupler. A second end cap and/or a second focusing lens may be disposed between the partially reflective output coupler and the second optical fiber.
In another aspect, embodiments of the invention feature a laser system that includes or consists essentially of an array of beam emitters each emitting a beam, focusing optics for focusing the beams toward a dispersive element, a dispersive element for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam, an optical fiber for receiving the multi-wavelength beam, and a partially reflective output coupler disposed optically downstream of the optical fiber. The partially reflective output coupler receives the multi-wavelength beam, reflects a first portion thereof back toward the dispersive element, and transmits a second portion thereof as an output beam composed of multiple wavelengths.
Embodiments of the invention may include one or more of the following in any of a variety of combinations. The dispersive element may include or consist essentially of a diffraction grating (e.g., a transmissive diffraction grating or a reflective diffraction grating). The focusing optics may include or consist essentially of one or more cylindrical lenses, one or more spherical lenses, one or more spherical mirrors, and/or one or more cylindrical mirrors. The optical fiber may be disposed within an external lasing cavity formed between the array of beam emitters and the partially reflective output coupler. The partially reflective output coupler may be disposed within a laser processing head detachable from at least a portion of the optical fiber and/or from a portion of the laser system that includes the diffractive element and/or the array of beam emitters. One or more focusing lenses may be disposed within the laser processing head. One or more focusing lenses may be disposed optically downstream of the optical fiber and optically upstream of the partially reflective output coupler.
These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. As used herein, the terms “substantially” and “approximately” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. Herein, the terms “radiation” and “light” are utilized interchangeably unless otherwise indicated. Herein, “downstream” or “optically downstream,” is utilized to indicate the relative placement of a second element that a light beam strikes after encountering a first element, the first element being “upstream,” or “optically upstream” of the second element. Herein, “optical distance” between two components is the distance between two components that is actually traveled by light beams; the optical distance may be, but is not necessarily, equal to the physical distance between two components due to, e.g., reflections from mirrors or other changes in propagation direction experienced by the light traveling from one of the components to the other.



BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:
FIG. 1A is a schematic of a wavelength beam combining (WBC) method along the array dimension of a single row of emitters in accordance with embodiments of the invention;
FIG. 1B is a schematic of a WBC method along the array dimension of a two-dimensional array of emitters in accordance with embodiments of the invention;
FIG. 1C is a schematic of a WBC method along the stack dimension of a two-dimensional array of emitters in accordance with embodiments of the invention;
FIG. 2 is a schematic showing the effects of smile in a WBC method along the stack dimension of a two-dimensional array of diode laser emitters in accordance with embodiments of the invention;
FIG. 3A is a schematic of a WBC system including an optical rotator selectively rotating a one-dimensional array of beams in accordance with embodiments of the invention;
FIG. 3B is a schematic of a WBC system including an optical rotator selectively rotating a two-dimensional array of beams in accordance with embodiments of the invention;
FIG. 3C is a schematic of a WBC system including an optical rotator selectively reorienting a two-dimensional array of beams in accordance with embodiments of the invention;
FIG. 3D illustrates output profile views of the system of FIG. 3C with and without an optical rotator in accordance with embodiments of the invention;
FIGS. 4A-4C illustrate examples of optical rotators in accordance with embodiments of the invention;
FIGS. 5A-5C illustrate related methods for placing combining elements to generate one-dimensional or two-dimensional laser elements;
FIG. 6 illustrates a WBC embodiment having a spatial repositioning element in accordance with embodiments of the invention;
FIG. 7 illustrates an embodiment of a two-dimensional array of emitters being reconfigured before a WBC step and individual beam rotation after the WBC step in accordance with embodiments of the invention;
FIG. 8 illustrates the difference between slow and fast WBC in accordance with embodiments of the invention;
FIG. 9A illustrates embodiments using an optical rotator before WBC in both a single and stacked array configurations in accordance with embodiments of the invention;
FIG. 9B illustrates additional embodiments using an optical rotator before WBC in accordance with embodiments of the invention;
FIG. 10 is illustrative of a single semiconductor chip emitter in accordance with embodiments of the invention;
FIGS. 11A and 11B are schematics of optical fibers in accordance with embodiments of the invention;
FIG. 11C is a schematic of a WBC laser system in accordance with embodiments of the invention;
FIGS. 12A and 12B are schematics of optical fibers in accordance with embodiments of the invention;
FIG. 12C is a schematic of a WBC laser system in accordance with embodiments of the invention;
FIG. 13 is a schematic of an optical fiber assembly in accordance with embodiments of the invention;
FIGS. 14A and 14B are schematics of optical fibers in accordance with embodiments of the invention; and
FIG. 15 is a schematic of a WBC laser system in accordance with embodiments of the invention.



DETAILED DESCRIPTION
Aspects and embodiments relate generally to the field of scaling laser sources to high-power and high-brightness using an external cavity and, more particularly, to methods and apparatus for external-cavity beam combining using both one-dimensional or two-dimensional laser sources. In one embodiment the external cavity system includes one-dimensional or two-dimensional laser elements, an optical system, a dispersive element, and a partially reflecting element. An optical system is one or more optical elements that perform two basic functions. The first function is to overlap all the laser elements along the beam combining dimension onto a dispersive element. The second function is to ensure all the elements along the non-beam combining dimension are propagating normal to the output coupler. In various embodiments, the optical system introduces as little loss as possible. As such, these two functions will enable a single resonance cavity for all the laser elements.
In another embodiment the WBC external cavity system includes wavelength stabilized one-dimensional or two-dimensional laser elements, an optical system, and a dispersive element. One-dimensional or two-dimensional wavelength stabilized laser elements, with unique wavelength, can be accomplished using various means such as laser elements with feedback from wavelength chirped Volume Bragg grating, distributed feedback (DFB) laser elements, or distributed Bragg reflector (DBR) laser elements. Here the main function of the optical system is to overlap all the beams onto a dispersive element. When there is no output coupler mirror external to the wavelength-stabilized laser element, having parallel beams along the non-beam-combining dimension is less important. Aspects and embodiments further relate to high-power and/or high-brightness multi-wavelength external-cavity lasers that generate an overlapping or coaxial beam from very low output power to hundreds and even to megawatts of output power.
In particular, aspects and embodiments are directed to a method and apparatus for manipulating the beams emitted by the laser elements of these external-cavity systems and combining them using a WBC method to produce a desired output profile. Wavelength beam combining methods have been developed to combine asymmetrical beam elements across their respective slow or fast axis dimension. One advantage of embodiments of the present invention is the ability to selectively-reconfigure input beams either spatially or by orientation to be used in slow and fast axis WBC methods, as well as a hybrid of the two. Another advantage is the ability to selectively-reconfigure input beams when there is a fixed-position relationship to other input beams.
FIG. 1A illustrates a basic WBC architecture. In this particular illustration, WBC is performed along the array dimension or slow dimension for broad-area emitters. Individual beams 104 are illustrated in the figures by a dash or single line, where the length or longer dimension of the beam represents the array dimension or slow diverging dimension for broad-area emitters and the height or shorter dimension represents the fast diverging dimension. (See also the left side of FIG. 8). In this related art, a diode bar 102 having four emitters is illustrated. The emitters are aligned in a manner such that the slow dimension ends of each emitted beam 104 are aligned to one another side by side along a single row—sometimes referred to as an array. However, it is contemplated that any lasing elements may be used and in particular laser elements with broad gain bandwidth. Typically a collimation lens 106 is used to collimate each beam along the fast diverging dimension. In some cases the collimation optics can be composed of separate fast axis collimation lenses and slow axis collimation lenses. Typically, transform optic 108 is used to combine each beam along the WBC dimension (or “WBC direction”) 110 as shown by the input front view 112. Transform optic 108 may be a cylindrical or spherical lens or mirror. The transform optic 108 then overlaps the combined beam onto a dispersive element 114 (here shown as a reflecting diffraction grating). The first-order diffracted beams are incident onto a partially reflecting mirror. The laser resonator is formed between the back facet of the laser elements and the partially reflecting mirror. As such, the combined beam is then transmitted as a single output profile onto an output coupler 116. This output coupler then transmits the combined beams 120, as shown by the output front view 118. It is contemplated creating a system devoid of an output coupler. For instance, a one-dimensional or two-dimensional system with wavelength stabilized laser elements and each having a unique wavelength may be accomplished in a few ways. One system or method uses laser elements with feedback from an external wavelength chirped Volume Bragg grating along the beam combining dimension. Another uses internal distributed feedback (DFB) laser elements or internal distributed Bragg reflector (DBR) laser elements. In these systems, the single output profile transmitted from the dispersive element would have the same profile as 118. The output coupler 116 may be a partially reflective mirror or surface or optical coating and act as a common front facet for all the laser elements in diode array 102. A portion of the emitted beams is reflected back into the optical gain and/or lasing portion of diode array 102 in this external cavity system 100a. An external cavity is a lasing system where the secondary mirror is displaced at a distance away from the emission aperture or facet (not labeled) of each laser emitter. Generally, in an external cavity additional optical elements are placed between the emission aperture or facet and the output coupler or partially reflective surface.
Similarly, FIG. 1B illustrates a stack of laser diode bars each having four emitters where those bars are stacked three high. Like FIG. 1A, the input front view 112 of FIG. 1B, which in this embodiment is a two-dimensional array of emitters, is combined to produce the output front view 118 or a single column of emitters 120. The emitted beams in external cavity 100b were combined along the array dimension. Here transform optic 108 is a cylindrical lens used to combine the beams along the array. However, a combination of optical elements or optical system may be used as such that the optical elements arrange for all the beams to overlap onto the dispersive element and ensure all the beams along the non-beam-combining dimension are propagating normal to the output coupler. A simple example of such an optical system would be a single cylindrical lens with the appropriate focal length along the beam-combining dimension and two cylindrical lenses that form an afocal telescope along the non-beam-combining dimension wherein the optical system projects images onto the partially reflecting mirrors. Many variations of this optical system can be designed to accomplish the same functions.
The array dimension FIG. 1B is also the same axis as the slow dimension of each emitted beam in the case of multimode diode laser emitters. Thus, this WBC system may also be called slow axis combining, where the combining dimension is the same dimension of the beams.
By contrast, FIG. 1C illustrates a stack 150 of laser diode arrays 102 forming a two-dimensional array of emitters, as shown by 120, where instead of combining along the array dimension as in FIGS. 1A and 1B, the WBC dimension now follows along the stack dimension of the emitters. Here, the stacking dimension is also aligned with the fast axis dimension of each of the emitted beams. The input front view 112 is now combined to produce the output front view 118 wherein a single column 120 of emitters is shown.
There are various drawbacks to all three configurations. One of the main drawbacks of configuration shown in FIGS. 1A and 1B is that beam combining is performed along the array dimension. As such external-cavity operation is highly dependent on imperfections of the diode array. If broad-area semiconductor laser emitters are used the spectral utilization in the WBC system is not as efficient as if beam combining is performed along the fast axis dimension. One of the main drawbacks of configurations shown in FIG. 1C is that external beam shaping for beam symmetrization is required for efficient coupling into a fiber. The beam symmetrization optics needed for a high power system having a large number of emitters may be complex and non-trivial. Another disadvantage of configuration 1C is that the output beam quality is limited to that of a single laser bar. Typical semiconductor or diode laser bars have 19 to 49 emitters per bar with nearly diffraction-limited beam quality in one dimension and beam quality that is several hundreds of times diffraction-limited along the array dimension. After beam symmetrization the output beam 120 can be coupled into at best a 100 μm/0.22 Numerical Aperture (NA) fiber. To obtain higher beam quality a small number of emitter bars is needed. For example to couple into 50 μm/0.22 NA fiber a five-emitter output beam is needed. In many industrial laser applications a higher brightness laser beam is required. For example, in some applications a two-emitter output beam is needed instead of 19 or 49. The two-emitter output beam can be coupled to a smaller core diameter fiber with much more engineering tolerance and margin. This additional margin in core diameter and NA is critical for reliable operation at high power (kW-class) power levels. While it is possible to procure five-emitter or two-emitter bars the cost and complexity is generally much higher as compared to standard 19 or 49 emitter bars because of the significantly reduced power per bar. In this disclosure, we disclose methods to remove all of the above shortcomings.
The previous illustrations, FIGS. 1A-1C, showed pre-arranged or fixed position arrays and stacks of laser emitters. Generally, arrays or stacks are arranged mechanically or optically to produce a particular one-dimensional or two-dimensional profile. Thus, fixed-position is used to describe a preset condition of laser elements where the laser elements are mechanically fixed with respect to each other as in the case of semiconductor or diode laser bars having multiple emitters or fiber lasers mechanically spaced apart in V-grooves, as well as other laser emitters that come packaged with the emitters in a fixed position.
Alternatively, fixed position may refer to the secured placement of a laser emitter in a WBC system where the laser emitter is immobile. Pre-arranged refers to an optical array or profile that is used as the input profile of a WBC system. Often times the pre-arranged position is a result of emitters configured in a mechanically fixed position. Pre-arranged and fixed position may also be used interchangeably. Examples of fixed-position or pre-arranged optical systems are shown in FIGS. 5A-C.
FIGS. 5A-5C refer to prior art illustrated examples of optically arranged one and two-dimensional arrays. FIG. 5A illustrates an optically arranged stack of individual optical elements 510. Mirrors 520 are used to arrange the optical beams from optical elements 530, each optical element 530 having a near field image 540, to produce an image 550 (which includes optical beams from each optical element 530) corresponding to a stack 560 (in the horizontal dimension) of the individual optical elements 510. Although the optical elements 500 may not be arranged in a stack, the mirrors 520 arrange the optical beams such that the image 550 appears to correspond to the stack 560 of optical elements 510. Similarly, in FIG. 5B, the mirrors 520 can be used to arrange optical beams from diode bars or arrays 570 to create an image 550 corresponding to a stack 560 of diode bars or arrays 575. In this example, each diode bar or array 570 has a near field image 540 that includes optical beams 545 from each individual element in the bar or array. Similarly, the minors 520 may also be used to optically arrange laser stacks 580 into an apparent larger overall stack 560 of individual stacks 585 corresponding to image 550, as shown in FIG. 5C.
Nomenclature, used in prior art to define the term “array dimension,” referred to one or more laser elements placed side by side where the array dimension is also along the slow axis. One reason for this nomenclature is diode bars with multiple emitters are often arranged in this manner where each emitter is aligned side by side such that each beam's slow dimension is along a row or array. For purposes of this application, an array or row refers to individual emitters or beams arranged across a single dimension. The individual slow or fast dimension of the emitters of the array may also be aligned along the array dimension, but this alignment is not to be assumed. This is important because some embodiments described herein individually rotate the slow dimension of each beam aligned along an array or row. Additionally, the slow axis of a beam refers to the wider dimension of the beam and is typically also the slowest diverging dimension, while the fast axis refers to the narrower dimension of the beam and is typically the fastest diverging dimension. The slow axis may also refer to single mode beams.
Additionally, some prior art defines the term “stack or stacking dimension” referred to as two or more arrays stacked together, where the beams' fast dimension is the same as the stacking dimension. These stacks were pre-arranged mechanically or optically. However, for purposes of this application a stack refers to a column of beams or laser elements and may or may not be along the fast dimension. Particularly, as discussed above, individual beams or elements may be rotated within a stack or column.
In some embodiments it is useful to note that the array dimension and the slow dimension of each emitted beam are initially oriented across the same axis; however, those dimensions, as described in this application, may become oriented at an offset angle with respect to each other. In other embodiments, the array dimension and only a portion of the emitters arranged along the array or perfectly aligned the same axis at a certain position in a WBC system. For example, the array dimension of a diode bar may have emitters arranged along the array dimension, but because of smile (often a deformation or bowing of the bar) individual emitters' slow emitting dimension is slightly skewed or offset from the array dimension.
Laser sources based on common “commercial, off-the-shelf” or COTS high power laser diode arrays and stacks are based on broad-area semiconductor or diode laser elements. Typically, the beam quality of these elements is diffraction-limited along the fast axis and many times diffraction-limited along the slow axis of the laser elements. It is to be appreciated that although the following discussion may refer primarily to single emitter laser diodes, diode laser bars and diode laser stacks, embodiments of the invention are not limited to semiconductor or laser diodes and may be used with many different types of laser and amplifier emitters, including fiber lasers and amplifiers, individually packaged diode lasers, other types of semiconductor lasers including quantum cascade lasers (QCLs), tapered lasers, ridge waveguide (RWG) lasers, distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, grating coupled surface emitting laser, vertical cavity surface emitting laser (VCSEL), and other types of lasers and amplifiers.
All of the embodiments described herein can be applied to WBC of diode laser single emitters, bars, and stacks, and arrays of such emitters. In those embodiments employing stacking of diode laser elements, mechanical stacking or optical stacking approaches can be employed. In addition, where an HR coating is indicated at the facet of a diode laser element, the HR coating can be replaced by an AR coating, provided that external cavity optical components, including but not limited to a collimating optic and bulk HR mirror are used in combination with the AR coating. This approach is used, for example, with WBC of diode amplifier elements. Slow axis is also defined as the worse beam quality direction of the laser emission. The slow axis typically corresponds to the direction parallel to the semiconductor chip at the plane of the emission aperture of the diode laser element. Fast axis is defined as the better beam quality direction of the laser emission. Fast axis typically corresponds to the direction perpendicular to the semiconductor chip at the plane of the emission aperture of the diode laser element.
An example of a single semiconductor chip emitter 1000 is shown in FIG. 10. The aperture 1050 is also indicative of the initial beam profile. Here, the height 1010 at 1050 is measured along the stack dimension. Width 1020 at 1050 is measured along the array dimension. Height 1010 is the shorter dimension at 1050 than width 1020. However, height 1010 expands faster or diverges to beam profile 1052, which is placed at a distance away from the initial aperture 1050. Thus, the fast axis is along the stack dimension. Width 1020 which expands or diverges at a slower rate as indicated by width 1040 being a smaller dimension than height 1030. Thus, the slow axis of the beam profile is along the array dimension. Though not shown, multiple single emitters such as 1000 may be arranged in a bar side by side along the array dimension.
Drawbacks for combining beams primarily along their slow axis dimension may include: reduced power and brightness due to lasing inefficiencies caused by pointing errors, smile and other misalignments. As illustrated in FIG. 2, a laser diode array with smile, one often caused by the diode array being bowed in the middle sometimes caused by the diode laser bar mounting process, is one where the individual emitters along the array form a typical curvature representative of that of a smile. Pointing errors are individual emitters along the diode bar emitting beams at an angle other than normal from the emission point. Pointing error may be related to smile, for example, the effect of variable pointing along the bar direction of a diode laser bar with smile when the bar is lensed by a horizontal fast axis collimating lens. These errors cause feedback from the external cavity, which consists of the transform lens, grating, and output coupler, not to couple back to the diode laser elements. Some negative effects of this miscoupling are that the WBC laser breaks wavelength lock and the diode laser or related packaging may be damaged from miscoupled or misaligned feedback not re-entering the optical gain medium. For instance the feedback may hit some epoxy or solder in contact or in close proximity to a diode bar and cause the diode bar to fail catastrophically.
Row 1 of FIG. 2 shows a single laser diode bar 202 without any errors. The embodiments illustrated are exemplary of a diode bar mounted on a heat sink and collimated by a fast-axis collimation optic 206. Column A shows a perspective or 3-D view of the trajectory of the output beams 211 going through the collimation optic 206. Column D shows a side view of the trajectory of the emitted beams 211 passing through the collimation optic 206. Column B shows the front view of the laser facet with each individual laser element 213 with respect to the collimation optic 206. As illustrated in row 1, the laser elements 213 are perfectly straight. Additionally, the collimation optic 206 is centered with respect to all the laser elements 213. Column C shows the expected output beam from a system with this kind of input. Row 2 illustrates a diode laser array with pointing error. Shown by column B of row 2 the laser elements and collimation optic are slightly offset from each other. The result, as illustrated, is the emitted beams having an undesired trajectory that may result in reduced lasing efficiency for an external cavity. Additionally, the output profile may be offset to render the system ineffective or cause additional modifications. Row 3 shows an array with packaging error. The laser elements no longer sit on a straight line, and there is curvature of the bar. This is sometimes referred to as “smile.” As shown on row 3, smile may introduce even more trajectory problems as there is no uniform path or direction common to the system. Column D of row 3 further illustrates beams 211 exiting at various angles. Row 4 illustrates a collimation lens unaligned with the laser elements in a twisted or angled manner. The result is probably the worst of all as the output beams generally have the most collimation or twisting errors. In most systems, the actual error in diode arrays and stacks is a combination of the errors in rows 2, 3, and 4. In both methods 2 and 3, using VBGs and diffraction gratings, laser elements with imperfections result in output beams no longer pointing parallel to the optical axis. These off optical axis beams result in each of the laser elements lasing at different wavelengths. The plurality of different wavelengths increases the output spectrum of the system to become broad as mentioned above.
One of the advantages of performing WBC along the stacking dimension (here also primarily the fast dimension) of a stack of diode laser bars is that it compensates for smile as shown in FIG. 2. Pointing and other alignment errors are not compensated by performing WBC along the array dimension (also primarily slow dimension). A diode bar array may have a range of emitters typically from 19 to 49 or more. As noted, diode bar arrays are typically formed such that the array dimension is where each emitter's slow dimension is aligned side by side with the other emitters. As a result, when using WBC along the array dimension, whether a diode bar array has 19 or 49 emitters (or any other number of emitters), the result is that of a single emitter. By contrast, when performing WBC along the orthogonal or fast dimension of the same single diode bar array, the result is each emitted beam increases in spectral brightness, or narrowed spectral bandwidth, but there is not a reduction in the number of beams (equivalently, there is not an increase in spatial brightness).
This point is illustrated in FIG. 8. On the left of FIG. 8 is shown a front view of an array of emitters 1 and 2 where WBC along the slow dimension is being performed. Along the right side using the same arrays 1 and 2, WBC along the fast dimension is being performed. When comparing array 1, WBC along the slow dimension reduces the output profile to that of a single beam, while WBC along the fast dimension narrows the spectral bandwidth, as shown along the right side array 1, but does not reduce the output profile size to that of a single beam.
Using COTS diode bars and stacks the output beam from beam combining along the stack dimension is usually highly asymmetric. Symmetrization, or reducing the beam profile ratio closer to equaling one, of the beam profile is important when trying to couple the resultant output beam profile into an optical fiber. Many of the applications of combining a plurality of laser emitters require fiber coupling at some point in an expanded system. Thus, having greater control over the output profile is another advantage of the application.
Further analyzing array 2 in FIG. 8 shows the limitation of the number of emitters per laser diode array that is practical for performing WBC along the fast dimension if very high brightness symmetrization of the output profile is desired. As discussed above, typically the emitters in a laser diode bar are aligned side by side along their slow dimension. Each additional laser element in a diode bar is going to increase the asymmetry in the output beam profile. When performing WBC along the fast dimension, even if a number of laser diode bars are stacked on each other, the resultant output profile will still be that of a single laser diode bar. For example if one uses a COTS 19-emitter diode laser bar, the best that one can expect is to couple the output into a 100 μm/0.22 NA fiber. Thus, to couple into a smaller core fiber, a lower number of emitters per bar is required. One could simply fix the number of emitters in the laser diode array to 5 emitters in order to help with the symmetrization ratio; however, fewer emitters per laser diode bar array generally results in an increase of cost per bar or cost per Watt of output power. For instance, the cost of a diode bar having 5 emitters may be roughly the same as that of a diode bar having 49 emitters. However, the 49 emitter bar may have a total power output of up to an order-of-magnitude greater than that of the 5 emitter bar. Thus, it would be advantageous for a WBC system to be able to achieve very high brightness output beams using COTS diode bars and stacks with larger number of emitters. An additional advantage of bars with larger number of emitters is the ability to de-rate the power per emitter to achieve a certain power level per bar for a given fiber-coupled power level, thereby increasing the diode laser bar lifetime or bar reliability.
One embodiment that addresses this issue is illustrated in FIG. 3A, which shows a schematic of WBC system 300a with an optical rotator 305 placed after collimation lenses 306 and before the transform optic 308. It should be noted the transform optic 308 may include or consist essentially of a number of lenses or mirrors or other optical components. The optical rotator 305 individually rotates the fast and slow dimension of each emitted beam shown in the input front view 312 to produce the re-oriented front view 307. It should be noted that the optical rotators can selectively rotate each beam individually irrespective of the other beams or can rotate all the beams through the same angle simultaneously. It should also be noted that a cluster of two or more beams can be rotated simultaneously. The resulting output after WBC is performed along the array dimension is shown in output front view 318 as a single emitter. Dispersive element 314 is shown as a reflection diffraction grating, but may also be a dispersive prism, a grism (prism+grating), transmission grating, and Echelle grating. This particular embodiment illustrated shows only four laser emitters; however, as discussed above this system could take advantage of a laser diode array that included many more elements, e.g., 49. This particular embodiment illustrated shows a single bar at a particular wavelength band (example at 976 nm) but in actual practice it may be composed of multiple bars, all at the same particular wavelength band, arranged side-by-side. Furthermore, multiple wavelength bands (example 976 nm, 915 nm, and 808 nm), with each band composing of multiple bars, may be combined in a single cavity. Because WBC was performed across the fast dimension of each beam it was easier to design a system with a higher brightness (higher efficiency due to insensitivity due to bar imperfections); additionally, narrower bandwidth and higher power output are all achieved. As previously discussed it should be noted that in some embodiments WBC system 300a may not include output coupler 316 and/or collimation lens(es) 306. Furthermore, pointing errors and smile errors are compensated for by combining along the stack dimension (in this embodiment this is also the fast dimension). FIG. 3B, shows an implementation similar to 3A except that a stack 350 of laser arrays 302 forms a 2-D input profile 312. Cavity 300b similarly consists of collimation lens(es) 306, optical rotator 305, transform optic 308, dispersive element 314 (here a diffraction grating), and an output coupler 316 with a partially reflecting surface. Each of the beams is individually rotated by optical rotator 305 to form an after rotator profile 307. The WBC dimension is along the array dimension, but with the rotation each of the beams will be combined across their fast axis. Fast axis WBC produces outputs with very narrow line widths and high spectral brightness. These are usually ideal for industrial applications such as welding. After transform optic 308 overlaps the rotated beams onto dispersive element 314 a single output profile is produced and partially reflected back through the cavity into the laser elements. The output profile 318 is now comprised of a line of three (3) beams that is quite asymmetric.
FIG. 3C shows the same implementation when applied to 2-D laser elements. The system consists of 2-D laser elements 302, optical rotator 305, transform optical system (308 and 309a-b) a dispersive element 314, and a partially reflecting mirror 316. FIG. 3C illustrates a stack 350 of laser diode bars 302 with each bar having an optical rotator 305. Each of the diode bars 302 (three total) as shown in external cavity 300c includes four emitters. After input front view 312 is reoriented by optical rotator 305, reoriented front view 307 shows the slow dimension of each beam aligned along the stack dimension. WBC is performed along the dimension, which is now the slow axis of each beam and the output front view 318 now comprises a single column of beams with each beam's slow dimension oriented along the stack dimension. Optic 309a and 309b provide a cylindrical telescope to image along the array dimension. The function of the three cylindrical lenses is two-fold. The middle cylindrical lens is the transform lens and its main function is to overlap all the beams onto the dispersive element. The two other cylindrical lenses 309a and 309b form an afocal cylindrical telescope along the non-beam combining dimension. Its main function is to make sure all laser elements along the non-beam combining dimension are propagating normal to the partially reflecting mirror. As such the implementation as shown in FIG. 3C has the same advantages as the one shown in FIG. 1C. However, unlike the implementation as shown in FIG. 1C the output beam is not the same as the input beam. The number of emitters in the output beam 318 in FIG. 3C is the same as the number of bars in the stack. For example, if the 2-D laser source consists of a three-bar stack with each bar composed of 49 emitters, then the output beam in FIG. 1C is a single bar with 49 emitters. However, in FIG. 3C the output beam is a single bar with only three emitters. Thus, the output beam quality or brightness is more than one order of magnitude higher. This brightness improvement is very significant for fiber-coupling. For higher power and brightness scaling multiple stacks can be arranged side-by-side.
To illustrate this configuration further, for example, assume WBC is to be performed of a three-bar stack, with each bar comprising of 19 emitters. So far, there are three options. First, wavelength beam combining can be performed along the array dimension to generate three beams as shown in FIG. 1B. Second, wavelength beam combining can be performed along the stack dimension to generate 19 beams a shown FIG. 1C. Third, wavelength beam combining can be performed along the array dimension using beam rotator to generate 19 beams as shown FIG. 3C. There are various trade-offs for all three configuration. The first case gives the highest spatial brightness but the lowest spectral brightness. The second case gives the lowest spatial brightness with moderate spectral brightness and beam symmetrization is not required to couple into a fiber. The third case gives the lowest spatial brightness but the highest spectral brightness and beam symmetrization is required to couple into an optical fiber. In some applications this more desirable.
To illustrate the reduction in asymmetry FIG. 3D has been drawn showing the final output profile 319a where the system of 300b did not have an optical rotator and output profile 319b where the system includes an optical rotator. Though these figures are not drawn to scale, they illustrate an advantage achieved by utilizing an optical rotator, in a system with this configuration where WBC is performed across the slow dimension of each beam. The shorter and wider 319b is more suitable for fiber coupling than the taller and slimmer 319a. 
Examples of various optical rotators are shown in FIG. 4A-4C. FIG. 4A illustrates an array of cylindrical lenses (419a and 419b) that cause input beam 411a to be rotated to a new orientation at 411b. FIG. 4B similarly shows input 411a coming into the prism at an angle, which results in a new orientation or rotation beam 411b. FIG. 4C illustrates an embodiment using a set of step mirrors 417 to cause input 411a to rotate at an 80-90 degree angle with the other input beams resulting in a new alignment of the beams 411b where they are side by side along their respective fast axis. These devices and others may cause rotation through both non-polarization sensitive as well as polarization sensitive means. Many of these devices become more effective if the incoming beams are collimated in at least the fast dimension. It is also understood that the optical rotators can selectively rotate the beams by various amounts, including less than 90 degrees, 90 degrees, and greater than 90 degrees.
The optical rotators in the previous embodiments may selectively rotate individual, rows or columns, and groups of beams. In some embodiments a set angle of rotation, such as a range of 80-90 degrees is applied to the entire profile or subset of the profile. In other instances, varying angles of rotation are applied uniquely to each beam, row, column or subset of the profile (see FIGS. 9A-B). For instance, one beam may be rotated by 45 degrees in a clockwise direction while an adjacent beam is rotated 45 degrees in a counterclockwise direction. It is also contemplated one beam is rotated 10 degrees and another is rotated 70 degrees. The flexibility the system provides may be applied to a variety of input profiles, which in turn helps determine how the output profile is to be formed.
Performing WBC along an intermediate angle between the slow and fast dimension of the emitted beams is also well within the scope of the invention (See for example 6 on FIG. 9B). Some laser elements as described herein, produce electromagnetic radiation and include an optical gain medium. When the radiation or beams exit the optical gain portion they generally are collimated along the slow and/or fast dimension through a series of micro lenses. From this point, the embodiments already described in this section included an optical rotator that selectively rotated each beam prior to the beams being overlapped by a transform lens along either the slow or the fast dimension of each beam onto a dispersive element. The output coupler may or may not be coated to partially reflect the beams back into the system to the laser element where the returned beams assist in generating more external cavity feedback in the optical gain element portion until they are reflected off a fully reflective mirror in the back portion of the laser element. The location of the optical elements listed above and others not listed with respect to the second partially reflective surface helps determine whether the optical elements are within an external cavity system or outside of the lasing cavity. In some embodiments, not shown, the second partially reflective mirror resides at the end of the optical gain elements and prior to the collimating or rotating optics.
Another method for manipulating beams and configurations to take advantage of the various WBC methods includes using a spatial repositioning element. This spatial repositioning element may be placed in an external cavity at a similar location as that of an optical rotator. For example, FIG. 6 shows a spatial repositioning element 603 placed in the external cavity WBC system 600 after the collimating lenses 606 and before the transform optic(s) 608. The purpose of a spatial repositioning element is to reconfigure an array of elements into a new configuration. FIG. 6 shows a three-bar stack with N elements reconfigured to a six-bar stack with N/2 elements. Spatial repositioning is particularly useful in embodiments such as 600, where stack 650 is a mechanical stack or one where diode bar arrays 602 and their output beams were placed on top of each other either mechanically or optically. With this kind of configuration the laser elements have a fixed-position to one another. Using a spatial repositioning element can form a new configuration that is more ideal for WBC along the fast dimension. The new configuration makes the output profile more suitable for fiber coupling.
For example, FIG. 7 illustrates an embodiment wherein a two-dimensional array of emitters 712 is reconfigured during a spatial repositioning step 703 by a spatial repositioning optical element such as an array of periscope mirrors. The reconfigured array shown by reconfigured front view 707 is now ready for a WBC step 710 to be performed across the WBC dimension, which here is the fast dimension of each element. The original two-dimensional profile in this example embodiment is an array of 12 emitters tall and 5 emitters wide. After the array is transmitted or reflected by the spatial repositioning element a new array of 4 elements tall and 15 elements wide is produced. In both arrays the emitters are arranged such that the slow dimension of each is vertical while the fast dimension is horizontal. WBC is performed along the fast dimension which collapses the 15 columns of emitters in the second array into 1 column that is 4 emitters tall. This output is already more symmetrical than if WBC had been performed on the original array, which would have resulted in a single column 15 emitters tall. As shown, this new output may be further symmetrized by an individually rotating step 705 rotating each emitter by 90 degrees. In turn, a post-WBC front view 721 is produced being the width of a single beam along the slow dimension and stacked 4 elements high, which is more suitable for coupling into a fiber.
One way of reconfiguring the elements in a one-dimensional or two-dimensional profile is to make ‘cuts’ or break the profile into sections and realign each section accordingly. For example, in FIG. 7 two cuts 715 were made in 713. Each section was placed side by side to form 707. These optical cuts can be appreciated if we note the elements of 713 had a pre-arranged or fixed-position relationship. It is also well within the scope to imagine any number of cuts being made to reposition the initial input beam profile. Each of these sections may in addition to being placed side by side, be on top and even randomized if so desired.
Spatial repositioning elements may be comprised of a variety of optical elements including periscope optics that are both polarized and non-polarized as well as other repositioning optics. Step mirrors as shown in FIG. 4a may also be reconfigured to become a spatial repositioning element.
Additional embodiments of the invention are illustrated in FIGS. 9A-9B. The system shown in 1 of FIG. 9A shows a single array of four beams aligned side to side along the slow dimension. An optical rotator individually rotates each beam. The beams are then combined along the fast dimension and are reduced to a single beam by WBC. In this arrangement it is important to note that the 4 beams could easily be 49 or more beams. It may also be noted that if some of the emitters are physically detached from the other emitters, the individual emitter may be mechanically rotated to be configured in an ideal profile. A mechanical rotator may be comprised of a variety of elements including friction sliders, locking bearings, tubes, and other mechanisms configured to rotate the laser element. Once a desired position is achieved the laser elements may then be fixed into place. It is also conceived that an automated rotating system that can adjust the beam profile depending on the desired profile may be implemented. This automated system may either mechanically reposition a laser or optical element or a new optical element may be inserted in and out of the system to change the output profile as desired.
System 2 shown in FIG. 9A, shows a two-dimensional array having three stacked arrays with four beams each aligned along the slow dimension. (Similar to FIG. 3C) As this stacked array passes through an optical rotator and WBC along the fast dimension a single column of three beams tall aligned top to bottom along the slow dimension is created. Again it is appreciated that if the three stacked arrays shown in this system had 50 elements, the same output profile would be created, albeit one that is brighter and has a higher output power.
System 3 in FIG. 9B, shows a diamond pattern of four beams wherein the beams are all substantially parallel to one another. This pattern may also be indicative of a random pattern. The beams are rotated and combined along the fast dimension, which results in a column of three beams aligned along the slow dimension from top to bottom. Missing elements of diode laser bars and stacks due to emitter failure or other reasons is an example of System 3. System 4 illustrates a system where the beams are not aligned, but that one beam is rotated to be aligned with a second beam such that both beams are combined along the fast dimension forming a single beam. System 4 demonstrates a number of possibilities that expands WBC methods beyond using laser diode arrays. For instance, the input beams in System 4 may be from carbon dioxide (CO2) lasers, semiconductor or diode lasers, diode pumped fiber lasers, lamp-pumped or diode-pumped Nd:YAG lasers, Disk Lasers, and so forth. The ability to mix and match the type of lasers and wavelengths of lasers to be combined is another advantage of embodiments of the present invention.
System 5 illustrates a system where the beams are not rotated to be fully aligned with WBC dimension. The result is a hybrid output that maintains many of the advantages of WBC along the fast dimension. In several embodiments the beams are rotated a full 90 degrees to become aligned with WBC dimension, which has often been the same direction or dimension as the fast dimension. However, System 5 and again System 6 show that optical rotation of the beams as a whole (System 6) or individually (System 5) may be such that the fast dimension of one or more beams is at an angle theta or offset by a number of degrees with respect to the WBC dimension. A full 90 degree offset would align the WBC dimension with the slow dimension while a 45 degree offset would orient the WBC dimension at an angle halfway between the slow and fast dimension of a beam as these dimension are orthogonal to each other. In one embodiment, the WBC dimension has an angle theta at approximately 3 degrees off the fast dimension of a beam.
As described above, the output beam of a WBC laser system may advantageously be coupled into an optical fiber for use in a variety of different applications. Embodiments of the present invention utilize the fiber itself (or a portion thereof or an interface therein or thereto) as the partially reflective output coupler, which in part establishes the WBC external lasing cavity, in order to reduce component count and system complexity. FIGS. 11A and 11B depict an optical fiber 1100 that may both (1) be utilized as a partially reflective output coupler in a WBC laser system and (2) deliver the multi-wavelength output beam. As shown, laser light 1105 is focused into fiber 1100 via an optional end cap 1110, which typically includes or consists essentially of glass. The end cap 1110 may have an anti-reflection coating 1115 on one or more surfaces thereof. End cap 1110 (and corresponding end cap 1120 at the opposite end of fiber 1100, which may also have an anti-reflection coating 1115 thereon) reduce the intensity (i.e., light power per unit area) at the surfaces of the fiber to minimize or substantially eliminate damage to any anti-reflection coatings and reduce the risk of surface contamination that might cause catastrophic failure of the fiber and/or laser system. The laser light 1105 is coupled into core 1125 and/or cladding 1130 of the fiber via end cap 1110, and one or more reflective structures and/or interfaces within the fiber 1100 reflect a first portion 1135 of the light 1105 back toward the dispersive element (e.g., a diffraction grating) of the WBC system while transmitting a second portion 1140 of the light 1105 as a multi-wavelength output beam. The end caps 1110, 1120 may include, consist essentially of, or consist of the same material as a material of at least part of the optical fiber (e.g., the core 1125 and/or the cladding 1130).
As shown in FIGS. 11A and 11B, the partially reflective structure within the fiber 1100 may include or consist essentially of one or more fiber Bragg gratings 1145. As known to those of skill in the art, the fiber Bragg grating 1145 includes or consists essentially of a periodic variation of the refractive index of a portion of the fiber 1100 (e.g., within the core 1125). The period variation may be, e.g., on the order of one-half of the wavelength (or one of the wavelengths) of light 1105, and the grating 1145 thus induces Fresnel reflection. The wavelength dependence and/or the magnitude of the reflection may be selected by the particular grating pattern and the refractive-index variation therein. As shown in FIG. 11B, multiple fiber Bragg gratings 1145 may be disposed within the fiber 1100, and each grating 1145 may have a different refractive-index variation and/or wavelength selectivity.
FIG. 11C depicts the fiber 1100 within an exemplary WBC laser system 1150. In WBC system 1150, beams are emitted by a one-dimensional or two-dimensional array of beam emitters 102. The beams may be manipulated by one or more (e.g., one for each beam, or one shared by all beams) optics 1155. Optics 1155 collimate, rotate, and/or otherwise manipulate the beams emitted by beam emitters 102, and optics 1155 may include or consist essentially of, for example, an optical rotator (as detailed above and/or as described in U.S. patent application Ser. No. 14/734,480, filed Jun. 9, 2015, the entire disclosure of which is incorporated by reference herein) or a micro-optics arrangement (e.g., a slow-axis collimation lens, an optical twister, and/or a fast-axis collimation lens, as described in U.S. patent application Ser. No. 14/667,094, filed Mar. 24, 2015, the entire disclosure of which is incorporated by reference herein).
The WBC system 1150 also includes focusing optics 108 (e.g., one or more cylindrical lenses and/or mirrors, and/or one or more spherical lenses and/or mirrors) that combine the beams emitted by the emitters 102 to form a combined beam that propagates toward a dispersive element 114. The dispersive element 114 (e.g., a diffraction grating, a dispersive prism, a grism (prism/grating), a transmission grating, or an Echelle grating) may be disposed approximately at the focal length of the focusing optics 108. As described herein, the dispersive element 114 receives the combined beam and transmits the beam as a multi-wavelength beam having a high brightness. The multi-wavelength beam is focused by optics 1160 (e.g., one or more spherical and/or cylindrical lenses) and transmitted to the optical fiber 1100, which, via fiber Bragg grating 1145, transmits a portion of the multi-wavelength beam and reflects a second portion of the multi-wavelength beam back toward the dispersive element 114 and thence to the emitters 102, forming an external lasing cavity. As shown in FIG. 11C, the grating 1145 may include or consist of regions, having a period 1160, of varying refractive index. For example, the grating 1145 may include regions having a refractive index n3 different from (e.g., larger than) the refractive index n2 of the core of fiber 1100. (Fiber 1100 is also depicted as having a cladding 1130 having a refractive index n1 and being surrounded by air having a refractive index n0.)
FIGS. 12A and 12B depict an optical fiber 1200 that may, in accordance with various embodiments of the present invention, both (1) be utilized as a partially reflective output coupler in a WBC laser system and (2) deliver the multi-wavelength output beam. As shown, laser light 1105 is focused into fiber 1200 and partially reflected by a partially reflective coating (e.g., a coating having a reflectivity to one or more wavelengths of the light 1105 ranging from about 1% to about 10%, or even more) or surface. As detailed herein, the reflected portion 1135 of light 1105 establishes the WBC external cavity in tandem with the source(s) of the light 1105, and the transmitted multi-wavelength portion 1140 of the light propagates through the fiber 1200 and may be utilized in any of a variety of applications.
In various embodiments of the invention, the partially reflective coating and/or surface may include or consist essentially of a partially reflective coating on a surface of the fiber and/or on the end cap 1110 (e.g., at the interface between the end cap and the fiber), or a coating or surface between two segments of fiber (or, equivalently, two fibers). In the configuration of FIG. 12A, a partially reflective coating 1210 is disposed between the fiber and the end cap 1110. For example, the coating 1210 may be disposed on the surface of the fiber core 1125 and fiber cladding 1130 facing the end cap 1110 and/or on the surface of the end cap 1110 itself. The coating 1210 may include or consist essentially of, e.g., one or more metallic layers and/or one or more dielectric layers. As in fiber 1100 described above, the coating 1210 may partially reflect the light 1105 via Fresnel reflection.
In the configuration of FIG. 12B, the coating 1210 is disposed between two fibers or fiber segments (which may be, for example, spliced together). As shown, this configuration may also include an optional adapter 1220 optically downstream of the coating 1210 in order to more efficiently capture and in-couple the transmitted multi-wavelength beam 1140 into the second fiber or fiber portion. For example, the adapter 1220 may include or consist essentially of a fiber taper (i.e., a coherent fiber optic plate that transmits either a magnified or reduced image from its input surface to its output surface) or mode field adapter (i.e., a splicing device to join together, within minimal light loss, fibers, particularly fibers with different mode fields (e.g., different core diameters and/or numerical apertures)). In various embodiments, the adapter 1220 may include or consist essentially of a portion of fiber core 1125 that is larger (i.e., has a larger diameter) than the core 1125 downstream of the adapter 1220.
FIG. 12C depicts the fiber 1200 within an exemplary WBC laser system 1250. In WBC system 1250, beams are emitted by a one-dimensional or two-dimensional array of beam emitters 102. As in WBC laser system 1150, the beams may be manipulated by one or more (e.g., one for each beam, or one shared by all beams) optics 1155. Optics 1155 collimate, rotate, and/or otherwise manipulate the beams emitted by beam emitters 102. The WBC system 1250 also includes focusing optics 108 that combine the beams emitted by the emitters 102 to form a combined beam that propagates toward dispersive element 114. The dispersive element 114 (e.g., a diffraction grating, a dispersive prism, a grism (prism/grating), a transmission grating, or an Echelle grating) may be disposed approximately at the focal length of the focusing optics 108. As described herein, the dispersive element 114 receives the combined beam and transmits the beam as a multi-wavelength beam having a high brightness. The multi-wavelength beam is focused by optics 1160 (e.g., one or more spherical and/or cylindrical lenses) and transmitted to the optical fiber 1200, which, via coating or surface 1210, transmits a portion of the multi-wavelength beam and reflects a second portion of the multi-wavelength beam back toward the dispersive element 114 and thence to the emitters 102, forming an external lasing cavity.
Various embodiments of the present invention incorporate a partially reflective output coupler 116 between two fibers (or fiber segments), as depicted in FIG. 13. FIG. 13 shows a fiber assembly 1300 in which light 1105 is focused and in-coupled into a first fiber 1310. The light 1105 subsequently exits the fiber 1310 and encounters the output coupler 116, where first portion 1135 is reflected back into fiber 1310 and back toward the beam emitter(s) (thus establishing the external WBC cavity), and multi-wavelength output portion 1140 enters a second fiber 1320 for delivery and use in any of a variety of different applications. As shown, the assembly 1300 may also incorporate end caps 1330 and/or focusing optics 1340 (e.g., one or more spherical and/or cylindrical lenses) on the ends of the fibers 1310, 1320 that face the output coupler 116. FIGS. 14A and 14B depict an optical fiber 1400 that may, in accordance with various embodiments of the present invention, both (1) be utilized as a partially reflective output coupler in a WBC laser system and (2) deliver the multi-wavelength output beam. As shown, laser light 1105 is focused into fiber 1400 and partially reflected by an interface having different refractive indices on either side. As detailed herein, the reflected portion 1135 of light 1105 establishes the WBC external cavity in tandem with the source(s) of the light 1105, and the transmitted multi-wavelength portion 1140 of the light propagates through the fiber 1400 and may be utilized in any of a variety of applications.
In various embodiments of the invention, the partially reflective interface 1410 may lie between the core 1125 and/or cladding 1130 of the fiber and an end cap 1420 that includes, consists essentially of, or consists of a material different from that of the core 1125 and/or cladding 1130, as shown in FIG. 14A. For example, the core 1125 and/or cladding 1130 may include or consist essentially of glass or fused silica, while the end cap 1420 may include or consist essentially of sapphire. While FIG. 14A depicts the entire end cap 1420 as being composed of a material different from that of the remaining parts of the fiber, in various embodiments only a portion of end cap 1420 (e.g., the portion facing and/or in contact with core 1125 and/or cladding 1130) is composed of the different material.
As shown in FIG. 14B, the partially reflective interface 1410 may also be established between the material of the end cap 1110 and/or core 1125 and/or cladding 1130 (e.g., silica) and a void (i.e., vacuum) or air region (e.g., bubble) 1430 disposed within the fiber 1400. In an exemplary embodiment in which the end cap 1110 is composed of silica and region 1430 comprises air, interface 1410 may have approximately 4% reflectivity. Other materials combinations may be utilized to tailor the reflectivity and/or wavelength dependence of partially reflective interface 1410. In various embodiments, the interface 1410 may be disposed on the exit surface of end cap 1120 (or of the fiber itself, if end cap 1120 is not present) when that surface lacks an anti-reflective coating (e.g., anti-reflective coating 1115) thereon. The resulting glass/air interface may have approximately 4% reflectivity and act as the partially reflective output coupler, reflecting light portion 1135 to establish the external cavity and transmitting multi-wavelength output beam 1140.
In various embodiments of the present invention, the partially reflective output coupler 116 is disposed optically downstream of the optical fiber, which is thereby disposed within the external WBC cavity. FIG. 15 depicts an exemplary WBC laser system 1500 that incorporates an optical fiber 1510 within the WBC external cavity. As shown, in WBC system 1500, beams are emitted by a one-dimensional or two-dimensional array of beam emitters 102. As in WBC laser system 1150, the beams may be manipulated by one or more (e.g., one for each beam, or one shared by all beams) optics 1155. Optics 1155 collimate, rotate, and/or otherwise manipulate the beams emitted by beam emitters 102. The WBC system 1500 also includes focusing optics 108 that combine the beams emitted by the emitters 102 to form a combined beam that propagates toward dispersive element 114. The dispersive element 114 (e.g., a diffraction grating, a dispersive prism, a grism (prism/grating), a transmission grating, or an Echelle grating) may be disposed approximately at the focal length of the focusing optics 108. As described herein, the dispersive element 114 receives the combined beam and transmits the beam as a multi-wavelength beam having a high brightness. The multi-wavelength beam is focused by optics 1160 (e.g., one or more spherical and/or cylindrical lenses) and transmitted to the optical fiber 1510, which may incorporate end caps as described herein. After propagating through the optical fiber 1510, the light is focused by optics 1520 (e.g., one or more cylindrical and/or spherical lenses) onto partially reflective output coupler 116, which transmits a portion of the multi-wavelength beam and reflects a second portion of the multi-wavelength beam back toward the dispersive element 114 and thence to the emitters 102, forming the external lasing cavity. In such embodiments, the output coupler 116 (and, in some embodiments, optics 1520) may be disposed within a processing laser head 1530 that is discrete from (and may be detachable from) the other components of laser system 1500 and that delivers the multi-wavelength output beam to a workpiece for applications such as cutting, welding, etc.
WBC laser systems in accordance with various embodiments of the present invention (e.g. WBC laser systems 1150, 1250, 1500) have advantages over conventional systems. First, the efficiency of the WBC laser may be increased due to reduced losses in fiber coupling, as the output coupler (and at least a portion of the fiber) is integrated into the external cavity. Second, the fiber may serve as a mode filter for the external cavity, selecting the desirable modes for efficient fiber coupling. For this reason the external cavity laser threshold may be reduced for lasers in accordance with embodiments of the invention (e.g., WBC laser system 1250). Third, the AR coating at the fiber input end may be omitted, which eliminates the risk of damage to the AR coating at this interface. AR coatings may be a failure point for optical components in general, since AR coatings often have a lower damage threshold as compared with bulk glass. A fourth advantage is a built-in safety mechanism at the input fiber connectorization point. If the fiber is disconnected from the laser and current is applied to the diode laser, owing to the low-reflectivity facet coating and the presence of the external cavity, the diode laser will not lase in the external cavity mode and instead, only weak incoherent emission will typically emerge from the output port. The WBC laser does not lase in the external cavity without the fiber (and any fiber connector(s)) attached. Thus, the laser may in general not be turned on and used to obtain significant output without having the fiber (and any fiber connector(s)) installed. This feature makes the WBC laser relatively safe to prevent high optical output power in free space when the output fiber is disconnected.
The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

## Claims

1. A laser system comprising:
an array of beam emitters each emitting a beam;
focusing optics for focusing the beams toward a dispersive element;
the dispersive element for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam;
an optical fiber for receiving the multi-wavelength beam; and
disposed within or on the optical fiber, a partially reflective interface for receiving the multi-wavelength beam, reflecting a first portion thereof back toward the dispersive element, and transmitting a second portion thereof as an output beam composed of multiple wavelengths.

2. The laser system of claim 1, further comprising an end cap attached to the optical fiber, the partially reflective interface being disposed between the end cap and the optical fiber.

3. The laser system of claim 2, wherein (i) the end cap comprises a first material, (ii) the optical fiber comprises a second material, and (iii) the first material is different from the second material.

4. The laser system of claim 3, wherein the first material comprises sapphire and the second material comprises glass.

5. The laser system of claim 2, wherein the partially reflective interface comprises a coating applied to a surface of the end cap and/or a surface of the optical fiber.

6. The laser system of claim 1, wherein (i) the partially reflective interface is disposed (a) within the optical fiber, and (b) between a portion of the optical fiber and a void disposed within the optical fiber, and (ii) a cross-section of the void is smaller than a cross-section of the optical fiber.

7. The laser system of claim 6, wherein the void comprises therewithin air or vacuum.

8. The laser system of claim 1, wherein (i) the optical fiber comprises first and second optical fiber portions joined together, and (ii) the partially reflective interface is disposed between the first and second optical fiber portions.

9. The laser system of claim 8, further comprising a mode field adaptor or a fiber taper disposed optically downstream of the partially reflective interface.

10. The laser system of claim 8, wherein (i) the first optical fiber portion comprises a core region optically upstream of the partially reflective interface, (ii) the second optical fiber portion comprises a core region optically downstream of the partially reflective interface, and (iii) a diameter of the core region of the second optical fiber portion is larger than a diameter of the core region of the first optical fiber portion.

11. The laser system of claim 6, wherein the void comprises vacuum therewithin.

12. The laser system of claim 6, wherein the void comprises a bubble disposed within the optical fiber.

13. The laser system of claim 1, wherein the dispersive element comprises a diffraction grating.

14. The laser system of claim 1, wherein the focusing optics comprises one or more cylindrical lenses, one or more cylindrical mirrors, one or more spherical lenses, and/or one or more spherical mirrors.

15. The laser system of claim 1, further comprising, disposed between the array of beam emitters and the focusing optics, second optics for rotating the beams emitted by the array of beam emitters.

16. The laser system of claim 5, wherein the coating has a reflectivity to at least one wavelength of the multi-wavelength beam ranging from about 1% to about 10%.

17. The laser system of claim 5, wherein the coating comprises one or more metallic layers.

18. The laser system of claim 5, wherein the coating comprises one or more dielectric layers.