Power and brightness scaling in fiber-coupled diode lasers using diodes with optimized beam size

Abstract

Being able to use an emission beam with a greater divergence increases the number of beams that can be coupled into the optical fiber, thereby providing increased beam power. Alternatively, with a fixed number of emitters, the total optical power can be maintained with fewer beams in the output beam at a smaller numerical aperture.

Description

[0001] This application is a divisional application of the invention patent application with the application date of January 10, 2018, entitled “Power and brightness scaling in fiber-coupled diode lasers using diodes with optimized beam size” and application number 201880011359.5.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 444,773, filed on January 10, 2017, the disclosure of which is hereby incorporated by reference in its entirety. Technical Field

[0004] The present disclosure relates to multi-emitter laser diode systems. Background Art

[0005] There is a great demand for fiber-coupled high-power, high-brightness diode lasers for applications such as fiber laser pumping and material processing. In some approaches such as those described in U.S. Pat. No. 8,891,579, U.S. Pat. No. 9,455,552, and U.S. Patent Publication No. 2016 / 0181762 (the disclosures of which are hereby incorporated by reference in their entirety), multiple single-emitter diode lasers are stacked along the fast axis to achieve power scaling and brightness improvement. One way to further scale the package power is to add more emitters in the fast axis direction. However, adding emitters increases the fast axis numerical aperture (NA) such that the power in the same NA does not increase further. Therefore, it is desirable to be able to increase the number of emitters stacked in the fast axis for power scaling and keep the fast axis image size and NA unchanged at the fiber aperture, or equivalently, keep the fast axis beam parameter product (BPP) unchanged, so as to achieve brightness improvement. Summary of the invention

[0006] The present disclosure generally relates to methods and apparatus that enable the use of an emitted light beam with a greater divergence to increase the number of light beams that can be coupled into an optical fiber, thereby providing increased coupled beam power. Alternatively, with a fixed number of emitters, the total optical power can be maintained with fewer light beams in an output light beam with a smaller numerical aperture.

[0007] In some examples, the apparatus includes a group of laser diodes having a predetermined spacing relative to a stacking axis, each laser diode having an emission region defining a fast axis and a slow axis, the emission region producing an emission beam having a beam fast axis full width of less than 1.62 μm. The fast axes of each of the laser diodes are mutually parallel and orthogonal to the stacking axis, and the slow axes of each of the laser diodes are mutually parallel and orthogonal to the stacking axis. A fast axis collimator (FAC) is associated with each of the laser diodes, and each of the fast axis collimators has a common focal length. The emission region of each of the laser diodes is coupled to a corresponding fast axis collimator to produce a corresponding beam collimated along the fast axis. An afocal telescope (also referred to as a fast axis telescope or FAT) is positioned to receive the fast axis collimated beam from the fast axis collimator and produce a fast axis beam spacing based on the predetermined spacing and a magnification M associated with the afocal telescope. An objective lens receives the fast axis collimated beam from the afocal telescope and delivers the combined beam to the core of an optical fiber. Typically, the FAC and the objective lens have focal lengths f FAC and f O , M is the magnification of the FAT in the fast axis direction, and the magnification of the emission region of the laser diode at the optical fiber is f O / Mf FA In a typical example, M is less than 0.8 and f FAC Between 200 μm and 1000 μm, between 250 μm and 600 μm, between 200 μm and 400 μm, and f O Between 5mm and 12mm. In a specific example, the group of laser diodes includes two groups of seven laser diodes, coupling at least 160W of total optical power into the optical fiber. The optical fiber has a core diameter of 50μm, 105μm or 200μm, and the fast axis spacing is between 400μm and 450μm. In a typical embodiment, the laser diodes are distributed with a fixed spacing along the slow axis direction. In another example, a group of reflectors corresponds to each of the laser diodes in the group of laser diodes, and the group of reflectors includes a corresponding reflector, and the corresponding reflector is positioned to receive a beam from a corresponding FAC and guide the beam to the objective lens so that the propagation axis of the guided beam at the objective lens is parallel. Typically, a group of slow axis collimators (SACs) are positioned to receive corresponding beams from the FACs and guide FA and SA collimated beams to corresponding reflectors.

[0008] The optical beam source includes a group of laser diodes positioned with a fixed spacing along respective fast axes parallel to each other, each laser diode being defined by a fast axis beam having a full width of less than 1.6 μm and having an emission region defining a fast axis and a slow axis, wherein the fast axes of each of the laser diodes are parallel to each other and the slow axes of each of the laser diodes are parallel to each other and orthogonal to the fast axis. A fast axis collimator (FAC) and a slow axis collimator (SAC) correspond to each of the laser diodes of the group and are positioned to collimate the optical beam emitted from the respective emission region of each of the laser diodes. Each of the FACs may have a common focal length f FAC , and each of the SACs may have a common focal length f SAC . An afocal telescope is positioned to receive the collimated beam and generate a fast-axis beam spacing based on the predetermined spacing and a magnification M associated with the afocal telescope. An objective lens is positioned to receive the beam from the afocal telescope and direct the beam to a focus. An optical fiber having an input surface is positioned at the focus of the objective lens. In a specific example, the optical fiber has a core diameter between 50 μm and 1.0 mm. In other examples, each of the laser diodes is secured to a corresponding step of a laser diode mount having a plurality of steps so as to be evenly spaced along a corresponding fast axis.

[0009] The method includes selecting at least one of an emission beam fast axis divergence, an emission beam fast axis size, and a fast axis waveguide thickness based on a predetermined step height of a laser diode mount so as to couple an emission beam from a selected number of laser diodes into an optical fiber. The selected number of laser diodes are positioned at the predetermined height, and slow axis and fast axis collimating lenses are positioned to produce a collimated beam from the emission beam from each of the laser diodes. A fast axis telescope and an objective lens are positioned to direct the collimated beam into the optical fiber. In some examples, at least one of the emission beam fast axis divergence, the emission beam fast axis size, and the fast axis waveguide thickness is selected based on the numerical aperture of the optical fiber. In a typical example, the step height is between 400 μm and 500 μm, and the magnification of the fast axis telescope is less than one. In some embodiments, the fast axis magnification of the emission beam is selected based on the core diameter of the optical fiber.

[0010] In other examples, the method includes selecting at least one of an emission beam fast axis divergence, an emission beam fast axis size, or a fast axis waveguide thickness based on a fixed spacing along a fast axis direction and a number of laser diodes to couple a beam having a selected power, a selected numerical aperture, or a combination of both into an optical fiber. The laser diodes are positioned at the fixed spacing, and an emission beam from each of the laser diodes is directed to the optical fiber using corresponding fast and slow axis collimators, a fast axis telescope, and an objective lens. In a typical example, the fast axis size of the emission beam is less than 1.60 μm.

[0011] The above and other objects, features and advantages of the disclosed technology will become more apparent from the following detailed description made with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A multi-emitter assembly is shown coupling light beams from multiple emitters into an optical fiber.

[0013] Figure 2A A representative laser diode assembly is shown, where three laser beam stacks are compressed by a beam compressor.

[0014] Figure 2B The input beam stack is shown.

[0015] Figure 2C A compressed beam stack is shown.

[0016] Figure 3A A laser diode assembly is shown in which two beam stacks are formed using polarization multiplexing and compressed using a beam compressor.

[0017] Figure 3B-3C The beam stack before and after compression is shown.

[0018] Figure 3D A representative stepped laser diode mount is shown.

[0019] Figure 3E-3F A beam stack without inter-beam gaps before and after compression is shown.

[0020] Figure 4 A method of coupling multiple laser diode beams into an optical fiber is shown.

[0021] Figures 5A-5C A multi-emitter arrangement is shown, where the beams from three laser diodes are combined in order to fill the objective aperture. DETAILED DESCRIPTION

[0022] Conventional multi-emitter laser diode systems typically attempt to achieve high output power by combining beams from multiple emitters. For high power operation, a relatively thick semiconductor gain region is used to reduce the power density to allow long-term, damage-free emitter operation. Beam combining can be limited by the relatively large fast-axis numerical aperture of the emitters, and increasing the gain region thickness to provide increased power and damage resistance produces a reduced fast-axis numerical aperture, simplifying the beam combining optics. The present disclosure takes an approach that recognizes that a thinner gain region and higher fast-axis numerical aperture allow power from more emitters to be coupled into the beam delivery fiber, thereby providing higher beam power, compared to conventional approaches. Thus, disregarding the conventional approach allows for superior multi-emitter laser diode systems as described below.

[0023] As used in this application and the claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. In addition, the term "comprising" means "including."

[0024] The systems, devices, and methods described herein should not be construed as limiting in any way. Rather, the present disclosure relates to all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and subcombinations with each other. The disclosed systems, methods, and devices are not limited to any particular aspect or feature or combination thereof, nor do the disclosed systems, methods, and devices require the presence of any one or more specific advantages or problems to be solved.

[0025] Although the operations of some disclosed methods are described in a particular sequential order for ease of presentation, it should be understood that this description includes rearrangement unless the specific language set forth below requires a particular order. For example, in some cases, the operations described in sequence can be rearranged or performed simultaneously. In addition, for simplicity, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and devices can be used in conjunction with other systems, methods, and devices. In addition, the description sometimes uses terms such as "produce" and "provide" to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the specific implementation, and can be easily discerned by those of ordinary skill in the art.

[0026] The theories of operation, scientific principles or other theoretical descriptions proposed herein with reference to the devices or methods disclosed herein have been provided for better understanding and are not intended to limit the scope. The devices and methods in the appended claims are not limited to those devices and methods that function in the manner described by such theories of operation.

[0027] In the following description, a laser beam or other propagating optical radiation is referred to as propagating along one or more axes. As used herein, such an axis refers to a linear axis that can be bent or folded using an optical element such as a prism or a reflector. In some examples, an optical element such as a cylindrical lens or a spherical lens is referred to as being centered or on an axis to indicate a lens having a central axis pointing through the center of curvature of the surface. In other examples, off-axis lenses or lens segments can be used. The lens center thickness refers to the lens thickness taken along the principal axis extending through the center of curvature of the surface. In the disclosed examples, the surface curvature generally corresponds to a cylindrical or spherical surface, but more complex curvatures can be used. Even for more complex surfaces such as aspherical surfaces, the surface curvature radius is generally defined based on the on-axis curvature, although the off-axis surface shape does not follow this curvature. Anti-reflection or other coatings can be provided to the optical surface, but these coatings are omitted from the disclosed examples. If within about 1 degree, 2 degrees or 5 degrees, the beam, surface and axis are generally referred to as parallel.

[0028] Disclosed herein are systems, methods, and apparatus that can allow for laser package brightness scaling based on selecting an appropriate fast axis dimension of the emission region of a single-emitter laser diode (typically by selecting an epitaxial waveguide design with a selected waveguide thickness, refractive index, and refractive index difference). Laser diodes are typically defined by one or more epitaxial layers such that the emission region is thin but elongated. The axis parallel to the epitaxial layer thickness dimension is referred to herein as the fast axis (FA) because the emitted beam has a greater divergence in this direction. Typically, the emission in the FA direction is diffraction limited, and the emitted beam numerical aperture is proportional to the ratio of the beam wavelength and the emission region thickness. The axis parallel to the epitaxial layer is referred to herein as the slow axis (SA) because the emitted beam has a smaller divergence in this direction than along the fast axis. In most examples, along the slow axis, the emitted beam does not correspond to a single mode and can have a complex intensity distribution, while along the fast axis, the beam intensity is approximately Gaussian. For convenience, the beam divergence and size relative to the fast and slow axes are referred to as fast / slow axis numerical apertures or fast / slow axis angular radii or diameters. Beam size is similarly referred to as fast / slow axis beam diameter, radius, or width. Beam size can be based on 1 / e 2 Intensity or amplitude or other value, and can be full width or half width. In the following figures and description, beam steering in the slow axis direction may not be discussed to emphasize fast axis characteristics.

[0029] In some examples, based on the equation corresponding to 1 / e 2 The emitted beam can also be characterized by reference to the dimensions of the effective emission area and size, or the angular dimensions corresponding to the ratio of the emission wavelength to the beam size or emission area size. In a typical example, the emission beam has a wavelength of >50° (FW, at 1 / e 2Laser diodes with fast axis divergence (at a power of 200 nm) are stacked in a diode laser array. For a fixed fast axis collimator (FAC) focal length and step height, a larger fast axis divergence allows a higher fill factor / smaller dead zone in the combined collimated beam produced by the FAC, slow axis collimator (SAC), and fast axis telescope (FAT). The beam divergence is typically a function of the beam wavelength, the size of the beam emission area, and the waveguide refractive index. A specific divergence angle can be selected based on the beam wavelength and emission area characteristics. In most examples, laser diodes with emission wavelengths between 800 nm and 1,000 nm are used, but laser diodes with any available wavelength range between 200 nm and 2,000 nm or any other range can also be used.

[0030] Figure 1 Portions of a representative multi-emitter system are depicted. For ease of illustration, three emitters and associated optical components are shown in detail, but additional emitters and associated optical components are typically included and are Figure 1 These additional transmitters and associated optical components are indicated by dashed lines in FIG. Figure 1 As shown in FIG. , the laser diodes 102A-102C are arranged along the fast axis direction ( Figure 1 The X direction is shown in the figure) separated by a common distance d stair , and emits a fast axis with an angular radius θ FA The laser diodes 102A-102C are typically offset in the Y direction (perpendicular to Figure 1 plane), but this displacement is not Figure 1 106A-106C receives the beams from the laser diodes 102A-102C and produces collimated beams 108A-108C directed along axes 107A-107C. An afocal telescope (also known as a beam expander or fast axis telescope (FAT)) 110 receives the beams and produces compressed output beams 112A-112C. An objective lens 116 then focuses or otherwise directs the output beams 112A-112C into a core 118 of an optical fiber 120. A combined beam formed by the focused beams corresponding to the compressed output beams 112A-112C is directed to the core 118, which has a numerical aperture (NA) of about mMd stair / 2f O proportional to, where M is the FAT magnification, m is the total number of emitters, and f O is the focal length of the objective lens 116. In a typical practical example, the magnification M<1 and is applied only along the fast axis using an afocal telescope based on a cylindrical lens. The fast axis angular radius of each focused compressed beam is approximately Mf FAC θ FA / f O, and the fast axis magnification of the emission region of each laser diode 102A-102C is approximately f O / Mf FAC , so the product of the angular radius and the fast axis beam height remains constant. Objective lens 116 is typically positioned to focus compressed beams 112A-112C at surface 122 of optical fiber 120. Figure 1 As shown in FIG. 1 , laser diodes 102A-102C generate beams with a common divergence, and FACs 106A-106C have a common focal length. The fast axis magnification of the emission region of each laser diode 102A-102C is approximately f O / Mf SAC ; The slow axis beam size is usually not changed by FAT.

[0031] exist Figure 1 In the example of FIG. 1 , the emission beams from the laser diodes 102A-102C are shown to fill the lens aperture, and in cases where the FA lens aperture is not filled, additional emitters and associated optics can be positioned to fill any gaps to increase the coupled optical power. In most cases, the beams are arranged to slightly underfill the FAC to avoid clipping.

[0032] In such Figure 1 In the multi-emitter system shown in , the fast axis image spot size at the input surface of the output fiber can be expressed as:

[0033]

[0034] Among them B FA is the emitting diode beam size in the fast axis direction, f O and f FAC are the focal lengths of the objective lens and FAC, respectively, and M is the magnification of the fast axis telescope (FAT). For the first order, the fast axis NA FA Independent of the diode fast axis beam size or the FAC lens focal length:

[0035]

[0036] where d stair is the diode laser step (stack) height, and m is the number of diode lasers stacked in the fast axis direction. Then the fast axis beam parameter product (BPP FA )for:

[0037]

[0038] To increase the fiber-coupled brightness in the fast axis, the number of stacked diode lasers can be increased, thus increasing the power, while keeping the fast axis BPP constant and below the maximum allowable fiber BPP.

[0039] Ability to reduce step height to d stair2 , so that d stair2 =d stair1 ×m1 / m2, where m1 and m2 are stair1 d stair2 Unfortunately, this approach requires a change in the fast axis spacing d stair These substrates may be difficult or expensive to manufacture, and it is often desirable to use substrates with a common fast axis spacing d for multi-emitter systems with varying numbers of emitters. stair The emitter fast axis spacing d stair Typically between about 250 μm and 1.0 mm, or between about 400 μm and 500 μm, and in practice manufactured to tolerances of ±10 μm, ±20 μm, ±50 μm, and in some cases made from expensive materials or materials that are difficult to machine. In some examples, fixed spacings of 100 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, 550 μm, 600 μm, 750 μm, 800 μm, 900 μm, or 1000 μm are used for various numbers of emitters, or other spacing ranges from about 75 μm to about 5 mm. Thus, an approach for increasing the stack of emitters along the fast axis without requiring a change in fast axis spacing is preferred. Alternatively, the FAC focal length can be increased and then the FAT magnification reduced to f FAC2 =f FAC1 × m2 / m1, and M2 = M1 × m1 / m2. This requires special FAC designs, which can be complex and may require different mounting and assembly materials and procedures in any case. Thus, it is preferred that there is no need to change the FAC approach, especially for manufacturing systems with different numbers of emitters using common optical and mechanical components.

[0040] To facilitate manufacturability and scalability, the fast axis beam size and FAT magnification can be reduced so that B FA2 =B FA1 ×m1 / m2 and M2=M1×m1 / m2. Compared to the other approaches described above, this requires almost no change to the optical design architecture or the optical alignment and manufacturing process; the fast axis beam size can be controlled by the emitter epilayer design. Reducing the fast axis beam size of the diode laser allows the fast axis BPP to be reduced for low beam NA, or allows the number of emitters to be increased with the same BPP to achieve high power / brightness. The product of the single emitter fast axis BPP and the number of fast axis stacked diode lasers is usually less than the maximum fast axis BPP allowed by the fiber, i.e. m×BPP SE <BPP FA, where m is the number of stacked emitters, BPP SE is a single emitter BPP, and BPP FA More individual emitters can be stacked and coupled into the fiber at the expense of beam clipping, which can result in power loss and reduced reliability.

[0041] Some representative examples are summarized in the table below, which also includes parameters for conventional multi-emitter systems based on larger initial fast axis beam sizes. The systems in the table are based on combining two sets of fast axis stacked emitters using polarization, wavelength or other multiplexing methods, and the total number of emitters is indicated as 2x7 or 2x8. The table also includes the slow axis (SA) beam size, where the same SA characteristics are used in the representative examples. The FA image size can be determined as B FA f O / (f FAC M). In these examples, the emission wavelength is about 0.9 μm, but similar systems can be provided for other wavelengths. Typical wavelength ranges are about 250 nm to about 2.0 μm.

[0042]

[0043] Table 1. Representative multi-emitter laser diode systems

[0044] As can be seen in Table 1, thin (or high NA) emitters can be used to increase fiber-coupled power by combining power from additional emitters, even if the laser diodes are fixedly spaced along the fast axis, or, without increasing the number of emitters, to reduce the beam NA at the output fiber while providing the same power. It is possible to use an emitted fast-axis beam size (1 / e 2.0) of less than 1.60 μm, 1.58 μm, 1.56 μm, 1.54 μm, 1.52 μm, 1.50 μm, 1.48 μm, 1.46 μm, 1.44 μm, 1.42 μm, 1.40 μm, 1.38 μm, 1.36 μm, 1.34 μm, 1.32 μm, 1.30 μm, 1.28 μm, or smaller. 2 The FAT magnification is 0.9, 0.85, 0.8, 0.75, 0.7, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.30 or less, or any value between 0.25 and 1.0. In one example, a magnification of 0.34 is used.

[0045] The above method of brightness scaling by reduced fast axis beam size does not require a specific slow axis size and can be applied to fiber coupled diode laser packages using laser diodes with various slow axis widths. According to conventional understanding, in a system such as the one described above, the probability of catastrophic optical mirror damage (COMD) will increase at a given power at each emitter. The maximum CW diode power limited by the cavity facet COMD can be expressed as:

[0046]

[0047] Among them, d QW is the active layer thickness, Γ is the fast axis mode confinement factor, d QW / Γ is the effective fast-axis modal size, W is the slow-axis beam width, R is the PR cavity facet (output coupler) reflectivity, is the COMD power density. Reducing the fast axis beam size alone will reduce the maximum diode power or increase the likelihood of COMD when operating at the same power level. In some examples, increasing the slow axis beam width can compensate for the reduction in fast axis beam size to obtain comparable cavity surface reliability. In order to achieve high cavity surface reliability without sacrificing slow axis brightness, a laser diode design with a wider slow axis beam width but with the same or even lower slow axis BPP is preferred. For example, a specific SA beam size can be selected in a so-called trumpet-shaped laser diode as described in U.S. Patent 9,166,369, which is hereby incorporated by reference in its entirety. Trumpet-shaped laser diodes allow selection from a variety of slow axis widths that all provide the same SABPP and are able to meet cavity surface reliability requirements when the fast axis beam size is reduced.

[0048] The examples in Table 1 are examples only and should not be considered limiting. For clarity, the dimensions and NA of the output fiber are calculated using a geometric optics model and are all at a wavelength of about 0.9 μm.

[0049] The low NA high brightness 2x7 emitter example (second column) uses a laser diode with a reduced FA beam size of 1.34μm, instead of the traditional FA beam size of 1.7μm. This results in a reduced fast axis BPP at the same number of diode lasers 2x7. The FAT magnification is modified to obtain the same fast axis image size at the fiber (for the same efficient fiber coupling). On the other hand, due to the reduction in fast axis BPP, the fast axis NA is reduced from 0.138 to 0.107. The 95% NA is 0.132, compared to 0.150. Although the fiber coupled power remains at a similar level due to the same number of emitters, the brightness is significantly improved. In some examples, both the FAT magnification and the objective lens focal length are varied.

[0050] As in the low NA package, the high brightness 2x9 emitters use the same laser diode with reduced FA beam size, and the same optics. Instead of aiming for a reduced fast axis BPP, two additional rows of emitters are stacked in the vertical direction for power scaling while maintaining the same fast axis BPP as in the conventional package (one column). The result is a 28% increase in fiber coupled power.

[0051] Table 2 lists the fast-axis modal limiting factors and slow-axis beam widths for several diode laser designs: a 95 μm rectangular design with a fast-axis beam size of 1.7 μm and a trumpet laser diode with a fast-axis beam size of 1.34 μm. The trumpet laser diode has a slow-axis beam width of 125 μm, but the BPP is similar or smaller than that of the 95 μm rectangular laser. The maximum CW COMD power (in W×d QW / Γ) is 7% higher than the rectangular design, even though it has a 21% smaller fast-axis beam size. Therefore, there should be no facet reliability concerns for flared laser diodes with reduced fast-axis beam size.

[0052]

[0053] Table 2. Beam parameters related to maximum COMD power for non-flared and flared laser diodes with fast axis beam sizes of 1.7 μm and 1.34 μm, respectively

[0054] exist FIG. 2A to FIG. 2C Another example of using a thinned waveguide is shown in . A packaged diode laser assembly 200 includes a housing 202, which is partially removed for ease of illustration. A stepped surface 204 includes a plurality of stepped surface portions 206A-206C, to which one or more diode lasers 208A-208C are respectively affixed so as to be located at different heights, typically so as to be monotonically descending or ascending. Typically, a single diode laser is affixed to each step, and the diode lasers are affixed to the steps so as to emit laser radiation along substantially parallel axes. As will be discussed below, deviations from a parallel arrangement can be compensated for as desired. In . Figure 2A , the fast axes of the laser diodes 208A-208C are perpendicular to the plane of the figure; the slow axes are in the plane of the figure. The light beams emitted from the laser diodes 208A-208C are received and collimated by fast axis collimating optics 212 and slow axis collimating optics 214 to produce collimated light beams. An optional volume Bragg grating element 213 can be located between the fast axis collimating optics 212 and the slow axis collimating optics 214 to provide wavelength locking of the laser diodes 208A-208C. A steering mirror 216 receives the collimated light beams, which propagate approximately parallel to each other after collimation. In Figure 2A In the example of FIG. 1 , the steering mirror 216 is positioned to reflect the collimated beam at a right angle so that the reflected beam is directed to the beam compressor 220 (i.e., the fast axis telescope). The propagation direction of each laser diode can be adjusted by corresponding adjustment of the corresponding reflector. At the beam compressor 220, the fast axes of the reflected beams are stacked one above the other to form separate stacked beams 222A-222C (corresponding to the diode lasers 208A-208C, respectively, and having heights based on the stepped surface portions 206A-206C). Figure 2B ). The turning mirror 216 and the slow-axis collimating optics 214 can be conveniently secured to a common surface 218.

[0055] The topmost part of each steering mirror 216 for each collimated beam is located at a certain height so that the reflected beam is not clipped by the subsequent steering mirror 216. Figure 2A , the bottom-most reflector in the figure has a maximum height to correspond to the maximum step height of the corresponding stepped surface portion 206A. The spacing of the stacked beams 222A-222C is adjusted using a beam compressor 220, and the compressed beams are then directed to an objective lens 224, which directs the compressed beams into the optical fiber 210. In some examples, the objective lens 224 is a single plano-convex lens, while in other examples, more complex multi-element lenses are used, including spherical and aspherical surfaces.

[0056] like Figure 2B-2C As shown in FIG. 1 , the input beam stack of stacked beams 222A-222C is output by the beam compressor 220 as a compressed beam stack of beams 232A-232C. It should be noted that the beam separation is compressed along with the beam diameter in the compression direction. In this example, the gaps between the stacked beams 222A-222 are also compressed.

[0057] refer to Figures 3A to 3D The diode laser assembly 300 is described with reference to a right-handed xyz coordinate system 380, wherein the z-axis extends upwardly out of Figure 3A The diode laser assembly 300 includes groups of laser diodes 302-305, which are arranged to emit corresponding groups of laser beams to corresponding groups of reflectors and fast-axis and slow-axis collimators 312-315. For example, the laser diodes of group 302 emit beams in the x-axis direction, and the beams are then redirected by corresponding reflectors of group 312 to propagate in the y-axis direction. The laser diodes of each group are displaced or offset from the other laser diodes of the same group along the z-axis, and the associated reflectors are positioned so that the laser beams from the group are not blocked by the reflectors. Figure 3DAs shown in FIG, laser diode group 302 is secured to a stepped mount 320 to provide the appropriate z-axis offset; similar mounts are provided for the remaining laser diode groups. In most cases, stepped mount 320 is made of a thermally conductive material. For convenience, the beam propagation axes 322-325 of the bottom-most laser diode of each group are shown; the beam propagation axes of the remaining laser diodes of each group are similar, but shifted along the z-axis.

[0058] The laser beam from laser diode group 302 is directed to half-wave retarder 330 and then combined with the laser beam from laser diode group 303 at polarization beam splitter 332, so that vertically stacked beam group 336A (at Figure 3B 315) is incident on the beam compressor 340. The laser beam from the laser diode group 305 is guided to the half-wave retarder 334 by the reflector of the group 315, and then combined with the laser beam from the laser diode group 304 redirected by the reflector of the group 314 at the polarization beam splitter 335, so that the vertically stacked beam group 336A is incident on the beam compressor 340.

[0059] Figure 3B Stacked beams 336A, 336B are shown incident on a beam compressor 340; Figure 3C Stacked beams 356A, 356B are shown exiting the beam compressor 340, where both the beam spacing and the individual beam heights are changed by the compression ratio M = H2 / H1. In this example, a cylindrical beam compressor is used, and there is compression in the z-direction, but not in the x-direction. Additional compressors can be provided to compress in other directions, or compressors utilizing spherical surfaces can be used. In some examples, the beams are arranged without gaps along the fast axis direction. Figure 3E shows stacked beams 360A, 360B exiting the beam compressor 340 with no gaps between the beams, Figure 3F Stacked beams 366A, 366B are shown exiting the beam compressor 340 with no gaps between the beams. The additional space available by positioning the beams without gaps can be provided with beams from additional emitters to provide higher output power, or to utilize multiple emitters by providing additional wavelengths, modulations, or other beam characteristics. Alternatively, the same number of beams can be packed into a smaller area at the FAT, thereby reducing the numerical aperture of the beam provided by the objective.

[0060] refer to Figure 4, method 400 includes selecting a fast axis emitter spacing at 402, typically based on an available stepped mount. At 404, an output power and an output NA are selected, and at 406, a number of emitters are determined based on the selected output power. At 408, an emitted FA beam width is selected to allow the necessary number of emitters to be combined into the selected NA. At 410, beams from the spaced emitters are coupled to an output fiber using a FAC, a FAT, and an objective lens to achieve the selected NA and power. Typically, the FAC, FAT, and objective lens are selected to magnify the emission area so as to fill the core of the optical fiber. In some cases, at 410, an emitted SA beam width or slow axis size is selected so that the slow axis BPP is less than the maximum allowable fiber slow axis BPP and COMD is avoided, and in some cases, the SA beam width is increased to provide a suitable beam intensity.

[0061] A representative three laser based system 500 is shown in FIG5 with reference to an xyz coordinate system shown in directions 550, 555. For ease of illustration, the unfolding mirrors fold the optical axis to remain straight. Thus, beam reflections from inclined planar surfaces are shown as continuous straight beam propagation. Typical folded propagation paths are shown above, such as Figure 2A as shown in .

[0062] The laser diodes 502A-502C are separated by a common distance d along the fast axis direction (the x direction as shown in FIG. 5 ). stair, and emits a beam collimated by the corresponding FAC 504A-504C. The FA collimated beams 506A-506C are directed to the corresponding SAC 507A-507C to produce FA and SA collimated beams 508A-508C. As shown in Figure 5, the laser diode emits a beam that propagates along the z-axis and has an emission axis shifted along the fast axis direction (x direction as shown in the figure). Typically, the laser diodes 502A-502C are positioned on a step mount so that the emission area of ​​the laser diodes 502A-502C is in a common plane parallel to the xy plane. Typically, the laser diodes 502A-502C are offset along the slow axis direction (y direction as shown). The beams 506A-506C appear to touch each other, but are shifted along the y-axis so that they do not need to be cut due to the FAC 504A-504C. As also shown, the emitted beam from each laser diode 502A-502C substantially fills the aperture of the FAC 504A-504C, respectively. The SACs 507A-507C are shown as being located in a common plane parallel to the xy plane, but can be distributed or conveniently propagated along the beam axis. In Figure 5, the SAC is represented as having no curvature because the SAC is typically a cylindrical lens with no curvature in the xz plane. The FA and SA collimated beams 508A-508C are directed to corresponding steering mirrors 509A-509C to produce collimated beams 515A-515C that propagate along the y-axis of the coordinate system 555. The FAT 513 reduces the beam size along the fast axis (x-direction) and produces output beams 512A-512C that are incident on the objective lens 516. In Figure 5A In the example of FIG. 5 , the FA and SA collimated beams 515A-515C are not separated by a gap and are incident on the FAT 513 without a gap. As a result, the output beams 512A-512C are not separated by a gap as input to the objective lens 516, as shown in FIG. Figure 5C In this arrangement, the individual beams are said to be combined so as to fill a portion of the aperture of the objective lens 516.

[0063] To couple the laser diode beam power into a selected fiber with a predetermined step step size, the fast axis beam size is controlled by a combination of the FAC focal length, the FAT magnification, and the objective focal length. These produce magnifications that are selected so that the individual beams are incident on an output fiber with a FA beam diameter that corresponds to the fiber core diameter. Similarly, the slow axis beam size is established based on the SAC focal length and the objective focal length, which are typically selected so that the SA beam diameter corresponds to the fiber core diameter. Representative combinations are listed in Table 3 below for a 2 by 9 configuration that provides a common excitation NA and a common SA / FA magnification.

[0064]

[0065] Table 3. Representative combinations of common combined beams

[0066] In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosure. We claim all arrangements that come within the scope and spirit of the following claims.

Claims

1. A device comprising: a group of laser diodes having a predetermined spacing relative to a stacking axis, each laser diode having an emission region defining a fast axis and a slow axis, the emission region producing an emission beam having a fast axis full width beam divergence of at least 51 degrees, wherein the fast axes of each of the laser diodes are parallel to each other and to the stacking axis, and the slow axes of each of the laser diodes are parallel to each other and orthogonal to the stacking axis; a fast axis collimator (FAC) associated with each of the laser diodes, each of the fast axis collimators having a common focal length, wherein an emission region of each of the laser diodes is coupled to a respective fast axis collimator to produce a respective light beam collimated along the fast axis; an afocal telescope positioned to receive the fast-axis collimated beam from the fast-axis collimator and to generate a fast-axis beam spacing based on the predetermined spacing and a magnification M associated with the afocal telescope; an objective lens positioned to receive the fast-axis collimated beam from the afocal telescope; and An optical fiber having a core positioned to receive the fast-axis collimated light beam from the objective.

2. The device according to claim 1, wherein: The FAC and the objective lens have focal lengths f FAC and f O , the magnification of the emission region of the laser diode at the optical fiber is f O / Mf FAC , and the fast axis numerical aperture of the fast axis collimated beam is NA FA =(d stair ×m×M) / (2×f O ), where m is the number of laser diodes in the group, and d stair is the predetermined distance.

3. The device according to claim 2, wherein: M is less than 1.

0.

4. The device according to claim 2, wherein: f FAC Between 200 μm and 400 μm, f O Between 5 mm and 12 mm, and M is less than 0.

9.

5. The device according to claim 1, wherein: Each diode in the group of laser diodes emits in a wavelength range of 800 nm to 1000 nm, and the groups of laser diodes include: 1) two groups of seven laser diodes that couple at least 160 W of total optical power into the optical fiber, 2) two groups of nine laser diodes that couple at least 200 W of total optical power in a beam having a numerical aperture of less than 0.15 into the optical fiber, wherein the optical fiber has a core diameter of 105 μm, 3) two groups of 15 laser diodes that couple at least 360 W of total optical power in a beam having a numerical aperture of less than 0.18 into the optical fiber, wherein the optical fiber has a core diameter of 105 μm, or 4) two groups of 24 laser diodes that couple at least 800 W of a beam having a numerical aperture of less than 0.18 into the optical fiber, wherein the optical fiber has a core diameter of 200 μm.

6. The device according to claim 5, wherein: The optical fiber has a core diameter of 105 μm or 200 μm.

7. The device according to claim 1, wherein: The predetermined spacing is between 200 μm and 1000 μm.

8. The device according to claim 1, wherein: The laser diodes are distributed along the slow axis direction to have a fixed slow axis interval.

9. The apparatus of claim 8, further comprising a set of reflectors corresponding to each of the laser diodes in the group of laser diodes, each reflector being positioned to receive a light beam from a corresponding FAC and direct the light beam to the objective lens so that a propagation axis of the directed light beam at the objective lens is parallel.

10. The apparatus of claim 9, further comprising a set of slow axis collimators (SACs) positioned to receive respective beams from the FACs and direct the FA and SA collimated beams to respective reflectors.

11. A method comprising: Based on the fixed spacing along the fast axis and the number of laser diodes, selecting a transmitted beam fast axis divergence of at least 51 degrees so as to couple a beam having a selected power, a selected numerical aperture, or a combination of both into the optical fiber; and positioning the number of laser diodes at the fixed intervals and directing an emission beam from each of the laser diodes to the optical fiber using corresponding fast-axis and slow-axis collimators, a fast-axis telescope, and an objective lens, wherein each of the laser diodes emits in a wavelength range of 800 nm to 1000 nm, and the number of laser diodes comprises: 1) two groups of seven laser diodes coupling at least 160 W of total optical power into the optical fiber, 2) two groups of nine laser diodes coupling at least 200 W of total optical power in a beam having a numerical aperture of less than 0.15 into the optical fiber, wherein the optical fiber has a core diameter of 105 μm, 3) two groups of 15 laser diodes coupling at least 360 W of total optical power in a beam having a numerical aperture of less than 0.18 into the optical fiber, wherein the optical fiber has a core diameter of 105 μm, or 4) two groups of 24 laser diodes coupling at least 800 W of a beam having a numerical aperture of less than 0.18 into the optical fiber, wherein the optical fiber has a core diameter of 200 μm.

12. A method comprising: Select the step height along the common fast axis; Based on the selected step height, selecting a common fast axis beam diameter and a common fast axis beam divergence of at least 51 degrees; shaping a beam from each laser diode in the group of laser diodes separated by the step height and having the common fast-axis beam diameter and the common fast-axis beam divergence along the common fast-axis direction using a corresponding fast-axis collimator, a fast-axis telescope, and an objective lens to produce a combined beam having a combined beam fast-axis beam diameter and a beam divergence corresponding to an output fiber core diameter and a numerical aperture, and The light beam from each laser diode in the group of laser diodes is shaped along a slow axis direction using a corresponding slow axis collimator and the objective lens so that the combined light beam has a combined light beam slow axis beam diameter and a beam divergence corresponding to the output fiber core diameter and numerical aperture, wherein the light beams of the group of laser diodes are combined to fill a portion of the aperture of the objective lens to enlarge the emission area filling the output fiber core diameter.

13. The method according to claim 12, wherein: The step height is between 200 μm and 1000 μm, and the magnification of the fast axis telescope is less than 1.

14. The method according to claim 12, wherein: The laser diodes are distributed along the slow axis direction to have a fixed slow axis interval.

15. The method according to claim 12, wherein: Each laser diode in the group of laser diodes emits in a wavelength range of 800 nm to 1000 nm, and the groups of laser diodes include: 1) two groups of seven laser diodes that couple at least 160 W of total optical power into the optical fiber, 2) two groups of nine laser diodes that couple at least 200 W of total optical power in a beam having a numerical aperture of less than 0.15 into the optical fiber, wherein the optical fiber has a core diameter of 105 μm, 3) two groups of 15 laser diodes that couple at least 360 W of total optical power in a beam having a numerical aperture of less than 0.18 into the optical fiber, wherein the optical fiber has a core diameter of 105 μm, or 4) two groups of 24 laser diodes that couple at least 800 W of a beam having a numerical aperture of less than 0.18 into the optical fiber, wherein the optical fiber has a core diameter of 200 μm.

16. The method according to claim 12, wherein: For each laser diode, a corresponding reflector is used to direct the light beam to the objective lens.

17. The method according to claim 16, wherein: The slow axis collimator is positioned to direct the fast axis and slow axis collimated beams to the respective reflectors.

18. The method according to claim 12, wherein: The focal length of the fast axis collimator is between 200 μm and 400 μm, and the focal length of the objective lens is between 5 mm and 12 mm.

19. The method according to claim 12, wherein: The output optical fiber has a core diameter of 105 μm or 200 μm.

20. The method according to claim 12, wherein: The laser diode is a flared laser diode.

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