CN107078466B

CN107078466B - Optical cross-coupling suppression system for wavelength beam combining laser system

Info

Publication number: CN107078466B
Application number: CN201580060011.1A
Authority: CN
Inventors: B·陈
Original assignee: Teradiode Inc
Current assignee: Panasonic Corp of North America
Priority date: 2014-12-10
Filing date: 2015-12-10
Publication date: 2020-01-14
Anticipated expiration: 2035-12-10
Also published as: WO2016094609A1; WO2016094609A8; CN107078466A; DE112015005587B4; JP2017539083A; CN110957637A; JP6585171B2; DE112015005587T5

Abstract

In various embodiments, a wavelength beam combining laser system includes an optical cross-coupling suppression system and/or a partially reflective output coupler designed to reduce or substantially eliminate unwanted back-reflections of stray light.

Description

Optical cross-coupling suppression system for wavelength beam combining laser system

RELATED APPLICATIONS

This application claims the benefit and priority of U.S. provisional patent application No.62/089,839, filed on month 12 and 10 of 2014. The entire contents of this application are incorporated herein by reference.

Technical Field

In various embodiments, the present invention relates to laser systems, in particular wavelength beam combining laser systems including systems for suppressing optical cross-coupling between beam emitters.

Background

High power laser systems are used in many different applications such as welding, cutting, drilling and material processing. Such laser systems typically include a laser transmitter, the laser light from which is coupled to an optical fiber (or simply "fiber"), and an optical system that focuses the laser light from the fiber onto the workpiece to be machined. The optical system is typically designed to produce the highest quality laser beam or equivalently the beam with the lowest Beam Parameter Product (BPP). BPP is the product of the divergence angle (half angle) of the laser beam and the radius of the beam at its narrowest point (i.e., beam waist, minimum spot size). BPP quantifies the quality of a laser beam and how it can be focused to a small spot, usually in units of millimeters-milliradians (mm-mrad). The Gaussian beam has the lowest possible BPP, which is the wavelength of the laser divided byThe circumference ratio is given. The ratio of the BPP of the actual beam to that of the ideal Gaussian beam at the same wavelength is denoted M²Or "beam quality factor", which is a wavelength-independent measure of beam quality, the "best" quality corresponds to the "lowest" beam quality factor of 1.

Wavelength Beam Combining (WBC) is a technique for adjusting the output power and brightness from a laser diode bar, a stack of diode bars, or other lasers arranged in a one-or two-dimensional array. WBC methods have been developed to combine beams along one or two dimensions of an array of emitters. A typical WBC system includes multiple emitters, such as one or more diode bars, that are combined using a dispersive element to form a multi-wavelength beam. Each emitter in a WBC system resonates individually and is stabilized by wavelength specific feedback from the output coupler, which is typically partially reflective, filtered by a dispersive element along the beam combining dimension. Exemplary WBC systems are described in detail in U.S. patent No.6,192,062, filed on 4/2/2000, U.S. patent No.6,208,679, filed on 8/9/1998, U.S. patent No.8,670,180, filed on 25/8/2011, and U.S. patent No.8,559,107, filed on 7/3/2011, each of which is incorporated by reference in its entirety.

Various WBC technologies are utilized to form high power lasers for many different applications. However, optical cross-coupling between beam emitters may result in a conventional WBC system with less than optimal brightness. Thus, a cross-coupling suppression configuration for WBC laser systems is needed.

Disclosure of Invention

According to embodiments of the present invention, a Wavelength Beam Combining (WBC) laser system features a plurality of emitters (or "beam emitters"), such as diode bars or individual diode emitters of a diode bar, that are combined to form a multi-wavelength beam. Each emitter in the laser system resonates individually and is stabilized via wavelength specific feedback from a common partially reflective output coupler that is filtered along the beam combining dimension by a dispersive element (e.g., a diffraction grating, a dispersive prism, a grism (prism/grating), a transmission grating, or an echelle grating). Advantageously, a non-slit cross-coupling suppression optical system is used to suppress crosstalk between the feedback beams. In various embodiments, the cross-coupling suppression system, or at least a portion thereof, is positioned within a rayleigh range of the multi-wavelength light beam transmitted by the dispersive element, and the output coupler is positioned within a rayleigh range of the multi-wavelength light beam transmitted by the cross-coupling suppression system (or at least a portion thereof). In this manner, the laser system according to the embodiment of the present invention produces a multi-wavelength output beam having high brightness and high power.

In various embodiments, the cross-coupling mitigation system includes or consists essentially of first and second optical elements (e.g., lenses), and the focal length of the first optical element is greater (or even much greater) than the focal length of the second optical element. In such an embodiment, the first optical element may be positioned within a rayleigh range of the multi-wavelength beam transmitted by the dispersive element, and the output coupler may be positioned within a rayleigh range of the multi-wavelength beam transmitted by the second optical element.

In various embodiments, optical cross-coupling is also reduced or substantially eliminated through the use of designed output couplers that minimize back-reflections of stray wavelengths that may be reflected back to individual optical emitters. Such output couplers may be used with or without other cross-coupling suppression systems described herein. In various embodiments, the partially reflective output coupler includes an anti-reflective coating on a surface thereof in an area other than the partially reflective portion, the partially reflective portion being sized and positioned to intercept only the multi-wavelength light beam. The partially reflective portion may protrude from the remainder of the output coupler, or the partially reflective portion may be substantially coplanar with the remainder.

In various embodiments of the present invention, the output coupler may comprise or consist essentially of an optical fiber, the core of which is sized and positioned to intercept only the multi-wavelength light beam. The surface of the core may be partially reflective and/or the core may include a fiber bragg grating therein to provide reflection of the optical beam that enables feedback. The cladding of the optical fiber may be coated with an anti-reflective coating to prevent stray light reflections and optical cross-coupling therefrom. End caps may be present on the optical fibers for, for example, environmental protection and/or to reduce the power density at the ends of the optical fibers. In various embodiments, the optical fiber may include and/or be used with a mode stripper that substantially eliminates unwanted modes from light propagating within the optical fiber.

Embodiments of the present invention couple a multi-wavelength output beam into an optical fiber. In various embodiments, the optical fiber has multiple claddings surrounding a single core, multiple discrete core regions (or "cores") within a single cladding, or multiple cores surrounded by multiple claddings. In various embodiments, the output beam may be delivered to a workpiece for applications such as cutting, welding, and the like.

Here, "optical element" may refer to any of a lens, mirror, prism, grating, etc., that redirects, reflects, bends, or otherwise optically manipulates electromagnetic radiation. Here, a beam transmitter, emitter, or laser, includes any electromagnetic beam generating device, such as a semiconductor element, that generates an electromagnetic beam, but may or may not be self-resonant. These also include fiber lasers, disk lasers, non-solid state lasers, Vertical Cavity Surface Emitting Lasers (VCSELs), and the like. Typically, each laser 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, which is not limited to any particular portion of the electromagnetic spectrum, but may be visible, infrared, and/or ultraviolet light. The emitter may comprise or consist essentially of a multi-beam emitter, such as a diode bar configured to emit multiple beams. The input beam received in embodiments herein may be a single wavelength or a multi-wavelength beam using various combinations of techniques known in the art.

Laser diode arrays, bars, and/or stacks such as those described in the general description below may be used in association with the inventive embodiments described herein. The laser diodes may be packaged individually or in groups, typically in one-dimensional rows/arrays (diode bars) or two-dimensional arrays (diode bar stacks). The diode array stack is typically a vertical stack of diode bars. Laser diode bars or arrays typically achieve substantially higher power and are cost effective than equivalent single broad area diodes. High power diode bars typically contain an array of wide area emitters, generating tens of watts with relatively poor beam quality; the brightness is usually lower than that of a broad area laser diode, despite the higher power. The high power diode bars may be stacked to produce high power stacked diode bars for generating extremely high power of hundreds or thousands of watts. The laser diode array may be configured to emit light beams into free space or into an optical fiber. Fiber-coupled diode laser arrays can be conventionally used as pump sources for fiber lasers and fiber amplifiers.

A diode laser bar is a type of semiconductor laser that contains a one-dimensional array of wide-area emitters or alternatively a sub-array with, for example, 10-20 narrow-band emitters. A wide area diode bar typically contains, for example, 19-49 emitters, each emitter having dimensions of the order of, for example, 1 μm x 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 multiple diffraction limited. Typically, diode bars for commercial applications have a laser resonator length on the order of 1-4mm, are about 10mm wide, and generate output power of tens of watts. Most diode bars operate in the wavelength region from 780 to 1070nm, with wavelengths of 808nm (for pumping neodymium lasers) and 940nm (for pumping Yb: YAG) being most prominent. The 915-976 nm wavelength range is used to pump erbium-doped or ytterbium-doped high power fiber lasers and amplifiers.

The diode stack is simply an arrangement of a plurality of diode bars that can deliver very high output power. Also known as a diode laser stack, a multi-bar module or a two-dimensional laser array, the most common arrangement of diode stacks is a vertical stack, which effectively is a two-dimensional array of edge emitters. Such a stack may be manufactured by attaching diode bars to a thin heat sink and stacking these components to obtain a periodic array of diode bars and heat sinks. Horizontal diode stacks as well as two-dimensional stacks also exist. For high beam quality, the diode bars should generally be as close to each other as possible. On the other hand, efficient cooling requires some minimum thickness of the heat sink mounted between the bars. This tradeoff in diode bar space results in a diode stack having a beam quality (and subsequently its brightness) in the vertical direction that is much lower than the beam quality of a single diode bar. However, several techniques exist to significantly alleviate this problem, such as spatial interleaving of the outputs of different diode stacks, by polarization coupling or by wavelength division multiplexing. Various types of high power beam shapers and related devices have been developed for this purpose. The diode stack may provide extremely high output power (e.g., hundreds or thousands of watts).

In one aspect, embodiments of the invention feature a laser system that includes or consists essentially of: an array of beam emitters (e.g., a one-dimensional array or a two-dimensional array) each emitting a beam, focusing optics for focusing the beam toward a dispersive element, a dispersive element for receiving and dispersing the focused beam thereby forming a multi-wavelength beam, and an optical fiber for receiving the multi-wavelength beam. The optical fiber comprises or consists essentially of: (i) a core for receiving the multi-wavelength light beam, reflecting a first portion thereof back toward the dispersive element, and transmitting a second portion thereof as an output beam comprised of a plurality of wavelengths, the core having a partially reflective surface, and (ii) a cladding surrounding the core having a reflectivity of less than 1% for the multi-wavelength light beam.

Embodiments of the invention may include one or more of the following in any of various combinations. A portion of the core may protrude from the cladding. The surface of the core may be substantially coplanar with the surface of the cladding. The optical fiber may be positioned such that, at the partially reflective surface of the core, the diameter (or other lateral dimension, e.g., width) of the core is not less than the diameter (or other lateral dimension, e.g., width) of the multi-wavelength light beam. The diameter of the core may be approximately equal to or greater than the diameter of the multi-wavelength beam. The end cap may be attached to the optical fiber and disposed optically upstream of the partially reflective surface of the core. An anti-reflective coating may be disposed on the cladding of the optical fiber. The mode stripper may be disposed around at least a portion of the core of the optical fiber. The mode stripper may be disposed around at least a portion of the cladding of the optical fiber. The focusing optics may include or consist essentially of one or more cylindrical lenses, one or more spherical mirrors, and/or one or more cylindrical mirrors. The dispersive element may comprise or consist essentially of a diffraction grating (e.g., a transmissive diffraction grating or a reflective diffraction grating).

The laser system may include a cross-coupling suppression system for receiving and transmitting the multi-wavelength optical beam while reducing cross-coupling thereof. The partially reflective surface of the core of the optical fiber may be disposed within a rayleigh range of the multi-wavelength beam transmitted by the cross-coupling suppression system. At least a portion of the cross-coupling suppression system may be disposed within a rayleigh range of the multi-wavelength optical beam transmitted by the dispersive element. The cross-coupling suppression system may be afocal. The cross-coupling suppression system may include or consist essentially of an afocal telescope. The cross-coupling mitigation system may include or consist essentially of a first optical element having a first focal length and a second optical element having a second focal length. The first optical element may be disposed optically upstream of the second optical element. The first focal length may be at least two times, at least three times, at least five times, at least seven times, at least ten times, or at least 100 times greater than the second focal length. Each of the first and second optical elements may comprise or consist essentially of a lens (e.g., a cylindrical lens or a spherical lens). The first optical element may be disposed within a rayleigh range of the multi-wavelength optical beam transmitted by the dispersive element. The partially reflective surface of the core of the optical fiber may be disposed within a rayleigh range of the multi-wavelength beam transmitted by the second optical element. The optical distance between the first and second optical elements may be approximately equal to the sum of the first and second focal lengths.

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 light beam towards the dispersive element; a dispersive element for receiving and dispersing the focused light beam, thereby forming a multi-wavelength light beam; a cross-coupling suppression system for receiving and transmitting the multi-wavelength optical beam while reducing cross-coupling thereof; an optical fiber disposed optically downstream of the cross-coupling suppression system for receiving the multi-wavelength light beam; and a fiber bragg grating disposed within the optical fiber for receiving the multi-wavelength optical beam, reflecting a first portion thereof back toward the cross-coupling suppression system, and transmitting a second portion thereof as an output beam comprised of a plurality of wavelengths.

Embodiments of the invention may include one or more of the following in any of various combinations. The end cap may be attached to the optical fiber and disposed optically upstream of the fiber bragg grating. The focusing optics may include a lens consisting essentially of one or more cylindrical lenses, one or more spherical mirrors, and/or one or more cylindrical mirrors. The dispersive element may comprise or consist essentially of a diffraction grating (e.g., a transmissive diffraction grating or a reflective diffraction grating). The mode stripper may be disposed around at least a portion of the optical fiber. The fiber bragg grating may be disposed within a rayleigh distance of the multi-wavelength optical beam transmitted by the cross-coupling suppression system. At least a portion of the cross-coupling suppression system may be disposed within a rayleigh range of the multi-wavelength optical beam transmitted by the dispersive element. The cross-coupling suppression system may be afocal. The cross-coupling suppression system may include or consist essentially of an afocal telescope. The cross-coupling mitigation system may or may include a first optical element having a first focal length and a second optical element having a second focal length. The first optical element may be disposed optically upstream of the second optical element. The first focal length may be at least two times, at least three times, at least five times, at least seven times, at least ten times, or at least 100 times greater than the second focal length. Each of the first and second optical elements may comprise or consist essentially of a lens (e.g., a cylindrical lens or a spherical lens). The first optical element may be disposed within a rayleigh range of the multi-wavelength optical beam transmitted by the dispersive element. The fiber bragg grating may be disposed within a rayleigh distance of the multi-wavelength optical beam transmitted by the second optical element. The optical distance between the first and second optical elements may be approximately equal to the sum of the first and second focal lengths.

These and other objects, together with the advantages and features of the invention disclosed herein, will become more apparent with reference to the following specification, drawings and claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. As used herein, the terms "substantially" and "about" mean ± 10%, and in some embodiments ± 5%. The term "consisting essentially of …" is meant to exclude other materials that contribute to function, unless otherwise defined herein. However, such other materials may be present in trace amounts, either collectively or individually. Herein, the terms "radiation" and "light" are used interchangeably unless otherwise indicated. Herein, "downstream" or "optically downstream" is used to refer to the relative placement of a second element that a beam of light impinges upon after encountering a first element, which is "upstream" or "optically upstream" of the second element. Here, the "optical distance" between two members is a distance between the two members through which the light beam actually passes; the optical distance may be, but need not be, equal to the physical distance between the two components due to, for example, reflection from a mirror or other change in propagation direction experienced by light traveling from one of the components to the other.

Drawings

In the drawings, like reference numerals generally refer to like parts throughout the different views. Moreover, 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 diagram of a Wavelength Beam Combining (WBC) method in a non-beam combining dimension according to an embodiment of the present invention;

FIG. 1B is a schematic diagram of a Wavelength Beam Combining (WBC) method in the beam combining dimension according to an embodiment of the present invention;

fig. 2 is a schematic diagram of a WBC laser system including an optical cross-coupling suppression system according to an embodiment of the present invention;

fig. 3 is a schematic diagram of an exemplary optical cross-coupling suppression system for a WBC laser system, in accordance with an embodiment of the present invention;

FIG. 4 is a schematic diagram of an optical cross-coupling suppression system and output coupler for a WBC laser system according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of an optical element and output coupler for a WBC laser system according to an embodiment of the present invention; and

fig. 6-8 are schematic diagrams of portions of an optical fiber used as an output coupler for a WBC laser system according to embodiments of the present invention.

Detailed Description

Aspects and embodiments relate generally to the field of tuning laser sources to high power and high brightness using external cavities, and more particularly to methods and apparatus for external cavity beam combining using one-dimensional or two-dimensional laser sources. In one embodiment, the external cavity system comprises a one-dimensional or two-dimensional laser element, an optical system, a dispersive element and a partially reflective element. An optical system is one or more optical elements that perform two basic functions. The first function is to overlap all laser elements onto the dispersive element along the beam combining dimension. The second function is to ensure that all elements along the non-beam combining dimension propagate orthogonally to the output coupler. In various embodiments, the optical system introduces as little loss as possible. Thus, these two functions will allow a single resonator to be used for all laser elements.

In another embodiment, a WBC external cavity system includes a wavelength stabilized one or two dimensional laser element, an optical system, and a dispersive element. One-or two-dimensional wavelength-stabilized laser elements having unique wavelengths may be implemented using various components, such as a laser element having feedback from a wavelength chirped volume bragg grating, a Distributed Feedback (DFB) laser element, or a Distributed Bragg Reflector (DBR) laser element. The main function of the optical system here is to overlap all the beams onto the dispersive element. Having parallel beams along the non-beam combining dimension is less important when there is no output coupling mirror external to the wavelength stabilized laser element. Aspects and embodiments further relate to high power and/or high brightness multi-wavelength external cavity lasers that generate overlapping or coaxial beams of light from very low output power to hundreds or even megawatts of output power.

Embodiments of the present invention suppress the amount of unexpected and/or undesirable feedback from non-originating emitters in a WBC laser system. For example, in a WBC system, two separate beam emitters share a common partial mirror (such as an output coupler), there is the possibility that feedback light from one emitter enters the other emitter. This unwanted feedback (or "crosstalk" or "cross-coupling") from the "non-originating" transmitter reduces the efficiency of the system. The methods and embodiments described herein may be applied to one-dimensional and two-dimensional beam combining systems in the slow divergence dimension (or "direction"), the fast divergence dimension, or other beam combining dimensions. For the purpose of this application, the emitted light beam has the following form: one dimension is nearly or completely diffraction limited, while the other dimension is multiple diffraction limited. Another way to describe this may be an axis and/or a dimension. For example, the output beam may have slow and fast divergence axes or dimensions.

Where the term substantially larger is used, reference to the focal length of one optical element being compared to the focal length of another optical element (f1> > f2) is to be understood as a factor of at least 2, 3, 4, 5, 7 or more. For example, the focal length of f1 may be 100mm or greater, while the focal length of f2 is 50mm or less. In another example, the focal length of f1 may be 200mm or greater, while f2 is 20mm or less. The term "angular filter" refers to creating a specified numerical aperture for the feedback beam. The size of the numerical aperture may limit the allowed feedback to correspond only to the originally emitted beam. That is, the angular filter prevents adjacent or nearby emitted light beams from returning to the original emitter (i.e., crosstalk). Stabilization of a transmitter refers to feedback received by each transmitter that has been clipped to a different wavelength. This may be in the form of seeding the emitter with a specific wavelength, which causes a portion of the emitted beam to be redirected back to the emitter and interfere with the feedback, such as placing a grating in the path to produce a different wavelength to be directed as feedback into the emitter. Typically, the feedback is reflected back to the original emission region where it passes through a dispersive element or diffraction grating before entering back into the optical gain medium portion of the original emitter. In some WBC embodiments, the feedback source may be a common reflective surface that provides feedback to multiple emitters, each of which is independently turned to a particular wavelength.

FIGS. 1A-1B illustrate an external cavity one-dimensional (1-D) WBC system comprising or consisting of: a one-dimensional beam emitter 102 (e.g., a diode bar) having a back reflective surface 104, a gain medium 106 having, for example, two or more diode emitters 105, a front reflective surface 108, combining optics 110, a dispersive element 112, and a partially reflective output coupler 114. In this embodiment, the combining optics or lens 110 is placed away from the focal length 120a from the front reflective surface 108 of the diode bar 102, while the dispersive element 112 is placed away from the focal length 120b on the back plate or other side of the lens 110. The output coupler 114 is placed at a certain distance from the dispersive element 112 and reflects a portion of the generated beam (feedback 116) back to the dispersive element 112.

In this embodiment, the placement of the combination lens 110 performs two functions. The first function is to overlap all chief rays from all diode elements to the dispersive element 112. The second function is to collimate each beam in both axes. Fig. 1A and 1B show schematic diagrams of a view in the non-beam combining dimension 130 (fig. 1A) and a view in the beam combining dimension 140 (fig. 1B). The transmitter 102 comprises or consists essentially of: a plurality of emitters (e.g., diode emitters) 105, a back reflective surface 104, a gain medium 106, and a front surface/facet 108.

In a WBC resonator, adjacent emitters may be optically cross-coupled to each other. This can severely degrade the output beam quality. Fig. 2 is a schematic diagram of a WBC resonator with two

adjacent emitters

202a and 202b, the

emitters

202a and 202b emitting their nominally on-

axis chief rays

260a and 260b (shown as solid lines) to a lens 210, the lens 210 focusing them on the center of a dispersive element (e.g., diffraction grating) 212. From here, the two chief rays are diffracted at their own unique wavelengths to propagate along the same axis 240 through cross-coupling suppression optics 250, which represent any and all lenses or optical elements between the grating 212 and the partially reflective coupler 214. The two rays are then partially reflected back onto themselves, propagating back to self-couple into their respective emitters. Dashed

lines

261a and 261b in fig. 2 show that the chief ray that would result in optical cross-coupling between the two emitters-i.e., the chief ray emitted from one emitter is coupled back to the other emitter.

The following parameters are defined as follows:

d is the distance between the two emitters (symmetrically arranged above and below the axis +/- (d/2)).

ε is the divergence angle (the angle between the solid and dashed chief rays at the emitter).

θ_1/2Half-divergence in the WBC direction of the emitter is the far field angle.

L₀Distance from the emitter to lens L1.

f₁The focal length of lens L1.

In fig. 2, the grating is shown as if it is operating at normal incidence. Here, it is assumed that the system operates in a Littrow (Littrow) configuration, where the angle of incidence and the angle of diffraction are equal (and non-zero). In the littrow configuration, small changes in the angle of incidence are matched to the first order by equal changes in the angle of diffraction. In the expanded schematic, any light operating at littrow will appear to travel straight through the grating. It is clear that only the central light ray 202c (the light ray emitted from the imaginary emitter between the two

emitters

202a and 202 b) is self-coupled at littrow.

The symmetry in fig. 2 is deliberate as it allows many important simplifications in the analysis of the unique divergence angle epsilon at which the chief ray can leave one emitter and return to another. The first simplification based on symmetry is that the deviating (dashed line) chief ray must strike the coupler at its center. The second simplification is that the cross-coupling wavelength must be the average of the two self-coupling wavelengths. This will in turn be the wavelength of the imaginary emitter between the two emitters, which will self-couple at littrow as described above. Thus, the dashed chief rays in fig. 2 must pass through the gratings at littrow, meaning that they will appear to travel straight through the gratings shown. Using this simplification, a conventional "y/y-bar" (chief ray height/chief ray slope) analysis can be used to track the top dashed chief ray:

exiting the top emitter:

entrance lens L1:

exit lens L1:

entering and leaving the grating (recall from the discussion above that the dashed chief ray does not change direction at the grating):

to complete the calculations at the coupler involving propagation through the cross-coupling suppression optics, recall that the dashed chief ray traverses the coupler at its center. Thus, the slope of the ray at the coupler only is non-zero, and note that both the height of the ray at the grating and the slope of the ray must be proportional to the slope of the ray at the coupler. This means that the ratio of height to slope at the grating must be constant. Also, this constant can be very intuitively interpreted as the negative of the effective distance of the coupler from the grating, as determined by the cross-coupling suppression optics. In other words,

wherein L is_{cplr_eff}Is the effective distance of the coupler beyond (the right of) the grating.

In fact, L_{cplr_eff}Can be calculated using ray tracing or using y/y-bar analysis of the lenticular rear lens. In any case, however, equation 9 allows equations 7 and 8 to be solved for the slip angle ε with the following results:

since the divergence angle epsilon that causes the cross-coupling has been determined, the amount of cross-coupling can be calculated. One reasonable definition of cross-coupling is the integral of the product of self-coupling strength and cross-coupling strength over the solid angle at the emitter, normalized by the integral of the square of the self-coupling strength. Before calculating this integral, it is important to note that in the name of simplified symmetry, the outgoing and incoming beams at the cross-coupled emitters are assumed to be equally divergent. Thus, for overlap integration, one beam that is not deflected (self-coupled beam) and another beam that is to be deflected by twice the angle epsilon (cross-coupled beam) are considered. Placing the segment into the form of an equation provides:

(Note that equation 11 relates to one-dimensional integration over a single angle, rather than two-dimensional integration over a solid angle. this is because integration over angle in the direction orthogonal to the beam deviation yields a constant that falls outside the ratio in equation 11.) equation 11 can be simplified to yield:

in summary, the correlation deviation angle ε may be calculated according to equation 10 according to known parameters. The resulting overlap (overlap) may then be calculated according to equation 12. This gives the ratio of the cross-coupling strength to the self-coupling strength, assuming that in the self-coupling case there is a perfect beam waist at the coupler.

When close to, but not exactly at, the usual configuration of placing the emitter one focal distance back from L1, there is a very interesting possibility of having a large effect on cross-coupling. If we just put the transmitter there, the first term in the denominator of equation 10 will be zero and equation 10 will be reduced to:

substituting equation 13 into equation 12 yields:

here, d' is the transmitter diameter at the near field, and Zr is the rayleigh distance of the beam. Thus, to reduce cross-coupling, the near field fill factor (d/d') should be high, the optical path length between the grating and the coupler should be long, and the rayleigh distance should be short. Typically, the near field fill factor is fixed. By way of example, if it is assumed that a WBC system comprises 20 diode bars and a transform lens with a focal length of 2000mm, the beam size at the grating is approximately 40mm (assuming a 20 milliradian full beam divergence). The rayleigh distance (1 μm wavelength and diffraction limited) of this beam is about 160 m. The distance between the grating and the output coupler should be as much as the rayleigh distance for cross-coupling suppression. Such a length would make a WBC system virtually impractical. However, if the beam is reduced by 40 x between the grating and the output coupler, the optical path length is reduced by 160 x or about 1 m. Further reduction of the optical path length can be achieved using a larger reduction in the beam size. Beam reduction may be achieved using various mechanisms such as lenses, prisms, or combinations thereof. Careful design must be considered so that the self-coupling of each emitter is not impaired so that the cavity performance suffers.

However, if the transmitter is slightly displaced from this position, the first term in the denominator of equation 10 may actually cancel the second term, so that the required divergence angle is infinite and the cross-coupling overlap is zero. Specifically, this occurs when the following is satisfied:

in other words, the effective distance L when to the coupler_{cplr_eff}Very large, equation 14 gives us a possible formulation to pull the emitter back slightly from the front focus of L1 to break the cross coupling.

Fig. 3 illustrates one example of a cross-coupling mitigation system 250 illustrated by a block in fig. 2. Here, the optical element 302 may be of focal length F ₁304. The second optical element 306 may also be a lens and have a focal length F ₂308. The distance between 302 and 306 is exactly or approximately the focal length F₁And F₂The sum of (1). As previously discussed, it is preferred that F₁And F₂Ratio (F)₁/F₂) Is at least two times greater. System 250 may be a afocal pantograph system. In other embodiments, multiple optical elements may be used, wherein the effect of the system still maintains the characteristics of an afocal telescopic system with a large ratio.

In various embodiments, it may be desirable to place the lens 302 within the rayleigh distance of the beam transmitted from the dispersive element (e.g., diffraction grating), while also placing the partially reflective output coupler or other reflective surface within the rayleigh distance of the beam exiting the lens 306.By appropriately providing F₁>>F₂With the lenses in relation placed in these positions, an effective system is created to reduce and in some cases eliminate any cross-coupled feedback from the incoming non-originating emitter or source.

Fig. 4 shows a stabilization system 400 (which may be part of a WBC laser system) in accordance with an embodiment of the present invention, in which an optical cross-coupling suppression system (which may include or consist essentially of optical elements 410, 420) is used in conjunction with a partially reflective output coupler 430, the partially reflective output coupler 430 being designed to minimize reflections that may lead to unwanted feedback. As shown, the output coupler 430 includes a partially reflected beam receiving portion 434 that is sized and positioned to receive the light beam from the optical element 420. In particular, the beam receiving portion 434 typically has a diameter (or other lateral dimension) that is approximately the same size as the diameter (or other lateral dimension) of the light beam it receives. The beam receiving portion 434, which may be substantially centered on the surface of the output coupler 430, is surrounded by a non-reflective portion (or surface) 432, the non-reflective portion 432 having a reflectivity of 1% or less of the wavelength of the received beam. For example, the non-reflective portions 432 may be coated with an anti-reflective coating to prevent undesirable back-reflection that may lead to optical crosstalk. Thus, any stray light propagating outside of the beam receiving portion 434 to the output coupler 430 will not be reflected back to the beam emitters of the WBC system. The beam receiving portion 434 may protrude from the remainder of the surface of the coupler 430 (i.e., may be raised relative to the non-reflective portion 432), as shown in fig. 4, or the beam receiving portion 434 may be substantially coplanar with the non-reflective portion 432.

The beam receiving portion 434 may have a reflectivity of less than about 15% for the wavelength of the light beam, for example in the range of about 2% to about 10%, to provide a desired wavelength stabilization of the light beam from the associated emitter. It will be appreciated that the remainder of the received beam will pass through the output coupler 430 and be transmitted to a downstream optical system component (e.g., an optical fiber or workpiece).

Fig. 5 illustrates a stabilization system 500 (which may be part of a WBC laser system) in accordance with an embodiment of the present invention, in which an optical element 510 (e.g., a cylindrical or spherical lens) simply focuses the beam on a partially reflective output coupler 430, and there is no second collimating optical element between them. In this manner, the output coupler 430 may be used in a WBC laser system without the use of an optical cross-coupling suppression system (e.g., comprising or consisting essentially of two or more optical elements such as lenses).

Fig. 6 illustrates a portion of a WBC laser system 600 in which an optical element 610 focuses light directly into the core 650 of an optical fiber that includes one or more features that operate as a partially reflective output coupler. As shown, the optical fiber may also have a cladding 640 surrounding a core 650; in general, cladding 640 has a refractive index that is less than the refractive index of core 650, such that light within core 650 is confined. The end surface of the core 650 may be substantially coplanar with the end surface 642 of the cladding, or the core 650 may protrude slightly from the surface 642. To provide wavelength stabilization, the end face of the core 650 may be partially reflective (e.g., between about 2% and about 10% reflective, or between about 4% and about 10% reflective) of the wavelength of the light beam. In various embodiments, the partial reflectivity may be provided by a coating on the end face of the core.

In various embodiments of the present invention, instead of or in addition to a partially reflective coating, a fiber bragg grating 654 may be disposed within the core 650 to provide a desired partial reflectivity. As known to those skilled in the art, a fiber bragg grating comprises or consists essentially of a periodic variation in the refractive index of a portion of the optical fiber (e.g., within the core 650). The periodic variation may be of the order of, for example, half the wavelength (or one of the wavelengths) of the received light beam, and the grating thereby causes fresnel reflections. The wavelength dependence and/or amplitude of the reflection may be selected by the particular grating pattern and the refractive index variation therein. In various embodiments, a plurality of fiber bragg gratings 654 may be disposed within the core 650, and each grating 654 may have a different refractive index variation and/or wavelength selectivity.

In various embodiments, the surface 642 of the cladding 640 may be coated with an anti-reflective coating to prevent any stray light from being deflected back to an adjacent emitter or light beam. For example, surface 642 may be coated to have a reflectivity of less than 1% for the wavelength of the light beam.

Fig. 7 shows a wavelength stabilization system 700 (which may be part of a WBC laser system) in which an optical element 710 focuses light into the core 650 of an optical fiber. In system 700, end cap 720 is disposed on core 650 and is in contact with core 650 (and in some embodiments, surface 642 of cladding 640); in some embodiments, end cap 720 is attached to the optical fiber with an index matching material therebetween. In other embodiments, at least a portion of the optical fiber (e.g., core 650) is fused directly to end cap 720. As shown in fig. 7, the presence of end cap 720 enables an effective interface between the optical fiber and the incident light beam (i.e., the point at which the light beam enters the end cap) to receive the light beam when it has a larger diameter (or width), thereby reducing the power density of the light beam as it enters the optical fiber. The presence of end cap 720 may also protect other portions of the optical fiber from heat, moisture, and/or other environmental contaminants.

Fig. 8 shows a wavelength stabilization system 800 (which may be part of a WBC laser system) in which an optical element 810 focuses light into the core 650 of an optical fiber. The system 800 includes a mode stripper 820 to further increase the purity and transmission capabilities of the fiber for the optical beam. It will be appreciated that as the light beam changes transmission medium, various refractive indices and angles of incidence of light into the fiber may cause cladding modes, i.e., light traveling within the material of the cladding. This cladding mode may be undesirable because such light may cause wavelength distortion and contamination of the original beam. As known to those skilled in the art, the mode canceller 820 may include, consist essentially of, or consist of: a material having a refractive index not less than (i.e., equal to or greater than) that of the cladding 640; in this way, light that may normally propagate in the cladding in cladding mode will preferentially enter the mode stripper and radiate out of the fiber. In various embodiments, the mode stripper 820 will have a refractive index greater than the refractive index of the cladding 640. As shown in fig. 8, an index matching material 830 may be disposed between the core 650 (or cladding 640 in some embodiments) and the mode stripper 820. (As used herein, the term "index matching material" refers to a material disposed between two other materials and having an index of refraction between the indices of refraction of the two materials or approximately equal to the index of refraction of one or both materials.) although FIG. 8 shows a mode stripper 820 immediately surrounding the core 650, in various embodiments, at least a portion of the cladding 640 is disposed between the core 650 and the mode stripper 820.

In any of the foregoing wavelength stabilization systems, it will be appreciated that the beam may be manipulated in various ways via the addition of optical and/or dispersive elements configured to achieve a desired beam quality. For example, optical elements such as gratings and/or collimators may be present in the WBC system and/or the stabilization system. It will also be appreciated that the partially reflective element may be provided with partially reflective properties by a number of means including, but not limited to, providing a grating, coating, etc. to achieve a desired transmission and a desired reflection quality.

The terms and expressions which have been 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 light beam towards a dispersive element;

a dispersive element for receiving and dispersing the focused light beam, thereby forming a multi-wavelength light beam; and

an optical fiber for receiving the multi-wavelength light beam, the optical fiber comprising (i) a core for receiving the multi-wavelength light beam, reflecting a first portion thereof back toward the dispersive element, and transmitting a second portion thereof as an output beam comprised of a plurality of wavelengths, the core having a partially reflective surface, and (ii) a cladding surrounding the core having a reflectivity of less than 1% for the multi-wavelength light beam.

2. The laser system of claim 1, wherein a portion of the core protrudes from the cladding.

3. The laser system of claim 1, wherein the optical fiber is positioned such that, at the partially reflective surface of the core, a diameter of the core is no less than a diameter of the multi-wavelength beam.

4. The laser system of claim 1, further comprising an end cap attached to the optical fiber and disposed optically upstream of the partially reflective surface of the core.

5. The laser system of claim 1, further comprising an anti-reflective coating disposed on the cladding of the optical fiber.

6. The laser system of claim 1, further comprising a mode stripper disposed around at least a portion of a core of the optical fiber.

7. The laser system of claim 1, further comprising a cross-coupling suppression system for receiving and transmitting the multi-wavelength optical beam while reducing cross-coupling thereof.

8. The laser system of claim 7, wherein the partially reflective surface of the core of the optical fiber is disposed within a Rayleigh distance of the multi-wavelength beam transmitted by the cross-coupling mitigation system.

9. The laser system of claim 7, wherein at least a portion of the cross-coupling suppression system is disposed within a Rayleigh distance of the multi-wavelength beam transmitted by the dispersive element.

10. The laser system of claim 7, wherein the cross-coupling suppression system comprises an afocal telescope.

11. The laser system of claim 7, wherein the cross-coupling suppression system comprises a first optical element having a first focal length and a second optical element having a second focal length, the first optical element disposed optically upstream of the second optical element.

12. The laser system of claim 11, wherein the first focal length is at least two times greater than the second focal length.

13. The laser system of claim 11, wherein the first focal length is at least seven times greater than the second focal length.

14. The laser system of claim 11, wherein each of the first and second optical elements comprises a lens.

15. The laser system of claim 11, wherein the first optical element is disposed within a rayleigh range of a multi-wavelength beam transmitted by the dispersive element.

16. The laser system of claim 11, wherein the partially reflective surface of the core of the optical fiber is disposed within a rayleigh distance of the multi-wavelength beam transmitted by the second optical element.

17. The laser system of claim 11, wherein an optical distance between the first and second optical elements is approximately equal to a sum of the first and second focal lengths.

18. The laser system of claim 1, further comprising:

a cross-coupling suppression system for receiving and transmitting the multi-wavelength optical beam while reducing cross-coupling thereof; and

a fiber Bragg grating disposed within the core of said optical fiber for receiving said multi-wavelength optical beam, reflecting said first portion thereof back toward said cross-coupling suppression system, and transmitting said second portion thereof as said output optical beam comprised of a plurality of wavelengths.

19. The laser system of claim 18, further comprising an end cap attached to the optical fiber and disposed optically upstream of the fiber bragg grating.

20. The laser system of claim 18, further comprising a mode stripper disposed around at least a portion of the optical fiber.

21. The laser system of claim 18, wherein the fiber bragg grating is disposed within a rayleigh distance of a multi-wavelength optical beam transmitted by the cross-coupling suppression system.

22. The laser system of claim 18, wherein at least a portion of the cross-coupling suppression system is disposed within a rayleigh distance of the multi-wavelength beam transmitted by the dispersive element.

23. The laser system of claim 18, wherein the cross-coupling suppression system comprises an afocal telescope.

24. The laser system of claim 18, wherein the cross-coupling suppression system comprises a first optical element having a first focal length and a second optical element having a second focal length, the first optical element disposed optically upstream of the second optical element.

25. The laser system of claim 24, wherein the first focal length is at least two times greater than the second focal length.

26. The laser system of claim 24, wherein the first focal length is at least seven times greater than the second focal length.

27. The laser system of claim 24, wherein each of the first and second optical elements comprises a lens.

28. The laser system of claim 24, wherein the first optical element is disposed within a rayleigh range of a multi-wavelength beam transmitted by the dispersive element.

29. The laser system of claim 24, wherein the fiber bragg grating is disposed within a rayleigh distance of a multi-wavelength optical beam transmitted by the second optical element.

30. The laser system of claim 24, wherein an optical distance between the first and second optical elements is approximately equal to a sum of the first and second focal lengths.