DE112015005587B4 - OPTICAL CROSS COUPLING ATTENUATION SYSTEMS FOR WAVELENGTH BEAM COMBINING LASER SYSTEMS - Google Patents

Abstract

A laser system comprising: an array of beam transmitters (102) each transmitting a beam; imaging optics (110) for focusing the beams on a dispersive element (112); a dispersive element (112) for receiving and scattering the focused beams, thereby forming a multi-wavelength beam; andan optical fiber for receiving the multi-wavelength beam, the optical fiber comprising (i) a core (650) for receiving the multi-wavelength beam reflecting a first portion thereof back onto the dispersive element (112) and a second portion thereof as one emits multi-wavelength composite output beam, the core (650) having a partially reflective surface, and (ii) surrounding the core, a cladding (640) having a reflectivity for the multi-wavelength beam of less than 1%, the scattering or The dispersive element (112) is selected from the group consisting of a diffraction grating, a scattering prism, a grism (prism / grating), a transmission grating or an echelle grating.

Description

Technical area

In various embodiments, the present invention relates to laser systems, in particular wavelength-beam-combining laser systems which include systems for attenuating optical cross-coupling between beam transmitters.

background

High-power laser systems are used for a variety of different applications such as welding, cutting, drilling and processing materials. Such laser systems generally include a laser transmitter or emitter that couples laser light therefrom into an optical fiber (or simply a "fiber"), and an optical system that focuses the laser light from the fiber onto the workpiece to be processed. The optical system is generally designed to produce the highest quality laser beam or, equivalent, the lowest beam parameter product (BPP) beam. The BPP is the product of the laser beam divergence angle (half angle) and the radius of the beam at its narrowest point (ie the beam neckline, the minimum spot size). The BPP quantifies the quality of the laser beam and how well it can be focused into a small spot and is generally expressed in units of millimeter-milliradians (mm-mrad). A Gaussian beam has the lowest possible BPP, given by the wavelength of the laser light divided by pi. The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is called M ² , or the "beam quality factor", which is a wavelength-independent measure of the beam quality, with the "best" quality being the " corresponds to the lowest “beam quality factor of 1.

Wavelength-Beam Combining (WBC) is a technique for scaling the output power and brightness of laser diode bars, stacking diode bars or other lasers arranged in a one- or two-dimensional array. WBC techniques were designed to combine beams together with one or both dimensions of an array of transmitters. Typical WBC systems include a variety of transmitters, such as one or more diode bars, that are combined using a dispersive element to form a multi-wavelength beam. Each transmitter in the WBC system resonates individually and is stabilized by wavelength-specific feedback from a conventional partially reflected output coupler which is filtered by the scattering element along a beam combining dimension. Exemplary WBC systems are detailed in US 6 192 062 B1 , US 6,208,679 B1 , US 8 670 180 B2 and US 8 559 107 B2 , with the entire disclosure of each being incorporated herein by this reference. In the US 6,665,471 B1 and the US 2013/0208361 A1 For example, a wavelength beam combination system having multiple emitters such as diode elements is combined using a dispersive element to form a multiple wavelength beam.

A variety of WBC techniques have been used to create high power lasers for many different applications. However, optical cross-coupling between beam transmitters can result in conventional WBC systems with sub-optimal brightness. Thus, there is a need for cross-coupling attenuation arrangements for WBC laser systems.

Brief description

In accordance with embodiments of the present invention, wavelength beam combining (WBC) laser systems feature multiple transmitters (or "beam transmitters"), e.g., diode bars or the single diode transmitters of a diode bar, that are combined to form a multi-wavelength beam . Each transmitter in the laser system oscillates individually and is stabilized via wavelength-specific feedback from a conventional, partially reflected output coupler, which is formed by a scattering or dispersive element (e.g. a diffraction grating, a scattering prism, a grism (prism / grating), a transmission grating or an echelle grating) is filtered along a beam combining dimension. Advantageously, the crosstalk attenuation between feedback beams is attenuated using a non-slot based cross coupling attenuating optical system. In various embodiments, the cross-coupling attenuation system or at least a region thereof is positioned in the Rayleigh region of the multi-wavelength beam transmitted by the scattering or dispersive element and the output coupler is in the Rayleigh region of the cross-coupling attenuation system (or at least a region thereof ) transmitted multi-wavelength beam is positioned. In this way, laser systems according to embodiments of the present invention produce multi-wavelength output beams with high brightness and high strength.

In various embodiments, the cross-coupling attenuation system comprises or consists essentially of first and second optical elements (eg lenses) and the focal length of the first optical element is greater (or also substantially greater) than the focal length of the second optical element. In such embodiments, the first optical element can be positioned in the Rayleigh region of the multi-wavelength beam transmitted by the scattering or dispersive element and the output coupler can be positioned in the Rayleigh region of the multi-wavelength beam transmitted by the second optical element.

In various embodiments, the optical cross-coupling is also reduced or substantially eliminated through the use of designed output couplers that minimize back-radiation of stray wavelengths which could reflect back to the individual beam transmitters. Such output couplers can be used with or without other cross-talk attenuation systems described herein. In various embodiments, the partially reflective output couplers incorporate an anti-reflective coating on their surface in regions other than a partially reflective area designed and positioned to intercept only the multi-wavelength beam. The partially reflective area can protrude from the remaining area of the output coupler or the partially reflective area can be substantially coplanar with the remaining area.

In various embodiments of the invention, the output coupler may include, or consist essentially of, an optical fiber whose core is sized and positioned to intercept only the multi-wavelength beam. The surface of the core can be partially reflective and / or the core can include a fiber Bragg grating therein to provide the feedback allowing reflection of the beam. The cladding of the optical fiber can be coated with an anti-reflective coating to prevent stray reflection and optical cross-coupling that may result. An end cap can be placed over the optical fiber for example to protect the environment and / or to reduce the current density at the end of the fiber. In various embodiments, the optical fiber may include a mode stripper and / or may be used in conjunction with a mode stripper that removes substantially undesirable modes of light from progressing in the optical fiber.

Embodiments of the present invention couple multi-wavelength output beams into an optical fiber. In various embodiments, the optical fiber has multiple cladding layers surrounding a single core, multiple discrete core regions (or “cores”) in a single cladding layer, or multiple cores surrounded by multiple cladding layers. In various embodiments, the output beams can be delivered to a workpiece for applications such as cutting, welding, etc.

As used herein, “optical elements” can refer to any of lenses, mirrors, prisms, gratings, and the like that redirect, reflect, diffract, or optically manipulate electromagnetic radiation in any other way. As used herein, beam transmitters, transmitters, or laser transmitters or lasers include any electromagnetic beam-generating device, such as semiconductor elements, that generate an electromagnetic beam, but may or may not be resonant themselves. These also include fiber lasers, disk lasers, non-solid state lasers, surface emitting diode lasers (VCSELs), and so on. In general, each transmitter includes a back reflective surface, at least one optical gain medium, and a front reflective surface. The optical gain medium increases the build-up of electromagnetic radiation, which is not limited to any particular region of the electromagnetic spectrum, but which can be visible, infrared, and / or ultraviolet light. A transmitter may include or consist essentially of multiple beam transmitters, such as a diode bar, designed to transmit multiple beams. The supplied beams received in the embodiments herein may be single-wavelength or multi-wavelength beams combined using various techniques known in the art.

Laser diode arrays, rods, and / or stacks such as those described in the general description below can be used in conjunction with embodiments of the innovations described herein. Laser diodes can be packaged individually or in groups, generally in one-dimensional rows / arrangements (diode bars) or two-dimensional arrangements (diode bar stacks). A diode array stack is generally a vertical stack of diode bars. Laser diode bars or arrays generally achieve significantly greater strength and cost effectiveness than an equivalent single wide area diode. High power diode bars generally contain an array of wide area transmitters generating tens of watts of relatively poor beam quality; despite the higher strength, the brightness is often lower than that of a wide area laser diode. High power diode bars can be stacked to produce high power stacked diode bars of extremely high strengths of hundreds or thousands of watts. Laser diode Arrays can be designed to send a beam into free space or into a fiber. Fiber-coupled diode laser arrays can commonly be used as a pump source for fiber lasers and fiber amplifiers.

A diode laser rod is a type of semiconductor laser that contains a one-dimensional array of wide area transmitters, or alternatively, sub-arrays containing, e.g., 10-20 narrow striped transmitters. A wide area diode bar generally contains, for example, 19-49 transmitters, each with dimensions on the order of, for example, 1 µm x 100 µm. The beam quality along the 1 µm dimension or fast axis is generally diffraction-limited. The beam quality along the 100 µm dimension or slow axis or arrangement dimension is generally limited in many ways by diffraction. In general, a diode bar for commercial applications has a laser cavity length on the order of 1 to 4 mm, is about 10 mm wide, and produces tens of watts of output power. Most diode bars operate in the 780-1070 nm wavelength region, with the 808 nm (for pumping neodymium lasers) and 940 nm (for pumping Yb: YAG) wavelengths being the most prevalent. The wavelength range of 915-976 nm is used for pumping erbium-doped or ytterbium-doped high-power fiber lasers and amplifiers.

A diode stack is simply an arrangement of multiple diode bars that can deliver very high output power, also called diode laser stacks, multiple bar modules or two-dimensional laser arrangements, the most common diode stack arrangement being that of a vertical one Stack is, which is effectively a two-dimensional array of edge transmitters. Such a stack can be created by attaching diode bars to thin heat sinks and stacking these structures so as to obtain a periodic arrangement of diode bars and heat sinks. There are also horizontal diode stacks and two-dimensional stacks. For the high beam quality, the diode bars should generally be as close to one another as possible. On the other hand, efficient cooling requires a certain minimum thickness of the heat sink attached between the rods. This trade-off in diode bar spacing results in beam quality from a diode stack in the vertical direction (and hence its brightness) that is much lower than that from a single diode bar. However, there are different techniques to significantly alleviate this problem, e.g., by spatially interleaving the outputs from different diode stacks by polarization coupling or by wavelength multiplying. Various types of high power beamformers and related devices have been designed for such purposes. Diode stacks can provide extremely high output powers (e.g. hundreds or thousands of watts).

In one aspect, embodiments of the invention characterize a laser system which has an arrangement (e.g. a one-dimensional arrangement or a two-dimensional arrangement) of beam transmitters, each transmitting a beam, imaging optics for focusing the beams on a scattering or dispersive element , a dispersive element for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam and including or essentially consisting of an optical fiber for receiving the multi-wavelength beam. The optical fiber comprises (i) a core for receiving the multi-wavelength beam, reflecting a first portion thereof back onto the dispersive element, and transmitting a second portion thereof as an output beam composed of multiple wavelengths, the core having a partially reflective surface , and (ii) the core, a cladding with a reflectivity for the multi-wavelength beam of less than 1% surrounds or consists essentially of it, wherein the scattering or dispersive element is selected from the group consisting of a diffraction grating, a scattering prism, a Grisma (prism / grating), a transmission grating or an echelle grating.

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

The laser system may include a cross-talk attenuation system for receiving and transmitting the multi-wavelength beam while reducing the cross-talk thereof. The partially reflective surface of the core of the optical fiber may be located in a Rayleigh region of the multi-wavelength beam transmitted by the cross-coupling attenuation system. At least one region of the cross-coupling attenuation system can be arranged in a Rayleigh region of the multi-wavelength beam transmitted by the scattering or dispersive element. The crosstalk attenuation system can be afocal. The crosstalk attenuation system can include or consists essentially of an afocal telescope. The crosstalk attenuation 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 can be arranged optically upstream of the second optical element. The first focal length can be at least two, at least three, at least five, at least seven, at least ten, or at least 100 times greater than the second focal length. Each of the first and second optical elements may include or consist essentially of a lens (e.g., a cylindrical lens or a spherical lens). The first optical element can be arranged in a Rayleigh range of the multi-wavelength beam, transmitted through the scattering or dispersive element. The partially reflective surface of the core of the optical fiber can be arranged in a Rayleigh region of the multi-wavelength beam transmitted through the second optical element. The optical distance between the first and second optical elements can be approximately equal to a sum of the first and second focal lengths.

These and other objects, along with advantages and features of the present invention disclosed herein, will become more apparent upon reference to the following description, accompanying drawings, and claims. Furthermore, it is 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 "approximately" mean ± 10%, and in some embodiments ± 5%. The term “consists essentially of” only means other materials that contribute to the effect, unless otherwise defined herein. Nonetheless, such other materials may be present in whole or in part in trace amounts. Here, the terms “radiation” and “light” are used interchangeably, unless otherwise indicated. As used herein, "downstream" or "optically downstream" is used to indicate the relative replacement of a second element that strikes a beam of light upon exposure to a first element, the first element being "upstream" or "optically upstream" of the second element. Here, "optical distance" between two components is the distance between two components that rays of light actually traverse; the optical distance may, but need not, be equal to the physical distance between two components due to, for example, reflections from mirrors or other changes in progressive direction experienced from the light directed from one of the components to the other.

Figure list

• 1A ist ein Schema von einem Wellenlänge-Strahl-Kombinations (WBC)-Verfahren in der Nicht-Strahl-kombinierenden Abmessung gemäß des Standes der Technik;
• 1B ist ein Schema von einem Wellenlänge-Strahl-Kombinations (WBC)-Verfahren in der Strahl-kombinierenden Abmessung gemäß des Standes der Technik;
• 2 ist ein Schema von einem WBC-Lasersystem, das ein optisches Kreuzkopplungs-Abschwächungssystem gemäß des Standes der Technik einbezieht;
• 3 ist ein Schema von einem beispielhaften optischen Kreuzkopplungs-Abschwächungssystem für ein WBC-Lasersystem gemäß des Standes der Technik;
• 4 ist ein Schema von einem optischen Kreuzkopplungs-Abschwächungssystem und einem Ausgangskoppler für ein WBC-Lasersystem gemäß Ausführungsformen der Erfindung;
• 5 ist ein Schema von einem optischen Element und einem Ausgangskoppler für ein WBC-Lasersystem gemäß Ausführungsformen der Erfindung; und
• 6-8 sind Schemata von Bereichen von optischen Fasern, angewendet als Ausgangskoppler für WBC-Lasersysteme gemäß Ausführungsformen der Erfindung.

In the drawings, reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead being placed generally on illustrating the principles of the invention. In the description below, various embodiments of the present invention are described with reference to the following drawings, in which:

• 1A Figure 13 is a schematic of a wavelength-beam-combining (WBC) process in the non-beam-combining dimension according to the prior art;
• 1B Figure 13 is a schematic of a wavelength-beam-combining (WBC) process in the beam-combining dimension according to the prior art;
• 2 Figure 13 is a schematic of a WBC laser system incorporating a prior art optical cross-talk attenuation system;
• 3 Figure 13 is a schematic of an exemplary optical cross-talk attenuation system for a WBC laser system according to the prior art;
• 4th is a schematic of an optical cross coupling attenuation system and output coupler for a WBC Laser system according to embodiments of the invention;
• 5 Figure 3 is a schematic of an optical element and output coupler for a WBC laser system in accordance with embodiments of the invention; and
• 6-8 are schematics of ranges of optical fibers applied as output couplers for WBC laser systems according to embodiments of the invention.

Description in detail

Aspects and embodiments generally relate to the field of scaling laser sources for high power and high brightness using an outer cavity, and more particularly, methods and apparatus for outer cavity beams combining the use of both one-dimensional and two-dimensional laser sources. In one embodiment, the outer resonator system includes one-dimensional or two-dimensional laser elements, an optical system, a scattering element, and a partially reflective element. An optical system consists of one or more optical elements that perform two basic functions. The first function is to overlap all laser elements along the beam combining dimension onto a scattering or dispersive element. The second function is to ensure that all elements along the non-beam combining dimension progress normally to the output coupler. In various embodiments, the optical system introduces as little loss as possible. As such, these two functions will enable a single resonant cavity for all laser elements.

In another embodiment, the WBC external resonator system includes wavelength stabilized one-dimensional or two-dimensional laser elements, an optical system, and a scattering element. One-dimensional or two-dimensional wavelength stabilized laser elements with the same wavelength can be produced using various means, such as laser elements with feedback of wavelength chirped volume Bragg grating, distributed feedback (DFB) laser elements or distributed Bragg reflector (DBR) laser elements, are executed. Here the main function of the optical system is to overlap all rays on a scattering or dispersive element. If there is no output coupler mirror outside of the wavelength stabilized laser element with parallel beams along the non-beam combining dimension, it is of little importance. Aspects and embodiments further relate to high power and / or high brightness multiple wavelength outer cavity lasers that produce an overlap or coaxial beam from very low output power to hundreds and even up to megawatts of output power.

Embodiments of the present invention mitigate the level of inadvertent and / or unwanted feedback from non-original transmitters in WBC laser systems. For example, in a WBC system where two individual beam emitters share a common partially reflective mirror (like an output coupler), there is the potential for the light from one emitter to be fed back to enter the other emitter. This unwanted feedback (or “crosstalk attenuation” or “cross coupling”) from a “non-native” transmitter reduces the efficiency of the system. The approaches and embodiments described herein can be applied to one- and two-dimensional beam combining systems along the slow diverging dimension (or “direction”), fast diverging dimension, or other beam combining dimensions. For purposes of this application, emitted rays have profiles in which one dimension is near or completely diffraction-limited while the other dimension is diffraction-limited many times. Another way of describing this can be in terms of axis and / or dimension. For example, an output beam can have a slowly and rapidly diverging axis or dimension.

When the term is used substantially larger when referring to the focal length of one optical element compared to the focal length of another optical element (f1 »f2), it is to be understood that a factor of at least 2.3 , 4, 5, 7 times or larger. For example, the focal length of f1 can be 100mm or more, while the focal length of f2 is 50mm or less. In another example, the focal length of f1 can be 200mm or more while f2 is 20mm or less. The term "angular filter" refers to a variety of optical elements that create a designated number of openings for feedback beams. The size of this number of openings can limit the allowable feedback to only that corresponding to the originally emitted beam. This means that the angular filter prevents neighboring or closely emitted beams from returning to the original transmitter (ie crosstalk attenuation). Transmitter stabilization refers to feedback received by any transmitter that has been narrowed to a noticeable wavelength. This can take the form of seeding the transmitters at a particular wavelength, causing a portion of the emitted beam to be redirected back into the transmitter, and intervening in the feedback, such as placing an optical grating in such a way as to bring one into the transmitters to generate individual wavelengths to be directed as feedback. Often times, feedback is reflected back onto the original emission surface, where it is through a scattering or dispersive element or diffraction grating gets back into the optical gain medium area of the original transmitter before entering. In some WBC embodiments, the feedback source may be a common reflective surface that provides feedback to multiple transmitters, with each of the feedback beams being individually tuned to a particular wavelength.

1A-1B illustrate a prior art one-dimensional (1-D) WBC system with an external cavity including or consisting essentially of a one-dimensional beam transmitter 102 (e.g. a diode bar) with a back reflective surface 104 , a gain medium 106 with, for example, two or more diode transmitters 105 , a front reflective surface 108 , a combining look 110 , a scattering or dispersive element 112 and a partially reflective output coupler 114 . In this embodiment, the combining optics or lens 110 at a focal distance 120a away from the front reflective surface 108 of the diode bar 102 placed while on the back plane or other side of lens 110 scattering or dispersive element 112 at a focal distance 120b is located away. The output coupler 114 becomes at a certain distance from the scattering or dispersive element 112 arranged and reflects a region of the generated beam (feedback 116 ) back to the scattering or dispersive element 112 .

In this embodiment, the arrangement of the combining lens is effective 110 two functions. The first function is to send all principal rays from all diode elements onto the scattering or dispersive element 112 to overlap. The second function is to collect each ray in both axes. 1A and 1B illustrate a schematic view of the non-beam combining dimension 130 view ( 1A) and the beam-combining dimension 140 view ( 1B) . Channel 102 includes multiple transmitters (e.g. diode transmitters) 105 , a back reflective surface 104 , Gain medium 106 and a front face / facet 108 or consists essentially of it.

In WBC resonators it is possible for neighboring transmitters to optically cross-couple with one another. This can severely degrade the output beam quality. 2 Figure 13 is a schematic of a prior art WBC resonator with two adjacent transmitters 202a and 202b which are their nominal on-axis principal rays 260a and 260b (shown as solid lines) to lens 210 send what the same to the center of the scattering or dispersive element (e.g. diffraction grating) 212 focused. From there, both main rays are diffracted at their own unique wavelengths to move along the same axis 240 propagate through the cross-coupling attenuation optics 250 any and all lenses or optical elements between the grating 212 and the partially reflective coupler 214 reproduces. Both beams are then partially reflected back onto themselves, spreading backwards to self-couple to their respective transmitter. The dotted lines 261a and 261b in 2 show the principal rays that would result in optical cross coupling between the two transmitters - that is, a principal ray originating from one transmitter couples back into another transmitter.

• d = Abstand zwischen den zwei Sendern (symmetrisch verschoben oberhalb und unterhalb der Achse um +/- (d/2)).
• ∈ = Ablenkungswinkel (der Winkel zwischen den Hauptstrahlen mit durchgezogener Linie und den Hauptstrahlen mit Punktlinie an den Sendern.
• θ_1/2 = Semi-Divergenz-Fernfeld-Winkel von einem Sender in der WBC-Richtung.
• L₀ = Abstand der Sender zur Linse L1.
• f₁ = fokale Länge von Linse L1.

The following parameters are defined as follows:

• d = distance between the two transmitters (symmetrically shifted above and below the axis by +/- (d / 2)).
• ∈ = deflection angle (the angle between the main rays with a solid line and the main rays with a dotted line at the transmitters.
• θ _1/2 = semi-divergence far-field angle from a transmitter in the WBC direction.
• L ₀ = distance of the transmitter to the lens L1.
• f ₁ = focal length of lens L1.

In 2 the grid is shown how it works in normal occurrences. It is assumed here that the system is operated in the Littrow configuration, with the angle of incidence and the angle of diffraction being equal (and non-zero). In the Littrow configuration, a small change in the angle of incidence matches first order with an equal change in the angle of diffraction. In the horizontal scheme, every ray operated with Littrow then appears to be propagating straight through the grid. It is clear that only the central ray 202c (the one from an imaginary transmitter halfway between two transmitters 202a and 202b would radiate) at Littrow self-couples.

The symmetry in 2 is careful as it provides some important simplifications in the analysis of the unique deflection angle ∈ at which a principal ray could exit from one transmitter and return to the other. The first symmetry-based simplification is that the deflected (dotted line) main ray has to hit the coupler at its center. The second simplification is that the cross-coupling wavelength must be the average of the two self-coupling wavelengths. Again, this would be the wavelength from an imaginary transmitter halfway between the two transmitters which, as indicated above, would be self-coupling at Littrow. Therefore the dotted line Main rays in 2 cross the grid at Littrow, meaning that they would appear to be spreading straight through the grid, as shown. Using this simplification, a standard "y / y-bar" (chief ray height / chief ray rise) analysis can be applied to trace the top dotted chief ray:

$${\overline{y}}_{Emitter}=\epsilon$$

Leaving the upper transmitter or emitter: $$y_{\mathrm{E.}mitter}=\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE112015005587B4\_0018.png,DE112015005587B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}$$

$${\overline{y}}_{\mathrm{E.}mitter}=\epsilon$$

$${\overline{y}}_{L1\_ein}={\overline{y}}_{Emitter}=\epsilon$$

Penetration in lens L1: $$y_{\mathrm{L.}1\_ein}=y_{\mathrm{E.}mitter}+{\mathrm{L.}}_0{\overline{y}}_{\mathrm{E.}mitter}=\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE112015005587B4\_0018.png,DE112015005587B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}+{\mathrm{L.}}_0\epsilon$$

$${\overline{y}}_{\mathrm{L.}1\_ein}={\overline{y}}_{\mathrm{E.}mitter}=\epsilon$$

$${\overline{y}}_{L{1}_{aus}}={\overline{y}}_{L{1}_{ein}}-\frac{y_{L{1}_{ein}}}{f_1}=\epsilon -\frac{\frac{d}{2}+L_0\epsilon }{f_1}=\epsilon \left(\frac{f_1-L_0}{f_1}\right)-\frac{d}{2f_1}$$

Exit from lens L1: $$y_{\mathrm{L.}1\_aus}=y_{\mathrm{L.}1\_ein}=\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE112015005587B4\_0018.png,DE112015005587B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}+{\mathrm{L.}}_0\epsilon$$

$${\overline{y}}_{\mathrm{L.}{1}_{aus}}={\overline{y}}_{\mathrm{L.}{1}_{ein}}-\frac{y_{\mathrm{L.}{1}_{ein}}}{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}=\epsilon -\frac{\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE112015005587B4\_0018.png,DE112015005587B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}+{\mathrm{L.}}_0\mathrm{<figure-callout\; id="1"\; label="\epsilon \; f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}{f_{}}=\epsilon \left(\frac{f_1-{\mathrm{L.}}_0}{f_1}\right)-\frac{d}{2{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}$$

$${\overline{y}}_{Gitter}={\overline{y}}_{L_1\_aus}=\epsilon \left(\frac{f_1-L_0}{f_1}\right)-\frac{d}{2f_1}$$

Entering and exiting the grid (recall the discussion above that the dotted principal ray does not change direction at the grid): $$y_{Gitter}=y_{\mathrm{L.}1\_aus}+f_1{\overline{y}}_{\mathrm{L.}1\_aus}=\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE112015005587B4\_0018.png,DE112015005587B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}+{\mathrm{L.}}_0\epsilon +\epsilon \left(f_1-{\mathrm{L.}}_0\right)-\frac{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE112015005587B4\_0018.png,DE112015005587B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}{2}=\mathrm{<figure-callout\; id="1"\; label="\epsilon \; f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}{f}_{}{}_1$$

$${\overline{y}}_{Gitter}={\overline{y}}_{{\mathrm{L.}}_1\_aus}=\epsilon \left(\frac{f_1-{\mathrm{L.}}_0}{f_1}\right)-\frac{d}{2{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}$$

Worin L_cplr__eff der effektive Abstand des Kopplers über (nach rechts von) das Gitter hinausgeht.To finish the calculations on the coupler that involve propagating through the cross coupling attenuation optics, recall that the dotted principal ray crosses the coupler at its center. Therefore, only the beam rise at the coupler is non-zero, and it should be noted that the beam height and beam rise at the grating must both be proportional. This implies that the ratio of the height to the slope on the grid must be constant. And one can constantly very intuitively interpret as the negative of the effective distance of the coupler from the grating, as determined by the cross-coupling attenuation optics. In other words $$\frac{y_{Gitter}}{{\overline{y}}_{Gitter}}\equiv -{\mathrm{L.}}_{cplr\_eff}$$

Wherein L _CPLR _ _{eff is} the effective distance of the coupler over (to the right of) beyond the grid.

Specifically, L _{cplr_eff} can be calculated either with a ray trace or with a y / y-bar analysis of the post-grating lenses. But in either case, Equation 9 allows equations 7 and 8 to be solved for the deflection angle ∈ with the following result: $$\epsilon =\raisebox{1ex}{$\left(\frac{d}{2f_1}\right)$}\!\left/ \!\raisebox{-1ex}{$\left[\left(\frac{f_1-{\mathrm{L.}}_0}{f_1}\right)+\left(\frac{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}{{\mathrm{L.}}_{cplr\_eff}}\right)\right]$}\right.$$

Now that the deflection angle ∈ that gives cross talk has been determined, the degree of cross talk can be calculated. A useful definition of cross-coupling is the integral over solid angles at the transmitter of the product of the self-coupled intensity and the cross-coupled intensity, normalized by the integral of the square of the self-coupled intensity. Before calculating the integral, it is important to note that in the name of simplistic symmetry, it is assumed that both the outgoing and incoming beams at the cross-coupling transmitters are the same. Thus, for the overlap integral, consider one ray (the self-coupled ray) which is undeflected and the other ray (the cross-coupled ray) to be deflected by doubling by angle ∈. Substituting this section into equation form gives: $$\ddot{\text{U}}berlappen=\frac{{\displaystyle \int exp}\left[-2{\left(\frac{\theta }{{\mathrm{<figure-callout\; id="1"\; label="\theta "\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\theta </figure-callout>}}_{1/2}}\right)}^2\right]exp\left[-2{\left(\frac{\theta -2\mathrm{<figure-callout\; id="1"\; label="\epsilon \; \theta "\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}{{\theta }_{}}\right)}^2\right]d\theta }{{\displaystyle \int exp\left[-2{\left(\frac{\theta }{{\mathrm{<figure-callout\; id="1"\; label="\theta "\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\theta </figure-callout>}}_{1/2}}\right)}^2\right]exp\left[-2{\left(\frac{\theta }{{\mathrm{<figure-callout\; id="1"\; label="\theta "\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\theta </figure-callout>}}_{1/2}}\right)}^2\right]d\theta }}$$

(Note that Equation 11 includes one-dimensional integrals over a single angle instead of two-dimensional integrals over solid angles. This is because the integration over angles in the direction orthogonal to the beam deviation gives a constant that falls out of the relationship in equation 11.) Equation 11 can be simplified to give: $$\ddot{\text{U}}berlappen=exp\left[-2{\left(\frac{2\epsilon }{{\mathrm{<figure-callout\; id="1"\; label="\theta "\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">\theta </figure-callout>}}_{1/2}}\right)}^2\right]$$

In summary, one can calculate the relevant deflection angle ∈ with regard to the known parameters according to equation 10. The overlap obtained can then be calculated according to equation 12. This gives the ratio of cross-coupled intensity to self-coupled intensity, assuming that in the self-coupled case there is a perfect neck at the coupler.

There is a very interesting way to have a big impact on the crosstalk if close, but not very, to the usual configuration of placing the transmitters a focal length back from L1. If we placed the transmitters exactly there, then the first term in the denominator of equation 10 would be zero, and equation 10 would reduce to: $${\epsilon }_{\left({\mathrm{L.}}_0=f_1\right)}=\frac{\left(d\right)\left({\mathrm{L.}}_{cplr\_eff}\right)}{2{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1{}^2}$$

Substituting equation 13 in equation 12 results in: $$\ddot{\text{U}}berlappen=exp\left[-\mathrm{4th}{\left(\frac{d*\left({\mathrm{L.}}_{cplr\_eff}\right)}{d\text{'}*Zr}\right)}^2\right].$$

Here d 'is the transmitter diameter at the near field and Zr is the Rayleigh region 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 coupler should be long, and the Rayleigh range should be short. In general, the near-field fill factor is fixed. As an example, assuming the WBC system 20th Contains diode bars and a conversion lens with a focal length of 2000 mm, then the beam size at the grating is roughly 40 mm (assuming 20 milli-radians of full beam divergence). The Rayleigh region of such a ray ( 1 µm wavelength and diffraction limited) is about 160 m. The distance between the grating and output coupler should be comparable to the Rayleigh range for cross-coupling attenuation. Such a length would make the WBC system essentially impractical. However, if the beam is reduced by 40x between the grating and the output coupler, the optical path length is reduced by 160x, or about 1 m. Furthermore, the reduction in optical path length can be achieved using greater reduction in beam size. Beam reduction can be accomplished using various mechanisms such as lenses, prisms, or a combination of both. Careful assembly must be observed so that the self-coupling of each transmitter does not degrade, so that the performance of the resonator suffers.

However, if the transmitters are a little out of this position then in the first term in the denominator of Equation 10 one can actually delete the second term, making the required deflection angle infinite and the cross talk overlapping zero. In particular, this happens when: $${\mathrm{L.}}_0={\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1+\left(\frac{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DE112015005587B4\_0022.png"\; state="\{\{state\}\}">f</figure-callout>}}_1{}^2}{{\mathrm{L.}}_{cplr\_eff}}\right)$$

In other words, if the effective distance to the coupler L _{cplr_eff is} very large, Equation 14 gives us a potential recipe for pulling the transmitters back slightly from the front focus of L1 to destroy cross-talk.

3 Figure 11 illustrates an example of a cross talk attenuation system 250 of the prior art, illustrated by a box in 2 . Optical element can be used here 302 a lens with a focal length F ₁ 304 exhibit. A second optical element 306 can also be a lens and have a focal length F ₂ 308 exhibit. The distance between 302 and 306 is exactly or approximately the sum of the focal lengths F ₁ and F ₂ . As discussed above, it is preferred that the ratio of F ₁ to F ₂ (F ₁ / F ₂ ) be at least two times or greater. The system 250 can be an afocal telescope system. In other embodiments, multiple optical elements can be used while the action of the system still retains the properties of a large ratio afocal telescope system.

In various embodiments it is desirable to have lens 302 to arrange in the Rayleigh range of rays that are transmitted by a scattering or dispersive element (e.g. diffraction grating), while also a partially reflective output coupler or other reflective surface in the Rayleigh range of rays that are transmitted from the lens 306 come out, is placed. By properly placing lenses with an F ₁ >> F ₂ relationship in these positions, an effective system is created to eliminate any cross coupling. To reduce feedback at the penetration of the non-parent transmitter or source and, in some cases, to remove it altogether.

4th illustrates a stabilization system 400 (which may be a portion of a WBC laser system) according to an embodiment of the present invention, in which a cross-talk attenuation optical system (which optical elements 410 , 420 may include or consist essentially of) in conjunction with a partially reflective output coupler 430 is used, designed to minimize reflections that could result in unwanted feedback. As shown, the output coupler closes 430 a partially reflective beam receiving area 434 , designed and positioned to receive the beam from optical element 420 , a. In particular, the beam receiving area 434 generally a diameter (or other lateral dimension) approximately the same size as the diameter (or other lateral dimension) of the beam that is receiving. The beam receiving area 434 , which is roughly centered on a surface of the output coupler 430 is from a non-reflective area (or surface) 432 that has a reflectivity of 1% or less to the wavelengths of the receiving beam. For example, the non-reflective area can 432 be coated with an anti-reflective coating to prevent unwanted reflection that could result in optical crosstalk attenuation. Thus, any stray light will propagate to the output coupler 430 outside the beam reception area 434 do not reflect back to the beam emitters of the WBC system. The beam receiving area 434 can be made from the rest of the surface of coupler 430 protrude (ie may in terms of the non-reflective area 432 be increased), as in 4th shown, or the beam receiving area 434 can be roughly coplanar with non-reflective area 432 be.

The beam receiving area 434 may have a reflectivity for the wavelengths of the beam of less than about 15%, for example in the range of about 2% to about 10%, so as to provide the desired wavelength stabilization of the beam from the associated transmitters. Care is taken that the rest of the received beam goes through the output coupler 430 and is transmitted to optical system components (e.g., an optical fiber or workpiece) downstream.

5 illustrates a stabilization system 500 (which may be a portion of a WBC laser system) according to an embodiment of the present invention, in which an optical element 510 (e.g. a cylindrical or spherical lens) simply direct the beam onto the partially reflective output coupler 430 focused, and there is no second converging optical element in between. In this way the output coupler 430 in a WBC laser system without the use of a cross-coupling optical attenuation system (eg, including or consisting essentially of two or more optical elements such as lenses).

6th Figure 3 illustrates a portion of a WBC laser system 600 in which an optical element 610 Light straight to the core 650 focused by an optical fiber that includes one or more features that act as the partially reflective output coupler. As shown, the optical fiber can also have a cladding 640 having the core 650 surrounds; generally exhibits the coat 640 a refractive index lower than that of the core 650 on so that light is in the core 650 is restricted. The end face of the core 650 may be substantially coplanar with the end face 642 the mantle or the core 650 can be easily removed from surface 642 stick out. To provide the wavelength stabilization, the end face of the core 650 partially reflective for the wavelengths of the beam (e.g., between about 2% and about 10% reflective, or between about 4% and about 10% reflective). In various embodiments, the partial reflectivity can be provided by a coating on the end face of the core.

In various embodiments of the invention, a fiber Bragg grating can be used instead of or in addition to a partially reflective coating 654 in the core 650 may be provided to provide the desired partial reflectivity. As known to those skilled in the art, the fiber Bragg grating involves a periodic variation in the index of refraction of a portion of the fiber (e.g., in the core 650 ) or essentially consists of it. The period variation can be, for example, of the order of half the wavelength (or one of the wavelengths) of the received beam and the grating thus introduces Fresnel reflection. The wavelength dependence and / or the height of the reflection can be selected by the particular grating pattern and the refractive index variation therein. In various embodiments, multiple fiber Bragg gratings 654 in the core 650 be arranged and each grid 654 may have a different refractive index variation and / or wavelength selectivity.

In various embodiments, the surface 642 of the coat 640 be coated with an anti-reflective coating so as to prevent any aberrant light that scatters from the beam from reflecting back into adjacent emitters or beams. For example, the surface can 642 be coated so as to have a reflectivity of less than 1% for the wavelengths of the beam.

7th illustrates a wavelength stabilization system 700 (which can be an area from a WBC laser system) in which an optical element 710 Light to the core 650 the optical fiber is focused. In system 700 is a graduation cap 720 over and in contact with core 650 (and in some embodiments surface 642 the coat 640 ) arranged; in some embodiments the end cap is 720 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 ) directly to the end cap 720 tied together. As in 7th shown allows the presence of the end cap 720 the effective interface between the optical fiber and the incoming beam (i.e., the point at which the beam enters the end cap) to accommodate the beam if it is of a larger diameter (or width), thereby increasing the current density of the beam after it has entered of the optical fiber is decreased. The presence of the end cap 720 can also protect other areas of the optical fiber from heat, moisture, and / or other environmental contaminants.

8th illustrates a wavelength stabilization system 800 (which can be an area from a WBC laser system) in which an optical element 810 Light to the core 650 the optical fiber is focused. system 800 includes a mode stripper 820 to further increase the purity and transmission capabilities of the optical fiber with respect to the beam. It is important that as rays change transmission media, the various indices of refraction and angles of incidence of light into an optical fiber can result in a cladding mode, that is, light deflected in the material of the cladding. This cladding mode can be undesirable as such light can result in wavelength interference and contamination of the primary beam. As known to those skilled in the art, the mode stripper 820 include, consists essentially of or consists of a material having an index of refraction not less than (ie, equal to or greater than) that of the clad 640 ; in this way, light that could generally propagate as a cladding mode in the cladding will preferentially enter the mode stripper and radiate out of the optical fiber. In various embodiments, the mode stripper 820 e has a refractive index greater than that of the clad 640 exhibit. As in 8th shown can be an index-matching material 830 between the core 650 (or in some embodiments the coat 640 ) and the mode stripper 820 be arranged. (As used herein, the term "index-matching material" refers to a material sandwiched 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 of the materials.) Although 8th the fashion stripper 820 than directly under surrounding the core 650 cited, is in various embodiments at least one area of the jacket 640 between the core 650 and the fashion stripper 820 arranged.

In any of the above-mentioned wavelength stabilization systems, emphasis is placed on the fact that the beam can be manipulated in various ways via the addition of optical and / or scattering or dispersive elements designed to achieve the desired beam qualities. For example, optical elements such as gratings and / or collimators can be present in the WBC system and / or the stabilization system. It is also appreciated that the partially reflective elements with partially reflective properties can be provided by any number of means including, but not limited to, the provision of gratings, coatings, etc., so as to provide the desired transmission and reflective qualities reach.

The terms and expressions used herein are used as terms of description and not of limitation, and there is no intent in the use of such terms and expressions to exclude any equivalents of the features or ranges shown and described, however, it is recognized that various modifications are possible are possible within the scope of the invention as claimed.

Claims (17)

A laser system comprising: an array of beam emitters (102) each transmitting a beam; Imaging optics (110) for focusing the rays on a scattering or dispersive element (112); a dispersive element (112) for receiving and dispersing the focused beams, thereby forming a multi-wavelength beam; and an optical fiber for receiving the multi-wavelength beam, the optical fiber comprising (i) a core (650) for receiving the multi-wavelength beam reflecting a first portion thereof back onto the dispersive element (112) and a second portion thereof as one composed of many wavelengths Emits output beam, the core (650) having a partially reflective surface, and (ii) surrounding the core, a cladding (640) having a reflectivity for the multi-wavelength beam of less than 1%, wherein the scattering or dispersive element ( 112) is selected from the group consisting of a diffraction grating, a scattering prism, a grism (prism / grating), a transmission grating or an echelle grating. Laser system according to Claim 1 wherein a portion of the core (650) protrudes from the cladding (640). Laser system according to Claim 1 wherein the optical fiber is positioned such that at the partially reflective surface of the core (650), a diameter of the core (650) is not less than a diameter of the multi-wavelength beam. Laser system according to Claim 1 further comprising an end cap (720) attached to the optical fiber and optically positioned upstream of the partially reflective surface of the core (650). Laser system according to Claim 1 further comprising an anti-reflective coating disposed over the cladding (640) of the optical fiber. Laser system according to Claim 1 further comprising a mode stripper (820) disposed around at least a portion of the core (650) of the optical fiber. Laser system according to Claim 1 , further comprising a cross-talk attenuation system (250) for receiving and transmitting the multi-wavelength beam while reducing the cross-talk therefrom. Laser system according to Claim 7 wherein the partially reflective surface of the core (650) of the optical fiber is located in a Rayleigh region of the multi-wavelength beam transmitted by the cross-talk attenuation system (250). Laser system according to Claim 7 wherein at least one area of the cross-coupling attenuation system (250) is arranged in a Rayleigh area of the multi-wavelength beam transmitted by the scattering element (112). Laser system according to Claim 7 wherein the cross talk attenuation system (250) comprises an afocal telescope. Laser system according to Claim 7 wherein the crosstalk attenuation system (250) comprises a first optical element (410) with a first focal length (304) and a second optical element (420) with a second focal length (308), the first optical element (410) is arranged optically upstream of the second optical element (420). Laser system according to Claim 11 wherein the first focal length (304) is at least two times greater than the second focal length (308). Laser system according to Claim 11 wherein the first focal length (304) is at least seven times greater than the second focal length (308). Laser system according to Claim 11 wherein each of the first and second optical elements (410, 420) comprises a lens (110). Laser system according to Claim 11 wherein the first optical element (410) is arranged in a Rayleigh region of the multi-wavelength beam transmitted through the dispersive element (112). Laser system according to Claim 11 wherein the partially reflective surface of the core (650) of the optical fiber is located in a Rayleigh region of the multi-wavelength beam transmitted through the second optical element (420). Laser system according to Claim 11 wherein an optical distance between the first and second optical elements (410, 420) is approximately equal to a sum of the first and second focal lengths (304, 308).

Images