Patent Publication Number: US-2022221663-A1

Title: Fiber laser insensitive aiming laser

Description:
RELATED APPLICATIONS 
     The present application is a National Phase entry under 35 U.S.C. § 371 of International Application No. PCT/US2020/036180, filed on Jun. 4, 2020, which claims priority to U.S. Provisional Application No. 62/857,537, filed on Jun. 5, 2019, the entire contents of these applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to methods and devices for backward-propagating radiation protection in fiber laser assemblies. 
     BACKGROUND 
     High-power industrial fiber laser users are accustomed to fiber lasers emitting a visible “aiming beam” on demand for use in tool alignment using the naked eye. Regulatory requirements for such visible beams typically limit their output power to &lt;1 mW. It is desirable that this power is transmitted through a user&#39;s choice of materials processing optics with little attenuation. The low attenuation requirement encourages incorporation of the alignment beam in the core of the fiber laser. 
     In an example fiber laser assemblies, a visible beam is injected into an output beam through a combiner. For example, as depicted in  FIG. 1 , laser assembly  100  produces a laser output beam  124  coaxial with a visible beam  134 . 
     In this embodiment, assembly  100  includes pump laser beam sources  110 ,  112  that produce beams  111 ,  113  respectively. Beams  111 ,  113  propagate in respective fibers  114 ,  116 . Fibers  114 ,  116  are spliced to combiner input fibers  120 ,  122 . Combiner  102  receives and combines beams  111  and  113  to form a combined output beam  124  that is coupled into the cladding of combiner output fiber  126 . Visible light source  132  produces visible beam  134  which is coupled to additional input fiber  140  and combiner  102  via Wavelength Division Multiplexer (WDM)  136 . Combiner  102  couples visible beam  134  in the core of output fiber  126  that includes laser  154  comprising active fiber between high reflector fiber Bragg grating (HR FBG)  152  and partial reflector fiber Bragg grating (PR FBG)  150 . Fiber  126  delivers laser output beam  124  to a laser head  128  that directs beam  124  to workpiece  130  to perform processing operations such as cutting, welding, brazing, additive manufacturing, or the like. Visible beam  134  is coaxial with beam  124  and can be used for guiding and alignment of beam  124  on workpiece  130 . 
     During active operation the laser output beam  124  can reflect from a surface of workpiece  130  or cause workpiece  130  to emit radiation in response to incident beam  124 . Both the emitted and reflected radiation may be coupled backward into the laser fiber core. This backward-propagating radiation  140  can travel back through input fibers and combiner  102  to reach and potentially damage upstream components. Damage caused by backward-propagating radiation can cause catastrophic failure. For example, backward-propagating radiation may damage or disable the source  132  of the visible aiming beam  134 . One way to protect the visible light source  132  is to inject visible beam  134  through WDM  136  designed to transmit the aiming beam  134  into the fiber laser core and transmit backward-propagating radiation  140  from the fiber laser into an unused port such as WDM rejection port  138 , where it may be safely dissipated. However, such a device is expensive and can add undesirable cost to a fiber laser. 
     The problem is to find a cost-effective method of injecting a visible aiming beam into the output of a high-power industrial fiber laser that is reliable under anticipated backward-propagating radiation. 
     SUMMARY 
     Disclosed herein are assemblies, apparatus&#39; and methods for reducing deleterious effects of backward-propagating radiation in a fiber laser. Such assemblies, apparatus&#39; and methods include a laser assembly comprising multi-clad fiber optically coupled to a light source (e.g., a laser diode) configured to emit optical radiation at a first wavelength (e.g., in the visible spectrum) and a protective element disposed between the light source and the multi-clad fiber so as to prevent a portion of backward-propagating optical radiation at a second wavelength from coupling into the light source. 
     In an example, the multi-clad fiber may be double-clad fiber comprising a core, a cladding and a buffer layer, wherein the core has a higher refractive index than the cladding and the cladding has a higher index than the buffer layer. In a different example, the multi-clad fiber may be a triple-clad fiber comprising a core, a first cladding, a second cladding and a buffer layer, wherein the core has a higher refractive index than the first cladding, the first cladding has a higher index than the second cladding and the second cladding a higher index than the buffer layer. 
     The protective element may be a reflector or an absorber or a combination thereof. In an example, the protective element may be a dichroic filter configured to transmit optical radiation at the first wavelength and to reflect optical radiation at the second wavelength. Further, the protective element may reflect optical radiation at the second wavelength in such a way as to couple a portion of the backward-propagating optical radiation into one or more cladding layers of the multi-clad fiber and/or reflect optical radiation at the second wavelength in such a way as to direct the optical radiation away from a core of the multi-clad fiber. 
     In an example, the protective element may be a dichroic filter. The dichroic filter may be applied to an output end of the multi-clad fiber. In some cases the output end of the multi-clad fiber may be angled, curved or spherical. Additionally, or alternatively, a dichroic filter may be applied on the surface of a window that forms a portion of a package encapsulating the light source or on the surface of a protective element disposed adjacent to an output end of the multi-clad fiber. 
     The laser assembly may also include a focusing optic configured to focus the optical radiation at the first wavelength into the multi-clad fiber. In such a case, a dichroic filter may comprise a coating applied to a surface of the focusing optic. In an example, the multi-clad fiber may be a fiber pigtail configured to be optically coupled to an input fiber of a fiber laser. Such a pigtail may be further configured to couple the optical radiation at the first wavelength from the light source to the input fiber, wherein the first wavelength is in the visible spectrum and the fiber laser is configured to propagate the optical radiation through an output fiber to a workpiece. In an example, the fiber laser may be a diode pumped fiber laser or a counter-pumped fiber laser. 
     The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures which may not be drawn to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, wherein like reference numerals represent like elements, are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the presently disclosed technology. In the drawings, 
         FIG. 1  illustrates an example laser assembly for directing a visible light beam coaxially with an output high-power laser beam including a WDM for back-reflection protection; 
         FIG. 2A  illustrates an example laser assembly for generating and directing a high-power output laser beam with a coaxial visible light beam comprising a visible light source protection element; 
         FIG. 2B  illustrates an example visible light source protection assembly for protecting a visible light source from damage by backward-propagating radiation; 
         FIG. 2C  illustrates an example refractive index profile of a double-clad fiber configured for protecting a visible light source from damage by backward-propagating radiation; 
         FIG. 2D-2H  illustrate a number of example visible light source protection assemblies for protecting a visible light source from damage by backward-propagating radiation; 
         FIG. 3A  illustrates an example visible light source protection assembly for protecting a visible light source from damage by backward-propagating radiation; 
         FIG. 3B  illustrates an example refractive index profile of a triple-clad fiber configured for protecting a visible light source from damage by backward-propagating radiation; and 
         FIG. 4  illustrates an example counter-pumped laser assembly for generating and directing a high-power output laser beam with a coaxial visible light beam comprising a visible light source protection element. 
     
    
    
     DETAILED DESCRIPTION 
     As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” does not exclude the presence of intermediate elements between the coupled items. 
     The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation. 
     Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art. 
     In some examples, values, procedures, or apparatus&#39; are referred to as “lowest”, “best”, “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections. Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation. Moreover, in the following examples, laser components and assemblies are described at a high level of abstraction and do not include a complete description of all mechanical, electrical and optical elements necessary for operation. 
     As discussed above, a reliable and cost effective method of injecting a visible aiming beam into an output of a high-power industrial fiber laser is a desirable alternative to using expensive WDM devices to handle backward-propagating radiation at the visible light source. The approach proposed herein is to use a visible light source that reflects enough of the backward-propagating radiation back into the fiber laser to reduce the power of the radiation incident on the light source itself to a safe level without destabilizing the operation of the fiber laser. 
       FIG. 2A  illustrates an example a fiber laser assembly  200  for generating and directing a high-power output laser beam  224  with a coaxial visible light beam  234 , wherein the assembly  200  incorporates a visible light source protection assembly  204  to protect a visible light source  232  from backward-propagating radiation  240 . In an example, assembly  200  includes pump laser beam sources  210 ,  212  that produce beams  211 ,  213  respectively. Beams  211 ,  213  propagate in respective fibers  214 ,  216 . Fibers  214 ,  216  are spliced to combiner input fibers  220 ,  222 . Combiner  202  receives and combines beams  211  and  213  to form a combined output beam  224  that is coupled into the cladding of combiner output fiber  226 . Combiner  102  couples visible beam  234  in the core of output fiber  226  that includes laser  274  comprising active fiber between HR FBG  272  and PR FBG  270 . 
     Visible light source  232  produces visible beam  234  which is coupled to additional input fiber  240  and combiner  202 . Combiner output fiber  226  delivers laser output beam  224  to workpiece  230  to perform a desired processing operation. Visible beam  234  is coaxial with beam  224  and can be used for guiding and alignment of beam  224  on workpiece  230 . 
     In an example, combiner  202  is a pump/signal combiner arranged to couple light from an external source (i.e., the visible light source  232 ) into a fiber laser core. Combiner  202  may also couple light from the fiber laser core back into the external source. During operation, workpiece  230  may reflect incident light when irradiated by beam  224  and may emit light in response to incident laser light; both reflected and emitted light may be coupled backward into a core of output fiber  226 . Such backward-propagating radiation  240  is propagated back through combiner  202  and into upstream components such as visible light source  232 . In an example, visible light source protection assembly  204  is configured to protect visible light source  232  from backward-propagating radiation  240  as will be explained in further detail below. 
     Visible light source  232  may be a visible laser diode coupled to combiner  202  by a visible light source pigtail  244  coupled via splice  260  to optical fiber  245 . Laser diodes are typically coupled in this way with single clad optical fiber, meaning the optical fiber confines light in a core of glass surrounded by a cladding of lower index glass that is itself surrounded by a cladding of higher index protective buffer material that does not propagate light in the glass cladding. Such a construction is not suitable for a fiber laser visible light source because the backward fiber laser radiation will not be confined only to the core but will also propagate in the cladding. If visible light source pigtail fiber  244  were single clad, backward-propagating radiation  240  coupled to pigtail fiber  244  would couple into the buffer causing fiber failure. To avoid such a failure mode, visible light source pigtail  244  comprises a double or triple-clad fiber. 
       FIG. 2B  illustrates an example visible light source protection assembly  204  for protecting visible light source  232  from damage by backward-propagating radiation  240 . In this example, visible light source pigtail  244  is a double-clad fiber comprising a low-loss buffer  256  with a lower index than the intermediate index glass cladding  254 . Cladding  254  is configured to propagate clad coupled light with low loss and thus minimal risk to the integrity of buffer  256 . The core  252  is a high-index glass comprising a higher index material than cladding  254 . 
     In an example, core  252 , cladding  254  and buffer  256  may comprise a variety of materials known to those skilled in the art to achieve the desired fiber structure and refractive index profile. As a non-limiting example, core  252  and cladding  254  may comprise SiO 2 , SiO 2  doped with GeO 2 , germanosilicate, phosphorus pentoxide, phosphosilicate, Al 2 O 3 , aluminosilicate, or the like or any combinations thereof. Buffer  256  may comprise glass and/or polymer materials such as fluoropolymers such as polyvinylidene fluoride (Kynar), polytetrafluoroethylene (Teflon), and polyurethane, or the like or any combinations thereof. 
     Pigtail fiber  244  may transmit backward-propagating radiation  240  sufficient to damage the visible light source  232 . In some cases even the backward-propagating light  240  guided in the core  252  could damage visible light source  232 . In an example, a protective element comprising a dichroic coating  248  configured to prevent backward-propagating radiation  240  from coupling into visible light source  232  may be applied to the end face  246  of fiber  244 . Such a protective element may reflect the incident backward-propagating radiation  240  back into the fiber laser  274 . The dichroic filter coating  248  may be designed to sufficiently transmit the visible light  234  to be injected into the core  252  while reflecting the potentially damaging wavelengths of the backward-propagating radiation  240  that is at wavelengths other than the visible light source wavelength. Any wavelength in the visible light spectrum will be suitable. Typically, the potentially damaging wavelengths will be the primary high-power laser wavelength (e.g., as a non-limiting example, 1000 to 1100 nm for Yb, 1900 to 2100 for Tm), with possible broadening due to non-linear effects such as self-phase modulation, and other wavelengths generated from the primary high-power laser wavelength by non-linear effects such as Stimulated Raman Scattering (SRS). 
     Light propagating backward from the fiber laser should not be coupled by reflection from the visible laser source into the core of the fiber laser or there is risk of seeding a destabilizing non-linear process like SRS or changing the output of the laser or amplifier with an unexpectedly broad seeding bandwidth. The core coupled Optical Return Loss (ORL) of the visible laser as measured from its fiber pigtail  244  should be low. Low ORL may be accomplished by angling the end-face  246  of the dichroically coated  248  optical fiber  244  interfacing to the visible laser source  232  so the reflection of backward-propagating radiation  240  off the end-face  246  is coupled out of the core  252  and into the fiber cladding  254 . 
     In this example, returned radiation  241  represents light that is reflected back into fiber  244  from one or more reflective components disposed in assembly  204 . Returned radiation  241  is reflected from the dichroically coated  248  end-face  246  of optical fiber  244 . It propagates primarily in the cladding  254  of the fiber pigtail  244  back toward the pump/signal combiner  202 . Returned radiation  241  will be safely propagated by the cladding of the double clad fiber (or triple-clad fiber, see  FIG. 3A ) back to the pump/signal combiner  202 . The pump/signal combiner  202  will couple returned radiation  241  primarily into the cladding of fiber  226  along with any pump light in fiber laser  274 . That cladding coupled returned radiation  241  will propagate through the fiber laser portion  274  and could be emitted at the output end of the fiber laser or could be stripped out and safely dissipated in a Clad Light Stripper (CLS) used to remove unwanted fiber laser cladding emission. 
       FIG. 2C  illustrates the relative refractive indices of core  252 , cladding  254  and buffer  256 . Refractive index profile  257  works together with reflective elements of assembly  204  to safely guide returned radiation  241  in the cladding back to the fiber laser. Other refractive index profiles known to those of skill in the art are possible and claimed subject matter is not limited by this or any other example. 
       FIG. 2D-2H  illustrate various examples of visible light source protection assemblies  280 - 288  for protecting a visible light source from damage by backward-propagating radiation. The following examples are illustrative and not intended to be exhaustive or limit claimed subject matter. In the examples, like reference numerals represent like elements described with respect to  FIGS. 2A and 2B  above. Further, in each of  FIG. 2D-2H , coating  258  comprises an optical filter or absorber such as for example a dichroic reflector. Coating  258  is configured to either absorb or reflect or otherwise filter backward-propagating radiation  240 . Generally, if coating  258  is a reflector, it will return incoming backward-propagating radiation  240  to fiber  244 . As the following examples illustrate, such returned radiation  241  can be substantially coupled into the cladding portion  254  of fiber  244  thereby protecting visible light source  232  from backward-propagating radiation  240 . Coupling returned radiation  241  into the cladding  254  also minimizes the risk of damage to other components of fiber laser assembly  200 . 
       FIG. 2D  illustrates an example visible light source protection assembly  280  wherein focusing lens  242  used to couple light from the visible source  232  into fiber  244  comprises a coating  258  on surface  259  that is an optical filter such as a dichroic filter. Coating  258  is depicted as facing fiber surface  246 . In another example, coating  258  could be applied to the portion of surface  259  facing visible light source  232 . Fiber surface  246  of assembly  280  may or may not be angled and may optionally comprise an optical filter coating  248  as well. Angling surface  246  may inhibit coupling of returned radiation into core  252 . 
       FIG. 2E  illustrates an example visible light source protection assembly  282  wherein an optical filter coating  258  is applied to a surface of a protective element such as a window  261  in package  262  protecting visible light source  232  from the surrounding environment. 
       FIG. 2F  illustrates an example visible light source protection assembly  284  wherein an optical filter coating  258  such as a dichroic filter is applied to a surface  265  of a protective element  264  dedicated to filtering out backward-propagating radiation  240 . Protective element  264  may be positioned at a variety of locations within assembly  284  to protect the visible laser source  232 , its fiber pigtail  244 , and any intervening optics  242 . 
     In another example, coating  258  may be an optical absorber configured to absorb radiation  240  rather than reflect a portion of the radiation  240 . This approach may have more limited power-handling capability than other approaches described herein. 
     Additionally, or alternatively, other reflective surfaces may be disposed between the end face  246  of the fiber pigtail  244  and the visible light source  232  so as to minimize returned radiation  241  reflecting back into the fiber core. 
       FIG. 2G  illustrates an example visible light source protection assembly  286  wherein protective element  264  may be tilted and/or shaped to help minimize coupling of reflected backward-propagating radiation  240  into core  252 . The surface may be curved or spherical (see,  FIG. 2H ). The tilt angle θ has to be slight enough (e.g., 3-12 degrees) to couple light back into the fiber cladding and close enough to the fiber (e.g., 75-125 um) to couple into the cladding  254 . If it is too far away the light will spread out and it won&#39;t couple well into the cladding  254 . Returned radiation  241  reflected back into fiber  244  by protective element  264  may be coupled into the cladding  254  to be safely carried back toward the fiber laser  274 . 
       FIG. 2H  illustrates an example visible light source protection assembly  288  wherein the end-face  268  of the fiber  244  may be shaped so the reflection of core guided light is poorly coupled back into fiber core  252 . The surface may be curved or spherical or another shape known to those of skill in the art to aid in controlling a reflection angle on backward-propagating radiation  240 . Further, optical filter coating  248  may be applied to curved end-face  268  and may allow visible light  234  to couple into fiber core  252  and reflect a portion of backward-propagating radiation  240  sending returned radiation  241  to the fiber laser  274 . In some examples, end-face  268  surface shape may be selected to promote coupling of visible beam  234  thus obviating the need for lens  242 . 
     In some examples, including those depicted in  FIG. 2A-2H , a visible light fiber pigtail may comprise a triple-clad fiber.  FIG. 3A  illustrates an example visible light source protection assembly  304  for protecting visible light source  332  from damage by backward-propagating radiation  340  wherein the visible light source pigtail  344  is a triple-clad fiber. 
     In an example, backward-propagating radiation  340  is reflected by dichroic coating  348  on angled end-face  346  and preferentially coupled back into cladding layers of fiber  344  as returned radiation  341 . 
     Visible light source pigtail  344  comprises a low-loss buffer  356  having a lower index than first glass cladding  354  and second glass cladding  358 . In an example, the refractive index of cladding  354  is lower than the refractive index of cladding  358  to confine a fraction of the clad light to second (inner) cladding  358  to prevent it from interacting substantially with buffer  356 . Core  352  comprises a higher index material than first cladding  358  and second cladding  354 . The triple-clad fiber also uses a buffer  356  with lower index of refraction than the first (outer) cladding  354  to promote low loss propagation of the fraction of light confined in the outer cladding  354 , further reducing the possibility of heating damage to the fiber buffer  356 . 
     In an example, core  352 , first cladding  358 , second cladding  354  and buffer  356  may comprise a variety of materials known to those skilled in the art to achieve the desired refractive index profile. As a non-limiting example, core  352 , first cladding  358 , and second cladding  354  may comprise SiO 2 , SiO 2  doped with GeO 2 , germanosilicate, phosphorus pentoxide, phosphosilicate, Al 2 O 3 , aluminosilicate, or the like or any combinations thereof. Buffer  356  may comprise glass and/or polymer materials such as fluoropolymers such as polyvinylidene fluoride (Kynar), polytetrafluoroethylene (Teflon), and polyurethane, or the like or any combinations thereof. 
       FIG. 3B  depicts an example refractive index profile  360  showing relative refractive indices of core  352 , first cladding  358 , second cladding  354  and buffer  356 . Refractive index profile  360  works together with reflective elements of assembly  304  to safely guide returned radiation  341  in the cladding back to the fiber laser. Other refractive index profiles known to those of skill in the art are possible and claimed subject matter is not limited by this or any other example. 
     A counter-pumped architecture would use a pump/signal combiner at the output end of the fiber laser to couple pump light propagating backward into the fiber laser relative to the intended fiber laser output direction. In such an architecture the visible light source pigtail could still be spliced into the fiber laser behind the high reflective Fiber Bragg Grating forming the back end of the fiber laser oscillator. 
       FIG. 4  illustrates an example counter-pumped fiber laser assembly  400  for generating and directing a high-power output laser beam  424  with a coaxial visible light beam  434 , wherein the assembly  400  incorporates a visible light source protection assembly  404  to protect a visible light source  432  from backward-propagating radiation  440 . In an example, assembly  400  includes pump laser beam sources  410 ,  412  that produce beams  411 ,  413  respectively. Beams  411 ,  413  propagate in respective fibers  414 ,  416 . Fibers  414 ,  416  are spliced to combiner input fibers  420 ,  422 . Combiner  402  receives and combines beams  411  and  413  to form a combined output beam  424  that is coupled into a combiner gain fiber  426 . Combiner gain fiber  426  includes laser  474  comprising active fiber between HR FBG  452  and PR FBG  450 . 
     Visible light source  432  produces visible beam  434  which is coupled to gain fiber core  426  via additional input fiber  445  and is from there coupled to output fiber  427  through combiner  402 . Combiner output fiber  426  delivers laser output beam  424  with coaxial visible aiming beam  434  to workpiece  430  to perform a desired processing operation. Backward-propagating radiation  440  is reflected and emitted from workpiece  430  and propagates back to visible light source pigtail  444  via fiber  427 , combiner  402 , gain fiber  426 , and fiber  445 . 
     In this example, visible light source pigtail  444  is a double or triple-clad fiber as described with respect to  FIG. 2B or 3A . Visible light source protection assembly  404  protects visible light source  432  from damage by backward-propagating radiation  440  by reflecting and/or absorbing all or a portion of radiation  440  according to the methods discussed above. Specifically, angled fiber end-face  446  comprises an optical filter material, coating  448  is configured to reflect backward-propagating radiation  440 . The reflection of backward-propagating radiation  440  off the end-face  446  is coupled out of the core  452  and into the cladding layers of visible light source pigtail  444 . Visible light source protection assembly  404  may include different or additional reflective elements configured to reflect radiation  440  back into pigtail  444 . In  FIG. 4 , returned radiation  441  represents such reflected radiation. Returned radiation  441  will be safely propagated by the cladding of fiber pigtail  444  back through laser  474  to combiner  402 . That cladding coupled returned radiation  441  may propagate through the fiber laser portion  474  and could be emitted at the output end or could be stripped out and safely dissipated in a CLS. 
     Having described and illustrated the general and specific principles of examples of the presently disclosed technology, it should be apparent that the examples may be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.