Abstract:
An exemplary embodiment of the disclosure provides a double fiber optic mode adapter including: a fiber core having a variable core diameter; a fiber cladding having a variable cladding size; a first input interface corresponding to a first core diameter and a first cladding size; a second input interface corresponding to a second core diameter and a second cladding size; a thermally-tapered region wherein the variable core diameter of the fiber core transitions from the first core diameter to the second core diameter and the variable cladding size of the fiber cladding transitions from the first cladding size to a third cladding size; and an etched tapered region wherein the variable core diameter of the fiber core is constant and the variable cladding size of the fiber cladding transitions from the third cladding size to the second cladding size

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 62/184,339, filed Jun. 25, 2015, and U.S. Provisional Patent Application No. 62/186,745, filed Jun. 30, 2015, which are both incorporated by reference in their entireties. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    This invention was made with Government support under Grant Number FA9451-13-M-0334 awarded by the Air Force Research Laboratory. The Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0003]    Fiber optic mode adapters are fiber devices that provide a transition that allows for the efficient transfer of optical power from one type of fiber to another. Single-core mode adapters, which, for example, match single mode fibers with 10 μm cores to those of 20 μm cores, are commercially available. 
       SUMMARY 
       [0004]    In an exemplary embodiment, the invention provides a double fiber optic mode adapter, comprising: a fiber core having a variable core diameter; a fiber cladding having a variable cladding size; a first input interface corresponding to a first core diameter and a first cladding size; a second input interface corresponding to a second core diameter and a second cladding size; a thermally-tapered region wherein the variable core diameter of the fiber core transitions from the first core diameter to the second core diameter and the variable cladding size of the fiber cladding transitions from the first cladding size to a third cladding size; and an etched tapered region wherein the variable core diameter of the fiber core is constant and the variable cladding size of the fiber cladding transitions from the third cladding size to the second cladding size. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following: 
           [0006]      FIG. 1  is a schematic diagram illustrating an exemplary double fiber optic mode adapter. 
           [0007]      FIG. 2  is a schematic diagram illustrating an exemplary double fiber optic mode adapter in an exemplary embodiment of the invention. 
           [0008]      FIG. 3  is a schematic diagram illustrating an exemplary environment for an exemplary double fiber optic mode adapter in an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    In fiber lasers and fiber amplifiers, it is desirable not only to match signal core light but also to match pump light into a double-clad fiber laser or amplifier gain fiber. Exemplary embodiments of the invention relate to a double fiber optic mode adapter for matching fibers having different core diameters as well as differing clad dimensions. 
         [0010]    A laser of any type is characterized by three key constituents: a gain medium that has an element with an atomic structure characterized by long lived upper electron states, a pump source whose energy is such that the substance in the gain medium can be excited to this upper state, and either a mirrored resonator structure to create an oscillator, or an input source to create an amplifier. A fiber laser or amplifier is a subset of this definition where the pump source is a laser diode and the gain medium is typically a Rare Earth doped glass with a fiber optic guiding structure. A fiber laser or amplifier can be created by way of a double clad fiber: there is a fiber optic core typically in the 5-50 μm diameter range with low numerical aperture such that a laser propagating mode of typical of near diffraction limited beam quality. Surrounding this central fiber signal propagation core is another guiding region typically in the 100 μm to 500 μm in diameter and is typically of high numerical aperture. In this way the pump light is injected into this cladding guided region of the fiber and as it propagates, portions pass through the rare earth doped core where it is absorbed. In this way the fiber laser or fiber amplifier is pumped to create the conditions necessary for laser action. 
         [0011]    A major challenge towards creating laser amplifiers is delivering both the signal input light and the pump light to the gain medium in an efficient manner where it can propagate down the length of the fiber. There is often a large mismatch in fiber core diameter between the input signal and the gain fiber. Simply splicing these two dissimilar fibers together will result in transition with unacceptable loss. To smooth this transition, exemplary embodiments of the invention provide a double mode adapter where the fiber with the larger core is thermally tapered. downward such that the diameter is reduced to match the diameter of the smaller fiber core. If this transition is made not too abruptly, the light from the smaller core can grow to match the larger core and transfer efficiently to the larger core fiber. Efficient coupling of pump and signal light into the gain portion of the fiber amplifier typically involves transitioning a small core fiber from the input into a larger core in the gain fiber. At the same time, exemplary embodiments of the invention provide for transforming the larger diameter low numerical aperture (NA) of the pumps into the smaller diameter and higher NA of the pump cladding. 
         [0012]    To accomplish this, exemplary embodiments utilize two techniques at different portions of a double mode adapter. The first involves thermal tapering where heat is applied to the fiber and when the fiber gets close to its melting point the fiber is pulled and both the core and the clad of the fiber are reduced by the same proportion. The other technique involves etching the outside of the fiber in a way that the outer diameter of the fiber is reduced while the core is left untouched. By using these techniques together, embodiments of the invention provide for the advantage of being able to transform the core and the clad independently of each other to satisfy the transition requirements for both the pump and the signal. 
         [0013]    One exemplary approach considered by the inventors for creating a double fiber optic mode adapter involved creating a single taper that combines both the tapering of the core and the opposite etching of the cladding to achieve both mode adaptations. In this approach, the transition fiber is thermally tapered to match the input signal, with the thermally-tapered cladding then being etched to provide the proper transition for the pump light in the cladding. 
         [0014]      FIG. 1  is a schematic diagram illustrating an exemplary double fiber optic mode adapter  100  based on this approach. The double fiber optic mode adapter may also be referred to as a double taper mode adapter or double taper pump combiner. In this example, a 20 μm core fiber from a prior amplifier stage is expanded to a mode field compatible with low loss coupling with a 40 μm core Photonic Crystal Fiber (PCF) fiber. The resulting tapered fiber is then etched with an adiabatic shape to achieve a desired pump taper. 
         [0015]    Studying the approach of  FIG. 1 , however, has led to the conclusion that it may be relatively difficult to etch the proper adiabatic taper on top of a thermal taper, and there may be certain instabilities associated therewith. 
         [0016]      FIG. 2  is a schematic diagram illustrating an exemplary double fiber optic mode adapter  200  in an exemplary embodiment of the invention, with separate thermal taper and etched taper regions. By providing separate regions, the difficulties of trying to etch an adiabatic taper on top of a thermal taper are avoided. The exemplary double fiber optic mode adapter  200  may further include an original fiber region, but it will be appreciated that the original fiber region is not required. 
         [0017]    In  FIG. 2 , in the thermal taper region  211 , the cladding  202  is tapered with the core  201  (via thermal tapering) such that the core  201  matches the core of an input signal fiber (not depicted) at input face  210 , In the etched taper region  213 , the cladding  202  is tapered to match the cladding dimensions of a PCF gain fiber (not depicted), which may be, for example, an air-clad PCF gain fiber, while the core  201  in the etched taper region  213  is unaffected by the tapering in the etched taper region  213  (via etching). The double fiber optic mode adapter  200  may further include an original fiber region  212  where neither the core  201  nor the cladding  202  is tapered. 
         [0018]    Based on the structure depicted in  FIG. 2 , pump light adiabatically expands in the thermal taper region  211  and contracts in the etched taper region  213 , while the signal light in the fiber core  201  also expands adiabatically in the thermal taper region  211  but is unchanged in the etched taper region  213 . By appropriate application of the thermal tapering and etching shown in  FIG. 2 , the resulting ratios of core to clad diameters can be achieved over a wide range of ratios. 
         [0019]      FIG. 3  illustrates an exemplary environment  300  in which the double fiber optic mode adapter  200  may be used. In this exemplary environment, for the input face  210  of double fiber optic mode adapter  200 , the cladding diameter of the double fiber optic mode adapter  200  is set to match an input pump bundle such that pump fibers  303  are configured to input pump light into cladding  202 , and the core diameter for core  201  of the double fiber optic mode adapter  200  is set to match the core diameter for core  301  of an input signal fiber of the input pump bundle. This may be achieved via thermal tapering in the thermal taper region  211  of the double fiber optic mode adapter  200 . It will be appreciated that the thermal tapering causes the dimensions of both the cladding  202  and the core  201  of the transition fiber forming the double fiber optic mode adapter  200  to become smaller. It will further be appreciated that the cladding diameter may set to be slightly larger than the diameter of the cores of the pump fibers to allow some tolerance (but not so much larger that the brightness or Etendue is unnecessarily lost). 
         [0020]    For the output face  214 , the cladding diameter for cladding  202  of the double fiber optic mode adapter  200  is set to match the cladding  312  of a PCF gain fiber  310 , and the core diameter for core  201  of the double fiber optic mode adapter  200  matches the core diameter for core  311  of the PCF gain fiber  310 . This may be achieved through etching in the etched taper region  213  of the double fiber optic mode adapter  200 . It will be appreciated that the etching is applied to the cladding of the transition fiber forming the double fiber optic mode adapter  200  and leaves the core of the transition fiber forming the double fiber optic mode adapter  200  unchanged. It will further be appreciated that the pump fibers  303  may be surrounded by a capillary sleeve  304 , which should be collapsed around the pump fibers  303  in a way that does not distort the fiber leading to loss of pump signal or poor transitioning of the signal light into the gain fiber. 
         [0021]    In the original fiber region  212 , the core diameter of the double fiber optic mode adapter  200  matches the core diameter of the PCF gain fiber at the output face  214 . In an exemplary implementation, the cladding diameter in the original fiber region  212  is simply the original diameter of the transition fiber used for the double fiber optic mode adapter  200  (i.e., the diameter of the transition fiber without being subjected to thermal tapering or etching), in this exemplary implementation, the size of the transition fiber used for the double fiber optic mode adapter  200  is selected based on the transition that needs to be accomplished between the input signal fiber and the core of the PCF gain fiber such that thermally tapering the transition fiber at the input face  200  causes both the cladding and the core of the transition fiber to match the input pump bundle. 
         [0022]    In an exemplary implementation, the input fiber bundle  301  may be a 20/240 μm signal core with six 220/240 μm 0.22NA pump fibers, and the PCF fiber may be a 30/400 μm 0.55NA Air Clad polarization-maintaining (PM) PCF fiber. In another exemplary implementation, a double mode adapter may provide an efficient transition between the pump and signal combined on a 10/125 μm 0.22 NA fiber to a 40/200 μm 0.55NA PCF type gain fiber. 
         [0023]    In an exemplary embodiment, to produce the double fiber optic mode adapter  200  depicted in  FIG. 2 , a transition fiber is provided that begins with a signal core matched to a PCF gain fiber core and that has a cladding that will match an input fiber bundle after thermal tapering. The transition fiber is then thermally tapered on the input side to match the input fiber bundle signal core (e.g., a 20 μm step index fiber (SIF). This transition fiber is then taper etched such that the output outer diameter will match the Air Clad diameter of the PCF gain fiber. 
         [0024]    All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
         [0025]    The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0026]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.