Abstract:
Exemplary embodiments of the disclosure include a fiber optic amplifier system and a fiber optic oscillator system having a first stage, comprising a first core fiber having a first core diameter and a first cladding size; a second stage, comprising a second core fiber having a second core diameter and a second cladding size; and a double mode adapter connecting the first stage to the second stage, wherein the double mode adapter is configured to provide transitions for the cores and the claddings of the first and second core fibers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation-in-part of copending U.S. patent application Ser. No. 15/191,829, filed Jun. 24, 2016, titled “Double Fiber Optic Mode Adapter,” which 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. 
         [0002]    This patent application claims the benefit of U.S. Provisional Patent Application No. 62/184,338, filed Jun. 25, 2015, and U.S. Provisional Patent Application No. 62/186,736, filed Jun. 30, 2015. 
         [0003]    All of the foregoing applications are incorporated herein by reference in their entireties. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0004]    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 
       [0005]    The semiconductor-pumped fiber laser has generated considerable interest for High Energy Lasers (HELs) in a number of Directed Energy (DE) Ballistic Missile Defense System (BMDS) applications, including long range sensing, communications, and missile defense. The potential efficiency, compact size, low weight, reliability and ruggedness are attractive for these and other DE applications. 
       SUMMARY 
       [0006]    Exemplary embodiments of the invention include a fiber optic amplifier system and a fiber optic oscillator system having a first stage, comprising a first core fiber having a first core diameter and a first cladding size; a second stage, comprising a second core fiber having a second core diameter and a second cladding size; and a double mode adapter connecting the first stage to the second stage, wherein the double mode adapter is configured to provide transitions for the cores and the claddings of the first and second core fibers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    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: 
           [0008]      FIG. 1  is a schematic diagram illustrating a pump combiner in an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    For semiconductor-pumped fiber lasers in DE applications, it would be advantageous for all-fiber semiconductor pumped fiber laser designs to be able to, in a single gain stage, produce narrow linewidth amplified signals at the multi-kW power level with near-diffraction-limited beam quality. An important component is thus the all-fiber pump power combiner. To achieve reliable operation at multi-kW power levels and efficient multi-kW pump integration, active gain fibers with large pump cladding diameters should be used. These large pump cladding diameters in turn utilize the combination of several pump fibers with an even larger input bundle diameter. In addition, the core is transferred through the combiner, adapting to the larger core of the active fiber while preserving the polarization extinction ratio of the input signal. Finally, the combiner is integrated with advanced gain fibers in a near-diffraction-limited counter-pumped configuration, and packaged in a robust manner that is thermally robust and stable. 
         [0010]    While certain all-fiber combiners exist commercially, they are of low input bundle diameters, are co-pumped, cannot provide mode adaption for large gain cores, and cannot handle multi-kW power levels. Embodiments of the invention, on the other hand, provide a systems approach to DE fiber laser pump integration that is able to seamlessly integrate multi-kW levels of pump light simultaneously in both the co- and counter-pumped directions in both Large Mode Area (LMA) step index fiber (SIF) and Photonic Crystal Fiber (PCF). 
         [0011]    Exemplary embodiments of the invention, which provide efficient, high-power-capable fiber-laser integration, may utilize the etched taper all-fiber pump combiner technology described in U.S. Pat. No. 8,818,151, which is incorporated by reference herein in its entirety. The efficient, high-power-capable fiber-laser integration provides for co- and counter-pumping along with a low loss transfer of energy between integrated gain stages. Exemplary embodiments of the invention may further utilize the double mode adapter technology described in U.S. patent application Ser. No. 15/191,829, filed Jun. 24, 2016, titled “Double Fiber Optic Mode Adapter,” which is incorporated by reference herein in its entirety. 
         [0012]    For fiber-based amplifiers in DE applications, it is advantageous for the pump to be tightly integrated into the amplifier design. Exemplary embodiments of the invention provide the ability to both co- and counter-pump, and to couple more than one gain stage together without the use of isolators, so as to provide power amplifiers with high gain, high thresholds for both Stimulated Brillouin scattering (SBS) and modal instability (MI), and high extraction efficiency. Pump combiners according to exemplary embodiments are highly efficient for both signal and pump, and have an efficient mode adapter to move from a small input core to a large output core, and have the signal core mode matching to microstructured fibers with large cores and advanced designs. 
         [0013]    Because SBS may be addressed by having a larger core, while MI favors having a smaller core, exemplary embodiments provide an architecture that allows for a smaller core fiber to transition into a larger core using a mode adapter. Exemplary embodiments further address pumping mismatches by utilizing a double mode adapter which provides for a transition for the fiber cladding in which co-pumped and/or counter-pumped light travels. 
         [0014]      FIG. 1  is a schematic diagram illustrating a pump combiner  100  in an exemplary embodiment. The pump combiner  100  includes input fiber  101 , co-pumped combiner  102 , core fiber  103 , double mode adapter  104 , core fiber  105 , counter-pumped combiner  106 , delivery fiber  107 , laser diode pump array  110 , combining tree  111 , and combining tree  112 . 
         [0015]    Input fiber  101 , for example, may correspond to a low-power precise seed source (e.g., in the mW levels where it is efficient to be able to modulate and control key parameters of signal light such as optical phase, and to be able to pulse the input signal light at very high speeds or very short pulses). 
         [0016]    Co-pumped combiner  102 , for example, may be a high-power co-pumped combiner with a built-in mode adapter from the input signal. 
         [0017]    Core fiber  103 , for example, may be a polarization-maintaining (PM) Yb doped 20-25 μm core SIF LMA fiber. This is an example of a relatively small core for the input where fiber non-linearities are not an issue because the signal level is small (i.e., the input has yet to be really amplified, which suppresses the onset of TMI). 
         [0018]    Double mode adapter  104  matches the two gain stages in both the core and cladding to allow for efficient transmission of the signal from the first stage pump combiner  100  to the second stage of the pump combiner  100 , and for bi-direction pump flow of the pump light between the stages to create dual-pumped configurations for both stages. Thus, the double mode adapter  104  provides for low-loss signal transport and exchange of pump light between stages. The double mode adapter facilitates both the use of differing gain stages with a large mismatch allowing the fiber amplifier to be better optimized for higher power performance. If only a signal mode adapter were used and there was a large pump clad mismatch, the pump light would have to be removed before the mode adapter leading to a far more complicated device. In an exemplary embodiment, the double mode adapter  104  includes a continuous mode adapter from a 20 μm input core to a 40 μm PCF gain fiber core. 
         [0019]    A general desire of fiber lasers and fiber amplifiers is to increase the power output of the laser while maintaining the properties that existed at low powers; namely, near diffraction limited beam quality, narrow line width, short pulses, etc. Working against this are fiber optical non-linearities such as Stimulated Brillioun Scattering (SBS), Stimulated Raman Scattering (SRS), Thermal Modal Instabilities (TMI), and Self Phase Modulation (SPM). All of these non-linearities except the TMI can be reduced by going to larger fiber core diameters. Avoiding TMI, on the other hand, requires smaller fiber diameters. It is therefore desirable to begin the amplifier with a smaller diameter (when the signal level is low and yet to be amplified) and then transition to a larger core fiber when the signal levels are much higher near the output of the amplifier. Exemplary embodiments of the invention thus provide amplifiers with two or more different gain sections where the mismatch in both core and pump clad can be considerable. The double mode adapter  104  provides a smooth, efficient transition of the signal fiber through the different types of gain fiber and also allows pump light to flow efficiently between the gain fiber sections in different embodiments, e.g., where there is co-pumping only, counter-pumping only, or pumping from both ends. 
         [0020]    Core fiber  105 , for example, may be a PM Yb doped 30-40 μm core PCF type fiber. This is an example of a relatively large diameter core fiber which reduces fiber non-linearities. 
         [0021]    Counter-pumped combiner  106 , for example, may be a high-power counter-pumped combiner for both PCF and LMA SIF fibers that provides low loss and high power capability. Many fiber non-linearities are reduced through counter pumping where the gain of the fiber better matches the signal level at a particular point along the amplifier. 
         [0022]    Delivery fiber  107 , may be, for example, an undoped piece of fiber that is used to connect the amplifier to the place where the amplified light is to be used (the needs for powering, cooling and coiling the gain fiber of a fiber laser or fiber amplifier often lead to the gain fiber being set apart from the place where the output light is being used to create a desired effect). 
         [0023]    Laser diode pump array  110 , for example, may include a plurality of pump laser diodes with a set output power and output fiber type, and may provide for powering and cooling of the pump laser diodes of the array. 
         [0024]    Combining trees  111 ,  112  facilitate matching the pump clad in the gain fiber in a manner that is governed by the Law of Conservation of Brightness. An example of this would be starting with a 100 W laser diode that emits light in a 105/125 um 0.15 NA fiber. Seven of these laser diodes can be combined into a single 220/240 um 0.22 NA fiber with approximately 95% efficiency. Then, six of these combined fiber would be placed into a pump bundle with the signal fiber in the center of the bundle into a 20/400 um 0.46 NA Yb doped gain fiber to allow for a total of 42 diode pump lasers (at a total of 4200 W minus combining efficiencies) in the laser diode pump array  110 , with the combining tree, to pump the gain fiber. 
         [0025]    Although the exemplary embodiment depicted in  FIG. 1  depicts both a first stage with co-pumping and a second stage with counter-pumping, it will be appreciated that in one alternative embodiment, the system may only provide the first stage being co-pumped without the second stage being counter-pumped and, in another alternative embodiment, the system may only provide the second stage being counter-pumped without the first stage being co-pumped. Thus, in the co-pumping only embodiment, counter-pumped combiner  106  and combining tree  112  (and the corresponding connections) may not be needed, and, in the counter-pumping only embodiment, co-pumped combiner  102  and combining tree  111  may not be needed. 
         [0026]    Adjusting fiber lengths and pump wavelengths allows for balancing of the co- and counter-pump levels. The resulting gain distribution in the fiber is determined by the input pump power, the absorption of the gain fiber (typically determined by the material absorption of the lasing ion (e.g. Yb2+) multiplied by the ratio of the doped core to pump clad areas), and the length of the gain fiber. The pump and hence gain distribution is an exponentially decreasing level from the pump input. This pump absorption is also wavelength dependent as an electron transition is the source of the pump absorption. Co-pumped light creates an exponentially decreasing gain profile beginning at the input of the fiber and decreasing towards the output end of the fiber. Counter-pumping similarly creates an exponentially decreasing gain profile beginning at the output of the fiber and decreasing towards the input end. Utilizing both types of pumping creates a profile that is the sum of the two gain profiles. By varying the wavelength of the pump light, and the concentration of the dopant ion, the core to clad area ratio, and the relative lengths of the two types of fiber, a gain profile that maximizes laser efficiency while minimizing non linear effects may be achieved. 
         [0027]    In an exemplary embodiment, by utilizing high-power counter-pumping, Stimulated Brillouin scattering (SBS) and modal instability (MI) thresholds may be increased. The counter-pumped combiner and double mode adapter interfacing to a PCF fiber, as well as the fiber pumping and balancing discussed above, may achieve high gain with high SBS and MI thresholds while maximizing overall efficiency. Differing gain fibers for the first section of the amplifier and the second section of the amplifier (power-stage) may be used to achieve an optimal combination of low gain in the second section of the amplifier (power-stage) to reduce MI and SBS effects and reduce the SBS overlap to increase SBS threshold. 
         [0028]    In an exemplary embodiment, a very short unpolarized taper section has no identifiable stresses such that light exiting a 25 μm PM core is transferred to the polarization-maintaining (PM) PCF core with &gt;20 dB extinction ratio. In many applications, it is desirable to have the polarization of the light maintained in the amplifier and have the light come out of the amplifier highly polarized. To achieve this, Polarizing (PZ) fiber, or Polarization Maintaining (PM) fiber, may be used for the gain fiber and for the input and delivery fibers. The thermally tapered or etched tapered regions do not generally work well with these polarized fibers, so a short section of unpolarized fiber that is held straight (not bent) may be used to maintain the polarization state from one section of fiber to the next. 
         [0029]    In an exemplary embodiment, an all-glass construction using the “optical heat spreading” technology described in U.S. patent application Ser. No. 14/158,469, which is incorporated by reference herein in its entirety, may be used to provide a reliable and robust combiner at multi-kW pump power levels. This is because combiners and double mode adapters are generally not 100% efficient. At kW levels, even 1 W of scattered light can be absorbed and turn into a significant amount of heat, potentially leading to burning of the fiber. Since the primary source of the heat is scattered light, actually insulating the device in a transparent material and spreading the light away from the fiber before it interacts with an absorbing material helps to protect the fiber. In this way, the density of the heat and hence the temperature is reduced. 
         [0030]    In practice, exemplary implementations of the pump combiner  100  have been produced using the following processes and components: capillary sleeve collapse and bundling, fiber etching, and splicing to PCF, with respect to 6+1 to 1 combiner with a 650 μm input bundle to 325 μm PCF co-propagating configuration, and with respect to a 1.3 mm to 500 μm PCF fiber co-propagating configurations, as well as with respect to a counter-pumping configuration. The fiber combiners are comprised of three different sections: (1) the fiber bundle that brings the pump fibers together with the signal fibers, (2) the output fiber that connects with the gain fiber and (3) a transition region that brightness transforms the pumps to the pump clad and mode adapts the input signal fiber core to the fiber core of the gain fiber. In the pump bundle the pump fibers are held in the desired relative positions (signal fiber in the center and pump fibers in a ring around the signal fiber) with very little stress on them so as not to distort the fibers and the light contained within those fibers. A glass capillary is collapsed on the bundle through a heating and pulling process, the bundle is then cleaved and or polished to create a flat and clean surface for splicing, and the transition fiber is created through either thermally tapering the starting transition fiber, etch tapering the transition fiber, or some combination of the two processes. Finally, the bundle is fusion spliced to the transition taper fiber and then this assembly is fusion spliced to the output fiber. 
         [0031]    These exemplary implementations have demonstrated that a &gt;99% efficient pump combiner, as well as LMA counter-pumped combiners with &gt;99% signal efficiency, can be produced according to exemplary embodiments of the invention. Further, pump transfer efficiencies of &gt;99%, core signal efficiencies of &gt;80% (in a co-propagating configuration with a continues mode field adapter (MFA)), and polarization extinction ratios of 20 dB, have been demonstrated. Additionally, such pump combiners, as well as completed amplifier harnesses incorporating such pump combiners, have been qualified at multi-kW levels and passed environmental tests for shock and vibration, and have been power-tested with 1.5 kW of cw pumps for tens of minutes with no adverse effects. 
         [0032]    It will be appreciated that the configuration shown in  FIG. 1  depicts an amplifier. In an alternative embodiment, this configuration may be modified to provide an oscillator by providing a mirror to the left of fiber  101  and a partially-reflecting mirror to the right of fiber  107 . 
         [0033]    Embodiments of the pump combiner discussed herein may be incorporated into various applications, including, for example, optimized two-stage fiber amplifier harnesses and balanced dual-pumped two-stage integrated power amplifiers. 
         [0034]    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. 
         [0035]    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. 
         [0036]    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.