Abstract:
An optical fiber comprising a core region embedded within a cladding. The core region of the optical fiber further comprises multiple sections, each doped with rare earth ions. The sections of the core region may be doped with different rare-earth ions or with different doping concentrations. The sections of the core region may also be made from different types of glass hosts. The optical fiber may further include multiple core regions embedded within the cladding, each core region having multiple sections doped with rare earth ions.

Description:
BACKGROUND 
       [0001]    Various implementations, and combinations thereof, are related to optical fiber, and more particularly to optical fiber with multi core sections. 
         [0002]    Generally speaking, an optical fiber is a fiber of glass or plastic capable of carrying light along its length and typically comprising a core section surrounded in a cladding, as illustrated in  FIG. 1 . An optical beam is propagated through the length of fiber  100  via a core  102 , confined therein by a cladding  104 , which has a lower refractive index than the core. 
         [0003]    The core itself can either be single mode or multimode. Multimode fibers support multiple transverse modes, whereas single mode fibers support only one. Typically, multimode fibers are used for communication links over short distances or where high power transmissions are desired. In contrast, single mode fibers are used for long-distance communication links. The relationship between the mode of fiber, the core, and the optical beam is given by the following equation: 
         [0000]        V= 2 π×NA×a/λ   
         [0000]    where NA is the numerical aperture, a is the radius of the core, and λ is the wavelength of the optical beam. When V is less than or equal to 2.405, the fiber is a single mode fiber. Otherwise, it is a multimode fiber. 
         [0004]    When doped with rare-earth ions, such as neodymium or ytterbium, optical fibers can be used as the gain medium in fiber lasers or fiber amplifiers. Such fiber is generally referred to as a gain fiber and the rare-earth ions are doped in the core and/or the cladding. Different laser wavelengths are generated from fibers with different doping ions. For example, approximately 1 and 1.3 microns are achieved with neodymium doped fibers, 1.55 and 2.7 microns from erbium doped fibers, 1 micron from ytterbium doped fibers, and 2 microns from thulium and/or holmium doped fibers. 
         [0005]    Different types of gain fibers are designed for use in different fiber lasers, the characteristics of the gain fiber effecting the resulting fiber laser. For example, the use of a double cladding gain fiber increases the output power of a fiber laser. Several existing patents focus on this relationship by attempting to affect the quality and nature of fiber lasers through the development of the gain fiber. By way of example, U.S. Pat. No. 4,829,529, issued to Kafka provides a single mode fiber laser pumped by a coherent high power laser diode source. Kafka attempts to address the issue that the small diameter of single mode fibers limits the ability to couple such fibers to high powered coherent laser diode sources, resulting in low powered lasers, whereas multimode fibers are not so limited, but the resulting lasers have poor beam quality output. Specifically, Kafka discloses laser diode pumped fiber lasers with double cladding.  FIG. 2  provides an exemplary embodiment of a fiber  200  as used in Kafka having a double cladding where a core  206  embedded within an inner cladding  204  and an outer cladding  202 . The optical fiber used has rare-earth ions doped into the core to provide an active gain medium. A multimode pump laser is coupled to the inner cladding to increase the pump power and excite the rare-earth ions in the core of the fiber. The larger cross section of the inner cladding, in comparison to the core, allows the multimode laser to be coupled to a single mode fiber. As a result, the high pump power of the inner cladding compared to the core pump produces a fiber laser having high output. 
         [0006]    By way of another example, U.S. Pat. No. 5,566,196, issued to Scifres, attempts to provide an optical fiber laser or amplifier medium using multimode fibers and having an increased output power without producing nonlinear optical effects such as Brillouin scattering. The fiber lasers and amplifiers of Scifres employ optical fibers with two or more generally parallel, nonconcentric doped core regions, each of which is capable of gain or lasing when optically pumped. An exemplary optical gain fiber according to Scifres is illustrated in  FIG. 3 . The fiber  300  may be single clad or double clad, the single clad fiber having only the inner cladding  304  where as the double clad fiber additionally has the outer cladding  302 . Multiple cores  306  may be embedded in a common cladding region, such as inner cladding  304 , or in separate cladding regions. The use of multiple cores spreads the light over a larger area of the fiber, compared with a single mode fiber, and thereby reducing or eliminating the non linear optical effects that would otherwise occur at high light intensities. 
         [0007]    The cores of the gain fibers of Kafka and Scifries are formed with a relatively uniform area. The laser is generated and/or amplified by one or more rare-earth ions doped into the core of the fiber. Other ions may be used to transfer energy to the lasing ions. For example, ytterbium ions are sometimes doped into an erbium doped fiber. The resulting lasing ions are erbium and receive energy transferred from the ytterbium after absorbing the pump. However, by forming gain fibers with a relatively uniform area, only one wavelength is generated from each fiber. In other words, the laser only occurs in one transition from the upper level to the lower level from one lasing ion. 
         [0008]    Clearly, the development of fiber lasers and fiber amplifiers, such as those described in either Kafka or Scifres, would benefit from optical fibers having non uniform cores, each core section being doped with different rare-earth ions and resulting in a gain fiber capable of generating more than one wavelength. 
       SUMMARY 
       [0009]    In one implementation, an optical fiber is presented having a core region embedded within a cladding. The core region further has multiple sections, each of which is doped with a rare-earth ion. 
         [0010]    In another implementation, an optical fiber is presented having multiple core regions embedded within a cladding. Each core region further has multiple sections, each of which is doped with a rare-earth ion. 
         [0011]    In another implementation, a fiber laser is presented. The fiber laser includes a laser optical fiber having a core region embedded within a cladding. The core region further has multiple sections, each of which is doped with a rare-earth ion. 
         [0012]    In yet another implementation, an ultra short pulse fiber laser is presented. The ultra short pulse fiber includes a laser optical fiber having a core region embedded within a cladding where the core region further has multiple sections. The laser optical fiber is additionally doped with a plurality of rare-earth ions. 
         [0013]    In still another implementation, a broad band amplified spontaneous emission (“ASE”) source is presented. The broad band ASE source includes a first core region embedded within a cladding. The core region further has multiple sections, each of which is doped with a rare earth ion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Implementations of the invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like elements bear like reference numerals. 
           [0015]      FIG. 1  is a cross sectional view of an exemplary embodiment of an optical fiber having a core and a single cladding; 
           [0016]      FIG. 2  is a cross sectional view of an exemplary embodiment of an optical fiber having a double cladding; 
           [0017]      FIG. 3  is a cross sectional view of an exemplary embodiment of an optical fiber having multiple cores; 
           [0018]      FIG. 4  is a cross sectional view of an exemplary embodiment of an optical fiber having two core sections; 
           [0019]      FIG. 5  is a cross sectional view of an exemplary embodiments of the configurations of a core having multiple sections; 
           [0020]      FIG. 6  is a cross sectional view of an exemplary embodiment of an optical fiber perform; 
           [0021]      FIG. 7  is a schematic depiction of an exemplary embodiment of a fiber laser using an optical fiber having multiple cores according to the present discussion. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Implementations propose an optical fiber having multiple core sections, which, when used, cause a laser to be generated and/or amplified simultaneously from more than one lasing ions and/or from more than one transition. Throughout the following description, this invention is described in preferred embodiments with reference to the figures in which like numbers represent the same or similar elements. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least on embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0023]    The described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are recited to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
         [0024]    Turning to the Figures,  FIG. 4  depicts a cross sectional view of an exemplary embodiment of an optical fiber  400  having core sections  404  and  406 , surrounded by a cladding  402 . A person of ordinary skill in the art will realize that, although  FIG. 4  depicts an optical fiber having two core sections, the following discussion is equally applicable to optical fibers having more than two core sections. 
         [0025]    In one embodiment, by dividing the core into sections, each core section  404  and  406  can be doped with different rare-earth ions. Thus, by way of example, core section  404  may generate a different wavelength when used in a fiber laser then core section  406 . 
         [0026]    In other embodiments, core sections  404  and  406  may be made from different glass host materials. By way of example, and not by way of limitation, core section  404  can be silicate glass while core section  406  is phosphate glass. In yet other embodiments, core sections  404  and  406  may be doped with the same rare-earth ions but with different doping concentrations, thus resulting in different spectroscopic properties and, therefore, affecting the performance of a fiber laser. 
         [0027]    As stated, the present discussion is applicable to optical fibers having any number of core sections. Further, the core sections can have a variety of configurations.  FIG. 5  presents a cross sectional view of exemplary embodiments of different configurations of segmented cores. A person of ordinary skill in the art will realize that  FIG. 5  is provided by way of illustration, and not by way of limitation, and that the present discussion encompasses configurations other than those depicted in  FIG. 5 . 
         [0028]    In one embodiment, although the core is formed having multiple discrete sections, from the point of view of a propagating optical beam, the sections form a single mode guide. In such an embodiment, each section of the core may be formed from glass having approximately the same refractive index. In other words, the difference between the refractive indices of the various sections is much smaller than the difference between the refractive index of the core and that of the cladding. 
         [0029]    A person of ordinary skill in the art will realize that although the present discussion focuses on single mode fibers and although a single mode fiber will be sufficient for most applications, the present discussion is equally applicable to multimode fibers. In such an embodiment, a cross sectional view of an individual multi section core of a multimode fiber will appear the same as a cross sectional view of a multi section core of a single mode fiber. However, the relationship between the mode of fiber, the core, and the optical beam is no longer designated by the equation: 
         [0000]        V= 2 π×NA×a/λ.    
         [0030]    By way of example, an optical fiber having a multi section core according to the present discussion may be fabricated by doping one core glass with erbium ions, having a refractive index of 1.50 at 1.55 microns. A second core glass may be doped with thulium ions, having a refractive index of 1.50±0.0005 at 1.55 microns. Thus, the difference between the two core glasses is less than 0.001. The cladding class may be fabricated having a refractive index of 1.4935, resulting in a numerical aperture of 0.14. 
         [0031]    Each core glass may be ground until they form semi-cylindrical rods, such that when the grounded surfaces are disposed in contact with one another, the two core glasses form a cylinder. The ground surface of each core glass may then be polished. 
         [0032]    A cladding glass tube can be fabricated from the cladding glass such that the inner diameter of the tube equals the outer diameter of the core glass, the outer diameter of the cladding being dependent upon the size of the fiber. Both the inner and outer diameters are polished. Once done, the semi-cylindrical rods of erbium doped glass and thulium doped class are inserted into the inner diameter of the cladding glass tube to form the fiber preform.  FIG. 6  is a cross sectional view of an exemplary fiber perform  600  fabricated according to the present discussion. 
         [0033]    The fiber preform is placed into a fiber drawing tower. During the fiber drawing process, the two core glasses are physically bonded to form the single mode core area. 
         [0034]    As stated, an optical fiber having a multi section core according to the present discussion can be used to generate multiple laser wavelengths simultaneously. For example, using the fiber of  FIG. 6 , a 1.55 micron fiber laser can be generated in the erbium doped core section and a near 2 micron fiber laser in the thulium doped core section. An exemplary embodiment of a fiber laser  700  using such an optical fiber is depicted in  FIG. 7 . 
         [0035]    The ability to generate multiple laser wavelengths in the same single mode core is highly beneficial. By way of example, and not by way of limitation, one wavelength can be used as the pump source and the other as the probe wavelength in a pump and probe experiment. As another example, two wavelengths can be used to generate a new laser wavelength through a nonlinear process such as, for example, different frequency generation, frequency summing, and frequency doubling. 
         [0036]    A fiber having a multi section core can also be used to generate an ultra short pulse fiber laser. The pulse width of a short pulse fiber laser is limited by the bandwidth of the gain medium. By doping multiple rare-earth ions into a fiber having a multi section core, gain bandwidth can be effectively extended thereby allowing an extremely short pulse fiber laser to be achieved. 
         [0037]    A fiber according to the present discussion can additionally be used to generate extremely broad band amplified spontaneous emission (“ASE”) source. By way of example, and not by way of limitation, one section of a bisected core can be doped with thulium ions and other with holmium ions. As is known by those of ordinary skill in the art, thulium emits emissions from 1.7 to 1.9 microns and holmium from 1.9 to 2.1 microns. By doping the stated sections with thulium ions and holmium ions respectively, an ASE source with emissions from 1.7 to 2.1 microns can be generated. 
         [0038]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described implementations are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
         [0039]    While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.