Abstract:
In one aspect, the invention relates to optical fibers and systems that include such fibers. In another aspect, the invention provides an optical fiber that includes a core, a cladding surrounding the core, a layer surrounding the cladding, and a region between the layer and the cladding. The region can comprise an index of refraction that is different than an index of refraction comprised by the cladding. In one embodiment, the region can include a void containing air or a liquid. The void can be evacuated. The region can include a solid, such as, for example, a polymer. The layer can contact the cladding. The fiber can comprise rare earth ions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/US02/21803, which has an international filing date of Jul. 10, 2002, and is entitled “Optical Fiber”, and which in turn claims priority to U.S. Provisional Patent Application Ser. No. 60/304,882, which was filed Jul. 12, 2001 and is also entitled “Optical Fiber”. The foregoing applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to optical fibers, and systems containing optical fibers. 
     BACKGROUND 
     Optical fibers can be used to transport and/or enhance signals at certain wavelengths. For example, pump energy at a wavelength λ p  can be emitted by an energy source, such as a laser, and coupled into an optical fiber having a core containing an active material that interacts with the pump energy, and undergoes certain electronic transitions to form energy at a different wavelength λ out . The optical fiber can include, for example, a pair of reflectors that form a lasing cavity at the wavelength λ out  so that the optical fiber can be used as a laser that converts energy at λ p  to energy at λ out . 
     SUMMARY 
     The invention generally relates to optical fibers and systems containing optical fibers. 
     In one aspect, the invention features an optical fiber that includes a core, a cladding contacting the core, and a region disposed in the cladding. The region has an index of refraction that is different than an index of refraction of the cladding. 
     In another aspect, the invention features an optical fiber that includes a core, a cladding contacting the core, a layer surrounding and contacting the cladding, and a region between the cladding and the layer. The index of refraction of the region is different than an index of refraction of the cladding. 
     In a further aspect, the invention features an optical fiber that includes a core, a cladding contacting the core, and a layer surrounding and contacting the cladding. The index of refraction of the layer is greater than an index of refraction of the cladding. 
     In one aspect, the invention features an optical fiber that includes a core, a cladding contacting the core, and a layer surrounding the cladding. The cladding is formed of a material capable of allowing energy at a desired wavelength to propagate therealong. The cladding and the layer define a void having a maximum dimension that is equal to or greater than the desired wavelength (e.g., at least twice the desired wavelength, at least five times the desired wavelength, at least 10 times the desired wavelength, at least 20 times the desired wavelength, at least 50 times the desired wavelength, at least 75 times the desired wavelength, at least 100 times the desired wavelength). 
     In another aspect, the invention features an optical fiber that includes a core, a cladding contacting the core, and a layer surrounding and contacting the cladding. The cladding and the layer define a void having a maximum dimension that is at least about one micron (e.g., at least about two microns, at least about five microns, at least about 10 microns, at least about 20 microns, at least about 50 microns, at least about 75 microns, at least about 100 microns). 
     In a further aspect, the invention features an optical fiber that includes a core and a cladding contacting the core. The cladding is formed of a material capable of allowing energy at a desired wavelength to propagate therealong. The cladding contains a void with a maximum dimension that is equal to or greater than a desired wavelength of propagation along the cladding. 
     In one aspect, the invention features, an optical fiber that includes a core and a cladding contacting the core. The cladding contains a void with a maximum dimension that is at least about one micron. 
     In another aspect, the invention features an optical fiber that includes a core, a cladding surrounding the core, and a first layer surrounding the cladding. The cladding has a substantially non-circular shape. The cladding and the first layer define a region between the cladding and the first layer. The region has an index of refraction that is different from the index of refraction of the cladding, and the region has an index of refraction that is different from an index of refraction of the first layer. 
     In a further aspect, the invention features an optical fiber that includes a core, a cladding surrounding the core, and a first layer surrounding the cladding. The cladding has a substantially non-circular shape, and the cladding and the first layer define a region between the cladding and the first layer that has an index of refraction that is less than an index of refraction of the cladding. 
     In a further aspect, the invention features an optical fiber that includes a core, a cladding surrounding the core, and a first layer surrounding the cladding. The cladding has a substantially non-circular shape, and the cladding and the first layer define a region between the cladding and the first layer that has an index of refraction that is less than an index of refraction of the first layer. 
     Embodiments of optical fibers can include one or more of the following features. 
     The region can have an index of refraction that is less than the index of refraction on of the cladding. The region can be formed of air. 
     The optical fiber can further include a layer surrounding the cladding. The index of refraction of the layer can be less than, greater than, or about the same as the index of refraction of the cladding. The layer can contact the cladding. 
     The core can be formed of an active material. The core can also include an additional material. The additional material can be a silica material. 
     The cladding can be formed of a silica material. The cladding can be formed of a material selected so that energy at a desired wavelength can propagate along the cladding. 
     The optical fiber can include a layer surrounding the cladding so that the region is between the cladding and the layer. 
     The region can be formed of a plurality of regions. The region can have a maximum dimension that is at least about two microns. The region can have a substantially non-circular cross-section. 
     The cladding can have a substantially square cross-section. 
     The optical fiber can further include a reflector configured to at least partially reflecting energy impinging thereon at a pump wavelength. 
     The optical fiber can further include a pair of reflectors with each of the pair of reflectors being configured to at least partially reflect energy impinging thereon at an output wavelength. 
     The optical fiber can have a numerical aperture of at least about 0.25 (e.g., at least about 0.3, at least about 0.35, at least about 0.4,at least about 0.5, at least about 0.6, at least about 0.7, at least about 0.8, at least about 0.9, about one). 
     The optical fiber can be included in a system with an energy source (e.g., a laser, such as a semiconductor diode laser) configured so that energy output by the energy source at the pump wavelength can be coupled into the optical fiber. The system can further include a Raman fiber laser configured so that energy output by the optical fiber at the output wavelength can be coupled into the Raman fiber laser. The system can further include an output cascade configured so that energy output by the Raman fiber laser can be coupled into the output cascade. The output cascade and the Raman fiber laser can be an integral unit. 
     In certain embodiments, the invention provides an optical fiber that has a cladding that does not have disposed thereon a lower refractive index layer. 
     In some embodiments, the invention provides an optical fiber that includes a cladding in contact with one or more regions having a lower refractive index than the cladding. In certain embodiments, one or more of the regions can be voids. A “void” as used herein, refers to a region within an optical fiber that is formed of one or more gases (e.g., air) or that is substantially evacuated. 
     In certain embodiments, the invention provides an optical fiber that includes a cladding having in contact therewith (e.g., fused therewith) a layer having a refractive index that is the same or higher than the refractive index of the cladding. For example, the layer can be formed of the same material as the cladding. The layer can, for example, enhance the mechanical integrity of the optical fiber, provide chemical protection for the cladding, and/or provide physical protection for the cladding. 
     In some embodiments, the invention can provide an optical fiber that can undergo three-level lasing with relatively high efficiency. For example, in certain embodiments, three-level lasing can be used to convert more than about 50% (e.g., more than about 60%, more than about 65%, more than about 70%, more than about 75%, more than about 80%, more than about 85%, more than about 90%, more than about 95%) of energy in an optical fiber (e.g., at a pump wavelength) to energy at a desired output wavelength (e.g., about 980 nanometers). As another example, in some embodiments, three-level lasing can be used to provide at least about 0.2 Watt (e.g., at least about 0.3 Watt, at least about 0.4 Watt, at least about 0.5 Watt, at least about 0.6 Watt, at least about 0.7 Watt, at least about 0.8 Watt, at least about 0.9 Watt, at least about 1 Watt, at least about 1.5 Watt, at least about 2 Watts, greater than about 2 Watts) of energy at about 980 nanometers. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are cross-sectional views of an embodiment of an optical fiber; 
         FIG. 2  is a schematic representation of an embodiment of a fiber laser system; 
         FIGS. 3A-3D  show an embodiment of making an embodiment of an optical fiber; 
         FIGS. 4A-4D  show cross-sectional views of embodiments of optical fibers; 
         FIG. 5  is a cross-sectional view of an embodiment of an optical fiber; 
         FIG. 6  is a cross-sectional view of an embodiment of an optical fiber; and 
         FIG. 7  is schematic representation of a fiber laser system. 
       Features, objects and advantages of the invention are in the description, drawings and claims. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  are cross-sectional views of an optical fiber  100  of the invention. Optical fiber  100  has a core  110  (e.g., a single mode core), a cladding  120 , and an exterior layer  140  that surrounds and contacts cladding  120 . Cladding  120  has sides  160   a ,  160   b ,  160   c  and  160   d  that form vertices  150   a ,  150   b ,  150   c , and  150   d . Vertices  150   a - 150   d  are fused to an inner surface  170  of layer  140 . Optical fiber  100  additionally includes regions  130   a ,  130   b ,  130   c , and  130   d  between portions of cladding  120  and layer  140 . 
     Generally, core  110  is provided to enhance pump energy absorption (e.g., to produce gain) by interacting with pump energy and/or to guide energy at a desired wavelength (λ out ) In certain embodiments, core  110  includes a first material (e.g., a silica material, such as fused silica) and at least one dopant (e.g., at least one rare earth ion, such as erbium ions, ytterbium ions, neodymium ions, holmium ions, dysprosium ions, and/or thulium ions, and/or at least one transition metal ion). In some embodiments, core  110  is formed of fused silica doped with ytterbium ions. 
     Core  110  can optionally include certain other materials. For example, core  110  can include one or more materials to increase its index of refraction (e.g., germanium oxide) or to decrease its index of refraction (e.g., boron oxide). As another example, core  110  can include one or more materials (e.g., aluminum oxide) that can enhance the solubility of the rare earth ion(s) within core  110  (e.g., within silica, such as fused silica). As a further example, core  110  can include one or more materials (e.g., phosphorus pentoxide) that enhance the homogeneity of the index of refraction within core  110 . Combinations of such materials can be used. In certain embodiments, core  110  can contain fluorine. Without wishing to be bound by theory, it is believed that fluorine present in core  110  can affect the viscosity of core  110  (e.g., at elevated temperature). It is believed that fluorine in core  110  can result in core  110  having enhanced homogeneity. 
     In general, cladding  120  is used to substantially confine the pump energy at wavelength λ p  so that the pump energy propagates along fiber  100  and can interact with core  110 . Cladding  120  is typically formed from a material having a lower refractive index than core  110 . In some embodiments, core  110  has a refractive index (n 110 ) and cladding  120  has a refractive index (n 120 ) so that ((n 110 ) 2 −(n 120 ) 2 ) 1/2  is less than about 0.2 (e.g., less than about 0.17) and greater than about 0.05 (e.g., greater than about 0.12), such as about from 0.12 to 0.17. Examples of materials from which cladding  120  can be formed include silica materials, such as fused silica materials. 
     Cladding  120  has a substantially square cross-section, including four substantially flat (e.g., optically flat) sides  160   a ,  160   b ,  160   c , and  160   d . The angle subtended by adjacent sides  160   a  and  160   b ,  160   b  and  160   c ,  160   c  and  160   d , and  160   d  and  160   a  is approximately 90°. Adjacent sides  160   a  and  160   b  meet at vertex  150   a , adjacent sides  160   b  and  160   c  meet at vertex  150   b , adjacent sides  160   c  and  160   d  meet at vertex  150   c , and adjacent sides  160   d  and  160   a  meet at vertex  150   d . Vertices  150   a ,  150   b ,  150   c , and  150   d  are fused to layer  140 . 
     Inner surface  170  of layer  140  partially defines regions  130   a ,  130   b ,  130   c , and  130   d , and can serve as a protective layer for cladding  120 . Layer  140  can also provide an outermost surface of fiber  100  for the subsequent coating of additional layers (e.g., layers providing mechanical strength, chemical protection and/or physical protection). Generally, the refractive index of layer  140  can vary as desired (e.g., the refractive index of layer  140  can be about the same as the refractive index of cladding  120 , the refractive index of layer  140  can be greater than the refractive index of cladding  120 , the refractive index of layer  140  can be less than the refractive index of cladding  120 ). Examples of materials from which layer  140  can be formed include silica materials, such as fused silica materials. Materials from which layer  140  can be formed can be, for example, fluorinated or nonfluorinated. 
     Regions  130   a ,  130   b ,  130   c , and  130   d  provide an optical interface at cladding sides  160   a ,  160   b ,  160   c  and  160   d  so that, when regions  130   a - 130   d  have a lower index of refraction than cladding  120 , regions  130   a - 130   d  can substantially confine pump energy inside cladding  120 . In some embodiments, regions  130   a  - 130   d  are substantially evacuated. In certain embodiments, regions  130   a - 130   d  contain a gas (e.g., air, nitrogen, argon), a liquid (e.g., one or more low refractive index oils) and/or a solid (e.g., one or more polymers). The refractive index of each region may be the same as or different than each other region. 
     In some embodiments, the maximum dimension of one or more of regions  130   a - 130   d  between cladding  120  and inner surface  170 , is about the same as or greater than the wavelength of the pump energy (λ p ) (e.g., about the same as the wavelength of the pump energy, at least about twice the wavelength of the pump energy, at least about three times the wavelength of the pump energy, at least about four times the wavelength of the pump energy, at least about five times the wavelength of the pump energy, at least about six times the wavelength of the pump energy, at least about seven times the wavelength of the pump energy, at least about eight times the wavelength of the pump energy, at least about nine times the wavelength of the pump energy, at least about 10 times the wavelength of the pump energy, at least about 20 times the wavelength of the pump energy, at least about 50 times the wavelength of the pump energy, at least about 75 times the wavelength of the pump energy, at least about 100 times the wavelength of the pump energy). 
     In certain embodiments, the maximum dimension of one or more of regions  130   a - 130   d  between cladding  120  and inner surface  170 , is at least 0.8 micron (e.g., at least about one micron, at least two microns, at least three microns, at least about four microns, at least about five microns, at least about six microns, at least about seven microns, at least about eight microns, at least about nine microns, at least about 10 microns, at least about 20 microns, at least about 35 microns, at least about 50 microns, at least about 60 microns, at least about 75 microns). 
     In certain embodiments, regions  130   a ,  130   b ,  130   c , and  130   d  are of sufficient dimension such that substantially no energy propagating in cladding  120  that is incident on sides  160   a ,  160   b ,  160   c , or  160   d  is coupled into layer  140 . 
     In some embodiments, pump energy can be efficiently coupled into cladding  120  (e.g., by end-coupling). The numerical aperture of a fiber describes the pump energy gathering efficiency of a fiber, and for fiber  100  the numerical aperture (NA) is given approximately by:
 
 NA =√{square root over (( n   120   2   −n   130   2 ))}, 
 
where n 120  is the index of refraction of cladding  120  and n 130  is the effective index of regions  130   a ,  130   b ,  130   c  and  130   d  surrounding cladding  120 . In some embodiments, fiber  100  can have a high numerical aperture (e.g., at least about 0.25, at least about 0.3, at least about 0.35, at least about 0.4, at least about 0.5, at least about 0.6, at least about 0.7, at least about 0.8, at least about 0.9, about one).
 
       FIG. 2  shows a fiber laser system  200  including fiber  100  and a pump source  220  (e.g., a laser, such as a semiconductor diode laser). Pump source  220  emits energy at wavelength λ p  and is configured so that this energy can be coupled into fiber  100  (e.g., by end-pumping or side-pumping). In addition to core  110 , cladding  120 , regions  130   a - 130   d , and layer  140 , fiber  100  includes reflectors  230 ,  240  and  250  (e.g., Bragg gratings). Reflector  230  is configured to reflect substantially all (e.g., about 100%) of the energy impinging thereon at wavelength λ p . Reflector  240  is configured to reflect substantially all (e.g., about 100%) energy impinging thereon at wavelength λ out  and reflector  250  is configured to reflect a portion (e.g., at least 98%, at least 95%, at least 90%, at least 80%, at least 70%, at least 60%, at least 50%, at least 40%, at least 30%, at least 20%, at least 10%, at least 5%) of the energy impinging thereon at wavelength λ out  so that reflectors  240  and  250  form a resonance cavity  260  for energy at wavelength λ out . 
     During operation of system  200 , pump energy at wavelength λ p  is emitted by source  220 , coupled into fiber  100  and propagates in fiber  100 . As the pump energy propagates along fiber  100 , it is substantially confined within the volume of fiber  100  defined by cladding  120 . A portion of the pump energy within cladding  120  intersects core  110 , and a portion of the pump energy intersecting core  110  interacts with the active material in core  110  to form energy at wavelength λ out  (e.g., via electronic transitions in the active material contained in core  110 , such as three-level lasing or four-level lasing). 
     λ out  is generally different from λ p  Examples of λ out  include about 1080 nanometers and about 1100 nanometers. Examples of λ p  include about 915 nanometers and about 975 nanometers. 
     Energy having wavelength λ out  that is formed in cavity  260  may experience gain (e.g., by stimulated emission) and grow in intensity. A portion of the energy at wavelength λ out  propagating in cavity  260  exits cavity  260  through reflector  250  and ultimately exits fiber laser  100  through end  215 . 
       FIGS. 3A-3D  show a method of making optical fiber  100 . Referring to  FIG. 3A , a preform  300  having a cylindrical cross-section with a core  310  and a cladding  320  (having a cylindrical cross-section) is prepared using, for example, modified chemical vapor deposition (MCVD). The outer surface of cladding  320  is ground and polished to yield a preform  330  having a square cross-section with core  310  and cladding  320   a  (FIG.  3 B). Cladding  320   a  has a square cross-section defined by sides  331 ,  332 ,  333  and  334 . Preform  330  is then inserted into layer  340  having an inner surface  350  (FIG.  3 C). Layer  340  is cylindrical in shape and can be formed from the same material as cladding  320 . As shown in  FIG. 3D , the air remaining between inner surface  350  and preform  330  is removed to ensure that the vertices  371 ,  372 ,  373  and  374  of cladding  320   a  contract inner surface  350 . Layer  340  and square preform  330  are then heated to fuse vertices  371 ,  372 ,  373  and  374  with inner wall  350  ,forming a final preform  380  with regions  361 ,  362 ,  363 , and  364  between layer  340  and cladding  320   a  (FIG.  3 D). Fiber  100  is then drawn from the final preform  380  (e.g., using a draw tower). 
     While  FIGS. 3A-3D  show a method of making an optical fiber preform, the invention is not so limited. Other methods can also be used. For example, in some embodiments a preform having a core and a cladding is formed, followed by boring holes into the preform (e.g., using a sonic drill) that run parallel to the preform axis. The optical fiber can then be drawn from the final preform. 
     While particular embodiments of optical fibers have been described, the invention is not limited to such embodiments. For example, while the core has been shown as being located substantially at the center of the cladding and the exterior layer, the core can be substantially eccentrically disposed with respect to the center of the cladding and/or with respect to the center of the inner surface of the exterior layer. 
     Moreover, in general, the cross-sectional shape of the cladding may be any two dimensional shape. For example, the cladding may be in the shape of any polygon. In some embodiments, the cladding may be in the shape of any four-sided polygon (e.g., a square, a rectangle, a parallelogram, a trapezoid etc.). As another example, the cladding may have fewer than four sides (e.g., three sides). As a further example, the cladding may have more than four sides (e.g., five sides, six sides, seven sides, eight sides, nine sides, 10 sides, etc.). 
     Furthermore, the sides of the cladding can be of substantially equal length, or different in length. In some embodiments, a plurality of sides of the cladding may be substantially equal in length, but may differ in length from other sides of the cladding. 
     In addition, the angles subtended by adjacent sides of the cladding may be substantially equal, or they can be different. In some embodiments, a plurality of the angles subtended by adjacent sides of the cladding may be substantially equal, but may differ from other angles subtended by adjacent sides of the cladding. 
     In some embodiments, the cross-section of the cladding may be in the shape of a convex polygon chosen so that the pump energy propagating within the optical fiber forms a substantially uniform radiation field. In certain embodiments, the shape of the cladding can be chosen such that substantially all possible modes of pump energy propagating in the optical fiber can intersect the core at least at one point in the fiber (e.g., modes, such as helical modes, that do not intersect the core are substantially unable to propagate along the cladding). 
     In general, the cladding can occupy any percentage of the area inside the inner surface of the exterior layer (e.g., inner surface  170  of layer  140 ) (e.g., at least about one percent, at least about two percent, at least about five percent, at least about 10 percent, at least about 20 percent, at least about 30 percent, at least about 40 percent, at least about 50 percent, at least about 60 percent, at least about 70 percent, at least about 80 percent, at least about 90 percent). In certain embodiments (e.g., for three-level lasing), the cladding occupies from about five percent to about 15 percent of the area inside the inner surface of the exterior layer (e.g., inner surface  170  of layer  140 ). In some embodiments, (e.g., for four-level lasing), the cladding occupies from about 75 percent to about 90 percent of the area inside the inner surface of the exterior layer (e.g., inner surface  170  of layer  140 ). 
     In general, the sides of the cladding can be straight or curved (e.g., convex or concave). In some embodiments, one or more sides of a cladding may have portions that are straight, convex and/or concave.  FIGS. 4A-4C  show examples of shapes that the sides of fiber claddings may have.  FIG. 4A  shows an optical fiber  414  having core  110 , a cladding  401  and layer  140 . Cladding  401  has a substantially flat side  403  and a convex side  402 . Cladding  401  and layer  140  define a region  404  (e.g., a D-shaped region).  FIG. 4B  shows an optical fiber  412  having core  110 , a cladding  410  and layer  140 . Cladding  410  has four sides, including a concave side  411 . Cladding  410  and layer  140  define regions  415   a ,  415   b ,  415   c  and  415   d .  FIG. 4C  shows an optical fiber  413  having core  110 , a cladding  420  and layer  140 . Cladding  420  has four sides, including a curved side  421  having portions that are convex and other portions that are concave. Cladding  420  and layer  140  define regions  416   a ,  416   b ,  416   c  and  416   d . In some embodiments, cladding  401 ,  410  and/or  420  can be formed of the same material as layer  140  (e.g., a silica material, such as fused silica). 
       FIG. 4D  shows an embodiment of an optical fiber  480  having core  110 , cladding  401 , layer  140 , region  404  and a layer  460 . Layer  460  can have any refractive index. For example, the refractive index of layer  460  can be substantially equal to the refractive index of layer  140 . Alternatively, the refractive index of layer  460  can be less than the refractive index of layer  140 . In some embodiments, the refractive index of layer  460  is less than the refractive index of layer  140 . In optical fiber  480 , the refractive index of layer  140  can be substantially equal to the refractive index of cladding  401 . 
     Layer  460  can be formed from, for example, silica and silica-containing materials (e.g., fused silica). In some embodiments, layer  460  can be formed from polymeric materials, for example, polymeric materials having a low refractive index (e.g., less than 1.50, less than 1.45, less than 1.40, from about 1.35 to about 1.38). Fluorinated, low index polymeric materials can be used in certain embodiments. 
     In some embodiments, a precursor to layer  460  can be included in the final preform from which fiber  480  is made. In alternative embodiments, layer  460  can be coated onto fiber  480 , at any time during or after fiber  480  is being made. 
     In general, the cladding contained in an optical fiber may be fused to the inner surface of the layer (e.g., surface  170  of layer  140 ) along the entire length of the cladding, or along one or more portions of the length of the cladding.  FIG. 5  shows a partial cross-sectional view of an embodiment of an optical fiber  500  having a core  510 , an exterior layer  540  and a cladding  520  that contacts layer  540  at points  580   a ,  580   b ,  580   c ,  580   d , and  580   e  without contacting layer  540  at points  570   a ,  570   b ,  570   c ,  570   d ,  570   e ,  570   f . In some embodiments, layer  540  and cladding  520  are fused at one or more of points  580   a ,  580   b ,  580   c  and/or  580   d . In certain embodiments, layer  540  and cladding  520  are not fused at points  580   a ,  580   b ,  580   c  and  580   d.    
     While regions have been described as having substantially D-shaped cross-sections, other cross-sections can be used. Generally, the regions can be any shape. In some embodiments, the regions may be substantially regularly shaped (e.g., oval, round, square, triangular, trapezoidal, etc.). In certain embodiments, the regions can be irregularly shaped. Different regions can have different cross-sectional shapes. For example, one region can be substantially D-shaped, while other regions are triangular. Combinations of different shapes can be used. 
     In some embodiments, one or more regions  130   a - 130   d  may be substantially continuous along the length of the optical fiber. In certain embodiments, one or more regions  130   a - 130   d  may be discontinuous along the length of the optical fiber. In some embodiments, adjacent regions may be at least partially continuous with adjacent regions (e.g., at points  570   a ,  570   b ,  570   c ,  570   d ,  570   e , and  570   f ). 
       FIG. 6  shows a cross-sectional view of an optical fiber  600 . Fiber  600  has core  610  and a cladding  620 . Cladding includes a region  630  having a different refractive index than cladding  620  (e.g., region  630  has a higher index of refraction than cladding  620  or region  630  has a lower index of refraction than cladding  620 ). Region  630  can be any two-dimensional shape (e.g., round, oval, irregularly shaped, polygonal, etc.). Although shown in  FIG. 6  as having only one region  630 , cladding  620  can contain multiple regions  630  (e.g., two regions, three regions, four regions, five regions, six regions, etc.). The region(s) can be continuous or discontinuous along the length of the optical fiber. Moreover, although not shown in  FIG. 6 , fiber  600  can include a layer disposed on the exterior surface of cladding  620  (e.g., a layer having a higher index of refraction than cladding  620 , a layer having a lower index of refraction than layer  620 , or a layer having substantially the same index of refraction as cladding  620 ). In some embodiments, a layer disposed on the exterior surface of cladding  620  can absorb a substantial amount of energy at the wavelength λ p  (e.g., a layer disposed on the exterior surface of cladding  620  can be substantially opaque to energy at wavelength λ p ). The ratio of the area of region  630  to the area of cladding  620  can be any value (e.g., at least about one percent, at least about five percent, at least about 10 percent, at least about 20 percent, at least about 30 percent, at least about 40 percent, at least about 50 percent, ay least about 60 percent, at least about 70 percent, at least about 80 percent, at least about 90 percent). 
       FIG. 7  shows a system  700  including an energy source  710  (e.g., a laser, such as a semiconductor diode laser) a fiber  701  (e.g., a fiber laser formed of an optical fiber and including reflectors as described herein), a Raman fiber laser  790  and an output cascade  770 . Energy source  710  is connected to a combiner  750  via fibers  720 ,  730  and  740 . Combiner  750  is connected to fiber  701  via a coupler  760 . Fiber  701  is in turn connected to a Raman fiber laser  790  via a coupler  780 , and Raman fiber laser  790  is connected to an output cascade  770  via a fiber coupler  775 . In certain embodiments, laser  790  and output cascade  770  are integrated into a single unit. 
     During operation, energy at wavelength λ p  is generated by source  710 , propagates along fibers  720 ,  730  and  740 , and is coupled into fiber  701  via combiner  750  and coupler  760 . A portion of the energy at λ p  is converted by fiber  701  into energy at wavelength λ out  Energy at λ out  exits fiber  701 , propagates along coupler  780  and is coupled into Raman fiber laser  790 . Some of the energy at wavelength λ out  entering Raman fiber laser  790  is converted to energy at one or more longer wavelengths. The energy at the longer wavelength(s) is coupled into output cascade  770  by coupler  775 . Cascade  770  optionally includes variable output couplers that can be dynamically adjusted to modulate the amount of energy allowed to exit system  700  at desired wavelengths. 
     While certain embodiments have been described, the invention is not limited to these embodiments. For example, in certain embodiments, an optical fiber can include a core (e.g., a single mode core) that does not contain an active material. As another example, an optical fiber may contain more than one lasing cavity. As a further example, the refractive index of a region (e.g., one or more of regions  130   a - 130   d ) can be equal to or less than the refractive index of the cladding. 
     Moreover, in certain embodiments, the optical fiber is substantially devoid of a support structure (e.g., a silica material, such as a silica web) between the portions of the optical fiber that define a region having a lower refractive index than the cladding. As an example, regions  130   a ,  130   b ,  130   c  and/or  130   d  can be substantially devoid of a support structure (e.g., a silica material, such as a silica web). As another example, region  630  can be substantially devoid of a support structure (e.g., a silica material, such as a silica web). 
     Other embodiments are in the claims.