WO1997033460A2

WO1997033460A2 - Multicore fiber optic amplifier

Info

Publication number: WO1997033460A2
Application number: PCT/US1997/002859
Authority: WO
Inventors: Robert R. Rice
Original assignee: Mcdonnell Douglas Corporation
Priority date: 1996-02-21
Filing date: 1997-02-21
Publication date: 1997-09-18
Also published as: GT199700002A; HN1997000026A; CO4750748A1; AU3282797A; WO1997033460A3

Abstract

An optical fiber (10) includes a rare-earth doped inner core (12) surrounded by an outer core (14), and a pump core (16) attached to the outer core (14). The pump core (16) is separated into branched regions each partially attached to the outer core (14). Excitation light, from an external pump source is coupled into each branched region (18) of the pump core (16), propagates into the outer core (14), without breaking open the outer core (14), and leaks into and is absorbed by the inner core (12). The rare-earth ions contained in the inner core (12) are capable of being stimulated by this received optical power, thereby providing the desired amplification of an optical signal propagating through the optical fiber (10).

Description

MULTICORE FIBER OPTIC AMPLIFIER

Field of the Invention

The present invention relates to double core optical fiber amplifiers, and, more particularly, to efficiently increasing fiber amplifier power from such double core optical fiber amplifiers.

Background of the Invention In recent years, rare earth doped optical fiber amplifiers have found wide use in the field of telecommunications. Such fiber amplifiers typically consist of doped, single mode cores approximately 6-10 microns in diameter surrounded by an undoped glass cladding having a lower refractive index. When a signal light beam is allowed to impinge on the rare earth atoms excited to the high energy level within the optical fiber, stimulated emission of light takes place causing transition of the rare earth atoms to the ground state, whereby the intensity of the signal light beam is amplified. While such doped fiber amplifiers are adequate for the needs of the telecommunications industry, for which signal levels' of +15-20 dB are not exceeded, the maximum power produced is insufficient for many important free space laser applications. The maximum power output is currently limited to a few watts because it is difficult to couple more than several watts into the doped core. The pump laser employed with such doped fiber amplifiers must operate in a fundamental mode and have high beam quality to enable efficient pump coupling.

A prior art improvement in fiber optic amplifier power occurred with the invention of the so-called double core optical fiber amplifier, such as illustrated in U.S. Patent No. 4,848,998, entitled "Selective Volitization Method for Preparing Fiber Optics," issued on July 18, 1989 to Elias Snitzer. As described in this reference, the prior art double core optical fiber amplifier consists of an inner rare earth doped single mode core as in the past which, however, is embedded in a second core before being embedded in the cladding. The function of the second core in this prior art arrangement is to allow the collection of pump light from low brightness laser diode arrays. This pump light is absorbed by the inner core to produce optical gain as in the prior art single core fiber optic amplifier, however, it does not require that the pump laser be in fundamental mode with high brightness coupled directly into the doped core as is required in prior art single core fiber optic amplifiers. Such fundamental mode pump lasers are limited in available power and hence the power performance of the single core fiber optic amplifier is also restricted. Being able to use high power low brightness pump arrangements, however, allows the amplifier output power to be greatly increased to a level where the physical mechanisms of the fiber limit the amount of power generated. Unfortunately, the pump power coupled into the pump core of the prior art double core optical fiber amplifiers cannot be arbitrarily increased because of the numerical aperture limits of the fiber, the size of the core, and the divergence of the light source, such as a two-dimensional laser diode array. Hence the output power from the fiber amplifier is necessarily limited. Prior art attempts to increase the power from such a fiber optic amplifier, have involved the use of multiple amplifiers in tandem such that the output beam of the first is coupled into the input of the second, and so on; however, this solution requires special coupling optics such as a lens to collimate the output beam of the first amplifier, a dichroic beam splitter to transmit the amplifier beam and allow the insertion of a pump beam for the second amplifier, and a lens to couple the amplifier beam into the doped core of the second fiber while coupling the pump beam into its outer core. This prior art solution, while feasible, still introduces undesirable power loss and is susceptible to undesirable mechanical misalignment. Such optical systems require careful alignment and are susceptible to environmental changes, vibration and thermal effects. Moreover, the optical surfaces are exposed to the high power of the amplifier beam and could initiate damage if not kept absolutely clean. These disadvantages of the prior art are overcome by the improvements of the present invention. Summary of the Invention

The present invention comprises a multicore fiber optic amplifier namely a double core optical fiber comprising an inner core, which is doped with a rare-earth element and is capable of absorbing optical power, with the inner core being surrounded by an undoped outer core having a refractive index less than that of the inner core. A pump core is attached to the outer core and is separated into branched regions along its length having unattached portions, each adapted to receive optical power from an external pump source. Excitation light, generated by an external pump source, is coupled into each of the unattached ends of the branched regions of the pump core.

This optical power propagates into the outer core and then leaks into the inner core and is absorbed by the rare-earth ions contained within the inner core. Thus, the improved double core optical fiber arrangement provides an effective method of coupling laser pump power into the doped core of a double core fiber optic device, without breaking open either the outer core or the inner core. In addition, the present invention provides an improved fiber optic amplifier which uses the double core optical fiber described above. According to this improved fiber optic amplifier, a pump source generating excitation light is coupled with the branched regions of the pump core. This excitation light propagates into the outer core and then is absorbed by the inner core. The rare-earth ions of the inner core are capable of being stimulated by this laser pump power thereby providing the desired ampli ication of an optical signal propagating through the optical fiber. In one preferred embodiment of the fiber optic amplifier, designed for very high power operation, the rare-earth-doped inner core contains diffraction gratings. Accordingly, the fiber optic amplifier suppresses the build-up of a Raman shifted signal and is capable of diffracting out stokes shifted radiation, thereby allowing efficient amplification of very high power signals. In addition, the present invention provides a fiber laser, which is also based upon the double core optical fiber. According to this fiber laser, the double core optical fiber, which includes a reflective coating, or a distributed feed¬ back Bragg grating, on each end, serves as a resonant cavity. The fiber laser also includes a light source for generating excitation light and optical means for directing the excitation light from the light source into the unattached portions of the branched regions of the pump core. The rare- earth doped inner core of the optical fiber is capable of absorbing laser pumped radiation introduced into the branched regions of the pump core, and contains ions capable of being excited by the laser pump radiation so as to cause laser emission therein. In the case of Bragg grating feedback, the precise frequency of laser oscillation is selected and controlled by the grating periodicity.

In addition, the present invention provides an improved fiber optic cable assembly which includes the double core optical fiber. According to this improved fiber optic cable, the double core optical fiber is surrounded by a cladding having a lower refractive index than the branched pump core and the outer core. The cable also has taps located adjacent the unattached ends of the branched regions of the pump core for coupling laser pump power into the branched regions.

Furthermore, the present invention provides a method for manufacturing the double core optical fiber, wherein the branched pump core is adhered to the outer core by fusing using a continuous drawing process or through the use of an organic-based adhesive.

Description of the Drawings These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

Figure 1 is longitudinal cross-sectional view of the improved optical fiber of the present invention; Figure 2 is a cross-sectional view of the optical fiber of Figure 1 taken along line 2-2 of Figure 1;

Figure 3 is a diagram showing the physical arrangement of the improved fiber optic amplifier of the present invention, which includes a cross-sectional view of the improved optical fiber;

Figure 4 is a diagram showing the physical arrangement of the fiber laser in accordance with the present invention, which includes a cross-sectional view of the improved optical fiber; and

Figure 5 is a cross-sectional view of the improved fiber optic cable assembly of the present invention. Detailed Description of the Invention

The improved fiber optic amplifier of the present invention preferably employs a double core optical fiber such as illustrated in Figures 1 and 2. The present invention preferably provides arbitrary increases in optical fiber amplifier power by fabricating arbitrarily long amplifiers or by coupling tandem amplifiers without the previous problems associated with the conventional prior art approach involving dichroic beam splitters and multiple coupling lenses. The principle of operation of the improved fiber optic amplifier of the present inventories is as follows. The mode diameter of the inner single mode core 12 of the optical fiber illustrated in Figures 1 and 2 is of the order of the core diameter, for example 6-10 microns. The mode size is determined by the core diameter and the refractive index difference between the inner core 12 and the outer core 14, just as the mode size in a conventional prior art single mode fiber is determined by the core size and the refractive index difference between its core and cladding. The modal field is concentrated in the core and does not extend very far into the material surrounding it; in fact, if the surrounding material were completely removed or substituted beyond a distance of a few core diameters, the mode would be essentially unaffected. Hence, the material surrounding the rare-earth doped single mode core can be modified at will beyond a few mode diameters without affecting a single mode beam propagating through that core. Hence, the pumping configuration can be optimized independently so that the amplifier signal level can be increased to a level where fundamental physical mechanisms become the limiting factor.

Referring further to Figures 1 and 2, an improved means for coupling excitation light into the active region is illustrated. The preferred embodiment of the double core optical fiber 10 includes an inner core 12, which is preferably a single mode inner core, surrounded by the outer core 14. The inner core 12 preferably comprises a silica- based fiber or glass and is doped with a rare-earth element such as, for example, erbium. As is well known in the art, a variety of other elements, including neodymium, ytterbium, thulium, holmium, and praseodymium may be used as the dopant.

The inner core 12 preferably has a circular cross-section having a diameter of 6-10 microns, and is uniform along its arbitrary length. Alternatively, elliptical shaped inner cores can be used in order to preserve a given polarization state for the signal mode.

The outer core 14 preferably consists of an undoped silica-based fiber or glass. Preferably, the outer core 14 is rectangular or elliptical in shape along its longitudinal axis and the inner core 12 is disposed along its longitudinal axis. A typical outer core 14 may be 30 microns in height and 300 microns in width.

The double core optical fiber 12 of the present invention preferably further includes an undoped silica-based fiber or glass pump core 16 attached to the outer core 14. Preferably, the pump core 16 is separated into spaced-apart branched regions 18 and is attached to one side of the outer core 14 as illustrated in Figure 1. Alternatively, if the desired application permits, the pump core 16 may be attached to two opposing sides of the outer core 14. Typically, each branched region 18 of the pump core 16 is preferably generally rectangular in shape and equal in width to the outer core 14, although different dimensions and shapes are still within the spirit and intent of the present invention. Preferably, the pump core 16 has a numerical aperture (NA) sufficient to allow the introduction of excitation light from an external pump source operating in a non-fundamental mode. Preferably, the NA of the pump core is at least 0.5. The pump core 16 may have a pre-determined cross-sectional size and shape designed to match the laser beam output by an external pump source, thus maximizing the efficiency with which the excitation light is coupled into the outer core 14.

The pump core 16 preferably contains a plurality of spaced-apart branched regions 18. Each branched region 18 is preferably partially attached to the outer core 14 with the unattached portion 20 extending outwardly from the outer core 14. The unattached portion 20 of each branched region 18 is adapted to receive excitation light from an external pump source. Each branched region 18 is open on one end of its unattached portion 20, thus allowing the introduction of optical pump power from a suitable light source to propagate into and along the pump core 16. Preferably, the refractive index of the pump core 16 is roughly equivalent to the refractive index of the outer core 14, thus allowing pump light coupled into the unattached portion 20 of the branched regions 18 to propagate freely from the pump core 16 into the outer core 14. Additionally, the refractive index of the outer core 14 is preferably less than the refractive index of the inner core 12. As a result, the pump light is confined to propagate through the outer core 14 and will be absorbed into the active inner core 12 and, thus, cause inversion of the electronic population of the inner core 12 material to permit an optical signal traveling through the optical fiber 10 to stimulate the emission of photons from the inner core 12 material.

The preferred interval between the branched regions 18 is preferably such that all the introduced pump light has propagated along the outer core 14 and leaked into the single mode inner core 12. Such a distance can be defined as the absorption length, and will vary depending upon the materials used in the three cores 12, 14, and 16, the dopant used for the inner core 12, and the wavelength of the light emitted from the pump source.

The pump core 16 may be attached to the outer core 14 using one of two preferred methods. Using the first method, the attached portions 22 of each branched region 18 are fused to the outer core 14 during a continuous drawing process. As is known to those skilled in the art, this fusing process requires a conventional drawing apparatus having a heat source and means to control the tensile elongation of the pump core 16 as it is fused to the outer core 14.

Using the alternative method to attach the pump core 16 to the outer core 14, each of the attached regions 22 are adhered to the outer core 14 with an organic-based adhesive, such as an optical epoxy, a glass frit, or any other optical cement, such as a transparent contact adhesive.

The optical fiber 10 described above can be applied to an optically active device such as a fiber optic amplifier or a fiber laser. Figure 3 illustrates an embodiment of an improved fiber optic amplifier 30 in accordance with the present inventions for receiving and amplifying an optical signal, incorporating the optical fiber 10 described above. The fiber optic amplifier 30 preferably includes, as its main elements, an active optical fiber 10 and a pump source 32 for generating excitation light. The optical fiber 10 used is preferably identical to the optical fiber described 10 above, having a rare-earth doped inner core 12, an outer core 14, and a branched pump core 16.

In accordance with the present invention, an appropriate signal from pump source 32 is preferably coupled into the open ends of each of the unattached portions 20 of the branches 18 of the pump core 16. Various arrangements may be used to accomplish this optical pumping. For example, the pump source 32 may comprise devices known in the art and capable of generating optical power, such as a semiconductor laser or a two-dimensional laser diode array having a plurality of individual emitters 33, as shown in Figure 3.

Preferably, the conventional two-dimensional laser diode array contains individual collimating lenses 34 for each emitter and condensing lenses 35 to focus the output of the laser diode array into the branched regions 18 of the pump core 16. As is known in the art, the laser diode array may preferably be a surface emitter array or a rack-and-stack edge emitter array.

The excitation light 36 generated by the pump source is preferably coupled into each of the branched regions 18 of the pump core 16, propagates into the outer core 14, and then leaks into the doped inner core 12. As is known to those skilled in the art, the pump source 32 provides optical signals of a known wavelength in order to excite the rare- earth elements contained within the inner core 12. When the optical power of the pumping light is sufficiently high, the rare-earth atoms in the doped inner core are excited to a high energy level, and if a signal light beam is introduced to the rare-earth atoms, then light is stimulated to emit from the rare-earth atoms. Consequently, the optical power of the signal light is amplified. The pump source 32 can include conventional collimating optics, such as an array of lenses, to shape the emitted laser beam to match the size and shape of the branched regions 18 of the pump core 16, thus improving the efficiency with which the excitation light is coupled into the pump core 16, thereby improving the overall magnitude of the amplification provided by the fiber optic amplifier. The pump source 32 may comprise a plurality of distributed pump sources, each coupled to a different branched region of the pump core. The pump source can also include a single pump source whose output is split and coupled to multiple branched regions of the pump core. Accordingly, depending on the power requirement, the number of pump sources can be reduced.

The fiber optic amplifier may further include a plurality of reflective devices 38, such as, for example, mirrors located within the outer core 14 and placed in alignment with the end of each attached region 22 of the pump core 16. The reflective devices 38 reflect the excitation light back into the outer core 14, thereby increasing the amount of optical power absorbed into the inner core 12.

The improved fiber optic amplifier 30 of the present invention may also include stress rods 37 located in the optical fiber 10 in order to preserve the polarization of the propagating amplifier signal. Preferably, the stress rods 37 include a glass composition having a refractive index lower than that of the outer core in order to prevent concentration of pump light in the stress rods 37. Preferably, the stress rods 37 are disposed axially along the outer core 14 and are place on opposing sides of the inner core 12. In addition, the improved fiber optic amplifier 30 may also include a method to suppress optical signals having wavelengths different than the desired wavelength of the amplified signal. As such, the scattering effects introduced by the fiber optic amplifier 30 are reduced or eliminated. The method may also suppress the build-up of a Raman shifted signal. One method for accomplishing such suppression provides for a wavelength selective absorber dopant, such as samarium, contained within the inner core 14. Another method provides for conventional diffraction gratings 39 written into the inner core 14 in order to dump the signal as it builds up.

Those skilled in the art will appreciate the purpose of the diffraction gratings 39, which is to remove undesired signals having wavelengths outside of a known wavelength range, thereby reducing the effects of scattering within the fiber optic amplifier 30. As is known in the art, diffraction gratings 39 may preferably be written in the glass of the inner core by a process of exposure to ultraviolet laser light through a patterned photomask. Figure 4 shows an embodiment wherein the optical fiber 10 is adapted for use in a fiber laser arrangement 40. The fiber laser 40 includes the optical fiber 10 described above, wherein the double core optical fiber 10 serves as a resonant cavity. The fiber laser 40 also includes a pump source 32 for introducing laser pump radiation into the branched regions 18 of the pump core 16 for absorption by the inner core 12. The ions contained within the inner core 12 of the optical fiber 10 are capable of being raised to an excited state by the laser pump radiation so as to cause laser emission therein.

The optical cavity or resonator is a region bounded by two or more reflecting surfaces 42 whose elements are aligned to provide multiple reflections. The reflecting surfaces 42 may preferably be made, for example, from a substrate of polished glass or silica that has an appropriate reflector coating of film applied. The pump source 32 may be of any known construction, such as the arrangements described above. The pump source 32 generates excitation light having a certain wavelength and is directed into the branched regions 18 of the pump core 16. The rare-earth ions contained in the inner core 12 of the optical fiber 10 are excited to a predetermined state by this excitation light, and light at a particular wavelength band can be emitted.

As is known in the art, if the fiber laser 40 constitutes an inner core 12 doped with a particular rare- earth element, such as erbium, and the pump source 32 generates compatible excitation light, such as light having a wavelength of 980 nanometers, the optical fiber will lase parasitically on its own without the use of a reflective coating, although the exact spectrum of the laser emission will generally be unpredictable.

In an alternative embodiment of the fiber laser 40, feedback to sustain laser oscillation is provided by distributed feedback from Bragg diffraction gratings 44 written in the ends of the inner core. These diffraction gratings 44 both control the level of feedback and establish the precise wavelength of laser oscillation. By changing the temperature of gratings 44 or, alternatively, by applying tensile stress to the gratings 44 of the fiber, the wavelength of laser emission can be tuned.

Because the pumping arrangement described above permits coupling high-power pump energy into the optical fiber, the fiber laser has a high power level, relative to prior art double core fiber lasers.

Figure 5 shows still another embodiment of the present invention, a fiber optic cable assembly 50, which includes the above-described optical fiber 10, and further includes a cladding 52 surrounding all three cores 12, 14, and 16, of the optical fiber 10, and an insulation layer 58 surrounding the cladding layer 52. The cladding 52 has a refractive index less than that of the pump core 16, thus permitting light coupled into the pump core 16 to propagate without being dispersed into the surrounding environment.

Preferably, the cladding 52 and the insulation layer 58 are formed from a low index transparent polymer or similar materials. The fiber optic cable assembly 50 further includes taps 54 located at the branched ends of each branched region 18 of the pump core 16 for coupling received optical power into the branched regions 18. Preferably, the taps 54 are exposed ends of the branched core 16 which permit excitation light to be coupled therein.

In a preferred embodiment, the fiber optic cable assembly 50 includes a strength member 56 displaced around the cladding layer 52 and beneath the insulation layer 58. Such a strength member 56 may be formed from steel or materials such as Kevlar, a trademark of the DuPont Company, wound around the cladding layer 52.

Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible without departing from the spirit and scope of the present invention. Therefore the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained therein.

Claims

ClaimsWhat is claimed is:

1. A double core optical fiber comprising: a rare earth doped inner core capable of absorbing optical power; an outer core surrounding the inner core having a refractive index less than that of the inner core; and a pump core adhered to the outer core and having a plurality of spaced-apart branched regions located along its length for receiving optical power through the branched regions and propagating the received optical power into the outer core along its length for absorption by the inner core; wherein the height of the branched regions of the pump core is substantially large compared to the height of the outer core to increase the efficiency with which the received optical power is absorbed by the inner core.

2. The optical fiber of claim 1, wherein the inner core is a single mode core.

3. The optical fiber of claim 1 wherein the refractive index of the pump core is approximately equivalent to the refractive index of the outer core, thereby enabling light to propagate freely from the pump core into the outer core.

4. The optical fiber of claim 1, wherein the rare earth dopant of the inner core is selected from the group consisting of neodymium, erbium, ytterbium, holmium, and praseodymium.

5. The optical fiber of claim 1, wherein the outer core is rectangular or elliptical in shape.

6. The optical fiber of claim 1, wherein the branched regions have a numerical aperture of at least 0.5.

7. The double core optical fiber of claim 1, wherein the branched regions are located on opposing sides of the outer core.

8. A method of manufacturing a double core optical fiber comprising: a rare earth doped inner core capable of absorbing optical power; an outer core surrounding the inner core having a refractive index less than that of the inner core; and a pump core adhered to the outer core and having a plurality of spaced-apart branched regions located along its length for receiving optical power through the branched regions and propagating the received optical power into the outer core along its length for absorption by the inner core, wherein the height of the branched regions of the pump core is substantially large compared to the height of the outer core to increase the efficiency with which the received optical power is absorbed by the inner core, the method, comprising the steps of: enclosing the rare earth doped inner core in the outer core, the outer core having a refractive index less than that of the inner core; and adhering the pump core having the plurality of spaced- apart branched regions located along its length for receiving laser pump power to the outer core.

9. The method of claim 8, wherein the pump core is adhered to the outer core by fusing using a continuous drawing process.

10. The method of claim 8, wherein the pump core is adhered to the outer core with an organic-based adhesive.

11. A fiber optic amplifier comprising: a double core optical fiber comprising a rare earth doped inner core and an outer core surrounding the inner core having a refractive index less than that of the inner core; a pump core adhered to the outer core and having a plurality of spaced-apart branched regions located along its length for receiving optical power through the branched regions and propagating the received optical power into the outer core along its length for absorption by the inner core; and means for introducing the optical power into the branched regions of the pump core for the propagation to the outer core and absorption by the inner core; the inner core being capable of absorbing the optical power and containing ions capable of being excited by the optical power for amplifying an optical signal passing through the double core optical fiber.

12. The fiber optic amplifier of claim 11 wherein the means for introducing the optical power comprises a light source for producing the optical power.

13. The fiber optic amplifier of claim 11, wherein the means for introducing the optical power comprises a two- dimensional laser diode array having a plurality of emitters, individual collimating lenses for each emitter, and a condensing lens to focus the output of the laser diode array into the branched regions.

14. The fiber optic amplifier of claim 13, wherein said laser diode array comprises a surface emitter array.

15. The fiber optic amplifier of claim 13, wherein said laser diode array comprises a rack-and-stack edge emitter array.

16. The fiber optic amplifier of claim 11, wherein the means for introducing the optical power comprises means to shape the introduced optical power to match the size and shape of the branched regions.

17. The fiber optic amplifier of claim 11, further comprising stress rods located in the optical fiber in order to preserve the polarization of the propagating amplifier signal.

18. The fiber optic amplifier of claim 11, further comprising a plurality of reflective devices located within the outer core to reflect the optical power back into the outer core, thereby increasing the amount of optical power absorbed into the inner core.

19. The fiber optic amplifier of claim 11, wherein the inner core further comprises means to suppress the build-up of a Raman shifted signal and to diffract out stokes shifted radiation.

20. The fiber optic amplifier of claim 19, wherein the means to suppress the build-up comprises a wavelength selective absorber dopant contained within the inner core.

21. The fiber optic amplifier of claim 19, wherein the means to suppress the build-up comprises diffraction gratings written into the inner core.

22. An optical fiber laser comprising: a double core optical fiber which serves as a resonant cavity comprising a rare earth doped inner core and an outer core surrounding the inner core having a refractive index less than that of the inner core; a pump core adhered to the outer core and having a plurality of spaced-apart branched regions located along its length for receiving optical power through the branched regions and propagating the received optical power into the outer core along its length for absorption by the inner core; and means for introducing the optical power into the branched regions of the pump core for absorption by the inner core; the inner core being capable of absorbing the optical power and containing ions capable of being excited by the optical power so as to cause laser emission therein.

23. The optical fiber laser of claim 22, further comprising means to provide feedback to sustain laser oscillation.

24. A fiber optic cable assembly comprising: a double core optical fiber comprising a rare earth doped inner core and an outer core surrounding the inner core having a refractive index less than that of the inner core; a pump core adhered to the outer core and having a plurality of spaced-apart branched regions located along its length for receiving optical power through the branched regions and propagating the received optical power into the outer core along its length for absorption by the inner core, wherein the height of the branched regions of the pump core is substantially large compared to the height of the outer core to increase the efficiency with which the received optical power is absorbed by the inner core; a cladding surrounding the inner, outer, and pump cores, having a lower refractive index than that of the inner, outer, and pump cores; an insulation layer surrounding the cladding; and taps located adjacent the branched regions for coupling the received optical power into the branched regions of the pump core.

25. The fiber optic cable assembly of claim 24, wherein the cladding is formed from a low index transparent polymer.

26. The fiber optic cable assembly of claim 24, further comprising a strength member surrounding the cladding.

27. The fiber optic cable assembly of claim 26, wherein the strength member comprises Kevlar or steel wrapped around the outer core.