Abstract:
A method and apparatus for coupling substantial optical power into an optical fiber from the side without encumbering ends of the fiber. A special optical fiber, having a gain medium preferably including rare earth dopants in the core, is provided. One or more launch sections are selected at intervals along the special fiber with absorptive loops therebetween. The launch sections are stripped to their inner cladding layer, which preferably has a rectangular cross section, and positioned adjacent each other to form a launch region having a convex side mated to a launch port shaped to conform thereto, and with a low refraction index material contacting the corresponding concave side of the launch region. The concave side is preferably supported by an upwardly convex support block. Pump light is directed by appropriate optics into one or more faces of a launch port, which has a refractive index substantially matching that of the inner cladding layer into which it transmits the pump light. The pump light entering each face is preferably provided by one or more stripe diodes about as long as the total width of the adjacent launch sections in the launch region, and may include combined light from a plurality of sources. As the pump light travels along the special fiber in the absorptive loops between the launch sections, it is significantly absorbed by the gain medium of the optical fiber and thereby contributes optical energy to generate or amplify light in the core. The launch port faces may reflect unabsorbed light back into the special fiber, even while transmitting pump light at a different wavelength into the launch port.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of fiber optics, more particularly to the field of pumping optical fibers to generate or increase output power from fiber optic based devices, and specifically to mechanisms for launching light for pumping into optical fibers. 
     BACKGROUND 
     In the field of fiber optic systems, fiber optic guides transmit light power from a light source to a utilization device. Referring to FIG. 1, light source  10  transmits light signal P S    11  at wavelength λ S  through fiber  12  to utilization device  14 . Couplings between light source  10 , utilization device  14  and fiber  12  are well known in the art and are not shown. Fiber  12  includes core  16 , cladding  18  and protective covering  20 . Light source  10  typically provides the optical signals carrying information which propagates in the core. This fiber is considered a single-clad fiber. There are also double-clad fibers. A double-clad fiber has a core, a first cladding, a second cladding and the protective coating. In the double-clad case, while a single-mode signal can propagate in the core, a multi-mode signal can be coupled into the inner cladding, whereupon the inner cladding acts as a core for the second cladding. 
     Numerous applications require the generation or amplification of optical signals. Fiber optics systems used in a large variety of commercial and military applications, such as in telecommunications, inter-satellite optical communications, and for missile radar tracking systems, require generation and amplification of optical signals. 
     Fiber optic guides (“fibers”) typically have at least two essential parts. One part is the core where light propagates. The other part is cladding surrounding the core which creates conditions whereby the light propagates only in the core. These fibers are capable of transmitting single mode optical signals in the core without amplification, and produce a small amount of background loss. These can be considered “regular” fibers. 
     “Special” fibers providing a gain medium typically include a core doped with rare earth atoms such as erbium (Er), ytterbium (Yb), erbium-ytterbium (ErYb), neodymium (Nd), thulium (Tm), etc., and are utilized in applications requiring the generation or amplification of optical signals. When subjected to optical energy (typically 800-1400 nm wavelength depending on the gain medium), these special fibers have atoms excited to their upper lasing level, and when thus excited they are capable of generating or amplifying optical signals. The special fibers providing the gain medium may be easily spliced to regular fibers, which then transmit the optical signals which have been generated or amplified in the gain medium. 
     A typical fiber amplifier has a source of optical signal coupled to a rare earth doped “special” fiber gain medium. Coupled also to the gain medium is an optical “pump” source to input optical power into the gain medium, and a utilization device to receive an amplified optical signal as output from the gain medium. Referring to FIG. 2, in a typical fiber optic amplification system gain medium  22  is coupled with fiber  12  to permit light signal P S    11  at wavelength λ S  to be amplified when combined with pump light signal P P  at wavelength λ P  to provide amplified signal AP S  at wavelength λ S  for use by utilization device  14 . 
     Those skilled in the art can appreciate that the more pump power that is coupled into a rare earth doped fiber, the more optical signal output is provided by the gain medium. One form of gain medium  22  is described in PCT Publication WO 96/20519, entitled “A Coupling Arrangement Between A Multimode Light Source and An Optical Fiber Through An Intermediate Optical Fiber Length”, wherein a progressively tapered fiber portion is fused to the inner cladding of a double clad fiber carrying an optical information signal in its core. This fused system is shown schematically in FIG. 3 of the present application. However, while the spliced coupling allows the ability to have multiple locations available to input the pump power into a single fiber and achieve power scalability with unrestricted access to both fiber ends, such fused fiber couplers are somewhat difficult to manufacture. 
     There are various ways to couple pump power into special fiber. In most applications, fiber lasers and amplifiers are end-pumped by single-mode diode lasers whose output is coupled directly into the core of the fiber. The maximum output power achieved with such pumping schemes is currently about 100 mW. This is partly because 100-200 mW is typically the maximum power level that can be coupled into a fiber core at the lowest transverse mode from a readily manufactured semiconductor laser. 
     However, there are applications, such as for space communications, which require multi-watt levels of pumping. Such higher output powers are generally achieved by using double-cladding fibers. These fibers have a doped single-mode core surrounded by a multi-mode inner cladding that guides pump radiation along the fibers. Typically, the pump radiation is launched into the inner cladding at one of the fiber ends with some kind of coupling optics. The maximum output power of such devices is limited by the brightness of available pump diodes, but tens of watts of output power have been demonstrated at specific wavelengths. The drawbacks of such configurations lie in stringent high-brightness requirements for the pump sources, limited accessibility of fiber ends, and in the difficulties in scaling to higher powers. 
     Efficient optical pumping of a single-mode fiber laser or an amplifier presents a serious challenge, especially when high output powers are required. Typical end pumping requires high-brightness-pump sources, limits scalability to higher powers, and restricts access to fiber ends, and known side-pumping techniques are difficult to manufacture. Accordingly, there exists a need for an effective, easy to manufacture method and apparatus for use in pumping fiber lasers and amplifiers which provides access to both fiber ends, enables scalability to high output powers, and is relatively straightforward and inexpensive to manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-identified needs, providing a method and apparatus to achieve efficient and scalable optical power pumping into a single fiber while allowing unrestricted access to both fiber ends. Moreover, devices embodying the invention may be made for use with either single- or double-clad fibers, and are relatively inexpensive to manufacture. 
     In accordance with the present invention, a pumping fiber section includes a doped optical fiber core surrounded by a cladding layer having a cladding index of refraction. The pumping fiber section has at least one launch section defined along the fiber and leading lengthwise into an absorptive section, but preferably includes a plurality of such launch sections separated from each other along the fiber by absorptive sections. The launch section(s) are given a convex side and a concave side, and if a plurality of launch sections are used then they are arranged proximate to each other, so that the one or more launch sections form a launch region having a convex side and a concave side. 
     A launch port having a port index of refraction which matches the cladding index of refraction is given a concave shape to match the convex side of the launch region. The launch port is then mated to the launch section(s) of the launch region. The launch port accepts optical pump power on one or more sides from a pump light source such as a laser diode stripe, and conveys the pump light into the launch section(s) of the doped optical fiber. 
     The concave side of the launch region is preferably in contact with a solid substrate which supports the shape and provides a surface having a lower index of refraction than that of the cladding. Many variations are possible for this concave side of the launch region, such as providing a support block with a low index surface, or using air or a low-index coating on that side. 
     In accordance with some preferred embodiments of the present invention the launch port guides light into the fiber coil from two directions. In one preferred embodiment, two diode stripes are provided, and appropriate optics direct light from each diode stripe into one of two sides of the launch port, which is generally trapezoidal in section. 
     In another preferred embodiment, a single diode stripe and optics direct light into a single side of the launch port, but residual light re-emerging from the fiber into the launch port is reflected back into the fiber using suitable reflection from another surface of the launch port. 
     In another preferred embodiment, one side of the launch port is transmissive at a first wavelength and reflective at a second, while another side is conversely transmissive at the second wavelength and reflective at the first wavelength. As a result, pump light may be guided into the launch port from two directions, while residual light from each source is reflected back into the fiber to enhance efficiency. 
     In yet another preferred embodiment, pump light from two laser diode stripes is first polarized transversely to each other, and is then combined in a beam-splitting device so that the input optical power is nearly doubled. The optical power thus obtained is delivered to one entry face of the launch port. In another preferred embodiment, optical power from a similar arrangement using two additional laser diode stripes may be delivered to the opposite side of the same launch port. In yet another preferred embodiment, two such combined sources providing light at two different wavelengths may be directed into two faces of a launch port, and each launch port face may be conductive at one of the wavelengths and transmissive at the other to form a high-power and efficient pumping mechanism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section of a prior art optical fiber, light source and utilization device. 
     FIG. 2 shows in schematic form a fiber optics system of the prior art wherein a fiber, light source and utilization device has a gain medium employing an optical pump. 
     FIG. 3 represents a tapered pump fiber connection to an information carrying fiber. 
     FIG. 4 is a side view of a double-clad doped fiber embodiment of the present invention. 
     FIG. 5 is a side view of a single-clad doped fiber embodiment of the present invention. 
     FIG. 6 is a top schematic view of the embodiment of FIG. 5 with dual pump light sources. 
     FIG. 7 is a detail view of the launch port area of the embodiment of FIG.  4 . 
     FIG. 8 is a single-source, return reflecting embodiment of the FIG. 7 launch port. 
     FIG. 9 is a two wavelength, dual source, conductive/reflective launch port. 
     FIG. 10 depicts combining two pump sources for one launch port entry face. 
     FIG. 11 shows an alternative polarizing combination of two pump sources. 
     FIG. 12 is a cross-section of the launch port interface using a rectangular fiber. 
     FIG. 13 is a cross-section of the launch port interface using a circular fiber. 
    
    
     DETAILED DESCRIPTION 
     The present invention preferably utilizes a rare earth-doped fiber as an active gain medium of the fiber laser or amplifier. Doped fiber  41  is arranged into coils as shown in FIG. 4 for one preferred configuration. Doped fiber  41  is preferably cladding pumped (CP) fiber, with inner cladding  40  having a rectangular cross-section; the active medium core is not shown in FIG.  4 . Outer cladding  42  is stripped off in the vicinity of launch port  44  and support block  46 . The stripped sections of the fiber are placed tightly adjacent on support block  46  (with an intervening substrate, if needed, as discussed later) to form an upwardly convex launch region thereupon. Launch port  44  is preferably shaped to match the contours of the upwardly convex fibers in the launch region, and then disposed and mated thereupon, as will be seen more clearly in FIGS. 7 and 8 which show detail area  48 . Launch port  44  has a first pump light entry face  43 , and may have a second pump light entry face  45 . Except in the vicinity of detail area  48 , the depiction in FIG. 4 of coils of doped fiber  41  is merely schematic, and the coils need not be particularly arranged. 
     Support block  46  may impose the fiber shape in the launch region. Inner cladding  40  typically is made of fused silica, n cl =1.45. In that case, to minimize losses in inner cladding  40  due to outer cladding  42  being stripped away above the support block, it is preferred that inner cladding  40  contact a low index substrate material, such as Dupont Teflon™ AF1600 (n s ˜1.3) or MgF 2  (n s ˜1.373). A thin layer of AF1600, for example, will form such substrate when applied to a support block  46  made of any compatible material. Alternatively, the entire support block  46  may be made of a low index polymer such as MgF 2 , and no further substrate is then needed. Indeed, support block  46  is optional. The concave side of the launch region may be left simply contacting air, or covered by a low-index coating if touching must be tolerated. 
     FIG. 5 shows an alternative embodiment of the present invention utilizing single-clad doped fiber  50  which preferably has a rectangular cross-section. In this embodiment single-clad fiber  50  may be wound tightly on support cylinder  56 . It should be noted that the support may have a non-cylindrical shape, such as oval or eccentric, which permits varying the loop length independently of the launch region curvature. It is primarily necessary to establish a proper curvature in the launch region in detail area  48 , so that launch port  44  will properly mate to the sections of fiber  50  upon which launch port  44  is disposed, as described in more detail with regard to FIGS. 7 &amp; 8. 
     It is preferred that fiber  50  have a rectangular cross-section in order to readily mate to the surface of support block  46  above the substrate. However, there are times when it will be convenient to utilize circular cross-section fiber  50 . In this case, if support block  46  is used it will preferably be adapted to mate to the lower semicircular portion of the circular fiber  50  cross-section. This may be accomplished, for example, by applying an optical glue (available from, e.g., Nye Optical Products of Fairhaven, Mass.) matching the low index substrate  70  (e.g. Dupont AF1600) before fiber  50  is disposed upon support block  46 . Alternatively, and as shown in FIG. 13, a low-index substrate  70  of a material more viscous than AF1600 can be applied to support block  46 , and fiber  50  may be disposed upon this substrate while the material is still compliant so that it conforms to fiber  50 . As a further alternative, support block  46 , whether coated with low refractive index material such as AF1600 or formed from a low refractive index material such as MgF 2 , may be machined to provide a conforming shape upon which to dispose fiber  50 . The support block may be omitted, if structural requirements permit, with the concave side of the launch region contacting air or a low-index coating. 
     FIG. 6 is a top view of the embodiment of FIG.  5 . Launch port  44  covers a plurality of launch sections of doped fiber  50 , which is wrapped around support  56 . Lens  62  schematically represents optics to focus the light from diode stripe  66  onto launch port  44 , and similarly lens  64  represents the optics to focus the light from diode stripe  68  onto launch port  44 . It can be seen that the launch sections of fiber  50  are best arranged laterally adjacent each other so as to maximize the efficiency of light transfer from diode stripes  66 ,  68  into the cladding of fiber  50 . 
     FIG. 7 shows, in side view, detail area  48  as referenced in FIGS. 4 &amp; 5. The trapezoidal cross section of typical launch port  44 , including pump light entry faces  43  and  45 , can be more clearly seen. Low index substrate  70  is used in this embodiment above support block  46 , which accordingly may be formed from a wide range of materials, so long as they are dimensionally stable at operating temperatures and are compatible with the substrate material used (e.g. Teflon™ AF1600). Fiber core  71  is doped with rare-earth elements to form the gain medium. In the preferred embodiment, the bottom of launch port  44  is contoured to fit fiber  50 , so that launch port  44  mates with cladding  40  of fiber  50  at interface  75 . Other mating geometries are possible; for example, cladding  40  may be polished flat, and the launch port interface  75  may be flat to match. In this embodiment, pump light entry faces  43  and  45  are preferably shaped perpendicular to the direction of the pump beam propagation, and given an anti-reflection (AR) coating. 
     The launch port has a port index of refraction which matches the cladding index; both port and cladding are preferably fused silica. The launch port may be attached to the fibers in the launch region by gluing, optical contact, or diffusion-bonding. When power densities are not excessive, low-absorption optical glue having an index of refraction matched to that of the cladding and launch port may be used (available commercially from e.g. Nye Optical Products of Fairhaven, Mass.). For higher power embodiments which are not subject to excessive vibration, optical contact may be used. Diffusion bonding may also be used for higher power, but has special design issues because the high temperatures needed for diffusion bonding are not compatible with Teflon™ AF1600, nor with the outer cladding of most double-clad fibers. 
     The light from first diode stripe  66 , seen here in end section, is gathered by first optics  62  to form first pump beam  76 , which traverses first pump light entry face  43  to focus upon first diode image plane  73 , and is thus reasonably well aligned within fiber cladding  40  after traveling through launch port/cladding interface  75 . Second diode stripe  68  pumps light through second optics  64  to form second pump beam  78 , which traverses second pump light entry face  45  and interface  75  to focus on second diode image plane  74 , well aligned with cladding  40  in a direction opposite that of first pump beam  76 . The pump power launched into fiber inner cladding  40  propagates along fiber  50 , being absorbed in the process by rare-earth dopants in core  71 . It is desirable that all pump power is absorbed, since this maximizes the device efficiency. However, pump radiation remaining after one round trip in the fiber will experience losses at the launch port, re-emerging into port  44 . Two general approaches are presented for minimizing such losses in order to increase the efficiency of the system. In a first approach, fiber loops long enough to absorb most of the pump light are employed, as discussed below. In a second approach, the launch port is modified to reflect, back into fiber  50 , that light which re-emerges into the launch port after traveling through an absorptive section, as discussed further below with respect to FIGS. 8 and 9. 
     Absorption through Loop Length 
     One means to achieve good pump light absorption is to select sufficient loop length for doped fiber  41 . The loop length of doped fiber  41  (FIG. 4) is preferably chosen such that 90% of pump power is absorbed per round trip. As an example, consider an Er/Yb—doped double cladding fiber amplifier operating at 1.55 μm in the embodiment shown in FIGS. 4 and 6. Pump absorption length (1/e) in the core l co =0.7 cm at 980 nm and 2.1 cm at 920 nm, respectively. The pump absorption length in the cladding is scaled by the ratio of the cladding and core areas A cl /A co , i.e., l cl =l co A cl /A co . Assuming 8 μm core diameter and 80 μm rectangular cladding, l cl =89 cm and 267 cm for 980 nm and 920 nm pump radiation, respectively. Therefore, 90% of 980 nm and 920 nm pump is absorbed in 205 cm and 615 cm long fibers, respectively. In high power applications, shorter fiber length is often desirable, since it reduces the chances of developing parasitic nonlinear effects. It may be seen from the above that reducing the ratio of cladding to core areas A cl ,A co  will permit the overall fiber length to be reduced. Preferable loop lengths for other gain mediums and pump light wavelengths may be calculated in a similar manner. Although 90% absorption is preferred, tradeoffs between absorption efficiency and parasitic losses may suggest that shorter lengths be used; and if parasitic losses are small, then exceeding the 90% absorption length may be useful. 
     Recycling Escaping Pump Light 
     It may sometimes be difficult, for some wavelength and gain medium combinations, to provide adequate loop length to absorb the bulk of the pump light within a single round trip. This is a particular problem for the configuration shown in FIG. 5, because there the diameter of the support cylinder is practically limited to tens of centimeters and thus the loop length is proportionally limited. FIG. 8 shows an alternative approach to enhance coupling efficiency, reducing losses at the port by redirecting escaping pump light back into the fiber. In FIG. 8, pump light is provided from diode stripe  68  as pump beam  78  into one side ( 45 ) of the launch port. Light  86  escaping the fiber after completing a trip around the fiber is returned to the fiber by means of mirror  82  placed on the other side of the port, opposite side  45 . Mirror  82  replaces the second pump light entry face to create single-entry launch port  80 . Mirror  82  is made by polishing a spherical surface on one side of the port and depositing a reflecting coating onto it. Output plane  84  of fiber  50 , which crosses the fiber at the right angle at edge  88  of interface  72  between launch port  80  and fiber cladding  40 , is imaged back onto itself by mirror  82 . Thus, residual pump light  86 , emerging into the launch port from fiber self-image plane  84 , is reflected back into cladding  40 . 
     FIG. 9 shows an embodiment employing the principles of both FIG.  7  and FIG. 8, utilizing a two-sided launch port which also reflects, back into the cladding, light which is re-emerging into launch port  90  after a round trip through an absorptive fiber loop. First pump beam  78  having a first wavelength λ 1  (which may be 915 nm, for example) is provided and focused through optics  64  and launch port  90  into fiber cladding  40  at focal plane  74 . Pump light which remains unabsorbed after one round trip through fiber cladding  40  may emerge back into launch port  90  beginning at plane  73  (where launch port  90  mates to fiber cladding  40 ) as escape light  86 , still at λ 1 . A first transmissive/reflective coating is provided on launch port face  92  to reflect light of first wavelength λ 1  back into fiber cladding  40 . This first coating also transmits light of second wavelength λ 2  (925 nm, for example) which is input from a source, through optics  62 , as second pump beam  76 . Such coatings, transmissive at a first wavelength and reflective at a second, are well known in the art. 
     Launch port entry face  92  is thus transmissive for light of λ 2  and reflective for light of λ 1 , while the coating of launch port entry face  94  is transmissive to light of λ 1  but reflective to light of λ 2 . Therefore, residual light  89  at λ 2  from pump beam  76  which re-emerges into launch port  90  after traveling around a loop of fiber will be reflected by launch port face  94  back into cladding  40 , while residual light  86  will be reflected by face  92 . 
     Ytterbium is a preferred rare-earth dopant for the embodiment shown in FIG. 9 due to its broad absorption band which enables it to efficiently absorb pump light at wavelengths differing by at least 10 nm, e.g. 915 vs. 925 nm. It is possible to use the same approach using other dopants, such as erbium, though erbium&#39;s narrower absorption band will make it more difficult to prepare a coating which is transmissive at one wavelength and reflective at a second, if both wavelengths are close enough to be efficiently absorbed by the erbium dopants. Another approach is to combine two different dopants in the fiber cladding, for example erbium and ytterbium, and provide one pump light source at the absorption center of each. The coating of one launch port entry face would thus transmit light having a wavelength centered at the absorption peak of erbium, and reflect light with a wavelength absorbed by ytterbium, while the coating of the other launch port entry face would behave conversely. By the same principle, the two pump sources may provide light at two different absorption peaks of the same dopant. 
     Geometry of Launch Port, Pump Source and Optics Versus Optical Power Transfer 
     The total optical power input to an optical fiber by the launch port is affected by the geometry of the pump light source and the port, as well as its efficiency. 
     The overall fiber length is the product of the length of absorptive sections (plus a small launch section) by the number of such absorptive sections or “turns”. The number of turns is determined by matching the useful width of the launch port to the desired pump source  66 ,  68 . The launch port useful width is determined by the total width of adjacent fibers, rather than by the width of the trapezoidal block bonded to the fibers, which may be enlarged for manufacturing convenience. Consider a 400 micron long single-stripe diode source emitting 3 W, such as is readily available commercially. Cylindrical optics  64 ,  66  with M=1 magnification in the stripe direction may be used for coupling the pump power into launch port  44 , which therefore should be at least 400 microns wide. A typical square cross-section inner cladding is 80 μm on a side. Five adjacent 80 μm fiber loops add to generate a total length of 400 μm, matching the source diode. In this configuration, a total 10.3 m fiber length is preferred if 980 nm pumping is chosen. Other lengths of stripe diodes are available; indeed, 10,000 micron long diodes are presently manufactured as a sequence of end-to-end diodes. Such longer diodes will generally have higher power output, and can accommodate a larger number of fiber turns. 
     To determine the appropriate magnification of the optics for the direction perpendicular to the source diode stripe, numerical aperture of the cladding beneath the launch port, NA=(n cl   2 −n s   2 ) ½ , should be considered so that the pump beam is captured by the cladding. It is always advantageous to have maximum possible numerical aperture, since it can accommodate beams of inferior quality or, alternatively, permit more efficient launch port geometry which reduces power scattering of the non-absorbed portion of the pump beam. Maximum numerical aperture is achieved by using substrates (see FIGS. 7,  8 ) having the lowest refraction index, e.g., those covered with Teflon™ AF1600. Since this substrate is not compatible with some assembly techniques, such as diffusion bonding, consider first a less optimal substrate, e.g. a support block  46  made of MgF 2 , which has a relatively high refraction index. 
     For fused silica cladding on a MgF 2  substrate, NA=0.45, which is somewhat less than the NA˜0.5 of the diode bars in the direction perpendicular to the stripe. Therefore, magnification M&gt;1.1 should be used for this direction. However, at this magnification the focal spot width of the beam is not much greater than the width of the diode stripe, typically about 1 micron, while the cladding into which the beam is focused is typically about 80 microns wide. There is thus a great deal of room to increase the focus spot size, permitting the use a larger magnification, e.g. M=5. Such a larger magnification reduces the NA of the pump beam at the launch port, thereby reducing system losses. Below, we assume M=5, which results in NA=0.1 for the pump beam at the port. 
     The height H of the trapezoidal launch port should be large enough to accommodate the whole beam at the pump light entry faces, which results in the restriction H&gt;2 NA L/2n, where L is the overall length of the trapezoid, and n is the refraction index of the launch port material. For H=1 mm, this gives L&lt;10 mm. The length L of the longest side of the trapezoid cross-section of launch port  44  should exceed the length l spanning the physical contact along interface  75  between launch port  44  and the fiber launch section(s). If the radius of the launch sections is R=10 cm, and the launch port is shaped to accept the launch section(s) to a depth d=30 μm, one gets length l of interface  72  contact=2(2 R d) ½ =4.9 mm, which is less than the maximum L determined above. If the launch sections have a smaller radius, or are mated with the launch port to a shallower depth, then the length of the launch block may be correspondingly reduced, which may result in smaller scattering losses. 
     Increasing Optical Power Input 
     FIG. 10 shows an approach for increasing the light coupled into fiber cladding  40  by a polarized combination of two source beams. Polarizing beam-splitting cube  104  directly transmits light from pump diode stripe  108 A which is collimated by optics  107 A and is polarized as shown by indication  103 . Diode stripe  108 B provides light polarized as shown by reference  101 , which polarization is then shifted by polarization half-wave plate  106  such that the polarization of the light emerging from half-wave plate  106  is as shown by reference  102 . The light thus polarized is reflected 90 degrees by polarizing beam-splitting cube  104 , such that it emerges in the same direction as the light from diode  108 A. Before entering launch port  44  (shown on support cylinder  56 ), the combined beams (polarized perpendicularly to each other) are refocused by lens  109 . 
     Variations are possible on the approach shown in FIG.  10 . For example, FIG. 11 shows diode stripes  108 A and  108 B oriented parallel to each other (perpendicular to the page), which is also parallel to the width of launch port  90 . Polarizing beam splitter  104  is then elongated to form a rectangular parallelpiped which extends at least the length of diode stripes  108 A and  108 B. Collimating lenses  107 A and  107 B, and polarization half-wave plate  106 , perform the same functions as in FIG.  10 . The combined pump light from diode stripes  108 A and  108 B form beam  78 . FIG. 11 shows beam  78  entering fiber cladding  40  only through face  92  of launch port  90  to focus on focal plane  74 . After traveling a loop of the fiber, residual light  86  re-entering launch port  90  at intersection plane  73  will be reflected from face  94 , which is reflectively coated. 
     The combining of beams shown in FIG. 11 may be used with a launch port  90  as shown in FIG. 9 to effectively input the light from four pump diodes into fiber cladding  40 . In this case, output power of the device may be estimated by assuming that the output of two 3 W diodes is combined with polarization coupling at each end of the launch port, thereby bringing the total available power to 12 W, less the inevitable losses. With 80% coupling efficiency, in excess of 9 W of pump power is coupled, and in excess of 8 W is actually absorbed by the active medium. Typically, 40% conversion efficiency is anticipated for an Er/Yb amplifier, resulting in more than 3 W output from a single amplifier stage. If more power is required, several stages may be employed with Faraday isolators spliced in between. 
     Mating of Launch Port and Fiber 
     FIG. 12 is a cross section of FIG. 11 (or any other port embodiment) emphasizing the interface between adjacent launch sections (shown with seven such sections) of a fiber having an inner cladding  40  with rectangular cross-section around doped core  71 . Material  70  on the concave side of the launch region may be a support block  46 , or a substrate on a support block, or simply air or a low-index coating. Interface  47 , between material  70  and the fibers of the launch region, may include index-matching optical glue. The launch sections are closely adjacent each other. Material  89  of launch port  90  mates to the launch region along interface  75 , which may include index-matching optical glue. The dotted line shows the bottom of launch port  90  where the launch region no longer intrudes. Since the cladding thickness is constant, each point of interface  75  is essentially plane-parallel to the corresponding point of interface  47 . 
     FIG. 13 shows the same interface using a fiber of circular cross section. Material  89  effectively conforms to cladding  40  of the adjacent fibers, which may be effected, for example, by machining launch port  90 , preferably in conjunction with index-matching optical glue along interface  75 . Material  70  may be, for example, a combination of optical glue on a substrate, or may be an optical material which compliantly conforms to interface  47 . Material  70  need not be solid to provide physical support, but may be air, or simply a low-index coating. 
     The present invention has been described in its preferred and alternative embodiments. It must be noted that each embodiment is further functional with a wide range of length for pump source diode, at least from 200 micron to 10,000 micron long. Moreover, the invention may be practiced with a wide range of gain mediums as are now or may become known in the art, and with a wide range of fiber sizes and materials. It is readily scalable to higher or lower powers, and is susceptible to numerous modifications and embodiments within the ability of those skilled in the art. Thus, it should be understood that various changes in form and usage of the present invention may be made without departing from the scope of this invention, and the invention is accordingly defined only by the claims which follow.