Tapered fiber laser

A tapered fiber laser having a multi-mode section, a single-mode section, and either a tapered section or fundamental mode matching junction therebetween. The multi-mode section has a large core to directly receive pump light from a broad stripe laser or diode bar, and a length preferably longer than the absorption length of the pump light (so optical amplification occurs predominantly in the multi-mode section). Doping levels can be increased to reduce the multi-mode length. The taper angle is sufficiently small to produce adiabatic compression of the fundamental mode from the multi-mode to single-mode sections, and acts as a cutoff filter favoring lasing of the fundamental mode within the multi-mode section. Alternately, the step junction may have a mode field diameter matched to the lowest-order mode, with laser light output via the single-mode section. The invention can be applied to waveguides (particularly those having an aspect ratio corresponding to a broad stripe laser source), doped with ytterbium or neodymium ions, and is particularly advantageous as a pump source for an erbium-doped fiber amplifier (EDFA).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to optical amplifiers and lasers,
 and in particular to fiber lasers.
 2. Technical Background
 Optical fiber is increasingly becoming the favored transmission medium for
 telecommunications due to its high capacity and immunity to electrical
 noise. Silica optical fiber is relatively inexpensive, and when fabricated
 as a single-mode fiber can transmit signals in the 1550 nm band for many
 kilometers without amplification or regeneration. However, a need still
 exists for optical amplification in many fiber networks, either because of
 the great transmission distances involved, or the optical signal being
 split into many paths.
 Erbium-doped fiber amplifiers (EDFAs) have been found quite effective in
 providing the required optical gain. As illustrated schematically in FIG.
 1, a conventional EDFA is interposed between an input transmission fiber
 and an output transmission fiber 14. Both transmission fibers 12, 14 need
 to be single-mode, because higher-order modes exhibit much greater
 dispersion (typically the limiting factor for the fiber transmission
 distance at high data rates). The EDFA includes a length (on the order of
 tens of meters) of an erbium-doped silica fiber 16, as is well known in
 the art. The doped fiber 16 should also be single-mode in order to
 maintain the transmission signal integrity. The doped fiber 16 is
 optically active due to the presence of Er.sup.3+ ions, which can be
 excited to higher electronic energy levels when the doped fiber 16 is
 pumped by a strong collinearly-propagating optical pump signal. Typically,
 an optical pump source 18 inputs the pump signal into the doped fiber 16
 through a pump source fiber 20 coupled to either the undoped upstream
 fiber or the doped fiber 16 through a wavelength-selective directional
 coupler 22, but downstream coupling is also known. Again, for integrity of
 the transmission signal, the pump source fiber 20 should be single-mode.
 An operative EDFA may contain some additional elements (such as an
 isolator) which are well known to the art but not relevant to the
 understanding of the background of the present invention.
 Conventionally, one typical pump source 18 has been an edge-emitting
 semiconductor laser that includes a waveguide structure (in what is called
 a "stripe" structure) that can be aligned with the single-mode pump source
 fiber 20 to provide effective power coupling. However, this approach has
 failed to keep up with modern fiber transmission systems incorporating
 wavelength-division multiplexing (WDM). In one approach to WDM, a number
 of independent lasers inject separately-modulated optical carrier signals
 of slightly different wavelengths into the transmission fiber 12. The EDFA
 has sufficient bandwidth to amplify carrier signals within about a 40 nm
 bandwidth. A large number of multiplexed signals to be amplified require
 in aggregate a proportionately large amount of pump power. Over the past
 decade, the number of WDM channels preferably utilized in a standard
 network has increased from about four to current levels of forty or more,
 but at best the output power from a single-stripe laser source has only
 doubled. Derivative designs such as a master oscillator power amplifier (a
 single-mode stripe followed by a broad stripe amplifier) or
 flared-semiconductor devices are capable of producing more than one watt
 of optical output power, but many of these designs have been subject to
 reliability problems (such as back-facet damage caused by feedback) that
 have hindered their practical deployment as fiber amplifier pumps.
 Another approach uses WDM technology to combine pump signals. Multiple
 single-stripe lasers are designed to emit light at narrowly-spaced
 wavelengths, usually within the wavelength bands of 970-990 nm or
 1460-1500 nm. Wavelength-dependent directional couplers combine these
 multiple optical waves into a single (somewhat broadband) pump signal.
 While this approach increases the power available for optical amplifiers,
 it greatly adds to the complexity of the pump source, and requires
 additional components such as thermoelectric coolers, fiber gratings, and
 directional couplers. As a result, this approach increases cost.
 An alternative approach for high-power pump lasers has involved fiber
 lasers that are pumped through their cladding. That is, a large outer
 cladding supports the pump signal from a primary pump source, and an inner
 cladding supports a single-mode output signal that is used as the
 secondary pump source for the EDFA. The core is typically doped to provide
 lasing capability. Typically, a neodymium- or ytterbium-doped double-clad
 fiber is pumped with a high-power diode optical source (at 800 nm or 915
 nm) to produce a single transverse mode (at 1060 nm or 1120 nm,
 respectively). One of these modes then pumps a cascaded Raman laser to
 convert the wavelength to around 1480 nm, which can then pump erbium. To
 date, such a design by itself (that is, without an additional Raman
 oscillator) does not produce an output in any of the appropriate
 absorption bands for EDFAs.
 Double-clad fiber lasers offer superior performance for four-level lasing
 (that is, where the lasing occurs in a transition between two excited
 states). In such a case, the doped core is still transparent at the laser
 signal wavelength when not being pumped. As a result, the power threshold
 for lasing depends essentially on the dimensions of the doped core, and
 the background loss in the fiber over the pump absorption length. However,
 ytterbium and neodymium ions (Yb.sup.+3 and Nd.sup.+3) provide three-level
 lasing systems at around 980 nm and 940 nm, respectively. In a three-level
 system, the lasing occurs from an excited level to either the ground state
 or a state separated from it by no more than a few kT (that is, thermally
 mixed at operating temperature). As a result, an unpumped doped core
 strongly absorbs at the laser wavelength, and the lasing power threshold
 can become a problem.
 Ytterbium has offered much promise as a pump for high-powered EDFAs. It is
 well known that Yb.sup.3+ ions exhibit gain in a narrow 6 nm-wide
 three-level transition at 976 nm, and in a broad quasi-three-level
 transition peaked at 1030 nm (but extending as far as 1140 nm). The latter
 transition requires a population inversion of only a few percent for
 transparency, while the former requires at least a fifty-percent
 inversion.
 Thus, a source based on the 976 nm Yb.sup.+3 transition has long been
 suggested as a pump for EDFAs. However, a single-stripe diode laser
 remains the most efficient pump structure. The problem potentially lies in
 the relationship between the gains in the two transitions and the pump
 absorption. As a representative example, the gains at the two wavelengths
 in Yb-doped germano-alumino-silicate glass (assuming homogeneous
 broadening) are related by the equation:
 ##EQU1##
 where G.sub.1030 and G.sub.976 are the gains at 1030 nm and 976 nm,
 respectively, .alpha.p is the partially-bleached absorption in decibels
 (dB), and .GAMMA..sub.S and .GAMMA..sub.P are the respective overlap
 factors of the signal mode and pump mode with the dopant profile.
 Double-clad fibers allow coupling from diode bars and other similar active
 structures. However, this is accomplished by a greatly-reduced pump
 overlap with the doping profile relative to the signal overlap, since the
 doping needs to be confined in or close to the signal core in order to
 obtain sufficient optical gain for the core mode at the signal wavelength.
 Typically, the core is uniformly doped, and the area ratio between the
 pump waveguide and the signal core is on the order of 100:1. As a result,
 .GAMMA..sub.S.apprxeq.1 and .GAMMA..sub.P &lt;0.01. Using these values in
 Equation (1), each 1 dB of pump absorption produces about 74 dB of gain at
 1030 nm. Even with weak pumping, amplified spontaneous emission (ASE) at
 1030 nm will saturate the amplifier and prevent a buildup of the
 population inversion necessary for lasing at 976 nm. In fact, even without
 an optical cavity, lasing at the longer four-level wavelengths is possible
 from the backscatter. Hence, high pump absorption will favor gain at 1030
 nm or longer even if the laser mirrors are tailored to 976 nm.
 If the fiber laser uses a single-clad fiber with both the pump and lasing
 signal confined to the one core, the ratio of the two overlap functions
 approaches unity, and the 976 nm transition can be selected simply by
 limiting the fiber length so that insufficient gain exists for lasing at
 1030 nm and longer. For a typical fiber laser with a round-trip end loss
 of about 14 dB (due to four percent reflectance at the cleaved output
 facet), 15 dB of pump absorption will cause the 976 nm transition to lase,
 but not the 1030 nm transition. However, this solution does not address
 the need to produce high output power into a single-mode fiber.
 It is thus desirable to find a more efficient method of pumping the 976 nm
 transition in an ytterbium-doped fiber amplifier.
 SUMMARY OF THE INVENTION
 The present invention may be summarized as a fiber (or other dielectric
 waveguide) laser pumpable by a diode laser of potentially large size. The
 fiber laser includes an optical fiber doped with an ion such as neodymium
 (Nd.sup.3+) or ytterbium (Yb.sup.3+) which can be optically pumped. The
 optical fiber includes a first section receiving the pump light and being
 multi-mode to the resultant lasing radiation. The multi-mode first section
 is connected to a second section having a core of decreasing diameter
 extending away from the first section so as to adiabatically decrease the
 fundamental mode spot size. The second section may be connected to a
 single-mode fiber to output the lasing light. Because the fundamental mode
 spot size of the multi-mode fiber or of the first section is not the same
 as the fundamental mode spot size of the single-mode fiber or of the
 second section, mode transformation occurs and is taught by the present
 invention.
 Alternatively, the tapered section can be replaced (in full or in part) by
 a junction between two fibers forming the multi-mode and single-mode
 sections, both designed to provide equal diameters for the lowest-order
 mode at the junction.
 The invention also applies these same concepts to other shapes of
 dielectric waveguides, whether freestanding or formed on a substrate. A
 representative example is a rectangular planar waveguide with an aspect
 ratio matching that of the diode laser pumping the laser.
 The invention is particularly advantageous when used as a pump source for
 an erbium-doped fiber amplifier (EDFA), such as may be found in
 single-mode fiber optic communication networks or systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The optically-active fiber, fiber amplifier, fiber laser, or dielectric
 waveguide laser of the present invention is shown in FIGS. 2 and 4-7, and
 is generally described and depicted herein with reference to several
 exemplary or representative embodiments with the same numbers referenced
 to the same or functionally similar parts.
 Referring particularly to FIG. 1, erbium-doped fiber amplifiers (EDFAs) are
 known to the art and are effective in providing the required optical gain
 within wavelengths operational for telecommunications or digital
 signal-processing applications. As illustrated schematically in FIG. 1, a
 conventional EDFA is interposed between an input transmission fiber and an
 output transmission fiber 14. Both transmission fibers 12 and 14 are
 preferably single-mode, because higher-order modes exhibit much greater
 dispersion (typically the limiting factor for the fiber transmission
 distance at high data rates). The EDFA includes a length (on the order of
 tens of meters) of an erbium-doped silica fiber 16, as is well known in
 the art. The doped fiber 16 should preferably also be single-mode, in
 order to maintain the transmission signal integrity. The doped fiber 16 is
 optically active due to the presence of Er.sup.3+ ions or other rare-earth
 metals, which can be excited to higher electronic energy levels when the
 doped fiber 16 is pumped by a strong collinearly-propagating optical pump
 signal. Typically, an optical pump source 18 inputs the pump signal into
 the doped fiber 16 through a pump source fiber 20 coupled to either the
 undoped upstream fiber 12 or the doped fiber 16 through a
 wavelength-selective directional coupler 22, but downstream coupling is
 also known. Again, for integrity of the transmission signal, the pump
 source fiber 20 should preferably be single-mode. An operative EDFA may
 contain some additional elements (not shown) which are well known to the
 art but not relevant to understanding the background of the present
 invention.
 One typical pump source 18 is an edge-emitting semiconductor laser that
 includes a waveguide structure (in a "stripe" structure or configuration)
 that can be aligned with the single-mode pump source fiber 20 to provide
 effective power coupling.
 As suggested above, efficient coupling between a large multi-mode pump
 source and a single-mode fiber laser can be facilitated according to the
 present invention by including both single-mode and multi-mode sections
 within the optical cavity of a fiber laser.
 As illustrated in the cross-sectional view of FIG. 2, an input side of a
 tapered fiber amplifier 30 is irradiated with a pump signal at wavelength
 .lambda..sub.P. The input side includes a length L.sub.MM of doped
 multi-mode fiber 32, preferably for our stated purposes doped with
 ytterbium ions (Yb.sup.3+). The multi-mode fiber 32 includes a core 34 and
 a cladding 36. No attempt has been made to accurately illustrate their
 relative diameters. Also, it is possible to use air (n=1) as the cladding.
 The output side includes a length of single-mode fiber 40, also composed
 of a core 42 and a cladding 44. The single-mode length is relatively
 unimportant beyond its being very long compared to the wavelengths
 involved so that any higher-order modes are adequately attenuated over its
 length. In most circumstances 1 cm of single-mode length is more than
 adequate. The multi-mode section 32 is joined to the single-mode section
 40 through an adiabatically tapered section 48 composed of a core 52 and a
 cladding 54. The tapered section 48 has a length L.sub.T, but a more
 relevant parameter is a taper angle .theta..sub.T which is of the order of
 tens of milliradians (1/2.degree.), as will be discussed later.
 As is well known, if a fiber is below a certain diameter it can support
 only a single transverse mode. Above that diameter, two or more transverse
 modes are supported. The larger the diameter, the larger the number of
 modes. For a simple optical fiber having a core of refractive index
 n.sub.core and having a thick cladding of refractive index n.sub.clad, the
 maximum core diameter d.sub.SM supporting only a single mode is given by
 the equation:
 ##EQU2##
 The relevant wavelength .lambda. is that of the lasing light, which is 976
 nm for a Yb.sup.3+ -doped fiber. Optical fibers of other designs have more
 complicated cross sections including additional layers at the
 core-cladding transition or a continuously graded refractive index across
 the transition. Numerical solutions for the maximum single-mode diameter
 are available for some of these. For others, whether the fiber supports
 one or more transverse modes at a particular wavelength can be determined
 experimentally.
 The tapered fiber laser 30 includes two mirrors 60, 62 defining the input
 and output ends respectively of the optical cavity. The multi-mode section
 32, the tapered section 50, and the single-mode section 40 are all
 included within the optical cavity. The input mirror 60 is made highly
 transmissive to an optical pump signal 64 at the pump wavelength
 .lambda..sub.P and highly reflective at the signal (lasing) wavelength
 .lambda..sub.S of the output signal 66 while the output mirror 62 is made
 partially reflective (partially transmissive) at the signal wavelength
 .lambda..sub.S. For fiber lasers, it is possible to use a cleaved output
 facet as the output mirror. Even its 4% reflectance across an air gap to a
 butt coupled output fiber 74 of FIG. 4 is sufficient to define the optical
 cavity. Thereby, the pump signal 64 is efficiently admitted into the
 optical cavity at the input mirror 60, an optical cavity is defined
 between the mirrors 60, 62, and some of the standing wave in the optical
 cavity is allowed to pass through the output mirror 62.
 Although interference filters can be used as one or both of the end mirrors
 60, 62, Bragg grating reflectors are conventionally written directly onto
 fibers by UV patterning. If the single-mode section 40 is fusion spliced
 to the output fiber 74, a low-reflectivity grating or a 4% reflection from
 a downstream pigtail can provide feedback. Laser efficiency is relatively
 insensitive to output coupling unless the cavity has high loss. It is
 further possible to discriminate against the 1030 nm mode by making one of
 the end reflectors preferentially transmissive to 1030 nm, but such
 discrimination is not necessary if the fiber length is limited so that
 insufficient gain exists for lasing at 1030 nm and longer, as has been
 discussed in the background section for single-mode fiber lasers.
 For the preferred ytterbium fiber laser, the signal wavelength
 .lambda..sub.P equals the 976 nm Yb.sup.3+ transition. The pump signal may
 be provided by AlGaAs or InGaAs broad stripes, arrays, or diode bar
 emitting at a wavelength shorter than 976 nm but within the ytterbium
 absorption band. The practical pump band extends from 850 to 970 nm with a
 more preferred range being 910-930 nm and a most preferred range being
 915-920 nm. The precise values of these bands and the lasing wavelength
 may shift by a few nanometers depending upon the dielectric host.
 The tapered section 50 acts as a cutoff filter passing the lowest-order
 mode but blocking any higher-order mode. It is important that the taper
 angle .theta..sub.T be kept small enough that the lowest-order mode passes
 through the taper without being mixed into other modes as the mode size
 shrinks from the size of the multi-mode fiber to the size of the
 single-mode fiber. This condition is called adiabatic coupling. Thereby,
 only the fundamental mode is coupled into the single-mode section with
 minimum loss of power. Also importantly, adequate lowest-order feedback
 from the output mirror 62 back into the multi-mode section 32 causes the
 multi-mode section 32 to lase only in the fundamental mode, thus greatly
 conserving power.
 Tapered optical fibers have previously been proposed to expand the mode
 field diameter coming from a single-mode fiber. That is, up-tapered fibers
 are known. These devices have been shown to operate with low loss, that
 is, with negligible coupling to higher-order modes up to spot sizes of 50
 .mu.m. The V-value at this spot size is approximately 30 for a typical
 0.2NA (numerical aperture) fiber, and this V-value corresponds to about
 500 guided modes supported in the multi-mode fiber. The limiting factor is
 mode coupling between the LP.sub.01 mode and the LP.sub.02 mode. This
 coupling can be kept to negligible values provided that the local taper
 angle .theta..sub.T (Z) be
 ##EQU3##
 where .beta..sub.01 and .beta..sub.02 are the propagation constants of the
 LP.sub.01 and LP.sub.02 modes and a(z) is the local core radius. The
 propagation constants vary strongly with the core radius resulting in the
 limiting taper angle .theta..sub.T,max in milliradians plotted as a
 function of core diameter d.sub.core in FIG. 3 for a 0.6NA fiber. Clearly,
 the condition becomes stricter as the fiber diameter increases, but even
 at 50 .mu.m, it is still 3.8mR (0.218 degree). The worst condition occurs
 for the core diameter of the multi-mode section. A constant taper over as
 short a length L.sub.T as 5 mm will show negligible loss to other modes. A
 parabolic rather than linear taper produces the shortest adiabatic length.
 Minute imperfections in the multi-mode fiber will cause some mode mixing
 from the fundamental to the higher-order modes. This type of mode mixing
 can be reduced by heavily doping the fiber with either Yb.sup.3+ or
 neodymium (Nd.sup.3+). In highly doped fibers, L.sub.abs
 &lt;&lt;L.sub.max, where Labs is the absorption length (inverse to doping
 concentration) and L.sub.max is the length corresponding to significant
 mode coupling due to the minute imperfections, the doping or fiber length
 is chosen such that absorption length nearly equals or is somewhat less
 than the length of the multi-mode fiber, L.sub.abs.apprxeq.L.sub.MM. If
 the multi-mode fiber were substantially longer, L.sub.MM
 &gt;&gt;L.sub.abs, lasing at 1030 nm would be favored, as was previously
 discussed. A length of no more than several centimeters is possible. An
 absorption of 15 dB in the multi-mode fiber means that a large fraction of
 the pump power is absorbed. This amount of absorption corresponds to about
 five absorption lengths, that is, L.sub.MM.ltoreq.5L.sub.abs. In practice,
 we may use a length L.sub.MM shorter than 5L.sub.abs, to trade off
 unabsorbed pump power against threshold.
 The illustration implies that the three sections 32, 50, 40 are drawn from
 a common doped preform. However, different fibers may be spliced together
 to form the illustrated structure. Because the pump light is mostly
 absorbed in the multi-mode section 32, only that section needs to be doped
 to absorb the pump light and provide the excitable states. Indeed, even
 the multi-mode section 32 can be divided into a doped and an undoped
 section.
 The most likely loss of the lowest-order mode is caused by coupling into
 higher-order modes at the end of the multi-mode section 32 next to the
 input mirror 60 as a result of an imperfect end face angle. For a 50m spot
 size, roughly the core diameter, a tolerance of .+-.0.23 degree is
 required to achieve less than 1 dB loss from the lowest-order mode. This
 tolerance is obtainable by careful cleaving or polishing.
 This design allows the major part of the optical gain to be obtained in the
 wide multi-mode section 32, particularly when the multi-mode length
 L.sub.MM is made longer than the absorption length L.sub.abs at the pump
 wavelength .lambda..sub.P, that is, longer than 1/.alpha.p. However, the
 output of the tapered fiber laser is a single, fundamental mode.
 As illustrated in the schematic view of FIG. 4, the similar aspect ratios
 of the diode laser 72 and of the input of the multi-mode section 32 (both
 vertically or horizontally aligned alike) allows a lens or fiber-optic
 coupler, optical exciter, or other beam shaper or focusing element 70 of
 FIG. 4 to focus the relatively large-size output of a wide stripe or
 "broad area" laser diode 72 or even a diode bar into the wide multi-mode
 core of the tapered fiber laser 30. The optical characteristics of a broad
 stripe laser may be good enough to allow direct coupling into the
 multi-mode fiber. A single-mode fiber 74 is butt coupled to the output end
 of the single-mode section 40. If the tapered fiber laser 30 is being used
 as a pump source for an EDFA or other doped optical amplifier, the
 single-mode fiber 74 is the pump fiber 20 of FIG. 1.
 The inclusion of the multi-mode fiber within the optical cavity avoids the
 problems of preferential lasing of 1030 nm radiation as discussed above
 with respect to Equation (1). That is, the fiber has only a single core
 and cladding and the overlap ratio .GAMMA..sub.S /.GAMMA..sub.P is close
 to unity. Almost all the optical gain can be confined to the multi-mode
 section 32 by requiring its length L.sub.MM be greater or at least not
 substantially less than the absorption length L.sub.abs =1/.alpha.p. Since
 the lasing predominantly occurs in the multi-mode fiber, the threshold
 power for lasing is increased over what it would be for a much smaller
 single-mode fiber.
 Preferably, the core 34 of FIG. 2, for example, is doped with optically
 excitable ions to form overlap factors .GAMMA..sub.S and .GAMMA..sub.P of
 the signal mode and pump mode with the dopant profile such that the
 overlap ratio .GAMMA..sub.S /.GAMMA..sub.P is between 0.1 and 10, and more
 preferably and usable between 0.2 and 5. Such overlap ratios should
 prevent low area ratio clad-pumping while allowing the dopant to be
 confined to the multi-mode core 34.
 Since almost all of the gain is obtained in the multi-mode fiber, the power
 threshold for lasing is increased over what it would be for single-mode
 fiber. The threshold power P.sub.t scales in proportion to the core area
 and the length of the device. It is well approximated by:
 ##EQU4##
 where .sigma..sub.a is the pump absorption cross section, .tau. is the
 fluorescent lifetime, .pi.a.sup.2 is the core area, and .alpha.p is the
 pump absorption in dB.
 The core area will be dictated by the brightness of the laser diode used as
 the pump source and the numerical aperture (NA) of a step-index fiber
 given by:
EQU NA=n.sub.core.sup.2 +L -n.sub.clad.sup.2 +L (5)
 For simple geometrical imaging of a broad stripe diode facet into the
 fiber, the product of the stripe width and the diode divergence angle in
 the plane of the junction sets the limits on the design. For a typical 100
 .mu.m broad stripe laser, the parallel divergence angle corresponds to an
 NA of approximately 0.2. For efficient coupling into a 30 .mu.m multi-mode
 core, a fiber NA of greater than 0.6 is desired. For a 15 .mu.m core, an
 NA of 1.3 is needed. These values represent very high contrast between the
 core and cladding and are higher than available in standard silica fiber.
 However, they can be achieved with a multi-component core and a silica
 cladding. Tantalum silicate and lanthanum alumina silicate fibers have
 been fabricated with a high refractive index relative to silica. Almost
 any multi-component fiber will give a high refractive index, for example,
 those based on phosphates, lead silicates, and germanates. However, the
 chemical and physical properties of the core must be compatible with the
 cladding. It is known that Yb.sup.3+ and Nd.sup.3+ can be doped into
 glasses other than silica and produce nearly the same transition levels.
 The use of beam-shaped laser diodes with reduced asymmetry in their M.sup.2
 values in the x- and y-directions eases the requirement for high values of
 NA as will be discussed later.
 Assuming a multi-mode core diameter of 30 .mu.m, the threshold power for 10
 dB of pump absorption is of the order of 1W. Two polarization multiplexed
 broad stripe lasers each producing 2W of output power can be coupled into
 a tapered Yb fiber laser to produce 2.7W of output power at 976 nm using
 some conventional assumptions.
 A second embodiment of the invention is illustrated in the cross-sectional
 view in FIG. 5 of a stepped fiber amplifier 80 having a uniform multi-mode
 fiber 82 with a core 84 and a cladding 86 and a uniform single-mode fiber
 88 with a core 90 and cladding 92. The multi-mode fiber 82, similarly to
 the multi-mode section 32 of FIG. 2, receives the pump light 64 and
 provides most of the optical amplification. The single-mode fiber 88 is
 butt coupled at a junction 94 to the multi-mode fiber 82, for example by a
 splice or other connection, and effectively outputs a lasing signal 66
 that is only the fundamental mode. The mode field diameters of the
 lowest-order modes are matched in the two fibers 82, 88, as is suggested
 in FIG. 5 by their cores 84, 90 having the same diameter although the mode
 size depends as well on the refractive indices of the core and cladding.
 As a result, the multi-mode fiber 82 has a larger contrast between its
 core 84 and cladding 86 and is thus a high-NA fiber while the single-mode
 fiber 88 has a lesser contrast between its core 90 and cladding 92 and is
 thus a low-NA fiber.
 The design of the stepped fiber amplifier 80 can be combined with that of
 the tapered fiber amplifier 30 to form a partially tapered fiber amplifier
 100, illustrated in cross section in FIG. 6. It includes the multi-mode
 section 32 of FIG. 2 and a partially tapered section 102 of construction
 similar to the tapered section 50 of FIG. 2, but it does not fully taper
 down to a single-moded end. Instead, its smaller end 104 continues to
 support multiple modes but fewer modes than its end 106 adjacent to the
 multi-mode section 32. Its smaller end 104 is butt coupled across a
 junction 108 to a single-mode fiber 110 having a core 112 and a cladding
 114. Similarly to the stepped design of FIG. 5, the mode field diameters
 for the respective lowest-order modes are matched across the junction 94
 between the smaller end 104 of the partially tapered section 102 and the
 single-mode fiber 110. As a result of the mode-matching requirement, the
 multi-mode section 32 and the partially tapered section 102 have larger
 contrast between their respective cores and claddings and thus form a
 high-NA fiber, while the single-mode fiber 110 has lesser contrast and
 forms a low-NA fiber.
 It is possible to pump directly into a tapered waveguide. However, the
 taper angle must be further reduced so that the pump light does not leak
 out of the tapered waveguide. It is generally preferred to not begin
 tapering until a significant fraction of the pump light is absorbed.
 Cylindrical fibers are only one example of dielectric waveguides. Fiber can
 be drawn into other shapes, for example, ellipses or rectangles.
 Furthermore, rectangular dielectric waveguides can be formed on planar
 substrates by techniques similar to those used in semiconductor
 fabrication or other techniques, such as ion-exchange, sputtering, plasma
 enhanced chemical vapor deposition, flame hydrolysis, and LiNbO.sub.3
 technology including diffusion doping. A simple example of such a planar
 waveguide is illustrated schematically in the orthographic view of FIG. 7.
 A rib waveguide 120 is formed on the top surface 122 of a dielectric
 substrate 124 having a lower refractive index than that of the rib
 waveguide 120. A separate unillustrated upper cladding may be formed over
 the rib waveguide 120 or air can serve as the upper cladding. The rib
 waveguide 120 includes a rectangular doped multi-mode section 126, a
 rectangular tapered section 128, and a nearly square single-mode section
 130. Other forms of planar waveguides are possible, including those
 tapered in two dimensions and those having a stepped transition between
 the multi-mode and single-mode sections. The formation of a rectangular
 waveguide on a planar substrate is additionally advantageous because it
 allows the integration of the laser diode on the same substrate. Similar
 structures can be drawn from a preform.
 The rectangular cross section of the multi-mode section 126 is particularly
 advantageous because its entrance face 132 can be more easily matched to
 the emission pattern of a wide stripe laser, which may have a
 height-to-width aspect ratio (AR) of 1:100. That is, the width of the
 entrance face 132 can be made substantially greater than its height, which
 will be defined as a low aspect ratio. Further, the height can be made to
 correspond to a single mode, thus eliminating the need for vertical
 tapering. The light emission from a broad stripe laser may be considered
 as an image being focused on the waveguide face 132. The image of a broad
 stripe diode laser is substantially wider in the plane of the diode chip
 than it is in the vertical direction, for example, tens of microns versus
 a few microns. Thereby, nearly all of the laser diode power can be easily
 coupled into the waveguide while maintaining a high optical pump power
 density. The high power density allows a lower power threshold for lasing
 than that available in circular or square waveguides.
 The threshold powers for various waveguides have been calculated as a
 function of the numerical aperture of the waveguide. In the example
 plotted in FIG. 8, the waveguide has a value of 20 .mu.m for the product
 of the length of the major axis and the numerical aperture, for example,
 an NA of 0.2 and a major axis of length 100 .mu.m. The top curve is
 calculated for a square multi-mode waveguide having a value of unity for
 the aspect ratio AR of the height to the width. The next lower curve is
 for a circular multi-mode waveguide. As the aspect ratio of the
 rectangular multi-mode waveguide drops, the threshold power is
 significantly decreased. For rectangular aspect ratios of less than .pi./4
 or 0.785, the rectangular multi-mode waveguide has a smaller threshold
 power than a circular one For example, for a waveguide with a numerical
 aperture of 0.6, the threshold power is reduced from 900mW for a circular
 33 .mu.m fiber to 200mW for a rectangular waveguide having an aspect ratio
 of 0.3 (33 .mu.m.times.11 .mu.m). These dimensions are consistent with
 image sizes of broad stripe diode lasers. This reduction in threshold
 power is greatly advantageous if a 1W diode is the limit of commonly
 available broad stripe pump sources.
 The inventive concepts presented above for cylindrical fibers can be
 applied as well to these other dielectric waveguides, that is, the
 waveguide being multi-moded on the pump end and single-moded on the output
 end and the various ways of achieving the transition in the two
 geometries.
 Although the invention is developed in view of Yb.sup.3+ doping, it is not
 so limited. The tapered fiber laser may be doped with other transitional
 or rare-earth ions, such as Nd.sup.3+. A combination of Yb and Nd doping,
 either by co-doping or by a sequence of differently doped fibers allows
 pumping at 800 nm rather than 920 nm.
 The invention has been described in terms of a fiber-based oscillator,
 however the invention is generally applicable to any multi-mode waveguide
 oscillator incorporating a transverse mode-selection filter as described
 and claimed herein.
 Referring to FIGS. 9-14, a modification of Corning Inc.'s multi-clad.TM.
 coupler technology is used to form a taper 30 or an asymmetric coupler 400
 as a mode transformer 300 by using tapering and core-diffusion for
 maximizing pump brightness in a brightness converter. Since the basic
 difference between a single-mode fiber and a multi-mode fiber is their
 core diameter size difference, the mode transformer 300 basically
 transitions itself from a larger core diameter to a smaller core diameter
 to match multi-mode to single-mode.
 In one embodiment of the invention, a rectangular input core 342 is
 employed to reduce the active area of the doped core without compromising
 the coupling efficiency between the pump diode 72 and a multi-mode fiber
 740 to match the elliptically-shaped pump radiation or emission. Other
 input core cross-sections of other shapes, for example, elliptical or any
 other matching-beam shape, can be used to match the shape of the pump
 emission area. However, it is desirable for the output of the fiber laser
 to have a substantially circular mode field as its output cross-section or
 core diameter 340. It is desirable for the output of the fiber laser to
 have a substantially circular mode field because a conventional fiber has
 a circular mode field and the better the mode field size and shape match,
 the lower the coupling loss. Even though the core 34 can be of other
 shapes, the rectangular aspect is preferred for this fiber laser
 application since the pump laser diode 72 also has a rectangular beam.
 A novel technique has been developed where the aspect ratio of the
 rectangle can be reduced by a dissolution and expansion process during
 tapering to form a mode transformer or optical transitioner 300 for
 transitioning multi-mode to single-mode. For example, if the multi-mode
 fiber 740 is made by the technique described in the pending patent
 application Ser. No. 09/103,655, entitled "Composition for Optical
 Waveguide Article and Method for Making Continuous Clad Filament" (now
 U.S. Pat. No. 6,128,430) or in the provisional patent application No.
 60/097,876, entitled "Methods and Apparatus for Producing Optical Fiber,"
 both by Chu et al. and assigned to the same assignee, a soft
 multi-component core glass 34 is surrounded by a silica cladding 36 by
 drawing the cladding tube filled with molten core cullets, filaments, rods
 or other shaped core material. The core material is placed within the
 cladding tube which has a lower refractive index than the core's
 refractive index. Optionally, as seen in FIGS. 12 and 13, for extra
 stability or for further aspect ratio control, an overclad tube, cane,
 sleeve, or jacket 360 made of the same cladding material as the first
 cladding tube or some other composition such that the overclad has a
 refractive index the same as the first cladding tube or lower to control
 the numerical aperture of the transformed fiber by building up more
 cladding material around the aperture or reduce the aperture,
 respectively. An exemplary composition of the overclad tube 360 is boron
 doped silica.
 The core glass is selected to melt at or below the softening point of the
 cladding tube and that the thermal expansion difference between the core
 and clad not be so large as to shatter the resultant fiber upon cooling.
 Hence, a difference in the softening point of the core and the softening
 point of the cladding is preferably at least 100 degrees Celsius.
 Furthermore, it is preferred that the core exhibit a viscosity of less
 than 10.sup.4 poise at a temperature at which the cladding exhibits a
 viscosity of 10.sup.7.6 poise.
 Since the core glass 34 is selected to be of a different material such that
 it is much softer than the pure silica cladding 36, the core glass 34 will
 be very fluid and conform to the geometry dictated by or conform to the
 cladding 36 when heated to the temperatures required to draw fiber and,
 according to the teachings of the present invention, to taper the
 resulting fiber 40. The shape of the core does not matter, it is more the
 inventive concept that the NA can be dropped and the shape can be made
 more circular with these soft glass cores. The fluid core 34 will begin to
 dissolve the silica clad 36, lowering the refractive index of the core 34
 and providing a better NA match to standard single-mode fibers 20 on the
 output end of the taper 340.
 Any time two dissimilar materials are placed in contact, there will be a
 driving force (created by the chemical potential difference between the
 two materials) for mixing. The speed at which the mixing will occur will
 depend on the diffusion coefficients of the migrating species and the
 viscosity of the glass. The more fluid the glass or more mobile the
 species, the faster the mixing will occur. In this inventive case, the
 core glass is quite fluid (viscosity&lt;100 Poise) during the taper 48 or
 coupler 400 (see FIG. 11) draw. The core glass begins to dissolve the
 silica clad, just as hot water will dissolve sugar faster than cold water,
 and ice will not dissolve sugar much at all (at least not in normal time
 scales) since the ice is now a solid and the diffusion coefficients are
 not very high.
 If the core is rectangular, the dissolution rate is faster in the direction
 of the minor axis due to mass transport constraints along the major axis.
 The core 34 can be initially made rectangular by starting with a cladding
 tube having a rectangular channel, groove, or other shaped aperture,
 inside the tube for depositing core cullets within, as described in
 60/097,876 or in a co-pending patent application (D14631) assigned to the
 same assignee, Ser. No. 09/377,926 entitled "Method for Making Fibers
 Having Cores with Non-Circular Cross-Sections, Fibers, Fiber Lasers and
 Amplifiers Formed Thereby, and Systems Incorporating Such Fibers as
 Brightness Converters" filed on the same date as the present invention and
 incorporated herein by reference.
 Because the core is much softer than the cladding, the core will become a
 liquid during the tapering process in between sealed ends of the fiber.
 The thermal expansion of a liquid glass is two to three times that of a
 solid glass, causing the core to become pressurized. The hydrostatic
 pressure in the core will exert a force proportional to the area on which
 it acts. Hence the larger sides of a rectangular or elliptical core will
 experience a greater outward force pushing the sides into a more circular
 output cross-section. Referring additionally to FIGS. 12 and 13, the
 aspect ratio of a mode transforming taper 48 can be reduced, in accordance
 with the teachings of the present invention, from the larger rectangular
 input cross-section 342 to the smaller circular output section 340, as the
 neckdown region 102 is drawn and the aspect ratio and NA of the core are
 both reduced by core diffusion/dissolution and expansion. More
 specifically, the taper or neckdown region 102 is made by heating a
 section of the fiber 740 and pulling under tension like pulling taffy. The
 tension is provided by an outer tube or sleeve (housing or preform) 360
 through which the rectangular core fiber 36 was originally inserted
 through so that it now surrounds and changes shape as the fiber 36 is
 heated and drawn to the desired configuration. Preferably, the material of
 the overclad tube or housing is made from the same or similar cladding
 material, such as boron doped silica.
 In yet another embodiment, a much stiffer overclad tube could be used to
 enhance diffusion and dissolution in a fiber with substantially matched
 core and clad softening points. Thus, at the high temperature required to
 draw the overclad tube, the viscosity of the core and clad would be
 reduced, and the diffusion/dissolution maximized to reduce the output
 aspect ratio and NA of the core material.
 The minimization of surface energy will also drive the rectangular core 342
 towards a circular cross section 340. Thus an initially rectangular
 multi-mode waveguide 740 can be transformed by tapering to a single-mode
 or a few-mode waveguide 40 of substantially circular geometry and with a
 reduced NA. This method has been successfully employed in transforming a
 30.times.10 micron rectangular core glass 34 of a multi-component silicate
 glass at the multi-mode rectangular input to an output mode closely
 matched to a CS980 single-mode fiber 20. Preferably, the multi-component
 silicate glass is 60SiO.sub.2 28Al.sub.2 O.sub.3 12 La.sub.2 O.sub.3 (in
 mole %). Even though other single-mode fibers are usable, the single-mode
 fiber 20 is the CS980 single-mode fiber made by Coming, Inc. for
 propagating wavelengths at 980 nm and having a standard 125 micron
 diameter for the overall fiber. A loss of less than 0.5 dB has been
 achieved with this mode transformer 48 mode-matched to the CS980 fiber 20.
 Since the core is molten and the cladding is softening, diffusional
 processes are relatively fast, so graded index profiles can be created in
 situ. With appropriate choices of cladding material, the transformed
 fibers produced can be fusion spliced to conventional fibers making the
 transformed fibers quite practical in existing fiber networks and easing
 device manufacturing.
 The refractive index versus radial distance graph of FIG. 14 shows how the
 peak index and effective graded NA can be decreased by drawing or tapering
 the core 34 from 18 to 5 microns. Further reduction in NA and increased
 circularity is achieved with more tapering or decreased core dimensions.
 Referring to FIG. 11, a further embodiment of the present invention is
 shown. A four port fused taper asymmetric coupler 400 is used as the mode
 transformer 300 for mode transformation. A 4-port asymmetric coupler 400
 (one arm 442 being multi-mode and the other arm 440 being single-mode) can
 replace the tapered section 30, as the mode transformer 300, in accordance
 with the present invention. In this instance tapering at both the
 multi-mode arm and the single-mode arm, each having one unused port,
 results in an equalization of propagation constants between the CS980
 single-mode fiber 74 and a low-order mode of the tapered multi-mode fiber
 442. This coupler differs from prior-art couplers because it is highly
 asymmetric and takes advantage of the decreased NA and reduced aspect
 ratio that occurs when tapering "soft core" fibers.
 The asymmetric coupler 400 couples the fundamental mode of the single mode
 fiber to one mode of the multi-mode fiber, ideally the lowest order mode.
 The order of the mode to which the CS980 LP01 mode of the single-mode
 fiber 74 couples is dependent on the taper ratio and the core-diffusion
 rate in the multi-mode fiber 442. A greater than 20 dB power exchange to
 the LP02 mode with an excess loss below 0.5 dB has been measured with this
 asymmetric coupler arrangement. In this example efficient operation
 requires that the LP02 mode suffers minimal mode conversion in the
 rectangular up-taper 440, in the straight multi-mode fiber section 740,
 and at the input mirror 701.
 The method to make the asymmetric coupler 400, as a fused taper coupler, is
 similar to the core diffusion and elongation process to make the tapered
 mode transformer 30. However, instead of using one multi-mode fiber 740 as
 the only material to be inserted through the outer cladding or housing
 360, a single-mode fiber 742 is also inserted through the outer cladding
 360 before heat is used to draw and diffuse the two separate multi-mode
 and single-mode fibers 740 and 742, respectively, into the desired tapered
 multi-mode arm 442. The other end of the multi-mode fiber 740 is
 unterminated to form the multi-mold fiber end 744 and tapered and fused
 along with the other end 74 of the single-mode fiber 742 through the same
 outer cladding 360 to form the single-mode arm 440. The cladding 360 can
 be a boron or fluorine doped capillary or overcladding-tube or jacket 360
 to hold the two inserted fibers, one single-mode and one multi-mode, in
 place and to build-up the cladding material, if necessary. The tube 360
 could be doped with other dopants as long as the tube 360 has a lower
 index than both claddings of the two fibers. The cladding tube 360 with
 the inserted fibers 74 and 744 are heated together and pulled at both ends
 to produce two tapered arms 442 and 440, similar to the process used in
 making couplers as shown in U.S. Pat. No. 4,902,324, assigned to the same
 assignee. Instead of having both fibers in a standard coupler made from
 single-mode fibers, the asymmetric coupler 400 couples one and only one
 mode of the multi-mode fiber 740 into a single-mode fiber 74. By the same
 diffusive and dissolution process, the resultant tapered coupler 400 will
 have matched propagation constants between the tapered CS980 single-mode
 fiber 74 and one of the modes of the tapered multi-mode at the single-mode
 port of the single-mode arm 440 formed from the diffusion of the
 multi-mode fiber 740.
 When the core and the clad of each of the two fibers 74 and 740 of FIG. 11
 are placed side-by-side, the fibers tend to cross-diffuse or dissolve to
 transform a rectangular core into a circular output, according to the
 present invention. Different thermal characteristics of core and clad
 glass lead to an interdiffusion during heating to force the diffusion to
 occur in the mm fiber so that the multiclad coupler would result to
 transition the multi-mode to single-mode.
 Referring to FIGS. 15-18, various implementation of the lens, power
 coupler, optical exciter or lensed-tapered fiber optics coupler 70 can be
 used as broad-area laser diode beam-shapers. As illustrated, several
 embodiments of anamorphic (circularly nonsymmetric) coupling optics beam
 shapers preferably provide the optimum power coupling for the tapered
 fiber laser 30. In accordance with the teachings of the present invention,
 techniques have been developed that enable efficient coupling of pump
 power from broad-area laser diodes having typical emitting apertures with
 dimensions of 100.times.1 .mu.m.sup.2 and NA's of 0.1/0.6 in the slow and
 fast axes, respectively, into a fiber with a rectangular core cross
 section of 30.times.10 .mu.m.sup.2 and effective numerical aperture of
 &gt;0.42. The terms "slow" and "fast" refer to the planes that are
 "parallel" and "perpendicular," respectively, to the laser diode junction
 plane. The laser diode etendu is 100.times.1.times..pi.0.6 0.1=18.8
 .mu.m.sup.2 sr, and the fiber output etendu is greater by an order of
 magnitude. Mathmaticallywise, etendu is given by the product of the
 emitter area and the emitter solid angle. SR is ster-radian and is a unit
 of solid angle. According to a fundamental principle of optics, the etendu
 of a source or emitter can not decrease upon propagation or manipulation
 by optical components such as a beam shaper. The design goal of a beam
 shaper is to maintain the etendu of a given laser diode beam as small as
 possible at the input facet of a receiving optical fiber; preferably, the
 receiving fiber has an etendu comparable in magnitude to that of the
 shaped beam.
 Referring to FIGS. 15 and 16, a single-element (2 surfaces) monolithic
 micro-optics power coupler 801 is illustrated as one embodiment of the
 optical exciter or lensed-tapered fiber optics coupler 70. In order to
 efficiently couple light from the broad-area semiconductor laser 72 with
 emitter dimensions of 100.times.1 .mu.m.sup.2 and NA's of 0.1/0.6 in the
 slow and fast axes (measured at 5% of the maximum far-field intensity
 points), respectively, a monolithic anamorphic micro-lens consisting of
 only two surfaces is made on fused silica; these two surfaces together
 produce an image of the emitter near field with dimensions of 30.times.10
 .mu.m.sup.2 and 5% NA's of 0.35/0.12 in the slow and fast axes,
 respectively. Referring to FIG. 16 or looking from the top, the first
 surface includes a cylinder with a non-circular cross sectional side-view
 810 which collimates the fast axis. The second surface includes a toroidal
 surface structure 820 which focuses the collimated fast axis with
 magnification greater than times five (.times.5) and images the slow axis
 near field with demagnification of .times.3.3. For the fiber laser to work
 optimally, this micro-power coupler 801 requires an alignment of twelve
 degrees of freedom with translational tolerances of 1-2 .mu.m and angular
 tolerances of 0.5.degree.. The shape of the cylinder 810 and toroid 820 is
 structured to minimize mis-alignment errors that could occur during the
 process of coupling light from the broad-area emitter into the fiber core.
 The coupling efficiency of this monolithic power coupler 801 is in the
 range between 75% to 90%. Typical dimension of the power coupler clear
 aperture, where the light is coupled through, is in the range of 100 to
 300 .mu.m. The power coupler has a total conjugate distance of about 1 mm,
 with front and back working distances of 30-60 .mu.m and 100-200 .mu.m,
 respectively.
 Referring to FIGS. 17A, 17B, and 17C, slightly larger than the micro-optics
 lens 801, but having a similar outside surface when assembled as one
 piece, a second embodiment of the lens or power coupler 70 is represented
 as a two-element (4 surfaces 906, 908, 910, and 912) mini-optics power
 coupler 900 for power coupling light into the input facet of the
 multi-mode fiber 32. This lens or power coupler 900 is formed from two
 crossed cylinders 901 and 902, made of a suitable glass material, with
 finite-conjugate imaging planes in each axis. The first optical coupling
 element 901 has first 906 and second 908 surfaces with cylindrical
 symmetry and a first axis of symmetry parallel to the slow axis of the
 multi-mode laser source 72. The second optical coupling element 902 has
 third 910 and fourth 912 surfaces with cylindrical symmetry and a second
 axis of symmetry parallel to the fast axis of the multi-mode laser source
 72, wherein the first and second optical coupling elements are crossed
 such that the first axis of symmetry of the first element 901 is oriented
 at right angles to the second axis of symmetry of the second element 902.
 As with other embodiments of the power coupler, the numerical aperture (or
 acceptance angle) of the power coupler in both orthogonal axes is larger
 than the numerical aperture (or divergence angle) of the output facet of
 the multi-mode laser source 72.
 Referring to FIG. 17A or looking from the side, a fast axis non-circular
 lens 901 with about .times.5 to .times.10 magnification followed by a slow
 axis telescope 902 with .times.3.3 demagnification, bonded and optically
 aligned. This lens system produces an image of the emitter nearfield with
 dimensions of 30.times.10 .mu.m.sup.2 and 5% NA's of 0.35/0.12 in the slow
 and fast axes, respectively. When coupled into a fiber with core
 dimensions of 30.times.10 .mu.m.sup.2 and NA.apprxeq.0.42, the etendu at
 the fiber output is 166 .mu.m.sup.2 sr. This mini-lens system preferably
 requires an alignment of eighteen degrees of freedom with translational
 tolerances of about 1 .mu.m and angular tolerances of about 0.1 degrees.
 The coupling efficiency of this coupler lens is in the range of 75% to
 90%. Typical dimension of the lens system aperture 904 is in the range
 between 0.5 to 2 mm. The power coupler has a total conjugate distance of
 about 2-5 mm, with front and back working distances of 100-150 .mu.m and
 150-350 .mu.m, respectively.
 Referring to FIGS. 18 and 19, a third embodiment of lens 70 is represented
 as a wedged/tapered fiber optics coupler 970 having two surfaces 942 and
 342. Instead of being separate pieces, such as the mini and micro-optic
 lens 900 and 801, the wedged/tapered coupler 970 can be a separate piece
 or made continuous with the multi-mode fiber 740. To form the coupler
 wedge or tapered fiber optics coupler 970, a fiber with typical core
 dimensions of 120.times.40 .mu.m.sup.2 and a core effective NA&gt;0.4 is
 down-tapered with a ratio of 4:1 to a 30.times.10 .mu.m.sup.2 core 342
 from a much larger core at the input 942 and fabricated from a similar
 method as the tapering process used in forming the tapered mode
 transformer 48.
 In order to avoid a loss penalty, an optional 20-30.degree. lens-wedge
 segment 181 is polished on the input core facet 942 of the coupler wedge
 970. The optional lens-wedge 181 reduces the launched numerical aperture
 of the laser diode beam into the fiber core 342, which allows the
 coupler-wedge 970 to butt-couple the multi-mode fiber 740 of the fiber
 laser such that the broad-area laser diode power has a launch NA in the
 range 0.1 to 0.2. The optional lens-wedge 181 allows a taper ratio in the
 range 4:1 to 2:1 of the mode transformer 48 without loss.
 Enabling larger taper ratios of the tapered fiber optics coupler 970,
 without incurring a loss penalty, would require replacing the lens-wedge
 181 as an input lens by an anamorphic lens profile 948 exemplified by
 various embodiments of FIGS. 22 and 23. In FIGS. 18 and 19, the input lens
 comprises a wedge-shaped microlens 181 on the input core end of the fiber,
 the wedge-shaped microlens including a first pair of surfaces that
 intersect at a line that substantially bisect the input core 942. The
 wedge-shaped microlens includes a pointed fiber endface with an apex on
 the axis of the adiabatically tapered optical fiber coupler 970. Instead
 of the planar surfaces of the wedge lens 181 of FIGS. 18 and 19, the lens
 profile 948 can be circular, have conic surfaces or surfaces of other
 shapes. In FIG. 20, microlens 948A is formed from a double-wedge-shaped
 lens profile having a steep angle 949 at a first radius and a shallow
 angle 950 at a second radius smaller than the first radius. In FIG. 21,
 microlens 948B comprises a multiple-wedge-shaped lens profile having a
 plurality of changing angles to approach a continuous surface, such as an
 ellipsoid. The method for making a lens profile 948 is taught in U.S. Pat.
 No. 5,455,879, assigned to the same assignee and incorporated herein. The
 number of degrees of freedom for alignment is minimized using the
 anamorphic lens profile 948.
 The coupler-wedge or tapered fiber optics coupler 970 is made similarly to
 the taper mode transformer 48, except that the temperature and pressure is
 controlled in such a way to minimize the diffusion, since the
 coupler-wedge 970 is designed to maintain its multi-mode characteristics
 and its rectangular shape at both input 942 and output 342. Hence, the
 same aspect ratio, or about the same, is preferably maintained both at the
 input and the output. Thus, a larger rectangular cross-section core would
 be at the input 942 while a smaller rectangular cross section core would
 be at the output 342 of the coupler wedge 970. The overcladding 960
 similarly builds-up the coupler-wedge 970 such that the desired aspect
 ratios can be formed.
 As fibers with very large numerical aperture (NA) and no diffusion are
 developed, the technique of using a wedge-coupler 970 allows a fiber beam
 shaper to be formed that has a brightness comparable to the laser diode
 (LD) brightness. As is known, brightness is defined as the ratio of
 optical power in Watts per unit of Etendu. Consider, for example, a
 circular core step-index fiber with NA=1; an input core with a diameter of
 110 .mu.m can be tapered to an output core with a diameter of 2.8 .mu.m
 with no loss provided that the launch NA in both planes is 0.025. Such a
 launch NA can not be achieved with the simple wedge lens 181 since the
 20-30 degree wedge could only produce launch NA's of about 0.1 with its
 planar surfaces. Instead an anamorphic lens profile 948 is formed on the
 fiber facet 942, as the optional lens-wedge 181 or is placed immediately,
 or otherwise coupled, in front of the wedge coupler 970, as seen in FIG.
 21 having more angular or curved surfaces. The resultant etendu is about
 20 .mu.m.sup.2 sr at the fiber output, nearly equal to the LD etendu. In
 another case, the fiber is tapered to an output core of only 8.8 .mu.m
 diameter which preferably uses a wedge-coupler 970 in the fast axis to
 decrease the 0.6 laser diode NA to a launch NA=0.08. As a result, the
 fiber output etendu is 190 .mu.m.sup.2 sr.
 As another example, a simple wedge 181 produces a launch NA=0.2. The
 receiving multi-mode fiber (30.times.10 sq. micron) 740 has an NA=0.6.
 This means that a taper ratio 3:1 in the wedge coupler 970 can exist and
 still have no loss to radiation modes. Thus an input core 942 with
 100.times.30 sq. micron can be reduced in the wedge coupler 970 through
 the taper ratio to an output core 342 of 33.times.10 sq. micron. For a
 taper 1 cm long in the wedge coupler 970, the angle of the lens wedge 181
 is (100-33)/10000=6 milli-radian, assuming a linear taper.
 Theoretically, the wedge angle is chosen by a complicated formula that
 gives for each ray of the input cone of light the launched NA into the
 coupling taper 970 as a function of wedge angle. The launched NA is given
 by:
EQU NA.sub.launch =n Sin[ArcSin[Sin[t+a Pi/180]/n]-a Pi/180] (6)
 where n is the index of the coupling wedge glass 970, a is the wedge angle
 in degrees (measured with respect to the normal to the optical axis) and t
 is the angle that the input ray makes with the optical axis.
 Because the wedge 181 also deviates the axial ray, the wedge angle is
 selected where the marginal and the axial ray curves intersect. This is
 about 22 degrees and thus provides that NAlaunch=0.2. Hence, the power
 coupler satisfies a desired condition where a lensed and/or a tapered
 fiber optics coupler embodiment, and other variations in between, has a
 taper ratio for exciting the tapered and core-diffused fiber segment or
 mode transformer, and has a launch numerical aperture equal to or less
 than the numerical aperture of the multi-mode fiber divided by the taper
 ratio of the lensed-tapered fiber optics coupler.
 It will be apparent to those skilled in the art that various modifications
 and variations to the lens, coupling scheme, fiber laser, and other
 components of the optical package can be made to the present invention
 without departing from the spirit and scope of the invention. Thus, it is
 intended that the present invention covers the modifications and
 variations of this invention provided they come within the scope of the
 appended claims and their equivalents.