System and method for a multi-mode pump in an optical amplifier

An optical amplifier includes a multi-mode pump laser module, a multi-mode waveguide, a multi-mode to multiple single-mode fiber converter module and a plurality of single-mode cores. The multi-mode pump laser module emits pump light having a plurality of modes to the multi-mode fiber or waveguide. The multi-mode waveguide propagates the emitted pump light to the converter module. The converter module receives the pump light and distributes the pump light approximately uniformly to a plurality of single-mode cores.

Not Applicable.

Not applicable.

BACKGROUND

Technical Field

This disclosure relates generally to optical nodes and more particularly, but not exclusively, to systems and methods for optical amplification of optical signals.

Description of Related Art

The statements in this section provide a description of related art and are not admissions of prior art. An optical amplifier is a device that amplifies an optical signal directly in the optical domain without converting the optical signal into a corresponding electrical signal. Optical amplifiers are widely used, for example, in the fields of optical communications.

One type of an optical amplifier is a doped-fiber amplifier, with a well-known example being the Erbium-doped fiber amplifier (EDFA). In operation, a signal to be amplified and a pump beam are multiplexed into the doped fiber. The pump beam excites the doping ions, and amplification of the signal is achieved by stimulated emission of photons from the excited dopant ions.

Another type of an optical amplifier is a Raman amplifier, which relies on stimulated Raman scattering (SRS) for signal amplification. A Raman amplifier uses the intrinsic properties of silica fiber to obtain signal amplification, such that transmission fibers themselves can be used as a medium for amplification, allowing the attenuation of data signals transmitted over the fiber to be mitigated within the fiber itself. More specifically, when a signal to be amplified and a pump beam are multiplexed into an optical fiber made of an appropriate material, a lower-frequency signal photon induces SRS of a higher-frequency pump photon, which causes the pump photon to pass some of its energy to the vibrational states of the fiber material, thereby converting the pump photon into an additional signal photon. An amplifier working on the basis of this principle is commonly known as a distributed Raman amplifier (DRA) or simply a Raman amplifier. The pump beam may be coupled into the fiber in the same direction as the signal (co-directional or co-pumping) or in the opposite direction (contra-directional or counter-pumping). The counter- and co-propagating Raman amplifiers are a marked improvement on this technology. In contrast to the standard Raman amplifier where a single counter-propagating Raman pump signal is responsible for the amplification of the traffic signals in the fiber, a counter-propagating and a co-propagating Raman amplifier may be used together. Together the co- and counter-propagating Raman pumps provide amplification to combat signal attenuation in fiber extending the reach of an optical span.

A multi-core fiber (MCF) increases a number of cores within a cladding of a single fiber. Multi-core fiber has the potential to increase data rates by using spatial division multiplexing (SDM). By increasing the number of cores within a single fiber, the information carrying capacity of the fiber is dramatically increased. Multi-core fibers have many times the signal-carrying capacity of traditional single-core fibers. Multi-core fibers may be employed in many applications ranging, e.g., from sensors to spatial division multiplexing to high density coupling.

A need exists for improved optical amplifiers to combat signal attenuation in multi-core fibers extending the reach of an optical span.

DETAILED DESCRIPTION OF THE INVENTION

An optical fiber may be a flexible filament of extruded glass (silica) capable of carrying information in the form of an optical signal, e.g. light. The two main elements of an optical fiber are a core, typically made of silica glass, and a cladding. The core includes the axial part of the optical fiber and is typically made of silica glass. The optical signal propagates primarily within the core, with a smaller portion propagating within the cladding via an evanescent component. The cladding typically surrounds the core, and may include one or more material layers. The refractive index of the core is generally higher than that of the cladding, so that light in the core intersects the interface with the cladding at a grazing angle, gets trapped in the core by internal reflection, and travels in the proper direction down the length of the fiber to its destination. Surrounding the cladding is usually another layer, called a coating, which typically includes protective polymer layers. Buffers may also be used as further protective layers applied on top of the coating. Other or additional layers may be employed in an optical fiber.

Fibers that support only one mode of transmission are called single-mode fibers (SMF), while those that support multiple propagation paths or transverse modes are called multi-mode fibers (MMF). Single-mode fibers have a smaller core diameter (e.g., diameter of 8.3 to 10 microns, depending on signal wavelength) designed such that the light travels substantially along one path or in one ray since in general only the lowest order bound mode propagates at the wavelength of interest. Single-mode (also sometimes called monomode) fibers typically have lower fiber attenuation than multi-mode fibers, and typically have lower wavelength-dependent dispersion. Thus the propagating signal usually retains better fidelity of the propagating signal in the single-mode fiber than in multi-mode fiber. As such, single-mode fibers are generally used for longer communication links.

For a given signal wavelength, multi-mode fibers have a larger core diameter, compared to single-mode fibers, allowing for a number of modes or paths of propagation through the core. Different guided modes of the multi-mode fiber are modulated with different modulated optical signals or different combinations of a given set of modulated optical signals. For a given wavelength λ, a multi-mode fiber is defined when the normalized frequency parameter (V, also referred to as the V number) is greater than approximately 2.405. Equation (1) gives the expression for V:
V=(πd/λ)NA(1)
wherein d is the diameter of the fiber core and NA is the numerical aperture of the fiber. NA is defined below in Equation (2):
NA=Square Root((n1)2−(n2)2)  (2)
Where n1is the core refractive index and n2is the cladding refractive index. From this equation, it is seen that the diameter of a multi-mode fiber core is larger than the wavelength of the light signal, thus allowing for multiple paths or rays through the fiber core. The number of modes, N, is defined by equations (3) and (4) below:
N=V2/2 for step index fiber  (3)
N=V2/4 for graded index fiber  (4)
Theory shows, and experiments confirm, that for single-mode operation, V equals approximately 2.405 or less. Therefore, a fiber designed to conduct only one mode is characterized by one or more of a smaller core diameter, d, a larger operating wavelength, and n2being as close to n1as possible.

In practice, a core diameter of a single-mode fiber is about 10 um or less, the range of operating wavelengths typically starts at about 1300 nm, and the relative refractive index A (Rcladding/Rcore−1), is less than about 0.4%. The result is a fiber that rejects all higher-order modes and conducts only one fundamental mode—a beam traveling exactly along the centerline of the fiber. The characteristics of typical multi-mode fibers include a core diameter of 50, 62.5, or even 1,000 μm; an operating wavelength range starting in the visible light region (˜390-˜700 nm); and a relative refractive index that is a minimum of 1% and, typically, 2% or higher.

A multi-core fiber (MCF) has more than one core within a cladding of a single fiber. Multi-core fiber has the potential to increase data rates by using spatial division multiplexing, e.g. multiplexing data among the several individual cores. By increasing the number of cores within a single fiber, the information carrying capacity of the fiber may be dramatically increased. The potential signal carrying capacity of multi-core fibers is many times that of traditional single-core fibers. Multi-core fibers may be employed in many applications ranging, e.g., from sensors to spatial division multiplexing to high density coupling.

Optical amplifiers for multi-core fibers include cladding-pumped or core-pumped amplifiers. Cladding-pumped amplifiers may use a high-power, uncooled, multi-mode pump laser that may be less expensive that more stable alternatives. The multi-mode lasers are operable to be directly coupled into the cladding and provide gain to the multiple cores of doped fibers. For example, a cladding-pumped amplifier includes one or more multi-mode laser diodes for cladding-pumping an Erbium (Er) doped multi-core fiber or Ytterbium (Yb) doped multi-core fiber or Er/Yb co-doped multi-core fiber to amplify the optical signals in the cores.

Core-pumped amplifiers for multi-core fibers may include several single-mode pumps for pumping light into the multiple cores of doped multi-core fibers. Single-mode pumps typically emit light in a narrow spectral band that increases coherence and enables the emitted pump light to be focused to a diffraction-limited spot size. The single-mode pumps in a core-pumped amplifier operating at higher powers often require a thermoelectric cooler (TEC) and thermistor, and are thus more complex and more expensive than a multi-mode pump. However, core-pumped amplifiers may have improved gain flatness and lower noise than cladding-pumped amplifiers.

As such, it would be advantageous to employ a high power but less complex and less expensive multi-mode pump laser in a core-pumped amplifier. However, multi-mode pump lasers emit light having a larger wavelength spectrum than single mode pump lasers. Multi-mode pump lasers output multimode light exhibiting power spectral lines around a center wavelength within a wavelength range. The multi-mode light may also exhibit divergence, e.g. different foci of light propagating in two perpendicular planes. For example, the output light may be divergent (such as approximately 30-40 degrees) in the vertical direction. The lack of coincidence of the foci limits the ability to focus the output multimode light to a small, sharp well-defined point. As such, multi-mode equipment may not efficiently launch (e.g. inject) light into a single-mode fiber. This inefficiency makes the use of multi-mode pumps in core pumping amplifiers more difficult.

These and other pertinent problems are addressed by various embodiments described herein of an optical amplifier operable to use a multi-mode pump laser in a core-pumped optical amplifier for multi-core fibers. In various embodiments, the optical amplifier distributes pump light from a multi-mode pump to a plurality of single-mode cores (e.g., either of a multi-core fiber or to a plurality of single-mode fibers) using a multi-mode to multiple single-mode converter module. Thus, various embodiments of the optical amplifier described herein enable the use of a high power, less expensive multi-mode pump laser in a core-pumped optical amplifier.

FIGS. 1A-Cillustrates schematic block diagrams of embodiments of an optical amplifier100. The optical amplifier100shown inFIG. 1Aincludes a multi-mode pump laser module110, multi-mode waveguide120, a multi-mode to multiple single-mode fiber converter module130and a plurality of single-mode cores140. The multi-mode pump laser module110includes a laser, e.g. a semiconductor laser such as a laser diode, configured to emit multi-mode pump light170. The multi-mode pump light170from the multi-mode pump laser module110generally exhibits multiple spectral lines around a center wavelength and may also exhibit divergence, e.g. different foci of light propagating in two perpendicular planes. The multi-mode pump laser module110may further include thermal and/or electrical management of the center wavelength and wavelength range. The multi-mode pump laser module110may also include automatic gain control (AGC), automatic level control (ALC), and automatic current control (ACC) to control the operation mode of the laser.

The multi-mode laser pump module110is operably coupled to the multi-mode fiber or waveguide120, e.g. a multi-mode fiber. The multi-mode waveguide120is operable to propagate the emitted pump light170from the multi-mode laser pump module110to the converter module130. In some embodiments, the multi-mode waveguide120is included in the multi-mode laser pump module110or converter module130. Converter module130receives the multi-mode pump light170and converts the multi-mode pump light170into single-mode pump light180. The converter module130may distribute the pump light170approximately uniformly to a plurality of single-mode cores140.FIGS. 2-5further describe embodiments of converter130herein below.

In an embodiment, the plurality of single-mode cores140includes one or more multi-core fibers150as shown inFIG. 1B. The multi-core fiber150includes a plurality of single-mode cores surrounded by a common cladding. In another embodiment, the plurality of single-mode cores140includes a plurality of multi-core fibers, either physically separate or with different claddings but included within a common coating and buffer.

In another embodiment, the plurality of single-mode cores140includes a plurality of single-mode fibers160a-nas shown inFIG. 1C. An instance of a single-mode fiber160includes a single-mode core and cladding. The single-mode fibers160a-nmay be physically separate or included within a common coating and buffer. In another embodiment, the plurality of single-mode cores140includes a combination of one or more multi-core fibers and one or more single-mode fibers160.

Various additional components, such as variable optical attenuators, optical add-drop multiplexers, optical filters, optical scramblers, optical couplers, etc., may be incorporated into optical amplifier100.

FIGS. 2A-Cillustrate a schematic block diagram of another embodiment of optical amplifier100. In this embodiment, the converter module130includes a plurality of laser inscribed three dimensional (3D) waveguides200in a glass cladding210, such as glass block or other shaped glass cladding. Though glass cladding210is described herein, similar or other types of cladding with inscribed or embedded waveguides may also be employed. In an embodiment, the glass cladding210is a glass block, though other shapes of glass claddings may also be employed.

The plurality of 3D waveguides200at input220are spaced to effectively behave as one multi-mode waveguide.FIG. 2Billustrates an example embodiment of a cross section of input220. The cores240of 3D waveguides200at input220are spaced in close proximity such that the effective normalized frequency parameter V of the plurality of cores240of 3D waveguides200is greater than about 2.405 to effectuate a multi-mode input. For example, due to the close spacing of cores240, the effective diameter of the 3D guides approximates the total diameter measured across the plurality of cores240or a sum of the diameters of the plurality of cores. Since the normalized frequency diameter V is proportional to the diameter of the core, the normalized frequency parameter V increases with increased diameter. With the effective diameter about equal to the diameter across the plurality of cores due to their close spacing, the 3D waveguides may approximately behave as a single multi-mode waveguide with a single core.

The 3D waveguides200then increasingly diverge or separate in both or either the X and Y axis of the glass cladding210to output230.FIG. 2Cillustrates an example embodiment of a cross section of the 3D waveguides200at output230. The 3D waveguides200at output230are separated such that individual 3D waveguides200collectively behave as a plurality of single-mode waveguides, e.g. the effective normalized frequency parameter V of individual ones of the plurality of the 3D waveguides200is approximately equal to or less than 2.405 thus effectuating a single-mode core at output230. For example, due to the increased spacing between the 3D waveguides, the effective diameter of the 3D guides no longer is approximated by the total diameter measured across the plurality of cores. In addition, each of the cores of the 3D waveguides has an individual diameter such that the effective normalized frequency parameter V of each core is approximately equal to or less than 2.405. So the plurality of 3D waveguides effectively behaves at output230as a plurality of single-mode waveguides.

In operation, the multi-mode pump laser module110pumps multi-mode pump light170into multi-mode waveguide120. The multi-mode waveguide120is optically coupled to converter module130to propagate multi-mode pump light170into the plurality of the 3D waveguide cores240through input port220. Preferably, pump light170is received substantially uniformly by the plurality of 3D waveguide cores200. To optimize the coupling, spot size and NA is approximately matched to the 3D waveguide cores240. As the 3D waveguides200diverge in the glass cladding210, the 3D waveguides200scramble the modes and polarization in the multi-mode pump light170. As the modes and polarization are scrambled, single-mode pump light180is expected to exhibit a more even distribution of power across the modes. Furthermore, due to the increased spacing between the cores240of the 3D waveguides, the plurality of 3D waveguides200effectively behave at output230as a plurality of single-mode waveguides. As such, the 3D waveguides200at output220effectively behave as a plurality of single-mode waveguides emitting single-mode pump light.

In an embodiment, a mode scrambler may be incorporated in the multi-mode waveguide120or converter module130to further scramble modes of the pump light170. Similarly, in an embodiment, a polarization scrambler may be incorporated in the multi-mode waveguide120or converter module130to further scramble polarization of the pump light170. In an embodiment, multi-mode pump laser module110emits pump light170exhibiting differing frequencies and/or modes over time to produce a time-dependent change of wavelengths and mixing of modes between wavelengths.

Multi-mode waveguide120may be optically coupled to converter130by slicing the multi-mode waveguide120to approximately align in size and shape the core of the multi-mode waveguide with the cores240of the 3D waveguides200at input220of converter module130. The multi-mode waveguide120and the converter module130may be cleaved, spliced together and then adhesively or otherwise mechanically attached, e.g. by fusing with heat. Optical fiber connectors or removable connections may be employed as well or alternatively.

In another embodiment, an optical coupler or one or more lenses are included between the multi-mode waveguide120and the converter module130, e.g. to change dimension (e.g. spot size) and/or focus of the multi-mode pump light170output by the multi-mode waveguide120. In some embodiments the dimension and/or focus are selected to approximately match the diameter of the cores240of the 3D waveguides200at input220.

FIGS. 3A-Cillustrate a schematic block diagram of another embodiment of optical amplifier100. In this embodiment, the converter module130includes an adiabatic taper module300having a multi-core fiber380within a capillary360. The multi-core fiber380includes a cladding310and single-mode cores340. The embodiment operates on the principle that multi-mode pump light170is initially guided within the cladding at an input320, and couples to the single-mode cores340as the light propagates to an output320.FIG. 3Billustrates a cross section of the multi-core fiber380. The single-mode cores340are closely packed with little or no space between the individual cores340. The cores340are embedded within the cladding310, which is bounded by the capillary360. The capillary360has a lower refractive index than the cladding such that the multi-core fiber380is operable to guide the pump light170within the cladding.FIG. 3Cillustrates a cross-section of an example embodiment of output330of the adiabatic taper module300. In this embodiment the diameter of the multi-core fiber380is greater than that of the input320. The diameter of multi-core fiber380is tapered such that the cores340are spaced in close proximity at input320, and are isolated at output330. In this context, “close proximity” means that at the wavelength of the pump light170, the light couples to more than one of the individual cores340. The term “isolated” in this context means that at the operating wavelength of the pump light170, a portion of the pump light170propagating within one core340does not substantially couple to any other of the cores340. Cores340that are in close proximity may touch neighboring cores340in some embodiments. Isolated cores340are separated from each other by cladding310.

At the input320end of the multi-core fiber380, the plurality of single-mode cores340are not expected to guide light individually. Moreover, the light is expected to substantially propagate within the cladding310due to the lower refractive index of the capillary360It is expected that the cores340will absorb a portion of the pump light170due to overlap of the cladding propagating modes with the core propagating modes.

During operation the multi-mode waveguide120couples the multi-mode pump light170from the multi-mode pump laser module110to cladding310of the adiabatic taper module300at the input320. Relatively little light is expected to couple to the cores340due to the small aperture of the cores340. Thus, the multi-mode pump light170propagates in the cladding310with essentially no leakage from the multi-core fiber380. This low refractive index creates internal reflection at the boundary between cladding and the capillary to substantially guide the pump light in the cladding.

As the diameter of the multi-core fiber380increases from the input320to the output330(positive Z direction in relation to the illustrated coordinate axes) the cores340diverge along the X and Y axes or in both the X and Y directions such that the distance between the cores340increases, e.g. monotonically. In some embodiments, and as illustrated isFIGS. 3B and 3C, the diameter of the cores340also increases from the input320to the output330. Due to the change of geometry of the taper module300the multi-mode pump light170that is initially guided within the cladding310at input320transitions to being substantially guided within the cores340due to change in the effective refractive index of the cladding310. It is thought that this transition occurs because, as the distance between the cores340increase, the multi-mode pump light170propagating within the cladding310intersects the boundary between the cores340and the cladding310at an angle greater than the confinement angle and is thereby captured by the cores340. In a preferred embodiment the increase of diameter of the multi-core fiber380and the length of the module300are sufficient to ensure that multi-mode pump light170is substantially transferred to the multiple cores340. The modes and polarization of multi-mode pump light170at input320are scrambled by the process, and pump light170exhibits a more even distribution of power across the modes. At output330, the cores340effectively behave as single-mode outputs emitting single-mode pump light180. The multi-core fiber380is operably, e.g. optically, coupled to multi-core fiber150at output330for propagation of pump light170through multi-core fiber150.

In an embodiment, a mode scrambler may be incorporated in the multi-mode waveguide120or adiabatic taper module300to further scramble modes of the pump light170. Similarly, in an embodiment, a polarization scrambler may be incorporated in the multi-mode waveguide120or adiabatic taper module300to further scramble polarization of the pump light170. A polarization scrambler typically includes a polarization controller that is operable to vary the state of polarization of the pump light170within the multi-mode waveguide120and so randomize the average polarization over time of the pump light170. One or more different polarization scramblers based on different technologies may be used, including LiNbO3 based scramblers, resonant fiber coil based scramblers, and fiber squeezer based scramblers.

In an embodiment, alternatively or in addition to the polarization scrambler, the multi-mode pump laser module110may emit pump light170that exhibits differing frequencies and modes over time to constantly change wavelengths and mix modes between wavelengths of the pump light170.

Multi-mode waveguide120may be operably coupled to multi-core fiber380by slicing the multi-mode waveguide120to approximately align in size and shape to input320of multi-core fiber380. The multi-mode waveguide120and multi-core fiber380may be cleaved, spliced together and then adhesively or otherwise mechanically attached or thermally fused. Optical fiber connectors or removable connections may be employed as well or alternatively.

In another embodiment, an optical coupler or one or more lenses are included between the multi-mode waveguide120and multi-core fiber380to alter the dimension and/or focus of pump light170output from the multi-mode waveguide120to the dimension of input320of multi-core fiber380.

FIGS. 4A-Cillustrate a schematic block diagram of another embodiment of an optical amplifier100. InFIGS. 4B and 4CXYZ coordinate axes provide a reference in the following description. In this embodiment, the converter module130includes an adiabatic taper module400with a plurality of single-mode waveguides480(FIGS. 4B and 4C), each including a core440and cladding410. The single-mode waveguides480are surrounded by a low refractive index capillary460.FIGS. 4B and 4Crespectively illustrate cross sections of an embodiment of input420and output430of the adiabatic taper module400. In the illustrated embodiment, from the input420to the output430the diameter of the waveguide cores440and the capillary460, and the thickness of the cladding410, increase. While the embodiments ofFIGS. 4B and 4Cshow space between the cladding410and the capillary460, in other embodiments the cladding410may be in physical contact with the capillary460. At the input420end light is thought to be substantially guided in the cladding410of the single-mode waveguides480due to the lower refractive index of the capillary460.

In operation, at input420, the pump light170from the multi-mode pump laser module110is expected to be substantially coupled to and propagated by the claddings410. As the diameter of the cores440increases in the X and Y directions as Z increases, the effective refractive index of the cladding410is thought to increase. The pump light170that is guided in the cladding410at input420therefore transitions to propagation modes within the cores440. In a preferred embodiment the length of the module400is sufficient to ensure that multi-mode pump light170is substantially transferred to the cores440. As the pump light170transitions to propagation modes within the cores440, the modes and polarization of pump light170is expected to be scrambled, resulting in a more even distribution of power across the modes at output430. The cores440at output420are expected to effectively behave as single-mode outputs emitting single-mode pump light180.

At output430of the adiabatic taper module400, the single-mode cores440interface to a plurality of single-mode cores140. In an embodiment, the single-mode cores440are operably, e.g. optically, coupled to the plurality of single-mode cores140to propagate single-mode pump light180to one or more single-mode fibers160at output430. In another embodiment, the single-mode cores440are operably, e.g. optically, coupled to the plurality of single-mode cores140to propagate single-mode pump light180to one or more multi-core fibers150at output430. In another embodiment, the single-mode cores440are operably, e.g. optically, coupled to the plurality of single-mode cores140to propagate single-mode pump light180to one or more single-mode fibers160and one or more multi-core fibers150at output430.

In an embodiment, a mode scrambler may be incorporated in the multi-mode waveguide120or adiabatic taper module400to further scramble modes of the pump light170. Similarly, in an embodiment, a polarization scrambler may be incorporated in the multi-mode waveguide120or adiabatic taper module400to further scramble polarization of the pump light170. In an embodiment, multi-mode pump laser module110emits pump light170exhibiting differing frequencies and modes over time to provide a time-dependent change of wavelengths mode mixing between the wavelengths.

Multi-mode waveguide120may be operably, e.g. optically, coupled to adiabatic taper module400by slicing the multi-mode waveguide120to approximately align in size and shape to input420. The multi-mode waveguide120and adiabatic taper module400may be cleaved, spliced together and then adhesively or otherwise mechanically attached or thermally fused. Optical fiber connectors or removable connections may be employed as well or alternatively.

In another embodiment, an optical coupler or one or more lenses are included between the multi-mode waveguide120and adiabatic taper module400to alter dimension and/or focus of multi-mode pump light180output from the multi-mode waveguide120. In this manner the dimension of the beam output by the multi-mode waveguide120may be matched to the dimension of input420.

FIGS. 5A-Cillustrate a schematic block diagram of another embodiment of optical amplifier100. InFIGS. 5B and 5CXYZ coordinate axes provide a reference in the following description. In this embodiment, the multi-mode pump laser module110includes a planar multi-mode laser500. The planar multi-mode laser500is a single transverse mode laser that exhibits a substantially single transverse mode behavior at one or more particular lasing wavelengths. For example, the output pump light180exhibits a substantially single-mode in the X direction and multiple modes in the Y direction. The output pump light180is propagated to a planar lantern510with scrambler520.

FIG. 5Billustrates an example embodiment of a cross section of input520to planar lantern510. The planar lantern510includes a plurality of 2D waveguides550with cores570and cladding560. The 2D waveguides550at input520are transversely spaced, e.g. along the X axis or other axis, to approximately align with the single transverse mode of the planar multi-mode laser500. Similarly toFIG. 5B, the 2D waveguides550at output530are transversely spaced along the X axis or other axis to approximately align with the single transverse mode of the planar multi-mode laser500. Since the 2D waveguides550at input520are transversely spaced along the X axis or other axis to approximately align with the single transverse mode of pump light180, the pump light180has substantially a single-mode at input520of 2D waveguides550. The 2D waveguides550at input520and output530are single-mode waveguides, e.g. the effective normalized frequency parameter V of individual ones of the plurality of the 2D waveguides200at input520and output530is approximately equal to or less than 2.405 to effectuate single-mode cores570.

The converter module130also includes scrambler540. When other modes leak or are absorbed at the input520of 2D waveguides550, scrambler540is operable to scramble polarization modes of the pump light180. The 2D waveguides at output530emit single-mode pump light180to the plurality of single-mode cores140. In this embodiment, the plurality of single-mode cores140includes a planar array of single-mode fibers160.

In another embodiment, the 2D waveguides550may diverge, or become increasingly separated, along the Z axis from input520to output530. Thus the spacing between waveguide centers at the output530is greater than at the input520. As with the first embodiment, the 2D waveguides550at input520are transversely spaced along the X axis or other axis to approximately align with the single transverse mode of the planar multi-mode laser500.FIG. 5Cillustrates an example embodiment of a cross section of the 2D waveguides550at output530in this second embodiment. As the 2D waveguides550diverge, the modes and polarization in the multi-mode pump light170are scrambled, and pump light170exhibits a more even distribution of power across any additional polarization modes. The 2D waveguides output substantially single-mode pump light180to a planar array of single-mode fibers160.

In an embodiment, planar multi-mode laser500is operably coupled to planar lantern510by a multi-mode waveguide or by slicing the planar lantern510to approximately align in size and shape to a waveguide output of planar multi-mode laser500. The planar multi-mode laser500and lantern510may then be adhesively or otherwise mechanically attached, or thermally fused. Optical fiber connectors or removable connections may be employed as well or alternatively. In another embodiment, an optical coupler or one or more lenses are included between the planar multi-mode laser500and lantern510to change dimension and/or focus of the output pump light170of the multi-mode laser500to the dimension of the cores240of the 2D waveguides550at input520.

FIG. 6illustrates a schematic block diagram of another embodiment of optical amplifier100. In this embodiment, a scrambler500is incorporated in the multi-mode waveguide120or in the adiabatic taper module400(FIG. 4A) or in both the multi-mode waveguide120and adiabatic taper module400. The scrambler400includes a mode scrambler or a polarization scrambler or both a mode and polarization scrambler to further scramble modes of the pump light170.

FIG. 7illustrates a schematic block diagram of another embodiment of optical amplifier100. In this embodiment, multi-mode waveguide120is operably coupled to converter130by optical coupler620. Optical coupler620includes one or more lenses to focus pump light170emitted from the multi-mode waveguide120to the dimensions of the input of converter module130.

FIG. 8illustrates a schematic block diagram of another embodiment of optical amplifier100. In this embodiment, converter module130is operably coupled to the plurality of single-mode cores140by optical coupler620. Optical coupler620includes one or more lenses to focus pump light170emitted from the converter130to the dimensions of the single-mode cores140.

FIG. 9illustrates a schematic block diagram of another embodiment of optical amplifier100. In this embodiment, both multi-mode waveguide120and the plurality of single-mode cores are operably coupled to converter module130by respective optical couplers620aand620b. Optical coupler620aincludes one or more lenses to focus pump light170emitted from the multi-mode waveguide120to the dimensions of the input of converter module130. Optical coupler620bincludes one or more lenses to focus pump light170emitted from the converter130to the dimensions of the single-mode cores140.

FIG. 10illustrates a schematic block diagram of an embodiment of an optical node700including optical amplifier100. The optical amplifier100in this embodiment is a doped-fiber amplifier, such as an Erbium-doped fiber amplifier (EDFA). The optical amplifier100includes a multi-mode pump laser module110, multi-mode waveguide120, a multi-mode to multiple single-mode fiber converter module130and a plurality of single-mode cores140. The plurality of single-mode cores140is operably coupled to doped fiber730by optical signal coupler720. The doped fiber730includes a plurality of single-mode cores, e.g. a multi-core single-mode fiber or a plurality of active single-mode fibers or a combination thereof.

In operation, the optical signal coupler720receives signal740from a multi-core fiber or a plurality of active single-mode fibers. The optical signal coupler720couples or multiplexes the single-mode pump light180from the plurality of single-mode cores140in a one to one manner to active cores of doped fiber730. The doped fiber730receives the single-mode pump light180and signal740. The pump light180excites the dopant ions, e.g. Er+3, and thereby amplifies the signal740by stimulated emission of photons from the excited dopant ions. The amplified signal740propagates to another optical node710. The multi-mode laser pump module110is thus operable to pump light into active cores of doped fiber730to achieve a core-pumped amplifier.

Optical node700and optical node710inFIG. 10may further include one or more of an add/drop multiplexer, optical transmitter, optical receiver, optical to electrical convertor, electrical to optical convertor, etc.

FIG. 11illustrates a schematic block diagram of another embodiment of an optical node700including optical amplifier100. In this embodiment, the optical amplifier100is operable for Raman amplification of optical signals. The optical amplifier100includes a multi-mode pump laser module110, multi-mode waveguide120, a multi-mode to multiple single-mode fiber converter module130and a plurality of single-mode cores140. The plurality of single-mode cores140is operably coupled to transmission fiber760by optical signal coupler720. The transmission fiber760includes a plurality of single-mode cores, e.g. a multi-core single-mode fiber or a plurality of active single-mode fibers or a combination thereof. In this embodiment, the Raman amplifier is a counter-propagating pump amplifier. In other embodiments (not shown) a co-pumped amplifier may be implemented instead or in addition to the counter-propagating pump.

In operation, in one embodiment, the optical signal coupler720de-multiplexes or decouples received amplified signal light740from the transmission fiber760. The optical signal coupler720also couples single-mode pump light180from the plurality of single-mode cores140in a one to one manner to active cores of transmission fiber760. When pumped by counter-propagating pump light180, transmission fiber760operates as a distributed Raman amplifier to amplify signal740. In some embodiments, the doped fiber730(FIG. 10) or transmission fiber760(FIG. 11) may be located within the optical node700. The multi-mode laser pump module110is thus operable to pump light into active cores of transmission fiber760to achieve a core-pumped amplifier, either by EDFA or Raman amplification.

Optical node700and optical node710inFIG. 11may further include one or more of an add/drop multiplexer, optical transmitter, optical receiver, optical to electrical convertor, electrical to optical convertor, etc.

In various embodiments, the optical amplifier described herein distributes pump light from a multi-mode pump to a plurality of single-mode cores (e.g., either of a multi-core fiber or to a plurality of single-mode fibers) using a multi-mode to multiple single-mode converter module. Thus, embodiments described herein are operable to employ a high power but less complex and less expensive multi-mode pump laser in a core-pumped optical amplifier.

The term “module” is used in the description of the various embodiments of the disclosure. A “module” indicates a device that includes one or more additional components, such as a single processing device or a plurality of processing devices, a functional block, hardware and software stored on memory for performing one or more functions as may be described herein. Note that, if the module is implemented via hardware, the hardware may operate independently and/or in conjunction with software and/or firmware. As used herein, a module may contain one or more sub-modules, each of which may be one or more modules. As may also be used herein, a module may include one or more additional components, such as a single processing device or a plurality of processing devices.

The boundaries and sequence of these functional building blocks may have been arbitrarily defined herein for convenience of description. Alternate boundaries and sequences can be defined so long as the specified functions and relationships are appropriately performed. Any such alternate boundaries or sequences are thus within the scope and spirit of the claimed invention. Further, the boundaries of these functional building blocks have been arbitrarily defined for convenience of description. Alternate boundaries could be defined as long as the certain significant functions are appropriately performed. Similarly, flow diagram blocks may also have been arbitrarily defined herein to illustrate certain significant functionality. To the extent used, the flow diagram block boundaries and sequence could have been defined otherwise and still perform the certain significant functionality. Such alternate definitions of both functional building blocks and flow diagram blocks and sequences are thus within the scope and spirit of the claimed invention. One of average skill in the art will also recognize that the functional building blocks, and other illustrative blocks, modules and components herein, can be implemented as illustrated or by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof.

The disclosure may have also been described, at least in part, in terms of one or more embodiments. An embodiment of the disclosure is used herein to illustrate the disclosure, an aspect thereof, a feature thereof, a concept thereof, and/or an example thereof. A physical embodiment of an apparatus, an article of manufacture, a machine, and/or of a process that embodies the disclosure may include one or more of the aspects, features, concepts, examples, etc. described with reference to one or more of the embodiments discussed herein. Further, from figure to figure, the embodiments may incorporate the same or similarly named functions, steps, modules, etc. that may use the same or different reference numbers and, as such, the functions, steps, modules, etc. may be the same or similar functions, steps, modules, etc. or different ones.

While particular combinations of various functions and features of the disclosure have been expressly described herein, other combinations of these features and functions are likewise possible. The disclosure is not limited by the particular examples disclosed herein and expressly incorporates these other combinations.