Patent Description:
The demand for Internet traffic and cloud computing places high demands on our current optical communications infrastructures. In particular, high bandwidth links are required for short-reach interconnects such as those found in data centers. Mode division multiplexing (MDM) over few-mode fibers (FMFs) is a promising approach to satisfy these data traffic demands. In MDM systems, the crosstalk between the spatial modes is one of the most critical impairments to signal quality.

In present MDM systems, multiple-input multiple-output (MIMO) digital signal processing (DSP) are generally required to compensate for channel crosstalk and receive information data at the receiver side. A drawback of this full-MIMO approach is its complexity. Although the complexity can be reduced to some extent by minimizing differential modal group delay, it still requires heavy computational resource and power consumption.

In order into reduce the DSP complexity, various design strategies in FMF have been proposed. One approach is to increase the effective index difference, δneff, between the spatial modes in order to reduce the complexity of the MIMO-DSP components.

Although a partial MIMO-DSP approach may reduce the system complexity it is still desirable to have a MIMO-free design. To achieve this, some solutions propose polarization-maintaining (PM) FMF designs in which the polarization degeneracies of the spatial modes are reduced by using elliptical ring-core fiber (ERCF), PANDA ring-core fiber, or ERCF with an inner air hole. These fiber designs aim to enhance the effective index difference between adjacent vector modes. However, these solutions suffer from the drawback that the number of modes is limited.

Therefore, there is a need for optical fiber, that is not subject to one or more limitations of the prior art.

The document <NPL> shows the construction of an optical fiber with an elliptical core. The document <CIT> also shows an optical fiber with an elliptical core. The document <CIT> shows an optical fiber with a depressed index layer made of silica with holes. The document BO-HUN CHOI ET AL: "New pump wavelength of <NUM>-nm band for long-wavelength-band erbium-doped fiber amplifier (I-band EDFA)", XP <NUM> shows a regular erbium-doped fiber with a circular core. The document <NPL> shows an optical fiber with an elliptical ring-core fiber.

The document <NPL> discloses an optical fiber with an elliptical core with an ellipticity between <NUM>-<NUM>.

An object of embodiments of the present invention is to provide an improved optical fiber for use in an MDM communications system.

An aspect of the invention as defined by independent claim <NUM> includes an optical fiber including a core having an elliptical cross section and an ellipticity between <NUM> and <NUM>. The optical fiber also includes a cladding having a circular cross section, with the cladding enclosing the core.

An aspect of the disclosure includes the core and the cladding having a common central axis.

An aspect of the disclosure includes an optical fiber wherein a difference of a refractive index of the cladding to a refractive index of the core is between <NUM> x <NUM><NUM> and <NUM> x <NUM>-<NUM>.

An aspect of the disclosure includes an optical fiber wherein a ratio of a refractive index of the cladding to a refractive index of the core is between <NUM> x <NUM>-<NUM> and <NUM> x <NUM>-<NUM>.

An aspect of the disclosure includes an optical fiber comprises a trench located between the core and the cladding. The trench has a uniform width and encircles the core. The refractive index of the trench is lower than the refractive index of the cladding.

An aspect of the disclosure includes an optical fiber wherein a width of the core along a y-axis allows for single mode transmission.

An aspect of the invention as defined by independent claim <NUM> includes an optical fiber wherein a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

An aspect of the invention as defined by independent claim <NUM> includes an optical fiber wherein each of the plurality of mode pairs have two orthogonal linear polarizations.

An aspect of the invention as defined by independent claim <NUM> includes an optical fiber wherein the plurality of mode pairs have an effective index separation between the adjacent vector modes greater than <NUM> x <NUM>-<NUM>.

An aspect of the invention as defined by independent claim <NUM> includes an optical fiber wherein the effective index separation is caused by thermal stress induced during the manufacture of the optical fiber and the elliptical shape of the core.

An aspect of the disclosure includes an optical fiber wherein the core is doped with rare earth ions.

An aspect of the disclosure includes an optical fiber amplifier (OFA) including a first WDM coupler receiving an input signal and an output from a first pump optical source. An optical fiber receiving an output from the first WDM coupler. The optical fiber includes a core having an elliptical cross section having an ellipticity between <NUM> and <NUM>. The optical fiber also includes a cladding, having a circular cross section, enclosing the core. The OFA also includes a second WDM coupler receiving the output of the optical fiber and an output of a second pump optical source. The second WDM coupler outputs an amplified optical signal.

An aspect of the disclosure includes an OFA wherein the core and the cladding have a common central axis.

An aspect of the disclosure includes an OFA wherein the core is doped with rare earth ions.

An aspect of the disclosure includes an OFA wherein the optical fiber includes a trench located between the core and the cladding. The trench has a uniform width and encircles the core. The refractive index of the trench is lower than the refractive index of the cladding.

An aspect of the disclosure includes an OFA wherein a width of the core along a y-axis allows for single mode transmission and a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

An aspect of the disclosure includes a method for manufacturing an optical fiber as disclosed in independent claim <NUM>. The method includes preparing a cylindrical preform having a cross section comprising an inner core and an outer cladding. The inner core has a circular profile. Cutting two opposing sides of the cylindrical preform along a length of the cylindrical preform to produce a cut preform with opposing parallel surfaces along a longitudinal axis of the cut preform. Heating the cut preform until the cut preform has a circular profile and an inner core of the cut preform has an elliptical profile. Pulling the cut preform to form the optical fiber having a core with an elliptical profile. The core has an ellipticity between <NUM> and <NUM>, and a cladding has a circular cross section, with the core being enclosed by the cladding.

An aspect of the disclosure includes the core and the cladding have a common central axis.

An aspect of the disclosure includes a trench portion situated between the core and the cladding. The trench portion is present in the optical fiber and isolates the core from the cladding.

An aspect of the disclosure includes a cylindrical preform that is fabricated using a modified chemical vapor deposition (MCVD) process.

In another aspect of the disclosure the preform further includes a trench portion situated between the core and the cladding. The trench portion is present in the optical fiber and isolates the core from the cladding.

In another aspect of the disclosure, the elliptical profile is formed due to surface tension and the flow of material during the heating.

Embodiments have been described above in conjunctions with aspects of the present invention upon which they can be implemented. Those skilled in the art will appreciate that embodiments may be implemented in conjunction with the aspect with which they are described but may also be implemented with other embodiments of that aspect. When embodiments are mutually exclusive, or are otherwise incompatible with each other, it will be apparent to those skilled in the art. Some embodiments may be described in relation to one aspect, but may also be applicable to other aspects, as will be apparent to those of skill in the art.

Embodiments of the invention comprise a polarization-maintaining highly elliptical core fiber (HECF). By combining the highly elliptical core shape and the thermal stress induced during the fiber fabrication, the birefringence values are elevated to be higher than 1x10-<NUM>, which reduces the mode coupling and in turn makes this fiber suitable for use in MIMO-free MDM transmission systems.

<FIG> illustrates a cross-section view of a highly elliptical core fiber (HECF) <NUM> according to a first embodiment of the invention. The optical fiber comprises a cladding <NUM>, which is typically the industry standard, <NUM> in diameter. At the center of the fiber is a solid core <NUM> surrounded by a trench <NUM>. Though not shown in <FIG>, in most embodiments, the cladding is further surrounded by a buffer, which is in turn covered by a jacket as is known in the art.

The cross-section profile of the core <NUM> is elliptical in shape. The core's cross section <NUM> has an x-axis <NUM> and a y-axis <NUM>. In one embodiment the core <NUM> has approximate cross-sectional dimensions of <NUM> <NUM> along the x-axis <NUM> and <NUM> along the y-axis <NUM>. The dimension along the short, y-axis <NUM>, is selected to allow for single mode operations, n = <NUM> for TEm,n/TMm,n modes. The dimension of the x-axis <NUM> is selected to allow for multiple spatial modes to be distributed in a one-dimensional array in the x-axis <NUM>. In other embodiments, the ellipticity, η, of the core, defined as the ratio of the cross-sectional length in the x-direction to the cross-sectional length in the y-direction, is between <NUM> and <NUM>. Those skilled in the art will appreciate that the x-axis <NUM> may also be referred to as the major axis of the elliptical core, while the y-axis <NUM> may be referred to as the minor axis of the elliptical core. Rotation of the fiber so that the major axis is no longer horizontal should not be considered to be changing the alignment of the X and Y axes. Those skilled in the art will also appreciate that the ellipticity, η, may also be considered as the ratio of the width of the core along the x-axis <NUM> to the width of the core along the y-axis <NUM>.

The core <NUM> may be thought of as elliptic cylinder, such that the length of the x-axis <NUM> is greater than the length of the y-axis and therefore, the x-axis <NUM> forms the major axis of the ellipse and the y-axis <NUM> forms the minor axis of the ellipse. The x-axis <NUM> and the y-axis <NUM> intersect at a right angle and the intersection of the x-axis <NUM> and the y-axis <NUM> defines the center of the elliptical cross-section. The core <NUM> has the shape of an elliptic cylinder with a central axis, also known as a longitudinal axis, running through the center of each elliptical cross-section of the core <NUM>. The cladding <NUM> may be viewed as having a circular cross section with a center and has a cylindrical shape with a central axis running through the center of each circular cross-section of the cladding <NUM>. The core <NUM> and the cladding <NUM> have a common central axis, such that the core <NUM> is centered within the cladding <NUM>. Accordingly, the HECF is formed so that the cladding <NUM> and the core <NUM> share a common central axis.

Optical fiber is manufactured using a silica glass material comprised mainly of SiO<NUM> and the cladding <NUM> is comprised of a substantially pure silica glass material. The core <NUM> is manufactured by doping the SiO<NUM> material with GeO<NUM>. In some embodiments, the core <NUM> is comprised of SiO<NUM> doped with <NUM> mol% of GeO<NUM>.

The trench <NUM> surrounds the core <NUM> and, in some embodiments is approximately <NUM> wide <NUM>. The trench is made of SiO<NUM> doped with F, fluorine. In some embodiments the trench is doped with <NUM> mol% of F. The bending loss experienced by the HECF increases as the ellipticity increases, however the trench serves to reduce the bending loss of higher-order modes in the optical fiber.

<FIG> shows the refractive index profile along the x-axis <NUM> for the optical fiber <NUM>. The core <NUM>, trench <NUM>, and cladding <NUM> have differing refractive indexes determined by their respective materials. In one embodiment the difference in refractive index between the core <NUM> and cladding <NUM>, Δn <NUM>, is approximately <NUM>×<NUM>-<NUM>. In embodiments, the refractive index difference between the core <NUM> and the cladding <NUM> is within the range <NUM>×<NUM>-<NUM> ≤ Δn ≤ <NUM>×<NUM>-<NUM>. In one embodiment the difference in refractive index between the trench <NUM> and the cladding <NUM>, -Δntrench <NUM>, is approximately <NUM>×<NUM>-<NUM>.

The geometry of the highly elliptical core leads to geometry-enhanced birefringence in the optical fiber. This source of birefringence is related to the geometric structure of the core, where a higher core ellipticity will cause a higher value of birefringence.

Referring to <FIG>, during manufacture of the optical fibre, inconstant, compressive thermal stress <NUM> is induced into the core <NUM>, mainly along the y-axis <NUM> direction. During the fiber drawing and annealing process, the fiber becomes stressed due to different thermal expansion coefficients of the core <NUM> and the cladding layers <NUM>. Different amounts of stress are applied along long, x-axis <NUM> and short, y-axis <NUM> due to the asymmetry of its core structure. With the increase of this stress, the isotropic glass starts to become anisotropic, with a consequent change of the refractive indices along the principal, x and y stress directions. The thermal stress has the effect to increase the effective index difference (δneff) between the two polarizations of the same spatial mode.

Optical fiber according to an embodiment of the invention may transmit data on five spatial mode groups with two-fold polarization degeneracy to obtain ten MDM channels. The supported modes are labeled as TE1n and TM1N, depending on the polarization state (respectively along the x-axis or the y-axis). In particular, the subscript "<NUM>" indicates the single mode operation along the short-axis (y-axis), while "n" is an integer indicating the number of the dark lines of the mode profile along the long-axis (x-axis).

The elliptical shape of the core combined with thermal stress created during manufacturing produces a relatively large birefringence so that the modal effective index separation, δneff is greater than <NUM> x <NUM>-<NUM>. This reduces the mode coupling inside the HECF so that the fiber modes can propagate without significant crosstalk up to a fiber length of approximately <NUM>. This allows for transmission systems to be designed that do not require MIMO DSPs.

In some embodiments the core may be doped with rare earth ions such as erbium and/or ytterbium in order to produce an optical active fiber.

<FIG> illustrates an optical fiber <NUM> according to another embodiment with an alternative structure. In comparison to the embodiment depicted in <FIG>, this embodiment comprised a cladding <NUM> and core <NUM>, but lacks a trench <NUM> between the core and cladding. The trench <NUM> helps to reduce the bending loss of higher-order modes in the fiber. However, in some embodiments, the bending loss may also be reduced by increasing the refractive index difference <NUM>, Δn, to approximately <NUM>×<NUM>-<NUM>.

<FIG> shows the refractive index profile along the x-axis <NUM> for the optical fiber <NUM>. The core <NUM> and cladding <NUM> have differing refractive indexes determined by their respective materials. In one embodiment the difference in refractive index between the core <NUM> and cladding <NUM>, Δn <NUM>, is approximately <NUM> x <NUM>-<NUM>.

<FIG> illustrates a system incorporating embodiments of the invention. The invention can be used in the MIMO-free mode-division multiplexing system. The outputs from WDM transmitters <NUM> are input to WDM couplers <NUM> and are multiplexed into a single output fiber. Each of the outputs from the WDM couplers <NUM> are then multiplexed by the mode multiplexer <NUM> and converted into a corresponding mode in the HECF <NUM> and. After transmission through the HECF <NUM>, the fiber modes are received and separated by the mode demultiplexer <NUM> following by a number of WDM couplers <NUM> to separate the wavelength channels and finally be detected by the receivers <NUM>. Due to the low crosstalk among the modes inside the fiber, a MIMO-DSP is not required.

<FIG> illustrates an embodiment of a mode division multiplexer utilizing HECF <NUM>. Optical inputs of different modes TE<NUM>, TE<NUM>,. , TE1n may be multiplexed onto the HECF <NUM> using fused fiber mode couplers <NUM>.

<FIG> illustrates an embodiment of a mode division demultiplexer utilizing HECF <NUM>. Fused fiber mode couplers <NUM> pass light of a predetermined mode to output optical fibers <NUM> in order to receive the signals from TE<NUM>, TE<NUM>,.

<FIG> illustrates an optical fiber amplifier (OFA) <NUM> comprising an HECF <NUM> according to an embodiment. The HECF <NUM> has a core that is doped with rare earth ions such as erbium and connects two WDM couplers <NUM>. Each of the WDM couplers receives the output of pump diodes <NUM>. The optical power of the input <NUM> is amplified by the OFA and output on fibre <NUM>.

<FIG> illustrates a sensing system <NUM> according to another embodiment. The HECF <NUM> passes through sensing elements <NUM>. The signal source and data acquisition module are co-located in device <NUM>. In some embodiments, sensing elements <NUM> are fiber Bragg gratings (FBG). When light from the optical signal source <NUM> passes through the sensing elements <NUM>, light of a predetermined fiber mode is back reflected to the source and received by the data acquisition module <NUM>.

<FIG> illustrates an optical transmitter <NUM> with an integrated mode-division multiplexing module. The transmitter is typically contained in a housing to enable it to be easily integrated into optical transmission equipment. A transmitter <NUM> may be packaged on its own or be integrated with a receiver to form a transceiver. In some embodiments a light source <NUM>, which may be a diode or laser, outputs at a continuous wavelength or mode. Data is applied to the light source <NUM> by a modulator <NUM> which is then multiplexed with other modulated light sources to be transmitted on the HECF <NUM>. In some embodiments, the light source and modulator may be integrated together.

<FIG> illustrates an optical receiver <NUM> with an integrated mode-division de-multiplexing module. The receiver is typically contained in a housing to enable it to be easily integrated into optical transmission equipment. A receiver <NUM> may be packaged on its own or be integrated with a transmitter <NUM> to form a transceiver. Mode multiplexed data is received from the HECF <NUM> and split into its modes in the mode demultiplexer <NUM>. Light of each mode is then received optical detectors <NUM> which may be PIN diodes or APDs.

<FIG> illustrates an optical transceiver <NUM> with an integrated mode-division multiplexer/demultiplexer module. The transceiver is typically contained in a housing to enable it to be easily integrated into optical transmission equipment. The transceiver <NUM> combines the functionality of a transmitter <NUM> and a receiver <NUM>. In some embodiments a light source <NUM>, which may be a diode or laser, outputs at a continuous wavelength or mode. Data is applied to the light source <NUM> by a modulator <NUM> which is passed through a circulator <NUM> and then multiplexed with other modulated light sources to be transmitted on the HECF <NUM>. In some embodiments, the light source and modulator may be integrated together. Received light passes through the mode MUX/DeMUX <NUM> and into the circulator <NUM> that sends the demultiplexed received light to optical detectors <NUM> which may be PIN diodes or APDs, to receive the data.

As shown in <FIG>, the HECF <NUM> may be fabricated using from a preform prepared using a modified chemical vapor deposition (MCVD) process. The preform has a cylindrical shape and includes a coaxial inner core <NUM> and outer cladding <NUM>. The inner core <NUM> has a circular cross section. Opposing, longitudal sides <NUM>, <NUM> are cut from the preform resulting in two opposing, flat, parallel sides <NUM><NUM>. The cut preform is then heated so that the flat surfaces disappear due to the surface tension and the flow of material. Consequently, the circular core <NUM> becomes elliptical during the heating process. The cut preform with an elliptical core is pulled to form the optical fiber. The resulting optical fiber includes a core <NUM> having an elliptical cross section having an ellipticity between <NUM> and <NUM>, and a cladding <NUM> having a circular cross section. The core <NUM> and the cladding <NUM> have a common central axis with the core <NUM> being enclosed by the cladding <NUM>. Since the preform was fabricated through MCVD, it was able to include a trench <NUM> surrounding the core.

Those skilled in the art will appreciate that it may be possible to make a plurality of cuts that are not necessarily aligned with each other (e.g. without creating longitudinal sides) but that taken as a whole, after the heating induced reshaping, result in the elliptical core discussed above.

<FIG> is a flowchart illustrating a method <NUM> of manufacturing a HECF as shown in <FIG>. In step <NUM>, a cylindrical preform having an inner core <NUM> and an outer cladding <NUM> is formed. The inner core <NUM> has a circular cross section profile. The cylindrical preform may be formed using an MCVD process. In step <NUM>, two opposing sides <NUM><NUM> of the cylindrical preform are cut along a length of the cylindrical preform to produce a cut preform with opposing parallel surfaces <NUM><NUM> along a longitudinal axis of the cylindrical preform. In various embodiments, the cutting of the preform is done in a manner that avoids cutting the inner core <NUM>. In step <NUM>, the cut preform is heated. The surface tension of the exterior surface of the cut preform, subjected to the softening caused by the heating, allows the cut preform to distort until the exterior of the preform is circular again. The surface tension causes a distortion in the entire preform, so that when the cut preform assumes an outer circular profile, the inner core <NUM> has been distorted and result in an elliptical profile for the core <NUM>. As discussed above, this generally elliptical core has an ellipticity of between <NUM> and <NUM>. The outer cladding, after enclosing (or enveloping) the elliptical core has a generally circular cross section. In step <NUM>, the cut and heated preform is pulled to form an optical fiber. In some embodiments, the core <NUM> and cladding <NUM> are generally co-axial, having a common central axis.

An aspect of the disclosure includes an optical fiber including a core having an elliptical cross section and an ellipticity between <NUM> and <NUM>. The optical fiber also includes a cladding, having a circular cross section, enclosing the core.

An aspect of the disclosure includes an optical fiber wherein a difference of a refractive index of the cladding to a refractive index of the core is between <NUM> x <NUM>-<NUM> and <NUM> x <NUM>-<NUM>.

An aspect of the disclosure includes an optical fiber comprising a trench located between the core and the cladding. The trench has a uniform width and encircles the core. The refractive index of the trench is lower than the refractive index of the cladding.

An aspect of the disclosure includes an optical fiber wherein a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

An aspect of the disclosure includes an optical fiber wherein each of the plurality of mode pairs have two orthogonal linear polarizations.

An aspect of the disclosure includes an optical fiber wherein the plurality of mode pairs have an effective index separation between the adjacent vector modes greater than <NUM> x <NUM>-<NUM>.

An aspect of the disclosure includes an optical fiber wherein the effective index separation is caused by thermal stress induced during the manufacture of the optical fiber and the elliptical shape of the core.

An aspect of the disclosure includes an optical sensor detecting an optical transmission of an optical mode. The optical sensor includes a plurality of sensing elements disposed along an optical fiber. The optical fiber includes a core having an elliptical cross section and an ellipticity between <NUM> and <NUM>. The optical fiber includes a cladding having a circular cross section. The core is enclosed by the cladding. The optical fiber receives an input signal including a plurality of vector modes. Each of the plurality of sensing elements reflects one of the plurality of vector modes. A data acquisition module receives one of the plurality of the vector modes reflected by one of the plurality of sensing elements. In some embodiments, the core and the cladding have a common central axis.

An aspect of the disclosure includes an optical transmitter module. The module includes a plurality of light sources. Each of the plurality of light sources outputs a constant light signal at one of a plurality of vector modes. A plurality of optical modulators receive one of the constant light signals and output a modulated light signal. A mode multiplexer receives each of the modulated light signals to produce a mode multiplexed optical output. An optical fiber receives the mode multiplexed optical output. The optical fiber includes a core having an elliptical cross section having an ellipticity between <NUM> and <NUM>. The optical fiber also includes a cladding having a circular cross section, with the core being enclosed by the cladding. In some embodiments. the core and the cladding have a common central axis.

An aspect of the disclosure includes an optical receiver module including an optical fiber. The optical fiber receives a mode multiplexed optical signal. The optical fiber includes a core having an elliptical cross section having an ellipticity between <NUM> and <NUM>. The optical fiber also includes a cladding. The core is enclosed by the cladding. The optical receiver module also includes a mode demultiplexer receiving the mode multiplexed optical signal from the optical fiber and outputting a plurality of modulated light signals. Each of the plurality of modulated light signals includes light at one of a plurality of vector modes. A plurality of optical detectors each receive one of the plurality of modulated light signals. In some embodiments. the core and the cladding have a common central axis.

An aspect of the disclosure includes an optical transceiver including a transmit path and a receive path. The transmit path includes a plurality of light sources outputting a constant light signal at one of a plurality of vector modes. A plurality of optical modulators receives one of the constant light signals and outputs a modulated light signal. A plurality of circulators receives one of the modulated light signals and outputs one of the modulated light signals. A mode multiplexer receives each of the modulated light signals from the plurality of circulators to produce a mode multiplexed optical output. An optical fiber receives the mode multiplexed optical output. The optical fiber includes a core having an elliptical cross section and an ellipticity between <NUM> and <NUM>. The optical fiber also includes a cladding having a circular cross section, with the core being enclosed by the cladding. In some embodiments. the core and the cladding have a common central axis.

The receive path includes a mode demultiplexer receiving the mode multiplexed optical signal from the optical fiber and outputting a plurality of received modulated light signals. Each of the plurality of received modulated light signals includes light at one of the plurality of vector modes. A plurality of optical detectors each receives one of the plurality of received modulated light signals via the plurality of circulators.

An aspect of the disclosure includes a method for manufacturing an optical fiber. The method includes preparing a cylindrical preform having a cross section comprising an inner core and an outer cladding. The inner core has a circular profile. Cutting two opposing sides of the cylindrical preform along a length of the cylindrical preform to produce a cut preform with opposing parallel surfaces along a longitudinal axis of the cut preform. Heating the cut preform until the cut preform has a circular profile and an inner core of the cut preform has an elliptical profile. Pulling the cut preform to form the optical fiber having a core with an elliptical profile. The core has an ellipticity between <NUM> and <NUM>, and a cladding has a circular cross section. The core being enclosed by the cladding. In some embodiments. the core and the cladding have a common central axis.

Claim 1:
An optical fiber (<NUM>) comprising:
a core (<NUM>) having an elliptical cross section, the core (<NUM>) having an ellipticity between <NUM> and <NUM>,
wherein ellipticity is defined as a ratio of a cross-sectional length along an x-axis (<NUM>) to a cross-sectional length along an y-axis (<NUM>),
wherein the x-axis (<NUM>) forms a major axis of the ellipse of the elliptical cross section and the y-axis (<NUM>) forms the minor axis of the ellipse of the elliptical cross section, and
a cladding (<NUM>), having a circular cross section, enclosing the core (<NUM>),
wherein a width of the core (<NUM>) along the x-axis (<NUM>) allows for the transmission of a plurality of mode pairs, wherein each of the plurality of mode pairs is comprised of two orthogonal linear polarizations,
wherein the plurality of mode pairs have an effective index separation between adjacent vector modes greater than <NUM> x <NUM>-<NUM>,
wherein the optical fiber is a polarization maintaining fiber;
and the optical fiber being characterized in that: the effective index separation is caused by thermal stress induced during the manufacture of the optical fiber and the elliptical shape of the core (<NUM>).