Mitigation of crosstalk in multi-core optical fibers under microbending

Abstract

In an optical fiber comprising a central axis (z) and a cladding extending along the z-axis and a coating disposed about the cladding, a twist having a twist period (τ) is imparted to the optical fiber about the z-axis. The twist mitigates microbend induced crosstalk. The cladding comprises a substantially circular axial cross-section. The substantially circular axial cross-section comprises a cladding center and an outer cladding diameter (ODclad). A plurality of cores (e.g., a first core, a second core, etc.) are disposed within the cladding. At least one core is helically disposed about the z-axis to form a helical core, the helical core comprising a helical pitch approximately equal to τ (p:p≈τ). The twist has a twist period (τ) of less than 9.1 centimeters (i.e., τ<9.1 cm).

Description

[Technical field]

[0001] The present disclosure relates generally to optical fibers, and more particularly to multi-core optical fibers. [Background technology]

[0002] Those skilled in the art of optical fiber will appreciate that there are significant differences between macrobending and microbending effects because the principles governing macrobending are different from those governing microbending, and design considerations for single-core fibers may differ from those for multicore fibers because different types of fibers may be affected in unexpected ways by various design parameters. Summary of the Invention

[0003] The present disclosure teaches an optical fiber having multiple cores (also referred to as a multi-core optical fiber or simply multi-core fiber (MCF)). In some embodiments, the optical fiber comprises a central axis (arbitrarily designated as the z-axis or simply z) along which an optical signal is transmitted. The optical fiber further comprises a cladding extending along the z-axis. The cladding comprises a substantially circular axial cross-section (cladding center and cladding outer diameter (ODclad)). In some embodiments, the ODclad is about 80 micrometers (μm) to about 300 μm (also referred to as 80 μm≦ODclad≦300 μm). Preferably, 100 μm≦ODclad≦240 μm, and even more preferred embodiments have 125 μm≦ODclad≦200 μm or 125 μm≦ODclad≦150 μm. In some preferred embodiments, the ODclad is about 100 μm or 125 μm (meaning ODclad≈100 μm or ODclad≈125 μm).

[0004] The optical fiber further comprises a coating disposed around the cladding. The coating has an outer diameter (ODcoat) that depends on ODclad. Preferably, the thickness of the coating ranges from 0 μm (meaning no coating) to about 200 μm, whereby the acceptable range of ODcoat is from about 80 μm to about 700 μm (meaning 80 μm≦ODcoat≦700 μm). In some embodiments (such as when ODclad≈125 μm), ODcoat≈245 μm or ODcoat≈200 μm, while in other embodiments (such as when ODclad≈100 μm), ODcoat≈200 μm or the coating thickness is further reduced such that ODcoat≈180 μm or even ODcoat≈160 μm. It should be noted that as ODcoat decreases or ODclad decreases, the optical fiber becomes more susceptible to microbending effects.

[0005] Since the optical fiber is an MCF, the optical fiber also comprises at least a first core disposed within the cladding and a second core disposed within the cladding. To comply with industry standards for single mode fiber (SMF), each core (e.g., first core, second core, etc.) of the MCF 100 has a maximum polarization mode dispersion (PMD) coefficient of 0.1 picoseconds per square root kilometer (ps / √km). Preferably, the maximum PMD coefficient is 0.04ps / √km (e.g., when the minimum length of the fiber removed from the spool is about 500 meters (m) or greater). Even more preferably, the maximum PMD coefficient is 0.02ps / √km (when the minimum length of the fiber removed from the spool is about 500m).

[0006] The optical fiber exhibits crosstalk from cross-coupling between the first and second cores. The crosstalk increases under microbending conditions. To mitigate the increase in crosstalk due to microbending, the optical fiber is twisted about the z-axis, thus exhibiting a twist (with a twist period of τ). With this twist, at least one core (e.g., the first core) is helically arranged about the z-axis to form a helical core having a helical pitch (p). Because the MCF is twisted about the z-axis, and because the first core is arranged within the cladding, p is approximately equal to τ, which means that p≈τ. In some embodiments, the second core is a central core that extends substantially along the z-axis and includes a spin with a period of τ (due to the twist on the optical fiber). In other embodiments, the second core is a second helical core that, like the first helical core, has a pitch of p≈τ.

[0007] The twist limits the maximum amount of increased crosstalk. In some embodiments, the maximum amount of increased crosstalk is limited to less than about 10 decibels (~10 dB) in the wavelength (λ) range of about 1260 nanometers (nm) to 1360 nm (1260 nm < λ < 1360 nm), known in the industry as the O-band (original band). In other embodiments, the maximum amount of increased crosstalk is limited to less than about 6 dB for 1530 nm < λ < 1565 nm, known in the industry as the C-band (conventional band).

[0008] To limit the maximum amount of increased crosstalk, some embodiments twist the optical fiber such that τ is less than 9.1 centimeters (meaning τ<9.1 cm). In other words, the optical fiber exhibits more than 11 (>11) twists / meter (or spins / meter (spins / m)), which is more than any previously recognized twist (or spin) number. This is because a high spin rate (or high twist rate), such as 50 spins / m (translating to τ≦2 cm), results in rapid degradation of fiber quality during fiber drawing and subsequent breakage.

[0009] For this reason, preferred embodiments of the disclosed optical fiber have a spin rate of τ>2.5 cm (which translates to less than 40 spins / m), with a preferred range of 2.9 cm<τ<6.7 cm (which translates to about 15≦spins / m≦about 35). More preferred embodiments include 3.3 cm<τ<5.0 cm (which translates to about 20≦spins / m≦about 30). Some embodiments of the disclosed twisted MCF have been tested with 3.4 cm<τ<4.0 cm (which means a narrow range of about 25≦spins / m≦about 29), which has shown success in mitigating microbending induced crosstalk.

[0010] Broadly speaking, a twist having a twist period (τ) is imparted to the optical fiber about the z-axis. The twist mitigates microbend-induced crosstalk. The cladding includes a substantially circular axial cross-section. The substantially circular axial cross-section includes a cladding center and an outer cladding diameter (ODclad). A plurality of cores (e.g., a first core, a second core, etc.) are disposed within the cladding. At least one core is helically disposed about the z-axis to form a helical core, the helical core including a helical pitch (p) where p≈τ and τ<9.1 cm.

[0011] Other systems, devices, methods, features, and advantages will be apparent to one with skill in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, be within the scope of this disclosure, and be protected by the accompanying claims.

[0012] Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numbers indicate corresponding parts throughout the several views. [Brief description of the drawings]

[0013] [Figure 1A]FIG. 2 is an axial cross-sectional view of an embodiment of a multi-core optical fiber with axes designated as the x-axis (horizontal) and the y-axis (vertical). [Figure 1B] 1B is a cross-sectional view of the multi-core optical fiber shown in FIG. 1A, with axes designated as the y-axis (vertical) and the z-axis (along the transmission path). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] As the demand for optical fiber transmission capacity increases with the growth of global data traffic, there is an ongoing effort to increase the number of optical fibers in a fiber optic cable. For example, by reducing the outer diameter (OD) of the optical fiber, a fiber optic cable can carry more fibers, thereby increasing the fiber density within the cable duct.

[0015] One approach to increase transmission capacity is to employ spatial division multiplexing (SDM) in multicore optical fibers (or multicore fibers (MCFs)). Unlike single-core optical fibers (or single-mode fibers (SMFs)), MCFs have multiple cores in the same cladding. Because the cores in MCFs are close together, cross-coupling between adjacent cores can result in inter-core crosstalk, an impairment that does not exist in SMFs. Inter-core crosstalk degrades signal quality and thus limits the reach or data capacity of MCFs.

[0016] As can be seen, inter-core crosstalk can be reduced by increasing the spacing between adjacent cores. However, increasing the spacing while keeping the outer cladding thickness (OCT) unchanged increases the OD of the MCF, thereby decreasing the fiber density. In other words, there is a trade-off between decreasing the OD in the MCF and increasing the spacing between the cores. Alternatively, if an increase in the core spacing is made without increasing the OD, the OCT decreases, thereby increasing the excess loss of the outer core, creating a trade-off between crosstalk and fiber loss. There are further trade-offs for improved crosstalk, such as attenuation, mode field diameter (MFD), cutoff wavelength, and manufacturing cost (in addition to fiber density).

[0017] One of the many confounding factors in inter-core crosstalk is the effect of both macro- and micro-bending on crosstalk. Fiber cables and actual field deployment conditions can have a strong effect on the actual crosstalk due to both macro- and micro-bending.

[0018] As will be appreciated by those skilled in the art, industry standards for optical fiber include macrobending loss as part of the fiber specifications. As a result, macrobending loss is routinely and easily tested for during factory fiber qualification. Unlike macrobending, microbending is not addressed in fiber specifications. Also, microbending loss is more difficult and time consuming to characterize than macrobending loss. Furthermore, unlike macrobending testing (performed by wrapping the optical fiber around mandrels of various diameters or bending the optical fiber in some other way), microbending testing is often performed on nominally straight optical fiber with no bends. Some examples of measuring microbending sensitivity are described in International Electrotechnical Commission (IEC) Standard Report (TR) 62221 2.0:2012 Edition, titled "Optical Fiber Measurement Methods - Microbending Sensitivity," which is familiar to those skilled in the art. Because of these differences between macrobending and microbending, approaches to mitigate macrobending loss are not directly translatable to microbending.

[0019] Because of the trade-offs between attenuation, MFD, cutoff wavelength, manufacturing cost, fiber count (or fiber density), macrobending effects on crosstalk, and microbending effects on crosstalk, as well as many other effects (e.g., polarization mode dispersion (PMD)), mitigating microbending crosstalk effects without adversely affecting other transmission impact parameters is not a simple or trivial task.

[0020] The disclosed embodiments seek to mitigate the microbending effect on crosstalk by imparting a twist (or spin) to the MCF. Specifically, the twist period (τ) applied to the MCF is less than 9.1 centimeters (cm), which translates to more than 11 twists / meter. Such a small τ<9.1 cm is much smaller than the spin rates traditionally used to mitigate macrobending losses. This is because a higher spin rate (or higher twist rate) increases the chance of breakage or undesired mechanical twist during fiber pull as the fiber is removed from the spool.

[0021] Providing a broad range of technical solutions to technical problems, reference is now made in detail to the description of the embodiments shown in the drawings. Although several embodiments are described in relation to these drawings, there is no intention to limit the present disclosure to one or more of the embodiments disclosed herein. On the contrary, it is intended to cover all alternatives, modifications, and equivalents. It should be understood that in this disclosure, spin and twist are used interchangeably, and similarly, spin rate and twist rate are used interchangeably in this disclosure unless otherwise expressly or by context dictates.

[0022] 1A and 1B (collectively designated as FIG. 1), the present disclosure illustrates an optical fiber 100 (denoted as MCF) having a central axis (arbitrarily designated as z-axis or simply z) along which optical signals are transmitted. Specifically, FIG. 1A illustrates an axial cross-section of one embodiment of MCF 100, and FIG. 1B illustrates a transverse cross-section of MCF 100. Optical fiber 100 includes a cladding 110 that extends along the z-axis. Cladding 110 includes a substantially circular axial cross-section (cladding center and cladding outer diameter (ODclad)). In some embodiments, ODclad is between about 80 micrometers (μm) and about 300 μm (also referred to as 80 μm≦ODclad≦300 μm). Preferably, 100 μm≦ODclad≦240 μm, and even more preferred embodiments have 125 μm≦ODclad≦200 μm or 125 μm≦ODclad≦150 μm. In some preferred embodiments, ODclad is about 100 μm or 125 μm (meaning ODclad≈100 μm or ODclad≈125 μm).

[0023] Since the optical fiber 100 is an MCF, the optical fiber 100 also includes multiple cores 120, 130a, 130b. In the embodiment of FIG. 1, the MCF includes a central core 120 and peripheral cores (such as a first peripheral core 130a and a second peripheral core 130b). The central core 120, the first peripheral core 130a, and the second peripheral core 130b are disposed within the cladding 110. In accordance with industry standards, each core (e.g., the central core 120 and the peripheral cores 130a, 130b (collectively designated as 130)) has a maximum polarization mode dispersion (PMD) coefficient of 0.1 picoseconds per square root kilometer (ps / √km). Preferably, the maximum PMD coefficient is 0.04 ps / √km (e.g., when the minimum length of the fiber removed from the spool is about 500 meters (m) or greater). In another preferred embodiment, the maximum PMD coefficient (when a minimum length of about 500 m is removed from the spool) is 0.02 ps / √km.

[0024] The optical fiber 100 further comprises a coating 140 disposed around the cladding 110. The coating 140 has an outer diameter (ODcoat). Preferably, the thickness of the coating ranges from 0 μm (meaning no coating) to about 200 μm, whereby the acceptable range of ODcoat is from about 80 μm to about 700 μm (meaning 80 μm≦ODcoat≦700 μm). In some embodiments (such as when ODclad≈125 μm), ODcoat≈245 μm, and in other embodiments (such as when ODclad≈100 μm), ODcoat≈200 μm. It should be noted that as the ODcoat decreases or the ODclad decreases, the optical fiber becomes more susceptible to microbending effects.

[0025] The optical fiber 100 exhibits crosstalk from cross-coupling between the first core 130a and the second core (whether central core to peripheral core crosstalk or peripheral core to peripheral core crosstalk). The crosstalk increases under microbending conditions. To mitigate the increase in crosstalk due to microbending, the optical fiber 100 is twisted around the z-axis and thus exhibits a twist (with a twist period of τ). This twist causes at least one core (e.g., the first core 130a) to be helically arranged around the z-axis, as shown in FIG. 1B, forming a helical core with a helical pitch (p).

[0026] Because the MCF 100 is twisted about the z-axis, and because the first core 130a is disposed within the cladding 110, p is approximately equal to τ, meaning that p≈τ. In some embodiments, the second core is a central core 120 that extends substantially along the z-axis and includes a spin with a period of τ (due to the twist on the optical fiber). In other embodiments, the second core is a second helical core 130b that, like the first helical core 130a, has a pitch of p≈τ.

[0027] The twist limits the maximum amount of increased crosstalk between adjacent cores. In some embodiments, the maximum amount of increased crosstalk is limited to less than about 10 decibels (~10 dB) in the wavelength (λ) range of about 1260 nanometers (nm) to 1360 nm (1260 nm < λ < 1360 nm), known in the industry as the O-band (original band). In other embodiments, the maximum amount of increased crosstalk is limited to less than about 6 dB for 1530 nm < λ < 1565 nm, known in the industry as the C-band (conventional band).

[0028] To limit the maximum amount of increased crosstalk, some embodiments twist the optical fiber 100 such that τ is less than 9.1 centimeters (meaning τ<9.1 cm). In other words, the optical fiber 100 exhibits more than 11 (>11) twists / meter (or spins / meter (spins / m)), which is higher than any previously recognized twist rate (or spin rate). This is because a high spin rate (or high twist rate), such as 50 spins / m (which translates to τ≦2 cm), results in rapid degradation of fiber quality during fiber draw and subsequent breakage.

[0029] Thus, preferred embodiments of the disclosed optical fiber 100 have a spin rate of τ>2.5 cm (which translates to less than 40 spins / m), with a preferred range of 2.9 cm<τ<6.7 cm (which translates to about 15≦spins / m≦about 35). More preferred embodiments include 3.3 cm<τ<5.0 cm (which translates to about 20≦spins / m≦30). Some embodiments of the disclosed twisted MCF have been tested with 3.4 cm<τ<4.0 cm (which means a narrow range of ∼25≦spins / m≦about 29), which has shown success in mitigating microbend induced crosstalk.

[0030] Spinning or twisting the optical fiber 100 provides additional benefits, such as further reduction and stabilization of PMD. Because MCFs have an asymmetric structure, the cladding 110 around the cores 120, 130 necessarily exhibits substantial frozen-in thermal stress from the drawing process, which is often orders of magnitude higher than the thermal stress in standard SCFs (single-core fibers). This stress induces optical birefringence, which is the cause of PMD. A spin period of less than 9.1 cm can reduce PMD while simultaneously reducing the microbending effect on crosstalk.

[0031] Experimentally, the disclosed MCF 100 embodiments were tested in the narrow range of 3.4 cm<τ<4.0 cm (meaning about 25≦spins / m≦about 29) and compared to non-spun (or untwisted) fibers. The non-spun (or untwisted) optical fiber showed an increase in inter-core crosstalk of about 17 dB / km in the O-band and about 6 dB / km in the L-band (for the long wavelength band, where 1565 nm≦λ≦1625 nm) under microbending conditions (compared to when no microbending is applied). Unlike the non-spun (or untwisted) optical fiber, the twisted optical fiber 100 showed an increase in inter-core crosstalk of only about 7 dB / km in the O-band and only about 4 dB / km in the L-band under microbending conditions (compared to when no microbending is applied). Thus, the experimental results showed an improvement of about 10 dB / km in O-band and about 2 dB / km in L-band for τ of about 25 to about 29 spins / m.

[0032] In another implementation, the microbending loss of the optical fiber 100, as averaged across all of the cores 120, 130, was reduced by approximately 0.04 dB / km.

[0033] Measuring PMD coefficient values ​​in both twisted and untwisted optical fibers, optical fibers with a twist of about 30 spins / meter (or τ ≈ 3.3 cm) showed improved PMD. For example, the untwisted optical fiber, when tested on a spool, showed a measured PMD coefficient of up to about 0.3 ps / √km (e.g., about 0.28 ps / √km), but when removed from the spool, showed a measured PMD coefficient of up to about 4.8 ps / √km (e.g., about 4.33 ps / √km). The latter value continued to increase proportionally with longer unspooled fiber lengths. This demonstrated that cable deployment can increase PMD-related signal impairments.

[0034] To compare untwisted optical fiber with twisted optical fiber, at a spin rate of about 30 spins / meter (τ≈3.3 cm), the PMD coefficient reaches from 0.28 ps / √km (for untwisted optical fiber on a spool) to 0.028 ps / √km (for twisted optical fiber on a spool), which is an order of magnitude improvement. As a result of the off-spool comparison, the off-spool PMD coefficient drops from about 4.33 ps / √km (for untwisted optical fiber) to about 0.03 ps / √km (for twisted optical fiber with τ≈3.3 cm), which is more than two orders of magnitude improvement. Importantly, the on-spool and off-spool PMD values ​​become essentially the same, which indicates that the low PMD in twisted optical fiber can be fully maintained during MCF cabling and field deployment. Furthermore, unlike most current transmission optical fibers, PMD on twisted MCF can be accurately measured on-spool, which no longer requires the low mode coupling approach typically used in production.

[0035] It is further noted that the high spin rate did not pose any significant impairment to other important parameters (e.g., core density, cutoff wavelength, MFD, attenuation, coating quality, fiber strength, etc.) as discussed above. Moreover, imparting such twist or spin to the MCF did not require any specialized equipment and was achieved by standard drawing tower equipment.

[0036] Finally, depending on the degree of birefringence in the MCF, spinning conditions may require additional optimization or tuning (compared to SMF), however, maximum spin rates (e.g., τ ≈ 2.5 cm) are achievable with standard draw tower equipment.

[0037] While exemplary embodiments have been shown and described, it will be apparent to those skilled in the art that many changes, modifications, or variations can be made to the present disclosure as described, and therefore, all such changes, modifications, and variations should be considered within the scope of the present disclosure.

Claims

1. A central axis (z); a cladding extending along said central axis (z) and having a substantially circular axial cross-section, said substantially circular axial cross-section comprising a cladding center, said substantially circular axial cross-section further comprising a cladding outer diameter (ODclad); a twist about said central axis (z) having a period of less than 9.1 centimeters (τ: τ<9.1 cm); a first core disposed within the cladding and helically arranged about the central axis (z) to form a helical core, the helical core including a helical pitch p approximately equal to τ (p≈τ); a second core disposed within the cladding; crosstalk from cross-coupling between the first core and the second core, where the increased crosstalk under microbending conditions increases up to a maximum amount of increased crosstalk, where the maximum amount of increased crosstalk is limited by the twist; and a coating disposed around the cladding and having an outer coating diameter (ODcoat).

2. 2. The optical fiber of claim 1, wherein the first core includes a maximum polarization mode dispersion (PMD) coefficient of 0.1 picoseconds per square root kilometer (ps / √km).

3. The optical fiber according to claim 2, wherein the maximum PMD coefficient is 0.04 ps / √km.

4. The optical fiber according to claim 2, wherein the maximum PMD coefficient is 0.02 ps / √km.

5. 2. The optical fiber of claim 1, wherein the maximum amount of increased crosstalk is limited to less than about 10 decibels (.about.10 dB) in the wavelength (.lambda.) range of about 1260 nanometers (nm) to 1360 nm (1260 nm<.lambda.<1360 nm).

6. 2. The optical fiber of claim 1, wherein the maximum amount of increased crosstalk is limited to less than about six decibels (.about.6 dB) in the wavelength (.lambda.) range of about 1530 nanometers (nm) to 1565 nm (1530 nm<.lambda.<1565 nm).

7. 2. The optical fiber of claim 1, wherein the cladding outer diameter (ODclad) is from about 80 micrometers (μm) to about 300 μm (80 μm≦ODclad≦300 μm).

8. The optical fiber according to claim 7, characterized in that the cladding outer diameter (ODclad) is approximately equal to 125 μm (ODclad≈125 μm).

9. 2. The optical fiber of claim 1, wherein the coating outer diameter (ODcoat) is from about 80 micrometers (μm) to about 700 μm (80 μm≦ODcoat≦700 μm).

10. The optical fiber according to claim 9, characterized in that the coating outer diameter (ODcoat) is approximately 245 μm.

11. The optical fiber according to claim 9, characterized in that the coating outer diameter (ODcoat) is approximately 200 μm.

12. 2. The optical fiber of claim 1, wherein τ is greater than 2.5 cm (τ>2.5 cm).

13. 13. The optical fiber according to claim 12, wherein τ is in the range of 2.9 cm to 6.7 cm (2.9 cm<τ<6.7 cm).

14. 14. The optical fiber of claim 13, wherein 3.3 cm<[tau]<5.0 cm.

15. 15. The optical fiber of claim 14, wherein 3.4 cm<[tau]<4.0 cm.

Images