Patent Application
20200241199
The invention relates to the field of fiber optic transmission, and, more specifically, to improved few-mode fiber designs for space division multiplexing.
An optical fiber is conventionally constituted of an optical core, which transmits an optical signal, and of an optical cladding, which confines the optical signal within the optical core. To that end, the refractive index of the core, nco, is greater than the one of the cladding, nCl. An optical fiber is generally characterized by a refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference Dn between the refractive index at radius r, n(r), and the refractive index of the optical cladding nCι is shown on y-axis.
Nowadays, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode fiber, the higher order modes (hereafter called HOMs) are cut-off or highly attenuated.
Single-mode fibers are commonly used for long-distance applications, metropolitan networks, access networks and premises applications in telecommunications, CATV and utility networks. To obtain an optical fiber capable to transmit a single-mode optical signal, a core with a relatively small diameter is required (typically between 5 μm and 11 μm). To meet requirements of high speed or bit-rate applications (for example ≥10 Gbps), standard single-mode fibers require use of a modulated single-mode laser emitter tuned to work typically at a wavelength of 1550 nm. However, single-mode fibers suffer from non-linearity problems, which are major limitations on fiber transmission capacity.
Multimode fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs), multi-dwelling units (MDUs) and Data Centers (DCs), more generally known as in-building networks. The core of a multimode fiber typically has a diameter of 50 μm, or 62.5 μm. The most prevalent multimode fibers in telecommunications are the refractive graded-index profile optical fibers. By minimizing the intermodal dispersion (i.e. the difference between the propagation delay times or group velocity of the optical modes along the optical fiber, also called DMGD for Differential Mode Group Delay), such a refractive index profile guaranties a high modal bandwidth for a given wavelength, typically 850 nm.
Since data traffic over fiber optic networks continues to grow exponentially, there is an increasing demand for increasing per-fiber traffic particularly across long distances. To this end, multiplexing techniques have been developed that allow a plurality of separate data streams to share the same optical fiber. Among these techniques, one promising approach is space division multiplexing (SDM), in which a plurality of data channels within a single optical fiber are provided by a respective plurality of optical signal modes guided by the fiber.
Such a technique has required the development of new types of optical fibers, called few-mode optical fibers, which support more than one spatial mode but fewer spatial modes than the multi-mode fibers. Such few-mode fibers, which are notably discussed in the PCT patent document WO2011/094400, support approximately 2 to 50 modes.
Space-division-multiplexed transmissions using Few-Mode Fibers (FMFs) have hence recently received considerable attention because of their potential to multiply the capacity of single-mode transmissions by the number of modes that will be used.
One approach to the design of Few-Mode Fibers consists in minimizing mode coupling, so that each LP (Linear Polarization) mode can be separately detected using simple 2×2 (non-degenerate LP modes) or 4×4 (two-time degenerate LP modes) Multiple Input Multiple Output techniques, regardless the number of LP modes.
Increasing the number of LP modes that can actually be used is a challenging issue, since mode coupling increases when increasing the number of LP modes used for transmission.
A careful design of Few-Mode Fibers is hence required in order to reduce mode coupling that will ultimately limit the transmission reach. One can reduce this mode coupling by increasing the effective index difference between any subsequent LP modes. Such an increase in effective index difference may be achieved by increasing the core refractive index, which however causes an unwanted increase in attenuation losses and decrease in effective area. Effective area should be kept high to limit intra-mode non-linearity.
Patent document US2002/0164140 discloses a family of fiber profiles which exhibit only three well guided modes in the operative “band”. The reduction in the number of modes is accomplished with a change in the refractive index in the core area. The change in refractive index in the core area changes the order of appearance of the modes, thus leading to fewer guided modes, and less multi-path interference (MPI). In one embodiment the refractive index ring comprises an area of depressed refractive index, and the null energy point of one of the guided modes is found therein. In another embodiment, the change in the refractive index in the core is located coincidentally with the null point of a desired mode. In some embodiments negative dispersion on the order of −400 ps/nm/km is experienced, while MPI is minimized. In another embodiment the fiber profile is further characterized by a negative slope suitable for compensating a link of transmission fiber.
Such a prior art FMF only supports 3 LP modes and is designed for dispersion compensation. A ring in the cladding is used to control dispersion and slope of dispersion of a high order mode.
Patent document US2014/0064686 discloses modified step index and GRaded INdex (GRIN) fibers with low core relative delta (near 0.8%) which have desirable properties for transmission. These lower delta fibers have lower attenuation losses due to reduced Rayleigh scattering, which is desirable to improve performance in multiple mode multiplexing. The fiber designs include optimized raised triangle profiles, and depressed cladding profiles, to support two and four LP modes.
Such a prior art design focuses on FMF with strong mode coupling between desired modes and elimination of undesired higher order modes.
None of these prior art designs allow the transmission capacity of a weakly coupled few mode fiber to be increased, while keeping attenuation losses low and Effective Area high.
There is therefore a need for an improved Few Mode Fiber profile, with increased transmission capacity, low attenuation losses and high Effective Area.
In an embodiment of the present disclosure, a few mode optical fiber is disclosed, which comprises an optical core and an optical cladding surrounding the optical core. The few mode optical fiber has a step-index profile (i.e. a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding interface so that the cladding is of a lower refractive index) and the optical cladding has at its outer edge (i.e. at the end of a glass part of the optical fiber) a refractive index nCl. The optical core has a core outer radius R1≥7.5 μm and a core refractive index difference Δn1 with respect to the optical cladding refractive index nCι such that 14.5×10−3<Δn1<24×10−3. The optical cladding comprises:
The present disclosure thus relies on a novel and inventive approach to the design of Few Mode Fibers. Actually, the structural features of such an optical fiber allow the few mode optical fiber to guide more LP modes than the LP modes used for transmission. The higher order modes (HOM), which are not used for transmission, are spatially separated from the modes used for transmission by adding an index ring in the cladding. Such an index ring minimizes the spatial overlapping between the HOM and the LP modes used for transmission. Reducing spatial overlapping between the modes either allows mode coupling to be further reduced, with the same constraints on the index difference between the modes, or constraints on the index difference between the modes to be released, while keeping the same level of mode coupling.
Designing such a few mode fiber in which more LP modes are guided than the number of LP modes used for transmission thus allows the properties of the LP modes used for transmission to be improved. The weakly coupled few mode fiber according to the present disclosure shows an increased transmission capacity, while keeping attenuation losses low and Effective Area high, as compared to prior art FMF.
According to an embodiment of the present disclosure, the core outer radius R1 and the inner cladding inner radius Ri1 are substantially equal, which means that Ri1−R1≤1 μm; and the inner cladding outer radius Ri2 and the ring inner radius Rr1 are substantially equal, which means that: Rr1−Ri2≤1 μm.
According to an embodiment of the present disclosure, the inner cladding refractive index difference Δnclad1 with respect to the optical cladding refractive index nCι is between −0.5×10−3 and 0.5×10−3 at λ=633 nm.
According to another embodiment of the present disclosure, the optical core comprises a depressed inner core surrounding an optical axis of the optical fiber, the depressed inner core having a depressed inner core outer radius Rc such that 0.8 μm<Rc<(R1−2)μm, and a uniform depressed inner core refractive index difference Δnc with respect to the optical cladding refractive index nCι such that 0<Δn1−Δnc<3.0×10−3, the refractive index differences Δnc and Δn1 being determined at λ=633 nm.
Hence, according to this embodiment, the core comprises a depressed inner core with a uniform depressed inner core refractive index difference Δnc, and an outer core with a uniform outer core refractive index difference Δn1. Adding such a depressed inner core has the additional advantage of increasing the minimum index difference between the LP21 and LP02 modes, while reducing the coupling between the modes.
According to an embodiment, the optical cladding comprises an intermediate cladding with an intermediate cladding inner radius Rint1 and an intermediate cladding outer radius R2, the intermediate cladding having an intermediate cladding refractive index difference Δnclad2 with respect to the optical cladding refractive index nCι between −1.0×10−3 and 1.0×10−3 at λ=633 nm, preferably between −0.5×10−3 and 0.5×10−3 at λ=633 nm.
According to an embodiment, the ring outer radius Rr2 and the intermediate cladding inner radius Rint1 are substantially equal, which means that Rint1−Rr2≤1 μm.
According to an embodiment, the intermediate cladding refractive index difference Δnclad2 is such that |Δnclad1−Δnclad2|≤1.0×10−3. It is also possible that both the inner and the intermediate cladding have the same refractive index difference (Δnclad1=Δnclad2).
According to an embodiment, the intermediate cladding outer radius is such that R2≤30 μm. Such a feature eases manufacturing of the optical fiber.
According to an embodiment, the few mode optical fiber of the present disclosure guides at least n+1 LP modes, where n is an integer such that 6≤n≤12, said at least n+1 LP modes comprising first n LP modes and at least one Higher Order Mode (HOM). A maximum Coupling-Overlapping Coefficient COCmax between any two modes of said first n LP modes is below 25%, a maximum Coupling-Overlapping Coefficient COCmax between any mode of said first n LP modes and any Higher Order Mode of said at least one Higher Order Mode is below 15%, where said Coupling-Overlapping Coefficient COCμν,κι between a LPμν mode and a LP mode is defined by the formula:
ψμν a mode field amplitude distribution of the LPμν mode at radius r and angle θ, i.e. at polar distance r and angle θ coordinates system of axes transverse to and centered relative to the fiber, at the operating transmission wavelength for which the optical fiber is intended λop,
ψκι a mode field amplitude distribution of the LPκι mode at radius r and angle θ, i.e. at polar distance r and angle θ in a coordinates system of axes transverse to and centered relative to the fiber, at the operating transmission wavelength for which the optical fiber is intended, at λop,
neff
neff
μ, ν, κ, ι being non-negative integers.
Hence, such a FMF guides at least six weakly-coupled LP modes used for transmission, and one or several unwanted higher order modes, which spatial overlapping with the used modes is minimized, thanks to the index ring added in the cladding. Moreover, the structural features of the ring index, width and position are properly chosen to get a Coupling-Overlapping Coefficient (COC) below 25% between the used LP modes, and below 15% between the unwanted HOM and the LP used modes.
The quantity ημν,κι represents the spatial overlapping of the energy of the fields ψμν for the LPμν mode, and ψκι for the LPκι mode. The boundaries of the integral range from 0 to 2π for dθ, and from 0 to the diameter of the optical fiber for dr, i.e. from 0 to 62.5 μm for example.
According to an embodiment of the present disclosure, the first n LP modes guided by the optical fiber have an effective area Aeff>80 μm2 at a wavelength λ=λop, where λop is an operating transmission wavelength for which said optical fiber is intended.
Effective Area is thus kept high, which limits non-linear effects in the fiber.
According to an embodiment of the present disclosure, bend losses BL of the first n LP modes guided by the optical fiber are such that |BL|<10 dB/turn, at 10 mm bend radius at λ=λop. BL can be characterized, for instance, by measuring the loss difference of a given mode selected by a mode multiplexer and injected in the few-mode fiber with and without applying a loop of 10 mm radius using a spectral attenuation bench.
According to an embodiment of the present disclosure, a minimum of effective index differences between any two modes LPμν and LPκι among the n+1 first LP modes Δneff
where Dneff(LPμν)=neff
Dneff(LPκι)=neff
and μ, ν, κ, ι being non-negative integers.
Mode coupling in the fiber is thus kept at a low level, which allows simple MIMO techniques to be used at reception.
According to an embodiment of the present disclosure, the fundamental LP01 mode guided by said optical fiber has an attenuation loss smaller than 0.28 dB/km at λ=λop. According to an embodiment of the present disclosure, λop is between 1300 nm and 1600 nm, preferably λop=1550 nm. It is noted that the FMFs described herein and throughout the document are suitable for use within, at a minimum, the entire “C-band” (1530 nm-1565 nm), but also with a suitable choice of the fiber structural features the S-(1460 nm-1530 nm), L-(1565 nm-1625 nm) and U-bands (1625 nm-1675 nm).
The present disclosure also concerns an optical link comprising at least one few mode optical fiber according to embodiments of the present disclosure.
Such an optical link may comprise any number of concatenated optical fibers, as long as one of them at least complies with the features set forth in the present disclosure. Such an optical link may also comprise several optical fibers, which would all comply with the features of the present disclosure.
The present disclosure also concerns an optical system comprising at least one few mode optical fiber or at least one optical link according to embodiments of the present disclosure.
The present disclosure can be better understood with reference to the following description and drawings, given by way of example and not limiting the scope of protection, and in which:
The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
The general principle of the present disclosure relies on the careful design of a Few Mode Fiber for transmitting n=6 to n=12 useful LP modes, according to which the fiber profile guides more than n LP modes and the higher order modes (i.e. modes above n) are spatially separated from the modes used for transmission by adding a ring in the cladding.
It is actually recalled that light travelling in an optical fiber forms hybrid-type modes, which are usually referred to as LP (linear polarization) modes. The LP0p modes have two polarization degrees of freedom and are two-fold degenerate, the LPmp modes with m≥21 are four-fold degenerate. These degeneracies are not counted when designating the number of LP modes propagating in the fiber. Hence, a few-mode optical fiber having two LP modes supports the propagation of all of the LP01 and LP11 modes, or a few-mode fiber guiding 6 LP modes supports the propagation of all of the LP01, LP11, LP02, LP21, LP12 and LP31 modes.
Reference will now be made in detail to embodiments of multimode optical fibers, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
One embodiment of a few-mode optical fiber according to the present disclosure is schematically depicted in isometric view in
Though not illustrated on
In embodiments of the present disclosure, the glass core 101 generally has an outer radius R1 greater than or equal to 7.5 μm. Moreover, the index ring 103 has an inner radius Rr1 between 12 μm and 19 μm. In order to satisfy manufacturing constraints, it is preferred to have R2≤30 μm.
In the embodiments shown and described herein, the core 101 and the cladding generally comprise silica, specifically silica glass. The cross-section of the optical fiber 10 may be generally circular-symmetric with respect to the center of the core 101. In some embodiments described herein, the radius of the glass portion of the optical fiber 10 is about 62.5 μm. However, it should be understood that the dimensions of the cladding may be adjusted so that the radius of the glass portion of the optical fiber may be greater than or less than 62.5 μm. The optical fiber 10 also comprises a coating surrounding the cladding. Such a coating may comprise several layers, and it may notably be a dual-layer coating, although these different layers are not shown on
The different portions in the cladding may comprise pure silica glass (SiO2), silica glass with one or more dopants, which increase the index of refraction (e.g. GeO2 or any other known dopant), such as when the portion of the cladding is “up-doped” “(e.g. for the index ring 103), or silica glass with a dopant, which decreases the index of refraction, such as fluorine, such as when the portion of the cladding is “down-doped” (e.g. for the slightly down-doped inner cladding 102 or intermediate cladding 104).
The refractive index profile of
More precisely, the optical core 101 has a core refractive index difference Δn1 with respect to the optical cladding refractive index nCι such that 14.5×10−3<Δn1<24×10−3. The optical cladding refractive index nCι is the refractive index of the optical cladding at its outer edge, i.e. at the end of the glass part of the optical fiber.
Radius R1 corresponds to the core-cladding interface. At distance R1 from the core center, refractive index sharply decreases, to reach a value Δnclad1, which corresponds to the refractive index difference of the inner cladding 102. The inner cladding refractive index difference Δnclad1 with respect to the optical cladding refractive index nCι is between −1.0×10−3 and 1.0×10−3, preferably between −0.5×10−3 and 0.5×10−3.
The cladding comprises an index ring 103, with a ring inner radius Rr1 between 12 μm and 19 μm, a positive ring refractive index difference Δnr with respect to the optical cladding refractive index nCι such that Δn1/Δnr is between 2 and 4, and a ring volume Vring=πΔnr(Rr22−Rr12) between 1.8 μm2 and 4.1 μm2 where Rr2 is the ring outer radius.
At radius Rr2, corresponding to the boundary between the index ring 103 and the intermediate cladding 104, the refractive index decreases sharply and reaches a value Δnclad2, which corresponds to the refractive index difference of the intermediate cladding 104. The intermediate cladding refractive index difference Δnclad2 with respect to the optical cladding refractive index nCι is between −1.0×10−3 and 1.0×10−3, preferably between −0.5×10−3 and 0.5×10−3.
In the embodiment of
In embodiments, the depressed inner core outer radius Rc and the outer core inner radius Rc1 are substantially the same, i.e. Rc1−Rc≤1 μm. In the following description of examples and figures, it is assumed, for sake of simplification, that Rc=Rc1.
Depressed inner core 1011 and outer core 1012 are not illustrated on
Hence, the refractive index profile of
Radius R1 corresponds to the core-cladding interface. At distance R1 from the core center, refractive index sharply decreases, to reach a value Δnclad1, which corresponds to the refractive index difference of the inner cladding 102. The inner cladding refractive index difference Δnclad1 with respect to the optical cladding refractive index nCι is between −1.0×10−3 and 1.0×10−3, preferably between −0.5×10−3 and 0.5×10−3.
The cladding comprises an index ring 103, with a ring inner radius Rr1 between 12 μm and 19 μm, a positive ring refractive index difference Δnr with respect to the optical cladding refractive index nCι such that Δn1/Δnr is between 2 and 4, and a ring volume Vring=πΔnr(Rr22−Rr12) between 1.8 μm2 and 4.1 μm2 where Rr2 is the ring outer radius.
At radius Rr2, corresponding to the boundary between the index ring 103 and the intermediate cladding 104, the refractive index decreases sharply and reaches a value Δnclad2, which corresponds to the refractive index difference of the intermediate cladding 104. The intermediate cladding refractive index difference Δnclad2 with respect to the optical cladding refractive index nCι is between −1.0×10−3 and 1.0×10−3, preferably between −0.5×10−3 and 0.5×10−3.
In the embodiment of
The structural features of the Few Mode fibers of
Such a Coupling-Overlapping Coefficient between a LPμν mode and a LPκι mode is defined by the formula:
ψμν the mode field amplitude distribution of the LPμν mode,
ψκι the mode field amplitude distribution of the LPκι mode,
neff
neff
μ, ν, κ, ι being non-negative integers.
The quantity ημν,κι represents the spatial overlapping of the energy of the fields ψμν for the LPμν mode, and ψ for the LP mode. The boundaries of the integral range from 0 to 2π for dθ, and from 0 to the diameter of the optical fiber for dr, i.e. from 0 to 62.5 μm for example. The mode field amplitude distributions are given at radius r and angle θ, i.e. at polar distance r and angle θ coordinates of a point in a system of axes transverse to and centered relative to the fiber.
Table 1 below lists the features of the refractive index profiles of four exemplary few mode fibers according to the present disclosure. More precisely, examples Ex. 1, Ex. 2 and Ex. 3 correspond to the exemplary embodiment of
All four examples in Table 1 fulfill the structural requirements of:
The detailed characteristics of the four examples Ex. 1, Ex. 2, Ex. 3 and Ex. 4 are disclosed in Table 2 below.
In Table 2, the first column corresponds to the list of characteristics which are measured and evaluated for each exemplary few mode fiber; the second column lists the LP modes; the third to sixth columns respectively correspond to exemplary fibers Ex. 1, Ex. 2, Ex. 3 and Ex. 4. Measurements and evaluations of Table 2 are achieved at an operating wavelength λop=1550 nm.
As may be observed, exemplary fibers Ex. 1, Ex. 2, Ex. 3 and Ex. 4 guide fourteen LP modes, among which the first six modes LP01, LP11, LP21, LP02, LP31 and LP12 are used for transmitting useful information. Eight more HOM, namely LP03, LP13, LP22, LP32, LP41, LP42, LP51, LP61 and LP71 are unwanted modes, which are also guided by the fiber.
Table 2 provides the refractive index difference of each guided LP mode with respect to the refractive index of the outer cladding: Dneff(LPμν)=neff
It thus appears that Δneff
Moreover, for all four examples, the first six modes LP01, LP11, LP21, LP02, LP31 and LP12 have an effective area Aeff greater than 101 μm2 for examples Ex. 1 to Ex. 3 (corresponding to the LP02 mode) and greater than 103 μm2 for example Ex. 4 (also corresponding to the LP02 mode), which limits intra-mode non-linearity. As used herein, the effective area of an optical fiber is the area of the optical fiber in which light is propagated and is determined at the specified mode, at a wavelength of 1550 nm, unless otherwise specified. The effective area Aeff
Where ψμν is the mode field amplitude distribution of the mode LPμν at the radius r, i.e. at the polar distance r in the polar coordinates of a point in a system of axes transverse to and centered relative to the fiber;
Table 2 also provides assessment of the bending losses per turn of 10 mm bending radius for the first six used LP modes for all four exemplary fibers Ex. 1 to Ex. 4. While characterization of FMFs is not standardized yet, the bending loss data illustrated in Table 2 are given according to measurements complying with the requirements of the IEC 60793-1-47 (ed.2.0), which is herein incorporated by reference. To properly characterize macrobending losses of the LP01 mode, 2 m of SMF can be spliced on the injection side of FMF under test to filter out the high order modes. For the high order modes, it is necessary to use mode converters at the input and the output of the FMF to correctly evaluate power in the desired modes.
As may be observed, bending loss remains very low, even for the LP12 mode for which it is below 1 dB/turn at 10 mm radius.
The attenuation loss for the fundamental LP01 mode is of 0.25 dB/km for exemplary fibers Ex. 1 to Ex. 3 and of 0.24 dB/km for exemplary fiber Ex. 4. Loss of LP01 mode can be measured according to IEC 60793-1-40 (ed1.0) standard (method A), which is herein incorporated by reference. However, in order to properly characterize the losses of the fundamental mode, 2 m of SMF can be spliced on the injection side of FMF under test to filter out the high order modes.
The maximum Coupling-Overlapping Coefficient COCmax between the first six LP modes is below 20% for exemplary fibers Ex. 1 to Ex. 3, and amounts to 22% for exemplary fiber Ex. 4. Although the presence of an index ring adds eight HOMs to the first six guided LP modes, its dimensions and position have been chosen so that the maximum Coupling-Overlapping Coefficient COCmax between the HOMs and the first six LP modes is below 15% for all examples.
Table 3 below illustrates in more details the Coupling-Overlapping Coefficient between the 6 guided LP modes of exemplary fiber Ex. 3 (LP01, LP11, LP21, LP02, LP31, LP12) and the two High Order Modes (LP41 & LP42) which have the highest COC values with the guided modes.
The maximum Coupling-Overlapping Coefficient COCmax between the first six LP modes is achieved between LP21 and LP11 and is 14%. The maximum coupling between one of the first six LP modes and the HOM modes not used for transmission is 10%, achieved between LP12 and LP41 modes. The coupling between HOMs can be very high (95% between LP41 and LP42), but this is not a problem, as these high-order modes are not used for transmitting information.
As a comparison, Table 4 provides the refractive index profiles of five other exemplary few mode fibers, namely Ex. 1o to Ex. 5o, which are all out of the scope of the present disclosure.
The detailed characteristics of the five examples Ex. 1o, Ex. 2o, Ex. 3o, Ex. 4o and Ex. 5o are disclosed in Table 5 below.
Like in Table 2, in Table 5, the first column corresponds to the list of characteristics which are measured and evaluated for each exemplary few mode fiber; the second column lists the LP modes; the third to seventh columns respectively correspond to exemplary fibers Ex. 1o, Ex. 2o, Ex. 3o, Ex. 4o and Ex. 5o. Measurements and evaluations of Table 5 are achieved at an operating wavelength λ=1550 nm.
Example Ex.1o is a step index FMF with a depressed inner core supporting 7 LP modes, i.e. LP01, LP11, LP21, LP02, LP31, LP12 and LP41. The requirements of the present disclosure as regards the core (R1=8.01 μm>7.5 μm and core index difference 14.5×10−3<Δn1=16.7×10−3<24×10−3) and the refractive index difference of the intermediate cladding (−1.0×10−3<Δnclad2=−0.2×10−3<1.0×10−3) are fulfilled, however, there is no index ring in the cladding. Table 6 below shows the Coupling-Overlapping coefficient in-between the different guided modes (LP01, LP11, LP21, LP02, LP31, LP12 and LP41).
The maximum coupling between the first six LP modes (COC=20%) is reached between LP11 and LP21 modes. Nevertheless, while Δneff
For this exemplary FMF Ex. 1o, the coupling between the first six LP modes and the HOM is thus too high. According to the present disclosure, a solution to this problem consists in adding a ring in the cladding of the exemplary fiber Ex. 1o, which leads to exemplary fibers Ex. 1 to Ex. 3 already discussed above in this document. As compared to Ex. 1o, the FMF of examples Ex. 1, Ex. 2 and Ex. 3 achieve approximately the same values of Effective Area Aeff, bend losses BL and attenuation losses (see Tables 2 and 5). However, as already discussed, the Coupling-Overlapping Coefficient COCmax between the first six LP modes is reduced below 20% for all three examples (see Table 2), although Δneff
The exemplary fiber Ex. 4o is a FMF supporting only six LP modes, and is also out of the scope of the present disclosure. It is a step-index fiber with a depressed inner core nut no index ring in the cladding. To achieve Δneff
Although the coupling-overlapping coefficient between the first six modes is low enough to achieve a weakly-coupled few mode fiber, the increase in core index causes an unwanted increase in attenuation losses and decrease in effective area Aeff (see Table 5).
As may be observed when comparing
However, if the index ring added in the cladding is not well designed, either in terms of dimensions or position, its effects can be null or even penalizing compared to the design of a FMF fiber profile with no ring. This is illustrated by examples Ex. 2o and Ex. 3o, which are two FMF profiles with a depressed inner core, a ring added in the cladding, yet out of the scope of the present disclosure.
In example Ex. 2o, the requirements of the present disclosure as regards the core (core outer radius R1=8.01 μm>7.5 μm and core index difference 14.5×10−3<Δn1=16.7×10−3<24×10−3) and the refractive index difference of both the inner and intermediate cladding (−1.0×10−3<Δnclad2=Δnclad1=−0.2×10−3<1.0×10−3) are fulfilled. The depressed inner core has an outer radius Rc such that 0.8 μm<Rc=2.8 μm<(R1−2)μm=8.01 μm−2.8 μm=5.21 μm, and a uniform depressed inner core refractive index difference Δnc=14.7×10−3 such that 0<Δn1−Δnc<3.0×10−3. The volume of the index ring is within the specified range: 1.8 μm2<Vring=2.5 μm2<4.1 μm2. The refractive index difference of the ring Δnr=6 is such that Δn1/Δnr is between 2 and 4.
However, the index ring is not properly positioned in the cladding, as it is too close to the core: Rr1=9.5 μm<12 μm.
As a consequence, the maximum Coupling-Overlapping coefficient COCmax between the first six LP modes on the one hand, and between the first six LP modes and the HOMs on the other hand, is increased, when compared to the same profile without ring of exemplary fiber Ex. 1o. Actually, COCmax between the first six LP modes is 20% for Ex. 1o and 22% for Ex. 2o, and COCmax with HOMs is 22% for Ex. 1o and 23% for Ex. 2o (see Table 5).
Adding a ring in the cladding is hence not enough, if it is not well positioned with respect to the core.
In example Ex. 3o, the requirements of the present disclosure as regards the core (core outer radius R1=8.01 μm>7.5 μm and core index difference 14.5×10−3<Δn1=16.7×10−3<24×10−3) and the refractive index difference of both the inner and intermediate cladding (−1.0×10−3<Δnclad2=Δnclad1=−0.2×10−3<1.0×10−3) are fulfilled. The depressed inner core has an outer radius Rc such that 0.8 μm<Rc=2.8 μm<(R1−2)μm=8.01 μm−2.8 μm=5.21 μm, and a uniform depressed inner core refractive index difference Δnc=14.7×10−3 such that 0<Δn1−Δnc<3.0×10−3. The refractive index difference of the ring Δnr=8 is such that Δn1/Δnr is between 2 and 4. The index ring is properly positioned in the cladding, as 12 μm<R1=15.5 μm<19 μm.
However, the volume of the index ring is too small: Vring=1.2 μm2<1.8 μm2.
As a consequence, the maximum Coupling-Overlapping coefficient COCmax between the first six LP modes on the one hand, and between the first six LP modes and the HOMs on the other hand, is increased, when compared to the same profile without ring of exemplary fiber Ex. 1o. Actually, COCmax between the first six LP modes is 20% for Ex. 1o and 23% for Ex. 3o, and COCmax with HOMs is 22% for Ex. 1o and 29% for Ex. 2o (see Table 5).
Adding a ring in the cladding is hence not enough, if its volume is not well designed.
Example Ex. 5o shows a refractive index profile which is similar to that of example Ex. 4, but with no ring added in the cladding. When comparing both profiles, it appears that adding a ring in the cladding for example Ex. 4 allows COCmax with HOMs to be reduced from 18% to 13% while Δneff
Table 8 presents the refractive index profiles of two other exemplary Few-mode fibers according to the present disclosure, namely Ex. 5 and Ex. 6. Both exemplary fibers are step-index fibers, with an index ring added in the cladding, and satisfy the structural requirements of:
Their refractive index profile corresponds to the one illustrated on
Exemplary fiber Ex. 5 guides seventeen LP modes, among which the first seven LP modes are used for transmission, and the ten others are HOMs, which are not used for transmission. The detailed characteristics of exemplary FMF Ex. 5 are disclosed in Table 9 below, which shows the same structure and content as Tables 2 and 5 discussed previously.
Exemplary fiber Ex. 6 guides sixteen LP modes, among which the first ten LP modes are used for transmission, and the six others are HOMs, which are not used for transmission. The detailed characteristics of exemplary FMF Ex. 6 are disclosed in Table 10 below, which also shows the same structure and content as Tables 2 and 5 discussed previously.
Tables 9 and 10 provide the refractive index difference of each guided LP mode with respect to the refractive index of the outer cladding: Dneff(LPμν)=neff
It thus appears that Δneff
Moreover, for example Ex. 5, the first seven LP modes have an effective area Aeff greater than 80 μm2 (corresponding to the LP02 mode); for example Ex. 6, the first ten LP modes have an effective area Aeff greater than 101 μm2 (also corresponding to the LP02 mode). This limits intra-mode non-linearity.
Tables 9 and 10 also provide assessment of the bending losses per turn of 10 mm bending radius for the first used LP modes for both exemplary fibers Ex. 5 and Ex. 6.
As may be observed, bending loss remains very low, even for the LP51 mode of exemplary fiber Ex. 6 for which it is below 1 dB/turn at 10 mm radius. As regards exemplary fiber Ex. 5, the bending loss remains below 0.001 dB/turn at 10 mm radius for all first seven LP modes used for transmission.
The attenuation loss for the fundamental LP01 mode is of 0.28 dB/km for exemplary fiber Ex. 5 and of 0.27 dB/km for exemplary fiber Ex. 6.
The maximum Coupling-Overlapping Coefficient COCmax between the first seven LP modes is 18% for exemplary fiber Ex. 5, and amounts to 22% between the first ten LP modes for exemplary fiber Ex. 6. Although the presence of an index ring adds HOMs to the first weakly-coupled guided LP modes, its dimensions and position have been chosen so that the maximum Coupling-Overlapping Coefficient COCmax between the HOMs and the first seven or ten LP modes is below 15% for all examples, namely of 13% for exemplary fiber Ex. 5 and of 3% only for exemplary fiber Ex. 6.
According to the first embodiment in
Such an optical system may comprise M optical links (or M spans of optical fibers).
In an example, M=1; in another example, M=2; in another example M=5; in yet another example, M=10. In case the optical system comprises M optical links or spans, there is only one mode multiplexer 82, optically connected between transceivers 81 an optical link 50, and only one mode demultiplexer 83, optically connected between optical link 50 and receivers 85.
According to the second embodiment in
Mode demultiplexer 83 demultiplexes the n multiplexed LP modes, and feeds each LP mode into an amplifier 84. At the output of amplifiers 84, LP modes enter receivers 85.
Such an optical system may comprise M optical links (or M spans of optical fibers).
In an example, M=1; in another example, M=2; in another example M=5; in yet another example, M=10. In case the optical system comprises M optical links or spans, it also comprises M mode multiplexers 82, M mode demultiplexers 83, and M amplifiers 84 for each LP mode guided by the optical system.
The embodiment in
The embodiments of
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/000349 | 3/10/2017 | WO | 00 |
