SI22065A - Multimode optical fibre with low differential delay between modes - Google Patents

Abstract

The invention improves the bandwidth, reliability and complexity of telecommunication systems, which are based on multi-mode optical fibres. The invention reduces the differential delay between the modes in reaching a bandwidth exceeding 5 GHz.km regardless of the type of inducing modes in the optical fibre. The invention thus enables the attaining of high bandwidths also in cases, when the highest modes have been induced. This has a favourable impact on the conditions, which the components like light source, connectors, fibre splitters, other optical components, cables etc. have to meet. The invention eliminates the unfavourable effects of deposits, which provide a reduction of the fibre core size and differences between the indices of refraction between the sheath and the core thus providing a higher bandwidth of the optical fibre while at the same time reducing the production cost. This way it is possible to reduce the delay between the fastest and slowest modes below 20 ps/km.

Description

MULTIPLE OPTICAL FIBER WITH LOW DIFFERENTIAL DELAY

The subject of the invention is an optical fiber which enables the bandwidth to be reached beyond 5 GHz.km with an arbitrary set of guided genera. Such a fiber is intended for fast data transmission mainly over shorter distances, but applications in other areas are also possible.

A technical problem solved by the invention is the reduction of differential mode delay (DMD), which limits the bandwidth of the telecommunications system. According to the invention, the shapes of multi-fiber optic profiles are presented, which enable the bandwidth exceeding 5 GHz.km regardless of the excited set of genera, including cases where higher genera or the entire set of aquatic genera are excited in a multifaceted fiber.

The objective of the telecommunications industry is generally to transmit a large amount of information over as short a distance as possible. With the increasing number of users and the complexity of services, the demands for the amount of information transferred are increasing daily.

There are many solutions known in the world that allow for the rapid transmission of information over long distances. Optical communications, which have been the subject of intense development over the last 20 years, make it possible to achieve unprecedented bandwidth in other technologies. Solutions that today use short-range optical transmissions are primarily used by multi-fiber. The advantage of multifaceted fiber lies in the possibility of assembling this fiber with simpler and cheaper sources. In the past, these were mainly LEDs with a wavelength of about 850 nm. Recently, low-cost vertical cavity surface emitting lasers (VCSELs) have emerged on the market, enabling more efficient fiber optic coupling and much higher modulation speeds.

Classical multipurpose fiber has been known for over 25 years and has been standardized by international standards such as eg. ITU G.651. According to the latest standards, such as OM3, such fibers allow bandwidth to be achieved with full excitation of the entire set of aquatic genera up to 1.5 GHz.km. Even when the total system bandwidth of 1.5 GHz.km was reached, the differential delay of the genera was still shown to be still about 1 ns / km, which greatly limits the further increase in bandwidth. The development of low-cost vertical resonator lasers has recently led to the development of a new concept in which the laser selectively excites only the lower genera of optical fiber. In this way, the delay between the fastest and slowest excited genera is reduced, allowing transmission speeds to be reached in excess of 1.5 Gbit.km. This principle has been the subject of much research and has recently been standardized with the IEEE 802.3z and IEEE 802.3ae standards. The selective excitation of genera in a multifaceted fiber brings with it a number of disadvantages, in addition to improving bandwidth. Optical connectors of poor quality, poor connections, some optical components, and in particular cabling and laying of fiber optic cables, can cause an assembly between genera, which can lead to a sudden and unwanted reduction in bandwidth. Selective excitation based systems also require more sophisticated field measurement equipment. It additionally requires the selective excitation of genera to use components with a narrower set of production tolerances, which reduces the cost-effectiveness of transmission systems. The potential biggest drawback of selective excitation in the future may stem from the lack of compatibility of this concept with the emerging VCSEL field technology. The VCSEL array consists of a large number of laser diodes with different wavelengths integrated on the same chip, allowing cost effective wavelength multiplexing. Such a field can be fully coupled to an optical fiber, provided that the size of the core area into which the optical field is coupled is sufficiently large. Therefore, a larger VCSEL field cannot be coupled to a multiple-stranded fiber in a way that would selectively excite only a smaller number of genera.

Therefore, the need for a multi-band fiber with a large bandwidth is evident, even when any or a whole set of aquatic genera is excited.

There are many known embodiments of systems that allow for the increase in the bandwidth of a multi-fiber. As mentioned above, most of these systems are based on standardized selective excitation of genera in a multilayered fiber. There are also individual non-standardized solutions for selective excitation systems, such as those described in U.S. Pat. Nos. 6,580,543 and 6,330,382. enable a bandwidth of up to 500 MHz to be achieved. Such solutions are disclosed by US Patents 6,434,309, US 6,438,303, US 6,618,534 and US 6,724,965. Alternatively, international dispersion compensation according to U.S. Pat. No. 6,363,195 is possible, but its effectiveness is very limited.

No known solution currently allows the bandwidth exceeding 5 GHz.km to be excited or fully excited by the entire set of aquatic genera. In the early years of the development of optical fibers, optical fiber profiles were proposed, which would allow the grouping speeds of higher and lower genera to be equalized. The first such example is the work of K. Okamoto and T. Okoshi, Analysis of Wave Propagation and Optical Fibers Having a Core with a-Power Refractive-lndex Distribution and Uniform Cladding IEEE Transactions on Microwave Theory and Techniques, 24 (7), pp.416 -421, March 1976. In this work, the authors proposed extensions of the optical fiber core so that the edges of the optical fiber core had a lower refractive index than the coating. The problem with this proposal was that it required a very large negative difference in the refractive index at the edge of the elongated nucleus, ie more than -0.5%, which is not compatible with optical fiber fabrication procedures. This work was then continued by Katsunari Okamoto and Takanori Okoshi in the Computer-Aided Synthesis of Optimum Refractive-lndex Profile for Multimode Fiber IEEE Transactions on Microwave Theory and Techniques, 25 (3), pp. 213-221, March 1977. Where, by means of experimentation, a slightly more favorable but still moderate for practical production has been calculated, owing to the large negative difference of the refractive indexes at the edge of the nucleus, an impractical profile. Later, B.Stolz and D.Yevick proposed in Correcting Multimode Fiber Profiles with Differential Mode Delay, J.Optical Communications, vol 4 (1983), no 4 pp. 139-147. The introduction of an additional structure at the edge of the nucleus, which should theoretically allow a partial correction of the differential gender delay to some extent, but the given proposal has never been put into practice for unknown reasons.

According to the invention, the problem is solved by the introduction of a special structure in the profile of a multifaceted optical fiber, which allows equalization of group velocities of higher and lower genera, while at the same time the profile is suitable for production with the currently known methods.

The invention will now be described in more detail by way of examples and figures showing:

FIG. 1 logarithm of relative time delay (RMS) of genera in a standard 50 pm optical fiber at a wavelength of 850 nm. Each point represents one linearly polarized genus. Many points overlap.

FIG. 2 is the logarithm of the relative time delay of the genera in a standard 50 pm optical fiber at a wavelength of 1310 nm. Each point represents one linearly polarized genus. Many points overlap.

FIG. 3 is an example of an improved optical fiber profile with a definition of depressed expanded nucleus and depression. FIG. 4 is the logarithm of the relative time delay of the genera in an improved 50 pm optical fiber at a wavelength of 850 nm. Each point represents one linearly polarized genus. Many points overlap.

FIG. 5 is the logarithm of the relative delay of genera in an enhanced 50 pm optical fiber at a wavelength of 1310 nm. Each point represents one linearly polarized genus. Many points overlap.

FIG. 6 absolute delay in an improved 50 pm optical fiber at a wavelength of 850 nm for different profile parameters. Each point represents one linearly polarized genus. Many points overlap.

FIG. 7 absolute delay in an improved 50 pm optical fiber at a wavelength of 1310 nm for various profile parameters. Each point represents one linearly polarized genus. Many points overlap.

FIG. 8 absolute delay in optical fibers with a reduced core wavelength of 850 nm for different profile parameters. Each point represents one linearly polarized genus. Many points overlap.

FIG. 9 absolute delay in optical fibers with reduced nucleus at a wavelength of 1310 nm for different profile parameters. Each point represents one linearly polarized genus. Many points overlap.

A detailed numerical analysis showed that bandwidth beyond 3 GHz.km could not be achieved in the case of complete waking of the entire set of aquatic genera in a standard multifaceted fiber. As is well known, minimal inter-atomic dispersion is achieved and thus the maximum fiber bandwidth is achieved by introducing an a-profile optical fiber defined by zn (r) = n _max (1-2A (r / a) ^a ) ^1/2 for r <a and n _max (1-2A) ^1/z for r> a, where r is the coordinate pointing in the radial direction of the cylindrical optical fiber, but is the radius of the optical fiber core, n (r) is the refractive index of the optical fiber at the radius r, n _min is the minimum refractive index (in ordinary fiber, this is the refractive index of the lining), n _{max is the} maximum refractive index of the fiber core, the α parameter of the profile, and Δ is defined as A = (n _max ² - nmin ² ) / ( 2nmax ² ). The profiles of standard multifilament fibers, therefore, consist of a gradient core defined by the equation above, which describes the α-alpha profile of the optical fiber and the coating with a constant refractive index n _min . The α-parameter of the profile is about 2 and depends on the wavelength for which the fiber is intended.

Detailed numerical analysis showed that the α-profile of the optical fiber does not allow minimization of group delay of higher genera compared to lower genera, which is shown in Figures 1 and 2. Figures 1 and 2 give the logarithm of relative time delay (RMS) as a function of the difference between the effective refractive index of individual genera and the lining. RCH is defined as:

RZZ delay of basic lineage - delay of observed higher lineage delay of observed higher lineage

Figures 1 and 2 show that the delays of the higher genera are significantly larger than the delays of the lower genera, causing strong inter-dispersion and severely limiting the optical fiber bandwidth upon excitation of the higher genera. The long delays are due to the negative influence of the lining with a constant refractive index on the propagation of higher genera. A considerable part of the evanescent field of the higher genera is spreading over the lining, where the effect of balancing the group velocities of the genera resulting from the wave properties of the gradient core is lost.

The invention solves the problem of the negative impact of the lining according to Figure 3 by introducing into the known profile of a multipart fiber consisting of a core 1 with a radius of 5 and a difference of refractive indexes 8 and a lining 3, with the additional structure:

a) depressed expanded core 2 with a radial dimension b 6 and a relative difference of the refractive indexes Δ ₂ 9 with respect to the lining 3, which is a continuation of the original core 1 with the shape of an a-profile

b) depression 4 with radial dimension c 7 and relative difference of refractive indexes Δ ₂ 9 with respect to the lining 3 located around the expanded core

This is achieved in two ways: the majority share of the evanescent field of the higher genera expands over the expanded depressed nucleus 2, which has the shape of an α-profile, allowing a good equalization of the group velocities of the higher genera, achieving better trapping of the higher genera into the primary nucleus 1 and the expanded nucleus 2 The final effect is that the fraction of the evanescent field of higher genera propagating in the coating with a constant refractive index 3 is negligible. This allows very good equalization of group velocities of higher genera, as shown in Figures 4 and 5. Both figures clearly show a significant reduction (improvement) in the relative delay between genera compared to the delays in Figures 1 and 2. The set of parameters of the depressed extended kernel 2 and depression 4, which allows the equalization of group velocities to be broad and depends on the wavelength. Of course, the minimum dimensions of both areas are of interest for practical fiber production. A larger expanded core 2 requires smaller dimensions of depression 4 and vice versa. In the extreme, depression 4 can be completely omitted, but in this case the required dimension of the expanded core 2 becomes larger and deeper, which is often disadvantageous from the point of view of fiber production.

Figures 6 and 7 give the absolute delays between each generic group for the fiber case with the following profile parameters: 2a = 50 pm, Δ ·, = 1%, Δ ₂ = −0.5%, b = 5.5, c = 5 pm, and a = 2.087 at 850 nm and a = 2.0 at 1310nm. Figures 6 and 7 show that the maximum delays between genera are 0.057 ns / km at 850nm and 0.053 ns / km at 1310nm, enabling a bandwidth of more than 10 GHz to be achieved with complete excitation of the entire set of aquatic genera

A series of systematic numerical calculations were performed to determine the practical minimum required radial dimension b (in Fig. 3 indicated by 6) of the expanded core 2, the radial dimension c (in Fig. 3 indicated by 7) of depression 4, and the refractive index differences J ₂ (in Fig. 3). 3 indicated by 9), which allow the effect of the lining to be effectively eliminated and thereby reduce the intergranular dispersion. Table 1 gives the practically calculated values of the parameters of the optical fiber profile, which limits the interannual dispersion resulting from the effect of the coating, below the theoretical limit, which is conditioned by the performance of an ideal α-profile multi-stranded fiber with a core size of 2a = 50 pm.

Table 1:

850 nm 1310 nm Δ ₂ = -0.3% b = 4.4 pm, c = 3 pm - Δ ₂ = -0.35% b = 4.9 pm, c-1 pm b = 4 pm, c = 5 pm Δ ₂ = -0.5% b = 5.35 pm, c-0 pm b = 5.5 pm, c = 1 pm

Successful handling of the adverse effects of the coating enables further reduction of the core a and J / sub dimensions of the standard fiber. If core radius a and A _1t were reduced without eliminating the effect of the coating, the effect of the coating would be pronounced and would limit the fiber bandwidth to values when reached by existing standard fibers. The introduction of extended depressed core 2 and depression 1 thus allows us to reduce the radius a of core 1 and the refractive index difference, which is reflected in the increase in bandwidth and the reduction in fiber. The decrease in Δι results in a decrease in the content of germanium, which is the most expensive raw material for the production of multifaceted fiber. In order to maintain sufficient insensitivity to curvature, such a fiber with a reduced nucleus must have adequate profile parameters for which the individual genera are sufficiently separated in the space of phase constants. The differences between the effective refractive indexes of each generic group must be at least as large as those in standard fibers. In practice, this means that the difference of the effective refractive indexes between adjacent genera must be at least 0.001 at a wavelength of 1310 nm and 0.0007 at a wavelength of 850 nm.

Practical examples of the profile parameters of such fibers with radii a = 19 pm and a = 75 / zm are given in Table 2.

Table 2:

Sail 850 nm 1310 nm a J2 [%] b [pm] c [pm] a δ ₂ [%] b [pm] c [pm] a = 15pm, Δ -, = 0.36% 2.1027 -0.25 4.24 3 2,015 -0.4 6.7 5 a = 19pm, Δ ^ Ο.578% 2.0965 -0.35 4.81 3 2.01 -0.45 6.27 5

Such fibers, despite their reduced core, can be effectively coupled to existing VCSEL sources.

The decrease in the radius of kernel a and Δ _Ί is reflected in a significant increase in the available bandwidth, as illustrated in Figures 8 and 9. The maximum delay between the fastest and slowest lineages is for fiber by = 19pm less than 18 ps / km and less than 5 ps / km for fiber for = 15 pm. This can theoretically achieve bandwidths between 50 and 100 GHz.km.

It is understood by a person skilled in the art that the parameter values in Tables 1 and 2 are only typical examples and that similar and slightly different parameters can be found that provide similar or identical performance to those shown in practical examples.

Claims (10)

PATENT APPLICATIONS Optical telecommunication system, characterized in that it consists of a multi-fiber linking the transmitter and the receiver, the system bandwidth exceeding 5 GHz.km, while transmitting the transmitter in a multi-line fiber to a complete set of guided genera. Optical telecommunication system according to claim 1, characterized in that it contains a multi-stranded fiber with a profile consisting of a gradient core (1) with a radius a (5) of greater than 22 μηη and less than 27 μηη and a refractive index difference Δ ₁ (8) greater than 0.7% and less than 1.5%, a depressed expanded gradient core (2) with a radial dimension b (6) and a negative refractive index difference Δ ₂ (8) between -0.1% and -0.5%, depression (3) with a refractive index difference of Δ ₂ (9) and a lining (4) extending to the outer edge of the fiber. 3. Optical telecommunication system, characterized in that it consists of a multi-fiber connecting the transmitter and the receiver, the system bandwidth exceeding 10 Gbit.km, while generating a full set of guided generators in the multi-fiber network. Optical telecommunication system according to claim 1, characterized in that it contains a multi-stranded fiber with a profile consisting of a gradient core (1) with a radius of a (5) greater than 15 μηη and less than 22 μηι and a refractive index difference Δι (8) greater than 0.5% and less than 1%, depressed expanded gradient core (2) with radial dimension b (6) and a negative refractive index difference Δ ₂ (8) of -0.1% and - 0.5%, depression (3) with a difference in refractive index Δ ₂ (9) and a lining (4) extending to the outer edge of the fiber. 5. Multiple optical fiber, characterized in that it supports the propagation of more than 7 linearly polarized genera and has a maximum differential delay between guided genera of less than 0.5 ns / km and consists of a gradient kernel (1) with radius a (5) , which is greater than 22 pm and less than 27 pm and a refractive index difference Δι (8) of greater than 0.7% and less than 1.5%, depressed expanded gradient core (2) with radial dimension b (6) and a negative difference refractive indexes Δ ₂ (8), ranging between -0.1% and -0.5%, depression (3), with the difference of refractive indexes Δ ₂ (9), and lining (4) extending to the outer edge of the fiber. Multiple fiber profile according to claim 5, characterized in that the difference between the effective refractive indexes of the adjacent genera is at least 0.001 at a light wavelength of 1310 nm and at least 0.0007 at a wavelength of 850 nm. 7. Multiple optical fiber, characterized in that it supports the propagation of more than 5 linearly polarized genera and has a maximum differential delay between guided genera of less than 0.25 ns / km and consists of a gradient core (1) with radius a (5) , greater than 22 pm and less than 27 pm and a refractive index difference Δ-, (8) greater than 0.7% and less than 1.5%, depressed expanded gradient kernel (2) with radial dimension b (6), and a negative difference in the refractive index Δ ₂ (8) of between -0.1% and -0.5%, depression (3) with the difference in the refractive index Δ ₂ (9) and the lining (4) extending to the outer edge of the fiber. Multiple fiber profile according to claim 7, characterized in that the difference between the effective refractive indexes of adjacent genera is at least 0.001 at a light wavelength of 1310 nm and at least 0.0007 at a wavelength of 850 nm. 9. Multiple-fiber optical fiber, characterized in that it consists of a gradient core (1) for the fracture shape, an expanded depressed gradient core (2) having a fracture shape and a continuation of the gradient core (1), the radial dimension of the depressed gradient kernel (6) is more than 0.4 pm and less than 15 pm, with a negative relative difference of the refractive indexes (9) between the outer edge of the expanded depressed kernel (2) and the liner (3) between -0.15% and 0.4%, depression (4), which surrounds an expanded gradient nucleus and has a radial dimension greater than 15 pm. 10. Multiple-fiber optical fiber, characterized in that it consists of a gradient core (1) for the fracture shape of radius a (5) greater than 15 pm and less than 26 pm, the relative difference of the refractive index zb (8) between the center of the gradient core and the cladding (3), which is between 0.3% and 1%, an expanded depressed gradient core (2) having a fracture shape a continuation of the gradient core (1) having the radial dimension of the depressed gradient core (2) more than 1 pm and less than 7 pm, of depression (4), which surrounds an expanded gradient nucleus and has radial dimensions between 2 pm and 35 pm, with a negative relative difference of the refractive index Δ ₂ (9) between depression ( 4) and the lining (3) between 0.1% and -0.5% and the lining (3) surrounding the depression (4).