WO2009003228A1

WO2009003228A1 - Multimode high bandwidth microstructured optical fibres

Info

Publication number: WO2009003228A1
Application number: PCT/AU2008/000959
Authority: WO
Inventors: Steven Manos; Leon Poladian; Maryanne Candida Jane Large
Original assignee: The University Of Sydney
Priority date: 2007-06-29
Filing date: 2008-06-30
Publication date: 2009-01-08

Abstract

The present invention relates to the design and manufacture of multimode microstructured optical fibres. One aspect of the invention provides a method of forming an optical fibre with multimode high bandwidth transmission characteristics, said method comprising forming a plurality of apertures at predetermined locations in a preform, and subsequently drawing said preform to form a length of optical fibre. Potential applications for optical fibres produced according to the present invention include situations where high bandwidth data communications are needed, such as computer local area networks, 'last mile' fibre to home high speed internet and multimedia connections, car communication systems, and aeronautical communications and control systems.

Description

MULTIMODE HIGH BANDWIDTH MICROSTRUCTURED OPTICAL FIBRES Field of the Invention

The present invention relates to the design and manufacture of multimode microstructured optical fibres. Background of the Invention

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Typically, multimode optical fibres are used for short distance, high bandwidth communications where characteristics such as a large fibre diameter and a large mode field diameter are desirable. As opposed to single-mode fibres with relatively small core sizes of 5 to 10 μm, multi-mode fibres can have much larger core sizes and higher numerical apertures (NA) which enhances connectivity. Silica multi-mode fibres have become popular for 1 Gb and 10 Gb ethernet applications. Polymer multi-mode fibres take this one step further by increasing the core size to 100 μm - 1000 μm. Polymer fibres also have a much higher index contrast Δ (and therefore NA) than silica fibres.

This improves the connection tolerances, reducing the required precision and therefore reducing the cost of connectors.

The high bandwidth characteristics in multimode optical fibres is typically achieved by equalising the velocities of the modes which make up a pulse of light. By ensuring that all modes arrive at the same time, more pulses can be sent down the fibre thereby resulting in a higher bandwidth. This equalisation is achieved in traditional silica and polymer multimode optical fibres by using graded refractive index profiles which help to equalise the group velocities of the modes. However, in microstructured multiniode optical fibres this equalisation is more difficult to achieve, particular in the case where other desired characteristics such as a large mode field diameter and large numerical aperture are to be maintained.

It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. Summary of the Invention

To this end, a first aspect of the present invention provides a method of forming an optical fibre with multimode high bandwidth transmission characteristics, said method comprising forming a plurality of apertures at predetermined locations in a preform, and subsequently drawing said preform to form a length of optical fibre.

More particularly, said plurality of apertures are located at predetermined locations within the preform so as to adjust the velocities of the modes. More particularly, said plurality of apertures are located at predetermined locations within the preform so as to filter via high attenuation those high order modes that have very different velocities to the lower order modes. More preferably, the modes are adjusted so as to arrive substantially simultaneously.

Preferably, said apertures extend through the preform in the direction in which the preform is to be drawn.

In one preferred embodiment, said preform is formed from an optically suitable polymer material.

A second aspect of the present invention provides an optical fibre with multimode high bandwidth transmission characteristics, said optical fibre comprising a plurality of apertures at predetermined locations.

More particularly, said plurality of apertures are located at predetermined locations within the preform so as to adjust the velocities of the modes. More particularly, said plurality of apertures are located at predetermined locations within the preform so as to filter via high attenuation those high order modes that have very different velocities to the lower order modes. More preferably, the modes are adjusted so as to arrive substantially simultaneously. More particularly, patterns of apertures can be designed such that multiple modes can be supported. Furthermore, the patterns can be adjusted to customise (1) the properties of the relationship between the effective index and group index (or velocities) of modes, (2) the density of states of the supported modes, and (3) the energy exchange properties of modes. An ideal property would be, for example, to have the energy distribution tend towards the fundamental mode, resulting in a smaller range of mode velocities, and therefore a higher bandwidth.

Advantageously, by designing fibres where properties (2) and (3) are controlled, microstructured fibres can support high bandwidth, while still maintaining other desirable optical characteristics such as good light acceptance properties (i.e. high NA) and low confinement loss.

Brief Description of the Drawings

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure Ia is a schematic diagram of a graded index fibre whilst Figure Ib is the manufactured fibre with a core diameter of 135 μm.

Figure 2 is a plot of effective index versus confinement loss of all modes in the range neff = [1.47,1.49] of a graded-index MPOF structure similar to that shown in Figure 1. An abrupt cutoff index at approximately neff = 1.48762 is evident. Approximately 1,800 modes lie within this range. Figure 3 illustrates plots of the 4 selected modes from Figure 2 are shown. The Sz component of the fields are plotted on a log scale with black indicating the most intense regions of light.

Figure 4 is a plot of effective index versus group index of all the bound modes shown in Figure 2.

The different colours indicate modes from the 5 different non-degenerate mode classes for the graded-index MPOF with symmetry 9 shown in Figure 1.

Figure 5a illustrates examples of the types of random designs with different symmetries and complexities which can be generated by the GA. Figure 5b is an example of a manufactured MPOF preform which was optimised with respect to attaining an average graded index profile.

Figures 6a, 6b and 6c illustrate designs of MPOF which exhibit high bandwidth characteristics.

Figure 7 is a graph of pulse broadening as a function of length for 500 μm graded index mPOF in comparison to a 1000 μm Optimedia GI-POF.

Figure 8 illustrates the cycle of optimization in evolutionary algorithms. The process starts with a population of individuals, in this case fibre designs.

Figure 9 is a schematic of the embryo geny used to generate fibre designs. Figure 10 is a photograph of a manufactured graded index. Detailed Description of the Invention

An area where MPOF has proven successful is in high-bandwidth LAN applications. In order to apply design algorithms such as Genetic Algorithms (GA) to the design of high-bandwidth fibres, it is necessary to approximate the bandwidth of complex microstructured fibre designs. Typically, with prior art methodologies the bandwidth evaluation of a multimode microstructured fibre involves evaluation times from hours to days. For a GA setting, this amount of computational work is impractical, and other more rapid estimates of bandwidth must be sought.

Advantageously, due to the prevalence of micro structured fibres with triangular arrays of holes, the automated design of microstructured fibres using optimisation schemes such as GAs has been relatively straight forward in terms of the parameterisation of the structures. This parametisation refers to a set of numerical values that can be used to characterise the structure. A human designer, or an automated design algorithm, can then alter these values over time to achieve a desired result in terms of the optical performance of the fibre. Typically the values Λ and d (i.e. spacing and hole diameter) are used for a hexagonal array which is fairly limited in the types of designs possible, Given that MPOF opens up this design space, allowing essentially arbitrary arrangements of holes, a new algorithm was devised which could design structures with different symmetries and different arrangements of holes. This resulted in an algorithm which can design microstructured fibres with more (or less) complexity as required for the design problem specified, rather than being limited to a particular type of design.

In order to design fibres for high-bandwidth communications applications using an automated algorithm such as a GA, a fast, efficient method to estimate the bandwidth of microstructured fibre designs is required. To gain insight into how this can be achieved, the ensemble of modes supported in structures such as Figures Ia to Ib are numerically evaluated. The ABCFDM algorithm² was used to evaluate the effective index, group index and confinement loss of all the modes supported. The plot of effective index versus group index is shown in Figure 2. The four labeled modes, including the fundamental mode labeled A, are shown in Figure 3. The cutoff of the modes is evident

² "Vector wave expansion method for leaky modes of microstructured optical fibres", Nader Issa and Leon Poladian, J. Lightwave Tech. 21(4), pp. 1005-1012 (2003). in Figure 2 at a value of approximately 1.4762. The effective index versus group index is shown in Figure 4, where we can see that modes mostly lie on a straight line. This correlation arises from the fact that as the effective index of the modes increase (i.e. the phase velocities decrease), the group velocity also decreases since the ray length of the light as it passes through the fibre is longer.

In order to maximise bandwidth, it is necessary to minimise the range of mode velocities. The correlated behaviour of the effective index and group index can then be exploited since we only need to consider the group index of a few modes in order to estimate the bandwidth. When considering the optimisation of fibres for high-bandwidth microstructured fibres, we have typically relied on other measures of fitness, such as using the microstructure as an approximation to an r² graded index. We now have more accurate method by which the evaluation of a few modes supported in the fibre can be used as an estimate of the bandwidth. This means that we are no longer relying on any pre-conceptions of particular 'types' of designs which might be better suited to multimode high bandwidth microstructured fibres.

One of the challenges in designing MPOFs was developing an algorithm which could match MPOF manufacture in terms of the complexity of designs possible. A GA was developed which uses a growth algorithm³ to generate designs of variable complexity and symmetry. This growth algorithm is also beneficial in that it automatically solves manufacturing constraints such as maintaining a minimum wall thickness between holes, etc. Some randomly generated examples of designs can be seen in Figure 4A. Using the GA, we can evolve MPOF fibres for different applications by

³ "A Genetic Algorithm with a Variable-Length Genotype and Emobryogeny for Microstructured Optical Fibre Design", Steven Manos, Peter Bentley, Maryanne Large, Leon Poladian, Submitted to Genetic and Evolutionary Computation Conference (GECCO-2006), Seattle, WA, USA, July Z^-U*¹, 2006. defining an appropriate fitness function. An example of a design which was optimised with respect to gaining an optimal graded average profile is shown in Figure 4B.

It was believed that the dominant effect which helped improve bandwidth performance was mode mixing. An example of a fibre with strong mode mixing was Fibre E. Both the independence of bandwidth with respect to launch NA, and the 4L pulse spread dependence, showed an equilibrium length L_e « 3 m. Using this assumption, the numerical modelling yields a bandwidth of « 4 GHz / 30 m, roughly corresponding to the measured bandwidth of 4 GHz / 29 m.

On the other hand, numerical modelling for a fibre similar in size to Fibre G4 (outer core diameter « 100 μm) yields a best case bandwidth of 3.5 GHz / 30 m. However, the experimental result gives a bandwidth of 26 G bit/s. This fibre also corresponds to one of the lowest fibre loss measurements of 0.5 dB / m. Although the reduction in scattering results in a lower loss and an equilibrium length longer than the length of fibre under test, differential mode attenuation plays an important role. The light launched into the fibre is re-distributed from low and higher-order modes to predominantly lower order modes, effectively decreasing δn_g to a much greater extent than mode-mixing can.

Figure 7 is a graph of pulse broadening as a function of length for 500 μm graded index mPOF in comparison to a 1000 μm Optimedia GI-POF. Both fibres demonstrate linear pulse broadening over the first 20 m. Further pulse broadening occurs for the GI- POF, while graded index mPOF experiences a reduced rate of pulse broadening between 20 m to 30 m, and almost no additional broadening to 50 m. As pulse broadening is occurring at a slower rate compared to theoretical inter-modal dispersion VL dependence, which suggest a dominant influence of DMA coupled with complete mode mixing helps lower the number of modes requiring equalization and hence less pulse broadening.

One aspect of the present invention is an evolutionary approach to more fully exploring diversity of fibre structures, and applying it to two specific problems. The first mimics one of the most important conventional fibres, the graded index multimode fibre for minimising intra-modal dispersion, and the second is an alternative single-mode design. The aim in the latter case is to try to find a design which preserves the single- mode property, but with a design that is either simpler (eg. fewer holes) or has other desirable qualities (such as better lateral access to the core). The approach to exploring the extended parameter space associated with non- periodic structures has two components. The first is to use evolutionary algorithms to find and optimize the designs, and the second is to use an embryogeny-based approach to generate a diversity of complex designs, which nevertheless satisfy all manufacturing constraints. A schematic of the evolutionary approach to design is shown in Figure 8. Each fibre design is treated as an individual within a population. A starting population evolves through a number of cycles of selection and recombination, until a pre-defined goal is achieved.

In comparison to classical optimization algorithms, which typically operate on single or few search points in a deterministic manner, the power of evolutionary algorithms arises from their inherently parallel search. Different positions in the search space are examined simultaneously for improvements in fitness. The design space is explored using the variation operators (recombination and mutation). Once areas of promising designs are found, these areas are exploited through the selection operator, favouring better designs and moving towards local and global optima. Important parameters in setting up the evolutionary cycle are the choice of representation of the fibre design, and the selection criteria.

A "genotype" is used to encode the parameters associated with a design. This representation provides a means by which candidate design descriptions can be stored. For example, a hexagonal array could be represented by two parameters, the spacing and radius of the holes. Candidates are selected according to their performance characteristics, rather than the genotype directly.

In order to fully exploit design complexity allowed by arbitrary hole positions and sizes, a new representation - an embryogeny - was developed. Embryogenies offer many advantages over simple representations. Using a direct representation, the location and size of every hole in a microstructured fibre would have to be described. As more holes are defined, the search becomes less efficient.

Embryogenies by contrast describe "growth rules". They can exploit hierarchy to re-use parts of the genotype, such as sets of holes which reappear at different locations in the structure. This results in a more efficient, and lower- dimensional search space. They also allow manufacturing constraints to be incorporated, so that all the designs produced by the algorithm can be fabricated.

The embryogeny chosen for this work is schematically illustrated in Figure. 9. In step 1 [top left] the binary genotype is decoded into symmetry nsymm and Nh triplets of xi,yi, ri values describe the positions and size of the holes. In step 2 [top right] the Nh hole positions xi,yi are symmetrized into a new symmetry nsymm. The holes are converted into polar coordinates and scaled by nsymm/4. In step 3, (nsymm- 1) copies of the holes are made to complete the fibre. In step 4 the holes are grown in a step-wise manner until the manufacturing constraints prohibit further growth. One the most important non-step index conventional fibres is the graded index fibre. Graded index designs minimise intermodal dispersion by allowing the group velocities of the fast and the slow modes to be approximately equalized. This allows relatively large core fibres to have improved bandwidth performance, and is one of the most important applications of polymer fibres. Thus, there is considerable motivation for exploring graded index structures in mPOF.

One of the most difficult aspects of implementing this however is defining the problem to be optimized. The behaviour of conventional graded-index fibres is, at least theoretically, well understood. The same cannot be said for microstructured GI fibres, and a certain amount of intuition is required in specifying the selection criteria. The most important requirements are for high bandwidth and low loss.

In optimizing the bandwidth, we assumed that mPOF have an analogous behaviour to those of conventional fibres. In conventional graded index fibres reducing the difference in velocity between the extreme modes is done by using a parabolic refractive index profile. We assume similar behaviour in microstructured fibres, and that the best performance will be obtained when the azimuthally averaged refractive index profile most closely matches: ri(r) = 1.49 - ar² (1) where 1.49 is the refractive index of PMMA. We assume that surface scattering is the dominant loss process in our fibres. We chose to approximate the loss by considering the overlap of the fundamental mode with the air-polymer interfaces. This overlap is calculated by taking the average intensity around the perimeter of each hole. Reducing this overlap:

^Σ -.lJVi*iWVø (2) effectively reduces the interaction of light at the interfaces.

A number of graded index designs resulted from this process. Figure. 10 shows an example of one of the fabricated structures.

The fibre shown in Figure. 10 was tested at Polymer Optical Fibre Applications Centre (POFAC), Nurnberg, Germany, and at the University of Auckland. Both organizations measure bandwidth by through pulse spreading. The major difference between their experiments is that the linewidth of the excitation source. POFAC uses a

3.5nm linewidth source, while the University of Auckland uses a 0.01 ran source, making material dispersion negligible. The experiments were done on a 25 m length of fibre and the extrapolated results for 100m give bandwidths of 2.5-5 GHz and 3.6-6.9

GHz respectively. The range of bandwidths reflects uncertainty as to the equilibrium length of the fibres. The smaller limit assumes no mode mixing, while the upper limit assumes equilibration has occurred. The loss of the fibres is <0.4 dB/m. These results are very competitive with commercially available fibres made from the same material in terms of bandwidth. The most closely analogous fibre, OM Giga, produced by

Optimedia⁴ has a bandwidth of 1.5 Gbit/s over 100 m, equivalent to 0.75 GHz. The loss performance is a factor of two higher. At least some of this loss is due to fabrication issues, which can potentially be improved. Improvements include better surface quality during preform production (the holes are currently drilled) and sleeving or jacketing to reduce micro-bending.

Potential applications for optical fibres according to the present invention include situations where high bandwidth data communications are needed. Examples include, computer local area networks, "last mile" fibre to home high speed internet and

Optimedia website: http://www.optimedia.co.kr/ multimedia connections, car communication systems, and aeronautical communications and control systems.

Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. A method of forming an optical fibre with multimode high bandwidth transmission characteristics, said method comprising forming a plurality of apertures at predetermined locations in a preform, and subsequently drawing said preform to form a length of optical fibre.

2. The method as claimed in claim 1 wherein said plurality of apertures are formed at predetermined locations within said preform so as to adjust velocities of the modes.

3. The method as claimed in claim 1 or 2 wherein said plurality of apertures are formed at predetermined locations within said preform so as to filter via high attenuation any high order modes that have substantially differing velocities to any lower order modes.

4. The method as claimed in claim 1 wherein said modes are adjusted so as to arrive substantially simultaneously.

5. The method as claimed in claim 1 wherein said plurality of apertures are formed at predetermined locations in said perform so multiple modes can be supported.

6. The method as claimed in claim 1 wherein said plurality of apertures are formed at predetermined locations in said perform so as to customise the properties of the relationship between the effective index and group index of the modes.

7. The method as claimed in claim 1 wherein said plurality of apertures are formed at predetermined locations in said perform so as to customise the density of states of the modes.

8. The method as claimed in claim 1 wherein said plurality of apertures are formed at predetermined locations in said perform so as to customise energy exchange properties of the modes.

9. The method as claimed in any one of claims 1 to 8 wherein said preform is formed from an optically suitable polymer material.