GB2355541A

GB2355541A - Optic fibre with monomode core and multimode core or layer

Info

Publication number: GB2355541A
Application number: GB9925043A
Authority: GB
Inventors: Peter John Duthie
Original assignee: Marconi Caswell Ltd; Marconi Electronic Systems Ltd
Current assignee: Marconi Caswell Ltd; BAE Systems Electronics Ltd
Priority date: 1999-10-23
Filing date: 1999-10-23
Publication date: 2001-04-25
Also published as: GB9925043D0

Abstract

An optical fibre (2) incorporates both a monomode core (4) and a multimode core (6) in which both cores are substantially transparent at the wavelength of light they are intended to guide. The optical fibre is intended for use in airframe or other applications in which it is impractical to replace the optical fibre and enable a single fibre to be used for both monomode or multimode communication. Interfaces 5, 7, cladding 8 and outer protective coating 10 are shown.

Description

2355541 1 P/62004.GBA OPTICAL FIBRES This invention relates to optical

fibres and more especially, although not exclusively, to optical fibres for use in airborne vehicles.

Optical fibres are well known and comprise a light guiding core which is clad by a material of differing refractive index, most typically a lower refractive index, such that the core is capable of transmitting light signals by total internal reflection. As is known optical fibres can be monomode or multimode types.

A monomode optical fibre is typically a stepped index optical fibre in which the diameter of the inner core is comparable with the wavelength of light it is to transmit; this results in there being only one mode of light propagation for each polarisation. In a multimode optical fibre, which is usually a stepped refractive index fibre, the core diameter is sufficiently greater than the wavelength of light to allow propagation of the light in a large number of different modes.

Monomode optical fibres are preferred in high speed and long haul optical communications systems as they minimise mode dispersion. However making low loss connections to monomode fibres whose central core is typically only a few microns can be difficult. As a result many applications use multimode optical fibres.

Due to the weight saving offered by optical fibres compared to conventional copper wiring it is known to use multimode optical fibres within the wiring looms of airborne vehicles such as aircraft and helicopters. Since monomode optical links are more 2 P/62004.GBA expensive for basic functionality as compared to multimode links it is preferred to use multimode optical fibres in airborne vehicles. The trend however in modem aviation is for an ever increasing complexity of control and navigation systems which requires an increased processing and communications capability. It is possible therefore that monomode optical fibre links will be preferred in the future for airborne vehicles.

Choosing between monomode or multimode optical fibres when installing the optical fibres within the wiring looms of an airframe constrains future upgrades. To replace an existing multimode optical fibre with a monomode fibre at a future date is not only prohibitively expensive but may also compromise the integrity of the airframe.

Installing two sets of optical fibres at the outset would be an unnecessary expense and unacceptable increase in weight.

The inventor has appreciated that a need exists therefore for an optical fibre for use within airborne vehicles which is capable of future upgrade without the limitations of the known arrangements.

According to the present invention an optical fibre incorporates a monomode core and a multimode core wherein both cores are substantially transparent at the wavelength of light they are intended to guide. In the context of this patent application substantially transparent is to be construed as meaning, low absorption at the intended operating wavelength and in practice this equates to an absorption of less than I dB per kilometre more typically less than 0.5dB per kilometre.

Preferably the monomode core is located within the multimode core and the multimode 3 P/62004.GBA core advantageously has a lower refractive index to that of the monomode core such that it comprises the cladding to the monomode fibre. Optical fibre amplifiers are known which comprise an optical fibre which comprises a monomode lasing core, an inner multimode pump cladding surrounding the lasing core and an outer cladding enclosing the core and pump cladding. The lasing core is doped with a laser active material often Erbium. In operation an optical signal to be amplified is guided by the lasing core and the outer cladding is pumped with optical energy of a suitable wavelength which couples into the core to produce amplification of the optical signal.

It will be appreciated that for such an amplifier to operate it is essential that optical energy is coupled from the inner cladding into the lasing core. This is achieved by the inner cladding being made of a material having a high absorption, typically 3 to 4 dB per metre, at the pump wavelength. In contrast to an optical fibre amplifier both cores of the optical fibre of the present invention are made of low absorption material whose absorption is three orders of magnitude lower.

For ease of fabrication and to assist in connecting and/or ten-ninating the optical fibre, the monomode core is preferably substantially co-axial with the multimode core and both cores are advantageously substantially circular in cross section. A particular advantage of such a configuration is that this minimises the coupling of optical energy from the multimode core into the monomode core since light can propagate in the multimode core in a screw or axial mode around the monomode core with a reduced probability of interacting with it. In contrast in optical fibre amplifiers and optical fibre lasers the inner lasing core is located off axis and/or the pump cladding has an asymmetrical cross section to increase the probability of light propagating in the 4 P/62004.GBA cladding interacting with the lasing core.

Preferably the optical fibre further comprises cladding around the multimode core which has a lower refractive index. In an alternative arrangement the monomode core is located within the cladding. To further ensure the future capability of the optical fibre L it advantageously further incorporates one or more additional monomode cores which can be incorporated within the multimode core or within the cladding.

Preferably the or each monomode and/or multimode core comprises silica doped with germanium and the cladding comprises undoped silica.

Advantageously the or each monomode, core has a diameter in the range 3 to I Otm and is preferably 8 tm for operation at 1550nm; the multimode core has a diameter which is in the range 5 0 to 100 tm and is preferably 62.5 Lm for operation at 900nm and the cladding has a diameter which is in the range 80 to 250 im.

z:

To provide mechanical strength and protection against an environment and the optical fibre further comprises an outer protective coating which preferably comprises a polymer material such as an ultra violet (uv) cured acrylic material.

According to a further aspect of the invention a cabling arrangement or communications equipment for use in an airborne vehicle incorporates one or more optical fibres as described above.

P/62004.GBA According to yet a further aspect of the invention there is provided use of an optical fibre as described above within an airborne vehicle.

In order that the present invention may be better understood two optical fibres in accordance with the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure I is a schematic representation of an end view of an optical fibre in accordance with a first embodiment of the invention and Figure 2 is a schematic representation of an end view of an optical fibre in accordance with a second embodiment of the invention.

Referring to Figure I there is shown an optical fibre 2 in accordance with the invention for use in an optical communications system within an aircraft or other airborne vehicle.

The optical fibre 2 comprises in order from its central axis, a central monomode core 4, a multimode core 6, an optical cladding layer 8 and a protective layer 10. The monomode core is intended for operation at a wavelength of 1550nm and the multimode core for operation at 900nm The monomode core 4 is of diameter 8pm and comprises silica which is doped with a few % germanium such that it has a refractive index of n=1.6. It will be appreciated that the diameter, and/or material, of the nionomode core 4 can be varied depending on the wavelength of the light it is intended to operate with and thus can typically have a 6 P/62004.GBA diameter in the range 3 to I Otm. The diameter of the monomode core 4 is selected to be comparable with the intended wavelength of operation such that it can guide light in only a single mode of propagation.

The multimode core 6 which surrounds and is co-axial with the monomode core 4 comprises silica doped with 1%germanium such that its refractive index (typically n=1.56 to 1.57) is lower than that of the monomode core 4. The interface 5 between the mono and multimode cores 4, 6 thus comprises a stepped refractive index change and the multimode core 6 therefore comprises the optical cladding to the monomode 10 core 4. The outer diameter of the multimode core 6 is selected to be substantially meater than the wavelength of light for which the core is intended to operate such that it is able to support propagation of light in a number of modes. In the specific embodiment described it has a diameter of 62. 5trn for operation with light of wavelength 900nm. It will be appreciated that the diameter, and/or material, of the 15 multimode core can be varied depending on the wavelength of light it is intended to guide and can thus have a typical diameter in the range 50 to I 001m The optical cladding layer 8 which surrounds and is co-axial with the multi mode core 6 is made of undoped silica (n=1.55) and has an outer diameter of 1251m. The interface 20 7 between the multimode core 6 and cladding 8 thus comprises a stepped refractive index change and the multimode core 6 is therefore capable of guiding light in a multimode fashion. To provide environmental protection and mechanical strength the outer protective 7 P/62004.GBA coating 10 is of a polymer material such as an ultra violet cured acrylic material and is typically of a thickness such that the overall diameter of the optical fibre 2 is between 250 and 900[im. It will be appreciated that the symmetrical, co-axial, nature of the optical fibre 2 enables such a fibre to be readily constructed using known manufacturing techniques such as using vapour axial deposition to form a preform and drawing out the preform, on a drawing tower, to the required diameter.

As described the optical fibre 2 is especially intended for use in the airframe of aircraft and helicopters and in use the optical fibre 2 would be incorporated within the wiring looms of the vehicle airframe. Initially it is intended that the optical fibre 2 is operated using the multimode core 6. However, as advances are made to monomode ancillary components and their reliability is proven to the extent that their use becomes accepted in the aerospace industry, it is then a simple matter to use the central monomode core 4 for such communication without the need to replace the optical fibre 2 and so compromise the integrity of the airframe. Whilst it is feasible to use the mono and multimode cores simultaneously this is unlikely to be of benefit in practice due to the possibility of optical coupling between the cores 4, 6 and the added complexity of separating and routing the various optical signals.

Preferably the connectors for coupling to the fibre 2 are initially toleranced for use with the monomode core 4 thereby eliminating the need to replace connectors when switching from multi- to monomode use. Alternatively when installing the fibre within an airframe sufficient additional free optical fibre is provided to allow for connector replacement when upgrading the system from multi- to monomode operation.

8 P/62004.GBA To reduce the amount of light which may be unintentionally guided in the multimode core 6 during monomode operation it is preferred to additionally provide a depressed index claddinR: around the central core 4 to prevent light re-coupling back into the monomode core 4 from the multimode core 6.

In yet a further alternative embodiment the central core 4 is constructed using a Bragg guiding structure of lower effective refractive index as opposed to a simple stepped refractive index profile.

Referring to Figure 2 there is shown an optical fibre in accordance with a further embodiment of the invention. The optical fibre is the same as that of Figure I except the monomode core 4 is located within optical cladding 8. Whilst such an arrangement improves the multimode performance and reduces mode coupling between the cores it is more difficult tojoin, connect and/or ten-ninate to such fibres as they additionally need 15 to be accurately angularly oriented. It will be appreciated that the optical fibre of the present invention is not restricted to the specific embodiments illustrated and described above and that variations can be made which are within the scope of the invention. Whilst it is particularly preferred for the 20 mono and multimode cores to be coaxially aligned for ease of manufacture, to assist in connecting and/or terminating the fibre and to enable the fibre to be readily fusion spliced without the need for accurate angular orientation of the fibres, it is also envisaged to have the monomode core radially disposed within the multimode core. Furthermore it is also envisaged to incorporate one or more additional monomode cores 9 P/62004.GBA within the optical cladding and/or multimode core.

It will be further appreciated that the dimensions, materials, refractive index and/or numbers of layers will depend upon the desired wavelength of operation and intended use of the fibre and can be readily modified by those skilled in the art to support a given application. For example it is envisaged to use polymer plastics material as the optical guiding medium for the mono and multimode cores. Additionally a separate optical cladding layer can be provided around the monomode core rather than utilising the multimode for this purpose. Furthermore graded refractive index variations between the respective layers can be provided. Whilst in the embodiments described the mono and multi- mode cores are circular in cross section, this being a particularly preferred form when the monomode core is located co-axially within the multimode core since it reduces the likelihood of optical coupling from multi to monomode core, other cross section cores are envisaged such as for example those oval or continuously curved.

Moreover, although the present invention has been described in relation to use within an airframe the optical fibre of the present invention has other applications such as for example in optical data links within ships or other vehicles; buildings or any application which uses optical fibre communication in which it is impracticable or too costly to replace the optical fibre.

P/62004.GBA

Claims

1. An optical fibre incorporating a monomode core and a multimode core wherein both cores are substantially transparent at the wavelength of light they are intended to guide.

2. An optical fibre according to Claim I in which the monomode core is located within the multimode core.

3. An optical fibre according to Claim 2 in which the monornode core is substantially co-axial with the multimode core.

4. An optical fibre according to any preceding claim in which the monomode and multimode cores are substantially circular in cross section.

5. An optical fibre according to any preceding claim in which the monomode core and multimode cores have an absorption at the wavelength of light they are intended to guide of less than I dB per metre.

6. An optical fibre according to any preceding claim and further comprising cladding around the multimode core having a lower refractive index.

7. An optical fibre according to Claim 6 when dependent on Claim I in which the monomode core is located within the cladding.

I I P/62004.GBA

8. An optical fibre according to any preceding claim and further incorporating one or more additional monomode cores.

9. An optical fibre according to any preceding claim in which the or each monomode core comprises silica doped with gen-nanium.

10. An optical fibre according to any preceding claim in which the or each monomode core has a diameter in the range 3 to I Ogm.

11. An optical fibre according to any preceding claim in which the multimode core comprises silica doped with germanium.

12. An optical fibre according to any preceding claim in which the multimode core has a diameter which is in the range 50 to 100 gm.

13. An optical fibre according to any preceding claim in which the cladding comprises unhoped silica.

14. An optical fibre according to any preceding claim in which the cladding has a diameter which is in the range 80 to 250 gm.

15. An optical fibre according to any preceding claim and further comprising an outer protective coating.

12 P/62004.GBA

16. An optical fibre substantially as hereinbefore described or substantially as illustrated by way of reference to Figure I or Figure 2 of the accompanying drawings.

17. An cabling arrangement for use in an airborne vehicle incorporating one or more optical fibres according to any preceding claim.

18. Communications equipment for use in an airborne vehicle incorporating one or more optical fibres according to any one of claims I to 14.

19. Use of an optical fibre according to any one of claims I to 14 within an airborne vehicle.