WO2016091635A1

WO2016091635A1 - Repeaters

Info

Publication number: WO2016091635A1
Application number: PCT/EP2015/078045
Authority: WO
Inventors: Silviu Puchianu; Keith David COVENTRY; James Evans
Original assignee: Ge Oil & Gas Uk Limited
Priority date: 2014-12-12
Filing date: 2015-11-30
Publication date: 2016-06-16
Also published as: GB2533160A

Abstract

A method of amplifying an optical signal, the signal being carried by an optical transmission path 4, comprises the steps of: i) providing a pair of electrical -optical data converters 6, 7 in the optical transmission path, including a first electrical -optical data converter for converting an input optical signal from the optical transmission path to an output electrical signal, and a second electrical -optical data converter; and ii) electrically connecting the first and second electrical -optical data converters, so that the second electrical -optical data converter is operable to convert the output electrical signal to an output optical signal for transmission on the optical transmission path.

Description

REPEATERS

This invention relates to a method of amplifying an optical signal, an optical signal amplifier, a cable for carrying an optical signal and a cable kit.

As is well-known in the field, underwater hydrocarbon extraction facilities, such as subsea oil or gas facilities, require both electrical power, which may be alternating current (AC) or direct current (DC), and communications signals to be transferred between a surface ("topside") and underwater (e.g. sea-bed) location or component, such as a subsea control module (SCM), via an "umbilical" cable. A typical umbilical configuration includes power carried by copper conductors, with optical communications signals carried by optical transmission paths such as optical fibres within the umbilical. The distances involved can be considerable, sometimes being many hundreds of kilometres. Longer distances are becoming more commonly used, however this causes problems since the optical communications signals can become overly weak over such distances. One target subsea application of this type is the subsea power & communications distribution system (PCDM) of a subsea production control system (SPCS).

The PCDM is a system for subsea distribution of power and communications from the subsea end of a long-offset umbilical to the various sections of the SPCS, as in known in the art. For longer offset oil and gas exploration/production systems, one proposed solution for lengthening the power and communications between the topside and the subsea side of an SPCS is to use at least one repeater, provided along the length of the signal path, for power and communications distribution.

Such repeaters typically comprise optical amplifiers such as Erbium-doped optical amplifiers, Raman amplifiers, etc.

However, these optical amplifier devices are complicated, expensive, power hungry, and of unknown reliability. It is an aim of the present invention to overcome these problems, and provide a repeater avoiding the use of such optical amplifiers.

The solutions proposed in accordance with the present invention are simple, cost effective and likely significantly more reliable than traditional solutions based on optical amplifiers.

This aim is achieved by using a pair of electrical - optical data converters (EODCs) provided in a back-to-back configuration to achieve amplification within a repeater connection, thus enabling amplification of optical communication signals in an otherwise typical SPCS. EODCs are existing, off-the-shelf, tried and tested components, which are widely used for bidirectional translation (optical-to-electrical, electrical-to-optical) of communication signals. As an example, small form-factor pluggable (SFP) devices may act as EODCs.

The repeater may be a standalone module for insertion within a cable, such as an umbilical, or a distributed device having components located at connecting ends of respective subsidiary cables.

The repeaters can be installed at distances estimated between 100 and 200 Km on the subsea umbilical.

The present invention also enables the provision of a splitter-repeater, i.e. a repeater which also provides wavelength division multiplexing of the optical communications signal.

The present invention provides various advantages over known solutions, including, but not limited to: a) Simple and reliable configuration - there is no need for expensive, complex, potentially unreliable, in need of both qualification and ruggedisation optical amplifiers (Erbium-doped optical amplifiers, Raman amplifiers, etc.) in the repeater. b) Better amplification (typically minimum 30dB) by comparison with typical optical amplifiers (typically maximum 30 dB); c) The removal of the need for Ethernet router modules (ERMs) in the repeater; d) The ability to convert dual-fibre to single-fibre links within the repeater. This allows for a lower number of wet-mate fibre optic connectors on a subsea control module (SCM - sometimes referred to as a "pod"), being the pressurised enclosure containing subsea electronic modules (SEMs) and directional control valves (DCVs) which is installed on the "Christmas tree" (XTREE) and receives power and communications from the topside via the umbilical and associated umbilical termination unit (UTU). The reduction in wet-mate fibre optic connectors makes the apparatus cheaper and simpler whilst retaining superior offset distances; e) Very long offset solutions for SPCS; f) For umbilical-embedded repeaters, very simple inter-umbilical copper connector (having no optical connections); g) Standardisation of repeater hardware; h) The use of existing proven technology for power and communication distribution; i) The proposed division in umbilical sections (between the repeaters) allows for easier umbilical installation and repair and possibly for umbilical cost savings; j) The use of pressure compensated transformers (which may be qualified by GE Oil and Gas for a maximum of 1650m seawater depth) shall minimise the repeater size; k) Significantly reduced complexity as compared to a system which requires routing; 1) Straightforward point-to-point, independent optical links; m) Removal of the need for a communications electronic module (CEM) in the PCDM; and n) The use of passive elements in certain embodiments.

In accordance with a first aspect of the present invention there is provided a method of amplifying an optical signal, the signal being carried by an optical transmission path, comprising the steps of: i) providing a pair of electrical - optical data converters in the optical transmission path, including a first electrical - optical data converter for converting an input optical signal from the optical transmission path to an output electrical signal, and a second electrical - optical data converter; and ii) electrically connecting the first and second electrical - optical data converters, so that the second electrical - optical data converter is operable to convert the output electrical signal to an output optical signal for transmission on the optical transmission path. In accordance with a second aspect of the present invention there is provided an optical signal amplifier module for amplifying an optical signal carried by a transmission path, the module adapted for connection between first and second sections of the transmission path, comprising: a pair of first and second electrical - optical data converters, each electrical - optical data converter being operable to interconvert optical and electrical signals, first connection means for optically connecting the first electrical - optical data converter and the first section of the transmission path to enable optical communication between the first electrical - optical data converter and the first section, second connection means for optically connecting the second electrical - optical data converter and the second section of the transmission path to enable optical communication between the second electrical - optical data converter and the second section, and wherein the first and second electrical - optical data converters are electrically connected, enabling electrical communication therebetween.

In accordance with a third aspect of the present invention there is provided a cable for carrying an optical signal on an optical transmission path, comprising, at an end thereof, an electrical - optical data converter being operable to interconvert optical and electrical signals, the electrical - optical data converter being optically connected to the optical transmission path to enable optical communication therebetween, and connection means for enabling electrical connection of the electrical - optical data converter to a separate, complementary, cable connection means.

In accordance with a fourth aspect of the present invention there is provided a cable kit comprising at least two cables in accordance with the third aspect.

The invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 schematically shows an optical signal amplifier module in accordance with an embodiment of the present invention for use with an AC electrical power signal;

Fig. 2 schematically shows an optical signal amplifier module in accordance with a second embodiment of the present invention for use with a DC electrical power signal;

Figs. 3a-c schematically show three systems incorporating exemplary optical signal amplifiers in accordance with the present invention;

Fig. 4 schematically shows a distributed optical amplifier arrangement in accordance with another embodiment of the present invention for use with a DC electrical power signal; and Fig. 5 schematically shows a splitter-repeater module in accordance with a further embodiment of the present invention for use with an AC electrical power signal.

A first embodiment of the invention is schematically shown in Fig. 1, having an optical signal amplifier module 1 for use with an AC electrical power signal. Here, the module 1 is arranged for use as an amplifying repeater module for insertion between first 2 and second 3 sections or lengths of an umbilical cable servicing a subsea hydrocarbon extraction facility. Each section of the umbilical cable contains both an optical signal transmission path taking the form of an optical fibre 4, used for communications data, and an AC electrical power signal carried on copper wiring 5, as is well-known in the art. Module 1 is adapted for connection between these first and second sections, to receive the electrical and optical signals at one end and output them from the other end. In use, all signals are transmitted bi-directionally, and the module 1 can similarly operate bi-directionally. For simplicity, the description will focus on the situation where transmission is occurring in a topside to subsea direction, e.g. from left to right in Fig. 1.

In this embodiment, module 1 is a discrete component, having a pair of first and second electrical - optical data converters (EODCs) 6, 7, each EODC 6, 7 being operable to interconvert optical and electrical signals, with first connection means 8 for optically connecting the first EODC 6 and the first section 2 of the transmission path to enable optical communication between the first EODC 6 and the first section 2, and second connection means 9 for optically connecting the second EODC 7 and the second section 3 of the transmission path to enable optical communication between the second EODC 7 and the second section 3.

The first and second EODCs 6, 7 are electrically connected, via copper links, in a back-to back configuration, enabling electrical communication therebetween. It should be noted that the ports of the EODCs 6, 7 are connected so that the transmit (TX) port of each EODC is electrically connected to the receive (RX) port of the other EODC.

Power for each of the EODCs is obtained from the electrical power signal via a power supply 10, which comprises power conversion hardware to provide correct input power characteristics for the EODCs used. The module 1 also includes a step-up transformer 11 located in the electrical signal path, to compensate for any voltage loss that is due to the inherent power losses on the umbilical cable.

For topside to subsea data traffic: - EODC 6 receives from the topside system (not shown) the optical communications signals off very low power (down to a minimum of -42dBm power with current GE Oil and Gas subsea systems);

EODC 6 passes the data to EODC 7;

EODC 7 sends to the subsea system (not shown) an optical communications signal of approximately 3dBm to either the next amplifier / repeater module down the line (if present), or to the subsea system.

For subsea to topside data traffic:

EODC 7 receives from the subsea system the optical communications signals off very low power (down to a minimum of -42dBm power with current GEOG subsea systems);

EODC 7 passes the data to EODC 6 via the copper links;

EODC 6 sends to the topside system an optical communications signal of approximately 3dBm to the next amplifier / repeater module up the line (if present), or the topside system. The resulting optical amplification can reach 44dB - 45 dB (due to EODC 6 + EODC 7 maximum amplification) minus ldB (i.e. two connectors, each with losses of approximately 0.5dB).

The module 1 may be connected upstream/downstream to other modules or to the topside/subsea SPCS by umbilical sections of around 100 to 200km for example. Fig. 2 schematically shows an optical signal amplifier module in accordance with a second embodiment of the present invention for use with a DC electrical power signal. This module 12 has many similarities to that shown in Fig. 1, and where possible like reference numerals are retained. The module 12 is arranged for use as an amplifying repeater module for insertion between first 2 and second 3 sections or lengths of an umbilical cable servicing a subsea hydrocarbon extraction facility. Each section of the umbilical cable contains both an optical signal transmission path taking the form of an optical fibre 4, used for communications data, and a DC electrical power signal carried on copper wiring 5, as is well-known in the art. Module 12 is adapted for connection between these first and second sections, to receive the electrical and optical signals at one end and output them from the other end. In use, all signals are transmitted bi-directionally, and the module 12 can similarly operate bi-directionally. For simplicity, the description will focus on the situation where transmission is occurring in a topside to subsea direction, e.g. from left to right in Fig. 2.

In this embodiment, module 12 is a discrete component, having a pair of first and second electrical - optical data converters (EODCs) 6, 7, each EODC 6, 7 being operable to interconvert optical and electrical signals, with first connection means 8 for optically connecting the first EODC 6 and the first section 2 of the transmission path to enable optical communication between the first EODC 6 and the first section 2, and second connection means 9 for optically connecting the second EODC 7 and the second section 3 of the transmission path to enable optical communication between the second EODC 7 and the second section 3.

Power for each of the EODCs is obtained from the electrical power signal via a power supply 10, which comprises power conversion hardware to provide correct input power characteristics for the EODCs used. Unlike for the embodiment shown in Fig. 1, here the module 12 does not require a step-up transformer located in the electrical signal path - there is very little voltage loss for the DC supply of power over long distances and therefore there is generally no need for voltage amplification. However, if the distances involved are very long, then a transformer may be included if necessary.

For topside to subsea data traffic:

EODC 6 receives from the topside system (not shown) the optical communications signals off very low power (down to a minimum of -42dBm power with current GEOG subsea systems);

EODC 6 passes the data to EODC 7;

EODC 7 sends to the subsea system (not shown) an optical communications signal of approximately 3dBm to either the next amplifier / repeater module down the line (if present), or to the subsea system. For subsea to topside data traffic:

EODC 7 passes the data to EODC 6 via the copper links; - EODC 6 sends to the topside system an optical communications signal of approximately 3dBm to the next amplifier / repeater module up the line (if present), or the topside system.

The resulting optical amplification can reach 44dB = 45 dB (due to EODC 6 + EODC 7 maximum amplification) minus ldb (2 off connector losses each approximately 0.5dB). Figs. 3a-c schematically show three systems incorporating exemplary optical signal amplifiers in accordance with the present invention.

As described above, two electrical to optical data converters, EODC 6 and EODC 7 are used to effect optical amplification. Where SFPs are used as the EODCs, it should be noted that most SFP manufacturers will make their products to a multiple source agreement (MSA) which will ensure that the electrical pin-out is the same on all the SFPs produced, to be MSA compliant. This then enables customers to plug and play with the SFPs across a variety of the different host modules. The optical end of the EODC however can be very different, for example some SFPs will be of a dual-fibre configuration (e.g. one fibre for an uplink and one fibre for a downlink). There is also another type of SFP, the single-fibre SFP, which uses two different frequencies, one frequency for the uplink and one frequency for the downlink, but these frequencies can travel on the same fibre without interfering. The present invention is not limited to any particular type of EODC, and Figs. 3a-c show systems configurations variously employing single-fibre and dual- fibre SFP EODCs, which may be employed depending on how many fibres are needed on a particular section of the optical transmission path. In all these figures, for clarity only the optical paths are illustrated, and the electrical signals may be either AC or DC. In all cases shown, a single optical amplifier (repeater) 100 is deployed in the optical transmission path between a topside unit 101 and an end unit 102, which could comprise a subsea control module (SCM) in the particular case of an underwater hydrocarbon extraction facility.

Fig. 3a schematically shows a system configuration in which the topside unit 101 comprises a single-fibre type SFP 103, and the end unit 102 also comprises a single-fibre type SFP 104, so that both sections 2, 3 of the optical transmission path (i.e. sections of an umbilical cable in the particular case of an underwater hydrocarbon extraction facility) comprise a single-fibre optical transmission path. In this case, both EODC 6 and EODC 7 comprise single-fibre type SFPs.

Fig. 3b schematically shows a system configuration in which the topside unit 101 ' comprises a dual-fibre type SFP 103', and the end unit 102' also comprises a dual- fibre type SFP 104', so that both sections 2, 3 of the optical transmission path comprise a dual- fibre optical transmission path. In this case, both EODC 6' and EODC 7' comprise dual- fibre type SFPs.

Fig. 3c schematically shows a system configuration in which the topside unit 101 " comprises a dual- fibre type SFP 103", but the end unit 102" also comprises a single-fibre type SFP 104", so that section 2 of the optical transmission path comprises a dual-fibre optical transmission path while section 3 comprises a single-fibre optical transmission path. In this case, EODC 6" comprises a dual- fibre type SFP, while EODC 7" comprises a single-fibre type SFP. In this example, if the repeater 100" is placed at the correct point in the span between the topside unit 101 " and the end unit 102", then the single-fibre SFP EODC 7" can be used to communicate to the SCM and so this will reduce the number of wet mate connections on the top of the SCM as it will only require one fibre instead of two.

Fig. 4 schematically shows a distributed optical amplifier arrangement in accordance with another embodiment of the present invention for use with a DC electrical power signal. With a distributed system, the amplification hardware is located embedded in the umbilical cable section ends 20, 21, rather than as a discrete module.

Here, one EODC 22, 23 is located at each cable end 20, 21, and optically connected to the optical transmission path of that cable section to enable optical communication therebetween, each EODC 22, 23 being operable to interconvert optical and electrical signals.

Each cable section end also includes connection means 24, 25 for enabling electrical, optical and mechanical connection of that section end to a separate, complementary, cable connection means, of a different cable. In the embodiment shown in Fig. 3, a single, separate connector 26 is provided for connecting the two cable ends 20, 21.

This connector 26 has four copper connections (power and copper TX/RX), though for simplicity only one power line is shown. In an alternative embodiment (not shown), physical connection means may be provided at each cable end section, enabling direct connection of the respective ends, without the need for a separate connector unit 26.

The other ends of the cable sections (not shown) may comprise similar hardware to the shown ends if additional amplification is required, or may link directly to the topside or subsea system as appropriate.

When connected, the cable end sections provide back-to-back connection of the EODCs 22, 23 via electrical connection, with the electrical ports of the EODCs 22, 23 being connected so that the transmit (TX) port of each EODC is electrically connected to the receive (RX) port of the other EODC .

Each cable end section also includes supply power conversion hardware 27 necessary for powering the EODCs 22, 23, which converts electrical power taken from the electrical transmission line into a suitable form for the EODC being used.

Although not shown, the other, distal end of the or each cable section may comprise an additional distributed optical amplifier arrangement, such that a plurality of umbilical sections may be linked, with an amplifying function provided at each link.

In an alternative embodiment (not shown), a single power supply hardware component could be used to provide power for both EODCs 22 and 23. In such an embodiment, the component would be located at one end only of the cable, with an additional copper line being provided to connect the component with the EODC in the complementary end. The electrical connection would be similar to that shown in Figs. 1 and 2 for example. A drawback to this approach is the need to provide the additional copper line at the connection means.

As DC electrical power is used, there is very little voltage loss over long distances and therefore there is no need for voltage amplification.

The functionality of this arrangement is similar as for the embodiment shown in Fig. 2. The resulting optical amplification can reach 44dB = 45 dB (due to EODC 22 + EODC 23 maximum amplification) minus ldb (2 off connector losses each approximately

0.5.B).

The EODCs 22, 23 may for example comprise single-fibre type or dual-fibre type SFPs as required, or a combination thereof.

Fig. 5 schematically shows a discrete splitter-repeater module 30 in accordance with a further embodiment of the present invention, here for use with an AC electrical power signal. It is generally similar to the embodiment described with reference to Fig. 2. The system can readily be adapted for DC systems, as set out below. Here the module 30 is connected to a single section of upstream umbilical cable 31 leading topside, but to a number n of separate downstream umbilical sections 32i__n leading subsea, with the module 30 performing a splitter / multiplexing function in addition to amplification of the optical signal. All the umbilical sections include both an optical transmission path for carrying an optical communications data signal, and a copper electrical transmission path for carrying an electrical power signal.

The module 30 comprises various components:

1. Power boost module 33 a. For AC power distribution, the power boost module is a transformer, to compensate for the voltage loss that is due to the inherent power losses on the long-offset upstream (topside) umbilical 31. b. For DC power distribution, there is very little voltage loss for the supply of power over long distances and therefore usually there is no need for voltage amplification.

2. Front-end wavelength distribution module (WDM) 34 The WDM 34 has optical TX/RX links to/from the upstream (topside) umbilical 31. The optical signal coming from the upstream umbilical 31 contains a number of optical signal carriers of different wavelengths (λι, λ₂, ... λη), with one wavelength per SPCS section, i.e. for each of the downstream umbilicals 32.

The WDM 34 is a passive system and does not need powering.

The optical data signals distributed to the subsea systems by WDM 34 can utilise either dense wavelength division multiplexing (DWDM) or coarse wavelength division multiplexing (CWDM), as is known in the art.

3. Upstream EODCs 35i__n

There are two or more upstream EODCs 35, one for each wavelength (i.e. one for each downstream umbilical 32). These EODCs 35 are configured as follows: a. Optical TX/RX links to/from the front-end WDM 34; b. Copper TX/RX links to/from the downstream EODCs 36 (see below). These links are used to transfer and amplify data for the downstream umbilicals 32.

4. Downstream EODCs 361 __n

There are two or more downstream EODCs 36, one for each wavelength (i.e. one for each downstream umbilical 32). These EODCs 36 are configured as follows: a. Copper TX RX links to/from the upstream EODCs 35; b. Optical TX/RX links to/from the downstream umbilicals 32.

5. Power supply module 37

This module 37 supplies power to the EODCs 35, 36, converting electrical signal from the upstream umbilical 31 via the power boost module 33 as required for the EODCs being used.

Functionality:

A) For upstream (topside) to downstream (subsea) data traffic: 1. EODC 35_n (n = 1, 2...N) receives from the topside system the optical communications signals of very low power (down to a min of -42dBm power with current GEOG subsea systems).

2. EODC 35„ (n = 1, 2...N) passes the data to EODC 36„ (n = 1, 2...N) on the copper data links.

3. EODC 36_n (n = 1, 2...N) sends to the subsea system "n" an optical communications signal of approximately 3dBm.

B) For downstream (subsea) to upstream (topside) data traffic:

1. EODC 36_n (n = 1, 2...N) receives from the subsea system "n" an optical communications signals of very low power (down to a min of -42dBm power with current GEOG subsea systems).

2. EODC 36„ (n = 1, 2...N) passes the data to EODC 35„ (n = 1, 2...N) on the copper data links.

3. EODC 35_n (n = 1, 2...N) sends to the topside system an optical communications signal of approximately 3dBm.

The resulting optical amplification can reach 44dB = 45 dB (due to EODC 35_n + EODC 36_n maximum amplification) minus IdB (2 off connector with losses each approximately 0.5dB).

The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art.

Claims

CLAIMS:

1. A method of amplifying an optical signal, the signal being carried by an optical transmission path, comprising the steps of: i) providing a pair of electrical - optical data converters in the optical transmission path, including a first electrical - optical data converter for converting an input optical signal from the optical transmission path to an output electrical signal, and a second electrical - optical data converter; and ii) electrically connecting the first and second electrical - optical data converters, so that the second electrical - optical data converter is operable to convert the output electrical signal to an output optical signal for transmission on the optical transmission path.

2. A method according to claim 1, comprising the step of: iii) providing an electrical operating power supply for each of the first and second electrical - optical data converters.

3. A method according to either of claims 1 and 2, wherein the optical transmission path is located within a cable, the cable also including an electrical conductor for carrying an electrical signal.

4. A method according to claim 3 when dependent on claim 2, wherein the electrical signal provides an operating power supply for the first and second electrical - optical data converters.

5. A method according to either of claims 3 and 4, wherein the cable comprises an umbilical cable.

6. A method according to any preceding claim, wherein the first and second electrical - optical data converters are provided located in a common module.

7. A method according to any of claims 1 to 5, wherein the first and second electrical - optical data converters are provided located in respective modules, each module comprising a complementary connection means for interconnecting the respective modules.

8. A method according to any of claims 3 to 7, wherein the electrical signal comprises an alternating current signal.

9. A method according to claim 8, comprising the step of providing a step-up transformer for increasing the voltage of the electrical signal.

10. A method according to any of claims 3 to 7, wherein the electrical signal comprises a direct current signal.

11. A method according to any preceding claim, comprising the steps of: iv) providing at least one additional pair of electrically interconnected first and second electrical - optical data converters; v) providing a wavelength division multiplexer in the optical transmission path; vi) optically connecting the wavelength division multiplexer to each pair of electrical - optical data converters.

12. A method according to any preceding claim, wherein each electrical - optical data converter comprises a small form- factor pluggable device.

13. A method according to claim 12, wherein at least one of the pair of electrical - optical data converters comprises a single-fibre type small form-factor pluggable device.

14. A method according to either of claims 12 and 13, wherein at least one of the pair of electrical - optical data converters comprises a dual-fibre type small form-factor pluggable device.

15. A method according to claim 12, wherein the first electrical - optical data converter comprises a dual- fibre type small form-factor pluggable device and the second electrical - optical data converter comprises a single-fibre type small form-factor pluggable device.

16. An optical signal amplifier module for amplifying an optical signal carried by a transmission path, the module adapted for connection between first and second sections of the transmission path, comprising: a pair of first and second electrical - optical data converters, each electrical - optical data converter being operable to interconvert optical and electrical signals, first connection means for optically connecting the first electrical - optical data converter and the first section of the transmission path to enable optical communication between the first electrical - optical data converter and the first section, second connection means for optically connecting the second electrical - optical data converter and the second section of the transmission path to enable optical communication between the second electrical - optical data converter and the second section, and wherein the first and second electrical - optical data converters are electrically connected, enabling electrical communication therebetween.

17. An optical signal amplifier module according to claim 16, wherein the first and second sections are located within respective sections of a cable, the cable also including an electrical conductor for carrying an electrical signal.

18. An optical signal amplifier module according to claim 17, wherein the electrical signal is connected to each of the first and second electrical - optical data converters to provide an operating power supply for the optical data converters.

19. An optical signal amplifier module according to either of claims 17 and 18, wherein the cable comprises an umbilical cable.

20. An optical signal amplifier module according to any of claims 16 to 19, wherein the electrical signal comprises an alternating current signal.

21. An optical signal amplifier module according to claim 20, comprising a step-up transformer for increasing the voltage of the electrical signal.

22. An optical signal amplifier module according to any of claims 16 to 19, wherein the electrical signal comprises a direct current signal.

23. An optical signal amplifier module according to any of claims 16 to 22, comprising at least one additional pair of electrically interconnected first and second electrical - optical data converters, and a wavelength division multiplexer in the optical transmission path, the wavelength division multiplexer being optically connected to each pair of electrical - optical data converters.

24. An optical signal amplifier module according to any of claims 16 to 23 wherein each electrical - optical data converter comprises a small form-factor pluggable device.

25. An optical signal amplifier module according to claim 24, wherein at least one of the pair of electrical - optical data converters comprises a single-fibre type small form-factor pluggable device.

26. An optical signal amplifier module according to either of claims 24 and 25, wherein at least one of the pair of electrical - optical data converters comprises a dual- fibre type small form-factor pluggable device.

27. An optical signal amplifier module according to claim 24, wherein the first electrical - optical data converter comprises a dual-fibre type small form-factor pluggable device and the second electrical - optical data converter comprises a single-fibre type small form-factor pluggable device.

28. A cable for carrying an optical signal on an optical transmission path, comprising, at an end thereof, an electrical - optical data converter being operable to interconvert optical and electrical signals, the electrical - optical data converter being optically connected to the optical transmission path to enable optical communication therebetween, and connection means for enabling electrical connection of the electrical - optical data converter to a separate, complementary, cable connection means.

29. A cable according to claim 28, wherein the cable comprises an umbilical cable.

30. A cable according to either of claims 28 and 29, comprising an electrical conductor for carrying an electrical signal.

31. A cable according to claim 30, wherein the electrical signal is connected to the electrical - optical data converter to provide an operating power supply for the optical data converter.

32. A cable according to claim 30, wherein the electrical - optical data converter is electrically connected to the connection means for receiving an operating power supply for the optical data converter when connected to the complementary cable connection means in use.

33. A cable according to any of claims 28 to 32 comprising, at the other end thereof, a second electrical - optical data converter being operable to interconvert optical and electrical signals, the second electrical - optical data converter being optically connected to the optical transmission path to enable optical communication therebetween, and connection means for enabling electrical connection of the second electrical - optical data converter to a separate, complementary, cable connection means.

34. A cable according to any of claims 28 to 33 wherein the or each electrical - optical data converter comprises a small form-factor pluggable device.

35. A cable according to claim 34 wherein the or each electrical - optical data converter comprises a single-fibre type small form-factor pluggable device.

36. A cable according to either of claims 34 and 35, wherein the or each electrical - optical data converter comprises a dual-fibre type small form-factor pluggable device.

37. A cable according to claim 34 when dependent on claim 33, wherein the first electrical - optical data converter comprises a dual-fibre type small form-factor pluggable device and the second electrical - optical data converter comprises a single-fibre type small form-factor pluggable device.

A cable kit comprising at least two cables in accordance with any of claims 28