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Figure 1 - Fiber Versus Copper

Fiber Optic Telemetry For Deep ROV Operations

Terence M. Dwyer, Ian B. MacKay, Graham A. J. SmithFocal Technologies Inc.40 Thornhill Dr., Unit 7

Dartmouth, NS, B3B 1S1, Canada

1. INTRODUCTION weight and drag in the water cause higher cable

In response to demands for operation in deeper cable cross-section is required, which in turnwater, and the associated need for longer ROV makes the cable even heavier and larger inumbilical cables, new technologies are being diameter. The small size of optical fiber allows aadopted by offshore energy operators. Among reduction in the diameter of the “payload” core,these are singlemode optical fiber and digital while its reduced weight is also helpful. Insignal multiplexing. This paper reviews the state conjunction with high-modulus structuralof the art of aspects of these important materials, such as aramid fiber, the size andtechnological advances. weight saving achieved by using fiber optics for

2. FIBER OPTICS in lengths as great as 10 km. Reducing cable

In recent years, trends in ocean activity have of an ROV Tether Management System (TMS).required operations in increasingly deep water. Inthe offshore oil industry, this is generally for theexploration and development of deeper and/or more remote petroleum reservoirs. ROV systemsdeployed in such areas require longer andstronger umbilical cables that can also handle theever-increasing data rates required for digitizedvideo, high resolution imaging sonars, and digitalnetworks where total data rates often exceed1 Gbit/s. The beneficial signal characteristics and

physical properties of optical fiber enable muchlonger telemetry systems than are possible withcopper alone.

The dominance of fiber in telecommunicationssystems is a strong indicator of its significantadvantages over copper for data signals. Ascable lengths increase, the high signal attenuation In the past, most fiber optic ROV tethersof copper, even coaxial cable, quickly becomes employed multimode fiber, since it was the firstunacceptable. Optical fibers provide not only commercially available fiber, the associatedmuch lower attenuation, as shown in Figure 1, but components were available, and connectors for italso smaller size, lower weight, and much larger were relatively easy to install. The large core sizesignal bandwidth than copper. Fiber’s low of multimode fibers, typically either 50 or 62.5

attenuation is also constant with signal frequency microns, couples large amounts of power fromup to the microwave band, eliminating the need inexpensive LED transmitters. But althoughfor equalizers in the end equipment. Its inherent multimode fiber is an improvement over metallicimmunity to electro-magnetic interference (EMI) is coaxial conductors, its bandwidth and attenuationanother valuable property in electrically noisy limit practical cable lengths to 3 or 4 km. Modalenvironments. dispersion, i.e. pulse spreading caused by the

Optical fiber has an important size and weight accounts for most of multimode fiber’s bandwidthadvantage over insulated electrical conductors. limitation.

As umbilical cables get longer, their increased

loads. More area for strength members in the

signals allows ROV cables to remain manageable

size and weight also reduces the size and weight

differences in transit time for different modes,

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Singlemode fiber is superior to multimode fiber in for cabled fiber are typically under 0.5 dB per several ways and has become the standard for kilometer. The optical transmission system mustlong cables. It is distinguished from multimode be designed with sufficient optical power andfiber by its relatively small core diameter, typically receiver sensitivity to accommodate the total8 microns, which permits only one mode of light to optical losses in the system. A sample opticalpropagate, thus eliminating modal dispersion and power budget is shown in Figure 2.increasing bandwidth. The smaller core size isbetter at containing the optical energy in the fiber, Chromatic dispersion, rather than attenuation, is

reducing attenuation from 1-3 dB/km for normally the distance-limiting parameter for highmultimode fiber to about 0.5 dB/km for speed digital signals on singlemode fiber. Pulsesinglemode. This allows singlemode fiber to spreading results from the slight difference insupport telemetry over longer lengths than velocity for photons of different wavelengths, amultimode. Macro-bending losses and losses parameter known as dispersion that is arelated to cable manufacturing processes are characteristic of the glass itself and is a functiontypically much lower than for multimode fiber. of wavelength. Excessive pulse spreading results

Singlemode fiber also has an extremely large data stream. Longer cables exhibit proportionallybandwidth. Using multiple wavelengths, it is more such interference.possible to transmit 20 Gbit/s over a single100 km fiber without repeaters. With so much Standard fiber has a low dispersion region near bandwidth available, all of the analog and digital 1300 nm. This is occasionally shifted by

signals in an ROV system can typically be manufacturers to the 1550 nm window, for itsmultiplexed onto a single fiber, although slightly lower attenuation. Ideal lasers would emitadditional fibers are normally used for reasons at a single wavelength, thus eliminating the pulsegiven in section 4. spreading caused by dispersion. Typical laser

The primary design considerations for singlemode The resulting chromatic dispersion must belinks are total optical attenuation, chromatic accounted for in the system design. In mostdispersion, and return loss. The glass used in cases, dispersion can be calculated andfiber has low attenuation windows around incorporated as a power penalty of 1 or 2 dB in1300 nm and 1550 nm. Losses in both windows the flux budget.

in interference between adjacent bits in the serial

sources, however, have 1-3 nm spectral widths.

ITEM POWER TYPICAL LOSS TOTAL LOSS

Transmitter 0 dBm

Connector at pressure case 0.5 dB 0.50 dB

Tether (500 m typically) 0.25 dB 0.75 dB

TMS winch reel JB connectors 0.5 dB 1.25 dB

Model 242 SM FORJ 3.0 dB 4.25 dB

TMS winch cage JB connectors 0.5 dB 4.75 dB

Umbilical (10 km) 5.0 dB 9.75 dB

Deck winch reel JB connectors 0.5 dB 10.25 dB

Model 242 SM FORJ 3.0 dB 13.25 dB

Deck winch shipboard JB connectors 0.5 dB 13.75 dB

Deck cable (100 m) 0.05 dB 13.80 dB

Connectors 0.5 dB 14.30 dB

Dispersion Penalty 1.0 dB 15.30 dB

Total Received Power -15.3 dBm

Receiver sensitivity -20.0 dBm

Margin 4.7 dB

SM=singlemode; JB=junction box.

Figure 2 - Sample Optical Power Budget

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Figure 3 - Model 903 Multiplexer (Remote Unit)

Return loss, or back reflection, is a measure of configuration. TDM usually requires morehow much light is reflected back by an optical bandwidth than FDM, but this is a readilycomponent, such as a connector. Higher return available commodity with optical fiber.loss, or lower back reflection, means lessreflected light. Too much reflected power can The rapid and ongoing improvements in digitaldegrade a laser transmitter’s performance, integrated circuitry, and associated decreasingresulting in an undesirable power penalty. costs, have established TDM as today’s dominant

Commercially available PC (physical contact) established protocols available, such as FDDI,fiber optic connectors have return losses of 40 to SONET, Fibre Channel and various forms of 50 dB, while APC (angled physical contact) Ethernet. Many proprietary data link schemes,connectors can achieve return losses as high as such as used in the Model 903 shown in Figure 3,70 dB. Use of low power transmitters minimizes are based on high speed parallel-to-serialthe effects of back reflection. conversion chip sets.

3. MULTIPLEXING AND DIGITALTECHNOLOGY

When multiple signals need to be combined on asingle fiber, three types of multiplexing arecommonly used: time division multiplexing (TDM),

frequency division multiplexing (FDM), andwavelength division multiplexing (WDM). Eachtechnique allows many separate data channels tobe carried by a single transmission signal.Channels are kept separated by time slot,frequency band, or by the wavelength of the lightused. Since fiber allows simultaneous andbidirectional transmission of many wavelengthswithout interference, wavelength divisionmultiplexing can be used in conjunction with FDMand TDM to multiply the effective capacity of the Analog signals can be incorporated in TDMfiber. schemes by digital sampling. Video signals, for

FDM is an analog technique that is broadly used subcarrier frequency (3.58 MHz for NTSC) with 8in CCTV and CATV systems as well as over-the- to 10 bit resolution. The digitization andair radio and television broadcasting. The signal reconstruction process can be well controlled toto be transmitted modulates a carrier frequency, maintain signal quality over the transmissionhence shifting the information to a portion of the system. Because of TDM’s inherent digitalfrequency spectrum around the carrier. By using nature, signal degradation through the fiber cabledifferent carrier frequencies, spaced sufficiently itself is eliminated, provided that the digital signalfar apart, multiple channels can be conveyed on is received properly. This effectively isolates thethe same transmission medium. FDM has limited desired signal from the adverse effects of ability to handle changes in the number of dispersion and attenuation described earlier. Inchannels, or their bandwidth requirements, contrast, analog transmission schemes generallywithout adding equipment. suffer from signal degradation as a function of

TDM is a flexible digital technique whereby existing standards for transmission quality, suchmultiple parallel digital channels are combined in as EIA/TIA-250-C for video, allow evaluation of a single high-speed serial bit stream. Each end-to-end system performance.channel uses only certain bits, or time slots, in theserial sequence. One of TDM’s major Even relatively slow data signals can be sampledadvantages lies in the ability to assign more bits to avoid synchronization issues. Commonper second to the channels that need it, and fewer formats, like RS-232, RS-422, and RS-485, maybits per second to slower channels. This flexibility be converted to TTL voltage levels and combinedyields very efficient use of the transmission by a microprocessor or other digital circuits. Thebandwidth and allows easy changes in “sampling” process does not involve an analog-to-

multiplexing technique. There are many well

example, are typically digitized at four times the

distance transmitted. Regardless of the method,

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Figure 4 - Block Diagram of Model 903 Fiber Optic Transmission System

Figure 5 - Model 242 Singlemode FORJ(2 pass version shown)

digital converter, since the signals are already bidirectional with insertion losses under 1 dB anddigital. It is sufficient to simply accept the binary isolations of greater than 30 dB.state of each channel in sequence. Withasynchronous protocols, this “bit sampling” can A typical WDM multiplexing scheme is shown inbe as slow as 3 or 4 times the channel data rate Figure 4. Bidirectional transmission on a singlewithout significant degradation. fiber is provided with a 1310 nm uplink and

Wavelength division multiplexing places channels

on separate wavelengths of light. The technique Newer dense WDM systems (DWDM) enable 8 or is not limited to analog or digital signals and can 16 separate wavelengths closely spaced near be combined with both FDM and TDM signals. 1550 nm. Currently used in advanced tele-The entire signal capacity of the fiber is multiplied communications systems, DWDM is an expensiveby the number of wavelengths employed; technique requiring highly stable lasers withnormally 1300 nm and 1550 nm are used in narrow spectral widths and special non-zerosinglemode fiber. The combination and dispersion-shifted fiber (NZ-DSF) to minimizeseparation of wavelengths is actually done non-linear mixing effects. In future, though, thispassively by an inexpensive optical device, also technique may provide a viable upgrade path for known as a WDM (wavelength division ROVs from conventional singlemode systems.multiplexer). Typical singlemode WDMs are

1550 nm downlink.

4. HOW MANY FIBERS? loss of control of the vehicle. For this reason

Although the technology is available to combine an inexpensive optical switch, the multiplexer all control and communications channels for even system can be manually or automaticallythe largest ROV system on a single optical fiber, transferred from a bad fiber link to a good onemore than one fiber is generally used. This may with minimal loss of data. Most umbilical cablesbe to accommodate a preferred system using singlemode fiber have from 4 to 10 fibersconfiguration, a future upgrade, or to allow for a available.failure in the optical fiber link. The loss of a singlefiber in a multiplexed system could cause total Multi-pass singlemode Fiber Optic Rotary Joints

alone, availability of a spare fiber is critical. Using

(FORJs), needed in the surface and TMSwinches, are now standard components. Focal’sModel 242 multi-pass FORJ, shown in Figure 5,accommodates up to three singlemode passeswith insertion losses typically under 3 dB and

rotational variations less than 0.5 dB.

5. CONCLUSION

The advances in fiber optic and digitalmultiplexing technologies have enabled theextension of ROV systems to deeper waters.Ongoing development in the technologies willprovide even greater bandwidths and ranges inthe near future.

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