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Practical Techniques to Increase Fiber-Optic Performance
Operations Manager, Institute for Exploration
When fiber optics first appeared in the marine industry in the early 1980s, it changed the playing field. Massive quantities of data could be sent over long distances using a single fiber. As a result, cables and umbilicals progressively reduced in size and weight. ROV designers and manufacturers quickly caught on, and fiber is now the standard telemetry backbone for deep-diving tethered vehicles.
Fiber optics are high-tech but basic in principle; light is used to transmit signals. Users of the technology tend to view it in black-and-white terms; either it works or it does not. The truth is there are many shades of gray related to fiber performance, and the more information you try to push down the pipe, such as high-definition video or high-resolution sonar data, the more sensitive the system becomes to attenuation. Poor polishing techniques, environmental exposure and improper cleaning methods can all greatly affect light transmission.
For example, the quality of a fiber-optic connection from a tethered ROV can be degraded or made less reliable by the use of isopropyl alcohol, which most technicians and engineers regard as superior to regular denatured alcohol because it leaves less residue on the endface during cleaning. Unfortunately, all alcohols are hydrophilic; they absorb moisture from the air just sitting on the shelf in a bottle. Over a short period of time, go-to cleaning fluids can become contaminated, resulting in streaks in and on the endface of the connector.
Static-dissipative fluorinated fluids, which can drastically reduce the presence of streaks and residue, have become popular among fiber-optic users over the past few years, but as these products have not found their way into many ROV shops, they are not widely used by ROV technicians. I have found a night-and-day difference in using static-dissipative fluorinated or other purpose-made fluids, rather than isopropyl alcohol. After the Institute for Exploration began cleaning its ROV connectors using static-dissipative fluorinated fluids, approximately 6 decibels of loss in each fiber pass were eliminated—a 50 percent improvement.
Cleaning the connectors this way also allowed the institute's technicians to see defects caused by hand-polishing and realize that new polishing tools were needed, as poor polish on connector endfaces are another common source of optical loss. Technicians who terminate their own fibers know how complicated the process can be. Hot-melt connectors make this process much easier, but the final polish is what really makes the difference. Good results are possible by hand but require someone who is more of an artist than a technician. Portable fiber polishing machines have become more affordable and are standard equipment in network installation vans; some do not require a level surface to operate (a requirement for anyone going offshore). These machines can take a lot of guesswork out of the equation and result in a professional polish, saving hours of labor.
Improving the performance of a fiber-based system can be as easy as removing unnecessary connections. The Institute for Exploration's custom-built, 4,000-meter-rated dual ROV system, Hercules and Argus, had more than 50 connections, with one path totaling 22 individual connection points. Considering that barrel connectors produce about 1 decibel of loss per union, it is amazing a signal could get through. After streamlining the system, loss measurements were cut by almost half.
The land-based fiber-optic industry is an active community that is continually advancing. The ROV industry, however, always seems to be several years behind, despite existing in a more critical and demanding environment. Practical lessons, such as cleaning and polishing methods, have dramatically improved the optical performance of the Institute for Exploration's ROV system; every technical team should evaluate its tools and techniques. This might mean digging into some deep and dark places in your ROV system and an initial investment in additional tools, but in the long run it can save time, money and frustration.