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Precision Electro-Optomechanical Cable Assembly for a Towed Array
EOM Project Includes New Deep Submergence Hybrid Connector Pair


Amy Brown

Michael Einhorn

Carl Stevens

BIRNS ETA-I certified photonics technician terminates the assembly, applying epoxy on each optical ferrule to secure the optical fiber in place.
Electro-optical (EO) cable terminations provide the advantage of combining a variety of sophisticated capabilities into a single subsea connection. These custom cable assemblies must simultaneously pass power and data and withstand extreme environmental and performance requirements, while preserving the integrity of delicate optical fibers. Among the most challenging EO assemblies are those that also require mechanical termination for a tow cable to a towed body, i.e., electro-optomechanical (EOM) terminations. These applications demand a termination strong enough to bear all towing and handling loads and still be able to perform reliably over time in the extremes of the marine environment.

EOM Assembly Requirements
In 2012, BIRNS Inc. (Oxnard, California) was asked to develop a unique EOM cable assembly for the tow cable of a high-performance passive towed array, Northeastern University's (NU) Ocean Acoustic Waveguide Remote Sensing (OAWRS) receiver system. The OAWRS receiver project was initiated in 2009 when a team of NU scientists and engineers from the commercial sector commenced developing the receiver array suite as a lightweight acoustic system for rapid, wide-area monitoring and measurement of the ocean environment. The system was tailored as a general purpose, mobile, remote-sensing tool to study marine life, oceanography, geologic and geophysical properties of the ocean environment, as well as anthropogenic noise in the ocean and its effect on marine life.

Einhorn Engineering PLLC (Seattle, Washington) was selected to provide the mechanical engineering services required to design and oversee fabrication of the handling system. This included specification of a custom tow cable and wet side junction box subassembly. The OAWRS system's EOM tow cable consisted of two single-mode fibers, four power conductors and a rotation-resistant, over-armor package. The specialized mechanical and electrical terminations required for the project presented unique technical challenges, as the wet side junction box design required very tight length tolerances for the hybrid electro-optical connector termination. In response, BIRNS created a fabrication plan to address proper installation of the existing ball-and-socket mechanical termination, as well as termination and over molding of the EO connector, while maintaining the demanding finished-assembly tolerances. Additionally, BIRNS developed a new deep submergence hybrid BIRNS Millennium 3O-2F2-FR (flanged receptacle) and mating CP (cable plug) connector pair, incorporating two single-mode optical fibers and two 16-AWG electrical pins. The connector pair has a 6-kilometer depth rating.

As in any EOM assembly, it is critical to create the system so that the viability and performance of both the electrical and optical conductors are ensured. Since the OAWRS array was to be operated at depths of 200 meters for weeks at a time, a particularly robust solution was necessary. The mechanical termination had to provide defense for the delicate conductors of power and information as they transited from the protection of the armored strength members to the connection point on the wet side junction box. One challenge with the OAWRS design was the very tight tolerance on the subcable terminated length, which was only 12 inches (+/-.25 inches). The design dictated a severe space constraint between the ball joint of the mechanical termination and the connector due to the small package size (26 inches long by 8 inches diameter) and the location of the pressure vessel (9.44 inches long by 7.50 inches diameter) within the housing, which required finesse and precision during the complex termination process.

The OAWRS system was to be applied in both stand-alone, passive sensing applications, as well as paired with the OAWRS Source Array in active sensing deployments. The receiver array equipment suite included the synchronized hydrophone array, which allowed for high-resolution directional sensing over wide areas and a handling system designed to be quickly mobilized on a vessel of opportunity. Therefore, the system called for one single-mode fiber to transmit the array data with a second, backup fiber. The connector design was finalized with the inclusion of two optical fibers to allow for seamless transition to the backup and two 16 AWG electrical power conductors. To continue this article please click here.

Amy Brown is the director of corporate communications for BIRNS Inc. and is responsible for developing and managing a comprehensive set of strategic external marketing, media relations and internal communications programs. She has held marketing leadership roles for companies in the marine electronics industry since 2004.

Michael Einhorn worked as a mechanical engineer for the Naval Civil Engineering Laboratory and Naval Facilities Engineering Service Center (NFESC) before starting Einhorn Engineering PLLC in 2008.

Carl Stevens, a civil engineer, worked at NCEL and NFESC and has been with Einhorn Engineering since its founding. Einhorn Engineering has consistently been involved in ocean engineering research and development projects, with an emphasis on marine launch and recovery systems for both vehicles and towed arrays.

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