3.1 DEPLOYMENT CONCEPT 



The present deployment concept for a fiber-optic supervisory-controlled submersible 

 calls for paying out an optical fiber tether cable from a vehicle-mounted prewound, pre- 

 twisted, stationary free-standing spool as the vehicle moves along its underwater path. Once 

 the cable leaves the spool, the intent is not to have the vehicle pull it through the water but 

 rather to have the tether lie relatively motionless in the water column. Thus, in concept, the 

 vehicle with its spool of cable will move through the water column while laying its tether in 

 its track. This concept assumes that the link ultimately will be low enough in cost to be 

 expendable. To achieve this cost, the intrinsic expense of the cabling process associated 

 with the packaging of the optical fiber must be low. In addition, if it is to be capable of 

 extended track ranges, say 10 km, the cable must be small and light because of packaging 

 and payload Hmitations. The only strengthening allowed, therefore, is the minimum required 

 to permit the cable to be handled and deployed without breakage for the duration of the 

 intended mission; typically on the order of several hours. In addition, the tether deployment 

 mechanism must be inexpensive and reliable. 



3.1.1 FIBER-OPTIC LINK 



To achieve desired design objectives, several low-loss optical fiber link designs are 

 presently under consideration. The physical characteristics of each generic type are listed in 

 table 2. A Mk 37 torpedo wireguide is also listed for comparison purposes. 



Design 1, the "unarmored fiber-optic element," consists of the glass optical fiber 

 surrounded by a polymer buffer layer which prevents surface abrasion and a resulting 

 weakening of the glass (see figure 3). Essentially, all the tether's strength is provided by the 

 glass itself; for a 0.005-inch-diameter fiber proof-tested to a stress level of 200 000 psi, a 

 proof strength of approximately 4 pounds results. The buffer layer increases the total 

 tether diameter to approximately 0.020 inch. Experimentation with winding and deploy- 

 ment of this fiber cable has shown that such an unruggedized design is not practical. A cable 

 design with no stiffness other than that provided by the 5-mil-diameter fiber must be wound 

 or spooled with a binding agent of extremely low adhesive strength such as lacquer or rubber 

 cement. Unfortunately, binding agents of this type create a very slow winding rate and 

 result in a structurally unstable package from a handling standpoint. In addition to practical 

 spool fabrication problems, the unruggedized cable design may not necessarily result in the 

 lowest cost, as might be assumed. The reason is that a high proof-test specification of 

 150-200 kpsi results in a very high cost per meter for long, continuous (5000-m) lengths. 

 For example, in FY 80, 5-km lengths of 100-kpsi proof-tested fiber were bought from ITT 

 for $ 1 .25/m. ITT would not bid on fiber of this length at higher proof tests, or at a 1 00-kpsi 

 proof test in longer than 5-km lengths. Sumitomo, on the other hand, supplied 5-km 

 lengths of fiber proof-tested to 1 50 kpsi for $2.50/m. The capabiHty of the optical fiber 

 manufacturers to supply fiber to high proof tests, low loss, and great lengths is characterized 

 by "yield" curves. Such a family of curves is depicted in figure 4. Note that these curves 

 vary from one manufacturer to another and are determined by the level of manufacturing 

 technology attained by the company. 



Payout experiments in FY 80 determined that a stiff tether was required to bend- 

 limit the cable at the spool peel-off point. In addition, a minimum tensile strength of 6-10 

 lb would be required to achieve a reasonable safety factor. For these reasons, it became 

 apparent that some sort of "ruggedization" would be required for the cable. A standard 



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