device at the time of manufacture, prior to integration witii tiie cable and 

 optoelectronics. Ideally, the pressure-barrier subcomponent should be 

 standardized and type-certified, and should lend itself to integration with 

 any type of optical fiber or connector when incorporated into its final 

 penetrator configuration. 



3. The design should be fully demountable from both the higli- and low- 

 pressure sides of the penetrator unit. Ideally, the high-pressure side should 

 be capable of underwater make/break operation, a valuable asset for some 

 important system applications. Full demountability obviates the requirement 

 for treating the penetrator, the optoelectronics assembly, and the cable as a 

 single unit, and allows separation for the purpose of repair, exchange, trans- 

 port, and storage of the individual subsystems over the life and mission 

 profile of the system. 



4. The design should be compatible with a wide variety of optical fiber and 

 connector types. This reduces the certification burden, and also minimizes 

 inventory requirements; one penetrator type should be capable of operation 

 with a wide variety of optical fiber styles by means of a standardized, type- 

 accepted connector body. 



5. The design should be straightforward to manufacture in a repeatable fashion 

 with reasonable manufacturing tolerances. It should not be so complex that 

 reliability and cost-effectiveness are compromised. 



6. The design should be rugged and robust, wliich would permit operation over 

 a wide range of temperature and pressure conditions, and should be resistant 

 to damage by vibration and explosive shock. Overall system performance 

 must not be compromised in any way by limitations of the fiber-optic 

 penetrator. In the case of the NOSC free swimmer, the penetrator must be 

 capable of an operating pressure of 1000 psi over a temperature range of 

 0°C to 40° C. 



7. The design should lend itself to hermetic sealing for applications which 

 require sustained exposure to high hydrostatic pressures, and should 

 tolerate no vapor intrusion for reliability reasons. Class-to-metal or glass-to- 

 ceramic seals should be usable to form a hermetic vapor barrier without 

 requiring a major redesign or recertification effort. 



Historically, epoxy-filled hypodermic needles, epoxy pottings, and elastomeric 

 squeeze bushings have been employed when it was required to transfer light from an optical 

 fiber across a pressure gradient. These techniques all realize their light-transfer function by 

 means of physical sealing to the external cable sheath or to the optical fiber itself. Such 

 approaches fail to satisfy all or even most of the criteria that describe a viable penetrator. 

 While these techniques sometimes serve as adequate solutions when applied to the test and 

 evaluation of developmental fiber-optic components, reliance on them would result in un- 

 desirable engineering and operational compromises if applied to Navy systems. 



The penetrator realization reported here is based on the concept disclosed in ref 

 14. This approach utilizes a Graded Refractive Index Rod Lens (GRIN) of one-half pitch 

 length as a combination pressure barrier and imaging device (schematically depicted in fig 17). 



34 



