Fiber In Metal Tube (FIMT)

Fiber In Metal Tube (FIMT) is the process of encapsulating very long lengths of optical fibers within a hermetically sealed metal tube. This rugged construction is particularly effective in protecting against the hydrostatic pressures, high temperature effects and corrosive environments associated with down-hole fiber optic sensing applications.

The metal tube is defined by the tubes outer diameter (OD) and the effective wall thickness. The most commonly used metals for this process are 304 Stainless Steel and 316 Stainless Steel. Other specialty alloys, such as Incoloy 825 or Inconel 625, are also utilized. FIMT’s are available in sizes ranging from 0.84mm OD containing a single optical fiber to tubes with a 3.65mm OD holding up to 72 individual fibers. Constructions may include individual or multiple optical fibers contained within concentric metal tubes. These concentric metal tubes consist of an inner core containing single or multi-fiber micro cores, a protective buffer layer, captured in an encapsulated outer macro tube. Buffer and encapsulation materials typically used are FEP, PFA, PVC, Teflon and others. Single, double and triple walled FIMT solutions are available based on the application specific requirements.

Additionally, the tubes may be filled with a thixotropic gel that is used as a filling compound. This viscous gel protects the fibers from many environmental concerns, prevents damage from microbending conditions and helps to minimize the forces applied during spooling and deployment. The addition of a hydrogen scavenging/absorbing version of this gel is generally used in down-hole sensor based applications to prevent the potential darkening of the optical fibers due to the natural ingression of small amounts of hydrogen which are present in these extreme conditions. In hydrogen-rich environments, the use of a hermetically sealed, carbon coated optical fiber(s) is strongly recommended.

Another important parameter to consider with FIMT is the Excess Fiber Length (EFL). EFL is defined by the percentage of excess fiber loaded into the innermost metal tube during processing to relieve induced stresses directly related to the differing thermal coefficient of expansions of each material contained in the construction. FIMT is exposed to drastic and often rapid changes in temperature and pressure, so the EFL must be considered to ensure continuous, robust performance across all operational conditions.

Verrillon offers a series fibers encapsulated in a variety of metallic tubes such as SS304, SS316L and Incoloy 825. Polymeric tubes types are FEP, Hytrel, PEEK, PFA and Tefzel.