Harsh environment fiber diagnostics

From cryogenic to high temperature environments, different polymer and metallic coatings are available in Exail’s portfolio for optical fibers. The coating is a multi-purpose element, some of its key functions are to protect the mechanical integrity of the fiber, to shield the silica glass from chemicals, and to limit micro-bending.

Most of the coatings in use nowadays are made of polymer. Polymer coatings are relatively easy to manipulate, provide good mechanical and chemical resistance and are very well suited to handle moderate temperatures found in outdoor applications. Metal coatings are also available for temperatures higher than 350 °C.

• Acrylate is the standard coating for telecom fibers, with a maximum operating temperature of +85°C. It remains the preferred coating for standard environmental conditions, but it is not suitable for high temperature and/or radiation levels.
• High temperature acrylate (acrylate HT) was specifically developed to offer an extended operating temperature up to +150 °C with good radiation resistance.
• Polyimide is the preferred choice for the most demanding applications, with a maximum operating temperature of +300°C and excellent radiation resistance. It is worth noting that polyimide fibers typically have an outer diameter of only 150 – 160 µm, compared to 250 µm for acrylate coated fibers. It is also adapted for cryogenic environments (-196 °C)
• Metal coatings such as aluminum or copper can handle long term temperatures beyond 400 °C and are currently under development to complete Exail’s fiber portfolio for high temperature applications.

The coating is deposited during the drawing process before the bare silica fiber touches any hard external surface. This ensures a proper protection of the fiber before it touches the metallic drawing pulleys that pull the fiber at the bottom of the drawing tower.

For polymer coatings, the fiber goes through a bath of liquid polymer and exits through a die of calibrated diameter which sets the outer diameter of the coated fiber. The fiber is then cured in-line using a UV or thermal furnace to polymerize the coating. Depending on the coating material and recipe, several layers of coatings can be applied on top of each other to achieve the desired opto-mechanical characteristics. Each layer may have a different chemical composition and thickness.

Polyimide coating is also adapted for cryogenic temperatures, even though the fiber should remain still once installed in the cryogenic environment because of the brittle nature of polymers well below their glass transition temperature. Nevertheless, tests performed on polyimide coated fibers cooled at liquid nitrogen temperature (-196 °C) for 24 hours showed no degradation of the optical transmission and excellent resistance to mechanical traction.

Radiation Induced Attenuation (RIA) is the most noticeable effect of radiation on optical fibers and consists in the decrease of optical transmission (expressed in dB/km/Gy) due to the creation of color centers by the incoming radiation.

Rad-Hard fibers were developed to mitigate the RIA and extend the fiber’s lifetime when used in radiative environments. An example of such fiber is the PSC (Pure Silica Core) fiber with Fluorine doped (F-doped) cladding, available in singlemode or multimode. A matching Rad-Tolerant fiber (Germanium-doped) is available for low to moderate radiation level.

Exail rad-hard fibers can be used for data transmission, sensing or experiment monitoring. Some fibers are available on stock and custom design are available based on your specific application.

Example of applications: highly specific rad-hard fibers are used for diagnostics in inertial laser fusion facilities (such as LLNL NIF and CEA LMJ).

Exail offers Rad-Hard singlemode fibers for 1550 nm operations (but also in other wavelengths like 900, 1000, 1310), step-index multimode fibers, or graded-index multimode fibers:

• Exail single mode fibers are available with NA between 0.12 and 0.37. High-NAs fibers provide exceptional robustness against bending-loss and are also commonly used for FBG inscription thanks to their improved photosensitivity without hydrogenation. They can be coated with standard, high temperature acrylate or polyimide. Rad-hard versions can operate in harsh radiative environments.

• Exail step-index multimode fibers (MMSI) are made of a pure silica core with a fluorinated cladding. Standard 105-125 fiber with 0.22 NA is available off the shelf. In addition, a wide range of preforms can be manufactured and drawn with an on-demand cladding diameter up to 600 µm. They can be drawn with standard acrylate, high temperature acrylate or polyimide coatings.

• Exail graded-index multimode fibers (MMGI) exist with various core size and NA. Standard versions have a Ge-doped core surrounded by a silica cladding, radiation resistant MMGI fibers are also available and have a Fluorinated core. The doping composition of the core, the cladding diameter and coatings can be tailored.

Example of applications: Singlemode fibers are typically used for low-loss transmission over a reduced wavelength range. FBG can be written on singlemode fibers for strain and/or temperature sensing. Multimode fibers have a much larger core than singlemode fibers and can transmit signals over a broad wavelength range. The large core of multimode fibers is advantageous to collect a lot of light for temporal and spectral monitoring such as plasma spectroscopy.