Harsh environment fibers for Sensing

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 300 °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 (HT acrylate) was specifically developed to offer an extended operating temperature up to +150 °C with good radiation resistance.
• Polyimide offers an extended operating temperature range up to +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)
• Aluminum-coated fibers can handle long term temperatures of +400 °C. The thin metallic layer is also hermetic to hydrogen and water vapor, greatly reducing H2 induced absorption peaks and improving the mechanical reliability of the fiber.
• Carbon-coated fibers with either acrylate, HT acrylate or polyimide are currently under development. These advanced coatings combine both low loss and easy handling of the polymer coatings, together with a thin hermetic layer of carbon.

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, by the incoming radiations, of absorbing point defects inside the silica matrix, also called color centers.

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. PSC F-doped fibers are designed and optimized according to the customer’s specifications, especially with regards to the operating wavelength. 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).

Leveraging Exail’s expertise in Radiation-Hardened fibers, Rad-Hard fibers with polymer and metallic coating are available with both singlemode and multimode designs. They are available for operations for UV, visible or NIR wavelengths (900, 1000, 1310, 1550 nm):

• Exail singlemode 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.