Harsh environment fiber diagnostics

# #

Harsh environments usually refer to extreme temperature and/or radiation level, and encompass environments such as nuclear facilities, high energy physics laboratories, and space. Leveraging decades of continuous improvements made on standard telecom fibers, specialty optical fibers are now widely used in harsh environments for data transmission, sensing or in-vitro diagnosis.

Optical fibers are well suited for operation in extreme temperatures and radiative environments as they benefit from a light weight and low volume, low loss and high bandwidth, an immunity to electro-magnetism, and an ability to operate under a wide temperature range. Exail is a designer and manufacturer of passive or active specialty fibers, offering virtually unlimited design possibilities and optimization for UV, visible or NIR wavelengths through the careful selection of raw material, dopants and doping profile.

Coatings for harsh environments: from acrylate to polyimide and metal

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.


(left) Aluminum-coated fiber / (right) Copper-coated fiber (Source: Photonics Bretagne)
Radiation resistance Max Temperature Vacuum compatibility
Standard telecom acrylate Low +85 °C Medium
High temperature acrylate Medium +150 °C Medium
Polyimide High +300 °C Good
Aluminum High +400 °C Excellent
Copper High +600 °C Excellent

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.

Rad-hard fibers: leveraging a decade of investment in R&D

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.

Overview of radiation induced point defects in silica-based optical fibers, S. Girard and al (2019)

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.

The LabH6 Joint Laboratory was created between Exail and Hubert Curing Lab (CNRS/IOGS/St-Etienne Univ.) in 2019 to study optical fibers and optical fiber-based sensors in harsh environments.

Studying the effects of radiation on silica fibers led to continuous fiber improvement to reach better Radiation Induced Attenuation (RIA), at the forefront in the field. R&D efforts within the LabH6 joint laboratory are also opening the way to new developments in optical fibers for sensors in nuclear environment or for distributed radiation sensing, but also Fiber Bragg Grating for sensors in harsh environments.

LabH6 logo

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

Single to multimode fibers, according to each customer needs

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.
Singlemode fiber: <10 µm core diameter / Low-loss / Pure Silica Core (Rad-Hard), or Ge-doped (Rad-tolerant)
  • 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.
Step-index multimode (MMSI) fiber: Pure Silica Core, F-doped cladding / Low-OH, Mid-OH or High-OH core / Large core diameter (up to Ø400 µm) / Wide operating wavelength range
  • 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.
Cleave of a Rad-Hard Graded-index multimode (MMGI) fiber: Low temporal dispersion at the design wavelength / F-doped core (Rad-Hard), or Ge-doped (Rad-tolerant) / Custom geometry (core up to Ø400 µm)


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.

How to choose the best optical fiber for harsh environments, according to temperature and radiation levels?

The table below summarizes the recommended singlemode fiber depending on the temperature and radiation levels. The references listed are compatible with standard SMF-28 fiber and can be spliced with low loss.

Ø clad = 125 µm,
NA = 0.14
Temperature ≤ 150 °C 150 °C ≤ Temperature ≤ 300 °C
Low-moderate radiation levels Singlemode fiber with acrylate HT coating, 125 µm core diameter


Singlemode fiber with polyimide coating, 125 µm core diameter


Rad Hard Fibers

High radiation levels Singlemode Rad-hard fiber with acrylate HT coating, 125 µm core diameter


Rad Hard Fibers

Singlemode Rad-hard fiber with polyimide coating, 125 µm core diameter


Rad Hard FibersRad Hard Fibers

Polyimide coating can be applied indifferently on single-mode, multimode or custom fibers.

With more than 20 years of experience in manufacturing specialty optical fibers, Exail has all the expertise and tools to address custom developments and high-volume requirements.


  • Phosphosilicate Multimode Optical Fiber for Sensing and Diagnostics at Inertial Confinement Fusion Facilities

    IEEE, O. Duhamel, A. Morana, Member, IEEE, D. Lambert, Senior Member IEEE, V. De Michele, C. Campanella, G. Mélin, T. Robin, J. Vidalot, A. Meyer, A. Boukenter, Y. Ouerdane, E. Marin, V. Yu. Glebov and G. Pien

    IEEE Sensors Journal ( Volume: 22, Issue: 23, 01 December 2022)

  • Overview of radiation induced point defects in silica-based optical fibers

    Sylvain Girard, Antonino Alessia, Nicolas Richard, Layla Martin-Samos, Vincenzo De Michele, Luigi Giacomazzi, Simonpietro Agnello, Diego Di Francesca, Adriana Morana, Blaž Winkler, Imène Reghioua, Philippe Paillet, Marco Cannas, Thierry Robin, Aziz Boukenter, Youcef Ouerdane

    S. Girard et al., Reviews in Physics (4) 100032, 2019.

  • Combined effect of radiation and temperature: towards optical fibers suited to distributed sensing in extreme radiation environments

    G. Mélin, A. Barnini, A. Morana, S. Girard, P. Guitton & R. Montron

    30th Conference on Radiation and its Effects on Components and Systems (RADECS 2019) – September 2019

    Combined effect of radiation and temperature on the response of polyimide coated radiation hardened single-mode fibers is investigated in the context of distributed monitoring of large nuclear infrastructures. Radiation induced attenuation (RIA) is evaluated for doses ranging from 1 to 10 MGy(SiO2) and temperatures up to ~250 °C.

    Learn more
  • Radiation resistant single-mode fiber with different coatings for sensing in high dose environments

    G. Mélin, P. Guitton, R. Montron, T. Gotter, T. Robin, B. Overton, A. Morana, S. Rizzolo & S. Girard

    IEEE Transactions on Nuclear Science – 10 December 2018

    A radiation single-mode optical fiber has been specifically developed for distributed sensing in harsh environments associated with MGy(SiO2) dose radiation. Different types of coating have been used: acrylate, polyimide, aluminum that allow extending the range of accessible temperatures up to 400°C…

    Learn more
  • Radiation hardened temperature measurement chain based on femtosecond laser written fbgs in a specific optical fiber

    G. Mélin, L. Lablonde, T. Robin, J. Kuhnhenn, U. Weinand, A. Morana, S. Girard, E. Marin, J. Périsse, J.S. Genot, J. Grelin, L. Hutter, J.R. Macé, A. Boukenter & Y. Ouerdane

    5th Workshop on Specialty Optical Fiber and Their Applications – October 2017

    A radiation resistant temperature measurement chain based on femtosecond laser written FBGs is described. First results confirm a suitable design to withstand foreseen harsh nuclear envirnoment both in term of temperature and cumulated dose.

    Learn more