Temperature Resistance
Pure silica, that is the base material commercial for single-mode optical fibres, has a maximum usability temperature of 1100°C, that is far beyond the needs of most D-FOS industrial applications. In standard fibres however, silica is always coated by some polymeric materials in order to protect it from surface scratches that are the main source of fibre mechanical failure, and such polymer coating is what restricts in practice the usable temperature range.
The typical temperature limits for polymers commonly used for fibre coating are:
For further information you may refer to:
Optical fiber cables for Brillouin distributed sensing applications
Moisture Resistance
Moisture has been demonstrated to have an aggressive behaviour for silica optical fibres due to the reaction of water H+ and OH- ions with the Si-O-Si bonds on the glass surface and at the tip of surface cracks, that creates new surface flaws and increases the size of existing cracks even at zero-loading.
Moisture-induced damage builds-up with ageing on very long term, hence it is difficult to recognize especially for unstrained cables and it is sometimes erroneously considered a false myth.
According to the cited literature, moisture ageing can bring a significant (~30%) reduction of the fibre failure strength yet at moderate (~3 months) ageing time, so moisture protection appears to be a wise precaution at least for sensing cables that are designed for long-term durability.
The type of moisture protection that is required for a D-FOS application is stronger for strain sensing applications and also depends on the deployment environment, being maximum when the sensing cable is known to be installed in a wet or submersed environment.
For further information you may refer to:
Optical fiber cables for Brillouin distributed sensing applications
Resistance to Chemicals and Radiation
Chemicals and water contaminants have to be taken into account for the durability of sensing cables: the fibre glass in particular can be degraded by chemicals such as acetylene, ammonia and dimethyl sulfoxide with a reduction of the failure strength with ageing that is faster than that for water. Other chemicals may react with the coating or strengthening members of structured sensing cables reducing their durability in the actual deployment environment.
A small increase of Brillouin shift (0.5 MHz/MGy) and linewidth induced by exposition to gamma radiation is also reported, although having negligible impact on D-FOS usability.
