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