H-parameter, or holding parameter, is important because it is another widely-used measure of the polarization-maintaining performance of an optical fiber. It is simply a measure of how well polarization is preserved in one axis; expressed linearly (per unit length of fiber), and is related to the extinction-ratio. One axis is illuminated, using a polarized fiber source, and the power transmitted through a polarized detector is considered, looking for the maximum and minimum.
The minimum power is divided by the maximum power and the fiber length under test, and a lower value for H-parameter indicates better polarization extinction.
For example an extinction-ratio of -30dB achieved over 1000m of fiber is equal to an H-parameter of 1/1000,000 per metre - which is often written as just 10-6.
H-parameter is dependant upon the environmental conditions under which the fiber is used. Coiling the fiber, using with potting compounds, otherwise introducing micro bending or changing temperature and other environmental parameters can all have an impact on the h-parameter achieved. Standard coating packages can adversely affect h-parameter quite significantly below freezing. For this reason Fibercore have developed their applications specific gyroscope fiber coating package to preserve H-parameter over a wider operating temperature range.