  This paper presents a simplification design for the fiber cavity ring-down
technique. The new configuration is no longer in ring cavity and is converted
to a linear cavity. The proposed linear cavity is created with a single fibre
coupler of 99:1 and two thin-film mirrors located at the end of the fibre arms.
This configuration can be interrogated in reflection or in transmission
(whether if two fibre couplers are used in conventional configuration), however
the reflection interrogation presents better resolution when compared
signal-to-noise ratio in the decay time measurement. The mirrors can be also
replaced by fibre Bragg gratings and fibre loop mirrors. One of the main
advantages of this simplification is its use for remote sensing. However, it is
necessary to include amplification in the cavity to eliminate fiber length
losses. This configuration allows using any intensity-based optical fibre
sensor in the literature with twice improvement in the sensitivity and can be
easily adapted to a commercial system. A proof-of-concept for strain sensing is
demonstrated. This simplification opens new paths in physical, chemical or
biological parameters using this simple fibre configuration.
