Application of π/2 phase shifted Bragg grating for the longitudinal mode selection of single-frequency fibre lasers

In this paper we propose a new type of fibre laser configuration which consists of closing an erbium-doped fibre laser cavity by two Bragg gratings (similarly to DBR laser). One of the grating has a Π/2 phase shift at the centre. In the case of passive fibre long Bragg grating with a Π/2 phase shift has narrow transmission peak in the centre of the reflection spectrum. In an active fibre, the spontaneous emission spectrum below threshold exhibits a maximum at the place of the peak in the centre of the reflection spectrum. That would give possibilities to obtain single-frequency oscillation with high output power provided by the long length of the laser cavity