To eliminate adverse effects on a signal beam by a laser beam oscillated in a resonator by generating stable inverted distributing condition in an optical amplifier of an optical resonator with a laser beam and then extracting the amplified signal beam with an optical isolator in such a direction that the laser beam does not leak within the optical resonator.
A first and a second wavelength multiplex optical isolators 8, 12 are provided. An optical amplifying fiber 1 is an erbium-doped fiber(EDF). When a signal beam is input to an optical fiber 10b, this signal beam is transmitted in the direction of arrow mark a within an optical resonator 2. Adequate amplification is executed, not depending on an input intensity in the EDF 1 where the inverted distributing condition is fixed and an output is provided from an optical fiber 10d via the second wavelength multiplex optical isolator 12. In the second wavelength multiplex optical isolator 12 from which the amplified signal beam can be extracted, the laser beam in the optical resonator in different running direction from the signal beam is hardly isolated, and therefore a laser beam which causes noise, etc., does not enter the signal beam.
FUJISAKI AKIRA
OGOSHI HARUKI
