PURPOSE: To emit laser light preferably having a narrow spectral width while suppressing multimode oscillation and intensity noise by bonding a ring resonator to an optical fiber feedback system.

CONSTITUTION: A laser light from a semiconductor laser 1 is also emitted to a feedback type optical fiber 2, and the laser 1 is controlled to be fed back to increase the narrow spectrum of the emitted light. A ring resonator 7 formed of an optical fiber is bonded to the fiber 1 to compositely feedback it. The mode interval is increased from 333MHz to 1.33GHz to emit the laser light preferably having a narrow width while substantially suppressing multimode oscillation and intensity noise.