PURPOSE:To reduce the noise due to return light by reduction of the amount of return light by a method wherein a pig-tail containing a fiber type polarization filter and a birefringence optical fiber is arranged in such a manner that the output end of a semiconductor laser optically couples with one end of the pig- tail. CONSTITUTION:When a semiconductor laser beam is made incident to the pig- tail 14 so that the polarization direction 22 of an emitted beam of the semiconductor laser 11 may become coincident with the stress direction of the polarization filter 18 of the pig-tail 14, it passes through the first section 18 as it is because of no attenuation of TE waves. Next, this beam is made incident to the second section 19, where the length is 2n+1L/4; therefore, the beam comes to the state of circular polarization 24 at the emission end. A return light reflected on this emission end, on returning to the connection part of the first section 18, becomes TM waves perpendicular to the stress direction of this section 18. Since the TM waves are attenuated by a metal film 17 during passing through the secion 18, the light hardly returns to the semiconductor laser 11.
展开▼
