| 摘要 |
PURPOSE:To prevent isolation from being deteriorated by specifying the length of a constant polarizing fiber while adjusting the fiber so that the polarization direction of incident light is at a specific angle to the orthogonal characteristic polarization directions of the constant polarizing fiber. CONSTITUTION:Laser output light is made into parallel light by a lens 2 and the light is transmitted through an isolator 3 and then made incident on the constant polarizing fiber 10. Further, the constant polarizing fiber 10 is provided so that the polarization direction 9 of light incident on the fiber 10 is at 45 deg. to the orthogonal characteristic polarization directions 7 and 8 of the constant polarizing fiber 10; and its length is set to (n+1/4)lambda.1/B or (n+3/4)lambda.1/B (n: integer, lambda: wavelength of light, B: beat length). The constant polarizing fiber 10 serves as a lambda/4 plate and reflected light from the projection terminal 10b of the constant polarizing fiber 10 crosses the incident polarized light when returning to an incidence point 10a as if it were transmitted through the lambda/2, so that the polarization direction of the backward feedback light is controllable. |
