| 摘要 |
PURPOSE:To reduce an insertion loss and to enhance an isolation characteristic by a method wherein a semiconductor photodetector having a wave-guide path of multiple quantum well structure is used as a photodetector for a light isolator. CONSTITUTION:A beam of light from a semiconductor laser 1 is converged by a lens 11 and is injected into a semiconductor photodetector 14 after its polarization plane has been turned by 45 deg. with a Faraday rotor 13. The photodetector 14 is equipped with a wave-guide path, of multiple quantum well structure, whose incident and outgoing faces are coated to be nonreflective, and amplifies a TE polarization component selectively. After the laser beam has been amplified with the photodetector 14, it passes through a lens 12 and enters an optical fiber 2. By this setup, an insertion loss of a light isolator composed of the Faraday rotor 13 and the photodetector 14 can be reduced. Only the TE component of the beam of light returned from the optical fiber 2 is amplified with the photodetector 14; furthermore, the beam of returned light is turned by 45 deg. by the rotor 13. Because the beam of returned light is returned to the laser 1 in the form of the TE component which has been turned by 90 deg. with reference to the TE component of the laser 1, it does not disturb a laser oscillation.
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