| 摘要 |
PROBLEM TO BE SOLVED: To provide a tunable filter with which a wide variable wavelength range can be obtained by using a diffraction grating as a wavelength selecting element and a high diffraction efficiency can be obtained however the polarization state of an incident light varies. SOLUTION: The tunable filter includes a means 6 which polarizes and separates the incident light, a means 7 which rotates the plane of polarization of one of separated lights by 90 deg., the diffraction grating 5 on which the light having its plane of polarization rotated by 90 deg. and the other separated light are made incident, and a means which adjusts the angle of the incident light to the diffraction grating.
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