PURPOSE: To measure the difference of transmittance with high accuracy by feeding the light from a nonpolarizing light source to a optical wave guide to be tested, measuring the degree of polarization of the transmitted light through the optical wave guide, and determining the polarization-dependent loss based on the obtained result.
CONSTITUTION: The polarized constituent of the outgoing light from an optical fiber 4 is elliptically polarized, and the rotation angles of a 1/2-wavelength plate 16 and a 1/4-wavelength plate 17 are adjusted so that the polarized constituent fed into a polarization modulator 18 is linearly polarized in the 45° direction against the X-axis of the modulator 18. When the linearly polarized light is fed in the 45° direction against the X-axis which is the main axis of the modulator 18, the outgoing light from the modulator 18 is not linearly polarized, thus the outgoing light from the modulator 18 is linearly polarized by a 1/4-wavelength plate 19. When voltage is applied to the modulator 18 to change the polarization direction by 90°, the light power transmitting a polarizer 20 is made maximum. The nonpolarizing constituent is not changed after passing through the modulator 18. The output signal from a photo-detector 21 is detected by a lock-in amplifier 22, and the voltage of the polarized constituent is obtained.
YAMADA HIROAKI
HORIGUCHI MASAHARU
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