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A low-loss compact single-mode fiber optical directional coupler (SMF-ODC) is developed for use in SMF reflectometers or gyroscopes. The SMF-ODC is made of two sapphire ball lenses, a half-mirror, and two ferrules. Two fibers (endfaces are concave) are aligned parallel to each other in the ferrule. The optical and mechanical axes are tilted. Cross-talk light from fibers or lenses is avoided. The SMF-ODC excess loss is 1.5-2.5 dB at the 1.2-1.5-microm wavelength range. The directivity is 53 (51) dB at the 1.3 (1.53)-microm wavelength. The light polarization dependency is or =43 dB.
Masuda et al. (Fri,) studied this question.
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