Single-mode fiber-optic directional coupler

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Abstract
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-μm wavelength range. The directivity is 53 (51) dB at the 1.3 (1.53)-μm wavelength. The light polarization dependency is ≤0.1 dB. The return loss is ≥43 dB.