Wavelength-flattened 2 × 2 splitters made of identical single-mode fibers

Abstract
The fabrication of a new design of fused single-mode-fiber wavelength-flattened 2 × 2 couplers is reported. The device consists of a slightly unbalanced all-fiber Mach–Zehnder interferometer made with identical single-mode fibers; it is compact (≈2 cm) and shows good coupling-ratio uniformity (50 ± 2.5% over 300 nm) with low excess loss (<0.1 dB).