We have demonstrated a low-voltage Mach–Zehnder (MZ) modulator with a multiple InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) for 1.55 µm wavelength. The FACQW is expected to exhibit a large electrorefractive index change owing to its unique behavior of the quantum-confined Stark effect. The MZ modulator with ridge waveguides was fabricated by molecular beam epitaxy and wet etching. The product of a half-wave voltage and a phase shifter length (V
π
L) was as low as 1.2 V·mm in DC modulation. In addition, we discussed the degradation of the electrorefractive index change due to the nonuniformity of an electric field in the core layer and proposed a novel combined multiple FACQW structure to prevent the degradation.
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