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We developed an optical input module consisting of built-in metal-semiconductor-metal photodiodes (MSM-PDs) and co-planar waveguide transmission lines fabricated on an InP substrate, which is flip-chip bonded with a single-flux-quantum (SFQ) circuit fabricated on a Si substrate. To improve photosensitivity of the MSM-PD for a wavelength above 1530 nm at 4 K, we tested the MSM-PDs made of lattice mismatched In1-xGaxAs films on InP substrates. By employing In1-xGaxAs with x=0.44, an apparent reduction of photosensitivity for a wavelength above 1530 nm was suppressed. We also tested newly designed DC/SFQ converters with higher input current sensitivities. The observed current sensitivity was improved to 41 μA, while that in the conventional DC/SFQ converter was 140 μA.