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We investigated the improvement in the wavelength conversion performance of a semiconductor optical amplifier-based delayed interference signal-wavelength converter with the phase offset of a Mach-Zehnder delay interferometer and band pass filter (BPF) detuning. In the experiments, the power penalty of the wavelength converted output was improved using negative phase offset due to the suppression of double pulse and reduction of zero level power, and the transmission characteristics were improved using negative detuning of a BPF because of the elimination of the degraded components in the output signal. Moreover, the power penalty was optimized using a -0.12 rad phase offset and -0.15 nm BPF detuning at a bit rate of 10 Gb/s. We also investigated its effect via simulations by solving rate equations. The waveform and Q-factor of the wavelength converted signal were in good qualitative agreement with the experimental results. We studied the principle of the impact of both phase offset and BPF detuning on output signal improvement by a comparison of the optical spectrum of the wavelength converted output with that of the return-to-zero signal. Waveform improvement can be achieved by positioning the intensity of the two sidebands symmetrically on both sides of the carrier.