Differential phase shift keying are today considered key solutions for the WDM long-haul transmission infrastructure, and a Mach-Zehnder interferometer (MZI) for its demodulation needs to have very low polarization dependent frequency shift (PDf). MZIs based on silica-based planar lightwave circuits can offer a compact reliable demodulator, but it suffers from a large PDf. In particular, they exhibit a large wavelength dependence on the PDf. It results in a narrow wavelength range where the PDf is sufficiently low, and this limits the operating range of the demodulator. In this paper, we propose a novel method for reducing the PDf over a wide wavelength range. We reveal that the large wavelength dependence of the PDf is due to both the polarization crosstalk of the half-wave plate and the birefringence of the waveguide. Using a mathematical approach, we derived a way to suppress the PDf by controlling the birefringence and the direction of the optic axis of the half-wave plate in the MZI. We confirmed the effectiveness of our proposed method experimentally and achieved a low PDF of $< 1\%$ for an FSR of over 140 nm in the wavelength.
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