Reference wave source based on silicon nitride waveguide in point diffraction interferometer.

Reference wave source (RWS) is the key component of the point diffraction interferometer, which determines the quality of the reference wave. The silicon nitride waveguide RWS is proposed to efficiently overcome the drawbacks of the existing RWSs, aimed at providing a spherical reference wave with high numerical aperture (NA) and high accuracy. The waveguide RWS consists of the straight waveguide, the bend waveguide, and the Y-branch edge coupler. The straight waveguide determines the accuracy and the NA of the reference wave, whereas the latter two determine the light transmittance of the RWS. Simulation results show that the peak-to-valley (PV) and the rms of the deviation from an ideal spherical wave are 2.86×10 (=532) and 4.83×10, respectively, and the maximum light transmittance could reach 24%. Experiment results show that the NA of the reference wave reaches up to 0.58, its spot has a good circular symmetry, and its intensity has Gaussian distribution. Although the light transmittance is only 0.2%, it is expected to improve with the development of experimental conditions and waveguide fabrication technology.