2D refractive index measurement for direct laser writing waveguides with complex structures

Direct laser writing (DLW) waveguides are promising for next-generation integrated photonic devices. Accurately measuring the two-dimensional (2D) refractive index distribution (RID) of DLW waveguides is crucial for their implementation. However, the complex structures of DLW waveguides present real challenges in measuring their 2D RIDs. This paper proposes a method for measuring the 2D RID of DLW waveguides with complex structures by combining the modified near field (MNF) method and the quantitative phase imaging technique. The qualitative RID of waveguides is calculated according to the near field intensity profile using the MNF method, from which the corresponding qualitative phase distribution can be derived. Based on digital holography (DH), a Mach-Zehnder interferometer is used to record the digital hologram of waveguides, and the quantitative phase distribution is extracted by the hybrid reconstruction algorithm. Finally, the linear coefficient is determined based on the qualitative and quantitative phases of waveguides. Consequently, the 2D RID of waveguides is quantitatively calculated. The 2D RIDs of two types of DLW waveguides with different positive and negative RID changes are measured. Meanwhile, the 2D RIDs of their corresponding waveguide arrays are obtained efficiently. The proposed method is significant for the precise characterization and wide application of DLW waveguides.