Passive achromatic phase shifter fabricated using ultrafast laser inscription

Phase shifters are a key component in photonic integrated circuits. Passive phase shifters utilize a path length difference or an effective refractive index difference to generate a phase delay. However, these methods are wavelength-dependent, making them unsuitable for broadband applications. This manuscript demonstrates, for the first time, the fabrication of achromatic phase shifters using ultrafast laser inscription. The designed achromatic phase shifters employ a multi-segmented technique that leverages differential waveguide dispersion to correct for the wavelength dependency. To characterize the waveguide dispersion, point-by-point Bragg gratings are inscribed within waveguides of different widths. After determining the multi-segment design, adiabatic tapers are added to minimize losses and to ensure only the fundamental mode propagates. The tapers are designed using a new multi-sectional structure, enabling the sections to be re-arranged according to the waveguide width/diameter creating smooth structures, and allowing unnecessary sections to be removed, minimizing the overall device length. The 180° phase shifter is 3683.5 μm long with a measured phase shift of 171.5 ± 4° from 1440 to 1640 nm. The phase shift is polarization-independent, as the fabricated waveguides exhibit minimal birefringence. The device is ≈ 4 times shorter than a comparable low-index contrast lithographic phase shifter and 401.7 μm longer than one fabricated using a high-index contrast lithographic platform. The compact length is attributed to the use of multi-mode waveguides that increase the dynamic range of the differential dispersion.