Ultrafast Laser Inscription of Achromatic Phase Shifters

We report the first demonstration of an achromatic phase-shifter fabricated using Ultrafast Laser Inscription. Phase shifters are essential in integrated photonic circuits for coherent applications such as optical communication and astronomical interferometry [1]. Phase shifters typically use a path length or an effective index difference to generate a differential phase shift. As both methods are chromatic, devices are only effective over a narrow bandwidth. Current methods to create a broadband achromatic phase shift include subwavelength structures [2], and differential waveguide dispersion [3], [4]. The latter technique uses three segments of different width combinations to create an achromatic phase shift. Adiabatic tapers are then added between the different widths to reduce losses. These tapers are added to each waveguide to maintain the differential phase shift. Devices are fabricated using the three-dimensional ultrafast laser inscription technique that uses multiphoton absorption at the focal point of an ultrafast laser to induce a refractive index change. The focal point is then translated at 16.7 mm/s and the laser pulse energy is varied to inscribe waveguides of different widths.