Femtosecond laser direct writing (FLDW) is developing rapidly but to date, there is no native optical isolator (needed to
mitigate reflections in any optical system) for the platform. As a step towards integrated glass isolators, we have
investigated FLDW in kHz and MHz pulse rate regimes for two magneto-optical glasses (TG20 and MR3-2) to
ultimately create one-way structures based on the Faraday effect.
Previously, we fabricated basic waveguides obtaining single-mode guidance at 632 nm (the Faraday effect is strongest
near the Tb<sup>3+</sup> resonance at 485 nm) in both regimes. kHz regime waveguides were isotropic but had high propagation
loss due to associated photodarkening (which could be post-annealed). The propagation loss of the MHz regime
waveguides was acceptable due to lower photodarkening, but the waveguides were too narrow to confine light properly
because of the very strong focus of the writing beam.
To try to combine the lower loss with larger waveguide width, we created overlapping structures using a series of
superposed waveguides arranged in rings in MHz regime. The confinement in these multi-ring structures was indeed
improved and the structure propagation loss was intermediate between that of one-path waveguides created in kHz and
MHz regimes. For most other glasses, MHz FLDW systems operate in a heat-accumulation regime, producing
waveguide diameters much larger than the writing laser spot size and superposed waveguides that merge into one by
melting. Here, the sub-unit waveguides maintained their individual identity indicating that the heat-accumulation effect
Since the discovery, that a tightly focused femtosecond laser beam can induce a highly localized and permanent refractive index change in a wide range of dielectrics, ultrafast laser inscription has found applications in many elds due to its unique 3D and rapid prototyping capabilities. These ultrafast laser inscribed waveguide devices are compact and lightweight as well as inherently robust since the waveguides are embedded within the bulk material. In this presentation we will review our current understanding of ultrafast laser - glass lattice interactions and its application to the fabrication of inherently stable, compact waveguide lasers and astronomical 3D integrated photonic circuits.