Control of the refractive index change in fused silica glasses induced by a loosely focused femtosecond laser

We demonstrate waveguide fabrication using of a loosely focused femtosecond laser that induces both nonlinear ionization and nonlinear propagation in silica glasses without any scanning process. The numerical aperture is chosen to be 0.007 to avoid spatial splitting of the laser pulses during the nonlinear propagation of the femtosecond laser pulses inside the fused silica glass. It also enables a uniform cylindrical refractive index change, which acts as an optical waveguide, to be induced. We found that the induction of irregular refractive index changes is related to the free electron density of the focused area and is controlled by decreasing the NA. The length, position, and core diameter of the fabricated waveguide can be controlled by the pulse-width, energy, and irradiation time of the incident laser. By using this technique, we varied the length of the fabricated waveguides between 20 μm and 6 mm, while keeping the core diameter at around 5 μm.