Direct writing of optical waveguides in bulk glass using a femtosecond laser oscillator

In recent years, femtosecond lasers have proven to be extremely useful for micromachining the surface and bulk of transparent materials. When a femtosecond laser pulse is focused into a transparent material, the intensity in the focal volume can become high enough to cause absorption through nonlinear processes, leading to optical breakdown and permanent structural change to the material. Because the absorption is nonlinear, this structural change can be localized in the bulk of the sample, allowing a three-dimensional structure to be micromachined. In this paper, we show that by focusing a femtosecond laser pulse very tightly, one can micromachine bulk glass using an unamplified laser. A 1.4 numerical aperture microscope objective is used to focus 5 nJ, sub-100-fs pulses from a 25-MHz Ti:Sapphire laser oscillator to a 0.5-µm diameter spot inside bulk glass. The intensity at the focus reaches 3 X 1017 W/m2, causing optical breakdown and structural change in the material. Micromachining with a high repetition-rate, unamplified laser enables a new kind of multiple-shot, thermal effect to be utilized for waveguide fabrication. Successive laser pulses deposit a small amount of heat (< 1 nJ) at a rate faster than energy can diffuse out of the focal volume. Over thousands of pulses, a micrometer-sized volume of the glass melts. By scanning the sample perpendicular to the incident direction of the laser beam, single-mode optical waveguides with a near-gaussian output profile are formed inside the glass. From the divergence of the waveguide output we determined the refractive index change in the core to be about 5 X 10-4. The ability to directly write waveguides in three dimensions into bulk optical materials enables the fabrication of a wide variety of passive and active optical devices for the telecommunications industry. Here we have shown that this task can be achieved using only a femtosecond laser oscillator.