Femtosecond micromachining of waveguides in a Faraday material

The use of femtosecond lasers for micromachining purposes has grown significantly over the past decade. Femtosecond micromachining of photonic devices has been demonstrated using only a few nanojoules of energy from a laser oscillator. This oscillator-only technique has recently become increasingly valuable as a micromachining tool.

Faraday isolators have not yet been realized using this ultrafast micromachining technique. A Faraday isolator is an important device because it functions as an optical diode, allowing signals to propagate only in one direction. It is particularly useful in circulators, where the signal selected by a Bragg grating is dropped to an alternate path. When a Faraday material is exposed to a magnetic field, a non-reciprocal index change linear with respect to the applied magnetic field is induced on the material. A Faraday isolator can be constructed by adding polarizers at the entrance and exit of the Faraday material.

We used an extended-cavity laser oscillator delivering pulses with an energy 20 nJ and a pulse duration of 55 fs at a 25-MHz repetition rate to micromachine waveguides into a Faraday glass sample. The laser is focused into the sample with a 1.4-NA oil-immersion objective and the sample is translated at a speed of 10 mm/s. The sample has a Verdet constant of –4300 degrees/T-m at 632.8 nm. The waveguides are single-mode at 632.8 nm and 1550 nm. We are studying the feasibility of integrating these microstructures into small Faraday isolators.