Bragg-grating waveguides (BGWs) are 3-D devices that provide low-loss light guiding and sharp Bragg resonances. They are usually written point-by-point with low-repetition-rate (1 kHz) Ti:sapphire femtosecond lasers inside transparent materials. However, this two-step process typically first forms the waveguide and later forms the grating lines, and does not cause sufficient refractive-index change to support light guiding that could lead to successful creation of 3-D optical circuits. But thanks to the capabilities of femtosecond fiber lasers, researchers at the University of Toronto (Toronto, Ont., Canada) have developed a single-step process that forms 3-D BGWs in silicon that produce sharp spectral resonances tunable over the 1300 to 1550 nm telecommunications band.
A fiber pulse-chirped-amplification laser system frequency-doubled to 522 nm with 300-fs-duration pulses and a pulse-repetition rate tunable from 100 kHz to 2 MHz was directed through an acousto-optic modulator driven with a 500 Hz square waveform. The output was focused approximately 75 µm below the surface of a bulk silica sample mounted on a translation stage to form an array of refractive-index voxels (3-D pixels). The resulting 3-D BGW structures have potential applications in telecommunications, sensing, and biophotonics. Contact Haibin Zhang at [email protected].