Multi-shot near-infrared femtosecond laser tuning of silicon microring resonators

We studied multi-shot femtosecond laser surface modification as a permanent tuning technique for obtaining the desired resonance wavelengths for silicon microring resonators. In this multi-shot tuning approach, each microring resonator was irradiated with 10 or 100 laser pulses in the same location of the waveguide with selected laser fluence. The laser beam has 800 nm wavelength, 130 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 13.1μm, and pulse energies of 12 nJ to 186 nJ to generate the applied range of laser fluences. The silicon microring resonators used in the study have a 15μm diameter and are coupled to a waveguide of 500 nm in width with a gap of 260 nm. We found that the multi-shot tuning curves (resonance wavelength shifts as a function of laser fluences) do not monotonically increase with laser fluences: they first increase, then plateau, and finally decrease. We observed that the laser energies required for the multi-shot case to obtain the onset of crystalline modifications and ablation are several times below the single-shot case because of the incubation effect. With proper control we can achieve both positive and negative resonance wavelength shifts with acceptable induced roundtrip waveguide losses.