High refractive index contrast in fused silica waveguides by tightly focused, high-repetition rate femtosecond laser

A new domain of optical waveguide writing with record high refractive index contrast (0.022) is reported in fused silica by strong focusing of a 522 nm wavelength, 500 kHz repetition rate femtosecond laser with oil-immersion optics. The strongly confining waveguide supports a mode of only 7 μm mode field diameter at 1550 nm wavelength, opening the door for higher density integration in photonic circuits formed by femtosecond lasers. It is found that green and fundamental wavelengths have similar absorption in femtosecond laser waveguide writing in fused silica and that the advantage of the second harmonic is simply from an increased fluence through a smaller focal volume.

Research Highlights► Femtosecond laser waveguide writing in fused silica with tightly focused green pulses explored. ► Record high refractive index contrast of 0.022 leading to record low 7 micron mode size at 1550 nm. ► Highly confined waveguides can be used in optical circuits with bend radii of 15 mm. ► Benefit of green wavelength is higher possible fluence, not a more efficient nonlinear absorption.