Recently, we proposed using embedded nanogratings to change the polarization state in fused silica femtosecond laser direct written optical circuits. Full control over the elements’ birefringence properties can be attained by changing the inscription parameters and using a suitable writing geometry. Therefore, these structures can be used to arbitrarily transform the polarization state on an optical chip. Due to the intrinsic birefringence of these structures, the required length of the functionalized section is only a few hundred micrometers. We demonstrated four single qubit quantum gates based on these structures (Hadamard, Pauli-x, Pauli-z and Pi-8th). However, the overall losses of these structures are still rather high. We present our endeavour to reduce the losses by using adapted beam shaping. The improved performance and their potential for optical quantum computing will be presented.
The laser inscription of waveguides into the volume of crystalline silicon is presented. By using sub-ps laser pulses at a wavelength of 1552 nm highly localized light guiding structures with an average diameter ranging from 1 – 3 μm are achieved. The generated waveguides are characterized in terms of mode field distribution, damping losses and permanent refractive index modification. First investigations indicate an induced increase of the refractive index in the order of 10-3 to 10-2. Depending on the applied laser pulse energy single-mode to multimode like propagation behavior can be observed. At optimized processing parameters, the damping losses can be estimated below 3 dB/mm.