Reducing noise in optical telecommunications links
To eliminate one of the main sources of noise in optical telecommunications links, it is essential to protect the laser source from light being reflected back into the laser diode. With current commercial bulk optical isolators, packaging of the laser diode module necessitates precise, sub-micron alignment techniques that have a significant impact on its overall cost. Moreover, mass fabrication techniques are rendered impossible. A waveguide-based version of the optical isolators has therefore been a long-time pursued objective in the field of photonics. Research with the ISOLASER project focused on applying a transversely magnetised ferromagnetic film to introduce nonreciprocal modal absorption in a semiconductor optical amplifier. The interest in this class of materials came from their unique combination of low optical loss at telecommunications wavelengths and considerably strong magneto-optical Kerr effect, the source of non reciprocity. The result was a device, which being transparent in the forward while providing loss in the opposite direction, is isolating. Once this configuration was experimentally demonstrated, improved understanding of the contact structure's impact on the optical and magneto-optical parameters resulted in enormous advances in the device's performance. Optimisation of the device's design was based on the selection of a low-absorbing ferromagnetic metal with strong magneto-optical features. To compensate for the remaining optical loss in the ferromagnetic metal contact, a multiple quantum well active region from amplifying material with the highest possible transverse magnetic-polarised gain was designed. One-dimensional simulations with the Cavity modelling framework, extended with a perturbation algorithm for magneto-optical waveguide calculations, were used for the design of the geometric device parameters. The experimental isolation ration reached approximately 99dB/cm, showing that practical implementation of this amplifying waveguide optical isolator is within reach. Importantly, as this optical isolator has the same structure as the semiconductor laser it is to be integrated with, monolithic integration would be straightforward.