Integrated optics is analogous to the integrated electronics that has had such a profound impact on society over the last 50 years through the phenomenal growth in the use and capabilities of silicon chip based electronics. It replaces wires with waveguides, and can integrate light sources, active and passive light processors, detectors, etc to enable full optical system on a chip capability. It exploits the same potential economies of scale as integrated silicon electronics to reduce costs and increase functionality, whilst in the case of optics providing other side benefits such as mechanical stability and robustness that make possible types of optical systems that cannot be implemented any other way. There is also another important distinction, in that optical integrated circuits operate directly on the photons themselves to process them (and so also on encoded data if present) whereas electronic circuits operate solely on the data. This presents a range of capabilities not present in the electronic domain. Whilst there are direct analogues in the optical domain for devices such as transistors for switching, frequency mixers, etc, with the exception of a few restricted application areas, the idea with integrated optics is not to build an optical equivalent of an electronic circuit (so no optical computers!). Such capabilities make applications in telecommunications, defence, sensing, metrology, instrumentation, astronomy, quantum processing, etc viable.
Figure 1 - conceptual diagram of multi-material hybrid integration platform
One major challenge in accomplishing the full range of capabilities on a chip is that by definition, no single optical material can implement every desired optical functionality well due to conflicting requirements. Different materials each have their individual strengths, and so to maximise system performance, multiple materials need to be integrated onto a single chip in a so called heterogeneous or hybrid integration process. To enable a full range of optical functions, materials with diverse physical properties must be integrated and processed together and interconnected with low loss. Examples of desirable materials include: germanosilicate glass (fibre compatible, low loss, passive, and Bragg grating capable), chalcogenide glasses (very high optical nonlinearity), InGaAsP (diode pump lasers, semiconductor amplifiers, photodiodes), tellurite glass (rare earth doped gain for sources/amplifiers), 2-D materials (e.g. graphene for saturable absorbers, photodiodes, modulators), etc. To date only limited levels of hybrid integration have been accomplished, typically only two materials.
The specialised materials themselves and the co-integration process is a major area of research strength at the Laser Physics Centre in the Research School of Physics and Engineering. We are the acknowledged world leaders in the fields of chip based chalcogenide and tellurite waveguide devices, and the first to demonstrate a high performance hybrid integration platform for glass waveguides. This is now being extended (see figure 1) based upon a germanosilicate base layer to integrate silica, tellurite, and chalcogenide guiding layers with low loss interconnection and that is compatible with InGaAsP and 2-D materials.
Currently work is progressing to extend this platform to build for the first time chip based devices for applications such as Brillouin scattering based RF signal processors for 5G and defence applications, ultrafast fs resolution/THz bandwidth instrumentation, and mid-infrared (remote) chemical analysis. Chip based implementations of these devices and others will revolutionise some aspects of science of technology by providing low lost high performance capabilities not available now that can be field deployed anywhere from the lab to the doctor’s surgery, to your car, to find alien planets, or to the battlefield.