This paper discusses some of the remaining challenges for silicon photonics, and how we at Southampton University have approached some of them. Despite phenomenal advances in the field of Silicon Photonics, there are a number of areas that still require development. For short to medium reach applications, there is a need to improve the power consumption of photonic circuits such that inter-chip, and perhaps intra-chip applications are viable. This means that yet smaller devices are required as well as thermally stable devices, and multiple wavelength channels. In turn this demands smaller, more efficient modulators, athermal circuits, and improved wavelength division multiplexers. The debate continues as to whether on-chip lasers are necessary for all applications, but an efficient low cost laser would benefit many applications. Multi-layer photonics offers the possibility of increasing the complexity and effectiveness of a given area of chip real estate, but it is a demanding challenge. Low cost packaging (in particular, passive alignment of fibre to waveguide), and effective wafer scale testing strategies, are also essential for mass market applications. Whilst solutions to these challenges would enhance most applications, a derivative technology is emerging, that of Mid Infra-Red (MIR) silicon photonics. This field will build on existing developments, but will require key enhancements to facilitate functionality at longer wavelengths. In common with mainstream silicon photonics, significant developments have been made, but there is still much left to do. Here we summarise some of our recent work towards wafer scale testing, passive alignment, multiplexing, and MIR silicon photonics technology.
Test points are essential in allowing optical circuits on a wafer to be autonomously tested after selected manufacturing steps, hence allowing poor performance or device failures to be detected early and to be either repaired using direct write methods, or a cessation of further processing to reduce fabrication costs. Grating couplers are a commonly used method for efficiently coupling light from an optical fibre to a silicon waveguide. They are relatively easy to fabricate and they allow light to be coupled into/out from any location on the device without the need for polishing, making them good candidates for an optical test point. A fixed test point can be added for this purpose, although traditionally these grating devices are fabricated by etching the silicon waveguide, and hence this permanently adds loss and leads to a poor performing device when placed into use after testing. We demonstrate a similar device utilising a refractive index change induced by lattice disorder. Raman data collected suggests this lattice damage is reversible, allowing a laser to subsequently erase the grating coupler.
In this paper we will discuss recent results in our work on Silicon Photonics. This will include active and passive devices for a range of applications. Specifically we will include work on modulators and drivers, deposited waveguides, multiplexers, device integration and Mid IR silicon photonics. These devices and technologies are important both for established applications such as integrated transceivers for short reach interconnect, as well as emerging applications such as disposable sensors and mass market photonics.