We reviewed our recent developments on the post-fabrication trimming techniques and programmable photonic circuits based on germanium ion implanted silicon waveguides. Annealing of ion implanted silicon can efficiently change the refractive index. This technology has been employed to fine-tune the optical phase, and therefore the operating point of photonic devices, enabling permanent correction of optical phase error induced by fabrication variations. High accuracy phase trimming was achieved with laser annealing and a real-time feedback control system. Erasable waveguides and directional couplers were also demonstrated, which can be used to implement programmable photonic circuits with low power consumption.
We review our recent developments of the trimming techniques for correcting the operating point of ring resonator and Mach-Zehnder Interferometers (MZIs). This technology has been employed to fine-tune the effective index of waveguides, and therefore the operating point of photonic devices, enabling permanent correction of optical phase error induced by fabrication variations. Large resonance wavelength shift of ring resonators was demonstrated, and the shift can be tuned via changing the laser power used for annealing. A higher accuracy trimming technique with a scanning laser was also demonstrated to fine-tune the operating point of integrated MZIs. The effective index change of the optical mode is up to 0.19 in our measurements, which is approximately an order of magnitude improvement compared to previous work, whilst retaining similar excess optical loss.
Ion implantation into silicon causes radiation damage. If a sufficient dose is implanted, complete
amorphisation can result in any implanted part of an optical device. Amorphous silicon has a refractive index
that is significantly different higher than that of crystalline silicon (~10<sup>-1</sup>), and can therefore form the basis of
a refractive index change in optical devices. This refractive index change can be partially or completely
removed by annealing. In recent years we have presented results on the development of erasable gratings in
silicon to facilitate wafer scale testing of silicon photonics circuits. These gratings are formed by amorphising
selected areas of silicon by utilising ion implantation of Germanium. However, we have now used similar
technology for trimming of integrated photonic components. In this paper we discuss design, modelling and
fabrication of ring resonators and their subsequent trimming using ion implantation of Germanium into silicon
followed by annealing.
A crucial component of any large scale manufacturing line is the development of autonomous testing at the wafer scale. This work offers a solution through the fabrication of grating couplers in the silicon-on-insulator platform via ion implantation. The grating is subsequently erased after testing using laser annealing without affecting the optical performance of the photonic circuit. Experimental results show the possibility for the realisation of low loss, compact solutions which may revolutionise photonic wafer-scale testing. The process is CMOS compatible and can be implemented in other platforms to realise more complex systems such as multilayer photonics or programmable optical circuits.
In recent years, we have presented results on the development of erasable gratings in silicon to facilitate wafer scale testing of photonics circuits via ion implantation of germanium. Similar technology can be employed to develop a range of optical devices that are reported in this paper. Ion implantation into silicon causes radiation damage resulting in a refractive index increase, and can therefore form the basis of multiple optical devices. We demonstrate the principle of a series of devices for wafers scale testing and have also implemented the ion implantation based refractive index change in integrated photonics devices for device trimming.
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.
We present three main material platforms: SOI, suspended Si and Ge on Si. We report low loss SOI waveguides (rib, strip, slot) with losses of ~1dB/cm. We also show efficient modulators and detectors realized in SOI, as well as filters and multiplexers. To extend transparency of SOI waveguides, bottom oxide cladding can be removed. We have fabricated low loss passive devices in a suspended platform that employ subwavelength gratings. Ge on Si material can have larger transparency range than suspended Si. We have designed passive devices in this platform, demonstrated all optical modulation and carried out two photon absorption measurements. We have also investigated theoretically free carrier optical modulation in Ge.
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.
In this paper we discuss silicon-based photonic integrated circuit technology for applications beyond the
telecommunication wavelength range. Silicon-on-insulator and germanium-on-silicon passive waveguide circuits are
described, as well as the integration of III-V semiconductors, IV-VI colloidal nanoparticle films and GeSn alloys on
these circuits for increasing the functionality. The strong nonlinearity of silicon combined with the low nonlinear
absorption in the mid-infrared is exploited to generate picosecond pulse based supercontinuum sources and optical
parametric oscillators that can be used as spectroscopic sensor sources.
We have investigated several material platforms for the mid-infrared including silicon on insulator (SOI), polycrystalline
silicon, and suspended silicon structures. We review photonic devices based on these platforms including splitters,
ring/racetrack resonators, Mach-Zehnder interferometers, and spectrometers.
In this paper we present silicon photonics devices designed for the 3-4μm wavelength region including waveguides,
MMIs, ring resonators and Mach-Zehnder interferometers. The devices are based on silicon on insulator (SOI) platform.
We show that 400-500 nm high silicon waveguides can have propagation losses as low as ~ 4 dB/cm at 3.8μm. We also
demonstrate MMIs with insertion loss of 0.25 dB, high extinction ratio asymmetric Mach-Zehnder interferometers, and
SOI ring resonators. This combined with our previous results reported at 3.4μm confirm that SOI is a viable platform for
the 3-4 μm region and that low loss mid-infrared passive devices can be realized on it.
We review our recent work on waveguide grating couplers, including an apodized grating coupler with engineered
coupling strength to achieve Gaussian-like output profile, which greatly improves the fiber-chip coupling efficiency. We
will also discuss a new class of grating couplers involving the use of sub-wavelength nanostructures to engineer the
optical properties. Effective medium theory can be used in the design of sub-wavelength structures, which, when
properly engineered, can offer broadband coupling and polarization independence. Other applications of waveguide
gratings, for example bi-wavelength two dimensional gratings coupler for (de-)multiplexing two different wavelengths,
fiber-waveguide hybrid lasers and mid-infrared grating couplers on silicon-on-sapphire wafer will also be briefly
We review our recent work on chirped waveguide gratings for efficient coupling between standard single mode optical
fibers and silicon photonic wire waveguides. The use of a linear chirp in grating period reduces the second order Bragg
reflection from the waveguide gratings and increases the coupling efficiency for perfectly vertical optical fibers.
Measurement results obtained from devices fabricated using deep UV lithography yielded coupling efficiencies of over
34%. Techniques to further improve the coupling efficiency will be discussed. The use of chirped waveguide gratings
for low cost photonic packaging and the application of waveguide gratings for splitting/combining light will also be