Silicon optical modulators

Optical technology is poised to revolutionise short reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such interconnect is the optical modulator. Modulators have been improved dramatically in recent years. Most notably the bandwidth has increased from the MHz to the multi GHz regime in little more than half a decade. However, the demands of optical interconnect are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimising metrics such as the energy per bit, and device footprint, whilst maximising bandwidth and modulation depth are non trivial demands. All of this must be achieved with acceptable thermal tolerance and optical spectral width, using CMOS compatible fabrication processes. Here we discuss the techniques that have, and will, be used to implement silicon optical modulators, as well as the outlook for these devices, and the candidate solutions of the future

Integrated nano-plasmonics: reflecting a waveguide mode with a single nano-antenna

Integrated plasmonic systems have the potential to revolutionise telecom devices [1]. However, the large losses and poor efficiency of plasmonic systems [2] have limited the use of metallic systems as optical circuits. Here we present a hybrid silicon-metallic system in which a single nano-antenna embedded in a single-mode silicon waveguide acts as a tuneable and narrow-band switch. Nanoantennas interact very strongly with light [3], such that when the nano-antenna is at resonance with the waveguide mode up to 80% of the light is blocked by it. This drop in transmission is a consequence of scattering and interference between the dipole and the waveguide mode, which is efficient over the nano-rod resonance bandwidth. The high performance and simplicity of this hybrid device holds great promise for future optoelectronic circuits

Group IV functionalization of low index waveguides

Low fabrication error sensitivity, integration density, channel scalability, low switching energy and low insertion loss are the major prerequisites for future on-chip WDM systems and interfacing with optical fibres. A number of device geometries have already been demonstrated that fulfil these criteria, at least in part, but combining all of the requirements is still a difficult challenge.Two contenders that could fulfil these criteria are the low loss nitride waveguiding platform and the high index group IV compounds for active photonic devices. Silicon Oxynitride (SiON) and Silicon Nitride (SiN) based waveguides are extremely powerful and central to today’s optical communications networks. The intermediate refractive index provides low footprint devices but eases the fabrication demands that can result in phase errors and repeatability problems in the all silicon approach. This enables multiplexers and demultiplexers with very low crosstalk and insertion loss and extremely low loss long range waveguides, making them very attractive for the optical backplanes and rack to rack links inside supercomputers and data centers. Group IV Photonics GeSi has a number of attractive optical characteristics for modulation, absorption and detection in a small volume area enabling low power and high density integration.Here, we propose and demonstrate a novel architecture consisting of the interfacing of a range of deposition method using low temperature PECVD and HWCVD nitride waveguides, Photonic crystal modulators [1] but also detectors [2] connected by a silicon nitride bus waveguide. The architecture features very high scalability due to the small size of the devices (~100 micrometre square) and the modulators operate with an AC energy consumption of less than 1fJ/bit

Coarse wavelength division (de)multiplexer using an interleaved angled multimode interferometer structure

We have demonstrated a coarse wavelength (de)multiplexing structure on the silicon-on-insulator platform. It comprises two 4-channel angled multimode interferometers interleaved with an imbalanced Mach-Zehnder interferometer (MZI) leading to an 8-channel multiplexing device. The device requires only single lithography and etching steps for fabrication and has a good tolerance to fabrication errors in terms of waveguide width. The insertion loss and crosstalk achieved are 3-4 dB and -(15-20) dB, respectively. Potential is shown for achieving improved performance using larger waveguide bending radii in the MZI arms and/or (a) local heater(s) for refractive index tuning

Locally erasable couplers for optical device testing in silicon on insulator

Wafer scale testing is critical to reducing production costs and increasing production yield. Here we report a method that allows testing of individual optical components within a complex optical integrated circuit. The method is based on diffractive grating couplers, fabricated using lattice damage induced by ion implantation of germanium. These gratings can be erased via localised laser annealing, which is shown to reduce the outcoupling efficiency by over 20 dB after the device testing is completed. Laser annealing was achieved by employing a CW laser, operating at visible wavelengths thus reducing equipment costs and allowing annealing through thick oxide claddings. The process used also retains CMOS compatibility

A monolithically integrated silicon modulator with a 10 Gb/s 5 V pp or 5.6 V pp driver in 0.25 μm SiGe:C BiCMOS

This paper presents as a novelty a fully monolithically integrated 10 Gb/s silicon modulator consisting of an electrical driver plus optical phase modulator in 0.25 μm SiGe:C BiCMOS technology on one chip, where instead of a SOI CMOS process (only MOS transistors) a SiGe BiCMOS process (MOS transistors and fast SiGe bipolar transistors) is implemented. The fastest bipolar transistors in the BiCMOS product line used have a transit frequency of f t ≈ 120 GHz and a collector-emitter breakdown voltage of BV CE0 = 2.2 V (IHP SG25H3). The main focus of this paper will be given to the electronic drivers, where two driver variants are implemented in the test chips. Circuit descriptions and simulations, which treat the influences of noise and bond wires, are presented. Measurements at separate test chips for the drivers show that the integrated driver variant one has a low power consumption in the range of 0.66 to 0.68 W but a high gain of S 21 = 37 dB. From the large signal point of view this driver delivers an inverted as well as a non-inverted output data signal between 0 and 2.5 V (5 V pp differential). Driver variant one is supplied with 2.5 V and with 3.5 V. Bit-error-ratio (BER) measurements resulted in a BER better than 10 −12 for voltage differences of the input data stream down to 50 mV pp . Driver variant two, which is an adapted version of driver variant one, is supplied with 2.5 and 4.2 V, consumes 0.83 to 0.87 W, delivers a differential data signal with 5.6 V pp at the output and has a gain of S 21 = 40 dB. The chip of the fully integrated modulator occupies an area of 12.3 mm 2 due to the photonic components. Measurements with a 240 mV pp electrical input data stream, 1.25 V input common-mode voltage and for an optical input wavelength of 1540 nm resulted in an extinction ratio of 3.3 dB for 1 mm long RF phase shifters in each modulator arm driven by driver variant one and a DC tuning voltage of 1.2 V. The extinction ratio was 8.4 dB at a DC tuning voltage of 7 V for a device with 2 mm long RF phase shifters in each arm and driver variant two

Ultra-sharp asymmetric Fano-like resonance spectrum on Si photonic platform

In this paper, we report the generation of an ultra-sharp asymmetric resonance spectrum through Fano-like interference. This generation is accomplished by weakly coupling a high-quality factor (Q factor) Fabry–Pérot (FP) cavity and a low-Q factor FP cavity through evanescent waves. The high-Q FP cavity is formed by Sagnac loop mirrors, whilst the low-Q one is built by partially transmitting Sagnac loop reflectors. The working principle has been analytically established and numerically modelled by using temporal coupled-mode-theory (CMT), and verified using a prototype device fabricated on the 340 nm silicon-on-insulator (SOI) platform, patterned by deep ultraviolet (DUV) lithography. Pronounced asymmetric resonances with slopes up to 0.77 dB/pm have been successfully measured, which, to the best of our knowledge, is higher than the results reported in state-of-the-art devices in on-chip integrated Si photonic studies. The established theoretical analysis method can provide excellent design guidelines for devices with Fano-like resonances. The design principle can be applied to ultra-sensitive sensing, ultra-high extinction ratio switching, and more applications

Silicon photonics for optical interconnects

Integration density, channel scalability, low switching energy and low insertion loss are the major prerequisites for on-chip Wavelength Division Multiplexing systems. A number of device geometries have already been demonstrated that fulfil these criteria, at least in part, but combining all of the requirements is still a difficult challenge. Photonic crystal cavities provide the ultimate confinement of light in space and time, giving the potential to realise devices with the lowest energy consumption and highest integration densities. Here, we demonstrate Photonic Crystal cavity modulators with very low switching energies and high speed Mach Zehnder interferometer modulators and discuss their relative merits

Self-aligned silicon ring resonator optical modulator with focused ion beam error correction

Data interconnects are on the verge of a revolution. Electrical links are increasingly being pushed to their limits with the ever increasing demand for bandwidth. Data transmission in the optical domain is a leading candidate to satisfy this need. The optical modulator is key to most applications and increasing the data rate at which it operates is important for reducing power consumption, increasing channel bandwidth limitations and improving the efficiency of infrastructure usage. In this work silicon based devices of lengths 3.5mm and 1mm operating at 40Gbit/s are demonstrated with extinction ratios of up to 10dB and 3.5dB respectively. The efficiency and optical loss of the phase shifter is 2.7V.cm and 4dB/mm (or 4.5dB/mm including waveguide loss) respectively