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

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

A high efficiency input/output coupler for small silicon photonic devices

Coupling light from an optical fibre to small optical waveguides is particularly problematic in semiconductors, since the refractive index of the silica fibre is very different from that of a semiconductor waveguide. There have been several published methods of achieving such coupling, but none are sufficiently efficient whilst being robust enough for commercial applications. In this paper experimental results of our approach called a Dual-Grating Assisted Directional Coupler, are presented. The principle of coupling by this novel method has been successfully demonstrated, and a coupling efficiency of 55% measured

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

InGaAs/AlGaAsSb avalanche photodiode with high gain - bandwidth product

Increasing reliance on the Internet places greater and greater demands for high -speed optical communication systems. Increasing their data transfer rate allows more data to be transferred over existing links. With optical receivers being essential to all optical links, bandwidth performance of key components in receivers, such as avalanche photodiodes (APDs), must be improved. The APDs rely on In0.53Ga0.47As (grown lattice-matched to InP substrates) to efficiently absorb and detect the optical signals with 1310 or 1550 nm wavelength, the optimal wavelengths of operation for these optical links. Thus developing InP -compatible APDs with high gain-bandwidth product (GBP) is important to the overall effort of increasing optical links’ data transfer rate. Here we demonstrate a novel InGaAs/AlGaAsSb APD, grown on an InP substrate, with a GBP of 424 GHz, the highest value reported for InP -compatible APDs, which is clearly applicable to future optical communication systems at or above 10 Gb/s

Improving the efficiency of fibre-chip grating couplers near 1310 nm

We present our recent work on fibre-chip grating couplers operating around 1310 nm. For the first time, we demonstrated the combination of dual-etch and apodization design approaches which can offer state of the art performance. Initial tests from fabricated structures show a -2.2dB loss

Normal mode analysis for scalar fields in BTZ black hole background

We analyze the possibility of inequivalent boundary conditions for a scalar field propagating in the BTZ black hole space-time. We find that for certain ranges of the black hole parameters, the Klein-Gordon operator admits a one-parameter family of self-adjoint extensions. For this range, the BTZ space-time is not quantum mechanically complete. We suggest a physically motivated method for determining the spectra of the Klein-Gordon operator.Comment: 6 pages, no figure, late

Classical and quantum properties of a 2-sphere singularity

Recently Boehmer and Lobo have shown that a metric due to Florides, which has been used as an interior Schwarzschild solution, can be extended to reveal a classical singularity that has the form of a two-sphere. Here the singularity is shown to be a scalar curvature singularity that is both timelike and gravitationally weak. It is also shown to be a quantum singularity because the Klein-Gordon operator associated with quantum mechanical particles approaching the singularity is not essentially self-adjoint.Comment: 10 pages, 1 figure, minor corrections, final versio

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