Hamdam Nikkhah

Electro-optic up-conversion mixer amenable to photonic integration

A novel electro-optical up-conversion mixer architecture comprising four electro-optical phase modulators situated in the arms between an interconnected 1 × 4 distribution tree and a complementary 4 × 2 combination tree is proposed. The distribution and combination trees are based on multi-mode interference couplers (MMI). The novelty lies in the use of the intrinsic phase relations between the MMI ports to realize a broadband and free of drift design requiring no static phase shift elements. A transfer-matrix approach is followed to represent the main building blocks in the proposed design, and hence to describe the operation of the entire optical up-conversion mixer. The concept is demonstrated by computer simulations. A single side-band modulation with carrier suppression is obtained at the output of the proposed architecture, which is in agreement with the analytical development. Scenarios considering both ideal and imperfect power balances and phase relations in the MMIs, as well as imperfect phase relations of the electrical drives to the phase modulators are analyzed.

Radio-over-Fibre access for sustainable Digital Cities

Pervasive broadband access will transform cities to the net social, environmental and economic benefit of the e-City dweller as did the introduction of utility and transport network infrastructures. Yet without action, the quantity of greenhouse gas emissions attributable to the increasing energy consumption of access networks will become a serious threat to the environment. This paper introduces the vision of a ‘sustainable Digital City’ and then considers strategies to overcome economic and technical hurdles faced by engineers responsible for developing the information and communications technology (ICT) network infrastructure of a Digital City. In particular, ICT energy consumption, already an issue from an operating cost perspective, is responsible for 3 % of global energy consumption and is growing unsustainably. A grand challenge is to conceive of networks, systems and devices that together can cap wireless network energy consumption whilst accommodating growth in the number of subscribers and the bandwidth of services. This paper provides some first research directions to tackle this grand challenge. A distributed antenna system with radio frequency (RF) transport over an optical fibre (or optical wireless in benign environments) distribution network is identified as best suited to wireless access in cluttered urban environments expected in a Digital City from an energy consumption perspective. This is a similar architecture to Radio-over-Fibre which, for decades, has been synonymous with RF transport over analogue intensity-modulated direct detection optical links. However, it is suggested herein that digital coherent optical transport of RF holds greater promise than the orthodox approach. The composition of the wireless and optical channels is then linear, which eases the digital signal processing tasks and permits robust wireless protocols to be used end-to-end natively which offers gains in terms of capacity and energy efficiency. The arguments are supported by simulation studies of distributed antenna systems and digital coherent Radio-over-Fibre links.

Free space and waveguide Talbot effect: phase relations and planar light circuit applications

Optical fields that are periodic in the transverse plane self-image periodically as they propagate along the optical axis: a phenomenon known as the Talbot effect. A transfer matrix may be defined that relates the amplitude and phase of point sources placed on a particular grid at the input to their respective multiple images at an image plane. The free-space Talbot effect may be mapped to the waveguide Talbot effect. Applying this mapping to the transfer matrix enables the prediction of the phase and amplitude relations between the ports of a Multimode Interference (MMI) coupler– a planar waveguide device. The transfer matrix approach has not previously been applied to the free-space case and its mapping to the waveguide case provides greater clarity and physical insight into the phase relationships than previous treatments. The paper first introduces the underlying physics of the Talbot effect in free space with emphasis on the positions along the optical axis at which images occur; their multiplicity; and their relative phase relations determined by the Gauss Quadratic Sum of number theory. The analysis is then adapted to predict the phase relationships between the ports of an MMI. These phase relationships are critical to planar light circuit (PLC) applications such as 90° optical hybrids for coherent optical receiver front-ends, external optical I-Q modulators for coherent optical transmitters; and optical phased array switches. These applications are illustrated by results obtained from devices that have been fabricated and tested by the PTLab in Si micro-photonic integration platforms.

Metamaterial Lüneburg Lens for Fourier Optics On-a-chip

A planar metamaterial Lüneburg lens that enables Fourier optics on-a-chip can be implemented in an SOI slab waveguide structure by patterning the silicon core with variable sized holes. The subwavelength patterning of binary nanocomposite material to form the metamaterial offers the major advantage of fabrication by a single etch step while demanding feature sizes that can be accessed by deep UV lithography in addition to e-beam lithography. A numerical calibration procedure is described that is used to find the relation between fill factor and the local homogenised effective refractive index and which improves upon the predictions of analytic effective media theory used by other researchers. The concept and designs were verified by the 2D FDTD simulation of a two lens telescope system with waveguide feeds implemented in a metamaterial that shows a low insertion loss of -0.45 dB with a reliable field profile at exit. A 3D FDTD simulation of the same two lens telescope system that takes full account of the SOI layers, their finite thickness, and the ridge waveguide feeds also predicts a low loss of -0.83 dB. Less reliance however can be placed on this result due to the coarseness of the computational grid that was necessary. Nevertheless both results are encouraging for planned fabrication trials. This structure can be used in optical transpose interconnection systems in optical switching architectures with the advantage of avoiding large number crossover waveguides in optical communication systems.

Metamaterial Lüneburg waveguide lenses for switch fabric on-a-chip applications

Optical information processing has traditionally been demonstrated using 3D free-space optical systems employing bulk optical components. These systems are bulky and unstable due to the stringent alignment tolerances that must be met. Taking advantage of the alignment accuracy offered by planar light circuits, these issues may be overcome by confining the light in a planar slab waveguide. The limitation on scaling, consequent on the loss of one dimension is offset by the nanoscale component footprints attainable in a silicon integration platform. A key component of this free-space-opticson- a-chip concept is a waveguide lens. Waveguide lenses are of general utility but our specific application is their use to implement the complex crossover interconnections of a switch fabric. The graded refractive index of the lens is engineered by patterning the silicon layer of silicon on insulator slab waveguides into a dense distribution of cylinders; either solid (silicon) or voids (air); using a single etch step. The cylinders have variable diameters and are placed on a regular square or hexagonal grid with sub-wavelength pitch. In the case of voids, the patterned silicon may be suspended in air to form the core of a symmetric slab waveguide. Solid cylinders must be supported by the Si02 layer leading to an asymmetric waveguide of reduced effective index range. Advantageously, the patterning of the metamaterial region within the slab-waveguide requires only a single etch step. Photonic wire feeder waveguides at different positions around the lens may be used to launch light into the lenses or collect light from the lenses. A method is developed to determine the local effective media index of a periodic metamaterial in terms of the parameters of its unit cell. This method is used as a calibration to lay out a metamaterial with graded parameters. The operation of a metamaterial Lüneburg lens telescope is verified by FDTD simulations and shown to be capable of near zero insertion loss and crosstalk. The careful approximation of the graded index of the Lüneburg lens by a metamaterial introduces minimal impairments.

Effect of the degree of phase-correlation of laser sources on the transmission and optical coherent detection in radio-over-fibre systems

The deployment of high capacity Radio-over-Fiber (RoF) systems rely, among many aspects, on the capability to efficiently generate, transport, and detect millimeter-wave carriers modulated at high data rates. Photonic approaches based on the heterodyne beating of two free-running laser sources have been proposed as an alternative to generate multi-Gbps quadrature phase modulated signals imposed on millimeter wave carriers. Implementing photonic approaches in the down-link avoids the need for electronic generation of high frequency carriers and decreases the requirements at the base band electronics. In addition, implementing complex modulation formats overcomes some of the typical issues found in intensity modulation direct detection approaches such as non linearity, receiver sensitivity and dynamic range. In this work, the performance improvement of a coherent RoF system carrying 10 Gbps QPSK signals is numerically analyzed in terms of both the frequency linewidth and the degree of phase correlation between the lasers utilised at the down-link (for the optical heterodyne beating) and at the up-link (for the optical coherent detection). Relative to phase correlated lasers featuring linewidths of 5 MHz, the peak power of the 60 G Hz carrier generated at the down-link is reduced by 8 dB for un-correlated lasers. In addition, the error vector magnitude of the received signal at the up-link is improved from over 20% (for un-correlated lasers and linewidths of 5 MHz) to around 15% (for correlated lasers) at an optical received power of -30 dBm. The results obtained reinforce the idea of using coherent comb laser sources with phase correlated modes located at the Central Office. It also motivates the eventual deployment of techniques to control the degree of phase correlation between the lasers used as signal and local oscillator at the optical coherent receivers.

A transpose optical interconnect utilising metamaterial Luneburg waveguide lenses for switch fabric on-a-chip applications

Advances in silicon photonics motivate the consideration of on-chip switch fabrics that combine switch elements into larger port-count switches. A major challenge is the large number of inter-stage waveguide crossovers. A novel freespace architecture utilising micro-lenses, capable of near zero insertion loss and crosstalk, is described and its principles of operation explained. The architecture is mapped to a planar implementation by the substitution of propagation in a slab-waveguide for free-space propagation and Luneburg lenses for the micro-lenses. Simulations show the careful approximation of the graded index of the Luneburg lens by a metamaterial introduces minimal additional crosstalk.

A photonic circuit for complementary frequency shifting, in-phase quadrature/single sideband modulation and frequency multiplication: analysis and integration feasibility

Abstract A novel photonic integrated circuit architecture for implementing orthogonal frequency division multiplexing by means of photonic generation of phase-correlated sub-carriers is proposed. The circuit can also be used for implementing complex modulation, frequency up-conversion of the electrical signal to the optical domain and frequency multiplication. The principles of operation of the circuit are expounded using transmission matrices and the predictions of the analysis are verified by computer simulation using an industry-standard software tool. Non-ideal scenarios that may affect the correct function of the circuit are taken into consideration and quantified. The discussion of integration feasibility is illustrated by a photonic integrated circuit that has been fabricated using ‘library’ components and which features most of the elements of the proposed circuit architecture. The circuit is found to be practical and may be fabricated in any material platform that offers a linear electro-optic modulator such as organic or ferroelectric thin films hybridized with silicon photonics.