Ramón Maldonado-Basilio

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.

Comments on X. Yin, A. Wen, Y. Chen, and T. Wang, ‘Studies in an optical millimeter-wave generation scheme via two parallel dual-parallel Mach–Zehnder modulators’, Journal of Modern Optics, 58(8), 2011, pp. 665–673

Yin et al. have described an innovative filter-less optical millimeter-wave generation scheme for octotupling of a 10 GHz RF oscillator, or sedecimtupling of a 5 GHz RF oscillator using two parallel dual-parallel Mach–Zehnder modulators (DP-MZMs). The great merit of their design is the suppression of all harmonics except those of order (octotupling) or all harmonics except those of order (sedecimtupling), where is an integer. A demerit of their scheme is the requirement to set a precise RF signal modulation index in order to suppress the zeroth order optical carrier. The purpose of this comment is to show that, in the case of the octotupling function, all harmonics may be suppressed except those of order , where is an odd integer, by the simple addition of an optical phase shift between the two DP-MZMs and an adjustment of the RF drive phases. Since the carrier is suppressed in the modified architecture, the octotupling circuit is thereby released of the strict requirement to set the drive level to a precise value without any significant increase in circuit complexity.

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.

Dual-function photonic integrated circuit for frequency octo-tupling or single-side-band modulation

A dual-function photonic integrated circuit for microwave photonic applications is proposed. The circuit consists of four linear electro-optic phase modulators connected optically in parallel within a generalized Mach-Zehnder interferometer architecture. The photonic circuit is arranged to have two separate output ports. A first port provides frequency up-conversion of a microwave signal from the electrical to the optical domain; equivalently single-side-band modulation. A second port provides tunable millimeter wave carriers by frequency octo-tupling of an appropriate amplitude RF carrier. The circuit exploits the intrinsic relative phases between the ports of multi-mode interference couplers to provide substantially all the static optical phases needed. The operation of the proposed dual-function photonic integrated circuit is verified by computer simulations. The performance of the frequency octo-tupling and up-conversion functions is analyzed in terms of the electrical signal to harmonic distortion ratio and the optical single side band to unwanted harmonics ratio, respectively.

Theoretical analysis and modeling of a photonic integrated circuit for frequency 8-tupled and 24-tupled millimeter wave signal generation: erratum

A novel photonic circuit design for implementing frequency 8-tupling and 24-tupling was presented [Opt. Lett.39, 6950 (2014)10.1364/OL.39.006950OPLEDP0146-9592], and although its key message remains unaltered, there were typographical errors in the equations that are corrected in this erratum.

Low polarization-sensitive asymmetric multi-quantum well semiconductor amplifier for next-generation optical access networks

A broadband and low-polarization-sensitive multi-quantum well semiconductor optical amplifier with an asymmetric structure is reported for operation in the E-band wavelength range. A gain peak of 20 dB for a bandwidth of more than 50 nm is measured for both TE and TM polarizations. A maximum polarization sensitivity of 3 dB is measured for a broad wavelength range from 1340 to 1440 nm.

Numerical Analysis of a Multi-Function Photonic Integrated Circuit for Coherent Radio-Over-Fiber

A digital coherent radio-over-fiber (RoF) system comprising a microwave photonic integrated circuit (MPIC) as the main building block is proposed. The multifunction MPIC circuit is analyzed to operate as a single-sideband (SSB) modulator and as an in-phase and quadrature modulator (IQM) of a baseband signal in the RoF downlink. The circuit is also analyzed in the uplink as an SSB of a 60-GHz frequency-shifted input. The performance of the MPIC is assessed through numerical simulations, computing the harmonic distortions in the SSB, as well as the error vector magnitude in the IQM and coherent RoF system. Obtained results illustrate the advantage of the proposed MPIC for heterodyne mixing by providing the upper and lower sidebands spatially located in distinct ports. Moreover, simulation results yield insights into the feasibility of the circuit as an IQM of both baseband and low-power intermediate frequency signals. A comparison of an orthogonal frequency division multiplexing system and a single carrier system employing the proposed circuit in the end-to-end transport of high data rate is presented through error vector magnitude calculations.

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.

Characterization of an Asymmetric InGaAsP/InP Multi Quantum Well Semiconductor Optical Amplifier

An experimental characterization of broadband semiconductor optical amplifiers (SOAs) at 1360 nm is reported. In addition to their inherent small size, fast dynamics, and feasibility of integration with other optoelectronic components, the relevance of the multi quantum well (MQW) asymmetric SOAs here reported relies on the achievement of a flat and broad 3 dB amplification bandwidth. SOAs are composed of nine In1-xGaxAsyP1-y 0.2% tensile strained MQW layers separated by latticed matched InP barriers. The asymmetry of the active region is based on the difference of the molar concentrations, with Ga (x) ranging from 0.46 to 0.47 and As (y) ranging from 0.89 to 0.94. Devices under test have 7 degrees tilt cleaved facets and feature different geometries: ridge widths from 2 to 4 μm in steps of 0.25 μm, and cavity lengths of 600, 900, 1200, and 1500 μm. Fabry-Pérot (FP) lasers with the same material composition as the SOAs and within the same wafer are used as test structures for parameters extraction, providing a feedback mechanism for further design improvement. The ridge width of the FP lasers varies from 2 to 8 μm, in steps of 2 μm. All the devices have been designed and characterized at the Photonics Technology Laboratory, Centre for Research in Photonics, fabrication was done at Canadian Photonics Fabrication Centre (CPFC), Canada and supported by CMC Microsystems. Devices under test are DC-biased and temperature controlled at 25°C. A single pass gain of 13.5 dB is measured for a 3 dB bandwidth of 60 nm centred at 1360 nm. Light-current plots obtained from the FP lasers show that the threshold current varies with the cavity length, with a minimum of 80 mA for a cavity length of 600 μm and a ridge width of 2 μm. A thermal roll-off occurring at high injection currents is observed, especially with the smallest cavity length. In conclusion, asymmetric MQW SOAs featuring different ridge widths and cavity lengths have been

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.