Bimodal grating coupler design on SOI technology for mode division multiplexing at 1550 nm.

In this paper, we evaluate by means of simulation and experimentally the simultaneous coupling of the LP01x-LP11ax fiber modes and the TE0-TE1 nanophotonic SOI waveguide modes using a grating coupler for a two-mode fiber at 1550 nm. Both the grating width (ranging from 10 µm to 15 µm) and the grating vertical profile have been considered in the design procedure. The optimum design (14 µm width and 609 nm grating period) has been selected in terms of coupling efficiency (both LP01x-TE0 and LP11ax-TE1), compactness and tolerance to lateral misalignments between fiber and coupler. The LP01x-TE0 and LP11ax-TE1 modes achieved coupling efficiencies of 49% and 45%, respectively.

Real-time 20.37 Gb/s optical OFDM receiver for PON IM/DD systems.

This paper presents the hardware architecture of an OFDM receiver suitable for optical communications. The receiver has been implemented in an FPGA device and used to demonstrate experimentally an optical OFDM transmission in a passive optical link with a directly modulated DFB laser and DAC/ADC at 5 GS/s. A bit rate of 20.37 Gb/s is achieved using an OFDM signal with subcarrier modulation format up to 512-QAM, it has been successfully transmitted over 10 km SSMF with a spectral efficiency of 8.38 bit/s/Hz.

Design of asymmetrical directional couplers on ridge and strip SOI technology with high-dimensional variation tolerance.

The supermode analysis of asymmetrical directional couplers (ADC) based on SOI technology for strip and ridge structures at 1550 nm is herein reported, targeting to reduce ADC device fabrication requirements. The reported analysis based on supermodes permits us to assess the sensitivity of the ADC coupling efficiency by calculating the index difference between even and odd supermodes. Optimum designs have been found for 100 and 400 nm gaps, respectively, capable of converting and (de) multiplexing both TE0 and TE1 modes taking into account the width, gap, height, and slab thickness variations produced with respect to the nominal design.

Dimensional variation tolerant mode converter/multiplexer fabricated in SOI technology for two-mode transmission at 1550 nm.

The use of an integrated asymmetrical directional coupler for two-mode transmission at 1550 nm is analyzed. The design is based on silicon-on-insulator (SOI) technology and permits mode conversion and mode multiplexing/demultiplexing. In the nominal design, mode conversion and mode (de)multiplexing are achieved with 97% efficiency and a 23.4 dB crosstalk level in the 1540-1560 nm band using a 0.1 µm gap. The dimension tolerance of the SOI process has been taken into account in the selection of the optimum design, and the coupling efficiency would remain above 82.3% (corresponding to 0.8 dB excess loss) with 3σ accuracy. A 90% efficiency has been experimentally obtained.

Evaluation of optical ZP-OFDM transmission performance in multimode fiber links.

In this paper, the performance of Zero Padding Orthogonal Frequency Division Multiplexing (ZP-OFDM) on intensity modulation-direct detection (IM-DD) multimode fiber (MMF) links is assessed by means of numerical simulations. The performance of ZP-OFDM is compared to classical Cyclic Prefixed form of OFDM (CP-OFDM) which is known to offer a limited performance in terms of symbol recovery in subcarriers suffering severe fading. Simulations results show that ZP-OFDM is able to reach 29 Gbps in 99.5% of all installed MMF links up to 600 meters compared to 14 Gbps for CP-OFDM when a 64 points fast Fourier transform (FFT) size is used. The use of ZP-OFDM makes it possible to increase the link length up to 1200 and 2400 m with a 25 Gbps data rate if the FFT sizes are increased to 128 and 256 points, respectively; whereas the CP-OFDM scheme will offer a maximum data rate of 10 Gbps in both cases. ZP-OFDM can be an alternative to adaptive loading OFDM schemes without the need of a negotiation between transmitter and receiver, reducing the system deployment complexity and increasing the flexibility in scenarios with multiple receivers.

Generation of pure electrical quadrature amplitude modulation with photonic vector modulator.

A photonic vector modulator architecture for generating pure quadrature amplitude modulation (QAM) signals is presented. An electrical quadrature-modulated signal at microwave-millimeter-wave frequencies is generated from its corresponding baseband in-phase (I) and quadrature (Q) components. In the proposed scheme, no electrical devices apart from the electrical tone oscillator are needed in the generation process. In addition, the purity of the generated signal is increased, and the hardware requirements are reduced when compared with previously proposed architectures so a highly compact low-cost architecture can be implemented. A pure 1.25 Gbit/s 4-QAM signal has been experimentally generated at a 42 GHz carrier frequency.

All-optical WDM multi-tap microwave filter with flat bandpass.

An incoherent photonic microwave filter implementing multiple positive and negative coefficients based on two sets of optical carriers and dispersive media is proposed and demonstrated. Positive and negative coefficients are obtained thanks to the pi phase inversion in a single electro-optic Mach-Zehnder modulator as well as the modulator Vpi dependence with wavelength. To show the feasibility of this technique to implement practical filter transfer functions, the Parks-McClellan algorithm has been used to design a 5-tap flat bandpass filter. Experimental results show an excellent agreement with theory.