Guillaume Tremblay

Optimizing fiber Bragg gratings using a genetic algorithm with fabrication-constraint encoding

We propose an advanced genetic algorithm (GA) to design fiber Bragg gratings (FBGs) with given fabrication constraints. Our GA is enhanced by a new Fourier-series-based real-valued encoding to obtain more degrees of freedom and a rank-based fitness function. The new GA enables us to remove phase shifts in the gratings. The designed minimum-dispersion bandpass grating has a dispersion of /spl plusmn/28 ps/nm in the 0.33-nm flat-top passband. The grating is fabricated using a phase mask without phase shift.

Improving imaging performance of a metallic superlens using the long-range surface plasmon polariton mode cutoff technique

The metallic superlens is based on excitation and amplification of coupled surface plasmon polariton (SPP) modes through a metal slab. However, the narrow and too-high peaks of the SPP resonance modes in the transfer function can jeopardize imaging performance, such that high sidelobes occur in the image of isolated subwavelength patterns. We propose to design a metallic superlens by approaching the cutoff condition of the long-range SPP mode to flatten the transfer function and to improve imaging performance significantly.

Designing the metallic superlens close to the cutoff of the long-range mode.

The metallic superlens typically shows two peaks in its transfer function related to the long- and the short-range surface plasmon polariton (SPP) modes. These peaks are necessary to amplify the evanescent waves compensating the exponential decays, but enhance the spatial frequencies disproportionally, resulting in strong sidelobes in the image. We propose to design the metallic superlens with close to the cutoff condition of the long-range SPP mode to balance the SPP amplification and the flatness of the transfer function, and thus eliminating the sidelobes in the image. The design experiments for the Al superlens at 193 nm with both the transfer-matrix approach and the numerical finite difference in time domain method are shown.

Effects of the phase shift split on phase-shifted fiber Bragg gratings

In proximity side-writing fiber Bragg gratings (FBGs) using a phase shifted phase mask, the phase shift in the phase mask is split into two half-amplitude phase shifts. The effects of the phase-shift split on FBGs is investigated. A FBG with a split π-phase shift is modeled as two cascaded and tightly coupled grating-based Fabry-Perot filters. A compact expression of its reflectivity spectrum is obtained showing spectral asymmetry errors and a shift of the transmission peak. Our numerical results are in good agreement with previous experimental results. These effects decrease with increasing FBG length and become negligible for gratings more than a few millimeters long.

Monitoring the Quality of Signal in Packet-Switched Networks Using Optical Correlators

The increasing demand in the Internet network for real-time multimedia data traffic with high quality of service (QoS) is pushing the limits of existing network structure. Optical packet switching (OPS) is considered as a possible technology for future telecommunication networks due to its compatibility with bursty traffic and efficient use of the network resources. But OPS brings about new challenges to the research in optical performance monitoring (OPM). In this scenario, each packet follows its own path along the network depending on the routing information contained in the label and thereby packets suffer from different signal degradations. Therefore, a definitive goal for OPM is to provide comprehensive signal quality information as part of QoS implementation to keep the level of QoS promised to customers. In this paper, a novel optical SNR (OSNR) monitoring technique based on the use of optical correlation is presented. A fiber Bragg grating-based correlator was constructed and used to experimentally demonstrate the successful correlation. Experiments performed on a 40 Gb/s system confirm the viability of this approach. By measuring statistics from the autocorrelation peak, the monitor is capable of direct OSNR monitoring with an error of less than 0.5 dB.

OSNR monitoring at high-speeds using a FBG-based correlator in optical packet-switched networks

A novel OSNR monitoring technique based on use of optical pulse correlation is demonstrated experimentally. By measuring statistics extracted from the autocorrelation peak, the OSNR is estimated with an error of less than 0.5 dB.

Modeling and designing metallic superlens with metallic objects

When the metallic near-field superlens is to image a planar object, which is itself metallic, such as that in the near-field lithography applications, the object nanometer features will act as the Hertzian dipole sources and launch homogeneous and evanescent waves. The imaging system can be modeled as a dielectric Fabry-Perot cavity with the two surface plasmon resonant mirrors. We show the expressions of the transfer function and optimize the imaging system configuration using the genetic algorithm. The effectiveness of the design is confirmed by the image intensity profile computed with the numerical finite difference in time domain method.

Metallic superlens designed with close-to-cutoff of long-range SPP mode

The metallic superlens of a single negative permittivity is easier to implement than the double negative material superlens and can be applied to nano-scale resolution lithography. The metallic superlens amplifies by the resonance the surface plasmon polariton (SPP) waves, which carry the sub-wavelength detail information of the object. However, the excitations of the long- and the short-range SPP modes of the metal slab lead to two peaks in the transfer function which enhance the spatial frequencies disproportionally, resulting in strong sidelobes in the image. Conventionally the metallic superlens is designed by trials without rules to follow. We propose to design the metallic superlens by approaching the cutoff condition of the long-range SPP mode in order to flatten the transfer function and to improve the imaging performance significantly. Design experiments of Al and Ag superlens with both the transfer-matrix approach and the numerical Finite Difference in Time Domain method are shown.

Imaging performances of the metallic superlens

The imaging properties of the single-layer metallic superlens are studied systematically. We computed the impulse response and the transfer function of the lens which are critical for understanding the imaging performance of this superlens and for possible improvements in the future. Furthermore, we relate the single layer superlens structure to the well known surface plasmon polariton (SPP) waveguide, and find out that the metal slab superlens is a particular type of SPP waveguide, where the field distribution is mainly determined by the long- and short-range modes. We show that the excitation of the long-range mode causes large sidelobes in the impulse response. We propose to improve the imaging performances of the superlens by designing a structure for which the long-range mode is cutoff.

Numerical Optimization of Passband Fiber Bragg Gratings

Real-valued encoding genetic algorithm is used to optimize fiber Bragg gratings. One may pre-choose number of phase-shifts in grating according to fabrication conditions. The fabricated grating achieves designed performance with minimum dispersion in flat-top passband.