Transient reflection of light near excitonic resonances

The authors present an experimental analysis of the temporal structure of picosecond light pulses reflected from GaAs crystals near the lowest excitonic resonance at normal and Brewster angle incidence. The reflected pulses exhibit nonexponentially decaying tails, measured over five orders of magnitude, which follow the dynamics of the coherent polarization in the reflecting layer. Model calculations, including spatial dispersion effects and dead layer effects, provide a general understanding of the data. The numerical fit, however, fails to describe the fast initial decay observed experimentally. The latter can be explained by taking into account an inhomogeneous transition layer with a depth-dependent damping constant. Strong intensity dependence of the transients is observed     

Wavelength selective optical fiber couplers based on longitudinalFabry-Perot structures

A new approach for the development of wavelength selective optical fiber couplers is proposed. It is based on a standard polished fiber coupler in which an intermediate planar multilayer structure, which acts as a transversal optical filter, is included between the two fibers. The simplest multilayer structure, with interesting potential applications, is constituted by high refractive index single-layer Fabry-Perot filters. Experimental results for several single-layer Fabry-Perot filters, as well as a phenomenological model that provides accurate predictions about the wavelength selective coupling ratio, are presented     

Calculation of the propagation constant of optical modes inmultiquantum-well structures

A simple mathematical formalism is used to calculate the propagation constant β for optical modes (TE₀, TM₀ ) in multiquantum-well semiconductor laser structures. The energy correction due to narrow quantum wells, the energy dependence of the refractive indexes, and the energy dependence of electron and hole masses are taken into account. The data show that there is a considerable difference between the calculated values and the averaged values for β, especially for a small number of narrow quantum wells     

Damping of cavity-mode resonances in PCB power-bus structures using discrete capacitors

In this paper, the impact of lumped elements connected to the power-bus of high-speed printed circuit boards is investigated and a computationally efficient expression of the resulting input impedance is derived. This result builds the theoretical basis of a novel technique suppressing cavity-mode resonances within the power-bus using discrete capacitors. Guidelines for an optimized selection and placement of these capacitors are specified. The theoretical results are well confirmed by measurements.     

Wavelength detection in FBG sensor network using tree search DMS-PSO

As the number of fiber Bragg gratings (FBGs) increases, the conventional peak detection method will be unsuitable to detect Bragg wavelengths of FBG sensors in a wavelength-division-multiplexed (WDM) network. To solve this problem while achieving a higher accuracy at reduced computation cost, a novel tree search dynamic multiswarm particle swarm optimizer (TS-DMS-PSO) is developed. The TS-DMS-PSO yields better results with less computation cost based on the simulations (codes are available from the second author).     

Wavelength selective optical logic and interconnects

A wavelength selective optical logic (WSOL) element that uses monolithically integrated wavelength selective optical input and output elements is described. Input optical signals are detected by photothyristors situated in an optical cavity which provides a highly selective response of a wavelength determined by the fabrication process. Output signals are generated by vertical cavity surface emitting lasers, whose lasing wavelengths can also be specified during the fabrication process. A vertical integration of these input and output elements that is suitable for wavelength selective optical logic and wavelength selective optical interconnect applications is proposed. The proposed circuitry is easily cascadable so that arbitrarily complex optical logic functions can be performed by WSOL devices in series. Several of the possible logic functions are described, including OR and AND gates, an adder, and a flip-flop     

Efficient detection of resonances in anechoic chambers using thematrix pencil method

In a previous paper we have presented a method for evaluating the performance of anechoic chambers by analyzing the S-parameters of a system comprising two antennas facing each other in an anechoic chamber using the matrix pencil method. In this work, we present an improvement of this resonance detection technique using only the transmission parameter S₂₁. The propagating components of the transmission parameter S₂₁ are derived over small frequency intervals using the matrix pencil method and then removed from S₂₁ in a two-level decomposition procedure. The resonances are clearly identified from the residual signal. Two examples of resonance detection in two different anechoic chambers illustrate the proposed method     

Novel receiver structures for systems using differential detection

Novel receiver structures are introduced which improve the bit error rate performance of differentially detected MSK and QPSK systems. These new receiver structures are based on 1) combining with feedback, 2) a simple Viterbi decoder which uses one- and two-bit detector outputs for maximum-likelihood sequence estimation, and 3) using the correlation of noise for the partial removal of its effect prior to decision. The new receiver structures presented are useful in mobile radio and mobile satellite communications where power efficiency, synchronization, and implementation complexity are of primary concern.     

Wavelength conversion using semiconductor optical amplifiers

This paper reports a detailed theoretical study of the dynamics of wavelength conversion using cross-gain and cross-phase modulation in semiconductor optical amplifiers (SOA's) involving a large signal, multisection rate equation model. Using this model, recently reported experimental results have been correctly predicted and the effects of electrical and optical pumping on the conversion speed, modulation index, and phase variation of the converted signal have been considered. The model predicts, in agreement with experimental data, that recovery rates as low as 12 ps are possible if signal and pump powers in excess of 14 dBm are used. It also indicates that conversion speeds up to 40 Gb/s may be achieved with less than 3 dB dynamic penalty. The employment of cross-phase modulation increases the speed allowing, for example, an improvement to 60 Gb/s with an excess loss penalty less than 1 dB     

Wavelength conversion using a T-gate laser

Wavelength conversion from 830 nm to 837 nm of a 250 Mb/s NRZ optical signal has been demonstrated at 10/sup -9/ bit-error rate and a detector sensitivity of -29 dBm using a semiconductor power amplifier monolithically integrated at 90/spl deg/ with a GaAs-AlGaAs SQW semiconductor laser. This device offers a high degree of isolation between the input and the output and has the potential for high-speed operation with a wide continuous-wavelength conversion range.     

Wavelength add-drop switching using tilting micromirrors

This paper describes a single-mode optical fiber switch which routes individual signals into and out of a wavelength multiplexed data stream without interrupting the remaining channels. The switch uses free-space optical wavelength multiplexing and a column of micromechanical tilt-mirrors to switch 16 channels at 200 GHz spacing from 1531 to 1556 nm. The electrostatically actuated tilt mirrors use an 80 V peak-to-peak 300 KHz sinusoidal drive signal to switch between ±10° with a 20 μs response. The total fiber-to-fiber insertion loss for the packaged switch is 5 dB for the passed signals and 8 dB for added and dropped signals, with 0.2 dB polarization dependence. Switching contrast was 30 dB or more for all 16 channels and all input and output states. We demonstrate operation by switching 622 Mb/s data on eight wavelength channels between the two input and output ports with negligible eye closure     

Wavelength add-drop node using silica waveguide integration

We integrated a wavelength-selective cross-connect, de-interleaver, interleaver, power combiner, and variable attenuators on one silica waveguide chip and a band demultiplexer, band multiplexer, and optical monitor on another. These two chips are used to create an eight-channel add-drop node that is expandable up to 80 channels with minimal traffic interruption by adding more of these chips.     

Room temperature excitonic nonlinear absorption and refraction in GaAs/AlGaAs multiple quantum well structures

We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.     

Computation of cavity resonances using edge-based finite elements

The eigenvalues of a cavity resonator are computed using edge-based elements, and it is shown that these elements offer significant improvements in accuracy, in addition to being suitable for modeling arbitrarily shaped inhomogeneous regions. A performance comparison between the edge-based tetrahedra and rectangular brick elements is also carried out     

Multiwavelength sources using laser diodes frequency-locked toatomic resonances

It is shown that low-cost compact disk AlGaAs laser diodes make it possible to realize manifold optical sources with precise and stable wavelengths when locked to rubidium vapor resonances. Many optical frequency references can be obtained with the use of absorption cells containing a foreign gas or by placing cells in static magnetic fields. The same approach can be used with other wavelengths required by optical fiber communications     

Wavelength switching and routing using absorption and resonance

A resonator side coupled to a pair of waveguides can switch an optical signal between two ports by means of absorption. The absorptive mechanism is used to suppress the resonant power transfer, rather than to promote loss. Thus, the input signal only suffers small attenuation, provided that the mode volumes of the resonators can be made small enough. Multiply-coupled resonators lead to improved crosstalk performance for both the ON and OFF switched states. The performance of such devices are analyzed analytically, and universal switching curves are derived     

Wavelength demultiplexing using bends in a single-mode opticalfiber

A method of wavelength demultiplexing is developed that makes use of the wavelength and bend-radius dependence of the pure bend loss at single radius bends in a single-mode fiber. The light leaving the fiber at a bend forms one output of the demultiplexer. Equations are derived that predict the bend radii for the most-efficient operation of the demultiplexer and the power extracted from the fiber at each wavelength. The demultiplexer operates over the wavelength range corresponding to V numbers from 2.4 to 1.6. A two-wavelength demultiplexer formed from 90° bends is constructed and tested. Collection efficiencies of 70% and insertion losses of less than 2 dB are shown to be possible. Crosstalk isolation in excess of 40 dB can be achieved with the aid of optical wavelength filters. The performance of a three-wavelength demultiplexer is also considered     

A selective decision–fusion rule for cooperative spectrum sensing using energy detection

Increasing the number of terminals in a cognitive radio network is known to improve the accuracy of cooperative spectrum sensing at the cost of reducing the useful communication time. This downside can be partially mitigated using decision‐based fusion and/or sequential reporting. This paper proposes a novel selective decision‐based cooperative spectrum sensing strategy that limits the reporting time to a single reporting slot with a possibility for retransmissions using automatic repeat request. The terminal with the highest energy estimate sends its local decision to the fusion center to make a final decision. Potential decoding errors are mitigated using threshold‐based automatic repeat request. The performance of the proposed strategy is studied using rigorous mathematical analysis and intensive computer simulations. Results show observable performance enhancements compared with some benchmark strategies in terms of detection accuracy and agility. Copyright © 2015 John Wiley & Sons, Ltd.     

Wavelength division two-way fibre-optic transmission experiments using micro-optic duplexers

Experimental two-way transmission of high-speed digital pulse signals over a single 10 km optical fibre line has been realised at 800 nm band, employing newly developed small size and low-loss micro-optic duplexers, AlGaAs d.h. lasers of different emission wavelengths, a spliced graded-index high-silica fibre and Si-a.p.d.s.