Minimum Channel Spacing Research Articles

Turbidity-tolerant underwater wireless optical communications using dense blue–green wavelength division multiplexing

Underwater wireless optical communication (UWOC) has demonstrated high-speed and low-latency properties in clear and coastal ocean water because of the relatively low attenuation ‘window’ for blue–green wavelengths from 450 nm to 550 nm. However, there are different attenuation coefficients for transmission in ocean water at different wavelengths, and the light transmission more seriously deteriorates with fluctuations in the water turbidity. Therefore, traditional UWOC using a single wavelength or coarse blue–green wavelengths has difficulty tolerating variations in water turbidity. Dense wavelength division multiplexing (WDM) technology provides sufficient communication channels with a narrow wavelength spacing and minimal channel crosstalk. Here, we improve the UWOC in clear and coastal ocean water using dense blue–green WDM. A cost-effective WDM emitter is proposed with directly modulated blue–green laser diodes. Dense wavelength beam combination and collimation are demonstrated in a 20-metre underwater channel from 490 nm to 520 nm. Demultiplexing with a minimum channel spacing of 2 nm is realized by an optical grating. Remarkably, our WDM results demonstrate an aggregate data rate exceeding 10 Gbit/s under diverse water turbidity conditions, with negligible crosstalk observed for each channel. This is the densest WDM implementation with a record channel spacing of 2 nm and the highest channel count for underwater blue–green light communications, providing turbidity-tolerant signal transmission in clear and coastal ocean water.

Tunable narrow band add-drop filter design based on apodized long period waveguide grating assisted co-directional coupler.

Tunable add/drop filter based optical interconnects are an integral part of data centers as well as optical communications. Although add/drop filters based on ring resonators and waveguide Bragg gratings are well developed, long period waveguide grating (LPWG) based add/drop filters have little been investigated so far. In this article, we propose an apodized LPWG assisted co-directional coupler for narrow band add/drop filtering by combining silicon (Si) waveguide with titanium dioxide (TiO2) waveguide geometry. The proposed structure has been analyzed by combining the finite element method (FEM) and transfer matrix method (TMM), showing a good side lobe suppression ratio (SLSR) equal to 25.7 dB and an insertion loss of 0.6 dB. Owing to the high group index difference of Si and TiO2 waveguides, a narrow band response of 1.4 nm has been achieved with 800µm long LPWG. The opposite thermo-optic coefficients of Si and TiO2 ensures a good thermal tunability of the central wavelength. Considering a thin metallic heater of titanium nitride (TiN) the thermal tuning efficiency is found to be 0.07 nm/mW. Further, two LPWGs have been cascaded to realize a tunable dual channel filter with a minimum channel spacing of 185 GHz and a channel crosstalk better than 20 dB, showing its potential application towards dense wavelength division multiplexing.

Numerical study of mixed convection in a horizontal no parallel-plates channel with an unheated moving plate

PurposeThe purpose of this paper is to analyze the thermal and fluid dynamic behaviors of mixed convection in air because of the interaction between a buoyancy flow and a moving plate induced flow in a horizontal no parallel-plates channel to investigate the effects of the minimum channel spacing, wall heat flux, moving plate velocity and converging angle.Design/methodology/approachThe horizontal channel is made up of an upper inclined plate heated at uniform wall heat flux and a lower adiabatic moving surface (belt). The belt moves from the minimum channel spacing section to the maximum channel spacing section at a constant velocity so that its effect interferes with the buoyancy effect. The numerical analysis is accomplished by means of the finite volume method, using the commercial code Fluent.FindingsResults in terms of heated upper plate and moving lower plate temperatures and stream function fields are presented. The paper underlines the thermal and fluid dynamic differences when natural convection or mixed convection takes place, varying minimum channel spacing, wall heat flux, moving plate velocity and converging angle.Research limitations/implicationsThe hypotheses on which the present analysis is based are two-dimensional, laminar and steady state flow and constant thermo physical properties with the Boussinesq approximation. The minimum distance between the upper heated plate of the channel and its lower adiabatic moving plate is 10 and 20 mm. The moving plate velocity varies in the range 0-1 m/s; the belt moves from the right reservoir to the left one. Three values of the uniform wall heat flux are considered, 30, 60 and 120 W/m2, whereas the inclination angle of the upper plate θ is 2° and 10°.Practical implicationsMixed convection because of moving surfaces in channels is present in many industrial applications; examples of processes include continuous casting, extrusion of plastics and other polymeric materials, bonding, annealing and tempering, cooling and/or drying of paper and textiles, chemical catalytic reactors, nuclear waste repositories, petroleum reservoirs, composite materials manufacturing and many others. The investigated configuration is used in applications such as re-heating of billets in furnaces for hot rolling process, continuous extrusion of materials and chemical vapor deposition, and it could also be used in thermal control of electronic systems.Originality/valueThis paper evaluates the thermal and velocity fields to detect the maximum temperature location and the presence of fluid recirculation. The paper is useful to thermal designers.

Analyzing DWDM System with Different Modulation Formats and Channel Spacing

Abstract Different modulation formats for 64×20-GB/s dense wavelength division multiplexed (DWDM) system has been compared and simulated using Optsim. Various performance parameters including Q-factor, bit error rate, jitter, eye opening & eye closure are observed and analyzed. It is reported that RZ Soliton format is a better choice among the tested formats. Also, for the proposed system, minimum channel spacing should be 50 GHz.

Non-intrusive OSNR measurement of polarization-multiplexed signals with spectral shaping and subject to fiber non-linearity with minimum channel spacing of 37.5GHz.

A non-intrusive OSNR measurement technique relying on the detailed spectral comparison of an optical signal with its "noise-free" spectrum is described, including mathematical basis, validity conditions and algorithmic steps. The technique's performance is experimentally demonstrated with 100G PM-QPSK and 200G PM-16QAM signals subject to fiber non-linearity induced by 100G PM-QPSK and 10G NRZ-OOK neighbors. The OSNR measurement performance is also demonstrated when root-raised cosine spectral shaping is applied to the signals, with channel spacings of 50GHz and 37.5GHz. Experimental results for OSNR levels up to 30dB and launch powers up to 3dB above the optimum BER launch conditions are shown for different system and signal configurations.

Relaxed laser control of uncooled multiple‐input and multiple‐output dense wavelength‐division multiplexing systems for low power consumption data communication links

A simulation-based assessment is made of the benefits of using advanced modulation formats including pulse amplitude modulation (PAM) and carrierless amplitude/phase modulation (CAP) to ease laser control requirements of a recently proposed novel uncooled multiple-input and multiple-output wavelength-division multiplexing (WDM) system. At a per channel bit rate of 25 Gb/s, the spectral efficiency improvements inherent to these schemes provide additional optical system power margin over non-return-to-zero (NRZ) modulation (0.4 dB for PAM-4 and 4.4 dB for CAP-16) for a link distance of 20 km. This allows the minimum channel spacing between channels to be reduced from 0.4 nm (NRZ) to 0.3 nm for PAM-4 and to 0.14 nm for CAP-16, this in turn allowing for a more stable operating region to be realised. Compared with a traditional 16 × 25 Gb/s dense WDM system, a power consumption saving of 30% can be realised using NRZ whereas PAM-4 and CAP-16 offer 29 and 27% savings, respectively.

Closely spaced dual-wavelength fiber laser using an ultranarrow bandwidth optical filter for low radio frequency generation

A dual-wavelength (DW) fiber laser with a closely spaced single longitudinal mode (SLM) output is proposed and demonstrated. The proposed fiber laser utilizes a conventional fiber Bragg grating with a center wavelength of about 1546.8nm in conjunction with an ultranarrow bandwidth tunable optical filter to generate the desired DW SLM output. Observations with a very high resolution optical spectrum analyzer, which was capable of achieving resolutions up to 0.16pm, revealed detailed spectral characteristics not characteristically seen before. A channel spacing of up to 58nm was realized, and spacing as small as 2pm was achieved. The minimum channel spacing and its resulting beat frequency are the narrowest observed yet to the best of our knowledge for a DW fiber laser at room temperature.

Optimization of Super Dense WDM systems for capacity enhancement

We investigate the performance of the Super Dense Wavelength Division Multiplexed (SDWDM) systems with high spectral efficiency and narrow spacing of the channels and optimization in terms of bit rate up to 15Gbps, channel spacing as low as 12.5GHz, number of channels up to 64 and repeater less transmission distance up to 100km and report high capacity SDWDM systems. We demonstrate the minimal allowed channel spacing and provide recommendations for future SDWDM solutions. The simulation results have shown that the minimum channel spacing for 15Gbps, 32 channel system need to be not less than 0.35nm and that for a 10Gbps system it should be not less than 0.25nm. The 5Gbps system gives acceptable results at spacing of 0.1nm for maximum up to single span of 80km.

Radiative Effects on Mixed Convection in a Uniformly Heated Vertical Convergent Channel with an Unheated Moving Plate

AbstractFluids engineering is extremely important in a wide variety of materials processing systems, such as soldering, welding, extrusion of plastics and other polymeric materials, Chemical Vapor Deposition (CVD), composite materials manufacturing. In particular, mixed convection due to moving surfaces is very important in these applications. Mixed convection in a channel, as a result of buoyancy and motion of one of its walls has received little research attention and few guidelines are available for choosing the best performing channel configuration, particularly when radiative effects are significant. In this study a numerical investigation of the effect of radiation on mixed convection in air due to the interaction between a buoyancy flow and an unheated moving plate induced flow in a uniformly heated convergent vertical channel is carried out. The moving plate has a constant velocity and moves in the buoyancy force direction. The principal walls of the channel are heated at uniform heat flux. The numerical analysis is accomplished by means of the commercial code Fluent. The effects of the wall emissivity, the minimum channel spacing, the converging angle and the moving plate velocity are investigated and results in terms of air velocity and temperature fields inside the channel and wall temperature profiles, both of the moving and the heated plates, are given. Nusselt numbers, both accounting and not for the radiative contribution to heat removal, are also presented.

Correlations for Natural Convection in Vertical Convergent Channels With Conductive Walls and Radiative Effects

Natural convection in air, in vertical convergent channels, is analyzed to carry out thermal design and optimization criteria. A scale analysis is developed to estimate the optimal geometrical configuration in terms of total volume and average wall temperature. The best geometrical configuration obtained by this analysis is the parallel-plates channel. New correlations for mass flow rate, radiative heat flux, and dimensionless maximum wall temperature are proposed in the emissivity range from 0.10 to 0.90, convergence angle ranging from 0° to 10°, ratio between minimum and maximum channel spacing in the range from 0.048 to 1.0, aspect ratio, the ratio between wall length and minimum channel spacing, in the range from 10 to 58, and average channel Rayleigh number in the range from 5.0 to 2.3 × 105. For the same convergence angle and ratio between minimum and maximum channel spacing ranges, new average Nusselt number correlations are also given. These correlations are evaluated for emissivity value equal to 0.90, for aspect ratio, referred to the minimum channel spacing, ranging from 10 to 80 and average channel Rayleigh number ranging from 2.5 × 10−2 to 2.3 × 105.

Radiative effects on natural convection in vertical convergent channels

Natural convection in air, in a convergent channel, uniformly heated at the principal walls, is experimentally investigated, in order to analyze the effects of the radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls inclination angle, the spacing between the walls, the heat flux, are given for two values of the wall emissivity. Flow visualization is carried out to show the peculiar pattern of the flow between the plates in several configurations. The comparison between two wall emissivity values, 0.10 and 0.90, shows that the effect of thermal radiation is more pronounced for larger convergence angles. For a wall emissivity equal to 0.90 and for small values of the minimum channel spacing, heat transfer in slightly convergent vertical channels is stronger than in a vertical parallel channel. Flow visualization points out a recirculating zone in the upper part of the channel for small values of the minimum channel spacing and for converging angles equal to 5° and 10°. Nusselt numbers and dimensionless maximum temperatures are then evaluated and correlated to the Rayleigh number, in the investigated range from 5 to 5 × 10 8 and 0° ⩽ θ ⩽ 10°. A very good agreement between experimental data and correlations is observed for the dimensionless parameters based on the maximum channel spacing. Comparisons between experimental and numerical data are also performed and a good relationship is observed.

Optimization of fractal-like branching microchannel heat sinks for single-phase flows

Fractal-like branching flow networks in disk-shaped heat sinks are numerically optimized to minimize pressure drop and flow power. Optimization was performed using a direct numerical search, gradient-based optimization, and genetic algorithm. A previously validated one-dimensional pressure drop and heat transfer model, with water as the working fluid, is employed as the objective function. Geometric constraints based on fabrication limitations are considered, and the optimization methodology is compared with results from a direct numerical search and a genetic algorithm. The geometric parameters that define an optimal flow network include the length scale ratio, width scale ratio, and terminal channel width. Along with disk radius, these parameters influence the number of branch levels and number of channels attached to the inlet plenum. The geometric characteristics of the optimized flow networks are studied as a function of disk radius, applied heat flux, and maximum allowable wall temperature. A maximum inlet plenum radius, minimum interior channel spacing, and ranges of terminal channel widths and periphery channel spacing are specified geometric constraints. In general, all geometric constraints and the heat flux have a significant influence on the design of an optimal flow network. Results from a purely geometrically derived network design are shown to perform within 15% of the direct search and gradient-based optimized configurations.

WDM Performance and PMD Tolerance of a Coherent 40-Gbit/s Dual-Polarization QPSK Transceiver

We report the measured wavelength-division-multiplexing (WDM) performance and polarization-mode dispersion (PMD) tolerance of a coherent 40-Gbit/s dual-polarization quadrature phase shift keying (DP-QPSK) transceiver at 50-GHz minimum channel spacing in a 40-channel 40-span test bed comprised of 3200 km of uncompensated G.652 fiber. We also evaluate the impact of polarization-dependent loss (PDL) on system performance and present the measured tolerance to frequency misalignment between the transmitter and a multiplexer/demultiplexer filter pair.

Comparative analysis of the traffic performance of fiber-impairment limited WDM and hybrid OCDM/WDM networks

Optical code-division multiplexing (OCDM) is a technique that is currently generating considerable research interest. This paper analyzes and compares the traffic performance of wavelength-division multiplexing (WDM) and hybrid OCDM/WDM-based optical networks. The analysis considers the influence of the limitations of fiber-induced signal impairments on traffic performance and comparisons are performed for an example network utilizing different standardized fiber types. Furthermore, comparisons of traffic performance are also made between different lightpath schemes used in WDM and OCDM/WDM networks. The analysis results show that the OCDM/WDM lightpath schemes significantly outperform the WDM lightpath schemes for given blocking probability criteria. Moreover, the analysis indicates that fiber nonlinearity (which limits the minimum channel spacing) affects the traffic performance more severely compared to fiber dispersion (limits code cardinality).

Comparison of Nonlinear Pulse Interactions in 160-Gb/s Quasi-Linear and Dispersion Managed Soliton Systems

The understanding and development of 160-Gb/s transmission systems requires the study of the impact of different dispersion compensation schemes on pulse propagation in nonlinear fiber. In this paper, we present an investigation of 160-Gb/s optical transmission systems, focusing on optimal propagation regimes, and in particular, we analyze different transmission limitations and dominant nonlinear effects by comparing quasi-linear and dispersion managed soliton systems. Two quasi-linear systems, one using nonzero dispersion-shifted fiber (NZDSF) and the other single-mode fiber (SMF), and one short-period (1 km) dispersion managed soliton (DMS) system are studied, both for single-channel and wavelength-division-multiplexed (WDM) transmission. First, the performance of the two quasi-linear systems in single-channel transmission are compared and it is shown that the NZDSF and SMF systems allow similar error-free transmission distances with only small differences in the intrachannel four-wave mixing (IFWM) induced amplitude jitter. The effect of pulsewidth on transmission performance in this regime was investigated and the use of shorter pulses was found to result in lower amplitude jitter. We analyzed the behavior of the DMS system and showed that the reduced pulse broadening during transmission allowed a significantly longer single-channel transmission distance with a smaller impact of nonlinearities compared to quasi-linear propagation. The sensitivity of the DMS system performance to statistical fluctuations in the fiber dispersion was studied and the results show the level of accuracy in the dispersion management map which must be ensured in these systems. Finally, the performance of the DMS in WDM transmission was investigated and it was found that it was subject to very large penalties increasing the minimum channel spacing possible because of the strong impact of interchannel cross-phase modulation (XPM).

Phase-Domain Fractional-<tex>$ N $</tex>Frequency Synthesizers

The concept of phase-domain fractional-N frequency synthesis is presented. Synthesizers using this architecture can achieve fast frequency switching without limiting the minimum channel spacing. In this architecture, a numerical phase comparator is used in conjunction with weighting coefficients, as a linear weighted phase-frequency detector. The synthesizer output spur level is determined by two factors. Namely, the delay of the numerical phase comparator, and the accuracy of the digital-to-analog convertor (DAC) used to convert the phase error to the analog domain. A novel second-order timing-error cancelation scheme is proposed to eliminate the effect of the phase comparator delays. Using this technique together with a 10-bit accuracy DAC, a maximum spur level of less than -65 dBc is simulated for a 900-MHz synthesizer. The settling time of the simulated synthesizer is less than 7 /spl mu/s, and is independent of the channel spacing. The details of the synthesizer architecture, design considerations, and system-level simulations are presented. Implementation issues including the DAC accuracy and timing-error effects are discussed extensively throughout the text.

Broad-band Dynamic Dispersion Compensation in Nonlinear Fiber-Based Device

In this paper we report on the design, numerical simulation and experimental testing of a novel dynamic dispersion compensation device based on self-phase modulation (SPM) in nonlinear fiber. The proposed all-fiber device is inherently simple and presents several unique advantages, most notably the potential for a broad-band operation covering all wave-length division multiplexing (WDM) channels of a system and the ability to address variable amounts of residual dispersion in each individual channel. Dynamic compensation ranges of up to 140 ps/nm for a single-stage and 240 ps/nm for a two-stage device are demonstrated with 40 Gb/s CS-RZ signal. It is shown that the device can operate with a minimum channel spacing of 200 GHz. For a two-stage device with inter-stage spectral filtering, simultaneous dynamic dispersion compensation (130 ps/nm for 1 dB penalty) and 2R regeneration (2 dB receiver sensitivity improvement) are demonstrated.

Performance characteristics of a WDM channel monitor based on an all-fiber AOTF with an on-fiber photodetector

We demonstrated an all-fiber wavelength-division-multiplexing channel monitor based on a narrow-band acoustooptic tunable filter (AOTF) with a built-in on-fiber photodetector. By employing a frequency-shift keying (FSK) modulation scheme, intrinsic wavelength accuracy of 0.02 nm is obtained for single-channel detection and 0.05 nm for multiple-channel detection. The minimum resolvable channel spacing is 1.0 nm. The measured channel power accuracy is <1 dB. The demonstrated channel monitor showed a polarization-dependent wavelength shift of /spl sim/0.15 nm, which is limited by the polarization mode splitting of the AOTF itself. The influence of FSK frequency spacing and the effect of channel crosstalk are also discussed.

On the use of optical spreading codes to design DWDM systems with reduced FWM crosstalk

Four-wave mixing (FWM) is a fiber nonlinear effect that limits the maximum reach and minimum channel spacing of a dense wavelength-division multiplexing (DWDM) communication system. We propose an alternative frequency assignment scheme to reduce FWM crosstalk by allocating frequencies to a set of unevenly spaced channels determined by an optical spreading code. This assignment scheme provides several advantages over previously proposed frequency assignment schemes, these include, a reduced optical bandwidth requirements, ease of computing, unlimited channel counts and the complete avoidance of FWM intrachannel crosstalk. Further analysis of the spreading code-based scheme carried out on a 50 GHz spaced 16×2.5 Gbit/s DWDM system, indicates a consistent improvement in Q-factor by over 2 dB compared to similar equal spaced channel DWDM systems.

Design of an efficient channel block retuning

Cellular networks must be updated very often. Due to technical and economical reasons, the complete channel retuning of an urban network has to be done in several steps. The objective is then to define the steps in such a way that the increase in the interference level is minimum, and do not let it go further than a given threshold of minimum quality for the cellular network. We propose a greedy heuristic and an ascent---descent method including a Tabu Search module to retune a network in a given number of steps. All constraints of the channel assignment problem are taken into account (co-channel, adjacent channel, minimum channel spacing on antennas) as well as retuning constraints such as the number of steps and limits on the maximum number of cells which can be retuned at each step. Co-channel and adjacent channel constraints are expressed through compatibility matrices produced with a discretization of the signal-to-interference ratio. Numerical results are presented on a Bell Mobilite urban network of 359 cells.