Wavelength Division Multiplexing Channels Research Articles

4$\,\times\,$44 Gb/s Packet-Level Switching in a Second-Order Microring Switch

We demonstrate simultaneous switching of wavelength-division-multiplexed (WDM) data consisting of four 44-Gb/s channels (176 Gb/s total) through an electro-optically active second-order microring switch with a 0.7-ns rise and a 3.4-ns fall time. The higher order microring device allows fast simultaneous switching of multiple high data rate WDM channels. We verify the correct active switching operation and low resultant power penalties on both switch output ports. The ability to switch multiple high data rate channels simultaneously at high speed with low power consumption makes higher order ring switches attractive components for silicon photonic switching fabrics.

Influence of guard band on performance of absolute polar duty cycle division multiplexing over a single wavelength and wavelength division multiplexing system

For the first time to the best of our knowledge the effect of guard band (GB) on the performance of 40Gb/s Absolute Polar Duty Cycle Division Multiplexing (AP-DCDM) over a single wavelength and wavelength division multiplexing (WDM) are investigated and reported. It is demonstrated that the spectral width occupied by 40Gb/s AP-DCDM with GB is 100GHz (with minimum spectral efficiency of 0.4b/s/Hz) whereas, this value can be reduced to around 80GHz for AP-DCDM without GB (with minimum spectral efficiency of 0.5b/s/Hz). In addition to better spectral efficiency, this amount of saving in the spectral width leads to ∼60ps/nm improvement in chromatic dispersion tolerance. In this paper, characteristics of AP-DCDM with and without GB over WDM system are compared at the speed of 40Gbit/s per WDM channel, for the tolerance to narrow optical filtering and minimum allowed channel spacing. The AP-DCDM without GB has narrower spectral width than AP-DCDM with GB, which makes its implementation in WDM system advantageous.

Performance Comparisons of DP-16QAM and Duobinary-Shaped DP-QPSK for Optical Systems With 4.1 Bit/s/Hz Spectral Efficiency

This paper, for the first time, experimentally presents direct comparisons of two 25 GHz spaced wavelength division multiplexing (WDM) systems using conventional dual-polarization (DP) 16-ary quadrature amplitude modulation and duobinary-shaped DP-quadrature phase shift keying (QPSK) modulation. Both systems operate at the same bit rate per channel of 112 Gbit/s, yielding a spectral efficiency of 4.1 bit/s/Hz. The comparisons are conducted for three different cases, i.e., the back-to-back sensitivity, the nonlinear tolerance over a 640-km standard single-mode fiber link, and the phase-noise tolerance (by means of simulations). The results show that the duobinary-shaped DP-QPSK system not only provides a 3.4 dB superior back-to-back sensitivity, but also exhibits a 3 dB higher tolerance against nonlinear impairments after 640 km transmission with three WDM channels. In addition, the numerical results indicate that both investigated systems provide similar tolerances to the laser phase noise given that the block length used in the carrier phase estimation is optimized.

In-Line Polarization-Insensitive Parametric Tunable Dispersion Compensator for WDM Signals

We develop frequency-preserving parametric tunable dispersion compensator (P-TDC) with polarization diversity for simultaneous dispersion compensation of wavelength division multiplexing channels. The in-line polarization-insensitive P-TDC is composed of cascaded parametric tunable frequency converters based on degenerate four-wave mixing process in polarization-maintaining highly nonlinear fibers with a low dispersion slope. Dispersion-managed error-free transmission of four channels of 43 Gb/s nonreturn-to-zero ON-OFF keying signal is successfully demonstrated over 126 km dispersion-shifted fiber with the in-line polarization-insensitive P-TDC.

Optimal Design of Dispersion Filter for Time-Domain Split-Step Simulation of Pulse Propagation in Optical Fiber

The nonlinear Schrodinger equation can be solved by split-step methods, where in each step, linear dispersion and nonlinear effects are treated separately. This paper considers the optimal design of an FIR filter as the time-domain implementation for the linear part. The objective is to minimize the integral of the squared error between the FIR frequency response and the desired dispersion characteristics over the band of interest. This least square (LS) problem is solved in two approaches: the normal equation approach gives the explicit solution, whereas the singular value decomposition approach, which is based on the theory of discrete prolate spheroidal sequences, provides geometrical insights and reveals that the normal equation could be ill-conditioned. In addition, the frequency response might exhibit singular behaviors such as overshoot. We propose two filters that both can mitigate these shortcomings: the regularized LS filter achieves this by adding a regularization term to the objective function; the quadratically constrained quadratic programming-based filter addresses overshooting more efficiently by imposing a maximum magnitude constraint on the frequency response. Numerical results show that these filters can suppress the overshoots, control the squared error, reduce the filter length and lower the computational complexity. For both single channel and wavelength-division multiplexing channels, the proposed methods generate similar outputs as the standard split-step Fourier method.

Performance improvement of on-off-keying free‑space optical transmission systems by a co‑propagating reference continuous wave light

In this paper, we analytically investigate an optical signal detection scheme to mitigate the scintillation effect with the assistance of a co-propagating reference continuous wave (CW) light. Using the correlation coefficient between the intensities of the data light and the reference CW light, we mathematically derive their joint intensity distributions under two widely used atmospheric turbulence channel models, namely log-normal distributed channel model and Gamma-Gamma distributed channel model, respectively. We also carry out the Monte-Carlo (MC) simulation and show that theoretical results agree with simulation results well. Our analytical results reveal that when the correlation coefficient is 0.99, the power reductions to achieve BER of 10⁻³ are 12.3 dB and 20.4 dB under moderate and strong atmospheric turbulence conditions (i.e., Rytov variances of 1.0 and 4.0), respectively. In addition, the feasibility of the scheme applied to wavelength-division-multiplexed (WDM) free-space-optical (FSO) transmission systems is also investigated, where only a single reference CW light could be used to mitigate the scintillation effects on all WDM channels.

Reconfigurable All-Optical OTDM-to-WDM Conversion Using a Multiwavelength Ultrashort Pulse Source Based on Raman Compression

We propose and demonstrate a reconfigurable all-optical format conversion from optical time-division multiplexing (OTDM) to wavelength division multiplexing (WDM) providing full flexibility in terms of channel spacing and wavelength allocation. The system consists of a Raman amplification-based multiwavelength pulse compressor and a parametric sampling gate. Four conventionally generated WDM 10 GHz pulse trains are compressed at the same time to around 2.5 ps and, then, used for multiwavelength sampling of a 40 Gb/s 3.49 ps OTDM signal. Simultaneous extraction of all four 10 Gb/s tributaries to WDM channels in WDM grid with tunable channel spacing is demonstrated. Controlling time delays among the WDM clock channels makes it flexible to map OTDM tributaries onto any desired wavelength order, which is also called wavelength- and time-selective function. The reconfigurable functions can be realized independently on the coming OTDM signal by simply setting corresponding parameters of the WDM synchronous pulse trains at the terminals. This is also the key for the good overall performance obtained for all demultiplexed channels. Low power penalties below 1.9 and 2.1 dB are achieved with small penalty variations, 0.3 and 0.5 dB, among channels as the OTDM tributaries are converted to WDM grid with spacing of 400 and 200 GHz, respectively.

45.8 and 125 Gb/s CP-QPSK/CP-BPSK Field Trial Over Installed Submarine Cable

We show successful transmission of 45.8 and 125 Gb/s coherent-detected polarization-multiplexed quadrature phase-shift keying (CP-QPSK) over both 2054 and 4108 km of 10 Gb/s nonreturn-to-zero-optimized field-deployed submarine cable. Moreover, we report successful transmission of 45.8 Gb/s coherent-detected polarization-multiplexed binary phase-shift keying (CP-BPSK) over 9420 km of legacy submarine fiber. We present single-channel transmission results, as well as wavelength-division multiplexed (WDM) transmission results with copropagating WDM channels placed on a 100 GHz and on a 50 GHz grid, where we optimized the transmission performance by sweeping the launch power and precompensation. For 45.8 Gb/s CP-QPSK, a margin with respect to the forward error correction (FEC) limit of 1.7 dBQ was obtained after transmitting over a 4108 km distance in a 50 GHz grid WDM configuration. Considering the transmission of 125 Gb/s CP-QPSK over the same distance and with the same channel configuration, a 1.1 dBQ margin was measured, showing the feasibility of a tenfold increase in transmission capacity over legacy submarine fiber. This significantly delays the costly alternative of deploying a new submarine cable. For the transmission of a 45.8 Gb/s CP-BPSK modulated signal on a 50 GHz WDM grid over a 9420 km distance, a margin of 1.2 dBQ with respect to the FEC limit is reported. This underlines the great robustness of CP-BPSK toward nonlinear fiber impairments and thereby the feasibility to cross transoceanic distances with this modulation format.

High-speed wavelength-division multiplexing quantum key distribution system

A high-speed quantum key distribution system was developed with the wavelength-division multiplexing (WDM) technique and dedicated key distillation hardware engines. Two interferometers for encoding and decoding are shared over eight wavelengths to reduce the system's size, cost, and control complexity. The key distillation engines can process a huge amount of data from the WDM channels by using a 1 Mbit block in real time. We demonstrated a three-channel WDM system that simultaneously uses avalanche photodiodes and superconducting single-photon detectors. We achieved 12 h continuous key generation with a secure key rate of 208 kilobits per second through a 45 km field fiber with 14.5 dB loss.

Weighted Regularized Sparse Recovery Method for Optical Power Monitoring

Monitoring optical power from the broadened optical spectrum acquired by scanning a low-cost tunable optical filter (TOF) is a practical and challenging task for the management of wavelength-division-multiplexing (WDM) networks. Existing methods, such as least squares (LS) and regularized sparse recovery (RSR) methods, suffer from performance deterioration when the power discrepancies between adjacent channels are significant. In this letter, we formulate it as a maximum a posteriori (MAP) estimation problem under Bayesian framework. A weighted-RSR algorithm is, therefore, proposed to estimate the optimal channel power. Experimental results demonstrate that, using commercial low-cost TOFs, the weighted-RSR can accurately monitor the power of mixed 10-G/40-G WDM channels with 50-GHz channel spacing and 10-dB power difference.

Dispersion-Free SOI Interleaver for DWDM Applications

Integrated optical asymmetric Mach-Zehnder interferometer-based dense wavelength division multiplexing (DWDM) channel interleaver structures (2 × 2) have been theoretically studied using various single-mode waveguide geometries in silicon-on-insulator platform. Subsequently, a dispersion-free interleaver design has been proposed, fabricated, and characterized. It was designed to separate alternate DWDM channels with 100 GHz spacings. Some of the fabricated prototype devices have been fiber pigtailed and packaged in a suitably designed metal housing. The transmitted spectra (1520 nm <; λ <; 1600 nm) at each of the output ports reveal that alternate DWDM channel wavelengths are separated with a uniform inter-channel extinction of ~ 18 dB. Each of the channel passbands shows their 3-dB bandwidth of ~ 40 GHz.

Spectral Efficiencies of Channel-Interleaved Bidirectional and Unidirectional Ultradense WDM for Metro Applications

We measure spectral efficiencies of ultra-dense (UD) wavelength-division multiplexing (WDM) channels for metro applications. We change the channel spacing in 500-MHz step using a beat-frequency-locking method. Applying both a polarization control (PC) and a forward-error correction (FEC), we achieve spectral efficiencies as high as 1.54 and 0.91 b/s/Hz in back-to-back operations for channel-interleaved (CI) bidirectional and unidirectional UD-WDM, respectively. If we use the PC and the FEC separately for the CI-bidirectional UD-WDM, the PC enhances the spectral efficiency over the FEC by 4.6%-21.2% as the transmission distance is increased.

Locating Regions of Polarization Activity in WDM Systems

We propose a method for determining the spatial position of fast polarization changes in a WDM system. In particular, we demonstrate that the PMD (polarization mode dispersion) vector and Stokes parameters for each WDM (wavelength division multiplexing) channel can be employed to find with an accuracy, shown to be less than 15% in most simulated cases, the position of the sources of rapid polarization fluctuations in a link with a mean DGD (differential group delay) greater than 15 ps. The technique can also be applied to locate hinge positions provided that only a single hinge is active at any given time.

A millimeter-wave WDM-ROF system based on supercontinuum technique

In this paper, a new millimeter-wave (mm-wave) wavelength division multiplexing (WDM) system based on radio-over-fiber (ROF) technology is proposed. In this approach a multi-wavelength light source is obtained by supercontinuum (SC) technique, and mm-wave signals are obtained by using optical heterodyning method. We experimentally demonstrate the generation of optical carriers for 6-WDM channels, obtain 40 GHz mm-wave signals by employing optical heterodyne technique, and successfully achieve low error rate transmission of 2.5 Gbit/s in WDM channels over a distance of 25 km in a G.652 fiber. The experimental results verify that the proposed solution is feasible and cost effective.

Empirical modeling and simulation of phase noise in long-haul coherent optical transmission systems

An empirical phase noise channel model suitable for performance evaluation of high spectrally efficient modulations in 100G long-haul coherent optical transmission systems using polarization-division multiplexed and wavelength-division multiplexing channels is presented. The derivation of the model is worked out by exploiting the similarity between the power spectral density of the carrier extracted from the analysis of propagation measurements and the Lorentzian spectrum that is usually adopted to describe instabilities of semiconductor lasers. The proposed channel model is characterized by only two parameters: the linewidth of the carrier and the signal-to-noise ratio. We show that in the case of quadrature phase-shift keying transmission a good agreement exists between quantitative measures of performance extracted by processing experimental data and those obtained from simulations based on the use of the empirical model.

A long-reach WDM passive optical network enabling broadcasting service with centralized light source

We propose a long-reach wavelength-division-multiplexed (WDM) passive optical network (PON) to provide conventional point-to-point (P2P) data and downstream broadcasting service simultaneously by superimposing, for each WDM channel, the differential-phase-shift-keying (DPSK) broadcasting signal with the subcarrier multiplexing (SCM) modulated downstream P2P signal, at the optical line terminal (OLT). In the optical network units (ONUs), by re-modulating part of the downstream signal with a reflective semiconductor optical amplifier (RSOA), we realize color-less ONUs for upstream data transmission. The proposed scheme is numerically verified with a 5 Gb/s downstream P2P signal and broadcasting services, as well as 2.5 Gb/s upstream data through a 60 km bidirectional fiber link. In particular, the influence of the downstream lightwave's optical carrier–subcarrier ratio (OCSR) on the system performance is also investigated.

U-Band Wavelength References Based on Photonic Bandgap Fiber Technology

In this paper, we surveyed potential wavelength references in U-band wavelength region. The hollow-core photonic bandgap fiber (PBGF) filled with methane (CH4) gas is used as a gas cell. The experimental results clearly indicate that the weak absorption lines of methane can be used as U-band wavelength division multiplexing (WDM) channel references due to the long interaction path length provided by PBGF. The absorption lines of CH4 are useful as wavelength references in the 1625-1700 nm region. In addition, we have measured the pressure broadening and the wavelength stability of absorption lines, and find that absorption lines are particularly insensitive to external perturbation. This technique is useful for wavelength calibration of components of communication system, or as monitor the wavelength of the channels. It may be implemented in next-generation WDM communication systems.

Tunable wavelength division multiplexing channel isolation filter based on dual chirped long-period fiber gratings

We propose and demonstrate a wavelength tunable wavelength division multiplexing channel isolation filter based on two concatenated chirped long-period fiber gratings (LPGs). An intergrating space (IGS), deliberately introduced between the two gratings, provides an extra phase difference between the core and cladding modes. Changing this phase by heating the IGS without affecting the gratings tunes the channels. A theoretical account of the filter action is also presented and the results are found to be in excellent agreement with the experiments. Unlike the filters based on normal concatenated chirped LPGs without an IGS, the current filter shows a linear tuning over an increased spectral range.

Transmission of 10-Gb/s Directly Modulated RSOA Signals in Single-Fiber Loopback WDM PONs

We demonstrate the transmission of 10-Gb/s upstream signals over 20-km fiber using a 1.3-GHz-bandwidth reflective semiconductor optical amplifier and a 40-ps delay interferometer (DI) in a loopback configured wavelength-division-multiplexed (WDM) passive optical network. We show that a single DI can be used in a set-and-forget mode to equalize 34 WDM channels anchored at the 100-GHz spaced ITU grid, without any help of postdetection electronic processing.

Optical Solutions to Improve PDM-QPSK Resilience Against Cross-Channel Nonlinearities: A Comparison

We compare by simulation different optical methods to improve the resilience of coherent 112-Gb/s polarization-division multiplexing (PDM)-quadrature phase-shift keying (QPSK) wavelength-division-multiplexing (WDM) transmissions against cross-channel nonlinearities. Such methods consist of 1) increasing the line group velocity dispersion (GVD), or 2) the line polarization-mode dispersion (PMD), or 3) inserting in-line cross-phase modulation (XPM) suppressors. Such methods are tested using nonreturn-to-zero (NRZ), aligned return-to-zero (aRZ), and interleaved RZ (iRZ) pulse formats. We show that the nonlinearity-mitigating effect underlying all three methods is an increase of the interchannel decorrelation, obtained by either increasing walk-off (methods 1 and 3) or by depolarizing the WDM channels (method 2). Interchannel decorrelation improves performance and reduces the difference among the three pulse formats. We find that the best performance is achieved by iRZ-PDM-QPSK in a dispersion-managed link with an XPM suppressor at each span.