Multichannel Wavelength-division Multiplexing Research Articles

Joint linear and nonlinear noise monitoring techniques based on spectrum analysis.

As one of the key indicators of signal quality in fiber communication systems, optical signal-to-noise ratio (OSNR) needs to be accurately monitored to ensure reliable network planning, operation, and reconfiguration. OSNR monitoring techniques considering only accumulated amplified spontaneous emission (ASE) noise are no longer suitable for dispersion unmanaged long-haul and dense wavelength division multiplexing (WDM) systems, where the contribution of fiber nonlinearity to total SNR cannot be neglected. In this paper, we propose a modulation-format-transparent, accurate joint linear and nonlinear noise monitoring scheme based on calculation of correlation between two spectral components at the upper and lower sideband of the signal spectrum. Different characteristics of flat linear noise spectrum and non-flat nonlinear noise spectrum are used to distinguish the influences on the correlation value from both noise sources. Simulation results show that the proposed scheme can accurately monitor SNRlinear and SNRnonlinear within a wide launch power range from -5 dBm to 5 dBm per channel for multi-channel WDM systems with a 915-km single mode fiber (SMF) link. The performance of the proposed scheme is further experimentally verified in up-to-7 channel WDM systems over a 915 km SMF link.

Linear optical sampling technique for simultaneously characterizing WDM signals with a single receiving channel.

Wavelength-division multiplexing (WDM) schemes have greatly increased optical transmission capacity, especially when using massively parallel optical frequency combs (OFCs) as carriers. In this paper, we show that linear optical sampling (LOS) is a kind of multiheterodyne process and it can be performed with arbitrary-shaped periodic waveforms. Meanwhile, it can be used for characterizing WDM signals with only a single receiving channel. We successfully demonstrate an observation of 40 WDM channels with 1 THz total bandwidth and total 800 Gbit/s data rate by only using one sampling channel. We show that the adoption of the concept of multiheterodyne detection can improve the performance of LOS technique, and can also greatly simplify multi-channel WDM monitoring systems.

Investigation of in-band transmission of both spectral amplitude coding/optical code division multiple-access and wavelength division multiplexing signals

The transmission of both optical code division multiple-access (OCDMA) and wavelength division multiplexing (WDM) users on the same band is investigated. Code pulses of spectral amplitude coding (SAC)/optical code division multiple-access (CDMA) are overlaid onto a multichannel WDM system. Notch filters are utilized in order to suppress the WDM interference signals for detection of optical broadband CDMA signals. Modified quadratic congruence (MQC) codes are used as the sig- nature codes for the SAC/OCDMA system. The proposed system is sim- ulated and its performance in terms of both the bit-error rate and Q-factor are determined. In addition, eavesdropper probability of error-free code detection is evaluated. Our results are compared to traditional nonhybrid systems. It is concluded that the proposed hybrid scheme still achieves acceptable performance. In addition, it provides enhanced data confiden- tiality as compared to the scheme with SAC/OCDMA only. It is also shown that the performance of the proposed system is limited by the interference of the WDM signals. Furthermore, the simulation illustrates the trade- off between the performance and confidentiality for authorized users.

Multi-channel WDM based on coupling between waveguide and micro-cavity

Based on the coupling between photonic crystals (PCs) waveguide and micro-cavity, a new method to design wavelength division multiplexing (WDM) is proposed. By changing the position of border dielectric rods of micro-cavity and adopting the output waveguide with a 60° bend, the property of micro-cavity is optimized. The new WDM is designed based on the model. And by modulating the border dielectric rods, different wavelengths can be coupled into the micro-cavity, and then are output from the load waveguide selectively. Only by adding a reflective layer in the bus waveguide, the transmission efficiency of output wavelengths is greatly improved.

Multichannel silicon WDM ring filters fabricated with DUV lithography

We have fabricated 9-channel silicon wavelength-division-multiplexing (WDM) ring filters using 193nm deep-ultraviolet (DUV) lithography and investigated the spectral properties of the ring filters by comparing the transmission spectra with and without an upper cladding. The average channel-spacing of the 9-channel WDM ring filter with a polymeric upper cladding is measured about 1.86nm with the standard deviation of the channel-spacing about 0.34nm. The channel crosstalk is about −30dB, and the minimal drop loss is about 2dB.

Assessment of gain saturation in cascaded semiconductor optical amplifier for high-bit-rate wavelength-division-multiplexing signals over long transmission distance

In this paper, the investigation of 10 channels at 80-Gb/s wavelength-division-multiplexing (WDM) transmission over 910-km single-mode fiber and dispersion compensating fiber using cascaded in-line semiconductor optical amplifiers at a span of 70 km for soliton return zero differential phase-shift keying (RZ-DPSK) modulation format is achieved. With the narrow channel spacing (i.e., 200 GHz), the results of quality is more than 15 dB the received signals after covering 910-km transmission distance without any power drop. The semiconductor optical amplifier (SOA) structure parameters for in-line amplifier is optimized to obtain low cross talk in multichannel WDM systems at nil power penalty with sufficient gain. For the differential gain 200 atto cm 2 and length 750 mm of SOA has minimum SOA-induced cross talk is observed. The impact of optical power received and the Q factor at different differential gain, carrier lifetime, and length has been illustrated. It is found that for a transmission distance of 700 km, with decrease the channel spacing, there is an increase in cross talk among channels, hence the signal quality continues to decrease. A clear eye diagram and a good optical spectrum are observed at a transmission distance of 910 km in a soliton RZ-DPSK system.

Secure transmission over an existing public WDM lightwave network

We develop a spread-spectrum-based approach to security-enhanced transmission over an existing public wavelength-division-multiplexing (WDM) lightwave network. Secure transmission for a dedicated user is achieved by overlaying a covert channel onto a multichannel WDM network. The covert channel is spectrally encoded using the technique of frequency hopping and then temporally spread by a dispersive element, with the resulting average power below the noise floor in the fiber. The presence of the WDM channels provides an ad hoc security expansion and increases the difficulty for an eavesdropper to intercept and decode the secure signal

A Dispersion Management Scheme for Reducing SOA-Induced Crosstalk in WDM Links

The penalty imposed on the multichannel wavelength-division multiplexing (WDM) link employing cascaded semiconductor optical amplifiers (SOAs) caused by the cross-gain modulation (XGM) in the SOAs is analyzed and shown to depend on the dispersion management scheme utilized in the link. Based on the results of the analysis, a novel dispersion management scheme is proposed to prevent the repetition of the bit patterns at each SOA stage. Numerical calculations show that with the proposed dispersion management scheme, the crosstalk is mitigated and the performance is significantly improved.

A Novel Bi-wavelength Method for Accurately Measuring Gain andNoise Characteristics of an Erbium-Doped Fibre Amplifier forMulti-Channel Wavelength Division Multiplexing Transmission

Gain and noise figure (NF) are the most important two parameters of anerbium-doped fibre amplifier (EDFA) for a multi-channel wavelengthdivision multiplexing (WDM) transmission system. A simple bi-wavelengthmethod for accurate gain and NF spectrum measurement of EDFA for WDMapplications is proposed. A saturating input signal, whose power equalsto the sum of all the WDM signal power and whose wavelength isdetermined by the channel numbers, and the wavelength of the WDM inputsignals saturates the EDFA in a degree as the same as the WDM signalsinput. Meanwhile, a small power probe signal scans and measures thegain and the NF value at every wavelength of the WDM input signals.Investigative results by numerical simulation show that the gain andthe NF spectra measured by this method have good agreement with thereal spectra of the WDM signal input in a large total input powerrange. The maximum errors of the gain and the NF are less than 0.2 dBand 0.16 dB, respectively, for a 50-channel input case. The method iscompetent for the accurate gain and the NF spectrum measurement of thefibre preamplifier and the line-amplifier for WDM applications and hasthe advantages of simplicity, convenience and easy implement.

Reducing crosstalk and signal distortion in wavelength-division multiplexing by increasing carrier lifetimes in semiconductor optical amplifiers

A simple theory for the crosstalk in multichannel wavelength-division multiplexing (WDM) communication systems caused by the cross gain saturation in semiconductor optical amplifier (SOA) is developed. It is shown that increasing the carrier recombination lifetime while reducing the differential gain in the SOA greatly mitigates the crosstalk. An SOA using type-II quantum wells with reduced recombination probability is designed, and its characteristics are numerically modeled for a number of different WDM systems. It is shown that when used as a preamplifier or in local/metro networks with a small number of amplifiers, error-free transmission can be achieved with the type-II SOA.

Spectra of dual overwritten fiber Bragg grating

The perturbation to the refractive index induced by dual overwritten fiber Bragg grating (FBG) is analyzed, and the coupled-mode equations are obtained. The spectral characteristics that can be achieved in dual overwritten fiber gratings are investigated by numerical simulation. It is found that the spectral characteristics are affected significantly by period difference ΔΛ, primary phase difference Δφ and grating length L. Moreover, multiwavelength reflection and strong second harmonics will occur in dual written FBG when the nonlinear UV-induced index perturbation exists, which is very useful for multichannel wavelength-division multiplexing. The results concluded in this paper are conducive to the fabrication and applications of dual overwritten FBGs.

Fabrication of a multichannel wavelength-division multiplexing–passive optical net demultiplexer with arrayed-waveguide gratings and diffractive optical elements

A novel, to our knowledge, integrated wavelength-division multiplexing-passive optical net demultiplexer that uses an arrayed-waveguide grating and diffractive optical elements is presented. The demultiplexer is used to distribute 1.3-microm wavelength signals and to multiplex an eight-channel wavelength-division multiplexer spectrum at a 1.55-microm wavelength. The device shows high functionality and good optical performance. The measured cross talk was less than -21 dB, and the 3-dB bandwidth was determined to be 97 GHz, which is close to the theoretical value of 93 GHz. Average losses of 4.5 and 8 dB were measured for the 1.3- and the 1.55-microm signals, respectively.

Spectrum-sliced fiber amplifier light source for multichannel WDM applications

A potentially inexpensive light source for multichannel WDM applications is proposed. The high-power amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA), which is already in the single-mode fiber, can be efficiently divided into many channels by using an integrated optic wavelength-division-multiplexing (WDM) demultiplexer. This spectrum-sliced ASE can be used as light sources for WDM systems in place of several wavelength-selected DFB lasers. To demonstrate the principle, the 40-nm-wide ASE spectrum of an EDFA was sliced using a narrow optical filter (3-dB bandwidth: 1.3 nm), and the resulting source was used for the transmission of up to 1.7 Gb/s of data. The problem of spontaneous-spontaneous beat noise in these sources is dealt with. It is estimated that the total capacity would be about 40 Gb/s, realistically, since the channel spacing should be at least three times the optical bandwidth of each channel to avoid crosstalk.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The impact of optical amplifiers on long-distance lightwave telecommunications

The potential of erbium-doped fiber amplifiers (EDFA) and wavelength-division multiplexing (WDM) technology for expanding transmission capacity in long-distance telecommunications is examined. Properties of EDFA are nearly ideal for application in lightwave long-haul transmission. Nonlinear effects in the transmission fiber and amplifier spontaneous emission noise limit the performance and therefore dictate the design of long-distance amplified systems, especially those employing WDM. The next-generation transoceanic system will use EDFA as repeaters, yielding a capacity almost ten times larger than what is available today. Multichannel WDM soliton transmission promises further substantial enhancement. Terrestrial long-haul networks will also benefit greatly from amplified WDM systems designed to mine the large inherent bandwidth in the embedded fiber. The ten- to fifty-fold capacity increase over present systems not only will provide for ample growth, but also will enable network operators to enhance operational flexibility and network functionality, and to facilitate a fast-recovery self-healing capability through cost-effective redundant routing. >

The analysis of crosstalk in multichannel wavelength division multiplexed optical transmission systems and its impact on multiplexer design

An analysis of crosstalk in multichannel wavelength-division multiplexing (WDM) systems is presented, deriving new expressions for system degradations and taking into account both receiver design and the practical limitations of regenerator electronics. This theoretical study is corroborated by measured results obtained on a commercially available digital transmission system and is then used to determine the optimum multiplexer design for multichannel wavelength division multiplexed systems. From this study, the optimum multiplexer design for giving acceptable transmission performance is determined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

WDM applications in broadband telecommunication networks

Network applications of multichannel wavelength-division multiplexing (WDM) techniques are considered. The photonic technologies that will be critical to future systems using WDM are reviewed. Both interoffice and subscriber loop architectures that utilize the unique properties of WDM to achieve important advantages over more traditional network designs are presented. Several experimental demonstrations that illustrate how these architectures can be realized with currently available components are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Technology and system issues for the WDM-based fiber loop architecture

A fiber-optic subscriber loop architecture employing multichannel wavelength-division multiplexing (WDM) techniques is discussed. The architecture, which is called the passive photonic loop (PPL), eliminates the need for outside-plant multiplexing and routing electronics while maintaining complete compatibility with broadband ISDN. An experimental demonstration of the PPL employing 32-channel WDM and a combination of distributed-feedback (DFB) laser and light emitting diode (LED) transmitters is described. A family of alternative PPL implementations is identified. Tradeoffs between these alternatives are explored, and related hardware issues such as wavelength alignment and WDM-component temperature sensitivity are discussed. An economic analysis comparing the PPL with the use of dedicated fiber between the central office and customer premises is presented. The analysis predicts that the PPL can achieve a cost advantage for at least 80 to 90% of the subscribers served in today's network, given DFB-laser and dense-WDM-component cost reductions of approximately one order of magnitude from today's unit-quantity prices. >