Extending transmission distance of high-density WDM systems usingpost transmitter fiber Raman amplifiers

The authors investigate the behavior of multichannel signals in a high-density wavelength division multiplexing (WDM) system with the consideration of stimulated Raman scattering. A parameter to evaluate the depth of Raman crosstalk is provided, and the transmission distance limitation imposed by Raman crosstalk is analyzed. To extend the system transmission distance, a post-transmitter fiber Raman amplifier is used. The allocation of signal channels on the pump gain profile is discussed. It is found that there exists an optimum pump power which results in a maximum amplifier gain. The example shows that the transmission distance limitation imposed by Raman crosstalk can be extended by 20 km for a 30-channel system with 0.25-dB/km fiber loss     

Signal light amplification by stimulated Raman scattering in anN-channel WDM optical fiber communication system

The use of forward and backward Raman amplification in an N -channel wavelength-division-multiplexing (WDM) optical-fiber communication system is analyzed. Analytical expressions for the signals, the pumps, and the amplified spontaneous scattered power (ASSP) are presented. The crosstalk among the signal channels is analyzed in terms of system parameters. It is found that the crosstalk is negligible while low pump power is used and becomes significant if high pump power is used. The signal-to-ASSP ratio can be improved by increasing pump power; however, the improvement is small when pump power is already high. A 300-km repeater spacing with more than 20-dB signal-to-ASSP ratio is calculated     

Statistical properties of stimulated Raman crosstalk in WDM systems

The crosstalk variance of stimulated Raman crosstalk in wavelength-division-multiplexing (WDM) systems is derived analytically in a closed-form formula for all systems with different walkoff length. The probability density function (pdf) of stimulated Raman crosstalk is found to be lognormal distribution (Gaussian distribution in decibel scale). Both power penalty and power limit induced by Raman crosstalk are evaluated and can be applied to single- and multispan WDM systems     

Theoretical investigation of 8 /spl times/ 10-Gb/s WDM signal transmission performance based on gain-equalized SOAs using backward Raman pumping at DCF

We have theoretically investigated 8 /spl times/ 10-Gb/s wavelength-division multiplexing (WDM) signal transmission characteristics based on semiconductor optical amplifiers (SOAs) with equalized gain using discrete Raman amplification (DRA). Gain equalization and low noise figures have been obtained by adjusting the backward Raman pumping power and wavelength at a dispersion compensating fiber (DCF) for each span. Bit-error-rate characteristics were calculated for 8 /spl times/ 10-Gb/s WDM signal transmission over 6 /spl times/ 40-km single-mode fiber (SMF) + DCF links with gain-equalized SOAs using DRAs at DCF. Approximately a 2.5-dB improvement of the receiver sensitivity was achieved by using SOAs and DRAs with optimized Raman pumping. One can easily upgrade the transmission length of a link based on SOAs with an appropriate backward pump laser at each DCF.     

Effects of In-Band Crosstalk in Wavelength-Locked Fabry–PÉrot Laser-Diode-Based WDM PONs

We investigated the impact of in-band crosstalk on the system's performance in wavelength-locked Fabry-Perot laser-diode (FP-LD)-based wavelength-division-multiplexed passive optical networks (WDM PONs). As expected, wavelength-locked FP-LD, which was injected by using an incoherent broadband light source, was more tolerant to in-band crosstalk than distributed-feedback (DFB) laser diode. A 1-dB power penalty in the wavelength-locked FP-LD-based PON system was observed when the crosstalk-to-signal ratio was ~-9 dB. We also compared the measured power penalties with the calculated power penalties. It has been shown that the in-band crosstalk-induced power penalty in wavelength-locked FP-LD-based WDM PON could be estimated properly by taking into account both effects of power addition and signal-crosstalk beat noise.     

Performance limitation due to statistical Raman crosstalk in a WDM system with multiple-wavelength bidirectionally pumped Raman amplification

A general theoretical model of statistical Raman crosstalk and its impact on system performance in a multiwavelength bidirectionally pumped Raman fiber amplifier (RFA) is developed for the first time, where we have taken modulation statistics, dispersion-induced pulse walk-off and signal-induced pump depletion into account. Two kinds of statistical Raman crosstalk, from signal-induced forward-pump depletion and from signal-signal Raman interaction, are included in one model. Formulas for normalized Raman crosstalk, Raman crosstalk-induced relative intensity noise spectral density, and its variance and system performance impact in terms of Q penalty are presented for both a single-span system and a dispersion-compensated multispan wavelength-division-multiplexed (WDM) link. Based on these formulas, we numerically investigate the impact of Raman crosstalk on system performance in a three-wavelengths bidirectionally pumped 40 /spl times/ 40-Gb/s WDM system for various fiber types. In addition, Raman crosstalk in a four-wavelength bidirectionally pumped RFA was experimentally measured. The results agree well with our theory.     

All-fiber WDM optical crossconnect using ultrastrong widely tunableFBGs

We demonstrate an all-fiber wavelength-division-multiplexed optical crossconnect using ultrastrong fiber Bragg gratings (FBGs) with wide tunability. These FBGs have 0.2-nm bandwidths and can be tuned over ~52 nm due to a novel grating preparation technique to reduce mechanical strength degradation. There is negligible distortion in the transmission spectra while tuning the grating, and the crossconnect architecture uses cascades of these gratings for WDM operation. The ability to tune these >98% reflectivity gratings completely out of the entire WDM signal band minimizes the leakage of unwanted signal power and leads to the improvements of >10-dB crosstalk and >0.6-dB power penalty compared with conventional gratings     

Penalty-free 10 Gbit/s single-channel co-pumped distributed Ramanamplification using low RIN 14xx nm DFB pump

A penalty-free 10 Gbit/s single-channel transmission with co-pumped distributed Raman amplification using a novel high power, low relative integrity noise (RIN), multimode 14xx nm distributed feedback pump is reported. No penalty from pump to signal RIN transfer was observed with co-pumped Raman gains up to 6 dB large effective area fibre (LEAF). In contrast, 0.5 dB penalty was observed with a fibre Bragg grating stabilised pump at 6 dB of Raman gain     

Highly stable optical add/drop multiplexer using polarization beam splitters and fiber Bragg gratings

We demonstrate a novel wavelength-division add/drop multiplexer employing fiber Bragg gratings and polarization beam splitters. The multiplexer is easy to fabricate without any special technique such as UV trimming, and yet shows very stable performance with less than 0.3-dB crosstalk power penalty in a 0.8-nm-spaced, 2.5-Gb/s-per-channel wavelength-division multiplexing (WDM) transmission system.     

Semiconductor Raman amplifier for terahertz bandwidth opticalcommunication

Semiconductor Raman amplifiers are useful for frequency selection in terahertz bandwidth and wavelength division multiplexing (WDM) systems with terabit capacity, as well as direct terabit optical communication systems. We have developed GaP-AlGaP Raman waveguides with micrometer-size cross sections. We have reduced residual optical loss of the waveguide by improvement of the fabrication process and realized a low-loss waveguide that is 10-mm long, which has a continuous wave (CW) Raman gain of 3.7 dB. Also, the time-gated amplification with 80-ps pulse pumping is performed and 20-dB gain is obtained. These performances are very suitable for light frequency selection in terahertz bandwidth and WDM optical communication systems     

Design of Hybrid Optical Amplifiers for High Capacity Optical Transmission

This paper describes our design of a hybrid amplifier composed of a distributed Raman amplifier and erbium‐doped fiber amplifiers for C‐ and L‐bands. We characterize the distributed Raman amplifier by numerical simulation based on the experimentally measured Raman gain coefficient of an ordinary single mode fiber transmission line. In single channel amplification, the crosstalk caused by double Rayleigh scattering was independent of signal input power and simply given as a function of the Raman gain. The double Rayleigh scattering induced power penalty was less than 0.1 dB after 1000 km if the on‐off Raman gain was below 21 dB. For multiple channel amplification, using commercially available pump laser diodes and fiber components, we determined and optimized the conditions of three‐wavelength Raman pumping for an amplification bandwidth of 32 nm for C‐band and 34 nm for L‐band. After analyzing the conventional erbium‐doped fiber amplifier analysis in C‐band, we estimated the performance of the hybrid amplifier for long haul optical transmission. Compared with erbium‐doped fiber amplifiers, the optical signal‐to‐noise ratio was calculated to be higher by more than 3 dB in the optical link using the designed hybrid amplifier.     

Proposal of all-optical wavelength-selective switching using waveguide-type Raman amplifiers and 3-dB couplers

We propose a new optical switch that can enable wavelength-selective high-speed switching for applications to photonic wavelength-division multiplexing (WDM) Internet protocol (IP) packet routing. The switch utilizes characteristics of waveguide-type Raman amplifiers and 3-dB couplers, and so it operates all-optically at a few tens of picosecond or faster speed. The operation of the switch is studied by coupled-mode theory and is verified numerically using a beam-propagation method (BPM). As a result, we found that optical waves having arbitrary wavelengths among those of incident WDM waves are amplified and switched when the amplification rate of each waveguide-type Raman amplifier is set to the proper value by pump wave injection. Moreover, the dynamic range over 30 dB was also obtained from the simulation results of the switch.     

Structural modification of semiconductor optical amplifiers for wavelength division multiplexing systems

In this paper, the semiconductor optical amplifier is analyzed for in-line and pre-amplifier for wavelength division multiplexing (WDM) transmission having minimum crosstalk and power penalty with sufficient gain. It is evaluated that the cross gain saturation of the SOA can be reduced by settling crosstalk at lower level and also minimizing the power penalty by slight increase in the confinement factor. At an optimal confinement factor of 0.41069, high amplification is obtained up to saturation power of 20.804 mW. For this confinement factor, low crosstalk of −9.63 dB and amplified spontaneous emission noise power of 119.4 μW are obtained for −15 dBm input signal. It has been demonstrated for the first time that twenty channels at 10 Gb/s WDM can transmit up to 5600 km by use of this optimization. In this, cascading of in-line SOA is done at the span of 70 km for return zero differential phase shift keying modulation format with the channel spacing of 100 GHz. The optical power spectrum and clear eye are observed at the transmission distance of 4340 and 5600 km in RZ-DPSK system. The bit error rate for all channels increases more than 10⁻¹⁰ with the increase in launched input power.     

Transmission of optical communication signals by distributed parametric amplification

For the first time, to the authors' knowledge, distributed parametric amplification (DPA), i.e., the use of a transmission fiber itself for parametric amplification of communication signals is proposed and demonstrated. To account for the inevitable fiber loss, solutions were derived for the distributed amplifier, with either one or two pumps: They are obtained in terms of confluent hypergeometric functions. Low-penalty DPA of a 10-Gb/s nonreturn-to-zero (NRZ) signal over a 75-km dispersion-shifted fiber (DSF), is demonstrated by using only 66.5 mW of pump power. Three adjacent channels have been simultaneously transmitted, with little penalty due to nonlinear crosstalk. It is experimentally verified that DPA requires less pump power than distributed Raman amplification (DRA), for similar power penalties.     

Raman-enhanced regenerative ultrafast all-optical fiber XPM wavelength converter

The Raman gain enhancement of a regenerative ultrafast all-optical cross-phase modulation (XPM) wavelength converter (WC) is quantitatively investigated and experimentally demonstrated to operate error free at 40 and 80 Gb/s. The regenerative nature of the converter is shown by experimentally demonstrating a negative 2-dB power penalty at 80 Gb/s. It is also shown that the Raman gain greatly enhances the wavelength conversion efficiency at 80 Gb/s by 21 dB at a Raman pump power of 600 mW using 1 km of highly nonlinear fiber. An analytical theory based on nonlinear phase-shift enhancement of the fiber-effective length is presented and shows the relationship between a nonlinear enhancement and Raman gain as a function of pump power and fiber design parameters. Measured parameters are used in the analytical model, and a good fit between experiment and theory is shown for two different types of fiber: one dispersion-shifted and one highly nonlinear fiber. The ultrafast response time of Raman gain makes this technique applicable to fiber-based ultrafast WCs. In addition, the applicability to other nonlinear fiber wavelength conversion techniques is discussed.     

Transient gain dynamics in saturated Raman amplifiers

This paper provides a detailed analysis of transient gain dynamics in saturated Raman amplifiers fed by wavelength division multiplexed (WDM) signals. Such dynamics are due to a pump saturation effect, known as pump-mediated signal-to-signal crosstalk, which is equivalent to the well-known cross-gain modulation in EDFAs. We provide for the first time a simple block-diagram model of the Raman amplifier, whose state is represented by the relative pumps depletion sensed by the signals. With such model, we are able to prove that the time constants of the Raman gain transients are of the same order as the pump–signal walk-off times. For counter-propagating pumps, the model is very accurate in predicting both the steady-state gain and the transient gain dynamics, with more than an order of magnitude improvement in computation time with respect to the direct solution of the signals and pumps propagation equations. The value of such model is therefore in the simulation of dynamic WDM networking scenarios in which the input powers have large swings in time. The model also extends to the co-propagating pump and well captures the time constants involved in the transients, although the accuracy in the predicted power levels is worse than that of the counter-propagating pump case.     

光纤色散对WDM系统中受激喇曼散射串扰的影响

本文研究了波分复用系统中光纤色散对受激喇曼散射串扰的影响,提出了在考虑信号调制几率特性和脉冲走离效应后,系统误码率和功率代价的理论计算方法,并对典型系统进行了数值模拟。研究结果表明,由光纤色散引起的信号脉冲走离可以降低受激喇曼散射串扰,尤其是在复用信道较多、信号传输速率较高的系统中,光纤色散的作用更加明显     

Wavelength-Swept Super-Dense Wavelength-Division Multiplexing (SD-WDM) Transmitter: Theoretical and Experimental Study of Performance Degradation Caused by Interchannel Crosstalk

This paper presents a theoretical and experimental study of performance degradation caused by the interchannel crosstalk of a previously proposed scheme for a super-dense wavelength-division multiplexing (WDM) transmitter based on wavelength-swept light. In the theoretical study, we calculate the relationship between the optical filter bandwidth, the wavelength sweep range, and the power penalty based on an approximate modeling process. And we employ the results to clarify the number of channels that can be achieved without a serious power penalty for use as a practical criterion when the wavelength sweep range is given. Furthermore, we describe experiments on multichannel 100 Mb/s and 1.0 Gb/s WDM signal generation and selective detection using a fiber Bragg grating filter with a 3-dB bandwidth of 10.5 GHz. The results confirm that the theoretical and the experimental results agree well when the spectral broadening induced by data modulation is not very large.     

Compensating unflattened WDM chips spectra using dynamic backward-pumped fiber Raman amplifiers technology

Fiber Raman amplifiers (FRAs) with multiple pumps are proposed to realize dynamic gain equalization for a spectral chips signal with a non-flattened broadband light source (BLS) in a spectrum-sliced wavelength-division multiplexing (WDM) network. In FRAs with multiple pumps, the gain profile can be adjusted via appropriate specification of the relative position of the pump wavelengths and the power of the pump waves. This paper combines a pump-power control algorithm and a genetic algorithm (GA) to establish the optimal pump spectrum for any specified gain spectrum in the WDM system. The method flattens the power spectra of WDM chips by identifying the optimal pump wavelengths and pump power of backward-pumped FRAs. It avoids the conventional requirement for time-consuming trial-and-error adjustments or intensive numerical simulations. Simulation results show that the scheme is simple, effective, and applicable for various BLSs in a spectrum-sliced WDM transmitter.     

Amplifier induced crosstalk in multichannel optical networks

The effect of crosstalk introduced due to gain saturation in an optical amplifier when it is used for amplifying multiple channels in a wavelength division multiplexed (WDM) network employing ON-OFF keying with direct detection is studied. The system power penalty is quantified as a function of the amplifier input power, the number of channels, and the extinction ratio