Optimizing amplifier placements in a multiwavelength opticalLAN/MAN: the unequally powered wavelengths case

Optical networks based on passive-star couplers and employing WDM have been proposed for deployment in local and metropolitan areas. These networks suffer from splitting, coupling, and attenuation losses. Since there is an upper bound on transmitter power and a lower bound on receiver sensitivity, optical amplifiers are usually required to compensate for the power losses mentioned above. Due to the high cost of amplifiers, it is desirable to minimize their total number in the network. However, an optical amplifier has constraints on the maximum gain and the maximum output power it can supply; thus, optical amplifier placement becomes a challenging problem. In fact, the general problem of minimizing the total amplifier count is a mixed-integer nonlinear problem. Previous studies have attacked the amplifier-placement problem by adding the “artificial” constraint that all wavelengths, which are present at a particular point in a fiber, be at the same power level. This constraint simplifies the problem into a solvable mixed-integer linear program. Unfortunately, this artificial constraint can miss feasible solutions that have a lower amplifier count but do not have the equally powered wavelengths constraint. In this paper, we present a method to solve the minimum-amplifier-placement problem, while avoiding the equally powered wavelength constraint. We demonstrate that, by allowing signals to operate at different power levels, our method can reduce the number of amplifiers required     

EDFA-based DWDM lightwave transmission systems with end-to-end power and SNR equalization

An approximate analysis is presented which can be used to predict the performance of power and signal-to-noise ratio (SNR) equalization schemes when applied to dense wavelength-division multiplexing (DWDM) lightwave systems employing erbium-doped fiber amplifier (EDFA) cascades. Expressions are provided which relate the maximum number of amplifiers, EDFA gain imbalance, bit rate (R/sub b/), transmitter power, receiver dynamic range and number of channels. The relative advantages of these two equalization strategies are quantified by comparing the maximum number of amplifiers allowed by each scheme. It is shown that, while SNR equalization represents, on balance, the more desirable equalization strategy for future EDFA-based DWDM lightwave transmission systems, under certain conditions power equalization may be a better choice. When employing an APD receiver, for instance, power equalization can support 1.9 times more amplifiers than SNR equalization. However, when employing the more conventional preamplified PIN/FET receiver, SNR equalization can support 1.7 times more amplifiers than power equalization.     

Optimal design of flat-gain wide-band fiber Raman amplifiers

We present a novel method for designing multiwavelength pumped fiber Raman amplifiers with optimal gain-flatness and gain-bandwidth performance. We show that by solving the inverse amplifier design problem, relative gain flatness well below 1% can be achieved over bandwidths of up to 12 THz without any gain equalization devices. This constitutes a substantial improvement in gain flatness compared to the existing wide-band optical fiber amplifiers     

增益平坦光纤喇曼放大器的实现

介绍了一种独特的方法来实现具有最优增益平坦度和增益带宽的增益平坦喇曼光纤放大器.通过使用反向放大器设计,实现了不使用任何增益均衡器在12 THz带宽上的相对平坦度低于1%,这种放大器的结构比现有的宽带光纤放大器在增益平坦上有一定的进步.     

A Device Characterization and Circuit Design Procedure for Realizing High-Power Millimeter-Wave IMPATT-Diode Amplifiers

A measurement and characterization technique is presented which allows design and realization of IMPATT amplifiers operating at maximum generation efficiency. Diodes mounted in a stable reduced-height waveguide circuit are characterized by their complex reciprocal scattering coefficient as a function of frequency, dc bias, and RF drive power level. In particular, terminal conditions which correspond to a maximum power exchange between the active one-port network and the source are identified and then used to design and realize controlled-gain maximum generation-efficiency amplifiers. Simple equalization networks are shown to provide a wide range of available amplifier gains between limits set by stability requirements. As an example, the technique is effectively used with silicon diodes at K/sub a/-band (33-40 GHz) to realize a 7-dB gain 250-mW power amplifier with 5-GHz bandwidth.     

Application of preemphasis to achieve flat output OSNR intime-varying channels in cascaded EDFAs without equalization

In this paper, we present a new method for optical signal-to-noise ratio (OSNR) equalization of wavelength division multiplexed (WDM) channels at the end of a cascade of several erbium-doped fiber amplifiers (EDFAs) by use of preemphasis, as well as the proper choice of EDFA design parameters. Identical OSNR at the end of the cascade ensures better signal detection and quality of service. The dynamics of the equalizing method have been demonstrated by simulation for single- and double-stage amplifier designs using a numerical model incorporating time variation effects in EDFA. Calculations are based on the solution of a transcendental equation describing the dynamics of the reservoir, i.e., the total number of excited ions, for each EDFA. Traffic on eight WDM channels is modeled as statistically independent ON-OFF time-slotted sources. In addition, we investigate the effect of gain clamping of the first amplifier in the cascade-by implementing a ring laser and propagating the lasing power through the cascade-on the statistics of OSNR variation. We show that it is possible to achieve dynamic OSNR equalization for a WDM system by the use of preemphasis and an appropriate choice of EDFA parameters, without resorting to optical equalization filters. Most previous equalization methods are static with flat gain for a given inversion level in the amplifier. Changes in the input power (due to network reconfiguration or packetized traffic) will lead to a varying inversion level and hence non optimal equalization     

Tunable gain equalization using a Mach-Zehnder optical filter in multistage fiber amplifiers

Tunable signal gain equalization is demonstrated in three-stage Er/sup 3+/-doped fiber amplifiers using a waveguide type Mach-Zehnder (MZ) optical fiber. A 29-channel multiplexed system is examined where signal wavelengths are positioned from 1.548 to 1.555 mu m. By adjusting the MZ transmittance with the external control current, tunable gain equalization is achieved at the output of each amplifier.<>     

Gain optimization of germanosilicate fiber Raman amplifier and itsapplications in the compensation of Raman-induced crosstalk amongwavelength division multiplexing channels

Spectral characteristics of the stimulated Raman scattering (SRS) process were theoretically investigated for step-index silica optical fibers with various GeO₂ concentrations. Optimal-fiber lengths and germanium concentration, where the first Stokes power reaches maximum, were calculated at various pump power levels for application in Raman amplifiers. Based on this analysis, we proposed and experimentally demonstrated a new channel-equalizing technique to simultaneously compensate Raman-induced crosstalk and amplify wavelength-division-multiplexing (WDM) signals using a discrete Raman amplifier in the 1.5-μm range. As a further application of SRS in germanosilicate glass fibers, we introduce an all-optical variable attenuator for channel equalization that could be used in dynamic optical power tilt control in WDM systems     

Transmission performance of 10-Gb/s 1550-nm transmitters using semiconductor optical amplifiers as booster amplifiers

We have demonstrated the transmission performance of 10-Gb/s transmitters based on LiNbO/sub 3/ modulator using semiconductor optical amplifiers (SOAs) as booster amplifiers. Utilizing the negative chirp converted in SOAs and self-phase modulation induced by high optical power, we can successfully transmit 10-Gb/s optical signals over 80 km through the standard single-mode fiber with the transmitter using SOAs as booster amplifiers. SOAs can be used for booster amplifiers with a careful adjustment of the operating conditions. In order to further understand an SOA's characteristics as a booster amplifier, we model SOAs and other subsystems to verify the experimental results. Based on the good agreement between the experimental and simulation results, we can find the appropriate parameters of input signals for SOAs, such as extinction ratio, rising/falling time, and chirp parameter to maximize output dynamic range and available maximum output power (P/sub o,max/).     

Performance evaluation of the different types offiber-chromatic-dispersion equalization for IM-DD ultralong-distanceoptical communication systems with Er-doped fiber amplifiers

We have evaluated the transmission performance of different fiber-chromatic-dispersion-equalization methodologies for ultralong distance IM-DD optical communication systems that use Er-doped fiber amplifier repeaters. The experiment used a 1000 km fiber loop consisting of 30 dispersion-shifted fiber spans and 31 Er-doped fiber amplifiers. We changed the insertion point of the normal single-mode fiber for equalization to change the shape of the accumulated chromatic dispersion. Comparison of the longest transmission distance and the width of the 9000 km transmissible window are discussed for several types of dispersion equalization. The results indicate that the best type of the dispersion equalization for ultralong distance IM-DD optical communication systems is to install dispersion-shifted fibers with short sections of normal single-mode fibers to compensate the accumulated dispersion     

Effects of signal power control strategies and wavelength assignment algorithms on circuit OSNR in WDM networks

Software-defined networking is enabling wavelength-division multiplexed (WDM) networks to be programmable down to individual components. While taking into account typical gain and noise figure profiles of erbium-doped fiber amplifier (EDFA) components, the authors consider a number of signal power control strategies and compare their performance in terms of achievable lightpath optical signal-to-noise ratio (OSNR). These strategies are applied network-wide to concurrently control the gain of each individual amplifier and the signal power equalization at each reconfigurable optical add/drop multiplexer. Simulation and (in part) experimental results show that the lightpath OSNR is affected by three factors: the EDFA gain control strategy, power equalization strategy and wavelength assignment (WA) algorithm. A trade-off between lightpath average OSNR and OSNR variance across the WDM channels is also noted. Experimental work is conducted using a five-node meshed WDM network testbed proving both feasibility and effectiveness of a coordinated use of signal power control strategies and WA algorithms.     

Gain equalization of EDFA cascades

Investigates the impact of wavelength-dependent erbium-doped fiber amplifier (EDFA) gain spectrum on multichannel direct-detection lightwave transmission systems employing multiple amplifiers. An analysis is presented which quantifies the constraints imposed by received power imbalance, signal-to-noise ratio (SNR), and receiver sensitivity on an EDFA cascade. Expressions are derived which relate the system constraints to the EDFA gain imbalance, bit rate, number of channels, and receiver dynamic range. Results demonstrate that when four-wave mixing (FWM) is compensated in an 11-channel system, received power imbalance can impose a significant constraint on transmission distance when the EDFA gain imbalance is greater than 1 dB or when bit rate is less than 1.8 Gb/s. In addition, performance of the preemphasis gain equalization technique is studied for multichannel systems employing APD or p-i-n/FET direct-detection optical receivers. Simple expressions are derived which can be used to quantify the increase in transmission distance obtained when employing preemphasis equalization. Results indicate that equalization of the received power spectrum can provide a two- to four-fold increase in the transmission distance when using APD receivers, compared to a one- to two-fold improvement with p-i-n/FET receivers. Analytic results are compared with results obtained by proven simulation methods and found to be in good agreement     

Volterra series approach for optimizing fiber-optic communicationssystem designs

A new methodology for designing long-haul fiber-optic communication systems is presented. We derive the overall Volterra series transfer function of the system including linear dispersion, fiber nonlinearities, amplified spontaneous emission (ASE) noise from the fiber amplifiers, and the square-law nature of the direct detection (DD) system. Since analytical expressions for the probability of error are difficult to derive for the complex systems being used, we derive analytical expressions for an upper bound on probability of error for integrate-and-threshold detection at the receiver. Using this bound as a performance criterion, we determine the optimal dispersion parameters of each fiber segment required to minimize the effects of linear dispersion, fiber nonlinearities and ASE noise from the amplifiers. We study the dependence of optimal dispersion parameters on the average power levels in the fiber by varying the peak input power levels and the amplifier gains. Analytical expressions give us the freedom to choose system parameters in a practical manner, while providing optimum system performance. Using a simple system as an example, we demonstrate the power of the Volterra series approach to design optimal optical communication systems. The analysis and the design procedure presented in this work can be extended to the design of more complex wavelength division multiplexed (WDM) systems     

Long-haul coherent optical fiber communication systems usingoptical amplifiers

Studies on long-haul coherent optical fiber communication systems with in-line optical amplifier repeaters are made theoretically and experimentally. By theoretical calculation it was found that coherent systems can achieve wider dynamic range for an amplifier input power as compared with the intensity-modulation direct-detection (IM-DD) systems. The feasibility of such systems using traveling-wave semiconductor laser amplifiers (TWSLAs) and erbium-doped fiber amplifiers (EDFAs) was investigated, and 546 km, 140 Mb/s CPFSK transmission using TWSLAs and 1028-km, 560-Mb/s CPFSK transmission using EDFAs were successfully demonstrated     

Gain equalization of EDFA's with Bragg gratings

Flat-gain amplifiers are needed to ensure proper amplification of every channel in wavelength-division-multiplexed communication systems. Such amplifiers can be realized by combining a precisely tailored filter with an erbium-doped fiber amplifier (EDFA). In this letter, we show that Bragg gratings can lead to accurate EDFA gain equalization. Reflection and transmission gratings have been used to demonstrate gain equalization over 32 nm with excursion inferior to ±0.1 and ±0.3 dB, respectively. A maximum 0.04-dB noise-figure penalty results from this process. By extension, Bragg gratings could also equalize the gain of any arbitrary gain profile, over any arbitrary bandwidth     

All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10-Gb/s EPONs

This letter proposes a new power equalization scheme based on cascaded semiconductor optical amplifiers, for 10-Gb/s ethernet passive optical networks with uneven topology. For networks with a large number of subscribers, the proposed equalization scheme enables a 16-dB reduction of the receiver dynamic range.      

双包层塑料光纤放大器增益特性的研究

依据双包层光纤的截面特征,给出了适用于双包层结构光纤放大器的速率方程和功率传输方程,并用数值方法对不同结构的双包层塑料光纤放大器的增益特性进行了分析,结果表明:,偏心结构双包层塑料光纤放大器在提高信号增益的同时,又可有效地避免有机染料发生热漂白.     

Optical power equalization for multiwavelength fiber-amplifiercascades using periodic inhomogeneous broadening

Inhomogeneously broadened fiber amplifiers inserted periodically in an amplifier cascade are shown to provide significant interchannel power equalization in wavelength-multiplexed systems. Interchannel power variations in a six-amplifier cascade are reduced from 16 dB in a conventional system to about 5 dB when power equalizers are inserted     

Comprehensive Modeling of Single Frequency Fiber Amplifiers for Mitigating Stimulated Brillouin Scattering

A universal model capable of handling various operating conditions has been developed for designing of high power single frequency fiber amplifiers. The model analyzes the impacts of pump configurations, fiber lengths, and fiber temperatures on amplifier performance. It shows that counter-pumped amplifiers are capable of generating several times more output than co-pumped amplifiers. To fully take their advantages and deliver the laser output, a delivery fiber should be properly designed to avoid unwanted SBS which can significantly undermine the amplifier performance. On the other hand, for applications requiring delivery fiber at relative low power, the co-pumped amplifier can be an alternative since it can use the gain fiber as the delivery fiber and thus eliminate requiring an additional delivery fiber. The model also analyzes possible approaches for suppressing SBS, such as applying additional heating, the use of SBS suppressing fiber, and increasing core diameter. It shows that applying additional heating to a co-pumped amplifier can increase the output by a factor of 1.7. Finally, the model concludes that a single frequency amplifier can achieve multiple kilowatts output by carefully choosing amplifier design and utilizing SBS suppression technique.      

高功率双包层光纤放大器

高功率双包层光纤放大器在光纤传感、光纤通讯、光谱测量和惯性约束聚变等领域有广泛应用.介绍了两种获得放大激光输出的高功率双包层光纤放大器:单频双包层光纤放大器和脉冲双包层光纤放大器.分析了它们的工作原理及关键技术,并对国内外近期进展作了综述.