High sensitivity modulation/demodulation scheme for inter-orbit optical communication system with an optical amplifier

A numerical investigation of the performance of an open-loop optical chaotic communication system for the isochronous synchronization solution has been carried out, under strong optical injection conditions achieved using antireflective coating at the input facet of the receiver laser in combination with an optical erbium-doped fiber amplifier (EDFA). Different message encoding techniques have been considered and tested at multigigabit rates and for different levels of optical injection to the receiver. The effects induced by the amplified spontaneous emission (ASE) noise of the EDFA to the performance of the chaotic communication system have also been studied. The performance of all the examined encryption methods for the 1 Gb/s bit-rate messages was quite satisfactory and was characterized by Q-factor values that exceeded 10, after synchronizing in the strong injection regime. For higher message bit rates, the Q-factor values for all methods decrease considerably due to the residual frequency components of the chaotic carrier that are now more significant in the message spectral region. The effect of the amplifier's ASE noise to the system's performance was deteriorated as long as the EDFA chaotic input was kept in relatively high power levels.

While low loss and large bandwidth have allowed optical fibers to become essential in a large amount of telecommunication systems, this has not yet been true for video distribution systems where the cost per subscriber must be as low as possible. First optical video distribution systems have been developped using 1.3 pm distributed-feedback (DFB) lasers. Up to now, the modulation format used in such systems is analog amplitude-modulation vestigial-sideband (AM-VSB) scheme since it provides direct compatibility with cable TV industry and present television sets. However, this modulation scheme requires a very high signal-to-noise ratio on the receiver side so that it is necessary to detect large optical powers (above -10 dBm) to ensure good picture quality. This means that the power budget (budget one can afford for optical fibers and splitters losses) for 1.3 im systems, even with powerful optical sources, is not very important; in turn, this means that it seems difficult to cut the cost per subscriber with this technology. With optical Erbium-Doped Fiber Amplifier (EDFA) breakthrough, many suppliers of cable TV transmission systems moved to 1.5 .tm technology. Whereas this new component is revolutionizing most of the telecommunication systems as optical fiber did 20 years ago, the use of optical amplifiers for analog systems is not so impressive as for robust digital transmission systems. This is due to the fragility of analog signal in respect with noise and distortion. Because of the noise generated by an optical amplifier, it is not possible to cascade a large number of them while reaching a realistic signalto-noise ratio at the network termination. Optical amplifiers may also distort the analog signal when the transmitter consists of a directly-modulated semiconductor laser, suffering from chirp (unwanted wavelength modulation when power modulation is achieved through current modulation). Moreover, shifting from 1 .3 .tm to 1 .5 pm wavelength brings new problems : 1 .5 tm DFB lasers are not yet as linear as 1.3 im DFB lasers and analog system designers have to cope with chromatic dispersion at 1 .5 .tm in standard fibers (i.e. non dispersion-shifted fibers). It is clear that many of the technical issues for 1.5 im analog systems are far from being resolved. In this paper, we investigate the way, and to what extent, erbium-doped fiber amplifiers may improve analog systems power budget. The organization of the paper is as follows. In Section 2, we discuss what is the relevant noise parameter for optical amplifiers to predict the signal-to-noise degradation they cause. We also show that the same overall noise performance can be reached with amplifiers featuring different noise parameter and output power combinations. This section is completed with experimental measurements. Linearity definitions and requirements for AM-VSB systems are presented in Section 3. Included is a discussion of some specific factors that degrade the signal linearity within an optical amplifier in respect with optical source characteristics. Finally, the still opened technical matters and potential evolution for 1.5 jim analog systems are briefly summarized in Section 4

By the use of optical Erbium Doped Fiber Amplifiers (EDFA) in space communications it is possible to exploit the large capacity offered by the optical link, with a considerable reduction of transmitted power. A novel optical system for stratospheric communications is proposed. The link from a stratospheric platform to another one is carried out within the optical domain, thanks to the presence of a GEO that ensure the optical amplification. The optical preamplifier on the receiver end, is the key point for the system design: it must ensure the proper gain to the signal without introducing limitative Spontaneous Emission Noise.

In a wavelength-routed optical network, a transmitted signal remains in the optical domain over the entire route (lightpath) assigned to it between its source and destination nodes. The optical signal may have to traverse a number of crossconnect switches (XCSs), fiber segments, and optical amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs). Thus, while propagating through the network, the signal may degrade in quality as it encounters crosstalk at the XCSs and also picks up amplified spontaneous emission (ASE) noise at the EDFAs. Since these impairments continue to degrade the signal quality as it progresses toward its destination, the received bit error rate (BER) at the destination node might become unacceptably high. Previous work on the lightpath routing and wavelength assignment (RWA) problem assumed an ideal physical layer and ignored these transmission impairments. The main contribution of our work is to incorporate the role of the physical layer in setting up lightpaths by employing appropriate models of multiwavelength optical devices (XCSs and EDFAs) such that the BER of a candidate lightpath can be computed, in advance, to determine if this lightpath should be used for the call. Features from existing RWA algorithms are integrated with our on-line BER calculation mechanism. Our simulation studies indicate that employing BER-based call-admission algorithms has a significant impact on the performance of realistic networks.

Quadrature Phase Shift Keying (QPSK) is an important digital signal modulation method, which has the advantages of high spectrum efficiency and strong anti-interference. Coherent laser communication has the advantages of high speed, large capacity, light and small equipment as well as high sensitivity. In order to better introduce the QPSK modulation method into the space downlink coherent laser communication system, we give a bit error rate (BER) of QPSK modulation model with laser linewidth suitable for space downlink coherent laser communication system with QPSK modulation under atmospheric turbulence. Based on the model, we simulate and analyze the effects of atmospheric wind speed, zenith angle, gain of Erbium doped optical fiber amplifier (EDFA) and communication rate on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation. The results show that the linewidth tolerance of the system increases when the zenith angle decreases or the gain of EDFA increases or the wind speed decreases. And the effects of zenith angle, gain of EDFA and communication rate on the linewidth tolerance of the space downlink coherent laser communication system with QPSK modulation are large. The effect of wind speed on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation is smaller. And the larger the wind speed is, the smaller the effect of increasing wind speed on the linewidth tolerance. For the communication rate, the linewidth tolerance increases and then decreases with the increase of the communication rate. This work can be a reference for the design of space downlink coherent laser communication system with QPSK modulation.

In this paper, an analytical approach is presented to evaluate the performance of DWDM system with intensity modulation direct detection (IM/DD) due to the effects of amplified spontaneous emission (ASE) noise of optical amplifier, optical receiver noises and crosstalk. And a system has been proposed with an optimum number of amplifiers with higher gain and an improved optical receiver has been proposed. We have investigated the effects of optical amplifiers and optical receiver in the presence of crosstalk on the overall performance; in particular, Signal-to-Noise Ratio (SNR) and Bit-Error-Rate (BER) performance of a DWDM system. The system performances are evaluated for varying different amplifier gains, number of optical amplifiers, hop length, number of hops, receiver bandwidth and receiver gain considering the relationship between crosstalk and the number of wavelengths channel spacing. It is found that, the performance is highly degraded due to crosstalk and noises. As a result the system suffers significant power penalty at a given BER.

One of the most important technical advances in fiber optic telecommunication systems has been the optical fiber amplifier (OFA). The use of an all-optical fiber optic amplifier instead of a semiconductor optical amplifier has many inherent advantages. For example, the most popular erbium-doped fiber amplifiers (EDFAs) have a broad range of optical amplification in the 1550-nm window; these erbiumdoped fibers are easy to incorporate directly into an all—fiber optic telecommunication system. Today the erbium-doped silica fibers are the most common rare-earth (RE)-doped fiber amplifiers, as they have high gain and low noise. EDFAs are quite versatile in that they may be used as preamplifiers, in-line and booster amplifiers, as well as sources. They can provide more than 20 dB of gain across a broad wavelength band, which enables them to be extremely useful as components in dense wavelength-division multiplexed (DWDM) systems. Additionally, they may be pumped with 800-, 980-, and 1480-nm sources and their quantum efficiency can be quite high.

The erbium doped fiber amplifier (EDFA) is an imperative element in DWDM networks. This all-optical amplifier enables simultaneous amplification of multiple wavelengths. Nevertheless, uneven gain across the usable wavelength region remains a concern. Understanding and modelling the dynamic characteristics of an EDFA is an important step towards achieving a flat gain spectrum. A Simulink model for investigating EDFA dynamics has been developed by Novak and Gieske (2002). Following their work, we have developed an enhanced version of the EDFA Simulink model with more capabilities. Previous results were verified for the given doped fiber length without noise. In addition, the forward amplified spontaneous emission (ASE) noise is added to the model and the optimum length is determined (with and without considering ASE) by simulation and used in all subsequent stages. A significant addition to the model is the ability to handle multiple channels using the absorption and emission coefficients previously obtained experimentally. The resulting model accurately represents EDFA gain dynamics and forward ASE. Simulation results show a 12 dB gain fluctuation across a 40 mn window per EDFA.

The beat between an amplified optical signal and the amplified spontaneous emission (ASE) noise is typically the primary limitation to the performance of optical fiber amplifiers. A filtering technique which might reduce this beat noise by 3dB creates a paradox since the noise figure's quantum limit of 3dB could then be reduced to 0dB implying noiseless amplification, which is impossible. The conjecture that this paradox might be resolved by different attenuations of the quantum and semiclassical descriptions of signal-spontaneous noise has been experimentally rejected . Herein, we present a variation of that experiment and telecommunication' applications of this filtering technique are discussed.

High power semiconductor laser diodes and optical power amplifiers have important roles in solid state laser and optical fiber pumping, optical storage and recording, and can serve as efficient sources for medical and display applications. These high power devices must be mounted in the epitaxy-side down configuration for good heat transfer and so require a well-controlled, high yield, void-free, die attach method. We have developed a method which consistently yields absolute thermal resistances of 1.5/spl deg/C/W (/spl plusmn/4%) on tapered angle facet semiconductor optical amplifiers and specific thermal resistances of 0.004 K/spl middot/cm/sup 2//W for lasers and optical amplifiers mounted on oxygen-free high conductivity (OFHC) copper heatsinks. Our method has wide process margins, exhibits excellent yield (>95%, N=30) and repeatability, relaxes the requirements for highly polished heatsinks, and is well suited for laser die attach to heatspreading diamond submounts or advanced composite materials.

We propose an enhanced log-likelihood ratio (E-LLR) technique for soft-decision forward error correction (SD-FEC) in intensity-modulation direct-detection (IMDD) systems with optical amplifiers and demonstrate its effectiveness in 7-core weakly coupled fiber transmission. In amplified IMDD, square-law photodetection and modulator nonlinearities cause amplified spontaneous emission noise to produce non-Gaussian signal statistics, invalidating conventional LLR assumptions and degrading SD-FEC performance. To address this issue, we abandon the classic additive white Gaussian noise (AWGN) model and introduce a channel model based on multi-parameter chi-square distributions that more accurately reflect optical channel noise. Unlike previous Gaussian and chi-square approaches, E-LLR uses a training sequence to estimate the variance and non-centrality parameters for each pulse-amplitude modulation (PAM) level, generating <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> distinct noncentral chi-square distributions for precise LLR computation in FEC decoding. Experimental results show that the proposed approach improves the receiver sensitivity by 1.13dB and reduces the decoding iterations of FEC compared with AWGN-model based schemes. These improvements contribute to reduced system complexity and decoding latency, while still achieving the required bit error rate threshold.

Summary form only given. Summary form only given. Amplified spontaneous emission (ASE) noise is a major limit to the performance of modern day optical transmission systems. A way to alleviate system impairments due to ASE is the insertion of bandpass filters along the line. Soliton pulses have shown excellent stability to in-line narrow-band filtering, and also to sliding of the filter center frequency along the line. The dynamics of soliton signals in the presence of both fixed and sliding filters has been the subject of detailed studies. However, little attention has been given to the dynamics of the ASE growth in the presence of in-line filters. The analysis of the ASE noise growth in a chain of N amplifiers in the presence of in-line filters is the subject of this work. This analysis has been extended to the case of semiconductor optical amplifiers (SOA), where the noise growth, combined with the fast saturation of the amplifier gain, are more effective in degrading system performance. The analytical results have been compared with numerical simulations. Good agreement has been obtained for in-line erbium amplifiers, also considering a high inversion factor. For SOAs, on the other hand, the more complex amplifier dynamics makes the theory quantitatively less accurate, as expected. Even in this case, however, the analytical formulas can be taken as useful guidelines for the design of optical systems.

This paper presents the first unified wavelength division multiplexing (WDM) transmission model for systems incorporating cascaded optical amplifiers and a realistic demultiplexing (DEMUX) characteristic with periodic transfer function. The bit error ratio (BER) is evaluated accounting in rigorous form for the influence of non-Gaussian detected amplified spontaneous emission (ASE) noise, noise correlation between stochastic noise samples in the receiver, the bandwidth of the electrical receiver noise filter, the gain tilt and effective noise figure of optical amplifiers (with as well as without optical ASE noise filtering), channel crosstalk, signal extinction ratio and a one-or two-stage DEMUX implementation. The model is compared to the Gaussian receiver model in realistic design cases providing important information as to the validity of the Gaussian model. Practical design results indicate the link budget dependence on the DEMUX design and the ASE noise filtering.

This volume of trends in optical amplifiers and their applications includes such topics as: progess in optical fibre amplifiers; reliability of high-power pump lasers for erbium-doped fibre amplifiers; and inP-based optical switch array using semiconductor optical amplifiers.

A significant part of optical-fiber capacity can be tapped now that optical amplifiers can boost many high-speed channels simultaneously. The erbium-doped optical fiber amplifier opened the floodgates to wide-bandwidth, all-optical schemes for multiplexing many wavelengths. Until the advent of the erbium-doped fiber amplifier, no practical all-optical amplifier existed. Optical signals were instead regenerated electronically to overcome the attenuation inherent in the silica fiber as well as other losses due to optical components along the line.