Transient gain control in EDFA's by dual-cavity optical automaticgain control

We report a novel method for automatic gain control of optical amplifiers: dual-cavity optical automatic gain control. The method exploits inhomogeneous broadening in erbium-doped fibers to clamp the erbium gain spectrum with two oscillating lasers. The technique significantly reduces surviving channel steady-state and transient power excursions by automatic reduction of relaxation oscillations and spectral hole burning     

Theoretical analysis of spectral hole burning and relaxationoscillation in all-optical gain stabilized multichannel erbium-dopedfiber amplifier (EDFA)

The spectral hole burning effects and gain dynamics of all-optical gain clamped multichannel erbium-doped fiber amplifiers (EDFAs) are modeled. The two-level laser model is used to write the propagation and rate equations of the inhomogeneous laser medium. The governing equations are an uncountable system of partial differential equations (PDEs). After some mathematical manipulations, averaging over the fiber length and introducing an approximation method, the system of PDEs is converted to a finite system of ordinary differential equations (ODEs). The gain dynamics and hole burning of an all-optical stabilized multiwavelength EDFA and the transient response of an optical fiber inverter are analyzed by the solution of the system of ODEs. Theoretical results are in good agreement with the published experimental result     

Effects of intraband crosstalk on incoherent light using SOA-based noise suppression technique

We investigate the effects of intraband crosstalk on the intensity noise suppression of spectrum-sliced incoherent light sources using semiconductor optical amplifiers (SOAs). Both Q-factor and electrical spectrum measurements clearly show that intraband crosstalk deteriorates the SOA-based noise suppression by breaking the correlation between optical frequency components of light.     

Simple dynamic model of all-optical gain-clamped erbium-doped fiberamplifiers

A simple dynamic model of laser-gain clamped erbium-doped fiber amplifiers (EDFAs) using ordinary differential equations is presented. The model not only provides a fast means of calculating EDFA gain dynamics, but also shows explicitly how the laser relaxation oscillation frequency and damping constant are affected by design parameters through analytical expressions. It is shown that the dynamic power excursion caused by relaxation oscillations can be reduced by increasing the oscillation frequency. The simplified model has two limitations: first, the amplified spontaneous emission (ASE) spectrum is not fully resolved and the noise figure can not be studied; second, the effect of spectral hole burning, which also causes gain excursion, can not be modeled. The first limitation can be removed by ASE-resolved modeling, where the concept of average inversion is also utilized to save computing time     

Linewidth Enhancement Factor of Quantum-Dot Optical Amplifiers

The linewidth enhancement (alpha-) factor of quantum-dot (QD) semiconductor optical amplifiers in the small signal gain and nonlinear regimes is theoretically investigated. A microscopic polarization equation and a wave equation are used to model subpicosecond pulse propagation in the nonlinear regime. In addition, a population equation that takes into account spectral hole burning and carrier heating effects is used. A novel approach to obtain the alpha-factor from the output pulse amplitude and phase in the dynamic nonlinear regime is presented. An in-depth study reveals that the presence of excited states (ES) limits the alpha-factor to values greater than 1 except when the energy separation between the ground state and ES is large. The alpha-factor dependence on QD inhomogeneous broadening, carrier density, carrier temperature, energy level separation, and input pulse energy is analyzed. We find that these can change the alpha-factor considerably. In particular, the alpha-factor increases with increasing input pulse energy and can be greater than 10 for input pulse energies larger than the amplifier's input pulse saturation energy. In the light of our calculations, the optimum device engineering required to obtain a low alpha-factor is discussed     

A general and rigorous WDM receiver model targeting 10-40-Gb/schannel bit rates

We present a general and rigorously formulated dynamic receiver model aiming at 10-40-Gb/s wavelength division multiplexing (WDM) system design applications. A demultiplexing (DEMUX) characteristic with periodic transfer function has been treated in detail and it has been indicated how the model should be adjusted to take into consideration a general type of noise spectral density (NSD). The bit error ratio (BER) is evaluated accounting for the influence of non-Gaussian detected amplified spontaneous emission (ASE) noise, noise correlation between stochastic noise samples in the receiver, the gain and effective noise figure variation with wavelength of optical amplifiers, channel crosstalk, and intersymbol interference (ISI) effects caused by nonideal signal modulation, fiber dispersion, fiber nonlinearities, optical MUX, and DEMUX filtering and the impulse response of the electrical low-pass filter in the receiver. Also, the influence of shot and thermal noise is taken into account. Numerical results for the BER are presented considering a realistic 16-channel 10-Gb/s WDM system operating in the C-band using normal transmission fibers and including cascaded erbium-doped fiber amplifiers (EDFAs) with dispersion compensating fibers     

Differential Gain and Gain Compression in Quantum-Dot Lasers

The dynamics of optical gain in semiconductor quantum dots (QDs) is investigated. Simple analytical expressions are derived, which directly connect the laser dynamical response to capture and intradot relaxation rates. The effect of hole spreading in the valence band and spectral hole burning in the QD ensemble is also quantitatively assessed. The analysis shows that intradot relaxation constitutes the main limitation in the dynamics and points to possible routes towards the improvement of QD lasers     

Multidimensional Electro-Opto-Thermal Modeling of Broad-Band Optical Devices

This paper investigates the self-consistent modeling of broad-band optical devices such as super-luminescent light-emitting diodes (SLEDs) and semiconductor optical amplifiers. A multidimensional electro-opto-thermal approach using many-body gain theory, and temperature-dependent microscopic transport equations is presented. In addition, a Green's function based model for amplified spontaneous emission is derived from Maxwell's equations in a consistent way. To illustrate the model's validity it is implemented into an existing simulation tool and benchmarked with two InP-based edge-emitting SLEDs operating around 1310 nm, featuring nonidentical quantum wells as active region. A comparison between simulated and measured characteristics (both electro-thermal and spectral) proves the applicability of the novel model.     

Numerical Analysis of Gain Saturation, Noise Figure, and Carrier Distribution for Quantum-Dot Semiconductor-Optical Amplifiers

The gain saturation behaviors and noise figure are numerically analyzed for quantum-dot semiconductor optical amplifiers (QD-SOAs). The carrier and photon distributions in the longitudinal direction as well as the photon energy dependent facet reflectivity are accounted in the rate equations, which are solved with output amplified spontaneous emission spectrum as iterative variables. The longitudinal distributions of the occupation probabilities and spectral-hole burning are presented for electrons in the excited and ground states of quantum dots. The saturation output power 19.7 dBm and device gain 20.6 dB are obtained for a QD-SOA with the cavity length of 6 mm at the bias current of 500 mA. The influences of the electron intradot relaxation time and the QD capture time on the gain spectrum are simulated with the relaxation time of 1, 30, and 60 ps and capture time of 1, 5, and 10 ps. The noise figure as low as 3.5 dB is expected due to the strong polarization sensitive spontaneous emission. The characteristics of gain saturation and noise figure versus input signal power for QD-SOAs are similar to that of semiconductor linear optical amplifiers with gain clamping by vertical laser fields.     

Numerical Analysis of the Frequency Chirp in Quantum-Dot Semiconductor Lasers

We present a numerical model for the analysis of the chirp dynamics of quantum-dot (QD) semiconductor laser under large signal current modulation. The model is based on the multipopulation rate equation formalism, and it includes all the peculiar characteristics of the active QD material such as the inhomogeneous broadening of the gain spectrum, the presence of an excited state confined in the QDs and the presence of nonconfined states due to the wetting layer and the barrier. In this paper the model is applied to the analysis of the chirp of two QD single-mode lasers emitting from the ground state and from the excited state, respectively. In order to make comparisons of the chirp in various operating conditions, we define some equivalent parameters for quantifying the adiabatic and transient contributions to the chirp. These parameters are then used to analyze the chirp as function of the bias current, of the modulation depth and of the modulation frequency. All the various simulation results show that the carrier accumulation in the QD states, poorly involved in the stimulated emission process and the carrier dynamics in these states, can cause a nonzero chirp under current modulation even for the ideal condition of zero linewidth enhancement factor (or -parameter) at the laser threshold.     

Noise characteristics of nonlinear semiconductor optical amplifiersin the Gaussian limit

This paper addresses noise properties of nonlinear semiconductor optical amplifiers. From a basic point of view, noise properties of nonlinear optical amplifiers are sufficiently different from those of linear amplifiers to warrant detailed modeling which has not been formulated previously. From a practical point of view, nonlinear semiconductor optical amplifiers are important for future all-optical signal-processing applications which may involve the operation of these devices in a saturated regime. Nonlinear amplifiers are also common in systems operating near 1300 nm and in integrated booster amplifiers. Under nonlinear operating conditions, amplifier noise contains a narrow-band contribution that comes about due to the nonlinear coupling of noise and gain. The more conventional broadband spontaneous noise also changes as the inversion factor becomes power-dependent and varies along the amplifier axis. We analyze noise in nonlinear amplifiers in the Gaussian limit (meaning, for fields consisting of large photon numbers) for CW or NRZ modulated signals and separately for short pulses. We consider the case of a single input as well as configurations of multi-input signals interacting via four-wave mixing. Using a specific detection system for the calculations of electronic signal-to-noise ratios, we demonstrate a reduction in the narrow-band electronic noise due to saturation in the single input case. We also demonstrate a vast advantage of using short pulses in four-wave-mixing applications     

半导体光放大器的超快动态增益特性

提出了一种包括载流子密度脉动(CDP)、载流子加热(CH)和光谱烧孔(SHB)效应在内的半导体光放大器(SOA)的时域动态模型。利用该模型分析了半导体光放大器中的增益饱和、超快增益动态以及光脉冲在增益饱和半导体光放大器中的波形畸变,其中重点考虑了超短脉冲的情况。模拟计算表明,对于10ps量级以下的短脉冲,分析半导体光放大器的动态增益特性时,不能忽略载流子加热和光谱烧孔等带内超快非线性效应的影响。     

Theoretical and Experimental Study of High-Speed Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers

We numerically and experimentally investigate the high-speed small-signal cross-gain modulation (XGM) characteristics of a quantum-dot (QD) semiconductor optical amplifier (SOA). From a p-doped QD SOA operating at 1.3 $mu{hbox {m}}$, high-speed small-signal XGM responses up to 40 GHz are measured from low to high injection currents and improve at high injection currents. In the numerical model, we set up about six hundred coupled rate equations, where the carrier dynamics of QD electron and hole states are considered separately and the enhanced hole occupation due to p-type doping is included. The high-speed small-signal XGM spectra are calculated at various modulation frequencies and pump-probe detunings. We identify how the two separate XGM mechanisms of total carrier density depletion (TCDD) at low injection current and spectral hole burning (SHB) at high injection current affect the high-speed small-signal XGM behavior. From the measured and calculated results, we show that high-speed small-signal XGM responses of QD SOAs can be improved by injecting more carriers to the QD excited states, which enhances high-speed XGM induced by SHB rather than by TCDD.      

Measuring gain and noise figure of erbium-doped fiber amplifiers using a broad-band source and a transmission filter

We report a unique low-cost technique for measuring gain and noise figure (NF) in erbium-doped fiber amplifiers using a broad-band amplified spontaneous emission source and a transmission filter, by extracting the slope and intercept of output power spectral density versus input power spectral density at filter edges. The required filter depth is 30 dB. Discrepancies of /spl les/0.3 dB are found between gain or NF obtained from this technique and those measured using conventional spectral-interpolation technique employing multiple lasers at 100-GHz spacing across the C-band.     

High-Speed Mode-Locked Quantum-Dot Lasers and Optical Amplifiers

Recent results on GaAs-based high-speed mode-locked quantum-dot (QD) lasers and optical amplifiers with an operation wavelength centered at 1290 nm are reviewed and their complex dependence on device and operating parameters is discussed on the basis of experimental data obtained with integrated fiber-based QD device modules. Hybrid and passive mode locking of QD lasers with repetition frequencies between 5 and 80 GHz, sub-ps pulse widths, ultralow timing jitter down to 190 fs, high output peak power beyond 1 W, and suppression of Q-switching are reported, showing the large potential of this class of devices for O-band optical fiber applications. Results on cw and dynamical characterization of QD semiconductor optical amplifiers (SOAs) are presented. QD amplifiers exhibit a close-to-ideal noise figure of 4 dB and demonstrate multiwavelength amplification of three coarse wavelength division multiplexing (CWDM) wavelengths simultaneously. Modelling of QD polarization dependence shows that it should be possible to achieve polarization insensitive SOAs using vertically coupled QD stacks. Amplification of ultrafast 80 GHz optical combs and bit-error-free data signal amplification at 40 Gb/s with QD SOAs show the potential for their application in future 100 Gb Ethernet networks.     

Dynamic Spatiotemporal Speed Control of Ultrashort Pulses in Quantum-Dot SOAs

We present theoretical and experimental results on the propagation of ultrashort pulses in quantum-dot (QD) laser amplifiers. The propagation time of the light pulses is controlled by the pulse itself (self-induced speed control) or by injection of a second pump pulse (external speed control). Our simulations on the basis of spatially and temporally resolved QD Maxwell–Bloch equations reveal that the excitation and relaxation dynamics induced by the propagating pulse or a pump pulse within the active charge carrier system leads to a complex gain and index dynamics that may either speed up or slow down the propagating light pulse. The physical effects allowing for the dynamic speed control could be ascribed to complex (coherent and incoherent) level dynamics leading to dynamic gain saturation and index dispersion. The dependence of the propagation time on injection current density and pulse energy is discussed. The numerical results of pulse reshaping and propagation times in the gain and absorptions regime are compared to experimental results.     

WDM transmission system performance: influence of non-Gaussiandetected ASE noise and periodic DEMUX characteristic

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     

Theoretical and experimental investigation of a thin-film waveguide laser amplifier with a lossy cladding

In laser amplifiers using high gain materials such as dyes or semiconductors, the inherent broad-band amplified spontaneous emission (ASE) may strongly saturate the amplifier gain: this yields a severe limitation on the amplification of small signals. We show that this difficulty can be appreciably overcome in an optical waveguide amplifier with a lossy cladding. A theoretical analysis of gain saturation by the ASE noise in a lossy cladding waveguide amplifier is given, and the small-signal gain improvement is stressed. An experiment involving a metal-clad thin-film dye laser amplifier is reported, the results of which are in agreement with the theoretical predictions.     

Spectral Properties of Fabry-Pérot Laser Diodes and Conventional Semiconductor Optical Amplifiers

This paper presents an unified comprehensive model for the analysis of the spectral properties of Fabry-Pérot laser diodes and conventional semiconductor optical amplifiers. We develop the model by considering the wide-band amplified spontaneous emission fields and the input optical signal fields in a general frame. Specifically, this paper discusses theoretical aspects of the model in details, which are based upon the spectra of material gain and spontaneous emission power,nonlinear gain suppression, and longitudinal spatial hole burning. This paper also presents simulation results of the model for the case of conventional semiconductor optical amplifier and the case of Fabry-Pérot laser diode to demonstrate its capabilities.     

Distributed fiber Raman amplifiers with incoherent pumping

Distributed fiber Raman amplifier (DFRA) with incoherent pumping is investigated and its performance is compared to that with the conventional coherent pumping. It is shown that increasing the spectral bandwidth of incoherent pumping source can reduce the Raman gain ripple significantly, and degrade noise figure slightly at short wavelengths, compared to coherent pumping. To achieve the same gain flatness, the number of pumps for broad-band DFRAs with incoherent pumping can be significantly reduced compared to the coherent pumping.