Amplified spontaneous emission in pulse-pumped Raman amplifiers

We discuss amplified spontaneous emission (ASE) generated in Raman amplifiers that are counter-pumped with trains of pulses. Our experimental and theoretical results show that if the peak power of the pump pulses is too high, the ASE output from the amplifier can be significantly higher than that from a continuous-wave pumped amplifier providing the same gain. This effect places a lower limit on the duty cycle of pump pulses one can use for a given level of Raman gain. Furthermore, we report an additional ASE enhancement if there is insufficient walkoff between the pump pulses and copropagating ASE to average the effects of higher frequency pump intensity noise. As a result, less pump intensity noise can be tolerated when pulse-pumping a fiber having a zero-dispersion wavelength located midway between the pump and signal wavelengths.     

Amplified spontaneous emission in dye-doped sol-gel amplifiers

Transversely pumped dye doped slab waveguides are investigated. A set of coupled-rate equations for the excited-state population and for the amplified spontaneous emission (ASE) power, at all wavelengths, is presented. The equations are solved numerically for the steady-state case. The results describe the dependence of ASE output power on pumping power, the spectral narrowing process, and the effect of waveguiding. The theoretical calculations are compared with the results of experiments, and good agreement is found     

Amplified spontaneous emission and gain characteristics ofFabry-Perot and traveling wave type semiconductor laser amplifiers

Semiconductor laser amplifiers are investigated with respect to amplified spontaneous emission power and gain characteristics. The influence of the spontaneous emission coefficient, input power, and facet reflectivity on amplifier saturation characteristics is analyzed. It is shown that a small β, zero reflectance device has advantageous properties in terms of high gain and output power at the 3-dB compression point. The low reflectance contributes to a slow saturation, whereas a low β means a larger optical model and hence lower intensities and less saturation for a given output power or gain     

Spatio-temporal dynamics of light amplification and amplifiedspontaneous emission in high-power tapered semiconductor laseramplifiers

We investigate the spatio-temporal light field dynamics in high-power semiconductor lasers with continuous-wave optical injection. The amplification processes that characterize this system occur during the propagation of the injected signal within the active area and can be attributed to spatially dependent gain and refractive index variations. Those are shown to be determined by dynamic interactions between the light fields and the active charge-carrier plasma. This microscopic light-matter-coupling is described by a spatially resolved microscopic theory based on Maxwell-Bloch-Langevin equations taking into account many-body interactions, energy transfer between the carrier and phonon system and, in particular, the spatio-temporal interplay of stimulated and amplified spontaneous emission and noise. Results of our numerical modeling visualize the dynamic spatio-spectral beam shaping experienced by the propagating light in amplifiers of tapered geometry. This reveals the microscopic physical processes that are responsible for the particular amplitude and spatial shape of the light beam at the output facet     

Amplified spontaneous emission in cladding-pumped L-banderbium-doped fiber amplifiers

Propagation of amplified spontaneous emission (ASE) in cladding-pumped long-wavelength-band erbium-doped fiber amplifiers (EDFAs) is analyzed numerically. Forward and backward ASE power dependence on cladding area is analyzed for both pumping directions. ASE power propagating counterdirectionally to the pump is found to experience strong dependence on the cladding area. Increasing the cladding area results in more uniform pump distribution along the EDF, preventing short-wavelength gain and ASE buildup. Quantum conversion efficiency in cladding-pumped L-band EDFAs is discussed     

Amplified spontaneous emission in index-guided multimodal waveguide structures

This paper describes a theory of amplified spontaneous emission in index-guided multimodal waveguide structures. It is based on a perturbation approach taking into account the correlated excitation of the different guided modes of the unperturbed waveguide. The coupling between these modes due to the propagation through a perturbed waveguide is taken into account by deducing evolution equations of the cross correlations between the modal amplitudes. Eventually, a beam propagation method program, including the spontaneous emission model, has been used for a comparison of the noise characteristics of a single-mode SOA with a 1-by-1 multimode interference semiconductor optical amplifier. Experiments have been performed confirming the obtained simulation results.     

Performance predictions for vertical-cavity semiconductor laseramplifiers

Vertical-cavity surface-emitting laser structures are studied theoretically to examine their potential for optical amplification. The major advantages of such a device are the minimal insertion loss and the polarization insensitivity of the gain which the transverse geometry provides. Although single-pass gain is necessarily low owing to a short optical cavity, operation as a Fabry-Perot amplifier in either the reflection or transmission mode ran provide high-performance characteristics. A comparison between edge- and surface-emitting amplifiers shows where advantageous operation can be obtained, and suggests how the best use of the surface-emitting amplifier performance can be made. By considering contrasting device dimensions, we show that gain, saturation output power, and noise characteristics suitable for both fiber optic and array processing configurations are readily attainable     

Amplified spontaneous emission model for quantum-well distributedfeedback lasers

An amplified spontaneous emission model for quantum-well (QW) distributed feedback (DFB) lasers is presented, which takes into account local spontaneous emission, stimulated emission, and real refractive index change which are calculated from the Fermi-Dirac occupancy functions in a self-consistent manner. The local-normal-mode transfer-matrix method is used, which allows a coupling of the local DFB effect with the local QW spontaneous emission and gain. As an example, an analysis is given of a partly gain-coupled DFB laser with periodically etched QWs, which has a large discontinuity of spontaneous emission and gain in high- and low-corrugation regions. It is shown that the side-mode suppression improves with the increase of the number of etched QW's, due to the carrier-density-dependent gain-coupling     

Amplified spontaneous emission in soliton transmission systemsemploying sliding filters

The growth of the amplified spontaneous emission (ASE) noise in a soliton line employing sliding filters is studied. An analytical expression of the integrated ASE power is given for constant line dispersion. For large dispersion variations caused by the combined effect of sliding and third-order dispersion of the line, an optimal sliding rate exists that minimizes the amplified spontaneous emission. Methods to reduce the spontaneous emission at the line output are proposed     

Amplified spontaneous emission and carrier pinning in laser diodes

Theoretical and experimental results for the temperature dependence of amplified spontaneous emission (ASE) in laser diodes (LDs) and light-emitting diodes (LEDs) are presented. The theoretical model takes into account conduction band nonparabolicity and band-gap renormalization. The gain spectrum is calculated from the theoretical spontaneous emission spectrum, and both compare very well with experimental data. From a fit to the observed temperature dependence of ASE for an LED and the gain spectrum for an LD with a structure identical to that of the LED except for mirror reflectivity, it is possible to establish carrier density as a function of injection current for both devices. It is shown that photons fluctuating into cavity modes give rise to substantial subthreshold carrier pinning in laser diodes. These fluctuations extract an extra current from the device and play an increasingly important role with increasing temperature     

Amplified spontaneous emission and signal amplification in dye-laser systems

Transversely pumped dye-laser systems are investigated theoretically and experimentally. A set of coupled rate equations for the excited-state population densities and for the photon fluxes in both directions, at all wavelengths, is presented. Both the temporal and spatial dependence of these quantities are accounted for. The equations are solved numerically for a variety of practical situations, and analytical approximations for some limiting cases are discussed. The results describe the dependence of the amplified-spontaneous-emission (ASE) output flux on pumping rate, the spectral narrowing process, and the effects of gain saturation. It is found that under practical laboratory conditions the gain of such dye systems saturates rapidly. Consequently, at high pumping rates the output varies linearly with pump intensity, and the conversion efficiency from pump to ASE photons approaches unity. The performance of dyedaser amplifiers is described by the same set of equations, and the gain characteristics of such systems are analyzed as a function of input signal intensity and pumping rate. The theoretical calculations are compared with the results of a set of experiments, and good agreement is found. The operation characteristics of a dye-laser amplifier are evaluated and utilized in the design of a narrow-band oscillator-amplifier system.     

Application of communications theory to analyse carrier densitymodulation effects in travelling-wave semiconductor laseramplifiers

It is shown that standard communications phase modulation theory can be used to analyse the carrier density modulation caused by interchannel beat powers. Also, it is shown that spontaneous lifetime produces a phase shift which causes an interchange of power between channels     

Low residual reflectivity of angled-facet semiconductor laseramplifiers

Ridge-waveguide angled-facet semiconductor laser amplifiers for the 1.5 μm band have been fabricated with facet angles of 7° and 10°. Gain measurements performed with a stable, computer-controlled setup have revealed gain ripples as low as 0.025 dB at 22 dB gain for a 10° device. This corresponds to a residual reflectivity of 1×10^-5. Results demonstrate that the residual reflectivity of angled devices with one-layer antireflection coatings can be as low as that for normal facet devices with highly controlled double-layer antireflection coatings     

Numerical simulation of broad-area high-power semiconductor laseramplifiers

A comprehensive model is presented to study the electrical, thermal, and optical behavior of broad-area traveling-wave InGaAs-AlGaAs semiconductor amplifiers. Finite-element thermal analysis and carrier transport mechanisms are integrated into the beam propagation method to include thermal lensing and optically induced nonlinearities. A self-consistent iteration is developed to simulate the beam filamentation in broad-area semiconductor amplifiers with residual facet reflectivities. The experimentally observed periodic filamentation with intensity minima close to zero is investigated numerically     

磷酸盐光波导激光器放大自发辐射的理论研究

系统地研究了Er3 :Yb3 共掺磷酸盐玻璃波导的性质,在稳态情况下建立并求解速率方程.利用数值分析的方法模拟了980nm二极管激光器抽运时激光器的放大自发辐射(ASE)特性.模拟时用到的主要参数为:信号光的散射损耗α(1530)=8.2×10-2dB/cm,980nm时Yb3 的吸收和发射截面积分别为5.4×10-25m2和6.9×10-25m2,980nm时Er3 的吸收和发射截面积分别为1.65×10-25m2和1.21×10-25m2,Yb3 -Er3 间的能量传递系数为1.9×10-23m3/s,共谐上转换系数为8.09×10-25m3/s.     

Noise study of low-dimensional quantum-well semiconductor laseramplifiers

Low noise characteristics of semiconductor laser amplifiers (SLAs) consisting of low-dimensional quantum-well structures are obtained theoretically using density matrix theory. Due to a sharper gain spectrum as well as a smaller population inversion parameter in quantum-wire and quantum-box structures, predominant two beat noises of traveling-wave SLAs were found to be reduced in the lower dimensional quantum-well structures, even in solitary devices without a narrow bandpass filter. The noise figure can be reduced to 3.3 dB in a quantum-box structure, which nears the theoretical limit of 3 dB     

Amplified spontaneous emission in narrow-band pulsed dye laser oscillators--Theory and experiment

We have investigated the effect of amplified spontaneous emission (ASE) on the spectral and temporal characteristics of narrow-band pulsed dye laser oscillators. The space and time dependent rate equations for the molecular populations and photon fluxes have been solved numerically to study the dependence of ASE on various laser parameters and the effects of ASE on the spectral and temporal profile of the dye laser output. To account for the diffraction losses present in a real dye laser oscillator, appropriate feedback factors for the laser and ASE photon fluxes were introduced into the boundary conditions for the oscillator. These theoretical results have been substantiated by experimental measurements of ASE in a narrow-band pulsed dye laser oscillator. We show that a considerable reduction of ASE in a grazing incidence grating dye laser oscillator can be obtained by appropriately shaping the pump pulse. Oscillations observed in the temporal output of pulsed dye lasers are also discussed.     

Amplified spontaneous emission (ASE) in laser oscillators and amplifiers

Dye-laser oscillators and amplifiers are studied theoretically with inclusion of amplified spontaneous emission (ASE). Assuming pencil-like geometry in both cases, laser signal intensity and ASE intensity are described with appropriate photon-transport equations. In particular, there exist different boundary conditions fox both fluxes in the laser cavity case due to the different feedback behavior of an optical resonator for laser modes and ASE modes. The interaction of laser output and ASE intensity is discussed in detail, including laser output optimization without and with ASE effects. For single- and double-pass amplifiers closed-form solutions for gain saturation due to input signal and ASE flux axe also given. More complicated gain functions, such as inclusion of reabsorption in dye lasers, are also treated in some detail.     

Amplified spontaneous emission in α-phase and β-phase polyfluorene waveguides

Amplified spontaneous emission (ASE) of α- and β-phase poly(9,9-dioctylfluorene) (F8) based waveguides has been studied and ASE wavelengths have been determined as 451 nm and 468 nm, respectively. β-phase waveguides are found to be with higher net gain and lower light propagation loss as compared to α-phase waveguides. Such important improvement of ASE properties, making β-phase waveguides superior over α-phase counterparts is shown to be achieved at the expense of higher refractive index and photoluminescence quantum efficiency of β-phase F8.     

Theory of spontaneous emission noise in multisection semiconductor lasers

Calculations of spontaneous emission noise in semiconductor lasers are mainly based on a fundamental theory developed by Henry in 1986, which is useful for simple systems, together with a formulation in terms of transfer matrices by Makino and others, which facilitates application of the theory to more complicated multisection systems. The aim of this review is to present a unified account of this theoretical work in a transparent form intended to encourage its further use in complex systems. The opportunity is taken to strengthen the existing theory by including the effects of differing optical wave vectors in different sections and the consequent reflections at interfaces, which are important in some applications. Sample calculations are presented for a range of systems with one, two, three, and four sections and the predictions compared with other theoretical and experimental results.