Amplified spontaneous emission effects in semiconductor laseramplifiers

The analysis presented provides a quantitative method for predicting semiconductor laser amplifier performance in the presence of ASE (amplified spontaneous emission). It indicates that in order to increase the fraction of pump power that contributes to the amplification of the input laser field relative to that spent in overcoming internal losses, an amplifier should operate at as high an excitation level as possible. This may mean operating an amplifier above its free-running oscillation threshold. A limitation to the maximum pump power is the increase in ASE. With too high an excitation, ASE dominates over the amplified input laser field, resulting in a quenching of the amplifier gain, efficiency and coherence. ASE effects may be mitigated by increasing the input laser intensity, decreasing the amplifier facet reflectivities, or, in some cases, tuning the master oscillator so that it is resonant with the amplifier. The analysis indicates that minimizing the facet reflectivity is the most effective way to circumvent ASE limitations to power scaling semiconductor laser amplifiers     

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.     

Power scaling in quantum-well laser amplifiers

The effects of increasing excitation on the performance of quantum-well semiconductor laser amplifiers were investigated. Amplified spontaneous emission (ASE) and gain roll over at high injected carrier densities are two limitations to the power scaling of these devices. A Rigrod analysis was used to study the effects of these limitations on the gain, ratio of signal to ASE power, and efficiency for different values of injection current, facet reflectivity, and input laser intensity. Comparisons are made with an equivalent amplifier operating with a bulk semiconductor gain medium. This analysis suggests that quantum-well semiconductor amplifier performance improves with a double-pass configuration     

Numerical modeling of Tm-doped double-clad fluoride fiber amplifiers

Theoretical modeling of Watt-level average power Tm-doped fluoride glass fiber amplifiers operating at 1.87 /spl mu/m is presented. To characterize and optimize these devices a computer model has been developed taking into account the full spectral information of the laser transition as well as all important ionic levels, their decay schemes and important cross-relaxation rates, being capable of modeling steady-state and especially transient characteristics of an optically pumped fiber as is needed for the amplification of short pulses. As a result, optimum fiber lengths and core sizes for maximum output power can be determined. It is shown that the influence of amplified spontaneous emission (ASE) onto amplifier efficiency and gain strongly depends on the fiber length for a given amplifier geometry, thus realistic modeling of the ASE background and its wavelength shift with respect to the fiber length is a key issue for the layout of amplifier fibers. The model is compared with experimental results obtained by amplification of 20-30-ns pulses at repetition rates in the range of 5-60 kHz. A good agreement between experiment and numerical results was reached without a substantial adjustment on the input parameters concerning amplification as well as continuous-wave ASE output power of an unseeded fiber.     

A 1.6-μm pumped 1.9-μm thulium-doped fluoride fiber laser andamplifier of very high efficiency

Results are presented for color center, and semiconductor, laser pumping of the 1.82 μm transition in a thulium-doped fluoride fiber. As an amplifier small signal gain efficiencies of 8.1 dB/mN were attained with a maximum gain of 36.5 dB being achieved for around 17-18 mW of launched pump power. As a laser a maximum slope efficiency of 84% and a minimum threshold for oscillation of 330 μW was also demonstrated for this system. Furthermore by suppressing all reflections down to <36 dB superfluorescent or ASE output was observed. In achieving efficient operation with a diode laser pump source this system shows strong commercial potential     

Model for bidirectional transmission in an open cascade of opticalamplifiers

We derive a model for signal propagation and the generation and propagation of amplified spontaneous emission (ASE) in a completely open bidirectional cascade of optical amplifiers. The effect of single Rayleigh scattering of both signal and ASE is included. Our model predicts and our experiments substantially verify that the dominant gain saturating mechanism is the Rayleigh backscattered ASE centered around the 1535 nm peak in the ASE spectrum and that the spectral shape of the output ASE becomes much more sharply peaked around the 1535 nm gain peak. In an experimental open cascade of three EDFAs the ASE spectral density at the 1535 nm gain peak is increased by as much as 8-9 dB, resulting in gain compression ranging from 1-2 dB near 1550 nm to as much as 35 dB nearer 1538 nm. This combined with multiple Rayleigh scattering of the ASE around our 1538 and 1540 nm signal wavelengths results in Rayleigh scattering induced reductions of the optical signal to ASE ratio of 1-2 dB at the output of the first amplifier increasing to 36 dB at the output of the third amplifier each signal traverses     

A novel ASE self-pumping technique for gain-enhanced L-band erbium-doped fiber amplifiers

A novel method of enhancing gain in long-wavelength-band erbium-doped fiber amplifiers is described using the unwanted amplified spontaneous emission (ASE) self-pumping technique. The unwanted ASE from a first stage amplifier is recycled into a second stage amplifier as a secondary pump. Gain improvements in the vicinity of 0.4 and 1.2 dB are obtained in the wavelength range of 1570–1600 nm. This enables support for higher channels in multiwavelength systems with negligible noise figure penalties.     

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.     

ASE effects in small aspect ratio laser oscillators and amplifiers with nonsaturable absorption

A three-dimensional calculation of amplified spontaneous emission (ASE) is presented that allows rapid determination (with suitable approximation techniques) of extraction efficiency and power gain for small aspect ratio (length/width) laser amplifiers and oscillators. These calculations include nonsaturable absorption, ASE line-narrowing effects, and end mirrors that reflect ASE flux back into the active gain medium. The applicability of this technique has been verified for laser aspect ratios below unity, and results are presented that show the optimum scaling of extraction efficiency and power gain for large fusion class lasers. The technique is particularly suited to first-order design efforts where fast turnaround results are required. Nevertheless, the approach shows an accuracy of at least 10 percent when the laser medium is efficiently loaded with coherent flux.     

High-gain, V-band, low-noise MMIC amplifiers usingpseudomorphic MODFETs

State-of-the-art, 60-GHz, low-noise MMICs based on pseudomorphic modulation-doped FETs, with 0.25-μm×60-μm gates offset 0.3 μm from the source ohmic, are discussed. Single-state low-noise amplifiers (LNAs) exhibited minimum noise figures of 2.90 dB with 4.1 dB of associated gain at 59.25 GHz. Dual-state MMICs had minimum noise figures of 3.5 dB and 10.8 dB of associated gain at 58.50 GHz. Cascaded four-stage LNAs (two dual-stage MMICs) had minimum noise figures of 3.7 dB and over 20.7 dB of associated gain at 58.0 GHz. Finally, when biased for maximum gain, the four-stage amplifier exhibited over 30.4 dB of gain at 60.0 GHz     

Theoretical Investigations of TRAPATT Amplifier Operation

A device-circiut interaction program has been developed for the study of TRAPATT amplifiers. The device is simulated using the programs developed by Bauhahn. A slug-tuned coaxial circuit is simulated with the circuit parameters chosen to model an amplifier for which experimental results have previously been published. Results including diode waveforms over the entire amplifier frequency band are presented. Seperate mechanisms have been identified as being responsible for the fall off in gain and power output above and below the center frequency. The maximum bandwidth which can be attained with TRAPATT amplifier is also estimated.     

Demonstration of 184 and 255-GHz Amplifiers Using InP HBT Technology

In this letter, 184 and 255 GHz single-stage heterojunction bipolar transistor (HBT) amplifiers are reported. Each amplifier uses a single-emitter 0.4 mum 15 mum InP HBT device with maximum frequency of oscillation (f_max) greater than 500 GHz and of 200 GHz. The 183 GHz single-stage amplifier has demonstrated gain of 4.3 plusmn 0.4 dB for all sites on the wafer. The 255 GHz amplifier has measured gain of 3.5d B and demonstrates the highest frequency measured HBT amplifier gain reported to date. Both amplifiers show excellent agreement with original simulation.     

Modeling erbium-doped fiber amplifiers

Erbium-doped fiber amplifiers are modeled using the propagation and rate equations of a homogeneous two-level laser medium. Numerical methods are used to analyze the effects of optical modes and erbium confinement on amplifier performance, and to calculate both the gain and amplified spontaneous emission (ASE) spectra. Fibers with confined erbium doping are completely characterized from easily measured parameters: the ratio of the linear ion density to fluorescence lifetime, and the absorption of gain spectra. Analytical techniques then allow accurate evaluation of gain, saturation, and noise in low-gain amplifiers (G≲20 dB)     

X- and Ku-band amplifiers with GaAs Schottky-barriers field-effect transistors

During recent years significant progress has been made in GaAs technology and the GaAs Schottky-barrier field-effect transistor now shows outstanding microwave gain and noise properties. Two experimental microwave amplifiers demonstrate that the device is very well suited for broad-band applications and that large bandwidth in the X- and Ku-band can be obtained with simple circuits. The first of the two three-stage amplifiers realized was optimized with respect to noise and a noise figure of 3.8 dB was obtained at 8 GHz; the maximum gain is 17.5 dB at 8.3 GHz and the 3-dB bandwidth is 1.3 GHz. The second amplifier has a maximum gain of 11.5 dB at 11.5 GHz. The gain is greater than 8.5 dB in the range 9.5-14.3 GHz.     

Practical design of double-clad ytterbium-doped fiber amplifiers using Giles parameters

We present a simple and accurate method for measuring the Giles parameters of a double-clad ytterbium-doped fiber. The characterization is performed by cut-back on the doped fiber under constant pumping. Using nonlinear curve-fitting of the amplified spontaneous emission (ASE) power-density spectra, along with iterative solution of the photon balance model, we compute both the small-signal gain at complete population inversion and the small-signal absorption of the fiber. The method successfully predicts the extraction efficiency of an amplifier operating at 1064 nm. The ratio between the signal power and the out-of-band ASE power at the output of the amplifier is also accurately predicted by introducing spurious feedback from the fiber facets in the photon balance model. This work shows that a fiber facet reflectivity of a few thousandths of a percent (-40 to -50) dB can significantly enhance the out-of-band ASE power.     

The Effect of Parasitic Elements on Reflection Type Tunnel Diode Amplifier Performance

The effect of the tunnel diode series inductance and stray capacitance on the gain and bandwidth of broadband reflection type amplifiers is considered. General stability criteria imposed by these reactance are given together with realizability conditions for ideal (flat gain), Butterworth and Chebyshev responses. The main effect of the parasitic elements is to restrict the range of gain and bandwidth which may be achieved for a given number of elements in the matching network. The minimum gain is restricted together with both the maximum and minimum bandwidths. Comprehensive sets of curves are given which enable a rapid design of either Butterworth or Chebyshev response to be accomplished, and a procedure is given for conversion of the low-pass prototype network to band-pass form in the presence of the parasitic reactances. The frequency transformation is used to obtain an upper limit on the center frequency of the band-pass amplifier imposed by the parasitic. The use of the design data is illustrated by numerical examples.     

脉冲光纤放大器中放大自发辐射对光纤储能的影响

在高功率脉冲光纤放大器中,由于增益介质长,抽运功率高,脉冲间隔产生的放大自发辐射(ASE)严重限制了光纤储能能力和可提取能量的提高。针对低重复频率、强抽运的条件,以稳态速率方程为理论基础建立了脉冲放大器模型,利用理论模型对脉冲放大器性能进行了分析,着重讨论了不同的数值孔径、激光功率填充因子、端面反射率、纤芯直径、光纤长度、抽运功率等参数对放大自发辐射的影响。讨论了光纤的储能、增益和可提取能量等的变化规律,给出了掺镱光纤中最大可提取的单脉冲能量以及放大器增益。     

A black box model of EDFA's operating in WDM systems

A black box model is derived for an erbium-doped optical amplifier and is successfully applied to 1480 and 980 nm pumped devices operated under conditions which are typical for wavelength division multiplex (WDM) systems. It allows compound optical amplifiers with arbitrary passive optical circuitry (isolators, couplers, taps, and equalizing filters) to be modeled on the basis of “black box” characteristics. The gain model is based on an analytic solution for the effective two-level laser system, i.e., it is equivalent with the results of most numerical EDFA modeling tools. The model for amplified spontaneous emission (ASE) based on an equivalent ASE source in front of the amplifier can be applied over a wide range of operating points     

双电极半导体光放大器的放大自发辐射谱和增益特性的实验研究

设计并制作了双电极多量子阱半导体光放大器（SOA）,对其放大的自发辐射(ASE)谱和增益特性进行了测试和分析。结果表明,注入电流密度分布对多电极SOA的ASE谱和增益特性有非常大的影响。通过调节注入电流密度比,ASE谱的半高全宽、峰值波长、峰值功率以及增益特性能够得到很好的调控。     

Performance analysis of high-density WDM systems using Raman amplification

The applications of Raman amplifiers as repeater amplifiers or post-transmitter amplifiers in a high-density WDM system are theoretically investigated. There exists an optimum pump power which results in maximum amplifier gain. The result shows that amplifier gains up to 50 dB and 20 dB are achievable for use as a repeater and a post-transmitter amplifier, respectively.<>