Crosstalk in 1.3 μm praseodymium fluoride fiber amplifiers

Praseodymium-doped fluoride fiber amplifiers have been demonstrated to provide good gain, output power, and noise performance in the second telecommunications window. This paper presents a study of the crosstalk performance of these amplifiers. A theoretical analysis is presented from which a numerical model is developed and is used to predict the crosstalk performance of a typical amplifier. Experimental results, using two separate signals within the gain band, show good agreement between measurement and theory, indicating that it is unlikely that crosstalk will present a problem in anticipated amplifier applications. From the measured crosstalk, an upper limit is placed on the lower level lifetime, indicating that the population of this level will have a negligible effect amplifier gain. A novel technique for measuring the upper state lifetime in an amplifier, based on pump power dependence of crosstalk, is described and experimental results which are in agreement with fluorescent decay are presented     

Effects of time-delayed amplification in TEA CO2lasers

A novel technique is reported for studying the behavior of the time-dependent gain in a TEA CO2amplifier. The method involves the incorporation of an additional amplifier tube into a laser cavity already containing a laser gain tube. The two tubes are independently operated, but so arranged that they can be fired with a controllable time delay between the discharge current pulses. This system permits effects of the additional gain tube on the lasing properties to be investigated as the time delay is varied. In particular, there is a time delay between the discharge current pulse of the laser tube and the onset of lasing. The variations in this delay produced by the firing of the additional amplifier tube have been investigated. The observed time-delay changes can be related to a simple theory for the time-dependent gain. The analysis of the measurements can be used to determine parameters describing the time-dependent gain. This method has been used to measure decay times of the gain for various gas mixtures. The techniques reported here can also be used to study other time-dependent effects within laser amplifiers.     

The electron cyclotron maser as a high-power traveling wave amplifier of millimeter waves

The electron cyclotron maser instability has been exploited as the basis for a new type of traveling wave amplifier which operates at unusually high-power levels at millimeter wavelengths. The first experimental model of this amplifier has been operated at 35 GHz and has demonstrated a stable gain of 17 dB and an output power of 10 kW (unsaturated). The gain was linear over a dynamic range > 30 dB. The absolute value of the gain and its dependence on current and magnetic field were in excellent agreement with theoretical calculations. Bandwidth and saturated power have yet to be measured directly, but no fundamental problems were observed which will prevent successful achievement of the design predictions (viz., bandwidth ≃ 10 percent, power on the order of 10⁵W, efficiency > 10 percent).     

Semiconductor laser amplifier as optical switching gate

The properties of a semiconductor laser amplifier as optical switching gate are investigated. Particular attention is paid to gain, contrast ratio, and switching time of the device. These properties are studied experimentally and theoretically with respect to the injection current, optical input power, and cavity resonances. The experimental arrangements and the theoretical method are described. As an example of the various applications of semiconductor laser amplifier gates, packet switching experiments with self-routing, employing cascaded switching gates, are reported. In a theoretical analysis the restrictions that the properties of semiconductor laser amplifier gates impose on a larger switching system consisting of many such gates are investigated     

Why a circuit sever affects traveling-wave tube efficiency

The effect of a circuit sever on traveling-wave amplifier efficiency has been calculated from the large-signal traveling-wave tube equations. Measurements made on high-power traveling-wave amplifiers, which utilize a severed coupled-cavity interaction circuit, have confirmed the calculated results. The results show that a traveling-wave amplifier with C = 0.1 and QC = 0.25 must have 26 db or more growing-wave small-signal gain beyond the sever to obtain the maximum theoretical efficiency. If the growing-wave gain is less than this value, efficiency is reduced; for example, if the gain is 4 db less, the efficiency is reduced by one half. The mechanism which causes this reduction of saturated output power can be understood from the computed electron distribution in phase and velocity. From these phase-velocity diagrams, the degradation in efficiency is seen to be caused by the debunching of the beam by space-charge forces in the region between the sever and the point where the microwave signal builds up to the value it previously reached before the sever. Further study of the computer results shows that the loss in efficiency caused by insufficient gain beyond the sever cannot be made up by depressed collector operations, and that the use of over-voltage to obtain increased efficiency in a severed tube may not be effective because of the resulting reduction in gain that over-voltaging causes.     

Transfer characteristics of Brillouin fibre amplifiers for use in self-homodyne coherent optical transmission systems

Presents for the first time, the basic input-output characteristics of a Brillouin fibre amplifier, and shows that these are in good agreement with the first experimental results reported recently by other workers on a novel and potentially very important application of Brillouin amplification in a coherent optical fibre transmission system. For this type of application, theoretical results are also presented on the optical pump power dependence of the amplifier gain per unit optical pump power.<>     

Modeling the bias and temperature dependence of a C-class MESFETamplifier

In this paper, a complete bias and temperature-dependent large-signal model for a MESFET is determined from experimental S-parameters and dc measurements. This model is used in the analysis of the performance of a C-class amplifier at 4 GHz over a -50° to 100°C temperature range and for different bias conditions. The dependencies of the elements of the equivalent circuit, as well as the amplifier gain on the temperature and the operating point, are evaluated. The gain optimization and the analysis as a function of temperature of the MESFET amplifier are done by using the describing function technique. Optimum bias device conditions in the C-class are obtained for maximum gain and also the flattest gain versus input power rate. A comparison between theoretical and measured results over temperature and bias ranges is shown. Experimental results show an excellent agreement with the theoretical analysis     

1.5 µm GaInAsP traveling-wave semiconductor laser amplifier

This paper presents a theoretical and experimental study in terms of small-signal gain, signal gain saturation, and noise characteristics of a 1.5 μm GaInAsP traveling-wave amplifier (TWA), realized through the application of SiOxfilm antireflection coatings. This TWA, having a residual facet reflectivity of 0.04 percent, exhibits a wide, flat signal gain spectrum and a saturation output power of +7 dBm at a 20 dB signal gain. The TWA also has a noise figure of 5.2 dB, which is the smallest value reported for semiconductor laser amplifiers. The experimental results are confirmed to be in good agreement with the theoretical predictions based on the multimode traveling-wave rate equations in conjunction with the photon statistic master equation analysis, which takes into account the amplifier material and device structural parameters. Signal gain undulation, saturation output power, and noise figure are also theoretically evaluated as functions of the facet reflectivity. The superior performance of the TWA demonstrates that the device is favorable for use in linear optical repeaters in fiber transmission systems.     

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.     

Semiconductor laser amplifier optimization: an analytical andexperimental study

Long-wavelength semiconductor laser amplifiers are investigated with respect to spectral gain properties such as peak gain wavelength shift and width of gain curve, employing different structural parameters such as thickness of the active layer and amplifier length. The model takes into account Auger recombination, thermal effects, and spontaneous emission. It is shown that there exists an optimum thickness of the active layer with respect to current density for a given gain and that increased length of the amplifier allows higher gains and reduced variation of peak gain wavelength with respect to variation of peak gain at the expense of increased saturation by amplified spontaneous emission and increased excess noise. An experimental verification of the theoretical model is reported     

Noise reduction in long-haul lightwave all-amplifier systems

The overall gain of a chain of erbium-doped fiber amplifiers in a long-haul all-amplifier system would be automatically stabilized if each amplifier were operated slightly into saturation. However, with the required low level of amplifier output power which is imposed by nonlinearity in the transmission fiber, the resulting pump power becomes too low to effectively invert the gain medium of the amplifiers. Consequently, the amplifier output noise level becomes too high for proper system operation. This problem is solved by pumping the amplifiers harder so that a higher gain and higher output power are achieved. The excess gain is then counteracted by an appropriate value of post-amplifier loss. Because of the higher pump power in this case, the amplifier noise is reduced significantly. This technique is investigated theoretically, and experimental work that verifies it is reported     

基于新一代半导体GaN的高效率功率放大器的研制

近年来在无线通信、雷达等领域,对发射的功耗要求越来越苛刻,而产品可放置的空间越来越小,这就要求功率放大器要有更高的效率以及更高的工作结温,新一代宽禁带半导体材料GaN能够满足该要求。基于CREE公司的GaN功放管CGH40045研制了一款S频段的功率放大器,主要进行了功率匹配、散热考虑、杂散抑制的设计。最终的测试结果显示,在300 MHz的带宽内功率增益≥50 dB,饱和输出功率≥46 dBm,工作效率≥50%,比之前采用的工作效率为30%的GaAs功率放大器有了显著的提高。可见在今后的通信系统中,基于新一代半导体材料GaN的功率放大器有着非常好的应用前景。     

A High-Gain, 28GHz, 200kW Gyroklystron Amplifier

A design study of a high efficiency/gain gyroklystron amplifier is performed to demonstrate amplified radiation power of 200kW operating at 28GHz. A key design feature of the present gyroklystron amplifier is that the amplifier is designed to be high gain so that it can be saturated by a low power solid state power amplifier. A non-linear, time-dependent, large signal numerical code is used to predict tube performance. Simulations predict that a stable amplifier radiation power of 214kW is produced with a saturated gain of 54dB, an electronic efficiency of 37%, and a frequency bandwidth of 0.3% from a five-cavity gyroklystron amplifier. The amplifier gain is found to be very sensitive to a beam velocity spread.     

CW microwave amplification from circuit-stabilized epitaxial GaAs transferred electron devices

Stable c.w. reflection-type amplification at C- X-band frequencies has been obtained from circuit-stabilized GaAs transferred electron devices biased as high as three times threshold. An instantaneous fractional bandwidth exceeding 50% and a small-signal linear gain near 10 dB have been realized with a single device for center frequencies from 5.0 to 9.0 GHz. A -1-dB-gain compression-power output typically above 100 mW and a total saturated power output near 1 W have been measured. A narrow-band doubly tuned amplifier response with a linear gain of 40 dB has also been measured. With a noise figure of 15 dB, these amplifiers have a dynamic range in excess of 90 dB. Highly nonlinear effects have been observed for large-signal narrow-band operation. Both gain and hysteresis effects have been observed. Large-signal effects of bias, frequency, and power level on amplifier performance have been measured.     

High power cladding-pumped Er3+/Yb3+ fiber amplifiers: technologies, performances and impact of nonlinear effects

Yb³⁺-sensitized Er³⁺-doped fibers are attracting increasing interest because of the high achievable performances, such as high gain and pump efficiency. High output power can be obtained from a double clad (dc) Er³⁺/Yb³⁺ co-doped fiber pumped with broad area high power pump laser diodes. The principle of amplification in this kind of co-doped fibers is presented in this paper. Different solutions for the injection of pump power in the 1^st-cladding have been described. The energy transfer mechanism in a Er³⁺/Yb³⁺ co-doped system including cooperative-upconversion process is explained. Gain and absorption properties ofdc fibers have been determined experimentally and inserted in a theoretical amplifier model. Good agreement between measurements and modelling has been obtained. Hybrid Er³⁺-Er³⁺/Yb³⁺ amplifier architectures are suitable to obtain + 30 dBm output power. The gain bandwidth is in the 1535–1565 nm range for single wavelength operation. A spectral gain flatness is observed in a reduced C-bandWDM operation (i.e. 1545–1565 nm) without gain-flattening filter. Nonlinear effects such as the optical Kerr effect or the stimulated Brillouin scattering can be observed in high power amplifiers due to the high output peak power confined in the fiber core. These two nonlinear phenomena have been investigated for different high power amplifier configurations. Numerical modelling have also confirmed the observed signal distortions.     

GaInAsP semiconductor laser amplifiers for single-mode fiber communications

Gain, polarization sensitivity, saturation power, and noise characteristics of quaternary semiconductor laser amplifiers of the Fabry-Perot (FP) and traveling-wave (TW) types are reviewed. The status of antireflection coatings for TW amplifiers is presented. New results concerning the polarization sensitivity and output saturation power of a 1.5-μm buried-heterostructure (BH) amplifier are reported. A theoretical model is presented concerning the influence of the waveguide structure on the maximum internal gain of a CW-operating 1.5- μm BH amplifier, including thermal effects, and a comparison of this model with recent experimental results is reported. The influence of nonresonant losses on the noise factor of 1.5-μm amplifiers is discussed.     

Distributed amplification: A new approach

A general formula is derived for gain of a distributed amplifier using nonuniform lines but with image impedance match at each connection. A study of the gain formula shows that the rise of gain can be eliminated by having different propagation functions for the sections. Thus a new method of constructing amplifiers having constant gain is obtained. The new amplifier also shows pronounced improvement in transient response. In order to verify some of the theoretical predictions, an experimental amplifier of this type was built, and its characteristics studied. It is found that the theoretical calculations and the experimental results are nearly matched.     

Flashlamp-pumped Cr:LiSAF laser amplifier

A flashlamp-pumped Cr:LiSAF laser amplifier has been developed that is capable of producing ultrashort pulses with a peak power of ~TW. A systematic experimental study of laser oscillation, its small-signal gain, and chirped-pulse amplification has been performed under the same pumping conditions. The characteristic properties and performance of the Cr:LiSAF laser amplifier are reported     

On Theory and Performance of Solid-State Microwave Distributed Amplifiers

The performance characteristics of n-link distributed amplifiers employing GaAs MESFET'S are studied. At first, formulas of tie symmetrical amplifier using lumped circuit elements are developed for the case of an idealized FET model. The theoretical analysis is then extended to distributed line elements and later to an S-parameter derived transistor model. In efforts to optimize amplifier performance, the restriction of circuit symmetry is subsequently removed and the performance characteristics of two concepts, that of equal characteristic impedances and that of equal line lengths, are proposed and compared. Based on this analysis and practical considerations, several three-link hybrid amplifiers utilizing the equal line lengths approach have been assembled and test results are reported. A gain of G = 5.5+-0.6 dB was measured over the bandwidth of 2-20 GHz. Across this frequency band a maximum VSWR of 2.2:1 for the input and 2.5:1 for the output terminaf have been reafized, while a minimum output power at the l-dB compression points of 19.3 dBm was achieved from 2-18 GHz. Agreement between measured and computed small-signal gain as well as reverse isolation is excellent.     

Characterization of Brillouin fiber generator and amplifier for optimized working condition of distributed sensors

The Brillouin fiber amplifier (BFA) has been used in Brillouin optical time and frequency domain analyzer based sensors. For BFA based distributed sensor the optimized working condition is to have the highest Brillouin gain, i.e., highest SNR, which avoids high pump depletion induced gain saturation. We have found that the optimum working condition for distributed sensor system is associated with the stimulated Brillouin scattering (SBS) threshold for BFA, which can be experimentally determined with Stokes power inflexion and/or Stokes spectrum linewidth minimum methods. This threshold depends on both pump and probe power instead of just the pump power as in Brillouin fiber generator (BFG), as well as on sensing length as confirmed by our experimental results and theoretical simulation. This was achieved by introducing the concept of absorption coefficient of the sensing medium defined as the ratio of the total output power to the total input power. We find that the medium absorption is minimized when input Stokes power is an order of magnitude lower than BFA threshold. This minimum is a signature of the balance between maximum Stokes gain and pump depletion which is also the reason why Stokes spectrum linewidth goes through a minimum.