Impact of dispersion fluctuations on dual-pump fiber-optic parametric amplifiers

Effects of zero-dispersion wavelength (ZDWL) fluctuations on dual-pump fiber-optic parametric amplifiers are investigated analytically and numerically. It is found that the signal gain varies considerably from fiber to fiber even though each fiber may have the same ZDWL on average. Moreover, the gain spectrum becomes highly nonuniform for a given fiber because of such dispersion fluctuations. Numerical simulations show that this problem can be solved to a large extent by reducing wavelength separation between the two pumps, but only at the expense of a reduced amplifier bandwidth.     

Impact of pump-phase modulation on dual-pump fiber-optic parametric amplifiers and wavelength converters

Modulation of pump phases required for suppressing stimulated-Brillouin scattering is shown to cause large power changes in both the amplified and wavelength-converted signals when dual-pump fiber-optic parametric amplifiers are used. The physical origin of power changes is related to pump-amplitude modulations caused by fiber dispersion. Analytical as well as numerical calculations show that the signal-to-noise ratio of the signal or idler may reduce to below 23 dB when pump phases change rapidly with a sharp rise time.     

Noise characteristics of fiber optical parametric amplifiers

We derive an analytical theory for the noise figure of an undepleted and lossless fiber optical parametric amplifier (FOPA). Both the signal and the wavelength converted idler are investigated. Our theory is applicable for both an ideal pump power source, as well as a noisy one. We find that a noisy pump source can severely degrade the performance at high gain due to the stochastic gain-variations the signal and idler will experience. The theory is compared with Monte Carlo simulations of the FOPA and an excellent agreement is obtained. Simulations in the gain-depleted region show the possibility to reach below quantum-limited, phase-insensitive amplification for single channel transmission.     

Noise characteristics of semiconductor laser amplifiers

Noise characteristics of Fabry-Perot (FP) cavity type AlGaAs laser amplifiers are studied theoretically and experimentally. Noise power dependencies on pumping rate and input signal level are measured. Multimode rate equations with McCumber's fluctuation operator are solved for the parabolic band and the exponential bandtail models, both with the no k-selection rule. Experimental results are in good agreement with theoretical predictions.     

Gain characteristics of a dual-pump fiber optical parametric amplifier with Raman effect

We investigate the pump-depleted model of a dual-pump fiber optical parametric amplifier(FOPA) with Raman effect.As bandwidth increases,the gain profile of the distorted FOPA would be impacted seriously.Under the widebands,especially when the pump separation is large,zero dispersion wavelength(ZDW) fluctuation is another factor which can not be neglected.Numerical simulations with these comprehensive factors are mainly analyzed to obtain their influence on gain characteristics.Saturated gain spectrum is also discussed in detail.     

Influence of the zero dispersion wavelength fluctuation on the gain and noise performance in dual-pump fiber parametric amplifiers

Because of the zero dispersion wavelength (ZDWL) random fluctuations, the gain and noise figures deteriorate in optical parametric amplifiers based on highly nonlinear fibers (HNLFs). The detrimental role of ZDWL fluctuations on the gain and noise performance in dual-pump fiber optical parametric amplifiers (FOPAs) is presented. Numerical investigations of noise figure (NF) for different fibers with random fluctuation are analyzed. The noise figures for different fibers are varying even if the random fluctuation is small. Through choosing HNLFs with low ZDWL fluctuation and large nonlinear coefficient, high-gain and low-noise performance can be obtained.     

Simulating parametric amplifiers

In this paper, the problems of producing a general simulator model of a parametric amplifier are discussed. This model is to be simple, accurate, and applicable to many forms of parametric amplifiers. A brief discussion of the theory behind parametric amplification is followed by proposals for three models. Each of these is discussed in terms of its limitations and applications. The results of tests on two of these models are presented, and these results are discussed. Some emphasis is placed on the most sucessful of the models, which has been used to simulate a subharmonic oscillator.     

Two-diode parametric amplifiers

Analysis suggests that very useful performance may be achieved in parametric amplifiers incorporating two varactor diodes. The most interesting configuration is a bandpass amplifier in which the varactors are the shunt capacitive elements, operated with a phase shift between diodes of 90° at the centre signal frequency and with the diodes mounted with opposite polarities. With inphase pumping this gives unilateral operation and a large potential bandwidth.     

Intermodulation in parametric amplifiers

Third-order intermodulation products due to the charge-voltage nonlinearity of the varactor diode in a parametric amplifier have been computed. Some experimental data obtained from a two-carrier measurement have been reported. Finally, the noise power density in a telephone channel due to parametric amplifier intermodulation has been calculated as a function of total input power into the amplifier.     

Noise in amplifiers

In practically every type of research program in the physical sciences as well as in sophisticated engineering analyses, very small electrical signals must be measured and, in general, the limit of attainable precision and detectability is set by noise. This is true for the physicist and chemist performing nuclear magnetic resonance or spectroscopy experiments, for medical and biological researchers interested in evoked potentials, for geologists measuring small remanent magnetic fields in rock samples, for the metallurgist making Fermi surface measurements, and for the engineer performing vibration analysis and sensitive bridge measurements. These are only a few examples of applications in which noise plays a critical role in limiting measurement precision and signal detectability. This article discusses some of the inherent problems and describes techniques for improving signal-to-noise ratio.     

Gain stabilisation of parametric amplifiers

A method of stabilising the gain of a parametric amplifier from the pumped conduction current of a varactor diode is described. Long-term stabilities of 0.1dB/week have been achieved on a 13dB-gain amplifier with diode currents of less than 1 ?A.     

Subharmonic pumping of parametric amplifiers

The purpose of this paper is to present in some detail the operation of a parametric amplifier pumped subharmonically as compared to being directly pumped. As considered here, subharmonic pumping does not involve harmonic pump power generation (external to the varactor) but the utilization of higher-order time-dependent capacitances to yield parametric amplification by employing, basically, only a three-frequency system. The analysis given here is based on an evaluation of the Fourier series representation of the time-dependent capacitance resulting from large-signal ("hard") pumping of varactors. This evaluation indicates that significant values of higher-order time-dependent capacitances suitable for parametric amplification are obtained with relative pump swings in excess of about 90 per cent. Utilizing these higher-order time-dependent capacitances, the amplifier operation for various orders of subharmonic pumping is treated, including such factors as pump power requirements, gain, and noise figure. It is shown that, under certain conditions, less pump power is required to generate the same negative conductance than with direct fundamental pumping. Furthermore, for the same pump power and fundamental pump frequency, it is determined that significant improvements in amplifier noise figure are achieved by employing subharmonic pumping, provided varactor losses are small. From the results obtained by both analysis and experiment, it is concluded that subharmonic pumping, even without harmonic power generation, is not only feasible but can be very useful up to C-band signal frequencies with existing varactors.     

Investigation on gain characteristics in non-degenerate cascaded phase sensitive parametric amplifiers

We theoretically and experimentally show the impact of the ratio between the signal and idler generated from the PIA part on the gain characteristics in the continuous wave (CW) pump non-degenerate cascaded phase-sensitive fiber optical parametric amplifier (PS-FOPA). The results show that the length of highly nonlinear fiber (HNLF) used for generating the idler can cause the variation of power ratio between the idler and signal, which significantly affects the gain characteristics of the PS-FOPA under the small signal gain condition. To obtain high gain, it is better to choose long HNLF to generate idler. In our experiment, 5.5 dB gain and 18 nm bandwidth (on/off gain>10 dBm) in PS-FOPA can be achieved when 300 m-long HNLF instead of 200 m-long HNLF is used in PIA.     

Detuning effects in parametric amplifiers

Some detuning effects in parametric amplifiers have been observed when the pump frequency is above and below the optimum value. A possible explanation of these effects is suggested.     

Low-noise room-temperature wideband parametric amplifiers

Two room-temperature parametric amplifiers are described which have gains of 26 dB. One has an excess noise temperature of between 74 and 85 K across the band 3.7?4.2 GHz, and the other has an excess noise temperature of between 85 and 99 K across the band 7.25?7.75 GHz.     

Ion/electron-beam parametric amplifiers

Recent experiments with positive-ion beams suggest that an ion?electron beam, consisting of an electron beam superimposed on an ion beam, such that the two beams have different drift velocities, is realisable. A five-frequency parametric amplifier using an ion?electron beam is proposed.     

Monolithic GaAs transimpedance amplifiers for fiber-optic receivers

The authors describe the design of transimpedance amplifiers using GaAs MESFET technology. A GaAs transimpedance preamplifier for fiber-optic receivers has been fabricated with two gain stages and an inducer-FET load structure that reduces noise. The two-stage amplifier design provides increased open-loop gain as compared with a single-stage design, and greater closed-loop stability than a three-stage amplifier. An automatic-gain-control (AGC) circuit that varied the value of the feedback resistor was incorporated into the design     

Nondegenerate electron beam parametric amplifiers

Cyclotron wave amplifiers with a pump-to-signal frequency ratio of 5:1 (rather than the usual 2:1) have been built and have given a single-channel noise figure of 2 db. This paper reports on the difficulties which had to be overcome and the restrictions which had to be observed before achieving what, at the start, had looked deceptively simple. The paper begins with a discussion of the relative merits of assigning signal, idler and pump waves in various ways to beam and circuit. In the preferred arrangement the circuit carries only the pump wave; signal and idler are cyclotron waves on the beam. The validity of Tien's equations for the propagation constants and of the Manley-Rowe relations for the power-frequency ratio is demonstrated for the case of cyclotron waves. It is also shown that in a nondegenerate tube, idler noise must be extracted, no matter how high the idler frequency may be. Early experiments by C. B. Crumly, using interdigital couplers for signal and idler, gave very poor noise figures. The twisted coupler, a new structure with high coupling efficiency, was introduced but failed to reduce the high noise. It was then found that a signal carried by a cyclotron wave of low phase velocity was attenuated as it traveled along the beam, and that the noise extraction process worked poorly. This was traced to the spread of axial electron velocities, a source of trouble mentioned by C. P. Lea-Wilson and later by E. I. Gordon. P.G. Everett's measurements of this effect are described. It is then shown how the consequences of the effect may be minimized. Nondegenerate tubes are described in which the velocity spread effect is rendered almost harmless. The twisted coupler structure is particularly well adapted to experiments with synchronous waves; a brief report on such experiments is included.     

Active reactance compensation of parametric amplifiers

The increase in bandwidth potential of a simple parametric amplifier obtained by adding active rather than passive filter elements to the signal circuit is discussed. Measured performance data of an amplifier constructed for the 4GHz satellite-communication band is given; by using this technique, a sevenfold increase in bandwidth at 20dB gain was obtained