Loop gain measurement in feedback amplifiers

This paper presents a method of loop gain measurement for DC-coupled feedback amplifiers, which overcomes the short-comings of some earlier well-known methods. The method discussed in this paper is similar in principle to one presented by Middle-brook (1975), but has the advantage that the instrumentation which is required for its successful application is much simpler, This method can be applied fairly routinely to measure loop gain accurately.     

High gain traveling-wave amplifiers

Modern techniques make it possible to build traveling-wave amplifiers with stable gains as high as 60 or 70 db. Such tubes can have relatively flat frequency response, and high gain over frequency ranges well in excess of two to one. In design it is necessary to pay strict attention to adequate isolation of the input and output so that spurious feedback paths do not cause oscillation or excessive variation of gain with frequency. In many practical applications, such tubes are required to operate with source and load impedances which produce large reflections of energy. Special design techniques result in tubes which are stable under such conditions. Another feature which can be achieved in modern tubes is a high ratio of backward loss to forward gain, so that the amplifier can very effectively isolate an oscillator. The recent design trend is toward higher currents and lower voltages, which is of direct benefit to the equipment designer, since it results in shorter tubes as well as simpler power supply designs, With the use of highly convergent electron beams, cathode current density can be kept quite low, and ion bombardment effects minimized, so that excellent life is achieved. Traveling-wave amplifiers may be readily phase modulated since small variations in the beam voltage yield large phase excursions. This characteristic is used for high level mixing. By proper gun design, it is possible to obtain good mixer action with modulation frequencies as high as a few hundred megacycles. The problems involved in obtaining the above characteristics and typical experimental results are illustrated by data on the STS-69, a half-watt twt for the 2 to 4 kmc frequency range.     

Multiple-loop feedback filters using current feedback amplifiers

Novel high-order filter topologies realised by employing current feedback operational amplifiers as active elements are introduced in this article. These are constructed from multiple-loop feedback paths and could be utilised for the realisation of filters of arbitrary order and type. An offered benefit is the requirement for employing only grounded capacitors and resistors. Two design examples are provided and the correct operation of the corresponding topologies has been evaluated through experimental results.     

Antenna designer's notebook-maximum off-boresight gain, revisited

A derivation for the maximum off-boresight gain of a constant-width, variable-length rectangular aperture is presented. Also given are derivations for circular apertures, one with uniform illumination, and the other with an optimum tapered illumination     

Analysis of polarization-dependent gain in fiber amplifiers

Polarization-dependent gain (PDG) in fiber amplifiers is studied theoretically. A mathematical model is developed by treating the dopant ions in glass host as partially anisotropic oscillators, and both the signal and the pump effects are taken into account. It is illustrated that PDG includes two components. One arises from polarization-selective excitation of the dopant ions by the pump; the other originates from polarization-selective deexcitation of the lasing ions by the saturating signal, known as polarization hole burning (PHB). PDG depends strongly on the operating state of the amplifiers such as the level of gain compression, the anisotropy of the dopant ions, and the relative polarization orientations of the signal and the pump. In a small-signal regime, PDG is induced only by the pump while in large-signal condition it is the sum of the two components. The good agreement between the theoretical calculations and the reported results confirms the validity of the model     

Dynamic gain equalization in two-stage fiber amplifiers

Relative gain differences among channels in WDM-amplified lightwave systems can be corrected in two-stage fiber amplifiers having complementary gain spectra in each stage. With two channels spaced by 2.5 nm, relative gain corrections of 1 dB were demonstrated. Simulations show that this method can be used to dynamically equalize the channel gain in long-distance transmission systems     

Negative feedback revisited

Rules are derived for choosing one among the four kinds of basic amplifiers to be embedded in a particular passive feedback network. The loop transmission is identified and conditions for mismatch are obtained from a unified analysis of feedback circuits. Past experience gained with voltage mode basic amplifiers, the laws of duality and mismatch are applied to choose the building blocks employed in the design of the remaining three kinds of basic amplifiers. This intuitive synthesis is confirmed by formal network transformations employing nullors.     

Nonlinear gain in vertical-cavity semiconductor optical amplifiers

We report on the wavelength-dependent nonlinear-gain properties of a vertical-cavity semiconductor optical amplifier. A step-like output versus input transfer curve and bistability are found on the long wavelength side of the resonant peak. The characteristics are caused by a dispersive nonlinearity arising from gain saturation in the device. The nonlinear transfer of the amplifier could be used for logic regeneration and all-optical logic operations, while the optical injection power required to switch the device is an order of magnitude lower than previously reported with edge-emitting devices.     

Modeling of gain in erbium-doped fiber amplifiers

An analytic method is described for fully characterizing the gain of an erbium-doped fiber amplifier (EDFA) that is based on easily measured monochromatic absorption data. The analytic expressions presented, which involve the solution of one transcendental equation, can predict signal gains and pump absorptions in an amplifier containing an arbitrary number of pumps and signals from arbitrary directions. The gain of an amplifier was measured over a range of more than 20 dB in both pump and signal powers. The measured theoretical results agreed to within 0.5 dB. Although the results described apply explicitly to EDFAs pumped in the 1480-nm region, they are also applicable to EDFAs pumped in the 980-nm region. The method is valid whenever the gain saturation by amplified spontaneous-emission noise can be neglected, which is typically the case for amplifiers with less than about 20 dB of gain     

Polarization dependent gain in erbium doped-fiber amplifiers

Measurement of polarization hole burning is reported for an erbium doped fiber amplifier. This effect produces a dependence of gain, measured with a weak probe, on the polarization of the saturating signal; the maximum gain occurs for probe polarizations orthogonal to the signal. The pump also contributes to the gain dependence on polarization. Reduction of the degree of polarization of both pump and signal below 10% reduces the polarization dependent gain substantially     

Ultrafast gain dynamics in InAs-InGaAs quantum-dot amplifiers

The ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution. Both absorption and gain regions are investigated. While the absorption bleaching recovery occurs on a picosecond time scale, the gain compression recovers with ~100-fs time constant, making devices based on such dots promising for high-speed optical communications     

Excited-state gain dynamics in InGaAs quantum-dot amplifiers

The ultrafast gain recovery dynamics of the first excited state (ES) is studied in an electrically pumped InGaAs quantum-dot amplifier at room temperature and compared with the ground-state (GS) gain dynamics. Pump-probe differential transmission experiments are performed in heterodyne detection and the gain dynamics are investigated as a function of injection current. An ultrafast (<200 fs) initial gain recovery of both GS and ES transition is found, promising for optical signal processing at high bit rates. The obtained results suggest the occurrence of a fast recovery of the state occupation mediated by carrier-carrier scattering as long as a reservoir of carriers in the ESs and wetting layer is present.     

Transient gain dynamics in saturated Raman amplifiers

This paper provides a detailed analysis of transient gain dynamics in saturated Raman amplifiers fed by wavelength division multiplexed (WDM) signals. Such dynamics are due to a pump saturation effect, known as pump-mediated signal-to-signal crosstalk, which is equivalent to the well-known cross-gain modulation in EDFAs. We provide for the first time a simple block-diagram model of the Raman amplifier, whose state is represented by the relative pumps depletion sensed by the signals. With such model, we are able to prove that the time constants of the Raman gain transients are of the same order as the pump–signal walk-off times. For counter-propagating pumps, the model is very accurate in predicting both the steady-state gain and the transient gain dynamics, with more than an order of magnitude improvement in computation time with respect to the direct solution of the signals and pumps propagation equations. The value of such model is therefore in the simulation of dynamic WDM networking scenarios in which the input powers have large swings in time. The model also extends to the co-propagating pump and well captures the time constants involved in the transients, although the accuracy in the predicted power levels is worse than that of the counter-propagating pump case.     

Confinement factors and gain in optical amplifiers

A new identity is derived which relates the gain and the field distribution (or confinement factor) in a dielectric waveguide with complex refractive indices. This identity is valid for any guided mode of waveguides with an arbitrary cross section. It provides a new check of the accuracy of mode solvers. Also, it can be used in a variational approach to predict the gain or loss of a guided mode based on knowledge of confinement factors. It is shown that a previous analysis that is often used, is not correct. In addition, approximate expressions for the gain in slab waveguides are presented     

Automatic gain flattening in optical fiber amplifiers via clampingof inhomogeneous gain

An Er-doped fiber amplifier with a flattened gain spectrum is obtained by incorporating the amplifier into a ring laser and making use of inhomogeneous broadening at 77 K. The results indicate several potential benefits for doped-fiber material systems that are inhomogeneously broadened at room temperature     

Current gain high-frequency CMOS operational amplifiers

The performance of high unity gain-bandwidth current gain-based CMOS operational amplifiers fabricated in a 1.5-/spl mu/m CMOS digital process is discussed. High unity-gain bandwidth was achieved by using short-channel MOS transistors operating in the current gain mode. Stacked current mirrors have been utilized as current gain stages to minimize the effects of the channel-length modulation in short-channel MOS transistors. Open-circuit gain of 60 to 70 dB, a unity-gain bandwidth of 70 to 100 MHz, and slew-rate of 200 V//spl mu/s were demonstrated at a DC power dissipation of 1-2 mW.     

Dynamic gain compensation in saturated erbium-doped fiber amplifiers

Dynamic compensation of low-frequency gain fluctuations in saturated erbium-doped fiber amplifiers is demonstrated. This compensation, based on a simple feedback-loop scheme makes it possible to reduce transient gain fluctuations efficiently across the whole amplifier bandwidth using only a low-power optical feedback signal. Such an, automatic gain control technique could be applied to suppress data packet interference due to traffic bursts in multiple-access networks, as well as in the implementation of long-haul fiber systems using erbium fiber amplifiers.<>     

Modeling gain dynamics of thulium-doped fiber amplifiers

The transient response of single-(14xx nm) and dual-(14xx + 1550 nm) wavelength-pumped thulium-doped fiber amplifiers is modeled. With an optimally chosen method for numerical integration of rate equations involving several energy levels, we can estimate the transient response and frequency dependent characteristics during add-drop transitions. Results correspond to the experimental data with good accuracy.     

Very high gain Nd:YLF amplifiers

High-gain Nd:YLF rod amplifiers in which single-pass, small signal gains of over 1700 have been obtained along with stored energy densities ⩾0.4 J/cm² are discussed. The ability of Nd:YLF amplifiers to support such gains is a result of high parasitic oscillation thresholds, due primarily to the low refractive index of the material. These results suggest that Nd:YLF is an excellent candidate for amplifiers where high specific stored energies and/or very high gains are required