Four-wave mixing in traveling-wave semiconductor laser amplifiers

Nondegenerate four-wave mixing (NDFWM) effects on simultaneous amplification of copropagating and counterpropagating waves with a frequency spacing of a few gigahertz in a 1.5-μm nonresonant near traveling-wave semiconductor laser amplifier are discussed. Experimental results are in qualitative agreement with a theoretical model assuming the NDFWM process is mainly due to carrier pulsation produced by the beating of copropagating waves in the amplifier. The amplification of a phase-modulated wave is considered. Time-reversal properties of four-wave mixing are demonstrated     

Photon point process for traveling-wave laser amplifiers

The authors determine the evolution of the photon point process of a light beam as it passes through a traveling-wave laser amplifier (TWA). In particular, when coherent light is presented to the input of the amplifier, the output photon statistics are characterized by a marked-Poisson (MP) point process, which has a noncentral-negative-binomial (NNB) output photon-number distribution (PND). Using this distribution we calculate the probability of error (PE) for an ON-OFF keying (OOK) direct-detection photon-counting communication system, and show that the results differ somewhat from those obtained when the Gaussian-PND approximation is used. It is shown that receiver performance is optimized by filtering the amplifier output. Analysis of the point process is of interest because it permits the time response of the amplifier to be determined; this, in turn, allows the effects of intersymbol interference to be calculated     

Gain saturation in traveling-wave ridge waveguide semiconductor laser amplifiers

The saturation behavior of traveling-wave ridge waveguide diode amplifiers is studied both numerically and analytically. It is shown that an 'effective saturation power' can be used to characterize the saturation of these devices by up to 4 dB. The effective saturation power, which is sensitive to device geometry, carrier density, and operating wavelength, is useful in the design of the ridge waveguide optical amplifiers.<>     

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.     

On the bit error rate of lightwave systems with optical amplifiers

The problem of evaluating the performances of communication systems with optical amplifiers and a wideband optical filter is addressed. Exact probability of error expressions for optical signals in presence of amplifier spontaneous noise and photodetector shot noise are given and compared with those predicted by Gaussian approximations for amplitude shift keying (ASK), frequency shift keying (FSK), or differential phase shift keying (DPSK) modulations, both for ideal photodetectors and for the case where shot noise is significant     

Phase mismatch in traveling-wave parametric amplifiers

A misunderstanding is corrected regarding the effect of phase mismatch on the gain in traveling-wave parametric amplifiers. It is shown that gains greater than unity can be obtained even when the signal and idler waves do not grow exponentially.     

Four-wave mixing in traveling-wave semiconductor amplifiers

The traveling-wave equations of four-wave mixing in semiconductor amplifiers are solved analytically including saturation of the amplifier. Excellent agreement is obtained with numerical solutions of the traveling-wave equations. The analytical model is used to analyze a highly nondegenerate four-wave mixing experiment in a bulk semiconductor amplifier     

Effects of gain ripples in semiconductor optical amplifiers on veryhigh speed lightwave systems

An experimental and theoretical study of the effects of gain ripples in semiconductor optical amplifiers on lightwave systems at data rates in excess of 10 Gb/s is discussed. Significant system degradations, or even complete eye closure, occurred in an 11 Gb/s system due to the effect of large amplifier gain variations on the signals from a directly modulated distributed-feedback laser     

Optimized performance of erbium-doped fiber amplifiers insubcarrier multiplexed lightwave AM-VSB CATV systems

The authors study the effects of noise when erbium-doped fiber amplifiers are used as signal boosters for signal distribution in amplitude modulation, vestigial-sideband (AM-VSB) subcarrier multiplexed lightwave cable television (CATV) systems. A procedure for finding the amplifier length required to achieve a desired carrier-to-noise ratio at the receiver, while maximizing the allowed post-amplifier splitting ratios and transmission losses has been developed. A simple and easy-to-use approach that circumvents the computationally intensive optimization procedure and allows the design of systems whose performance is very close to optimal is discussed     

Gain saturation in traveling-wave semiconductor optical amplifiers

The gain saturation behavior of semiconductor traveling-wave optical amplifiers has been analyzed using a model that includes the specific dependence of gain on carrier concentration. Under the condition of a specific gain at a particular current, it is found that the saturation power strongly depends on the choice between quantum well (QW) or bulk amplifying medium but weakly on the detailed design of the device such as the number of QW's or the thickness of the bulk layer. The higher saturation power of the QW-based amplifier is caused by its logarithmic gain-current relation rather than its low optical confinement factor. Also, when the unsaturated device gain is specified, the designed saturation power can be obtained with the lowest drive current by using the highest optical confinement     

Gain saturation and propagation characteristics of index-guidedtapered-waveguide traveling-wave semiconductor laser amplifiers(TTW-SLAs)

A model is developed to investigate the propagation characteristics and gain saturation performances of tapered-waveguide traveling-wave semiconductor laser amplifiers (TTW-SLAs). The influences of several physical mechanisms are analyzed. Two taper geometries are studied in detail: the exponential TTW-SLA and the linear TTW-SLA. The first is shown to be more efficient for narrow tapers (output width <14 μm), whereas the linear TTW-SLA becomes superior for wide-taper geometries because of its better adiabatic propagation characteristics. Both tapered structures can provide a consistent improvement in the saturation output power compared to the conventional TTW-SLA, reaching absolute values on the order of ~18 dBm and ~23 dBm for the exponential and linear TTW-SLAs, respectively     

Solid-state laser pumping of 1.5-μm optical amplifiers andsources for lightwave video transmission

An optical power amplifier and a laser source are demonstrated at 1.5 μm. A diode-pumped Nd:YAG laser is used as the pumping source for an Er/Yb co-doped gain medium. The power scaling advantages of this approach are demonstrated. Up to +21 dBm of output power is obtained from the Er/Yb amplifier and up to +19 dBm is obtained from the laser source. The Er/Yb power amplifier was deployed in a 42-channel AM link with 40 km of fiber, and an optical loss budget of 18 dBm was demonstrated     

General design procedure for high-efficiency traveling-wave amplifiers

A general design procedure is developed for the design of both low-power and high-power high-efficiency traveling-wave amplifiers. The process is based on the selection of optimum values (for highest efficiency) of the design parameters C, QC, B and b from the large-signal curves and design of an amplifier with the particular type of RF structure specified by power and bandwidth requirements and operating parameters as near the optimum values as possible. In cases where the optimum design parameters cannot all be realized simultaneously, the design engineer will be able to select the parameter that he wishes to adjust. The procedure is first developed for helix-type tubes and then correction factors are derived that permit the design of amplifiers with any type of RF structure from the same set of curves.     

Large signal behavior of high power traveling-wave amplifiers

The results of an experimental investigation on the large signal behavior of a kilowatt power level helix type traveling-wave amplifier tube are presented. Operation with and without attenuators was investigated using a movable electromagnetic probe to measure power level along the tube. Quite different effects of drive power and beam voltage on the saturation level were found for operation with and without attenuators. The maximum power level is lower for attenuator operation. Also, power levels do not continue to increase with increasing beam voltage and drive power. In contrast, attenuator-less operation produces the highest efficiency, and the power levels continue to rise with increasing beam voltage and drive power. Conversion efficiencies as high as 25 per cent are obtained with an attenuator and as high as 40 per cent without an attenuator. Efficiency calculations based on small-signal theory can be made to agree reasonably well with the experimental attenuator-less operation efficiencies by assuming an appropriate ratio of the ac component of beam current to the dc component of beam current, i/I0.     

Theoretical power output and bandwidth of traveling-wave amplifiers

Expressions are developed to calculate the theoretical power output of traveling-wave amplifiers using any type of RF structure. Calculations are made for helix-type tubes and it is shown how to calculate the power output of tubes using more dispersive structures in terms of calculations made for helix tubes. The principal factors accounting for higher power output of dispersive structures are presented and discussed. The gain and bandwidth of forward-wave helix amplifiers are derived from the small-signal theory as functions of frequency and it is shown that the gain in db times the frequency bandwidth is a constant as a function of helix length for highgamma_{0}a'and the gain times the bandwidth squared is a constant for lowgamma_{0}a'.     

Performance degradations of multigigabit-per-second NRZ/RZlightwave systems due to gain saturation in traveling-wave semiconductoroptical amplifiers

A nonlinear model for a travelling-wave semiconductor optical amplifier has been used to determine eye closure degradations for 2.4 and 10 Gb/s NRZ/RZ lightwave systems due to gain saturation effects in the optical amplifier. At 10 Gb/s, with a carrier lifetime of 300 ps, the results indicate that the penalty is less than 1 dB for both NRZ and RZ systems provided that the ratio of the input power (P_in ) to the saturation output power (P_sat) is less than -17 dB. The NRZ system penalty is slightly larger than the RZ penalty when P_in/P_sat is larger than -17 dB. For example, with P_in/P_sat=-10 dB, the NRZ system penalty is about 2.8 dB versus 2 dB for the RZ system. The system penalty at 2.4 Gb/s is slightly less than that at 10 Gb/s. At P _in/P_sat=-10 dB, the NRZ system penalty is about 2.5 dB versus 1.5 dB for RZ     

Frequency characteristics of stationary optical pulses in lightwave communication systems with periodical gain created by semiconductor amplifiers

The propagation and evolution are studied of optical pulses in fiber optics transmission lines with both dissipation and periodical gain created by semiconductor optical amplifiers. Using numerical simulations, conditions that permit formation of averaged and dissipative optical solitons are considered. Their basic properties, such as shift of the frequency spectrum and mean frequency change are also examined. The qualitatively different tendencies are shown of the shift of the mean frequency for the two different types of stationary pulses. The influence is determined of the saturation on the change of the mean-frequency-shift velocity.     

High-capacity coherent lightwave systems

Recent theoretical work on coherent optical detection systems is reviewed and experimental results in high-speed coherent transmission are summarized. The theoretical advantages and limitations of the various modulation and detection formats are discussed and experimental progress towards the implementation of these systems is reviewed. The most significant obstacles to the attainment of quantum-noise limited detection at higher speeds are seen to be the requirement of uniform frequency response from electronic components and the local oscillator laser power requirement, which increases as the square of the bit rate. To make full use of the single-mode fiber bandwidth, frequency-division multiplexing of many moderate-rate channels is a very promising technique for local systems. For long-distance applications, frequency multiplexing is still possible but is limited by the need for optical amplifiers or wavelength-selective multiplexers