Saturated Amplifier Research Articles

Thulium-doped ZBLAN blue upconversion fiber laser: theory

A theory is presented for thulium-doped ZBLAN fiber amplifiers and lasers that are upconversion pumped by a single source at ∼1150 nm and emit in the blue spectrum at ∼480 nm. The performance of both heavily saturated traveling-wave power amplifiers and laser oscillators is studied with a view to understanding device optimization. The laser model is compared with published experimental data.

Source second-harmonic control for high efficiency power amplifiers

A novel technology that drastically improves output power and efficiency of amplifiers has been developed, where source and load second-harmonic impedances, as well as the fundamental impedances, are optimally terminated in the input and output matching circuits. A record high 74% power-added efficiency (PAE) with 31.4 dBm (1.4 W) output power at a frequency of 930 MHz has been achieved as a single-stage saturated amplifier using an ion-implanted GaAs MESFET under the low supply voltage of 3.5 V. As a single-stage linear amplifier, an excellent PAE of 59% with 31.5 dBm output power has been realized at V/sub d/=4.7 V and f=948 MHz. Saturated and linear two-stage power modules operating at 900 MHz band with 31 dBm (1.25 W) output power have been demonstrated for analog and digital cellular applications respectively, the volume of which is as small as 0.4 cc. The saturated power module has achieved a PAE of 66% at V/sub d/=3.5 V, and the linear one has realized a PAE of 50% at V/sub d/=4.7 V.

Amplification under the standard quantum noise limit.

We show that a saturable amplifier can beat the "3 dB" quantum noise figure of linear amplifiers, approaching the ideal "0 dB" Heisenberg limit. Noise suppression is due to nonlinear saturation effects which cut off the high-intensity tail of the photon distribution. A Monte Carlo simulation of an actual semiconductor laser amplifier leads to a minimum noise figure of 0.51 dB, with a still sizable gain of 10.8 dB. The best performance corresponds to an output signal photon number which equals 0.7 times the saturation number.

Transient modeling of pulsed phase conjugation experiments in a saturable Nd:YAG amplifier

We model the transient dynamics of degenerate four-wave mixing in a saturable gain medium. We confirm that the transmission gain grating case has a higher reflectivity than the reflection grating case in correspondence to experiments. A transient model of self-pumped phase conjugation in a saturable amplifier in a loop arrangement is compared to a pulsed experiment in a Nd:YAG amplifier. Results from the model show agreement in the behavioural characteristics of the experimental system. Dynamical evolution of the system is described with reference to temporal build-up of the gain gratings during the writing pulse and the subsequent erasure during the lasing output pulse.

Collisional-cross-relaxation effects in amplified-spontaneous-emission lasers

The role of collisional cross relaxation in saturated laser amplification is considered by using a simple strong-collisional model. Expressions for the particle-state density distribution and gain are derived for a four-level system. These are reduced to a simple analytic form, subject to an approximation of wide applicability. The role of collisions in changing an inhomogeneous-broadened line to homogeneous, with a change from rebroadening to narrowing on saturation, is demonstrated. The results are shown to be consistent with the existing limited experimental data.

The effect of transient gain compression in a saturated EDFA on optical time domain reflectometry testing

This paper examines optical-time-domain-reflectometry (OTDR) pulse distortion due to transient gain compression in a saturated erbium-doped fiber amplifier (EDFA) and the resultant effect on testing long-haul optical fiber systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mode-locked operation of a master oscillator power amplifier

We report the first mode-locked operation of a tapered amplifier MOPA to generate short optical pulses with record high average powers and pulse energies. The MOPA is constructed using a discrete semiconductor oscillator and amplifier operating at 940 mm wavelength. Pulses of 4.2 ps duration are generated with average powers of 296 mW and peak powers of 28.1 W. The energy of these pulses was 118 pJ, corresponding to energies more than an order magnitude greater than the gain saturation energy in the mode-locked laser itself, and also corresponds to energies internal in the amplifier in excess of the gain saturation energy. Although the amplifier saturation energy limits the pulse energy obtainable in a mode-locked laser, the MOPA configuration demonstrates that generation of pulses with energies in excess of the amplifier saturation energy is feasible in a post-amplification stage. >

Experimental and theoretical investigation of neonlike selenium x-ray laser spectral linewidths and their variation with amplification.

This paper describes our recent x-ray laser spectral linewidth measurements and discusses the consequences of these measurements. The experiments observed the 206.38- and 182.45-\AA{} laser transitions in neonlike selenium using an extremely high-resolution soft-x-ray spectrometer, and measured the spectral linewidths and relative spectral intensities of the laser lines from amplifiers of varying lengths. The data allow the intrinsic (unamplified) linewidths of the 206- and 182-\AA{} lasers to be extrapolated as 50 and 35 m\AA{}, respectively. The intrinsic spectral width of the 206-\AA{} laser is somewhat greater than the predicted 43-m\AA{} Voigt-profile width based on calculated transition rates and on the expected ion temperature in the plasma, and while the number of data points obtained with the 182-\AA{} laser is small and a conclusive comparison with calculations is not possible, the extrapolated intrinsic linewidth of this laser is clearly consistent with the predicted 37-m\AA{} Voigt-profile width and is not significantly broader or narrower than expected. The data also show significant gain narrowing of both lines with increasing amplifier length and show no significant rebroadening of the 206-\AA{} laser in long, saturated amplifiers. We discuss line broadening and line transfer calculations we have performed which show that the saturated 206-\AA{} laser line transfer behavior can be treated homogeneously due to non-negligible homogeneous lifetime broadening contributions, which significantly reduce inhomogeneous Doppler saturation rebroadening, and due to collisional redistribution rates which are large enough in any case to homogenize the otherwise inhomogeneous Doppler contributions. We also discuss the implications of these results with regard to earlier suggestions that Dicke narrowing effects could play a significant role in determining the intrinsic line profiles of these laser transitions, and we conclude that Dicke narrowing effects are not likely to be significant for either laser.

Periodic pulse generator employing a driven unlocked surface acoustic wave oscillator

A circuit for the generation of periodic pulses is presented, composed of a driven unlocked surface acoustic wave oscillator incorporating an overdriven loop amplifier, followed by a saturated output amplifier. The circuit is shown to behave chaotically. A plot showing the period bifurcation as a function of injected power demonstrates that the bifurcation period-doubling process collapses when the oscillator enters the locked state.

Transient degenerate four-wave mixing in a saturable Nd:YAG amplifier: the effect of pump-beam propagation

A transient model of degenerate four-wave mixing in saturable amplifiers is developed for calculation of the phase-conjugate reflectivity in the weak-probe-beam limit. Our model takes into account the laser amplification of all the interacting beams together with the wave-mixing process between the two counterpropagating pump waves. Experimental investigations are made in a flash-lamp-pumped Nd:YAG amplifier with nanosecond pulses at 1.06 microm. A reflectivity of 22% is achieved when all the waves interfere within the laser medium.

Suppression of polarisation holeburning-induced gain anisotropy in reflective EDFAs

Gain anisotropy resulting from polarisation holeburning in a saturated reflective erbium-doped fibre amplifier can be suppressed using a Faraday effect mirror as the signal reflector. Complete suppression of 0.28 dB gain anisotropy in an amplifier having 11 dB gain compression was demonstrated.

Modelling of saturated chirped pulse amplification and recompression

A model is presented of chirped pulse amplification and recompression which computes the effect on the recompressed pulse profile of both gain saturation and limited gain bandwidth in the amplifier. Results are presented for transform-limited 100 fs input pulses stretched to several picoseconds and amplified to different ratios of output fluence to saturation fluence in a KrF laser. The results can also be interpreted more generally in terms of amplification to a specified ratio of output fluence to saturation fluence.

Formation of the spatial coherence of amplified spontaneous emission. 1. Regularly inhomogeneous active medium

An equation for a transverse correlation function is used in a study of the spatial coherence of amplified spontaneous emission (ASE) at the exit from a plasma active medium of an x-ray laser. The case of large Fresnel numbers in the far-field zone is considered. If regular refraction occurs, in the far-field zone the correlation length of the ASE emerging through the end of the active medium rises exponentially with an increase in the length z of the medium, whereas the correlation length of the ASE emerging through the side surface of the active medium is relatively short and independent of z in the limiting cases of linear and saturated amplification. The region of the maximum correlation length in the ASE beam does not coincide with the region of maximum values of the flux density. Saturation of the amplification and nonresonant losses reduce the spatial coherence. A model calculation is reported of the spatial coherence of a Ne-like selenium x-ray laser in a direction perpendicular to the target. The agreement between the calculated coherence coefficient and the experimental value is obtained on the assumption that the ASE emitted through the side surface predominates at the measurement point.

Transient absorption and its influence on energy extraction in a discharge-pumped F2 laser

Small-signal transient absorption of a discharge-pumped F2 laser (157.6 nm) medium was measured by using a Raman-shifted F2 laser (168.6 nm). The small-signal total absorption was obtained for 6 atm laser media at an excitation rate of ∼26 MW/cm3, where a small-signal net gain was independently measured to ∼35%/cm, as the sum of a transient absorption of ∼2.5%/cm and a stationary absorption of ∼0.5%/cm. In spite of the relatively large gain-to-loss ratio a significantly low maximum laser intensity of ∼2 MW/cm2, which gives zero extraction efficiency in the F2 laser medium, measured in heavily saturated F2 laser amplification experiments, suggests that the energy extraction is limited by additional absorption.

Photonics applications in switching: An ATM optical node

Abstract An ATM optical node architecture that carries out cell routing and buffering by photonic means, controlled by electronic circuits, is presented. An evolutionary approach, where cell aggregation and compression are introduced to achieve a large throughput, with a high performance, is also described. And we report experimental results that demonstrate the feasibility of all the relevant photonic functions, based on advanced optoelectronic devices, namely: fast wavelength reassignment, suitable for multigigabit operation, based on fast tunable lasers and semiconductor optical amplifiers; high bit rate packet storage in a multiwavelength fiber loop memory including fast optical gates; dynamic wavelength selection, based on distributed feedback (DFB) filters with nanosecond tuning response; and all optical demultiplexing of multigigabit/s time division multiplexing (TDM) signals by means of saturated semiconductor optical amplifiers.

Dynamic compensation of transient gain saturation in erbium-doped fiber amplifiers by pump feedback control

Dynamic compensation of transient gain saturation in erbium-doped fiber amplifiers by pump feedback control is investigated both theoretically and experimentally. It is shown by the theoretical analysis that the gain saturation of 10 dB can be suppressed to less than 0.3 dB if moderately high pump power of 140 mW is available. Furthermore, the required response time of the feedback loop is as slow as 0.5 mu s, which is easily implemented by inexpensive electronics. It is demonstrated experimentally that the transient gain saturation of 3 dB is reduced to 0.3 dB, corresponding to a 10-dB extension of the linear dynamic range of the amplifier.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental comparison of 980 nm and 1480 nm-pumped saturated in-line erbium-doped fiber amplifiers suitable for long-haul soliton transmission systems

The performances of 980-nm- and 1480-nm-pumped erbium-doped fiber amplifiers applicable to high-bit-rate, long-haul soliton transmission systems are compared. Depending on the system parameters and preferred pumping configuration, 980-nm pumping is likely to offer a noise figure advantage of between 1.0 and 1.5 dB. However, the tolerances on pump wavelength are significantly tighter around 980 nm than at 1480 nm. >

Performance of pi /4 SQAM in a hard-limited channel in the presence of AWGN

A strategy that reduces the spectral spreading when an ideal hard-limiter is used as a first approximation to a fully saturated power amplifier is presented. This strategy combines superposed quadrature amplitude modulation (SQAM) filtering with the pi /4-shift quadrature phase-shift keying (QPSK) digital transmission format adopted for the first generation of US digital cellular systems. Simulation results showed that this pi /4 SQAM filtering strategy increased capacity by 35% in comparison to hard-limited pi /4 QPSK. Using computer-aided design, a receive filter that would limit the degradation of E/sub b//N/sub 0/ to less than 1.4 dB at a bit error rate of P/sub e/=10/sup -4/ was selected.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Wavelength flattened saturated erbium amplifier using multiple side-tap Bragg gratings

By writing several loss-inducing photosensitive side-tap Bragg gratings in a short length of optical fibre, the gain variation of ±1.6 dB over a bandwidth of 33 nm in a saturated erbium doped fibre amplifier is reduced to ±0.3 dB. It is shown for the first time that these gratings may be written with predictable ease.

Dependence of performance of saturated in-line erbium-doped fiber amplifiers on pump wavelength around 1480 nm

The effect of pump wavelength around 1480 nm on the performance of saturated erbium-doped fiber amplifiers (EDFAs) suitable for use as in-line amplifiers for terrestrial and submarine systems is investigated experimentally. The results show that for high-gain amplifiers (25 dB) with high output powers (15 dBm), operating around 10 dB in compression, the performance is relatively insensitive to pump wavelength. However, for lower gain amplifiers (12 dB), pumped at low (15 mW) powers, the compression of the amplifier is found to decrease significantly with increasing pump wavelength, while the noise figure shows a weak minimum for a pump wavelength near 1480 nm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>