Saturated Amplifier Research Articles

A 2.14-GHz Chireix outphasing transmitter

A Chireix outphasing system for 2.14 GHz including two saturated class-B pseudomorphic high electron-mobility transistor power amplifiers (PAs) and a Chireix power-combining circuit is reported in this paper. In an outphasing system, an arbitrary input signal is divided into two constant envelope branches. The phase-only modulated branches are then amplified with high-efficiency nonlinear PAs. By controlling the phases of these branches, the original signal waveform can be reconstructed, ideally with high efficiency and perfect linearity. In this paper, the design and implementation of an experimental Chireix outphasing system for a wide-band code division multiple access 2.11-2.17-GHz downlink band is presented. The measured system efficiency for 7-dB backed-off quadrature phase-shift keying signal was 42.2% with a channel power of 31.2 dBm.

On conservative and dissipative solitons in media with a periodic spatial modulation

A comparative theoretical analysis of properties of conservative and dissipative optical solitons in media with a periodic spatial modulation of optical characteristics is performed. It is shown that, in the case of modulation in the longitudinal (with respect to the axis of predominant propagation) direction, the mechanism of decay of conservative solitons because of the delocalization of their Fourier harmonics takes place, whereas, for dissipative solitons, this mechanism is absent. In the case of modulation in the transverse direction, the presence of discrete dissipative solitons in a set of optical fibers with nonlinear (saturable) amplification and absorption is shown, which, to a considerable extent, are similar to conservative discrete solitons.

Quantum noise measurements in a continuous-wave laser-diode-pumped Nd:YAG saturated amplifier

We present measurements of the power noise due to optical amplification in a Nd:YAG free-space traveling-wave amplifier as the amplifier transitions from the linear regime into the heavily saturated regime. The quantum noise behavior is demonstrated by saturating the gain of a 100-W class zigzag slab amplifier with a high-power beam and measuring the power noise detected by a single-spatial-mode probe beam traversing the same optical path through the amplifier.

A SYMMETRIC PIECEWISE-LINEAR CHAOTIC SYSTEM WITH A SINGLE EQUILIBRIUM POINT

In this paper, we propose a new autonomous electronic oscillator designed with some modifications of the well-known Wien bridge oscillator. In the mathematical model planned for such a circuit, the nonlinearity in the operational amplifier saturation is considered and reference is made to the only equilibrium point at the origin of phase-space. We show how the relation between the bifurcation parameters starts stable oscillations, providing an example for chaotic behavior and bifurcations diagrams. Finally, we conclude with a brief summary of the oscillators operation using a parameters plane.

Burst of strongly polarized maser emission at 1612 MHz in the proto-planetary nebula OH17.7–2.0

We report the discovery of a burst of highly polarized OH maser emission at 1612 MHz in the proto-planetary nebula candidate OH17.7-2.0. The total flux density of two red-shifted features at 72.8 and 73.3 km s -1 increased nearly threefold over a period of ∼430 days. This burst was not associated with any systematic changes of the other 1612 MHz maser features as well as of the 1665 and 1667 MHz masers. The burst was characterized by a linear growth of left-hand circularly polarized emission accompanied by considerable linearly polarized emission. During the burst, the degrees of circular and linear polarization increased up to -80% and 15% respectively. A relation found between the line width and the peak flux density of the two bursting features implies a saturated amplification. Among several possible causes of the burst, a local propagation effect due to Zeeman overlap initiated by evolutionary changes in the shell appears most plausible.

Design of amplifier- and modulator-integrated laser diode for 10-Gb/s 80-km transmission

We present the amplifier- and modulator-integrated laser diode designed for 10-Gb/s transmission over 80-km distance. An antireflection window structure is implemented at the tilt facet of the amplifier to reduce optical feedback by 40 dB. A monitor photodiode is monolithically integrated in the antireflection window region for accurate power regulation. To better control the amplifier input power and to reduce the feedback of amplified spontaneous emission, an attenuation inner-window is incorporated by removing a section of the waveguide. The reproducibility of the chip fabrication process is tested to confirm the design margin. Through amplifying the modulated signals and reducing modulator chirp by amplifier saturation, high-power optical transmission is demonstrated.

Grazing Incidence Pumping X-Ray Laser Scheme with Applicationof Paraboloid Mirrors

A grazing incidence pumping x-ray laser scheme using JIGUANG II isproposed. We investigate the characteristics of the focusing ofparabolas, and obtain the optimal intensity distributions in the targetsurfaces with paraboloid mirrors of different parameters by thevectorial ray-tracing. Kinetic simulations are carried out. The resultsshow that due to the enhancement of absorption by the selectable plasmaregion (gain region), saturated amplification of the x-ray laser can beachieved with 100 mJ energy in pre and pumping pulses, respectively, on3 mm Ti targets.

Statistical noise properties of an optical pulse propagating in a nonlinear semiconductor optical amplifier

We present a theoretical analysis and an experimental study of the statistical properties of the noise accompanying an optical pulse propagating in a nonlinear semiconductor optical amplifier. Several degrees of gain saturation corresponding to different levels of optical signal-to-noise ratios (OSNRs) are examined. We employ the Heun numerical procedure to ensure proper convergence to the Stratonovich solution of the multiplicative propagation equation. This algorithm takes also into account the effect of gain saturation due to the amplified spontaneous emission noise. Moreover, the multicanonical Monte Carlo algorithm is used to efficiently calculate the probability density functions (pdfs), including the tails, of the peak of a pulse which emerges at the output of a saturated semiconductor optical amplifier. The results are compared to the corresponding pdfs obtained in a linear amplification system (where the optical noise is additive and Gaussian) having the same gain and under identical OSNR levels. We demonstrate that the pdf of the saturated amplifier is shifted toward lower power levels and is narrower, or equivalently, the mean and variance for the saturated amplifier case are smaller. Also, the difference between the two configurations increases with the degree of gain saturation. The theoretical predictions are confirmed by a series of experiments in which the pdfs at the output of the linear amplification scheme and saturated semiconductor optical amplifier are measured for an optical pulse of /spl sim/ 70-ps duration.

INTRA-CHANNEL COLLISION OF KERR LAW OPTICAL SOLITONS

The intra-channel collision of optical solitons, with Kerr law nonlinearity, is studied in this paper by the aid of quasi-particle theory. The perturbation terms that are considered in this paper are the nonlinear gain and saturable amplifiers along with filters. The suppression of soliton-soliton interaction, in presence of these perturbation terms, is achieved. The numerical simulations support the quasi-particle theory.

Curvilinear Motion of Laser Soliton Complexes

Conditions under which the center of inertia of stable complexes of dissipative optical solitons moves curvilinearly have been determined. Such a character of the motion of dissipative structures is caused by asymmetry in the distribution of the intensity and energy fluxes, and it is pronounced for laser solitons with strong interaction. The results of the numerical simulation of these complexes in the model of surface emitting lasers or laser amplifiers with saturated amplification and absorption are presented. Such complexes may be observed in various spatially distributed nonlinear dissipative systems, in particular, in the form of discrete solitons.

Intra-channel collision of parabolic law optical solitons

The intra-channel collision of optical solitons, with parabolic law nonlinearity, is studied in this paper by the aid of quasi-particle theory. The perturbation terms that are considered in this paper are the nonlinear gain and saturable amplifiers along with filters. The suppression of soliton-soliton interaction, in presence of these perturbations terms, is achieved. The numerical simulations support the quasi-particle theory.

Recipe for Intensity Modulation Reduction in SOA-Based Interferometric Switches

This paper presents a theoretical and experimental analysis of saturated semiconductor optical amplifier (SOA)-based interferometric switching arrangements. For the first time, it is shown that such devices can provide enhanced intensity modulation reduction to return-to-zero (RZ) formatted input pulse trains, when the SOA is saturated with a strong continuous-wave (CW) input signal. A novel theoretical platform has been developed in the frequency domain, which reveals that the intensity modulation of the input pulse train can be suppressed by more than 10 dB at the output. This stems from the presence of the strong CW signal that transforms the sinusoidal transfer function of the interferometric switch into an almost flat, strongly nonlinear curve. This behavior has also been verified experimentally for both periodically and randomly degraded, in terms of intensity modulation, signals at 10 Gb/s using the ultrafast nonlinear interferometer as the switching device. Performance analysis both in the time and frequency domains is demonstrated, verifying the concept and its theoretical analysis.

1 ps, 3 mJ KrF laser pulses generated using stimulated Raman scattering and fast Pockels cell

A new simple picosecond KrF laser source was developed by the saturated amplification of sharp leading-edge pulses generated by fast Pockels cell and stimulated Raman scattering (backward Raman pulse compression and four-wave anti-Stokes generation in methane gas). Short-laser pulses of 1.1 ps and 3.2 mJ with a beam quality closed to that of a diffraction-limited pulse were generated at a repetition rate of 10 Hz. The measured brightness contrast ratio of the main pulse to ASE was more than 1010. A stable and long-life operation has been achieved using a fast Pockels cell.

Saturation and noise properties of quantum-dot optical amplifiers

Based on extensive numerical calculations, quantum-dot (QD) amplifiers are predicted to offer higher output power and lower noise figure compared to bulk as well as quantum well amplifiers. The underlying physical mechanisms are analyzed in detail, leading to the identification of a few key requirements that QD amplifiers should meet in order to achieve such superior linear characteristics. The existence of a highly inverted wetting layer or barrier region, acting as a carrier reservoir, is central to this performance enhancement. It is shown that amplified spontaneous emission acts to decrease the inversion of the wetting layer states, thus helping to quench the gain of these states, which might otherwise dominate.

Thermal Profiling: Locating the Onset of Gain Saturation in Semiconductor Optical Amplifiers

Spatially resolved thermal profiling of a semiconductor optical amplifier enables measurement of the gain and saturation characteristics without recourse to direct optical measurements. We observe the spatial onset of gain saturation, quantify both the saturated and unsaturated gain, and accurately predict the optical output power. Effects such as suboptimal operating conditions are identified. Since this method is independent of optical measurements, it is ideal for use with integrated devices.

The properties of amplified spontaneous emission noise in saturated fiber Raman amplifiers operating with CW signals

The properties of amplified spontaneous emission noise in saturated fiber Raman amplifiers operating with CW signals

Optical Preamplifier Using Optical Modulation of Amplified Spontaneous Emission in Saturated Semiconductor Optical Amplifier

This paper demonstrates a novel optical preamplifier using optical modulation of amplified spontaneous emission (ASE) emitted from a saturated semiconductor optical amplifier (SOA). Requirements on optical alignments and antireflection coating for SOAs can be relaxed and the elimination of an optical filter gives us a large tolerance of an input light wavelength in the proposed optical preamplifier. A small-signal gain of a fabricated preamplifier was over 13.5 dB for an input power of below -20 dBm. An optical gain bandwidth was over 60 nm. We measured the small-signal response of the optically modulated ASE. The 3 dB bandwidths at SOA bias currents of 200, 300, and 400 mA were 5.8, 12.6, and 16.5 GHz, respectively. We also investigated improvements in receiver sensitivities with the proposed optical preamplifier. Our calculation shows a possibility of 10 dB improvement in receiver sensitivities by using the optical preamplifier at 10 Gb/s. The measured receiver sensitivity was -22.7 dBm at 10 Gb/s with the optical preamplifier, which is corresponding to an improvement of 2.5 dB in the receiver sensitivity. Further improvements of the receiver sensitivity can be expected by optimizing the structure of SOAs for saturating ASE.

The properties of amplified spontaneous emission noise in saturated fiber Raman amplifiers operating with CW signals

This paper describes the properties of amplified spontaneous emission (ASE) noise in forward and backward pumped saturated Raman amplifiers operating with CW signals. Analytical and numerical calculations confirmed by experiments demonstrate that ASE noise propagating in the pump direction (the direction of increasing gain) is compressed faster than the gain. This is in clear contrast to saturated erbium doped fiber and semiconductor amplifiers where the gain is compressed faster than the noise under all circumstances.

Investigation of a highly saturated soft X-ray amplification in a capillary discharge plasma waveguide

The characterization of the laser beam intensity distribution of a highly saturated 46.9-nm soft X-ray laser excited by capillary discharges is reported. The laser produces a total output energy of 300 μJ/pulse by amplification in plasma channels having lengths up to 0.45 m. A regime of laser amplification, which is almost free from the effect of the refraction defocusing, is experimentally determined. This regime produces a soft X-ray laser beam with an intense sub-milliradiant component. In the longer active medium the laser intensity distribution reaches the divergence of 0.6 mrad, which approaches the limit of diffraction. A comparison of the experimental results with the simulations performed with a ray-tracing code shows that the small divergence of the beam could be attributed to the effect of a weak index waveguiding of the laser beam through the long plasma channels.

Quasi-stationary optical solitons with dual-power law nonlinearity

The multiple-scale perturbation analysis is used to study the perturbed optical solitons that are governed by the Nonlinear Schrödinger's equation, with dual-power law nonlinearity. We have considered the perturbations due to nonlinear damping and saturable amplification. A new definition of the phase of the soliton is introduced that captures the corrections to the pulse where the standard soliton perturbation theory fails.