Unidirectional Amplification Research Articles

Unidirectional amplification and shaping of optical pulses by three-wave mixing with negative phonons

A possibility to greatly enhance frequency-conversion efficiency of stimulated Raman scattering is shown by making use of extraordinary properties of three-wave mixing of ordinary and backward waves. Such processes are commonly attributed to negative-index plasmonic metamaterials. This work demonstrates the possibility to replace such metamaterials that are very challenging to engineer by readily available crystals which support elastic waves with contra-directed phase and group velocities. The main goal of this work is to investigate specific properties of indicated nonlinear optical process in short pulse regime and to show that it enables elimination of fundamental detrimental effect of fast damping of optical phonons on the process concerned. Among the applications is the possibility of creation of a family of unique photonic devices such as unidirectional Raman amplifiers and femtosecond pulse shapers with greatly improved operational properties.

Unidirectional Mechanical Amplification as a Design Principle for an Active Microphone

Amplification underlies the operation of many biological and engineering systems. Simple electrical, optical, and mechanical amplifiers are reciprocal: the backward coupling of the output to the input equals the forward coupling of the input to the output. Unidirectional amplifiers that occur often in electrical and optical systems are special nonreciprocal devices in which the output does not couple back to the input even though the forward coupling persists. Here we propose a scheme for unidirectional mechanical amplification that we utilize to construct an active microphone. We show that amplification improves the microphone's threshold for detecting weak signals and that unidirectionality prevents distortion.

Influence of feedback in wave based chaotic networks

The effective connectivity between input and output channels in complex coupled networks based on the transfer of electro-magnetic waves (RF, microwave or optical) plays an important role in many fields. The transfer of the waves between connected nodes in complex coupled networks is strongly influenced by the presence of non-reciprocal feedback channels with uni-directional amplification. The introduction of such non-reciprocal channels breaks the time reversal invariance of the connectivity matrix between the input and the output channels of the network. The feedback channels are intrinsically changing the spectral properties of the connectivity matrix and hence the associated static and dynamic transfer properties of the ingoing and outgoing ports of the network. Here we use a random matrix model to represent the complex and fully chaotic network without feedback. Extra feedback connections are introduced that change the reciprocity and time reversal invariance of the reference network and hence the connectivity. Exploiting a modern electrical engineering description of multi-port complex circuits in terms of the S-matrix of internal multi-port sections, we show by computer simulations the influence of feedback for increasing network complexity. The results are interpreted using a random matrix approach for the multi-port connectivity matrix. The effect of amplifying bi-directional or uni-directional connecting loop on the properties of the resulting S-matrix are computed for increasing network size. The signal transfer capacity of a multi channel the network based on the Smatrix shows for smaller networks a strong influence of the extra connected loop. In particular for the uni-directional amplifying feedback loop.

A Novel Dispersion-Free Interleaver for Bidirectional DWDM Transmission Systems

We propose a novel dispersion-free interleaver using optical delay lines by accurately locating the zeros in the transfer function. It has been implemented with the design of interleaver pairs with the same amplitude responses but opposite phase responses for bidirectional dense wavelength-division multiplexed (DWDM) transmission systems. The measured results are consistent with device simulation. We have further modified the original three-port design using unidirectional amplification, and a fourport interleaver has been built and demonstrated to achieve bidirectional DWDM transmission. In this paper, we fully studied and verified the applications of our four-port interleavers in bidirectional transmission. We demonstrated a bidirectional strain-line system over 210 km and a recirculating loop transmission over a 500-km standard single-mode fiber using 10-Gb/s on-off keying signals. Furthermore, we also demonstrated return-to-zero differential phase-shift keying (DPSK) and nonreturn-to-zero DPSK modulation formats for more than 230 km of transmission. For comparison, the different amplification functions, such as the erbium-doped fiber amplifier and the semiconductor optical amplifier, have also been probed in this paper. The experimental results have clearly illustrated the desirable functions of this novel bidirectional amplifier in this dispersion-free interleaver.

Wavelength-interleaving bidirectional transmission system using unidirectional amplification in a 5/spl times/100 km recirculating loop

This investigation presents a novel 50-GHz interleaved bidirectional transmission system with eight wavelengths that uses four-port interleavers in a bidirectional recirculating loop. This bidirectional configuration shares optical components in the fiber network and an interleaver is utilized to enable unidirectional amplification in an erbium-doped fiber amplifier to block noise associated with Rayleigh backscattering. After bidirectional transmission through 500-km LEAF fibers in the recirculating loop, power penalties of less than 2.5 dB were achieved at 10 Gb/s for all channels

Differential equation model of an Alfvén wave maser

A mathematical model describing the operation of an Alfvén wave maser in a laboratory environment is constructed from a continuous differential equation. The model incorporates the essential features of maser operation, namely, a resonator with a semitransparent boundary, a unidirectional amplification region, and a nonuniform magnetic field that provides frequency filtering. The spectral features and the parameter scaling predicted by the model are in agreement with laboratory measurements of an Alfvén wave maser [J. E. Maggs and G. J. Morales, Phys. Rev Lett. 91, 035004 (2003)]. The model provides a useful tool to explore a variety of operational scenarios for users of this novel wave source, including studies of Alfvénic interactions of relevance to space and fusion plasmas.

210-km bidirectional transmission system with a novel four-port interleaver to facilitate unidirectional amplification

A novel bidirectional transmission system is proposed and experimentally demonstrated using a four-port interleaver to enable unidirectional amplification. Following bidirectional transmission through 210 km of standard single-mode fibers, sensitivity variations of less than 0.2 dB, with a bit-error rate level of 10/sup -9/ at 10 Gb/s, were observed between bidirectional and unidirectional transmissions.

Angular and wavelength selectivity of parasitic holograms in cerium doped strontium barium niobate

The angular and wavelength selectivity of noise gratings in doped strontium barium niobate crystals for samples with two different thicknesses are investigated. We found that an offset in the optimum reconstruction angle with respect to the recording geometry occurs as a function of the thickness. This is a consequence of the unidirectional amplification. Thus the scattering intensity for the thinner sample is considerably lowered at the very recording conditions. We analyze the experimental results in terms of the kinematic theory of diffraction.

Mathematical models of RNA silencing: unidirectional amplification limits accidental self-directed reactions.

RNA silencing, found broadly throughout the eukaryotes, posttranscriptionally suppresses the expression of "aberrant" genes including those of many viruses and transposons. Similar to the specific immune system of vertebrates, RNA silencing works by generating specific responses against foreign elements and rapidly amplifying these responses to clear or otherwise inactivate the threat. Also like the vertebrate immune system, RNA-silencing systems risk making mistakes and mounting undesirable responses against the self. We develop a set of mathematical models of RNA silencing. We show that current models of RNA silencing do little to explain what prevents mistaken reactions from silencing vital organismal genes. We extend the basic models to show that the presumed unidirectional nature of the amplification process (namely, unidirectional RNA-directed RNA polymerase-mediated synthesis of secondary double-stranded RNA as observed in Caenorhabditis elegans) serves as a "safety mechanism" that safeguards against accidental generation of damaging self-directed reactions.

1.5 μm InGaAsP/InP vertically coupled semiconductor optical pre-amplifier

Vertical coupling of light output from a stripe InGaAsP/InP laser amplifier to the bottom of the substrate for detection by means of a 45° mirror has been demonstrated. The composite-cavity amplifier structure is shown to have an inherent low facet reflectivity and unidirectional amplification property. With a fiber-to-detector gain of 15.4 dB and a nearly diffraction-limited output spot size, the device is suitable for compact integration with a photodetector as an optical pre-amplifier in a high bit-rate direct detection system.

Summing-up of laser physics, laser technology, and industrial and scientific applications

Ultra short optical pulses of only a few wavelengths or a few femto seconds duration have proven to be very valuable for the understanding of elementary processes. The technology to generate tuneble short pulses with dye lasers in the colliding pulse configuration has been well developed. The further compression by means of self phase modulation in filters and dispersive cells to synchronize the frequency spectrum results in pulses of only a few wavelengths. The technique was reviewed by Kafka and Ippen. The measurements of the optical lengths are done by auto-correlations in a nonlinear cyrstal like KDP. A very important scientific application of these femto second pulses have been presented by Ippen. He reported on the electron relaxations in semi-conductors and metals. Using GaAs and heating the electrons with about 0.5 eV relaxation phenomena of the order of 200 fs are observed. In gold electron heating up to 1000 K and the speed of energy transport by the electrons of about 10 s cm/sec was observed. His measurements were based on an adjustable delay of a second probe pulse that follows the pump pulse. Although molecular laser systems may have many lasing transitions the linewdiths i.e. the continuous tunebility is in general poor compared with dye and solid state systems. However, the range of lasing frequencies is considerable due to optical pumping of many lasing gases. Especially the well devloped high power CO2 systems have shown to be very useful as pumping sources. A review of many optically pumped systems were presented by Gupta. The quantum efficiency can be as high as 28%. Several gases like NH 3 and CF 4 will also oscillate in continuous operation. The attractive features are the selective excitation, the non-destructive active medium and the adaptability. Furthermore, the gain anisotropy by pumping off center allows unidirectional amplification in ring lasers. To get full profit out of laser science the realization of reliabIe systems is a basic requirement. As long as laboratory studies are concerned with the prove of principle, the stability reproducibility may be poor. For further development also the engineering aspects with respect to scaling parameters, excitation efficiency, simplicity, optimization, etc. become important. Several aspects of laser construction and their performances have been presented by Witteman. He discussed the stability of a transition selective three mirror configuration, single discharge pulsed systems, and a table top e-beam pumped excimer laser construction.

Anisotropic gain in an optically pumped cw FIR laser

The observed asymmetry of tuning curves of a 70.6 μm CH 3 OH laser is interpreted as a consequence of unidirectional amplification in the optically pumped gas, a quantum mechanical double resonance effect. The occurrence of this effect. was verified by FIR gain measurements.

UNIDIRECTIONAL ULTRASONIC AMPLIFICATION IN A LASER-PUMPED MEDIUM

Unidirectional parametric amplification of an ultrasonic wave propagating in a laser-pumped birefringent medium was observed. Using x-cut quartz pumped by a ruby laser, power gains approaching 10 dB were measured at ultrasonic frequencies in the neighborhood of 75 MHz. The effect was identified as a ``coflow''-type parametric interaction.

A unidirectional amplifier with Esaki diodes

Two Esaki diodes can furnish unidirectional amplification when used in conjunction with a Hall-effect isolator. The input and output impedances of this amplifier are greater than the absolute value of the negative resistance of the diodes by a factor which is proportional to the voltage gain. The voltage gain-bandwidth product of such an amplifier is g_{T}/4\piRC , where g T is determined by the isolator and RC is the usual RC time constant of the Esaki diodes. Typically, g T is between 0.1 and 1. If one of the diodes is removed, the remaining network still amplifies unilaterally, but the impedance at the diode-less end drops to that of the isolator alone. In addition, a Hall-effect gyrator or circulator can be made essentially lossless by the addition of Esaki diodes.