Generation and detection of terahertz radiation by means of periodically and aperiodically poled crystals

The nonlinear optical methods for generating and detecting terahertz radiation using periodically and aperiodically poled crystals in the quasi-phase-matching mode are analyzed. Two radically different versions of optical pumping are considered: a quasi-continuous mode and pumping by Fourier-limited femto-second pulses. General relations are obtained, which determine the spectra of generated and detected waves through the optical pumping parameters and the crystal generator and crystal detector nonlinear transfer functions. It is shown that, by choosing the optimal spatial modulation of the crystal nonlinear susceptibility, one can control the shape of the generation and detection spectra. An optical scheme is proposed and the optimal parameters of domain structures for quasi-phase-matched generation and frequency-matched detection of forward and backward terahertz waves under quasi-continuous-wave optical pumping are reported.     

Terahertz-wave generation in a conventional optical fiber

We theoretically propose surface-emitted and collinear phase-matched terahertz (THz)-wave generation in a conventional optical fiber. The third-order nonlinear effect, four-wave mixing (FWM), is used to generate THz waves in an optical fiber. Surface-emitted THz-wave generation via FWM is realized using a single-mode fiber. Perfect phase matching is obtained at ~800 nm and 1.5 microm pumping, and it follows that third-order polarization in an optical fiber has the same phase at any point. In this situation, the optical fiber acts like a phased array antenna of the THz wave. Collinear phase-matching THz waves are obtained under the same conditions as for surface-emitted THz waves, and the THz wave is propagated in the silica cladding of the optical fiber. This is a promising method for realizing a reasonable THz-wave source.     

Nonlinear interference between two sets of frequency conversion processes with widely spaced pump wavelengths in an optical fiber

The processes of nonlinear interaction of two high-power pump waves with widely spaced frequencies are studied in an optical fiber. Substantially increased output power of the Stokes components, generated via stimulated Raman scattering or stimulated four-wave mixing by one of the pump waves have been obtained in the field of the other pump wave and its Stokes components. For the first time the possibility to excited new spectral lines in the spectrum of one of the pump waves with frequency shifts characteristic for the other one, is experimentally demonstrated.     

Mixing of electromagnetic waves by nonlinear crystals

Lasers have enabled researchers to observe a number of new nonlinear optical phenomena. These include stimulated Raman scattering, generation of harmonics, mixing of light waves, etc. [1]. Such nonlinear phenomena can be used for the modulation, detection, amplification, and mixing of light waves, as well as for other purposes. At the same time, they afford a means of gaining a deeper insight into the interaction of electromagnetic waves with matter.     

Pulse propagation in four-wave mixing process with group index difference of signal and idler waves

Four-wave mixing process with a large group index difference of the signal and idler pulses in a nonlinear optical fiber is theoretically investigated when the pump is continuous light. We prove that in the four-wave mixing process, the signal and idler waves would finally propagate in a common group velocity in spite of their different group indices. When the effective phase mismatch in four-wave mixing is not zero, their carrier frequencies shift in different directions. The asymptotic result of the signal and idler shape and carrier frequency shift are obtained. The theoretical prediction is validated by the numerical simulation.     

Resonant nonlinear optics of backward waves in negative-index metamaterials

The extraordinary properties of resonant four-wave mixing of backward and ordinary electromagnetic waves in doped negative-index materials are investigated. The feasibility of independent engineering of the negative refractive index and the nonlinear optical response as well as quantum control of the nonlinear propagation process in such composites is shown due to the coherent energy transfer from a control field to a signal field. Laser-induced transparency, quantum switching, frequency-tunable narrow-band filtering, amplification, and realizing a miniature mirrorless optical parametric generator of the entangled backward and ordinary waves are among the possible applications of the investigated processes.     

Multiwave mixing of singular light beams in resonant media

A method for transforming the topological structure of a singular light beam under multiwave mixing in the media with resonant nonlinearity is justified theoretically. The effect of self-and cross-modulation of the light waves under condition of nonlinear variation of the refractive index on stability of the optical vortices appearing in the multiwave mixing is studied.     

Four-wave mixing in a liquid suspension of transparent dielectric microspheres

The process of four-wave mixing in an artificial heterogeneous nonlinear medium—a liquid suspension of transparent dielectric microspheres—is considered. The dynamics of the concentration response to gradient forces that act on microspheres in the interference field of interacting waves are investigated on the basis of the Smolukhovskii equation. Kinetic equations for the amplitudes of light-induced concentration gratings that take part in the four-wave mixing are obtained with the use of the Fourier series expansion of the distribution function of microspheres. The ratios of the microsphere radius to the grating periods are obtained under which the resultant gradient force vanishes and, hence, a suspension of dielectric microspheres does not exhibit nonlinear properties irrespective of the intensities of the interacting waves. The kinetics of the process of four-wave mixing is investigated under efficient energy exchange between reference, signal, and reversed waves. It is shown that a liquid suspension of transparent dielectric spheres is a highly effective wideband nonlinear medium for reversing the wave front of low-intensity radiation of continuous-wave lasers.     

Determining the sea-roughness spectra from an optical image of the sea surface

An optical model of sea-surface imaging based on a two-scale presentation of sea roughness is developed. Using this model, an exact expression for the sea-surface image spectrum in diffuse sky light is obtained, which made it possible to estimate the nonlinear contribution of waves of different scales to the image spectrum. In particular, it is shown that the image spectrum is proportional to the wave tilt spectrum and the coefficient before the tilt spectrum is determined by the modulation of the short-wave contrast by long waves. Accuracy of determination of spectral contrasts of waves in surface-roughness anomalies (film slicks, internal-wave field, etc.) and accuracy of measurement of two-dimensional wave spectra from the sea-surface image spectrum are estimated. Examples of sea-roughness variability in the internal-wave field and examples of two-dimensional sea-roughness spectra are presented. All data were obtained by the optical method under full-scale conditions. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 1, pp. 53–63, January 2006.     

Evolution of first-order sidebands from multiple FWM processes in HiBi optical fibers

The generation of two idler waves inside a high birefringent (HiBi) optical fiber through three four-wave mixing (FWM) processes is studied theoretically. The coupled-equations for the field amplitudes are derived and analytically solved, in the co-polarized and orthogonal polarization schemes. The obtained solutions take into account the delayed Raman response of the medium. The polarization sensitivity of the generation of the idler waves is analyzed. Results show that the stimulated Raman scattering does not change the efficiency of the idler wave generation in the co-polarized scheme, whereas in the orthogonal polarization scheme that nonlinear process decreases the efficiency of the four-wave mixing processes. Results also show that this set of multiple four-wave mixing processes is physically quite different from the typical single or dual pump four-wave mixing configurations. Findings show that the power transfer from the pumps to the idler fields can lead to a monotonous growth, or a periodic evolution of the sidebands along the fiber. Results show that the process efficiency varies greatly with the angle between the two pump polarizations.     

Optical signal processing using nonlinear fibers

Ultra-fast optical signal processing is a promising technology for future photonic networks. This paper describes possible applications of nonlinear fibers to optical signal processing. The third-order optical nonlinearities in a fiber are discussed by analyzing the interaction of co-propagating optical waves. The properties of a nonlinear fiber are then considered in terms of optimizing the dispersion for achieving phase matching and decreasing walk-off. A highly nonlinear fiber (HNLF) is a practical candidate for an ultra-high-speed signal processor. Using HNLF, the following experiments are successfully demonstated: ultra-broadband wavelength conversion/optical phase conjugation by four-wave mixing, 160 Gb/s optical 3R-regeneration, and optical switching up to 640 Gb/s using a parametric amplified fiber switch. Steps for further improvements are also discussed.     

Spatial temporal wave mixing for space time conversion

A nonlinear optical processor that is capable of true real-time conversion of spatial-domain images to ultrafast time-domain optical waveforms is presented. The method is based on four-wave mixing between the optical waves of spectrally decomposed ultrashort pulses and spatially Fourier-transformed quasi-monochromatic images. To achieve efficient wave mixing at a femtosecond rate we utilize a cascaded second-order nonlinearity arrangement in a beta-barium borate crystal with type II phase matching. We use this ultrafast technique to experimentally generate several complex-amplitude temporal waveforms, with efficiency as high as 10%, by virtue of the cascaded nonlinearity arrangement.     

Simultaneous achievement of polarization attraction and Raman amplification in isotropic optical fibers

We present a theoretical analysis and experimental demonstration of the combined effects of polarization attraction and Raman amplification in isotropic optical fibers. The polarization attraction process is based on four-wave mixing interaction of counterpropagating pump and signal waves. We show that this technique can be used for the design of a highly efficient nonlinear system that permits the simultaneous processing of repolarization and amplification of light waves in dielectric media.     

The effectiveness of mixing differently polarized light waves at uniaxial nonlinear crystals

General expressions for the nonlinear interaction coefficients are obtained for three methods of mixing light waves at uniaxial crystals in the phase matching directions. Comparison of these coefficients enables one to choose the most effective method of mixing waves. Such a comparison is carried out for the generation of the difference frequency at a lithium niobate crystal and of the sum frequency at a KDP crystal in mixing the radiations of ruby and neodymium lasers.     

Influence of four-wave mixing and walk-Off on the self-focusing of coupled waves

Four-wave mixing and walk-off between two optical beams are investigated for focusing Kerr media. It is shown that four-wave mixing reinforces the self-focusing of mutually trapped waves by lowering their power threshold for collapse, only when their phase mismatch is small. On the contrary, walk-off inhibits the collapse by detrapping the beams, whose partial centroids experience nonlinear oscillations.     

Multi-purpose nonlinear optical microscope. Its principle and applications to polar thin-film observation

Multi-purpose nonlinear optical microscope is an optical microscope which images 2D distribution of the optical second harmonic (SH) waves from a specimen. Image contrast can be obtained either by inhomogeneous distribution of nonlinear optical tensor components or by the interference between SH waves from a specimen and a standard plate. This microscope also functions as a fluorescence (FL) microscope, and SH and FL images can be obtained from the same part of a specimen. Absorption and FL spectra from a specific part of a specimen are measured through an optical fiber which connects an ocular with a polychromator. These functions are especially useful for investigation of the J-aggregate state of polar dye molecules. Several photographs taken by the microscope revealed the structure of merocyanine dye/arachidic acid mixed monolayer and the role of bridge ions in the subphase. Fiz. Tverd. Tela (St. Petersburg) 41, 764–769 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Nonlinear nature of kinetic undamped waves induced by electrostatic turbulence in stimulated Raman backscattering

The influence of low-frequency waves of kinetic nature induced by electron trapping in backward Stimulated Raman Scattering (SRS) is investigated. Semi-lagrangian Vlasov-Maxwell simulations are carried out not only for periodic boundary conditions but also in the case of an open plasma with parabolic shape, in optical mixing. We provide a numerical example of generation of KEEN (kinetic electrostatic electron nonlinear) waves nonlinearly induced from the SRS through a mechanism we first here elucidate. In particular we identify a process of backward scattering of the SRS probe light from the so generated KEEN waves, which may provide a mechanism for the possible experimental observation and measurement of such nonlinear structures.     

Three-Wave Resonant Interaction in Optical Fibres on a Continuous-Wave Background

We predict that a three-wave resonant interaction (TWRI) for the excitations created from a continuous-wave background is possible in nonlinear optical fibres with a centro-symmetry. We show that in normal dispersion regime and near the zero-dispersion point of a single-mode optical fibre, the phase-matching condition for the TWRI can be satisfied by suitably choosing the wavevectors and frequencies of the exciting waves. The nonlinear envelope equations for the TWRI are derived by using a method of multiple-scales, and their explicit solutions for sum- and difference-frequency mixing are provided and discussed.     

Effects of weak localization of photons in nonlinear optics: Difference frequency parametric mixing

The difference frequency parametric mixing (DFPM) is considered in disordered nonlinear media. The angular distribution of difference frequency radiation intensity is shown to exhibit specific peaks due to the coherent interference of multiply scattered waves. The shape of the angular distribution of DFPM in a slab is calculated in the case of weak localization of the higher frequency wave. The limits of thick and thin slab is discussed as well as the prospects for future investigations of nonlinear optical phenomena in disordered media.     

Polarization-anisotropic scattering lines in LiNbO3

Lines of light-induced scattering with extraordinary polarization from two ordinary polarized pump waves in LiNbO₃:Fe crystals are observed. A nonlinear mixing of four copropagating waves in the crystal with photovoltaic charge transport is shown to be the reason for parametric amplification of the seed radiation scattered from optical imperfections of the sample.