Showing papers on "Four-wave mixing published in 2002"

Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers

Abstract: Supercontinuum generation is investigated experimentally and numerically in a highly nonlinear index-guiding photonic crystal optical fiber in a regime in which self-phase modulation of the pump wave makes a negligible contribution to spectral broadening. An ultrabroadband octave-spanning white-light continuum is generated with 60-ps pump pulses of subkilowatt peak power. The primary mechanism of spectral broadening is identified as the combined action of stimulated Raman scattering and parametric four-wave mixing. The observation of a strong anti-Stokes Raman component reveals the importance of the coupling between stimulated Raman scattering and parametric four-wave mixing in highly nonlinear photonic crystal fibers and also indicates that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond and nanosecond supercontinuum light sources.

Supercontinuum generation, four-wave mixing, and fission of higher-order solitons in photonic-crystal fibers

Abstract: The nonlinear propagation of femtosecond pulses in photonic-crystal fibers is investigated theoretically without the use of the slowly varying envelope approximation. Low-intensity supercontinuum generation caused by fission of higher-order solitons into red-shifted fundamental solitons and blue-shifted nonsolitonic radiation is studied in a large range of fiber and pulse parameters. It is shown that phase matching of degenerate four-wave mixing can be achieved in an extremely broad frequency range from the IR to the UV. Spontaneous generation of new frequency components and parametric amplification by four-wave mixing as well as its possible overlap with soliton fission are studied in detail.

Quantum-dot semiconductor optical amplifiers

Abstract: This paper provides prospects and current status of quantum-dot semiconductor optical amplifiers (SOAs) and their comparison with bulk and quantum-well technology, based on our pioneering work covering the proposal of their promising features, subsequent experimental demonstrations, and the design of all-optical quantum-dot switching modules. The proposed promising features are diverse; high-saturation power, high-speed amplification up to 160 Gb/s under gain saturation without pattern effect, high-speed cross-gain modulation up to 160 Gb/s without pattern effect, multiple-wavelength processing over broad gain spectra, and symmetric wavelength conversion by four-wave mixing. The operation theory of quantum-dot SOAs is provided in order to treat various aspects unique to quantum dots such as spatial localization, retarded carrier relaxation, and inhomogeneous and homogeneous broadening. Pattern-effect-free amplification at 10-40Gb/s, wavelength conversion by the cross-gain modulation at 10-40Gb/s, and symmetric wavelength conversion by four-wave mixing are experimentally demonstrated in 1.3-micron InGaAs/GaAs quantum-dot SOAs. All-optical quantum-dot switching modules are proposed, which we expect to work in the next-generation flexible all-optical photonic networks.

Symmetric highly efficient (/spl sim/0 dB) wavelength conversion based on four-wave mixing in quantum dot optical amplifiers

Abstract: Conversion efficiency to longer wavelengths in four-wave-mixing-based wavelength conversion in optical semiconductor amplifiers is generally much lower than that in the opposite direction. This study demonstrates experimentally that this feature is drastically improved, and the asymmetry between conversion directions is eliminated by using quantum dots in the active layer. We attribute this to a reduction in linewidth enhancement factor due to the discreteness of the electron states in quantum dots.

Opening Optical Four-Wave Mixing Channels with Giant Enhancement Using Ultraslow Pump Waves

Abstract: We show that by strongly modifying the dispersion properties of a four-level system, nonexisting wave mixing channels can be opened and significantly enhanced. Specifically, we show that coherent optical four-wave mixing with a pump wave mediated by electromagnetically induced transparency (thereby propagating with an extremely slow group velocity) will lead to many orders of magnitude enhancement in the amplitude of the generated wave. Contrary to common belief, a large transparency window, which causes a large propagation velocity, actually diminishes efficient mixing wave production.

Narrow-linewidth idler generation in fiber four-wave mixing and parametric amplification by dithering two pumps in opposition of phase

Abstract: Wide-bandwidth and high-gain fiber optical parametric amplifiers (OPAs) have been demonstrated recently. Their application as all-optical wavelength converters has been hampered by pump-induced converted-signal spectrum broadening, due to the required pump phase modulation. In this paper, we theoretically investigate and experimentally demonstrate a technique to cancel the converted-signal broadening by using four-wave mixing (FWM) or parametric amplification with two pumps phase-modulated 180/spl deg/ out of phase. The resulting converted-signal quality is comparable to that of the output signal.

Generation of macroscopic pair-correlated atomic beams by four-wave mixing in Bose-Einstein condensates.

Abstract: By colliding two Bose-Einstein condensates, we have observed strong bosonic stimulation of the elastic scattering process. When a weak input beam was applied as a seed, it was amplified by a factor of 20. This large gain atomic four-wave mixing resulted in the generation of two macroscopically occupied pair-correlated atomic beams.

Nonlinear and quantum atom optics

Abstract: Coherent matter waves in the form of Bose-Einstein condensates have led to the development of nonlinear and quantum atom optics - the de Broglie wave analogues of nonlinear and quantum optics with light. In nonlinear atom optics, four-wave mixing of matter waves and mixing of combinations of light and matter waves have been observed; such progress culminated in the demonstration of phase-coherent matter-wave amplification. Solitons represent another active area in nonlinear atom optics: these non-dispersing propagating modes of the equation that governs Bose-Einstein condensates have been created experimentally, and observed subsequently to break up into vortices. Quantum atom optics is concerned with the statistical properties and correlations of matter-wave fields. A first step in this area is the measurement of reduced number fluctuations in a Bose-Einstein condensate partitioned into a series of optical potential wells.

Generation of a 160-GHz transform-limited pedestal-free pulse train through multiwave mixing compression of a dual-frequency beat signal

Abstract: We report the experimental generation of a 160-GHz picosecond pulse train at 1550 nm, using multiple four-wave mixing temporal compression of an initial dual-frequency beat signal in the anomalous-dispersion regime of a nonzero dispersion-shifted fiber. Complete intensity and phase characterizations of the pulse train were carried out by means of a frequency-resolved optical gating technique, showing that 1.27-ps transform-limited pedestal-free Gaussian pulses were generated.

Large four-wave mixing of spatially extended excitonic states in thin GaAs layers.

Abstract: We study the size dependence of the nonlinear response of weakly confined excitons for the size region beyond the long wavelength approximation regime. The observed degenerate-four-wave mixing signal of GaAs thin layers exhibits an anomalous size dependence, where the signal is resonantly enhanced at a particular thickness region. The theoretical analysis elucidates that this enhancement is due to the size-resonant enhancement of the internal field with a spatial structure relevant to the nondipole-type excitonic state. These results establish the formerly proposed new type of size dependence of nonlinear response due to the nonlocality induced double resonance.

Continuous-wave parametric gain synthesis using nondegenerate pump four-wave mixing

Abstract: The authors demonstrate continuous-wave parametric amplification using modulational interaction between two spectrally distant optical pumps and a single section of uniform fiber. The proposed configuration is used to synthesize flat parametric gain over more than 22-nm bandwidth.

Suppression of intrachannel four-wave-mixing induced ghost pulses in high-speed transmissions by phase inversion between adjacent marker blocks.

Abstract: We propose a method for suppressing the intensity of the worst ghost pulse (a single ghost pulse between two long marker blocks) resulting from intrachannel four-wave mixing (FWM) in strongly dispersion-managed on-off keying optical transmissions by inverting the optical phases of the marker blocks surrounding the ghost pulse. We show both analytically and numerically that the method provides substantial suppression of the maximum ghost pulse energy. The suppression is experimentally verified in 40-Gbit/s transmission experiment over a 100-km non-zero-dispersion-shifted fiber. The proposed method may extend the reach of high-bit-rate optical transmission systems in which intrachannel FWM is the dominating nonlinear penalty.

Method for efficient four-wave mixing generation and short pulse generation equipment using the method

Abstract: The present invention provides a method for generating four-wave mixing to obtain idler light with high efficiency, in which the range of lengths of an optical fiber is appropriately set, and probe light and pumping light, having different frequencies, are launched into the optical fiber. When the nonlinear coefficient of the optical fiber, the loss per unit distance, and the wavelength and intensity of the probe light and pumping light are set to certain values, the idler light conversion efficiency at the output end of the optical fiber is a periodic function of an optical fiber having a maximal value and a minimal value. The maximum length of the optical fiber to be used to obtain four-wave mixing is set to be equal to or less than the length Lmax (Lmax=Lm+ΔL) which is given by adding the length of the optical fiber Lm, at which the idler light conversion efficiency takes on the first maximal value in the aforementioned periodic function and distance ΔL or 10% of L.

Phase matching in cascaded third-order processes

Abstract: We present an analytical, computational, and experimental study of frequency mixing and propagation that takes place in the guided-wave nonlinear frequency conversion of ultrashort pulses from the near infrared to the far and the vacuum ultraviolet. In addition to the standard phase-matching conditions for frequency mixing, we observe that cascaded frequency conversion, in which one of the input frequency components is generated simultaneously, is subject to two additional phase-matching conditions. In cascaded quasi-phase matching, one of the input fields oscillates in strength with a period that matches the phase mismatch. In gain-assisted phase matching, the growth of one of the input fields results in growth of the signal, in spite of a phase mismatch for the final mixing process.

Four-wave interaction in gas and vacuum. Definition of a third order nonlinear effective susceptibility in vacuum

Abstract: Semiclassical methods are used to study the nonlinear interaction of light in vacuum in the context of four wave mixing. This study is motivated by a desire to investigate the possibility of using recently developed powerful ultrashort (femtosecond) laser pulses to demonstrate the existence of nonlinear effects in vacuum, predicted by quantum electrodynamics (QED). An approach, similar to classical nonlinear optics in a medium, is developed in this article. A third order nonlinear effective susceptibility of vacuum is then introduced .

Consequences of induced transparency in a double-Λ scheme: Destructive interference in four-wave mixing

Abstract: We investigate a four-state system interacting with long and short laser pulses in a weak probe beam approximation. We show that when all lasers are tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM) field is strongly absorbed. The part that is not absorbed has the exact intensity required to destructively interfere with the excitation pathway involved in producing the FWM state. We show that with this three-photon destructive interference, the conversion efficiency can still be as high as 25%. Contrary to common belief, our calculation shows that this process, where an ideal one-photon electromagnetically induced transparency is established, is not most suitable for high-efficiency conversion. With appropriate phase matching and propagation distance, and when the three-photon destructive interference does not occur, we show that the photon flux conversion efficiency is independent of probe intensity and can be close to 100%. In addition, we show clearly that the conversion efficiency is not determined by the maximum atomic coherence between two lower excited states, as commonly believed. It is the combination of phase matching and constructive interference involving the two terms arising in producing the mixing wave that is the key element for the optimized FWM generation. Indeed, in this scheme no appreciable excited state is produced, so that the atomic coherence between states |0〉 and |2〉 is always very small.

Four-wave mixing of optical and microwave fields.

Abstract: We demonstrate degenerate four-wave mixing involving both optical and microwave fields. This four-wave mixing process, with fields that differ in frequency by 5 orders of magnitude, results from stimulated Raman scattering of the optical field from an atomic ground-state Zeeman coherence in warm rubidium vapor, which is induced and maintained by the microwave field.

Ultra-broadband high performance distributed Raman amplifier employing pump modulation

Abstract: A novel 110 nm, ultra-broadband distributed Raman amplifier has been constructed where pumps may be modulated to reduce non-linear interactions between pump wavelengths. This gave about I dB improvement in effective noise figure for shorter signal wavelengths channels and eliminated noise from an in-band four-wave mixing product produced by beating of pump wavelengths in the low dispersion transmission fibre.

Dynamics of Ligand Escape in Myoglobin: Q-Band Transient Absorption and Four-Wave Mixing Studies

Abstract: The dynamics of ligand escape from carboxymyoglobin are studied via Q-band transient absorption and diffractive optics-based four-wave mixing. The latter approach provides an interferometric method for following protein motions and energetics and allows unambiguous assignment of different signal components to specific dynamical processes, a problem that has hindered all photothermal, photoacoustic, and grating methods in the past. In particular, the real part of the four-wave mixing signal is isolated, and it is demonstrated that the excited-state population contribution to the signal can be identified and removed by tuning the probe to wavelengths where this contribution vanishes ("zero-crossings"). At these probe wavelengths, changes in the real part of the index of refraction are small compared to those of the imaginary part, making the heterodyne measurement sensitive to errors in setting the phase of the reference field. We solve this problem with a new balanced detection method that isolates the real part of the signal and is extremely robust against phase errors. The location of the zero-crossings in the population contribution to the index of refraction is found to be complicated by the presence of spectral shifts in the transient absorption that can be characterized and removed with a time-dependent Kramers-Kronig analysis. The spectral shifts are most apparent near the isosbestic points and show similar dynamics to the four-wave mixing signal, suggesting that both are sensitive to the same dynamical processes. These dynamics were observed previously by using four-wave mixing with an off-resonant probe and were assigned to CO migration out of the protein via a number of discrete channels. It appears that both the photoinduced protein conformational relaxation and CO migration through the protein contribute to the transient absorption signal, making the two effects difficult to separate. The direct coupling of the protein motions to the index of refraction changes in the four-wave mixing experiments aids in distinguishing the two processes.

A 10GBIT/S Tuneable Wavelength Converter Based on Four-Wave MIXING in Highly Nonlinear Holey Fibre

Abstract: We demonstrate a FWM based wavelength converter using a 15 m, highly nonlinear HF with a high SBS threshold. Error-free, efficient wavelength conversion of 10 Gbit/s NRZ signal over a ~10 nm bandwidth is reliably achieved

Four-wave mixing and parametric four-wave mixing near the 4P–4S transition of the potassium atom

Abstract: Potassium 4S1/2–6S1/2 two-photon excitation initiates the emission of several internally generated photons. For the first time two emission lines, one close to and one below the potassium 4P3/2 level, are reported for low pumping intensity. Radiation emitted below the 4P3/2 level is due to a parametric four-wave mixing process that uses the photons emitted at the 5P3/2–4S1/2 transition and a two-step four-wave mixing process generates the line emitted close to the 4P3/2 level.

All-optical picosecond-pulse packet buffer based on four-wave mixing loading and intracavity soliton control

Abstract: We report an all-optical packet buffer that is based on four-wave mixing loading and intracavity soliton pulse control. The target data packet consisting of picosecond pulses is loaded into the storage-buffer cavity by means of four-wave mixing with the pump pulses and stored at the corresponding idler wavelength. We stabilize the timing and amplitude of the stored idler pulses by applying intracavity all-optical soliton control. This is accomplished by combining the synchronous parametric-gain modulation achieved inside a pulse-pumped nonlinear-optical-loop mirror with the intensity discrimination provided by a nonlinear-amplifying-loop mirror. A 20 Gbit/s-packet pattern is loaded and stored for up to several seconds, corresponding to millions of circulations of the packet within the buffer.

Analysis of four-wave mixing between pulses in high-data-rate quasi-linear subchannel-multiplexed systems.

Abstract: We study four-wave mixing between pulses in two subchannels of a quasi-linear 40-Gbit/s subchannel-multiplexed system. For a pseudorandom bit string there are resonances in the mean of the ghost pulse energy and in the jitter of the energy in the marks as functions of the subchannel frequency spacing. However, away from these resonances the effect of four-wave mixing decreases as the subchannel spacing increases, permitting propagation over longer distances.

Ultra-Fast Radiative Decay of Spatially Extended Non-Dipole Type Excitonic State

Abstract: By means of the non-local theory of the pulse response and the analysis of the exciton-radiation coupled modes, we elucidate a recently observed short response time of degenerate four-wave mixing (DFWM) of weakly confined excitons in a thin film. Especially, it is expected that the excitons with non-dipole type of center-of-mass wavefunctions exhibit a peculiar thickness dependence owing to a spatial structure of the radiation field. The experimental results of the three-beam DFWM of GaAs thin layers strongly support our theoretical explanation of the radiative decay that is beyond the long wavelength approximation.

Raman-enhanced pump-signal four-wave mixing in bidirectionally-pumped Raman amplifiers

Abstract: We show that four-wave mixing (FWM) between pumps and signals, different from pump-pump FWM, can severely degrade the optical signal-to-noise ratio if the amplifier fiber has a zero dispersion wavelength lying midway between co-propagating pumps and signals.

Four-wave mixing in one-dimensional photonic crystals: inhomogeneous-wave excitation

Abstract: We present what is to our knowledge the first study of nonlinear four-wave mixing in one-dimensional photonic crystals. The possibility of inhomogeneous-wave excitation at the four-wave mixing frequency ω3=2ω1-ω2 for two noncollinear beams of femtosecond laser pulses is demonstrated. As a result of this effect the wave at the frequency of four-wave mixing may be guided along the surface of the photonic crystal. Experiments of this type open the possibility of development of a new tool based on photonic crystal devices for coherent Raman spectroscopy of surfaces and of nanometer and submicrometer scale layers.