Raman Nonlinearity Research Articles

Electronic anti-Stokes–Raman emission in quantum-cascade lasers

In this letter, the observation of electronic anti-Stokes–Raman emission in quantum-cascade lasers is reported. We present two distinct active region designs with enhanced anti-Stokes–Raman nonlinearity. The pump laser is monolithically integrated with the nonlinear region in a two-stack active core within the same waveguide. Electronic Raman emission was observed in both designs. Additionally, for the design with positive detuning, an incoherent upconversion signal resulting from optical pumping is detected.

Unsplit-Field PML Algorithm for Truncating Nonlinear FDTD Domains

In this paper, unsplit-field Perfectly Matched Layer (PML) formulations are presented for truncating nonlinear dispersive Finite Difference Time Domain (FDTD) grids. The proposed scheme is based on incorporating the nonlinear Z-transform FDTD algorithm into the Auxiliary Differential Equation PML (ADE-PML) formulations. Numerical example carried out in one dimensional domain which includes Lorentz dispersion as well as Kerr and Raman nonlinearities is included to show the validity of the formulations.

Parametric Raman wavelength conversion in scaled silicon waveguides

The benefits of using submicrometer modal-dimension silicon waveguides in realizing high-efficiency parametric Raman wavelength conversion are demonstrated theoretically and experimentally. The combined effects of Raman nonlinearities and free-carrier losses induced by two-photon absorption (TPA) are analyzed using the coupled-mode theory. The analysis indicates that scaling down the lateral dimensions increases the conversion efficiency of the Raman process and reduces the effective lifetime of free carriers and hence ameliorates the free-carrier losses. The feasibility of data conversion is demonstrated by coherent transfer of the analog radio-frequency (RF) signal from Stokes to anti-Stokes channels. The conversion efficiency, and hence signal-to-noise ratio (SNR), and bandwidth of the conversion process are found to be limited by the phase mismatch between the pump, Stokes, and anti-Stokes fields. The dispersion properties of submicrometer waveguides are also studied from the point of view of achieving phase matching and enhancing the conversion efficiency.

Optimization of nonlinear dispersive APML ABC for the FDTD analysis of optical solitons

We have investigated the parameter optimization for the nonlinear dispersive anisotropic perfectly matched layer (A-PML) absorbing boundary conditions (ABCs) for the two- and the three-dimensional (2D and 3D) finite-difference time-domain (FDTD) analyses of optical soliton propagation. The proposed PML is applied to the FDTD method of the standard and the high-spatial-order schemes. We first searched for the optimum values of the loss factor, permittivity, and the order of polynomial grading for particular numbers of APML layers in a two-dimensional (2-D) setting with Kerr and the Raman nonlinearity and Lorentz dispersion, and then we applied the optimized APML to a full three-dimensional (3-D) analysis of nonlinear optical pulse propagation in a glass substrate. An optical pulse of spatial and temporal soliton profile has been launched with sufficient intensity of electric field to yield a soliton pulse, and a reflection of -60dB has been typically obtained both for the 2-D and the 3-D cases.

Modeling of electromagnetic waves propagation in nonlinear optical media using HSCN-TLM method

In this paper, we propose a hybrid symmetrical condensed node TLM approach for the simulation of optical media under femtosecond regime. The formulation is based on the piecewise linear recursive convolution (PLRC) technique, voltage sources and the introduction of the variable admittance concept. Optical solitons with Kerr and Raman nonlinearities are simulated using this novel approach.

Propagation of self-focusing laser pulses in atmosphere: experiment versus numerical simulation

Numerical simulations of self-focusing laser pulses obtained via a slowly evolving wave approach (modified nonlinear Schrodinger equation) are compared with published experimental results in fused silica as well as with experimental results in air and fused silica obtained in our laboratory. The mathematical model includes group-velocity dispersion and third-order dispersion, optical shock, and both instantaneous Kerr and delayed Raman nonlinearities as well as a perfectly matched layer absorbing boundary condition. Second-harmonic frequency-resolved optical gating data taken after 10.91 m of propagation allow a direct comparison between experimental and computational envelopes at a number of pulse energies. FWHM measurements of the spectral width, intensity autocorrelation duration, and pulse radius at distances of 4.35, 10.91, 17.47, and 22.73 m provide a view of model fidelity across both energy and propagation distance variables. The magnitude of the nonlinear refractive index, n2, is inferred.

Raman induced wavelength conversion in scaled Silicon waveguides

Parametric Raman nonlinearities in Silicon waveguides is used to demonstrate wavelength conversion from Stokes to anti-Stokes channels. The effects of two photon absorption and free carrier nonlinear losses on the conversion process have also been analyzed. We find that scaling down the waveguide dimensions to submicron sizes is advantageous in terms of increasing the Raman nonlinearities and reducing the carrier lifetime and hence nonlinear absorption.

Multiwatt average power continua generation in holey fibers pumped by kilowatt peak power seeded ytterbium fiber amplifier

We demonstrate up to 3.8 W average power broad continuum generation in holey fibers pumped by a kilowatt peak, 10 W average power, single-mode, seeded ytterbium fiber amplifier at 1064 nm. Components of generated continua and spectral slicing indicate major contributions of Raman and Kerr nonlinearities. The proposed configuration opens the way to fully fiber integrated multiwatt white light sources.

Numerical simulations and coherence properties of supercontinuum generation in photonic crystal and tapered optical fibers

Numerical simulations have been used to study broad-band supercontinuum generation in optical fibers with dispersion and nonlinearity characteristics typical of photonic crystal or tapered fiber structures. The simulations include optical shock and Raman nonlinearity terms, with quantum noise taken into account phenomenologically by including in the input field a noise seed of one photon per mode with random phase. For input pulses of 150-fs duration injected in the anomalous dispersion regime, the effect of modulational instability is shown to lead to severe temporal jitter in the output, and associated fluctuations in the spectral amplitude and phase across the generated supercontinuum. The spectral phase fluctuations are quantified by performing multiple simulations and calculating both the standard deviation of the phase and, more rigorously, the degree of first-order coherence as a function of wavelength across the spectrum. By performing simulations over a range of input pulse durations and wavelengths, we can identify the conditions under which coherent supercontinua with a well-defined spectral phase are generated.

Stable laser-pulse propagation in plasma channels for GeV electron acceleration

To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in plasma without disruption. A 3D envelope equation for a laser pulse in a tapered plasma channel is derived, which includes wakefields and relativistic and nonparaxial effects, such as finite pulse length and group velocity dispersion. It is shown that electron energies of approximately GeV in a plasma-channel LWFA can be achieved by using short pulses where the forward Raman and modulation nonlinearities tend to cancel. Further energy gain can be achieved by tapering the plasma density to reduce electron dephasing.

Femtosecond two-beam coupling energy transfer from Raman and electronic nonlinearities

We present a study of two-beam coupling energy transfer arising from electronic and nuclear (i.e., Raman) contributions to the third-order refractive nonlinearity. Calculations of transient two-beam coupling in fused silica with no free parameters agree reasonably well with experiments. Numerical calculations of two-beam coupling in a medium with a purely electronic nonlinearity modeled by a Debye-type response are presented. The calculations indicate that it should be possible, although challenging, to measure electronic two-beam coupling for response times as short as ∼0.1 fs. Experiments designed to isolate the electronic response fail to produce two-beam coupling signals but do indicate that the time scale of the electronic response is well below 1 fs.

Mutual influence of the parametric effects and stimulated Raman scattering in optical fibers

The effect of Raman gain parametric suppression is investigated both theoretically and experimentally. Good agreement between theory and experiment is obtained. The analysis of the effect of Raman nonlinearity on the process of modulation instability in the region of negative group velocity dispersion of fibers is presented. New dependencies for the modulational instability frequency and increment are calculated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonlinear response theory—I

A general nonlinear response theory for the case of linear coupling of physical systems to arbitrary external fields is formulated for applications in different branches of physics. This is done within the framework of non-relativistic density matrix approach of quantum mechanics. Some simple properties of response functions and other related functions, which are introduced here for convenience, are studied to obtain suitable representations of the nonlinear response functions, including important sum-rules. As an example, the sum rule for the second-order response function is applied to electronic dipole nonlinearity at optical frequencies which includes both the Raman nonlinearity arising from perturbation to the electronic motion from external ionic displacement field and the usual optical sum, difference and harmonic generations. This immediately allows us to visualize a rigorous connection between these two types of non-linearities.

Wavefront reversal by four-wave mixing in a medium exhibiting Raman nonlinearity

A theoretical and experimental investigation is made of the feasibility of wavefront reversal by four-wave interaction in a medium exhibiting Raman nonlinearity. Theoretical dependences of the reflection coefficient of the four-wave mixer on the gain increment, phase detuning, and pump asymmetry parameter are obtained for the case of plane waves. In experiments using a ruby laser and quasiplane waves the maximum reflection coefficient of the nonlinear mirror was 0.5%, which agreed with the theoretical estimate. An experimental investigation was made of the wavefront reversal quality, and possible methods of improving this are discussed.

Nonlinear optical resonators (excited by external radiation) (review)

A review is given of theoretical and experimental investigations of optical resonators excited by an external light beam and filled with a nonlinear medium. An analysis is made of various types of nonlinearities (Kerr, Raman, Brillouin, and quadratic nonlinearities, saturation in a medium of two-level particles) and equations which describe the dynamics of optical oscillations in a resonator, are derived. Descriptions are given of the main properties such as limitation of the transmitted beam intensity, bistability and hysteresis, intensity pulsations, and amplification of amplitude-frequency deviations in the optical beam. Transient processes are also discussed. In the description of the experimental results, not only the resonator parameters and the observed properties are indicated but attention is also paid to identification of the type of nonlinearity. An analysis is also made of hybrid systems in which the nonlinearity is simulated (and deliberately increased) by radioelectronics.

Coherent generation of magnons by optical techniques

A new technique for the coherent excitation of magnons is proposed which is based on the magnetic Raman nonlinearity. The excitation is obtained by mixing two laser beams with different frequencies (ω1,ω2) under phase-match conditions [when (ω1−ω2) is equal to the frequency of one magnon mode and (k1−k2) is equal to the magnon wavevector]. The method allows selective excitation in frequency and wavevector and may by useful for magnon relaxation measurements. The proposed process is analyzed by quantum statistical methods. The number of excited magnons and the coherence properties of the generated states are investigated by using the representation of coherent magnon states. The results show that it is possible to excite a magnon population of the same order of magnitude as in nonlinear magnetic resonance experiments by using lasers with powers of hundreds of watts. The possibility of experimental observation is analyzed in detail for both ferromagnetic and antiferromagnetic materials.

Precise characterization of the Raman nonlinearity in benzene using nonlinear interferometry

Using a newly developed technique which we term two-beam nonlinear interferometry, we report accurate self-consistent determinations of both the real and imaginary parts of the third-order nonlinear optical susceptibility χ11113(−ω,ω,ωo,−ωo) in benzene including resonant Raman contributions from the 992-cm−1 mode. These measurements provide accurate values for the peak Raman cross section, nonlinear refractive index, and electronic hyperpolarizability, thus enabling the direct comparison of Raman, nonlinear refractivity, and frequency-mixing studies.