Order Sidebands Research Articles

Resonant enhancement of second order sideband generation for intraexcitonic transitions in GaAs/AlGaAs multiple quantum wells

We present an experimental study on efficient second order sideband generation in symmetric undoped GaAs/AlGaAs multiple quantum wells. A near-infrared laser tuned to excitonic interband transitions is mixed with an in-plane polarized terahertz beam from a free-electron laser. The terahertz beam is tuned either to the intraexcitonic heavy-hole 1s-2p transition or to the interexcitonic heavy-hole light-hole transition. We find strong evidence that the intraexcitonic transition is of paramount influence on n=±2 sideband generation, leading to an order-of-magnitude resonant enhancement of the conversion efficiency up to 0.1% at low temperature. At room temperature, the efficiency drops only by a factor of 7 for low terahertz powers.

A simplified model of frequency distortion in ocean acoustic signals by multiple interactions with a moving ocean surface and its use in oceanographic inversions.

Received omnidirectional spectra of low frequency narrowband tones measured in a shallow ocean waveguide exhibit Doppler-shifted sidebands at frequencies whose offsets are integer multiples of the dominant surface wave frequencies. These observations suggest that higher order scattering is important. However, higher-order perturbation theory involving only a single surface interaction does not predict the observed relative sideband levels. In particular, the higher order sidebands were often observed to increase in relative level to the main spectral peak during smaller surface wave, and, therefore, smaller kh, conditions. A simplified model is presented that simulates the surface wave-induced frequency distortions as path-length modulation caused by the vertical motions of the surface. The model incorporates multiple interactions with the moving surface, and quantitatively predicts the effects on the received spectra of the spatial coherence of the surface waves along the propagation path. The model results are identical to those from higher order perturbation with a single interaction. The simplified nature of this model allows it to be readily used to invert for various properties of the ocean surface wave directional spectrum. [Work supported by ONR, Code 321(US).]

Improving signal analysis for nonlinear time reversed acoustics: simulation results

Nonlinear time reversed acoustics have attracted attention as a tool to localize and identify nonlinear defects in solids [1-4]. The procedure is based on extracting the features due to the nonlinear scatterers from the signals received at the transducer array and back-propagate only the resulting portion of the signals. The usual approach is to filter the received signals to keep only contributions due to higher order harmonics or sidebands. As a result, the signals are very small in amplitude and can be not detectable if transducers are located far from the scatterers. It is evident the need for novel signal processing tools to increase the signal-to-noise ratio of the nonlinear contributions. We have recently proposed a scaling subtraction method [5], which is applied here to nonlinear time reversal virtual experiments, showing the improvements in the signal-to-noise ratio, with resulting better imaging of the nonlinear scatterer. Results will be presented for two approaches to nonlinear time reversal. [1] A. S. Gliozzi et al., J. Acoust. Soc. Am. 120, 2506-2517 (2006). [2] T. J. Ulrich et al., J. Acoust. Soc. Am. 119, 1514-18 (2006). [3] T. J. Ulrich et al., Phys. Rev. Lett. 98, 10430 (2007). [4] T. Goursolle et al., J. Acoust. Soc. Am. 122, 3220 (2007). [5] M. Scalerandi et al., submitted to Appl. Phys. Lett. (2008).

Optical mm‐wave DWDM signal generation with photonic frequency quadruple by only one external modulator

AbstractWe have proposed a novel scheme to generate optical mm‐wave DWDM signals with four time frequency of the local oscillator by using only one external modulator. By incorporating the proper direct current (DC) bias on the external modulator to suppress the first order sideband in central office (CO), and using optical filtering technique to separate the optical mm‐wave and optical carrier in the base station, the dense wavelength division multiplexing (DWDM) optical millimeter‐wave (mm‐wave) signals are generated with four times frequency of the LO signal. We have experimentally demonstrated four‐channel DWDM optical mm‐wave signal generation with a repetitive frequency up to 40 GHz by 10 GHz LO, and down‐stream signal delivery over 20‐km fiber with 0.7‐dB power penalty. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1152–1155, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23328

Highly Accurate Measurement of LN Optical Intensity Modulators by Small RF Inputs

A highly accurate measurement method of parameters of MZ-type LN optical intensity modulators is presented. In this method, a CW optical signal is input to an optical terminal and small CW RF signal is applied to an electrode of the modulator. Then sideband levels of an output optical signal at different bias points are measured by using optical spectrum analyzer. By using 1st order sideband levels which are measured at two different bias conditions, and using a compensation method to measured levels, we can obtain accurate chirp parameter even when very small power of RF signal is applied to the modulator. In this method, the chirp parameter can be obtained in good accuracy when the input RF voltage is only 3% of the halfwave voltage.

Vibrational molecular modulation in hydrogen

Detailed numerical modeling of using the vibrational coherence of H2 for molecular modulation is presented. The focus of the calculation is on a strongly driven system aimed at producing many sidebands in the presence of Doppler broadening and the effects of collisions at room temperature. It is shown that Dicke narrowing that reduces the Doppler width plays a critical role in high order sideband generation in room temperature H2. In addition, the calculation shows that generation of many sidebands favors the phased state as has been reported in all gas phase experiments and is primarily a consequence of the Stark shifts that result from the applied high intensities. The influence of self-focusing in the gas medium that has been conjectured in previous studies is only secondary. The numerical results agree with experimental data obtained in our laboratory, where we have succeeded in generating collinearly propagating Raman sidebands with wavelengths that range from 2216 nm in the infrared to 133 nm in the vacuum ultraviolet. The frequencies covered by these sidebands span over four octaves for a total of more than 70 600 cm �1 in the optical region of the spectrum.

Generalised dispersive phase and its effect on four wave mixing in fibers

A generalised method of calculating the efficiency of four wave mixing products in an optical fiber is presented in this paper. Apart from indicating the insufficiency of the existing theoretical model, this work brings out the importance of calculating the dispersive phase shift more precisely in combination with the nonlinear phase shift to predict the efficiencies of sideband generation. The experimental results for the generation of the first and second order sidebands are analysed. Contrary to expectations, higher powers and longer lengths of fiber do not result in larger number of sidebands. An attempt is made to understand this aspect as well as the experimentally observed fluctuations in the four wave mixing products.

DWDM optical millimeter-wave generation for radio-over-fiber using an optical phase modulator and an optical interleaver

We have proposed and experimentally demonstrated a novel scheme to generate a dense wavelength-division multiplexing (DWDM) optical millimeter-wave source by using an optical phase modulator and an optical interleaver. The stability of the DWDM optical millimeter wave generation is largely improved because we use an optical phase modulator without a dc-bias controller and an optical interleaver is subsequently employed to suppress the optical carrier of the DWDM source, which is not as temperature sensitive as a fiber Bragg grating. Moreover, the limitation of chromatic dispersion is greatly reduced due to avoiding the generation of higher order sidebands via driving the phase modulator with optimized RF signal

Efficient long-term laser excitation of a collimated beam of potassium atoms on the D2 line

With the aim to provide a reliable scheme for efficient laser excitation of the potassium D2 line over long periods of time, we have developed a robust stabilization of a single mode laser of frequency f 0 = (f 12 + f 23)/2 onto a crossover peak in the saturation spectrum of the 39K(D2) line (4s 2S $_{1/2}, F \to 4p ^{2}$ P $_{3/2}, F^{\prime} )$ . The two hyperfine transitions $F = 1\to F^{\prime} = 2$ (frequency f 12) and $F = 2\to F^{\prime} = 3$ (frequency f 23) are simultaneously excited by the first order sidebands $f_{\pm} = f_{0}\pm f_{EOM}$ of the laser beam (f 0), generated by its electro-optical modulation at the frequency f EOM = (f 12-f 23)/2. In this way stable excitation of the two transitions on their proper frequencies is achieved and hyperfine pumping compensated.

Generation of optical domain-wall structures from modulational instability in a bimodal fiber.

We study experimentally modulational instability in a normally dispersive bimodal fiber under modal group-velocity-matching conditions. In the strong pump depletion regime, higher order sideband harmonics detected in the output spectra as well as autocorrelation measurements reveal the formation of subpicosecond domain-wall structures. Across these temporal structures the electromagnetic field distribution switches abruptly between the two transverse modes of the fiber. These structures are reminiscent of the so-called domain-wall soliton. Our results constitute therefore an experimental indication of the existence of this fundamental soliton.

Higher order FM mode locking for pulse-repetition-rate enhancement in actively mode-locked lasers: theory and experiment

We present a novel higher order FM mode-locked technique for active mode-locked lasers which utilizes the higher order sidebands generated by an intracavity phase modulator to establish the mode locking. The resulting mode-locked output exhibits an enhancement of the pulse repetition rate over the modulation frequency by an integral multiple. The higher order FM mode locking is studied theoretically in a laser with a homogeneous gain medium, and simple analytical expressions are obtained to characterize the output pulses. It is shown that the scheme not only enhances the pulse repetition rate but also ensures chirp-free pulses and is effective in eliminating the output pulse phase-state instability, which are commonly observed in conventional FM mode-locked lasers. The effect of group velocity dispersion and cavity nonlinearity is also investigated through numerical solution of the self-consistency equation applied to a fiber ring laser. Finally, detailed experimental results on repetition rate enhancement in fiber lasers are presented and shown to be in good agreement with the theoretical results.

Generation of phase-coherent laser beams for Raman spectroscopy and cooling by direct current modulation of a diode laser

A setup for generation of GHz side-bands by direct current modulation of a diode laser is described. The first order side-bands are used to inject two slave power diode lasers, in order to produce phase coherent, controllable frequency shifted, light beams, the mutual phase coherence of which has been verified (beat-note FWHM of 1 Hz, instrumentally limited). Some possible applications in the context of laser Raman cooling of cesium atoms are briefly discussed.

Frequency measurement of accurate sidebands of an optical frequency comb generator

The accuracy of the optical sideband generation of an optical frequency comb generator (OFCG) was examined. The fundamental light and its second harmonics from a YAG laser were used as input to the OFCG and as pump light to an optical parametric oscillator (OPO), respectively. The frequency between the +15th order OFCG sideband and the OPO signal light was compared with that between the -15th order sideband and the idler. The frequency shift of the generated sideband was estimated to be smaller than 6 × 10 −13 at a frequency span of 276 GHz.

Coherent optical frequency domain reflectometry for a long single-mode optical fiber using a coherent lightwave source and an external phase modulator

Coherent optical frequency domain reflectometry (C-OFDR) is demonstrated for a long single-mode optical fiber at a wavelength of 1.55 /spl mu/m by using a highly coherent lightwave source and an electrooptical (EO) external phase modulator. Rayleigh backscattering and Fresnel reflection from a 16.4-km optical fiber are observed when the modulation frequency is swept from 4 GHz to 8 GHz for 40 ms. The insertion of an optical fiber delay line in a probe lightwave path makes it possible to distinguish the Rayleigh backscattering profile due to the first-order modulation sideband in the phase-modulated lightwave from those due to the higher order sidebands. A spatial resolution of better than 100 m is achieved for two neighboring Fresnel reflectors located near the far end of the test fiber. >

Continuous wave multiquantum electron paramagnetic resonance spectroscopy. IV. Multiquantum electron–nuclear double resonance

We report a theoretical and experimental investigation of the interaction of a coupled electron–nuclear spin system with three electromagnetic fields: two equally intense microwave fields resonant with the electron spin, and one radio-frequency field resonant with the nuclear spin. This is an electron–nuclear double-resonance experiment where the effect of the nuclear transition is detected via changes of the electron multiple photon transitions (MQ-EPR) rather than steady-state saturation and, therefore, is called multiquantum electron–nuclear double resonance (MQ-ENDOR). The theoretical framework previously developed for the description of multiple photon phenomena in a two level system is extended to the case of a four level system. The equation of motion of the density matrix is solved in the presence of three fields, which results in five master equations relating various populations and coherences. A ten photon approximation is used to study the functional dependence on spectral parameters and determine the sensitivity of this technique to spin relaxation rates. The experimental investigation is carried out on a sample of tri-t-butyl phenoxyl radical dissolved in mineral oil. At low values of the electron saturation factor Se, the rf-swept MQ-ENDOR is a de-enhancement of the 3Q-EPR signal (i.e., the first intermodulation sidebands). As the microwave field strength increases, the MQ-ENDOR signal changes phase by 180° due to dominance of generalized saturation. Higher order MQ-EPR signals (i.e., higher order sidebands) have larger negative enhancement and tend to display smaller positive enhancement. Line splitting results if the microwave frequency difference or field strength is increased. The dependence of MQ-ENDOR displays on various spectral parameters was found to be consistent with the general trends predicted by the theory. These displays provide a convenient way to estimate electron and nuclear relaxation rates.

A far infrared laser sideband spectrometer in the frequency region 550–2700 GHz

This paper describes a tunable far-infrared (FIR) spectrometer. Tunable radiation is obtained by frequency mixing, fixed frequency FIR laser radiation and tunable microwave radiation in Schottky barrier diodes. An optically pumped laser and an HCN discharge laser are used as FIR sources and klystrons in the frequency range of 22–114 GHz as microwave sources. This yields an 85% coverage of the frequency region between 550 and 2700 GHz. Up to sixth order sidebands have been generated and used for spectroscopy. The ultimate sensitivity corresponds to a minimum detectable fractional absorption of 10−5 at 1-s RC time. The applicability of the spectrometer in high-resolution spectroscopy of transient species has been demonstrated by the observation of spectra of OD and N2H+. New laser emissions of optically pumped CH2F2 have been found and accurate frequencies have been determined for some of them.

Determination of the sign of the order parameter from MAS spectra of oriented molecules

The use of the intensities of the spinning sidebands in the magic-angle spinning spectra of oriented molecules is proposed for the determination of the signs of the order parameters. The method is demonstrated for benzene and chloroform oriented in nematic phases of liquid crystals. On the basis of the theoretical expressions derived for the various order sidebands, the applicability of the method for different experimental conditions is discussed.

On the possibility of beat excitation of whistler sidebands in the magnetosphere via ponderomotive force

We propose a new mechanism to explain the observed sidebands of whistler waves. The mechanism based on nonlinear interaction has potential application to active experiments in the magnetosphere. The process requires two whistler waves with nearly equal frequency ω1‐ω2 and forward and backward traveling components. They exert a parallel ponderomotive force on the electrons at the difference frequency (ω1‐ω2). The modulation of the plasma density by the nonlinear mixing of these two waves, in conjunction with the oscillatory electron velocity at ω1 and ω2 generates a nonlinear current. This nonlinear current drives the first‐order sidebands. In the presence of low frequency density modulation, the oscillatory velocities at the first‐order sidebands generate second‐order sidebands. The process continues to generate higher order sidebands. The sidebands up to seventh order have been observed in VLF experiment at Siple station. The mechanism is effective for the large amplitude waves.

Extension of the spectral coverage range of frequency modulation spectroscopy by double frequency modulation

The performance of frequency modulation spectroscopic techniques is limited by the modulation cutoff frequency of optical phase modulators. A method which allows for overcoming this limit by using high order sidebands in a double-frequency modulation scheme is proposed. The usefulness of the proposed technique in relation with a recently developed frequency modulation spectroscopy is discussed in detail.

Slow molecular motion detected in the NMR spectra of rotating solids

We present an approach for detecting and analyzing slow molecular motions in solids based on changes in the zero order sideband of an inhomogeneously broadened spin system under magic angle spinning conditions. The effect is discussed theoretically from the standpoint of second order perturbation theory and the stochastic Liouville equation of Kubo, both for the case where the chemical shift is the main interaction. Experimental investigations of decamethylferrocene (resulting in Ea=3.3 kcal/mole) and hexamethyl benzene support the predicted trends. A five site jump model is used to interpret the results for decamethyl ferrocene, while the interpretation of the hexamethyl- benzene data is obfuscated by the presence of a chemical shift inequivalency presumably caused by a distortion of the aromatic ring above the phase transition (Tc=116 K).