Pulse Trapping Research Articles

Dynamics of a Rabi oscillation under a pulsed perturbation

We study how a Rabi oscillation is affected by a periodic pulsed perturbation. Unlike a continuous perturbation, which simply shifts the resonance frequency, a pulsed perturbation splits the Rabi resonance into two lines separated by the pulse repetition rate. When the perturbation integrated over one pulse is equal to $\ensuremath{\pi}$, the resonance splits into two symmetrically placed lines at \ifmmode\pm\else\textpm\fi{} half the pulse repetition rate. When it is $2\ensuremath{\pi}$, the Rabi oscillation is unaffected by the perturbation. We demonstrate these behaviors using the ground hyperfine transition of cesium atoms in a magneto-optical trap. The pulsed perturbation is introduced by a periodic magnetic field pulse. The $2\ensuremath{\pi}$ case is compared with the self-induced transparency, and its implications for optical frequency metrology using a pulsed optical trap is discussed.

Resonance Frequency Shift of a Two-Level Atom in a Pulsed Optical Trap

We consider the effect of a periodic pulsed perturbation on the shift of resonance frequency for a Rabi oscillation. Unlike a continuous perturbation, where the frequency shift is proportional to the perturbation strength, the shift under a pulsed perturbation shows periodic behavior. In fact, when the pulse height and the pulse duration are set such that a fictitious spin completes a 2π precession around the principal quantization axis, there is no frequency shift. This phenomenon can be used for optical frequency metrology by trapping reference atoms in a properly controlled pulsed optical trap. We estimate the residual systematic shift and line broadening due to practical limits from pulse energy fluctuation, atomic motion, and finite pulse duration and find that it is on the order of 100 Hz.

Ultrafast all‐optical switching using pulse trapping by ultrashort soliton pulse in birefringent optical fiber

AbstractUltrafast all‐optical switching by use of pulse trapping in birefringent optical fiber is demonstrated both experimentally and numerically. The wavelengths of the control soliton pulse and the trapped soliton pulse are shifted to satisfy the condition of group velocity matching. Furthermore, the energy of the trapped pulse is increased through Raman gain of the control pulse. Only a signal pulse in the pulse train with temporal separation of about 1.2 ps is successfully picked off. The repetition frequency corresponds to 0.83 THz. The spectrogram of the optical switching is directly observed using the X‐FROG technique. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(3): 38–44, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20470

Coherent backscattering under conditions of pulsed radiation trapping

Coherent backscattering of pulsed radiation emitted by optically dense atomic ensembles is considered. The diagrammatic technique is used for deriving analytic expressions for correlation functions of scattered light, which make it possible to take into account all main factors affecting the dynamics of the process, including the hyperfine and Zeeman structure of the ground and first excited states of atoms, polarization of probe radiation, the actual shape and size of an atomic cloud, its spatial inhomogeneity, motion of atoms, and angular-momentum polarization of atoms. On the basis of these relations, the time dependence of the total intensity and the dependence of enhancement factor of backscattered light on the pulse duration, type of polarization of the polarization system of observation, optical thickness of the scattering medium, and the carrier frequency of the pulse are investigated. The calculations are performed for an ensemble of rubidium-85 atoms cooled in magnetooptical traps.

Ultrafast All Optical Switching using Pulse Trapping by Ultrashort Soliton pulse in Birefringent Optical Fiber

Ultrafast all optical switching by use of pulse trapping in birefringent optical fiber is demonstrated both experimentally and numerically. The wavelength of the control soliton pulse and the trapped soliton pulse is shifted to satisfy the condition of group-velocity matching. Furthermore, the energy of trapped pulse is increased through Raman gain of control pulse. Only a signal pulse among three pulses with temporal separation of about 1.2 ps is successfully picked off. The repetition frequency corresponds to 0.83THz. The spectrogram of the optical switching is directly observed using the X-FROG technique.

High Contrast Ramsey Fringes with Coherent-Population-Trapping Pulses in a Double Lambda Atomic System

We report the observation of Raman-Ramsey fringes using a double lambda scheme creating coherent population trapping in an atomic ensemble combined with pulsed optical radiations. The observation was made in a Cs vapor mixed with N2 buffer gas in a closed cell. The double lambda scheme is created with lin perpendicular lin polarized laser beams leading to higher contrast than the usual simple lambda scheme. The pulsed trapping technique leads to narrow fringe widths scaling as 1/(2T) with high contrasts which are no longer limited by the saturation effect. This technique operates in a different way from the classical Ramsey sequence: the signal is done by applying a long trapping pulse to prepare the atomic state superposition, and fringe detection is accomplished by optical transmission during a short second trapping pulse without any perturbation of the dark state.

Nonlinear copropagation of two optical pulses of different frequencies in photonic crystal fibers

A theoretical investigation of the nonlinear copropagation of two optical pulses of different frequencies in a photonic crystal fiber is presented. Different phenomena are observed depending on whether the wavelength of the signal pulse is located in the normal or the anomalous dispersion region. In particular, it is found that the phenomenon of pulse trapping occurs when the signal wavelength is located in the normal dispersion region while the pump wavelength is located in the anomalous dispersion region. The signal pulse suffers cross-phase modulation by the Raman shifted soliton pulse and it is trapped and copropagates with the Raman soliton pulse along the fiber. As the input peak power of the pump pulse is increased, the red-shift of the Raman soliton is considerably enhanced with the simultaneous further blue-shift of the trapped pulse to satisfy the condition of group velocity matching.

Ultrafast all optical switching using pulse trapping in birefringent fibers

An ultrafast all-optical switching using pulse trapping by orthogonally polarized soliton pulse in birefringent fiber is investigated both experimentally and numerically. The signal pulse, which is polarized along the fast axis, is trapped by the control soliton pulse along the slow axis; they subsequently copropagate along the fiber. Their wavelengths are red-shifted by the soliton self-frequency shift. The trapped pulse is shaped as a sech2 ultrashort pulse. It is also amplified by the Raman gain of the control pulse. Temporal response of the pulse trapping is estimated as 250 fs for 150 fs signal pulses. Ultrafast all-optical switching is demonstrated for the 0.84 THz pulse train.

Enhanced dispersive wave generation by using chirped pulses in a microstructured fiber

A theoretical investigation on the nonlinear pulse propagation and dispersive wave generation in the anomalous dispersion region of a microstructured fiber is presented. By simulating the dispersive wave generation under different conditions, it is found that the generation mechanism of the dispersive wave is mainly due to the pulse trapping across the zero-dispersion wavelength. By varying the initial pulse chirp, the output spectrum can be broadened and the intensity of the dispersive wave can be obviously enhanced. In particular, there exists an optimal positive chirp which maximizes the intensity of the dispersive wave. This effect can be explained by the energy transfer from the Raman soliton to the dispersive wave due to the effect of the pulse trapping and the effect of the higher-order dispersion. From the phase aspect, the explanation of this effect is also included.

Laser pulse splitting and trapping in tenuous gases

The nonlinear propagation over long distances of moderate intensity laser pulses in tenuous gases is studied. The dynamics of these pulses will be affected by nonlinear focusing and dispersion due to the background gas, and by plasma induced refraction and dispersion. Laser propagation is studied numerically using the simulation code WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997)]. Different phenomena are found for different regimes of peak input power. For powers near the critical power, temporal pulse narrowing and splitting due to phase modulation and group velocity dispersion is seen. For slightly higher powers, plasma generation and the formation of a trailing pulse, which is guided off axis by plasma refraction and nonlinear gas focusing, is observed. For even higher powers, the laser pulse is partially trapped by the plasma and then exhibits a form of self interference.

Ultrafast all optical switching by use of pulse trapping across zero-dispersion wavelength

Ultrafast all optical switching by use of pulse trapping across zero dispersion wavelength in optical fiber is demonstrated both experimentally and numerically for the first time. Only an arbitrary single pulse among four pulses with temporal separation of about 1.5 ps is successfully picked off with almost perfect extinction ratio. The spectrogram of the optical switching is directly observed using the X-FROG technique. The characteristics of all optical switching are analyzed numerically by the use of strict coupled nonlinear Schrödinger equations and the numerical results are in agreement with the experimental ones. It is interesting to note that although the other pulses are also overlapped with the soliton pulse, they are not trapped.

Characteristics of pulse trapping by use of ultrashort soliton pulses in optical fibers across the zero-dispersion wavelength

Characteristics of pulse trapping by ultrashort soliton pulse in optical fibers across zero-dispersion wavelength are analyzed both experimentally and numerically. The spectrogram of pulse trapping is observed using the technique of cross correlation frequency resolved optical gating and the phenomenon of pulse trapping is confirmed directly. The pulse trapping is numerically analyzed using the coupled strict nonlinear Schrödinger equations and the numerical results are well in agreement with the experimental ones. It is clarified that the pulse trapping occurs due to the sequential cross phase modulation by the Raman shifted soliton pulse.

Trapped pulse generation by femtosecond soliton pulse in birefringent optical fibers

A novel phenomenon of trapped pulse generation by orthogonally polarized femtosecond soliton pulse is discovered in low birefringent optical fiber. As pulse propagation, wavelengths of soliton pulse and trapped pulse are shifted toward longer wavelength side due to effects of soliton selffrequency shift and pulse trapping. The energy of trapped pulse is increased exponentially through Raman gain of soliton pulse and orthogonally polarized and temporally overlapped two colored femtosecond twin pulses are generated. The spectrogram of output pulses is observed using crosscorrelation frequency resolved optical gating technique. The characteristics of this phenomenon are also analyzed numerically and numerical results are almost in agreement with experimental ones.

Pulse trapping by ultrashort soliton pulses in optical fibers across zero-dispersion wavelength

A new phenomenon of pulse trapping by the ultrashort soliton pulse of an optical fiber has been experimentally observed. The trapped pulse in the normal-dispersion region copropagates with the soliton pulse in the anomalous-dispersion region along the fiber, and the wavelength of the trapped pulse is shifted to satisfy the condition of group-velocity matching. The wavelengths of the soliton pulse and the trapped pulse change almost continuously as the power of the soliton pulse is varied. Almost perfect conversion efficiencies are observed for soliton self-frequency shift and pulse trapping.

相互相関周波数分解光ゲート法を用いた光ファイバにおける超短パルス光の伝搬特性の解析

Characteristics of ultrashort pulse propagation in optical fibers at the wavelength region of 1.55 μm are experimentally analyzed using the cross-correlation frequency resolved optical gating (X-FROG) technique. In the anomalous dispersive fibers, widely wavelength tunable femtosecond soliton pulse is generated. The spectrogram of almost transform limited soliton pulse around 2 μm is observed. In the dispersion shifted fibers, the wavelength tunable soliton pulse and anti-stokes pulse are generated. As the results of X-FROG measurement, it is clarified that the anti-stokes pulse is overlapped with the soliton pulse and trapped by the soliton pulse. In the highly nonlinear optical fibers, 1.2-2.1 μm ultra-widely broadened super continuum and 1.46-1.68 μm linearly chirped one are generated. The spectrograms of generated super continuum are observed. The phenomenon of pulse trapping is also observed in generated super continuum.

Short pulse generation and control in Er-doped frequency-shifted-feedback fibre lasers

Novel features of frequency-shifted-feedback (FSF) fibre lasers are demonstrated. Dual-wavelength-switching FSF Q-switching and picosecond pulse fibre laser is reported by exploiting bandwidth of erbium fibre amplifier for pulse trapping. Wavelength switching between the states that have the spectra stably trapped by edges of the gain spectrum was accomplished by modulating the RF power applied to the acousto-optic frequency shifter. Also, a FSF fibre laser is demonstrated that utilizes a Michelson interferometer for pulse repetition rate control. The laser enables obtaining 1.64 ps pulses with a repetition rate up to 150 GHz. Finally we report the generation of 620 fs full-width at half-maximum pulses from a Ti:sapphire pumped Er-doped FSF fibre laser by exploiting nonlinear polarization rotation effects. We believe that this correspond to the shortest pulse ever obtained with a FSF fibre laser.

Physiological monitoring of optically trapped cells: assessing the effects of confinement by 1064-nm laser tweezers using microfluorometry

We report the results of microfluorometric measurements of physiological changes in optically trapped immotile Chinese hamster ovary cells (CHOs) and motile human sperm cells under continuous-wave (CW) and pulsed-mode trapping conditions at 1064 nm. The fluorescence spectra derived from the exogenous fluorescent probes laurdan, acridine orange, propidium iodide, and Snarf are used to assess the effects of optical confinement with respect to temperature, DNA structure, cell viability, and intracellular pH, respectively. In the latter three cases, fluorescence is excited via a two-photon process, using a CW laser trap as the fluorescence excitation source. An average temperature increase of < 0.1 +/- 0.30 degrees C/100 mW is measured for cells when held stationary with CW optical tweezers at powers of up to 400 mW. The same trapping conditions do not appear to alter DNA structure or cellular pH. In contrast, a pulsed 1064-nm laser trap (100-ns pulses at 40 microJ/pulse and average power of 40 mW) produced significant fluorescence spectral alterations in acridine orange, perhaps because of thermally induced DNA structural changes or laser-induced multiphoton processes. The techniques and results presented herein demonstrate the ability to perform in situ monitoring of cellular physiology during CW and pulsed laser trapping, and should prove useful in studying mechanisms by which optical tweezers and microbeams perturb metabolic function and cellular viability.

A high density pulsed ion trap

By exploiting the synergy of two discharges in low pressure gases or gaseous mixtures, a pulsed electron beam with exceptional radial stability has been obtained having a diameter less than 50 μm over 40 mm. It acts as a pulsed high density ion trap for the ions created by electron impact along the beam. From the measured electron beam characteristics and using a simple model, an ionic density of about 10 16 cm −3 and a depth of the potential well of the order of a few keV has been inferred. The parameters of the trap can be tuned to assure a full ionization of the gas inside the tube current with a significant probability for multiple ionization, the recombination rate being reduced due to the mismatch between the electron and ion velocities. Preliminary results on Ar III and Ar IV emission lines from the plasma channel associated with the beam are reported.

Experimental spatial soliton trapping and switching

We show experimentally the trapping of two spatial solitons of the same wavelength but with slightly different propagation directions, which results from the collision of two initially overlapped solitons beams. This is the spatial analog of soliton pulse trapping in birefringent optical fibers. Adjustment of the phase shift between each beam permits the switching of one soliton to the other without disturbing their trajectory.

Trapping and annihilation in the antenna system of Photosystem I

The primary charge separation in Photosystem I of pea chloroplasts was measured as a photovoltage in the pico- and nanosecond time range by applying laser flashes at 532 nm of variable energy and different duration (12 ns and 30 ps, respectively). Contributions to the photovoltage from Photosystem II was eliminated by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The dependence of the photovoltage amplitude on the excitation energy could be described by an exponential saturation law when the excitation flash had a duration of 12 ns. Nearly the same dependence was found when the excitation source was the train of a mode-locked laser (approx. ten 30-ps flashes spaced by 7 ns; highest energy of a single flash, 80 μJ / cm −2). Even with single 30-ps flashes the photovoltage was only slightly smaller than the one elicited by 12-ns flashes of the same energy. These findings demonstrate that trapping of excitation energy by the reaction center of Photosystem I is much more effective than losses by annihilation and other loss processes. The photovoltage yield was nearly independent of the fraction of closed traps, thus demonstrating that the absorption cross section of Photosystem I is not altered by the closing of its reaction centers. By recording the rise time of the photovoltage with our highest time resolution we found that the trapping rate of the excitation energy in Photosystem I depended on the energy of the 30-ps flashes: at low excitation energies (less than 10 14 photons / cm 2 per pulse) trapping occurred within 90 ± 15 ps and at high excitation energy (10 15 photons / cm 2 per pulse) trapping and charge stabilization occurred within the time resolution of the apparatus, i.e., up to 50 ps. The trapping rate at low energies is in agreement with the one determined by fluorescence decay kinetics. Up to 50 ns there was no further detectable electrogenic phase (neither forward nor backward reactions). This demonstrates that all the electrogenicity, produced by the charge separation, takes place in less than 50 ps.