Gaussian Optical Pulse Research Articles

Optimal optical filter for a short pulse with a bandwidth-limited receiver

For an optical short pulse with white background noise, an optical filter preceding the photodetector can reduce the noise. The optimal optical filter is derived to balance the amount of the output noise with the energy of the output signal. As a numerical example, for a Gaussian optical pulse with a Gaussian receiver filter, the optimal optical filter is also a Gaussian filter with a bandwidth between that of the optical pulse and the receiver filter.

All-optical ultra-wideband pulse generation based on semiconductor optical amplifiers

Several all-optical methods for ultra-wideband (UWB) pulse generation based on various nonlinearities of single semiconductor optical amplifiers (SOAs), namely cross phase modulation (XPM), cross gain modulation (XGM), and a hybrid of self phase modulation (SPM) and XGM, have been demonstrated. In the first method, UWB doublet pulses are realized with XPM. The input optical Gaussian pulse will be converted to two polarity-reversed monocycle pulses by a blue shifted and a red shifted optical bandpass filters respectively. These two monocycle pulses are then combined with proper time delay to generate two polarity-reversed doublet pulses. Second, two polarityreversed monocycle pulses are obtained based on XGM of the SOA and group delay of the single mode fiber (SMF). In the scheme, two polarity-reversed Gaussian pulses are generated due to XGM, and then the mixed signal is converted to a monocycle shape due to the group delay of the SMF. Finally, we present UWB doublet generation based on SPM. The monocycle pulse is generated from a dark return-to-zero (RZ) signal and converted to a doublet pulse by injecting an additional probe signal with the SMF transmission. For the first time and to the best of our knowledge, we report that the generated doublet pulses are transmitted over 5 km SMF by proper dispersion compensation without distortion. The configuration of our alloptical methods is compact and simple. The feasibility to implement the pulse shape modulation and pulse polarity modulation is discussed.

Investigations on order and width of RZ super Gaussian pulse in pre-, post- and symmetrical-dispersion compensated 10 Gb/s optical communication system using standard and dispersion compensating fibers

We show the effect of varied order and width of super Gaussian pulse at 10 Gb/s in dispersion compensated optical communication system. The optical communication system consists of standard single-mode fiber of 16 ps/nm/km of a certain length, whose dispersion is compensated using pre-, post- and symmetrical-dispersion compensation schemes with proportionate length dispersion compensating fiber of −80 ps/nm/km. Performance of these three compensation schemes is compared at 14 dBm values of Er-doped fiber amplifiers (EDFA) power at 1st, 2nd and 3rd order RZ super Gaussian optical pulse. The pulse width, full width at half maximum (FWHM) is also varied from 5 to 30 ps to highlight the optimum performance. The graphical results obtained show a relationship among the attributes pulse width, order of RZ super Gaussian optical pulse and dispersion compensation scheme implemented. It shows that to decrease BER and timing jitter in the system, smaller width and 3rd order super Gaussian pulse should be used. It is recommended that to decrease dependency of BER and timing jitter in the communication system on the pulse width i.e. FWHM, the symmetrical compensation scheme should be implemented.

静磁表面波对高斯光脉冲的衍射特性研究

静磁表面波对高斯光脉冲的衍射特性研究

Magnetostatic-Wave-Based Magneto-Optic Pulse Compression by Control of Phase Mismatching

Microwave magnetostatic waves (MSWs) as moving gratings in magneto-optic (MO) film can lead to the Bragg diffraction of guided optical waves (GOWs). The MO coupling characteristics are responsible for the amplitude and phase frequency spectra of diffracted pulses and even result in the compression of chirped optical pulses in time domain. We theoretically investigate the noncollinear diffraction of linearly chirped Gaussian optical pulses by continuous magnetostatic forward volume waves in detail. For a given chirped optical pulse, with the increase of phase-mismatching slopes, the compression efficiency (CE) is gradually improved up to the maximum followed by the transition of diffracted pulses from single peak to multi peaks. The larger the chirp parameter is, the smaller the required phase-mismatching slope to achieve the maximal CE is. However, the rise of the chirp parameter or phase-mismatching slope reduces the relative peak intensity of the diffracted pulse. Lastly, it is pointed out that the phase-mismatching slope can be greatly increased by using the high-order modes of MSWs and GOWs.

Coherence of terahertz emission from photoexcited electron-hole plasma: Hydrodynamic model and Monte Carlo simulations

Terahertz emission from the electron-hole plasma excited by the femtosecond laser pulse in semiconductor structures is analyzed. It is shown that plasma excitations in the homogeneous structures are incoherent. The incoherence of the excitations essentially reduces the intensity of terahertz radiation. The -doped heterostruc- ture for coherent plasma excitations is suggested. The Monte Carlo simulations show that the efficiency of terahertz emission from the suggested structure exceeds the efficiency of terahertz emission from semiconduc- tor surface and from p-i-n structures by 2 orders of magnitude. Our analysis of the transient dynamics of photoexcited carriers shows that the surface and contactless p-i-n terahertz emitters have serious deficiencies that significantly reduce the intensity of terahertz radiation. First, only a minor part of the excited carriers contributes to the transient current if the inverse absorption coefficient exceeds the surface depletion width or the i layer thickness of the p-i-n structure. In this case, the major part of carriers is created in the region where the built-in electric field is screened by the extrinsic carriers. Second, the plasma frequency of the excited carriers is posi- tion dependent due to the exponential decay of the created carrier density. As a result, the transient response of the cre- ated carriers is incoherent in different regions of the structure because the frequency of current oscillations is position de- pendent. Therefore, the amplitude of oscillations of the re- sulting conductivity current is significantly reduced by the interference effects. In this work, we show that terahertz en- ergy radiated from the contactless structures is significantly enhanced when such deficiencies are eliminated. The -doped p-i-n heterostructure for the efficient terahertz gen- eration is suggested. The transient dynamics of photoexcited carriers is analyzed by using the hydrodynamic model and the ensemble Monte Carlo EMC simulations. The EMC simulations of the transient dynamics of photo- excited carriers are performed following Ref. 6. The models of GaAs and AlGaAs are taken from Ref. 9. The multichan- nel intercarrier scattering, the Pauli exclusion principle, and the photoexcitation of electron-hole pairs are considered. 6 The central photon energy 0 is taken to be 1.55 eV. The full width at half maximum F of the Gaussian optical pulse in- tensity is set to 100 fs. The exponential spatial profile of the density of photoexcited carriers is considered. The carrier thermionic emission over the potential barriers of heterojunc- tions and the tunneling through the barriers are taken into account. The triangular and trapezoidal shapes of the poten- tial barriers are considered for the carrier tunneling. 9 To investigate the role of the incoherence of plasma exci- tations, we consider the contactless GaAs p-i-n structure. 5,6 We use the hydrodynamic approach for the nonlinear analy- sis of the transient dynamics of photoexcited carriers taking into account the spatially nonuniform photoexcitation. The dynamics of the excited electron-hole plasma in the contact- less p-i-n structure is analyzed under the following simplify- ing assumptions. Let us assume that carriers are instantly generated, i.e., the intensity of the optical excitation fluence It can be approximated as It=I0t. This assumption is valid if the condition Ff1 is satisfied, where f is the fre- quency of plasma oscillations. Because the net current of the conductivity and displacement currents is equal to zero in the contactless structure, the spatiotemporal variations of the electric field Ez,t, electron and hole average velocities vez,t and vhz,t, and electron and hole densities nz,t and pz,t obey the equation

Ultrafast all-optical first- and higher-order differentiators based on interferometers

We demonstrate that a conventional two-arm interferometer can implement first-order temporal differentiation of ultrafast arbitrary optical waveforms. Straightforward extension of this technique to nth-order optical differentiation is also suggested. This approach is experimentally demonstrated by an efficient and accurate first- and second-order temporal differentiation of (sub-)picosecond Gaussian optical pulses.

Optical ultrawideband monocycle pulse generation based on cross-gain modulation in a semiconductor optical amplifier

A novel method for generating ultrawideband (UWB) monocycle pulses based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. Thanks to the XGM in the SOA, a pair of polarity-reversed optical Gaussian pulses is generated at the output of the SOA, to which a Gaussian pulse pump and a continuous-wave probe are applied. The two polarity-reversed optical pulses are then time delayed by two cascaded fiber Bragg gratings to introduce a time delay difference. A UWB monocycle pulse with a full width at half-maximum of 48 ps and a fractional bandwidth of 188% is generated at the output of a high-speed photodetector.

Optical integrator for optical dark-soliton detection and pulse shaping

The design and analysis of an Nth-order optical integrator using the digital filter technique is presented. The optical integrator is synthesized using planar-waveguide technology. It is shown that a first-order optical integrator can be used as an optical dark-soliton detector by converting an optical dark-soliton pulse into an optical bell-shaped pulse for ease of detection. The optical integrators can generate an optical step function, staircase function, and paraboliclike functions from input optical Gaussian pulses. The optical integrators may be potentially used as basic building blocks of all-optical signal processing systems because the time integrals of signals may sometimes be required for further use or analysis. Furthermore, an optical integrator may be used for the shaping of optical pulses or in an optical feedback control system.

Nonreciprocal effects in active nonlinear optical fibers with nonlinearity dispersion

The nonreciprocity of the dynamics of Gaussian optical pulses propagating in inhomogeneous nonlinear active optical fibers with nonlinearity dispersion is considered. A substantial nonreciprocity of the time of pulse propagation along the optical fiber in the forward and backward directions is revealed. The possibility of realizing the regime of superluminal propagation of a pulse in one of the directions of the optical fiber is indicated.

Quantum control via the dynamic Stark effect: Application to switched rotational wave packets and molecular axis alignment

Nonperturbative quantum control schemes in the intermediate field strength nonionizing regime are investigated. We restrict the matter-field interaction to the nonresonant dynamic Stark effect NRDSE as induced by infrared laser fields, which we argue is a new and general tool for quantum control of atomic and molecular dynamics. For the case of Raman coupled matter states, an effective Hamiltionian may be constructed, and quantum control via NRDSE may be thought of as reversibly modifying the effective Hamiltonian during system propagation, thus leading to control over dynamic processes. As an illustration, the creation of field-free “switched” wave packets through the adiabatic turn on and sudden turn off of the NRDSE is considered and experimentally demonstrated. Wave packets generated through the switched NRDSE interaction may be very different in form and content than wave packets generated via resonant transitions with Gaussian optical pulses. In order to provide an example, we discuss the specific case of rotational wave packet dynamics where the NRDSE manifests itself as molecular axis alignment. This technique is applied to the creation of field-free molecular axis alignment using an intense switched 1.064 m laser pulse. This switched laser pulse was generated via a plasma shuttering technique, giving a pulse with a rise time of 150 ps and a fall time of 170 fs. The temporal evolution of the molecular axis alignment is probed via the optical Kerr effect. Field-free alignment via the switched NRDSE is demonstrated for both linear CO2 ,C S 2 and symmetric top 1,2propadiene polyatomic molecules.

Analytical study of the optical spectrum shift in a modulating channel

In this work, an analytical model of the frequency shift experienced by an optical signal passing through a traveling wave modulating channel is derived. The model starts from the well-known coupled-mode theory but holds for an arbitrary temporal envelope of the optical signal and for any time dependence of the slowly varying modulating field; it provides an analytical and accurate explanation of various effects such as those due to losses or microwave-optical group velocity mismatch. The practical example of a modulated optical Gaussian pulse is provided.

Ultrafast all-optical differentiators

We report the experimental realization of an ultrafast all-optical temporal differentiator. Differentiation is obtained via all-fiber filtering based on a simple diffraction grating-assisted mode coupler (uniform long-period fiber grating) that performs full energy conversion at the optical carrier frequency. Due to its high bandwidth, this device allows processing of arbitrary optical signals with sub-picosecond temporal features (down to 180-fs with the specific realizations reported here). Functionality of this device was tested by differentiating a 700-fs Gaussian optical pulse generated from a fiber laser (@ 1535nm). The derivative of this pulse is an odd-symmetry Hermite-Gaussian waveform, consisting of two linked 500-fs-long, pi-phase-shifted temporal lobes. This waveform is noteworthy for its application in advanced ultrahigh-speed optical communication systems.

New method of controlling the power of a Gaussian optical pulse through an electro-optic modulator and a nonlinear waveguide for generation of solitons

Optical soliton propagation through an optical fiber is a well-established phenomenon in digital optical communication. This propagation can be exploited to secure many advantages in communication because of the unattenuated, unchirped, and undistorted Gaussian optical pulse propagated through the fiber. Here we propose a completely new scheme for controlling the power shape function of a Gaussian optical pulse propagated through an optical fiber in the form of soliton, leading to some new characteristics in connection with such propagation. To exploit this possibility an electro-optic modulator is used on the pulse before its introduction into the fiber.

Performance Analysis on Phase-Encoded OCDMA Communication System in Dispersive Fiber Medium

The performance of an asynchronous phase-encoded optical code-division multiple-access system is evaluated in a dispersive fiber medium. We derive an approximate analytical expression for the root mean square width of the phase-encoded signal (pseudorandom optical signal with low intensity) propagating in linear dispersive fibers. Bit-error rate analysis of the system is performed in the case of both ordinary single-mode fiber and dispersion-shifted fiber (DSF). The numerical results demonstrate that even though system performance improves due to the smaller width of initial Gaussian optical pulse, the effect from dispersion is higher. Larger code length reduces the effect of dispersion and the use of DSF greatly increases the transmission distance.

Photoacoustic point source.

We investigate the photoacoustic effect generated by heat deposition at a point in space in an inviscid fluid. Delta-function and long Gaussian optical pulses are used as sources in the wave equation for the displacement potential to determine the fluid motion. The linear sound-generation mechanism gives bipolar photoacoustic waves, whereas the nonlinear mechanism produces asymmetric tripolar waves. The salient features of the photoacoustic point source are that rapid heat deposition and nonlinear thermal expansion dominate the production of ultrasound.

Spatial and temporal dependence of single-pass parametric gain

The use of focused Gaussian beams and short optical pulses to pump single-pass optical parametric amplifiers is examined. A simple model is introduced to mimic the experimental conditions used, and the expected intensity profile of the output signal for degenerate parametric amplification in LBO and MgO:LiNbO3 is calculated. Experimental and theoretical results are compared for LBO. Second harmonic autocorrelation measurements were performed in order to determine the pulse duration of the output signal from the LBO parametric amplifier. These measurements showed that the amplified pulse was slightly shorter, and the deamplified pulse slightly longer, than the input signal—a result of the variation in the parametric gain over the 3 ps signal pulse. The implications of this spatial and temporal variation of the gain in squeezing are discussed.

Optical pulse propagation through a slab of random medium

Tractable analytic expressions are developed for the expected broadening that a Gaussian space-time optical pulse experiences as it propagates through a slab of random medium confined to a portion of the propagation path between the input and output planes. The analytic results developed in this paper are based on the modified von Karman spectrum for refractive index fluctuations and the Rytov method which restricts the validity of the results to the weak optical turbulence regime. It is further assumed that the Gaussian beam is collimated and that the Gaussian pulse is narrowband with respect to the carrier frequency which, for optical frequencies, requires pulse widths greater than 20 fs. The results in this paper are given for both near and far fields and it is shown that when the thickness of the slab of random medium is the total propagation distance, the results given in this paper agree with the results for an extended random medium. Examples are given for unmanned air vehicle (UAV) to low Earth orbit (LEO) satellite and LEO-to-LEO crosslinks.

SOLITON CHARACTERISTICS OF GAUSSIAN OPTICAL PULSES PROPAGATION IN THE NONLINEAR GAIN MEDIUM

In this paper, propagation of Gaussian optical pulses in the nonlinear gain medium is ana-lyzed theoretically. The mechanism of Gaussian optical pulses compression, broadening and optical soliton formation is also explained. Gaussian optical pulses with different parameters propagating in nonlinear gain medium are simulated with computer. The change of pulse width during the propagation is obtained. The results indicate that it is possible for the ini-tial Gaussian optical pulses with different peak power to form different order solitons in the medium, nevertheless, their propagation distance is limited due to the existence of gain.

Pulse response of a fiber dispersion equalizing scheme based on an optical resonator

Results of a theoretical simulation of the dynamic response of a dispersion equalizing scheme using a ring resonator when transmitting chirped Gaussian optical pulses over a long distance fiber are given. It is shown that two pulses which merge into each other due to dispersion are recovered by the resonator. Selection of the optimum resonator parameters in relation to the overall response is described. >