Picosecond Pulse Propagation Research Articles

Broadband self-pulsating fiber laser based on soliton self-frequency shift and regenerative self-phase modulation

We demonstrate experimentally and numerically the operation of a self-pulsating fiber laser based on the cascaded effects of soliton self-frequency shift and self-phase modulation spectral broadening. The combination of those two effects triggers and sustains the propagation of picosecond pulses in the cavity. At one of the outputs, the laser emits a supercontinuum with spectral width in excess of 150 nm at the repetition rate of 95 kHz.

Self-phase modulation and nonlinear loss in silicon nanophotonic wires near the mid-infrared two-photon absorption edge

We report an experimental study of picosecond pulse propagation through a 4-mm-long Si nanophotonic wire with normal dispersion, at excitation wavelengths from 1775 to 2250 nm. This wavelength range crosses the mid-infrared two-photon absorption edge of Si at ~2200 nm. Significant reduction in nonlinear loss due to two-photon absorption is measured as excitation wavelengths approach 2200 nm. At high input power, self-phase modulation is clearly demonstrated by the development of power-dependant spectral fringes. Asymmetry and blue-shift in the appearance of the spectral fringes at 1775 nm versus 2200 nm is further shown to originate from a strong reduction in the intra-pulse density of two-photon absorption-generated free carriers and the associated free-carrier dispersion. Analysis of experimental data and comparison with numerical simulations illustrates that the two-photon absorption coefficient β(TPA) obtained here from nanophotonic wire measurements is in reasonable agreement with prior measurements of bulk silicon crystals, and that bulk Si values of the nonlinear refractive index n(2) can be confidently incorporated in the modeling of pulse propagation in deeply-scaled waveguide structures.

Subnanosecond delay of light inCdxZn1−xTecrystals

We study excitonic polariton relaxation and propagation in bulk CdZnTe using time- resolved photoluminescence and time-of-flight techniques. Propagation of picosecond optical pulses through 0.745 mm thick crystal results in time delays up to 350 ps, depending on the photon energy. Optical pulses with 150 fs duration become strongly stretched. The spectral dependence of group velocity is consistent with the dispersion of the lower excitonic polariton branch. The lifetimes of excitonic polariton in the upper and lower branches are 1.5 and 3 ns, respectively.

Simulation investigation on supercontinuum generation and noise characteristics in the normal dispersion photonic crystal fiber with a flattened dispersion profile

The propagation of picosecond pulses in the normal dispersion photonic crystal fiber (PCF) with a flattened dispersion profile is numerically investigated. The characteristics of the amplitude and phase noise in the supercontinuum generation (SCG) are also analyzed through the coherent sliced supercontinuum (SC). The effects of self-phase modulation (SPM) and four-wave mixing (FWM) on broadening of the pulse spectrum are presented, and the best amplitude and phase noise performance with a specific fiber length is obtained. The SCG in the wavelength range of 1475 to 1625 nm has only fluctuation between ±1.5 dB, and both the amplitude and phase noise caused by the intensity noise of the input power is below 2.5% in the range of 1500 to 1600 nm.

Picosecond and Sub-Picosecond Flat-Top Pulse Shaping Using Abrupt Taper Interferometers

A picosecond flat-top pulse shaper using an abrupt taper interferometer is proposed and experimentally demonstrated. A theoretical model based on coupled mode equations is also derived to simulate the propagation of picosecond pulses through the abrupt taper interferometer. Through properly designing the abrupt taper interferometer, a 3.8-ps flat-top pulse with less than 4% ripple is successfully generated from a 2.2-ps Gaussian pulse. The proposed technique can also be used in the sub-picosecond regime.

Collisions and turbulence in optical rogue wave formation

We discuss optical rogue wave generation in terms of collisions and turbulence processes. Simulations of picosecond pulse propagation in optical fibres show rogue soliton generation from either third-order dispersion or Raman scattering independently. Simulations of rogue soliton emergence with dispersive perturbation in the long-distance limit are also presented.

Characterization of picosecond pulse nonlinear propagation in chalcogenide As_2S_3 fiber

We characterize the nonlinear propagation of picosecond pulses in chalcogenide As(2)S(3) single-mode fiber using a pump-probe technique. The cross-phase modulation (XPM)-induced sideband broadening and stimulated Raman scattering (SRS)-induced sideband amplification are measured in order to map out the Raman gain spectrum of this glass across the C-band. We extract the Raman response function from the Raman gain spectrum and determine the power and polarization dependence of the SRS. In contrast to previous work using As(2)Se(3) fiber, we find that the As(2)S(3) fiber does not suffer from large two-photon absorption (TPA) in the wavelength range of the telecommunications band. We achieved a 20 dB peak Raman gain at a Stokes shift of 350 cm(-1) in a 205 mm length of As(2)S(3) single-mode fiber. The Raman gain coefficient is estimated to be 4.3x10(-12) m/W and the threshold pump peak power is estimated to be 16.2 W for the 205 mm As(2)S(3) fiber. We also demonstrate that we can infer the dispersion of the As(2)S(3) fiber and justify the Raman response function by comparing simulation and experimental results.

Diffraction limited amplification of picosecond pulses in 1170μm^2 effective area erbium fiber

Robust fundamental mode propagation and amplification of picosecond pulses at 1.56 microm wavelength is demonstrated in a core-pumped Er fiber with 1170 microm2 effective area. Record peak power exceeding 120 kW, and 67 nJ pulse energy are achieved before the onset of pulse breakup. A small increase in input pulse energy results in a temporal collapse of the pulse center to 58 fs duration, with peak powers approaching 200 kW.

Nonlinear switching using long-period gratings in As_2Se_3 chalcogenide fiber

We present detailed experimental and numerical analyses of nonlinear all-optical switching in long-period gratings (LPGs) in As2Se3 chalcogenide fiber. We characterize the nonlinear propagation of picosecond pulses through the LPG and observe nonlinear switching at optical peak powers around 55 W. This is 2 orders of magnitude lower than previously demonstrated in silica fibers, owing to the very strong Kerr nonlinearity in As2Se3 chalcogenide glass. The results are discussed in the context of applications in passively mode-locked lasers and all-optical regenerators.

The picosecond pulse transmission on the conductor-backed coplanar waveguide with via holes

-The picosecond pulse propagation on the conductor-backed coplanar waveguide (CBCPW) with via holes is investigated. For this purpose, an empirical dispersion formula by fitting measurement data is proposed and Gaussian picosecond pulse transmissions on the CBCPW with via holes are simulated. By comparing the modeling data with the simulation result by the reported formula, we verify the usefulness and effectiveness of the proposed expression.

Investigation of pulse pedestal and dynamic chirp formation on picosecond pulses after propagation through an SOA

The authors investigate the propagation of picosecond pulses through semiconductor optical amplifiers using the measurement technique of frequency resolved optical gating. The work shows the generation of significant pulse pedestals and frequency chirp across the optical pulses, which initially have a duration of 2 ps. As the input peak power of the optical pulses is increased from 2.4 to 80 mW, the pulse pedestals increased by 20 dB and the chirp became significantly more nonlinear. The generated pedestals and the nonlinear output chirp may cause serious degradation in high-speed communications systems employing wavelength-division-multiplexing and optical time-division-multiplexing techniques.

Time-Frequency (Wigner) Analysis of Linear and Nonlinear Pulse Propagation in Optical Fibers

Time-frequency analysis, and, in particular, Wigner analysis, is applied to the study of picosecond pulse propagation through optical fibers in both the linear and nonlinear regimes. The effects of first- and second-order group velocity dispersion (GVD) and self-phase modulation (SPM) are first analyzed separately. The phenomena resulting from the interplay between GVD and SPM in fibers (e.g., soliton formation or optical wave breaking) are also investigated in detail. Wigner analysis is demonstrated to be an extremely powerful tool for investigating pulse propagation dynamics in nonlinear dispersive systems (e.g., optical fibers), providing a clearer and deeper insight into the physical phenomena that determine the behavior of these systems.

Numerical study on nonlinear pulse transmission in a fiber link with periodical dispersion slope compensation

In fiber links with a bit rate greater than 100 Gb/s per wavelength channel, the third-order dispersion (TOD), known as the dispersion slope, becomes a major factor that limits transmission capabilities. This paper presents a numerical study on the propagation of picosecond pulses in anomalous dispersion fibers, the dispersion slope of which is periodically compensated for by lumped compensators. In particular, pulse propagation is discussed in comparison with path-averaged solitons in a dispersion-flattened fiber. Simulations of single-pulse transmission reveal that pulses in the slope-compensated link can be easily dispersed. The dispersive component can be effectively eliminated by introducing inline bandpass filters, the bandwidth of which is smaller than that suitable for solitons in a dispersion-flattened fiber. The transmission of a pulse pair is also considered and is predicted to be stabilized for transmission power larger than that which is optimum for solitons in a dispersion-flattened fiber. These predictions are confirmed by demonstrating system analyses using a pseudorandom bit stream, showing that the penalty of the slope compensation is due to an increase in timing jitter.

Picosecond acoustic phonon pulse propagation in silicon

We report experimental and theoretical investigations of the behavior of coherent acoustic phonon pulses after propagation through millimeter-scale distances in crystalline Si. An ultrafast optical pump and probe technique is used to generate and detect the phonon pulses. The geometry of our experiment is such that we can make this measurement in either the far field or the near field of the acoustic source, allowing studies of diffraction effects in addition to dispersion and nonlinearity in the Si. We also present a rigorous derivation of the Korteweg--de Vries equation, which describes the behavior of these acoustic pulses in the nonlinear regime in one dimension. This one-dimensional (1D) model is combined with a 3D analysis of diffraction effects in anisotropic media in order to analyze far-field data.

Superluminal pulse propagation in linear and nonlinear photonic grating structures

Optical pulse propagation in photonic grating structures can show anomalous (i.e., superluminal or negative) group velocities under certain circumstances owing to the anomalous dispersive properties induced by the periodic grating structure. Such phenomena can be observed for either linear pulse propagation in passive dielectric grating structures, such as in fiber Bragg gratings (FBGs), as well as in frequency-conversion processes exploiting second-order cascading effects in quasi-phase-matched (QPM) nonlinear crystals. Engineering of the grating structure can be exploited to observe a wide variety of anomalous pulse transmission and reflection behaviors. In this article, we review the main recent experimental and theoretical achievements obtained by our group in this field. In particular, we report on superluminal propagation of picosecond optical pulses at the 1.5-/spl mu/m wavelength of optical communications in FBGs, both in transmission and reflection configurations, with the observation of group velocities as large as /spl sim/5c/sub 0/. We also show that the phenomenon of transparent pulse propagation at a negative group velocity in a gain doublet atomic amplifier, recently observed in cesium vapor by Wang and co-workers (L. J. Wang, A. Kuzmich, and A. Dogariu, Nature vol.406, p.277-9, 2000), can be achieved as well in a photonic parametric amplifier by exploiting the anomalous dispersive properties of the amplifier induced by a suitably designed QPM grating profile.

Propagation, manipulation, and control of picosecond optical pulses at 15 μm in fiber Bragg gratings

We review the main techniques and the most recent experimental achievements obtained by our group on manipulation and control of picosecond optical pulse trains at 1.5-µm based on fiber Bragg grating (FBG) devices of interest in ultrafast optoelectronics. In particular, all-optical techniques for repetition-rate multiplication of picosecond mode-locked pulse trains that use linearly chirped FBGs and pulse-shaping techniques based on Fourier spectral filtering in a novel class of dispersive FBGs are presented. Theoretical and experimental results on the control of the light speed in FBG structures, including superluminal transmission and reflection of picosecond pulses, are also discussed.

Evolution of ultra-short optical pulses in active DFB waveguides

We present a time-domain study on propagation of pico-second optical pulses in active semiconductor waveguides embedded with a Bragg grating. The nonlinear effects of gain saturation, limited carrier lifetime, as well as linewidth enhancement factor are taken into account. It is theoretically shown that the effective group velocity can be significantly enhanced by the grating, resulting in the so-called superluminal transmission. Moreover, the pulse propagation and reflection behaviors are greatly influenced by such parameters as Bragg coupling strength and optical gain in the waveguide.

A micromachined photoconductive near-field probe for picosecond pulse propagation measurement on coplanar transmission lines

For the first time, a micromachined near-field probe has been developed, based on picosecond photoconductive sampling using low-temperature-grown GaAs (LT-GaAs). By changing the direction of the probe, it is capable of scanning/mapping independent orthogonal components of free-space electric fields. The micromachined 1-/spl mu/m-thick LT-GaAs epilayer substrate of the probe provides a minimal loading effect for measuring picosecond electric fields. The use of optical fibers, for guiding laser pulses to the photoconductive switch on the probe and for electrical connections, enables the probe to be positioned freely with uniform sensitivity. We have demonstrated by finite-difference time-domain simulation that the measurement process of the developed probe is dominated by the switching action of the carriers generated in the photoconductive switch of the probe. Using the probe, propagating picosecond pulse waveforms and field distribution images on coplanar transmission lines were successfully obtained. The probe measurement provides a useful understanding, which cannot be obtained from conventional external-port access test instruments, of electromagnetic phenomena related to wave propagation.

Gain dynamics in traveling-wave semiconductor optical amplifiers

A theoretical analysis of the optical properties of multiquantum well semiconductor optical amplifiers (MQW-SOA) have been performed in the time-dependent regime. We describe the interaction between pulse propagation and gain compression within a pump-probe excitation in polarization insensitive MQW-SOA. We use a phenomenological model based on rate equations (describing the carrier dynamics in the different heterolayers along the growth axis for both bulk and MQW structures), coupled with the description of picosecond optical pulse propagation within a semiconductor slab (along the active waveguide axis), to describe the way gain dynamics determine the optical response of the SOA. Our model agrees well with available experimental data and also reveals the conditions for the validity of previous simpler approaches.

Inertial confinement fusion and fast ignitor studies

Laser imprinting has been studied and, in particular, saturation of areal density perturbations induced by near single mode laser imprinting was observed. Several issues important for the foam buffered direct drive scheme have been investigated. These studies included measurements of the absolute levels of stimulated Brillouin and Raman scattering observed from laser irradiated low density foam targets, either bare or overcoated with a thin layer of gold. A novel scheme is proposed to increase the pressure in indirectly driven targets. By heating a foam supersonically that is attached to a solid target the pressure generated is not only the ablation pressure but also the combined pressure due to ablation at the foam-foil interface and the heated foam material. Planar brominated plastic foil targets overcoated with a low density foam were irradiated by a soft X ray pulse. The pressure was obtained by comparing the rear side trajectory of the driven target observed by soft X ray radiography with one dimensional radiation hydrodynamic simulations. Observations were also carried out of the transition from supersonic to subsonic propagation of an ionization front in low density chlorinated foam targets irradiated by an intense soft X ray pulse. The diagnostic for these measurements was K shell point projection absorption spectroscopy. In the fast ignitor area the channelling and guiding of picosecond laser pulses through underdense plasmas, preformed density channels and microtubes were investigated. It was observed that a large fraction of the incident laser energy can be propagated. Megagauss magnetic fields were measured, with a polarimetric technique, during and after propagation of intense picosecond pulses in preionized plasmas. Two types of toroidal fields, of opposite orientation, were detected. In addition, the production and propagation of an electron beam through solid glass targets irradiated at intensities above 1019W/cm2 were observed using optical probingtechniques.