Short Optical Pulses Research Articles

Generating ultrashort flat-top optical pulses with a fiber-loop time-lens system

For generating short flat-top optical pulses, traditional methods are either bandwidth limited or have low controllability (in pulse width and shape). We propose a fiber-loop based time-lens system to realize ps-level flat-top pulses, and theoretically study their features under various parameter circumstances. This time-lens system, functioning as many stacked time-lenses, introduces great controllability for the generated pulse’s characteristics through tuning both the phase modulation device and the dispersion-compensating setup. To achieve high-quality narrow flat-top pulses (with steep pulse edges), several factors including phase modulation function, modulation strength and dispersion compensating status must be optimized simultaneously. In simulation, after 50 loops of parabolic phase modulation and chirp compensation, the flat-top pulse’s duration can be narrowed to 2.8 ps with edge widths of 1.7 ps. This kind of ps-scale flat-top pulse is very desirable for application in various areas.

Stochastic and deterministic switches in a bistable polariton micropillar under short optical pulses

Optical bistability of exciton polaritons in semiconductor microcavities is a promising platform for digital optical devices. Steady states of coherently driven polaritons can be toggled in tens of picoseconds by a short external pulse of appropriate amplitude and phase. We have analyzed the switching behavior of polaritons depending on the pulse amplitude, phase, and duration. The switches are found to change dramatically when the inverse pulse duration becomes comparable to the frequency detuning between the driving field and polariton resonance. If the detuning is large compared to the polariton linewidth, the system becomes extremely sensitive to initial conditions and thus can serve as a fast random-number generator.

The Influence of Metal Nanoparticles on the Propagation of Extremely Short Optical Pulses in Graphene

The propagation of two-dimensional extremely short optical pulses in graphene with metal nanoparticles has been investigated. The effects that are observed when changing the parameters of the effective Hamiltonian, which takes into account the exchange interaction, have been studied. It has been revealed that the introduction of metallic adatoms into graphene leads to an increase in the amplitude of the pulse. This effect can be enhanced by choosing a particular metal particle .

Spatiotemporal coupling of attosecond pulses

The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme UV range and bandwidths exceeding tens of electronvolts. They are often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely used approximation consisting of writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argument uses a simple analytical model based on Gaussian optics, numerical propagation calculations, and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring high-quality focusing while retaining the broadband/ultrashort characteristics of the radiation.

Propagation of two-dimensional extremely short optical pulses in photonic crystal with silicene

We consider the wave equation for an electromagnetic field propagating in silicene placed in photonic crystal (PC). We study the effects observed when the depth of the nonlinearity modulation are varied, as well as the initial amplitude of the electromagnetic pulse.

Влияние металлических наночастиц на распространение предельно коротких оптических импульсов в графене

AbstractThe propagation of two-dimensional extremely short optical pulses in graphene with metal nanoparticles has been investigated. The effects that are observed when changing the parameters of the effective Hamiltonian, which takes into account the exchange interaction, have been studied. It has been revealed that the introduction of metallic adatoms into graphene leads to an increase in the amplitude of the pulse. This effect can be enhanced by choosing a particular metal particle .

Световые пули в периодически неоднородной среде ориентированных углеродных нанотрубок в оптическом резонаторе

AbstractThe problem of the propagation dynamics of three-dimensional ultimately short optical pulses in an optical cavity in a periodically inhomogeneous medium of oriented carbon nanotubes is considered. It is shown numerically that such pulses demonstrate stable and sustainable propagation. In addition, it is shown that one can control for the pulse propagation rate and modify the pulse shape by varying parameters of the inhomogeneous medium.

Simulation of the effect of short optical pulses on graphene

The interaction of high-frequency pulsed electric fields with graphene is currently the subject of intense research. The paper presents the results of testing a software system for modeling such processes using the example of ultrashort laser pulses of the optical range with different polarizations. The authors develop the system on a base of a new theoretical approach based on the quantum kinetic equation. The approach contains a computational model for a new system of ordinary differential equations with non-linearly dependent on time and problem parameters coefficients. The need to analyze the behavior of solutions of this system of equations in the field of changing several parameters leads to the polynomial computational complexity. The lack of knowledge of the nature of the parametric dependence of solutions requires several iterations of the choice of covering grids. The paper describes the adaptation of this modeling system for use in massively parallel computing systems.

Three-dimensional Extremely Short Optical Pulses in Graphene with Inhomogeneities

Three-dimensional Extremely Short Optical Pulses in Graphene with Inhomogeneities

Characterization of laser ultrasound source signals in biological tissues for imaging applications.

Short optical pulses emitted from a tunable Q-switched laser (800 to 2000nm) generate laser ultrasound (LUS) signals at the surface of biological tissue. The LUS signal's acoustic frequency content, dependence on sample type, and optical wavelength are observed in the far field. The experiments yield a reference dataset for the design of noncontact LUS imaging systems. Measurements show that the majority of LUS signal energy in biological tissues is within the 0.5 and 3MHz frequency bands and the total acoustic energy generated increases with the optical absorption coefficient of water, which governs tissue optical absorption in the infrared range. The experimental results also link tissue surface roughness and acoustic attenuation with limited LUS signal bandwidth in biological tissue. Images constructed using 810-, 1064-, 1550-, and 2000-nm generation laser wavelengths and a contact piezoelectric receiver demonstrates the impact of the generation laser wavelength on image quality. A noncontact LUS-based medical imaging system has the potential to be an effective medical imaging device. Such a system may mitigate interoperator variability associated with current medical ultrasound imaging techniques and expand the scope of imaging applications for ultrasound.

Fast response organic photodetectors with a thick photoconversion region and fullerene transport layer

Organic photodetectors were prepared based on polymer-fullerene blended heterojunctions using Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and Phenyl-C61-Butyric-Acid-Methyl Ester (PCBM). In order to investigate and improve the sensitivity to short optical pulses, 5–10 micrometer thick layers were fabricated. A dual-solvent draw blade technique was developed to simplify the heterojunction production. The increased thickness resulted in a drastic reduction to the dark current. Both electrically inert nanoparticles and an evaporated fullerene interlayer were found to further improve the device performance. The external quantum efficiency was measured, as well as the fast pulse response in the limit of low photocarrier injection. Results showed a sensitivity to pulses of very small energy, 100 fJ, largely aided by a dark current that was below 10 nA/cm2.

Performance analysis of various pulse modulation schemes for a FSO link employing gain switched quantum cascade lasers

In this paper, we analyze the performance of an FSO communication link that incorporates gain switched Quantum Cascade Lasers (QCLs) operating at 9 µm and a Quantum-Well based Infrared Photodetector (QWIP). The performance of various modulation techniques like ON-OFF Keying (OOK), Pulse Position Modulation (PPM), Dual-Header Pulse Interval Modulation (DH-PIMα), Differential Pulse Interval Modulation (DPIM) are analyzed for the FSO link with short optical pulses. It is found that DH-PIM2 satisfies most of the properties required for an efficient pulse modulation. It provides highest power efficiency, normalized packet rate (3.8 times that of PPM), possesses the lowest bandwidth requirement of 3.8 GHz and also the lowest average symbol length of 11 bits. DPPM has the highest transmission capacity that is 2 times that of PPM while PPM performs significantly better among all the modulation techniques providing an astounding BER of 10−40 under soft decoding.

Numerical analysis of four wave mixing in photonic crystal semiconductor optical amplifier

Abstract In this paper, we present a theoretical analysis of four-wave mixing (FWM) between short optical pulses in a photonic crystal semiconductor optical amplifier (PC-SOA). The proposed model is based on a nonlinear propagation equation by taking into account self-phase modulation (SPM), carrier density pulsations (CDP), carrier heating (CH), and spectral-hole burning (SHB). To analyse the FWM in a PC-SOA, pulse evolution in a temporal and spectral domain are investigated. Also, the optical fluid method is used in the photonic structure for design and engineering an appropriate bandwidth with constant group index to analyse the FWM mechanism. The bandwidth is designed in the L-Band region of telecommunication optical networks. Based on calculated results, it is depicted that for low energy short pulses, the conversion efficiency (CE) of PC-SOA is higher than conventional SOA. Furthermore, it is shown that, by increasing the injection current the CE is increased.

Characteristics of selective and tunable wavelength converters using quasi-phase matched lithium niobate waveguide devices with dual pump configuration

We report selective and tunable wavelength converters (STWCs) from an arbitrary wavelength to another arbitrary one, which employ the cascaded second-order nonlinear effect of sum frequency mixing (SFM) and difference frequency mixing (DFM) in quasi-phase matched (QPM) lithium niobate (LN) waveguide devices. Through wavelength conversion experiments using short optical pulses for the QPM-LN waveguide devices having different crystal length, we investigate bandwidth limitation of the QPM-LN-based all-optical STWCs with dual pump configuration. We show that the critical pulse width to be wavelength-converted without waveform distortion is proportional to the length of the LN crystal, and also reveal that those ratio is 1.6 ps/cm. By utilizing this critical value as a performance metric, we successfully demonstrate highly efficient selective and tunable wavelength conversion of 40-Gbit/s data signals using the SFM–DFM cascade in a QPM-LN waveguide device with an optimum crystal length of 5 cm. The 5 cm-long device can be applied to channel-by-channel wavelength conversion in 100-GHz-spaced 40-Gbit/s dense wavelength-division multiplexed systems.

Vector solitons and dispersive waves in birefringent optical fibers

We develop a general formalism for investigating the evolution of arbitrarily polarized short pulses inside a birefringent optical fiber. We use it to numerically study the formation of a dispersive wave inside fibers exhibiting medium to high birefringence when a short optical pulse is launched such that it propagates as a vector soliton. We also investigate the polarization evolution of both the vector soliton and dispersive wave generated by it. The results show that, while the polarization of the dispersive wave is controlled by linear birefringence of the fiber, polarization of the vector soliton is affected considerably by the nonlinear birefringence. The coupled nonlinear equations that we solve include both the Raman and Kerr nonlinearities. Moreover, they include the cross-polarization Raman terms that couple the orthogonally polarized components of the vector soliton. Polarization of the vector soliton is found to be affected considerably by the Raman nonlinearity in the case of medium birefringence.

Particle Swarm Optimization approach to identify optimum electrical pulse characteristics for efficient Gain Switching in Dual Wavelength Quantum Cascade Lasers

Numerical simulation of Gain Switching in Dual Wavelength QCLs (DW-QCLs) for short pulse generation in the mid-Infrared region is reported. A four-level rate equation model is used to investigate the QCL which consists of 48 periods of injector and active regions, emitting at 10.5 μm and 8.9 μm. The device is simulated for various inputs such as square, haversine and tangential hyperbolic electrical pulses. The optimum parameters of electrical pulse pertaining to generation of short pulses with maximum power and minimum pulse width for single wavelength and simultaneously at both wavelengths are determined using Particle Swarm Optimization (PSO) technique. The physical nature of drive current and its time period dictates the duration and peak power in both the wavelengths. It is found that square pulse of amplitude 11.12 mA and 0.1 ns produces short optical pulses of width 15.89 ps with peak power of 2.2 mW at 10.5 μm. Similarly, short optical pulses with peak power 1.8 mW and width 15.92 ps are generated by square pulse of amplitude 13.12 mA at 8.9 μm. It is also observed that tangential hyperbolic pulse of amplitude 10.79 mA and 8.47 mA produces short optical pulses with equal power and equal pulse width respectively in both the wavelengths at 0.1 ns.

Pulse train interaction and control in a microcavity laser with delayed optical feedback.

We report experimental and theoretical results on the pulse train dynamics in an excitable semiconductor microcavity laser with an integrated saturable absorber and delayed optical feedback. We show how short optical control pulses can trigger, erase, or retime regenerative pulse trains in the external cavity. Both repulsive and attractive interactions between pulses are observed, and are explained in terms of the internal dynamics of the carriers. A bifurcation analysis of a model consisting of a system of nonlinear delay differential equations shows that arbitrary sequences of coexisting pulse trains are very long transients towards weakly stable periodic solutions with equidistant pulses in the external cavity.

Vertical cavity surface emitting laser based hybrid fiber-free space optic link for passive optical network applications

In this work, a hybrid architecture employing Vertical Cavity Surface Emitting Laser (VCSEL) based Single Mode Fiber (SMF) link followed by Free Space Optic (FSO) transmission, is investigated. The VCSEL is biased below threshold to generate short optical pulses, which is coded in NRZ format. After the 20 km SMF link, a splitter divides the received power into two branches. One portion is coupled to a PIN photodiode for receiver operation, whereas the remaining power is coupled to a lens for free space transmission. The VCSEL-SMF-FSO link is simulated and performance measures are computed and verified with link budget calculation. The maximum FSO link distances under different weather conditions and coupling ratio were found. For BER ≤10−12, under clear weather, the maximum attainable FSO link lengths for 1.25, 2.5, 5 and 10 Gbps data rates are found to be 0.205 km, 0.16 km, 0.14 km and 0.11 km respectively. The same procedure when extended to snowy conditions, yield FSO link length of 155 m, 125 m, 110 m and 90 m for 1.25, 2.5, 5 and 10 Gbps data rates under similar BER performance. The impact of fractional power coupled to FSO link was analyzed for 5 Gbps by varying the splitting ratio from 80:20 to 86:14. The optimum ratio up to which both SMF and the cascaded link system meet the required BER for Gigabit PON application is also determined.

Fast-Recovery of the Amplitude and Phase of Short Optical Pulses Using a Frequency-Swept Source Based Heterodyne Measurement

We propose a very fast heterodyne technique to recover the amplitude and phase of short optical pulses generated, e.g., by a mode-locked laser. A linearly swept frequency source is used to scan the entire optical spectrum of the mode-locked laser in a single continuous sweep. The beat signal is recorded on a fast oscilloscope and then digitally processed allowing the simultaneous recovery of the amplitude and the phase. This measurement is fast (less than ${2\,\mu \text{s}}$ ) and requires no prior spectral information about the signal under test.

On-the-Fly Control of High-Harmonic Generation Using a Structured Pump Beam.

We demonstrate experimentally a relatively simple yet powerful all-optical enhancement and control technique for high harmonic generation. This is achieved by using as a pump beam two different spatial optical modes interfering together to realize tunable periodic quasi-phase matching of the interaction. With this technique, we demonstrate on-the-fly quasi-phase matching of harmonic orders 29-41 at ambient gas pressure levels of 50 and 100 Torr, where an up to 100-fold enhancement of the emission is observed. The technique is scalable to different harmonic orders and ambient pressure conditions.