Optical vortices carrying orbital angular momentum and with durations of femtoseconds recently have attracted great interest due to their potential applications in ultra-fast spectroscopy, high-intensity field physics and ultrafast optics through a new degree of freedom that they add to nonlinear optics.Thus, developing compact and efficient tools for their generation is required. Here, we present a reflective multi-material phase converter with high conversion efficiency over a broad wavelength range (over 200 nm) and high damage threshold that works directly with a Gaussian beam, common to most mode-locked lasers. Additionally the device allows to compensate dispersion previously acquired by the incident pulse. Spatio-spectro-temporal numerical simulations showing viability of the device are presented.
There exist a variety of methods and platforms for generation of OAM beams. However, all these techniques imply that beams pass through an optical component and thus, when pulsed beam is used, the pulses can acquire dispersive broadening and distortion. The design of compact and efficient devices for OAM generation requires that these effects be characterized and quantified, and a set of parameters and techniques for their treatment should be developed. Here, the spatio-temporal properties of the ultrashort vortical pulses are analyzed numerically. Case studies will be presented. The results obtained are characterization of the effects of dispersion, geometry and discretization in the numerical modeling of ultrashort vortical pulses under various propagation conditions, and can serve as a basis for design of new optical devices.
Often during research and development a precise knowledge on derivatives is required. In many cases it is very difficult or impossible to obtain derivatives analytically. This usually occurs in situations when the data to be processed are from an experiment and, therefore, is discrete and with a mixture of noise. The same situation is observed when data to be processed are obtained from numerical simulations. Here we present a detailed comparison of four methods to obtain higher-order derivatives from digital/discrete data. Finite differences method, complex step method, Richardson's extrapolation method, and complex integration method are compared to get an accurate higher-order derivative approximation. Each of them has different properties which make them reliable for a variety of applications and can be easily implemented using software tools.
Ultrashort pulses (picoseconds or less), are characterized by a high peak intensity that usually leads to nonlinear interactions. Recently ultrashort pulses with orbital angular momentum have attracted great attention due to their wide field of applications, from super-resolution microcopy, optical tweezers and ultra-fast optical communications to quantum computing and astrophysics. A principal challenge here is the development of robust and effective methods for OAM generation. At the moment few techniques are known to obtain OAM beams in the free space arrangement cylindrical lenses, spiral phase plates or computer-generated holograms have been used. All those techniques imply that pulse is passing through an optical component and thus obtain dispersive broadening. We propose here a method and a device allowing formation of ultrashort optical vortices from an incident ultrashort pulse without its broadening or compensating the pulse chirp acquired previously. Potentially method and device will allow obtain ultrashort vortices shorter than incident pulse. Case study is presented based on spatio-temporal numerical simulation and analytical modeling.
We have investigated ultrashort parabolic pulse formation via passive nonlinear reshaping in normal dispersive optical fibers at 1550 nm. It was investigated parabolic pulse formation in the transient-state regime and in the steady-state regime. Numerical simulations have been made based on generalized nonlinear Schrödinger equation taking into account high-order dispersion terms and high order nonlinear terms. It was examined the applicability of different commercially available fibers for parabolic pulse formation at 1550 nm. It was found that small amount of positive second-order dispersion and quite sufficient third-order dispersion can restrict strongly the formation of parabolic pulses at 1550 nm. The most suitable fiber for pulse reshaping has been found.
Microstructured fibers have recently become popular due to their numerous applications for fiber lasers,1 super-continuum generationi2 and pulse reshaping.3 One of the most important properties of such fibers that is taken into account is its dispersion. Fine tuning of the dispersion (i.e. dispersion management) is one of the crucial peculiarities of the photonic crystal fibers (PCFs)4 that are particular case of the microstructured fibers.
During last years, there have been presented various designs of the PCFs possessing specially-designed dispersion shapes. 5-7 However, no universal technique exists which would allow tuning the PCF dispersion without using optimization methods.
In our work, we investigate the sensitivity of the PCF dispersion as respect to variation of its basic parameters. This knowledge allows fine-tuning the position of local maximum of the PCF dispersion while maintaining other properties unchanged.
The work is organized as follows. In the first section we discuss the dispersion computation method that is suitable for the global sensitivity analysis. The second section presents the global sensitivity analysis for this specific case. We also discuss there possible selection of the variable parameters.
Here we propose a simple design for a solid-core photonic crystal fiber made of silica by keeping the golden ratio
(1.618) between pitch and air hole diameter Λ /d in a subset of six rings of air-holes with hexagonal arrangement. In the
case when we have a pitch equal to one micron (Λ =1 μm), we need air-holes diameters d=0.618 μm in order to obtain
the golden ratio parameter (Λ/d=1.618), and achieve two zero dispersion wavelength (ZDW) points at 725 nm and 1055
nm; this gives us the possibility to use this fiber in supercontinuum generation using a laser emission close to that points.
We analyzed a series of fibers using this relation and show the possibilities of tunable ZDW in a wide range of
wavelengths from 725 nm to 2000 nm with low losses and small effective area. In agreement with the ZDW point
needed, the geometry of the structure can be modified to the point of having only three rings of air holes that surround
the solid core with low losses and good confinement mode. The design proposed here is analyzed using the finite
element method (FEM) with perfectly matched layers (PML), including the material dispersion directly into the model
applying the Sellmeier’s equation.
The supercontinuum (SC) generated by pumping in anomalous dispersion is sensitive to the input pulse fluctuations and pump laser’s shot noises and does not possess a single-pulse waveform, so the incident pulse becomes a noise-like train of spikes. A simple method of creating pulsed lasers with either pulse-maintaining ultrabroad SC or specially shaped pulse waveforms can be implemented using all-normal-dispersion microstructured optical fibers (ANDi-MOFs). An ANDi-MOF with a simple topology and dispersion profile maximum at 800 nm was designed using the effective index method. Its properties and suitability were characterized via numerical simulation of femtosecond parabolic pulse formation and generation of an octave-spanning pulse-maintaining SC using a generalized propagation equation. The designed ANDi-MOF is suitable for resolving both problems and allows some detuning of the pulse’s wavelength around 800 nm. However, a better choice for SC generation is pumping at or near the wavelength where the third-order dispersion becomes zero. This configuration benefits from the absence of pulse break-up under large pulse energies, which appears otherwise. The fiber can provide a low-cost method for developing supercontinuum sources and a solution to the problems of parabolic waveform formation to meet the needs of optical signal processing and pulse amplification and compression.
In the work, we have presented the technique based on the graphics processing unit accelerated finite-difference time-domain (FDTD) method for characterization of a single-mode photonic crystal fiber (PCF) with an arbitrary refractive index profile. In contrast to other numerical methods, the FDTD allows studying the mode propagation along the fiber. Particularly, we have focused attention on the method details that allowed us to reduce dramatically the computation time. It has been demonstrated that the accuracy of dispersion obtained by the FDTD method is comparable to the one provided by the finite elements method while possessing lower computation time. The method has been used to determine the fundamental mode cut-off of all-normal dispersion PCF and to find fiber losses beyond this wavelength.
In the work, we have presented the technique based on the FDTD method for the dispersion computation of a single-mode ber with an arbitrary refractive index pro le. In contrast to other numerical methods the FDTD allows studying the beam propagation along the ber taking into account material dispersion and the nonlinearity. Particularly, we have concentrated attention on the method details that allowed us to reduce dramatically the computation time and achieve the accuracy close to the one provided by the nite elements method.
We have designed an all-normal dispersion photonic crystal fiber optimized for pumping at 800 nm with initial pulses which are typical for conventional Ti:Sapphire lasers. Parabolic pulse formation and supercontinuum generation in this fiber is analyzed both in time and frequency domains.
Generation and applications of the optical pulses with a parabolic intensity profile has developed into the area of
dynamic research activity over recent years. Parabolic pulses can propagate remaining their parabolic profile.
Particularly these pulses resist to the deleterious effect of the optical wave breaking. They are of great interest for a
number of applications including the high power pulse generation, and all optical signal processing. Alternative methods
of generating parabolic pulses are of especial interest in the context of non-amplification usage, such as optical
telecommunications. It is found that Gaussian waveforms provide best quasi-parabolic pulses among others and within
shortest distance. There is a range of soliton numbers where the shape of quasi-parabolic pulse is closest to parabolic
one.
We investigate theoretically the influence of indium surface segregation in InGaN/GaN single quantum wells
on its optical properties. Obtained results show that the influence of the surface segregation on the dipole
matrix element is not equal for all optical transition. This effect results from the joint action of the piezoelectric
polarization and indium surface segregation which change selection rules. Quantum well structures having
different indium amount are analyzed and found that the influence of the indium surface segregation on absorption
spectra is more pronounced in quantum well structures with high indium amount.
Spectral characteristics of the interference optical filter based on a free-standing mesoporous silicon film containing
nematic liquid crystal E7 are studied experimentally. The porous structure represents two distributed Bragg reflectors
divided by a quarter-wave microcavity having resonance near 1600 nm. Transmission spectra of the filter are measured
in the temperature range from 27°C to 80°C. For the temperatures less than 62°C (clearing point of the liquid crystal),
we have observed continuous red shift of the microcavity resonant wavelength in the range of 11 nm. Measured thermal
dependence of the shift has sharply increasing slope near the clearing point. For temperatures exceeding 62°C the
microcavity resonant wavelength exhibits slow linear decrease. We have also investigated spectra of the filter using local
heating of the sample with laser. Our studies have shown, that laser beam with power of 100 mW provides total tuning
of the microcavity.
We have investigated a nonlinear pulse reshaping towards parabolic pulses in the passive normal dispersive optical
fibers. We have found that pulses with parabolic intensity profile, parabolic spectrum and linear chirp can be obtained
due to the passive nonlinear reshaping at the propagation distance exceeding a few dispersion lengths. These pulses
preserve parabolic profile during subsequent pulse propagation in a fiber. We have examined the influence of initial
pulse parameters and fiber parameters on the resulted pulse shape.
In this work we investigate the influence of extractor design and temperature on transport properties of quantum
cascade detector. For this purpose we realize numerical calculation of electron lifetimes considering electronphonon
and electron impurities scattering. Electron-phonon interactions are treated using Fermi Golden Rule
which allows to calculate lifetime of carriers with temperature and structure design taking into account. Transport
characteristics of the quantum cascade detectors have been computed using density matrix theory. As a result, we
have obtained the system of ordinary differential equations describing dynamics of electron distribution functions
and intersubband correlations. Managing carrier lifetime in quantum wells gives us possibility to control quantum
efficiency and response.
A numerical model for the investigation of the ultrafast gain properties in asymmetrical multiple quantum-well semi-conductor optical amplifiers (AMQW SOAs) has been developed considering propagation of ultrashort optical pulses with different wavelengths. The dynamics of the number of carriers and carrier temperature are investigated for each quantum well. The results agree with the experimental results of pump probe measurements with different wavelengths. It is shown that gain recovery is slower for higher energy wells for pump signals of all wavelengths.
Self-consistent computations of the potential profile in complex semiconductor heterostructures can be successfully applied for comprehensive simulation of the gain and the absorption spectra, for the analysis of the capture, escape, tunneling, recombination, and relaxation phenomena and as a consequence it can be used for studying dynamical behavior of semiconductor lasers and amplifiers. However, many authors use non-entirely correct ways for the application of the method. In this paper the versatile model is proposed for the investigation, optimization, and the control of parameters of the semiconductor lasers and optical amplifiers which may be employed for the creation of new generations of the high-density photonic systems for the information processing and data transfer, follower and security arrangements. The model is based on the coupled Schrödinger, Poisson's and drift-diffusion equations which allow to determine energy quantization levels and wave functions of charge carriers, take into account built-in fields, and to investigate doped MQW structures and those under external electric fields influence. In the paper the methodology of computer realization based on our model is described. Boundary conditions for each equation and consideration of the convergence for the method are included. Frequently encountered in practice approaches and errors of self-consistent computations are described. Domains of applicability of the main approaches are estimated. Application examples of the method are given. Some of regularities of the results which were discovered by using self-consistent method are discussed. Design recommendations for structure optimization in respect to managing some parameters of AMQW structures are given.
The semiconductor laser is commonly used as a light source in fiber-optical telecommunication systems. In order to send as much information as possible in a short time, it is important that the laser has a large modulation bandwidth, i.e., the turn-on and turn-off time should be as short as possible. In analogue fiber optic systems for transmission of radio or television signals, it is also important that the light from the laser increases linearly with driving current even at high modulation frequencies. Otherwise, the transmitted signal will become distorted. The modulation bandwidth and the modulation distortion are dependent both on the laser structure and the gain characteristics of the active material. One of the most useful approaches for the time-domain description of the response of optoelectronic devices is the so-called "rate equation model," which has been widely used to describe laser performance. Commonly, laser models with simple gain expressions are used for simulation of laser dynamics. In these models the small-signal dynamic parameters like the differential gain and gain saturation parameter are extracted from modulation response measurements. However, we show that in order to correctly calculate distortion, an accurate model of the dependence of gain on carrier density, n, and photon density, s, is needed. Commonly used gain models, fitted to give exactly the same modulation response can give significantly different distortion behavior.
We present development results of the web-oriented environment for distance education in the field of the semiconductor laser physics. The paper includes description of the Interactive Environment for Distance Education (IEDE) focused on connection of lecture courses and the laser simulation package LaserCAD III, which allows considering lasers directly during reading of lectures via Internet. Some examples of the package using are presented.
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