Optical Nyquist pulse generation in mode-locked fibre laser

We numerically show the feasibility of Nyquist optical pulse generation in a mode-locked fibre laser with an in-cavity flat-top spectral filter. The proposed scheme offers the possibility to generate high-quality sinc-shaped pulses with tunable bandwidth.

Design and applications of in-cavity pulse shaping by spectral sculpturing in mode-locked fibre lasers

We review our recent progress on the realisation of pulse shaping in passively-mode-locked fibre lasers by inclusion of an amplitude and/or phase spectral filter into the laser cavity. We numerically show that depending on the amplitude transfer function of the in-cavity filter, various regimes of advanced waveform generation can be achieved, including ones featuring parabolic-, flat-top- and triangular-profiled pulses. An application of this approach using a flat-top spectral filter is shown to achieve the direct generation of high-quality sinc-shaped optical Nyquist pulses with a widely tunable bandwidth from the laser oscillator. We also present the operation of an ultrafast fibre laser in which conventional soliton, dispersion-managed soliton (stretched-pulse) and dissipative soliton mode-locking regimes can be selectively and reliably targeted by adaptively changing the dispersion profile and bandwidth programmed on an in-cavity programmable filter. The results demonstrate the strong potential of an in-cavity spectral pulse shaper for achieving a high degree of control over the dynamics and output of mode-locked fibre lasers.

Control of the in-cavity dynamics in mode-locked fibre lasers

At the level of fundamental research, fibre lasers provide convenient and reproducible experimental settings for the study of a variety of nonlinear dynamical processes, while at the applied research level, pulses with different and optimised features – e.g., in terms of pulse duration, temporal and/or spectral intensity profile, energy, repetition rate and emission bandwidth – are sought with the general constraint of developing efficient cavity architectures. In this talk, we review our recent progress on the realisation of different regimes of pulse generation in passively mode-locked fibre lasers through control of the in-cavity propagation dynamics. We report on the possibility to achieve both parabolic self-similar and triangular pulse shaping in a mode-locked fibre laser via adjustment of the net normal dispersion and integrated gain of the cavity [1]. We also show that careful control of the gain/loss parameters of a net-normal dispersion laser cavity provides the means of achieving switching among Gaussian pulse, dissipative soliton and similariton pulse solutions in the cavity [2,3]. Furthermore, we report on our recent theoretical and experimental studies of pulse shaping by inclusion of an amplitude and phase spectral filter into the cavity of a laser. We numerically demonstrate that a mode-locked fibre laser can operate in dif- ferent pulse-generation regimes, including parabolic, flattop and triangular waveform generations, depending on the amplitude profile of the in-cavity spectral filter [4]. An application of technique using a flat-top spectral filter is demonstrated to achieve the direct generation of sinc-shaped optical Nyquist pulses of high quality and of a widely tuneable bandwidth from the laser [5]. We also report on a recently-developed versa- tile erbium-doped fibre laser, in which conventional soliton, dispersion-managed soli- ton (stretched-pulse) and dissipative soliton mode-locking regimes can be selectively and reliably targeted by programming different group-velocity dispersion profiles and bandwidths on an in-cavity programmable filter [6]. References: 1. S. Boscolo and S. K. Turitsyn, Phys. Rev. A 85, 043811 (2012). 2. J. Peng et al., Phys. Rev. A 86, 033808 (2012). 3. J. Peng, Opt. Express 24, 3046-3054 (2016). 4. S. Boscolo, C. Finot, H. Karakuzu, and P. Petropoulos, Opt. Lett. 39, 438-441 (2014). 5. S. Boscolo, C. Finot, and S. K. Turitsyn, IEEE Photon. J. 7, 7802008 (2015). 6. J. Peng and S. Boscolo, Sci. Rep. 6, 25995 (2016).

In-cavity pulse shaping and dissipative parametric instability mode-locking in fibre lasers

At the level of fundamental research, fibre lasers provide convenient and reproducible experimental settings for the study of a variety of nonlinear dynamical processes, while at the applied research level, pulses with different and optimised features – e.g., in terms of pulse duration, temporal and/or spectral shape, energy, repetition rate and emission bandwidth – are sought with the general constraint of developing efficient cavity architectures. In this work, we review our recent progress on the realisation of pulse shaping in passively- mode-locked fibre lasers by inclusion of an amplitude and phase spectral filter into the laser cavity. We present a fibre laser design in which pulse shaping occurs through filtering of a spectrally nonlinearly broadened pulse in the cavity. This strategy of pulse shaping is illustrated through the numerical demonstration of the laser operation in different pulse-generation regimes, including parabolic, flattop and triangular waveform generations, depending on the amplitude profile of the in-cavity spectral filter [1]. As an application of this general approach, we show that the use of an in-cavity flat-top spectral filter makes it possible to directly generate sinc-shaped Nyquist pulses of high quality and of a widely tunable bandwidth from the laser [2]. We also report on a recently-developed versatile erbium-doped fibre laser, in which conventional soliton, dispersion-managed soliton (stretched-pulse) and dissipative soliton mode-locking regimes can be selectively and reliably targeted by programming different group-velocity dispersion profiles and bandwidths on an in-cavity programmable filter [3]. Further, we report on our recent results on the passive mode locking of a Raman fibre laser by a recently predicted new type of parametric instability – the dissipative Faraday instability [4], where spatially periodic zig-zag modulation of spectrally dependent losses can lead to pattern formation in the temporal domain. High-order harmonic mode locking is achieved in a very simple experimental configuration, with the laser cavity including an optical fibre and two chirped fibre Bragg gratings, and no additional mode-locking elements. The results not only open up new possibilities for the design of mode-locked lasers, but extend beyond fibre optics to other fields of physics and engineering. References [1] S. Boscolo, C. Finot, H. Karakuzu, P. Petropoulos, “Pulse shaping in mode-locked fiber laser by in-cavity spectral filter,” Opt. Lett., vol. 39, pp. 438–441, 2014. [2] S. Boscolo, C. Finot, S. K. Turitsyn, “Bandwidth programmable optical Nyquist pulse generation in passively mode-locked fiber laser,” IEEE Photon. J., vol. 7, 7802008(8), 2015. [3] J. Peng, S. Boscolo, “Filter-based dispersion-managed versatile ultrafast fibre laser,” Sci. Rep., 2016, In press. [4] A. M. Perego, N. Tarasov, D. V. Churkin, S. K. Turitsyn, K. Staliunas, “Pattern generation by dissipative parametric instability,” Phys. Rev. Lett., vol. 116, 028701, 2016.

Effects of corrugation frequency and aspect ratio on natural convection within an enclosure having sinusoidal corrugation over a heated top surface

Natural convection of a two-dimensional laminar steady-state incompressible fluid flow in a modified rectangular enclosure with sinusoidal corrugated top surface has been investigated numerically. The present study has been carried out for different corrugation frequencies on the top surface as well as aspect ratios of the enclosure in order to observe the change in hydrodynamic and thermal behavior with constant corrugation amplitude. A constant flux heat source is flush mounted on the top sinusoidal wall, modeling a wavy sheet shaded room exposed to sunlight. The flat bottom surface is considered as adiabatic, while the both vertical side walls are maintained at the constant ambient temperature. The fluid considered inside the enclosure is air having Prandtl number of 0.71. The numerical scheme is based on the finite element method adapted to triangular non-uniform mesh element by a non-linear parametric solution algorithm. The results in terms of isotherms, streamlines and average Nusselt numbers are obtained for the Rayleigh number ranging from 10^3 to 10^6 with constant physical properties for the fluid medium considered. It is found that the convective phenomena are greatly influenced by the presence of the corrugation and variation of aspect ratios.

A 10 tesla table-top controlled waveform magnet

Controlled waveform magnets (CWMs) are a class of pulsed magnets whose pulse shape with time can be programmed by the user. With a CWM, the user gains control not only over the magnitude of the field but also over its rate of change. In this work we present a table-top CWM, driven by a capacitor bank, capable of producing virtually any user-shaped magnetic field waveform up to 10 tesla. Insulated gate bipolar transistor chips have been paralleled to form the high current switch and paralleled chips of SiC Schottky diodes form the crowbar diode module. Sample controlled waveforms including flat-tops up to 10 tesla and some triangular magnetic field pulses have been successfully generated for 10-20 ms with a ripple < 1%. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.3699316]

Dual gate photo-thin film transistor with high photoconductive gain for high reliability, and low noise flat panel transparent imager

In this presentation, we report excellent electrical and optical characteristics of a dual gate photo thin film transistor (TFT) with bi-layer oxide channel, which was designed to provide virgin threshold voltage (V T) control, improve the negative bias illumination temperature stress (NBITS) reliability, and offer high photoconductive gain. In order to address the photo-sensitivity of phototransistor for the incoming light, top transparent InZnO (IZO) gate was employed, which enables the independent gate control of dual gate photo-TFT without having any degradation of its photosensitivity. Considering optimum initial V T and NBITS reliability for the device operation, the top gate bias was judiciously chosen. In addition, the speed and noise performance of the photo-TFT is competitive with silicon photo-transistors, and more importantly, its superiority lies in optical transparency. © 2011 IEEE.

Assessing the image quality of pelvic MR images acquired with a flat couch for radiotherapy treatment planning

Objectives: To improve the integration of MRI with radiotherapy treatment planning, our department fabricated a flat couch top for our MR scanner. Setting up using this couch top meant that the patients were physically higher up in the scanner and, posteriorly, a gap was introduced between the patient and radiofrequency coil.

Droplets of structured fluid on a flat substrate

We study the equilibrium morphology of droplets of symmetric AB diblock copolymer on a flat substrate. Using self-consistent field theory (SCFT), we provide the first predictions for the equilibrium droplet shape and its internal structure. When the sustrate affinity for the A component, $\eta_A$, is small, the droplet adopts a nearly spherical shape much like that of simple fluids. Inside the spherical droplet, however, concentric circular lamellar layers stack on top of each other; hence the thickness of the droplet is effectively quantized by a half-integer or integer number of layers. At larger $\eta_A$ and smaller contact angle, the area of the upper-most layer becomes relatively large, resulting in a nearly flat, faceted top surface, followed by a semi-spherical slope. This geometry is remarkably reminiscent of the droplet shapes observed with smetic liquid crystals.

Evolução top-hat hidrodinâmica em campos de velocidades peculiares primordiais

We investigate the cosmology of the vacuum energy decaying into cold dark matter according to thermodynamics description of Alcaniz & Lima. We apply this model to analyze the evolution of primordial density perturbations in the matter that gave rise to the first generation of structures bounded by gravity in the Universe, called Population III Objects. The analysis of the dynamics of those systems will involve the calculation of a differential equation system governing the evolution of perturbations to the case of two coupled fluids (dark matter and baryonic matter), modeled with a Top-Hat profile based in the perturbation of the hydrodynamics equations, an efficient analytical tool to study the properties of dark energy models such as the behavior of the linear growth factor and the linear growth index, physical quantities closely related to the fields of peculiar velocities at any time, for different models of dark energy. The properties and the dynamics of current Universe are analyzed through the exact analytical form of the linear growth factor of density fluctuations, taking into account the influence of several physical cooling mechanisms acting on the density fluctuations of the baryonic component of matter during the evolution of the clouds of matter, studied from the primordial hydrogen recombination. This study is naturally extended to more general models of dark energy with constant equation of state parameter in a flat Universe

Evaluation of race distance, track surface and season of the year on exercise-induced pulmonary haemorrhage in flat racing Thoroughbreds in Brazil

Reasons for performing study: To our knowledge no studies evaluating risk factors to EIPH have been undertaken with such a population of horses in Brazil, which is one of the few countries in the racing community to accept the use of frusemide (FUR) as a prerace medication for bleeders. We also compared national results with those already in the literature. Objectives: To verify the influence of variables such as distance of the race, racetrack surface, going and season of the year in the recurrence and severity of EIPH and its effect on the finishing position of flat racing Thoroughbreds in Brazil. Methods: 2118 post race respiratory endoscopies were recorded, in a total of 1003 individual horses. All horses in the study were certified bleeders (deemed positive by registered veterinarians following one or more endoscopies) and were administered FUR prior to each race. Data was analysed using both ordinary logistic regression and multiple logistic regression. Results: The results suggest that race distance, season of the year and racetrack surface affect the recurrence of EIPH. Also, despite the use of FUR, the majority (62%) of horses continued to display some degree of haemorrhage. Conclusions: There was a tendency for horses that remained positive despite the use of FUR to finish unplaced in a race. Horses which had a significant reduction in bleeding and became HPIE negative after the use of FUR, tended to finish in top positions. It was not possible to determine a pattern of severity of EIPH in horses with more than one endoscopy. Further studies are required to improve our understanding of this syndrome and its correlations to performance in the racing Thoroughbred, together with the role of FUR and its possible enhancing performance effects and efficacy in consistently diminishing EIPH. Potential relevance: This study enabled a better understanding of the use of FUR in Brazilian Thoroughbred racehorses and showed there was a tendency towards improvement in performance after administrion of FUR in bleeders. The fact that a large number of horses remained positive for EIPH despite the use of FUR should encourage the scientific community to search for better ways of reducing, or preventing, this syndrome. © 2006 EVJ Ltd.

Shaping the PSF to nearly top-hat profile: CHEOPS laboratory results

Spreading the PSF over a quite large amount of pixels is an increasingly used observing technique in order to reach extremely precise photometry, such as in the case of exoplanets searching and characterization via transits observations. A PSF top-hat profile helps to minimize the errors contribution due to the uncertainty on the knowledge of the detector flat field. This work has been carried out during the recent design study in the framework of the ESA small mission CHEOPS. Because of lack of perfect flat-fielding information, in the CHEOPS optics it is required to spread the light of a source into a well defined angular area, in a manner as uniform as possible. Furthermore this should be accomplished still retaining the features of a true focal plane onto the detector. In this way, for instance, the angular displacement on the focal plane is fully retained and in case of several stars in a field these look as separated as their distance is larger than the spreading size. An obvious way is to apply a defocus, while the presence of an intermediate pupil plane in the Back End Optics makes attractive to introduce here an optical device that is able to spread the light in a well defined manner, still retaining the direction of the chief ray hitting it. This can be accomplished through an holographic diffuser or through a lenslet array. Both techniques implement the concept of segmenting the pupil into several sub-zones where light is spread to a well defined angle. We present experimental results on how to deliver such PSF profile by mean of holographic diffuser and lenslet array. Both the devices are located in an intermediate pupil plane of a properly scaled laboratory setup mimicking the CHEOPS optical design configuration. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Stilling Basin Scour Remediation Using Air Injection and Flat Plate Extension

The South Florida Water Management District (SFWMD) is responsible for managing over 2500 miles of waterways and hundreds of water control structures. Many of these control structures are experiencing erosion, known as scour, of the sediment downstream of the structure. Laboratory experiments were conducted in order to investigate the effectiveness of two-dimensional air diffusers and plate extensions (without air injection) on a 1/30 scale model of one of SFWMD gated spillway structures, the S65E gated spillway. A literature review examining the results of similar studies was conducted. The experimental design for this research was based off of previous work done on the same model. Scour of the riverbed downstream of gated spillway structures has the potential to cause serious damage, as it can expose the foundation of the structure, which can lead to collapse. This type of scour has been studied previously, but it continues to pose a risk to water control structures and needs to be studied further. The hydraulic scour channel used to conduct experiments contains a head tank, flow straighteners, gated spillway, stilling basin, scour chamber, sediment trap, and tailwater tank. Experiments were performed with two types of air diffusers. The first was a hollow, acrylic, triangular end sill with air injection holes on the upstream face, allowing for air injection upstream. The second diffuser was a hollow, acrylic rectangle that extended from the triangular end sill with air injection holes in the top face, allowing for vertical air injection, perpendicular to flow. Detailed flow and bed measurements were taken for six trials for each diffuser ranging from no air injection to 5 rows of 70 holes of 0.04" diameter. It was found that with both diffusers, the maximum amount of air injection reduced scour the most. Detailed velocity measurements were taken for each case and turbulence statistics were analyzed to determine why air injection reduces scour. It was determined that air injection reduces streamwise velocity and turbulence. Another set of experiments was performed using an acrylic extension plate with no air injection to minimize energy costs. Ten different plate lengths were tested. It was found that the location of deepest scour moved further downstream with each plate length. The 32-cm plate is recommended here. Detailed velocity measurements were taken after the cases with the 32-cm plate and no plate had reached equilibrium. This was done to better understand the flow patterns in order to determine what causes the scour reduction with the extension plates. The extension plate reduces the volume of scour, but more importantly translates the deepest point of scour downstream from the structure, lessening the risk of damage.