Pulse Energy Loss Research Articles

Pulse Picking PIV: A Single-Cavity Fiber Laser System For Highly Flexible Repetition Rates And Pulsing Schemes

A novel single-cavity laser system with flexible pulsing schemes suitable for particle image velocimetry (PIV) measurements is presented. The system is based on the concept of picking selected pulse pairs from an ultra-high repetition rate fiber laser which enables time-resolved and conventional dual-pulse PIV at highly flexible recording frame rates. Pulse picking PIV laser systems allow for the use of high-quality low speed dual-frame PIV cameras as well as highspeed cameras in both dual-frame and continuous recording modes, enabling the investigation of highly-transient phenomena at limited fields of view using time-resolved PIV as well as statistical PIV measurements with larger fields of view and higher spatial resolutions. The single-cavity output additionally avoids the alignment of laser beams of laser pulses, which is otherwise necessary in conventional dual-cavity PIV laser systems. Two pulse picking systems were set up and tested: An acousto-optic deflector selecting pulses using the diffraction across an acoustic wave and an electro-optic modulator rotating the polarization state of selected pulses in a Pockels cell. The performance of both systems was evaluated in terms of achievable inter-pulse durations, beam profile alterations, and pulse energy losses caused by the introduction of additional optical elements in the beam path.

Recovery of the laser-induced breakdown spectroscopy system using a ceramic microchip deteriorated by radiation for the remote elemental analysis

ABSTRACT The radiation-induced deterioration of ceramic microchip laser properties limits the applications of laser-induced breakdown spectroscopy (LIBS) systems. The deteriorated properties were recovered through thermal treatment of ceramics as derived by the spectroscopic comparison of ceramics and single crystals (SCs). The absorption in spectra was increased by gamma rays irradiation, which was higher for the ceramics than for the SCs in the infrared radiation region. Although the amount of absorption decreased by heating and increased via cooling ceramics, no significant thermal effects were observed for the SCs. The effects of irradiation on the laser properties of ceramics were examined, and the laser pulse energy loss was well recovered via heating. Because heating did not deteriorate the beam profile, affecting the LIBS efficiency, the procedure was effective in recovering the diminished LIBS signal. Although heating reduced the number of generated pulses, affects the burst-mode measurement, the reduction was recovered by increasing the pump laser power. Because the radiation-induced deterioration of the LIBS signal for single-pulse and multiburst mode measurements was well recovered, the procedures are considered effective for the application of LIBS systems using radiation-sensitive ceramics, particularly in harsh radiation environments.

Numerical study on an optimum Q-switching profile for complete multipeak suppression in an actively Q-switched Ytterbium fibre laser

We propose an optimum Q-switching profile for complete suppression of the multi-peak phenomenon (MPP) without loss of pulse energy in an actively Q-switched ytterbium (Yb)-doped fibre laser. Most of the previously demonstrated approaches to suppress MPP have been based on adjusting of the rise time, and these rely on the assumption that the switching profile of the Q-switch has a trapezoidal temporal shape with a linear leading edge. These approaches leave one fundamental question as to whether or not there exists an optimum leading edge profile other than a simple linear one for complete suppression of MPP without loss of pulse energy. Through comparison among four different leading edge profiles. i.e. linear shape, sinusoidal function shape (increasing slope), sinusoidal function shape (decreasing slope), and composite function shape profiles, the proposed leading edge shape of a sinusoidal function (increasing slope) is shown to be capable of achieving complete suppression of MPP without loss of pulse energy under particular rise time conditions. It is shown that the use of a sinusoidal function (increasing slope) with a rise time of approximately 11 times the cavity round-trip time allows for the complete suppression of MPP without pulse energy loss in an exemplary, Q-switched, Yb-doped fibre Fabry–Perot laser.

Numerical investigation into the impact of Q-switch rise time on the output pulse characteristics in an actively Q-switched ytterbium-doped fiber laser

We report the results of our theoretical investigation into the impact of the Q-switch rise time on three key parameters of the output pulses in an actively Q-switched fiber laser, namely the multipeak phenomenon, pulse energy, and pulse peak power. Our investigation is conducted over a Q-switch time range of up to 12 times the cavity round-trip time (τround). We show that when a longer Q-switch rise time is applied to the cavity, a trade-off exists between multipeak phenomenon suppression and pulse energy maintenance. The results show that the Q-switch rise time (τrise) for the efficient suppression of multipeak phenomenon without loss of pulse energy varies, depending on the cavity parameters. In an exemplary, actively Q-switched fiber laser operating under the condition of a repetition rate of 15 kHz, τrise is estimated to be approximately five times larger than τround.

Fivefold compression of 250-TW laser pulses

The pulse spectrum at the laser output was broadened due to self-phase modulation in fused silica and then the pulse was compressed by chirped mirrors. It was demonstrated that with an optimal choice of mirror dispersion a pulse with an energy of 17 J can be compressed from 70 to 14 fs. This compression after compressor approach has undoubted merits: simplicity, low cost, negligible pulse energy losses, and applicability to any high-power laser.

A cavity for enhancing the brightness of lasers with an induced thermal lens in the active element

This paper proposes the layout of a cavity with planoconvex mirrors for reducing the angular divergence of radiation due to the formation of a large diameter of the zeroth transverse mode. It is shown that installing the active element close to a flat mirror makes it possible to significantly reduce the length of the laser cavity by more rapidly reducing the Fresnel number for the focused beam. A discussion is presented of the optical layout and the results of a study of the characteristics of a laser system that combines a master oscillator and a two-pass, two-stage amplifier. At a relatively low pulse-repetition rate (100 Hz), about 30% of the pulse energy is lost at the output polarizer of the laser. The pulse-energy losses are eliminated by establishing a ninety-degree rotation of the polarization of the radiation between the amplifier stages.

Analysis and optimization of pulse dynamics for magnetic stimulation.

Magnetic stimulation is a standard tool in brain research and has found important clinical applications in neurology, psychiatry, and rehabilitation. Whereas coil designs and the spatial field properties have been intensively studied in the literature, the temporal dynamics of the field has received less attention. Typically, the magnetic field waveform is determined by available device circuit topologies rather than by consideration of what is optimal for neural stimulation. This paper analyzes and optimizes the waveform dynamics using a nonlinear model of a mammalian axon. The optimization objective was to minimize the pulse energy loss. The energy loss drives power consumption and heating, which are the dominating limitations of magnetic stimulation. The optimization approach is based on a hybrid global-local method. Different coordinate systems for describing the continuous waveforms in a limited parameter space are defined for numerical stability. The optimization results suggest that there are waveforms with substantially higher efficiency than that of traditional pulse shapes. One class of optimal pulses is analyzed further. Although the coil voltage profile of these waveforms is almost rectangular, the corresponding current shape presents distinctive characteristics, such as a slow low-amplitude first phase which precedes the main pulse and reduces the losses. Representatives of this class of waveforms corresponding to different maximum voltages are linked by a nonlinear transformation. The main phase, however, scales with time only. As with conventional magnetic stimulation pulses, briefer pulses result in lower energy loss but require higher coil voltage than longer pulses.

Effects of backward-propagating waves and lumped mirror losses on self-induced transparency modelocking in quantum cascade lasers

Work to date on self-induced transparency modelocking in quantum cascade lasers (QCLs) has neglected backward-propagating waves and lumped mirror losses. In this work, we remove these unrealistic assumptions. The qualitative features of the modelocking are unaffected by this improvement in the model, but the parameter regime in which stable modelocked pulses may be found is reduced. This reduction is due to incomplete gain recovery near the edges of the QCL when pulses pass through after reflecting from the mirrors, coincident with the loss of pulse energy at the mirrors. Spatial hole burning is observed in parameter regimes in which continuous waves can grow, but it does not affect the stability of the modelocking.

A novel confocal optical pulse stretcher for laser pulses

We present an analytical and experimental study of a novel confocal optical pulse stretcher (COPS). The simple and passive pulse stretcher consists of two concave mirrors and a scraper beam-splitter and its optical configuration ensures a perfect spatial overlapping of laser pulses at the beam-splitter. The pulse stretcher is compact and suitable for laser pulses of large divergences. The confocal optical pulse stretcher is demonstrated on a pulsed copper vapour laser to convert a 40 ns (1/ e 2 % points) pulse into a 55 ns with reduction of peak power by a factor of 1.375 without loss of pulse energy.

Efficient Pulse Stretching of Q-Switched Lasers

We present a new and simple technique for efficient stretching of Q-switched laser pulses using a novel ring resonator and a variable Pockels cell Q-switch driven by a simple circuit. The production of 10-mJ flat-topped Q-switched pulses with a duration that can be stretched by a factor of 4.5-1.8 mus without loss of pulse energy is demonstrated. We show that the pulse stretching agrees well with predictions of a numerical model.

Numerical simulation of non-invasive determination of the propagation coefficient in arterial system using two measurements sites

Literature shows a lack of works based on non-invasive methods for computing the propagation coefficient γ, a complex number related to dynamic vascular properties. Its imaginary part is inversely re- lated to the wave speed C through the relationship C = ω/Im(γ), while its real part a, called attenuation, represents loss of pulse energy per unit of length. In this work an expression is derived giving the propa- gation coefficient when assuming a pulsatile flow through a viscoelastic vessel. The effects of physical and geometrical parameters of the tube are then studied. In particular, the effects of increasing the reflection coefficient, on the determination of the propagation coefficient are investigated in a first step. In a second step, we simulate a variation of tube length under physiological conditions. The method developed here is based on the knowledge of instantaneous velocity and radius values at only two sites. It takes into account the presence of a reflection site of unknown reflection coefficient, localised in the distal end of the vessel. The values of wave speed and attenuation obtained with this method are in a good agreement with the theory. This method has the advantage to be usable for small portions of the arterial tree. PACS. 47.15.-x Laminar flows - 47.90.+a Other topics in fluid dynamics

Features of the femtosecond pulse propagation in air

The features of propagation of femtosecond laser pulses in the atmosphere are studied by numerical methods. The type of propagation is determined by the relation between processes of multiphoton ionisation and multiple small-scale self-focusing (filamentation). It is found that the process develops step by step and filamentation occurs without the pulse energy loss until the intensity threshold is achieved. After the achievement of the threshold, ionisation losses appear which are accompanied by the defocusing action of the electron plasma resulting in the intensity decrease. When the intensity becomes lower than the ionisation threshold, self-focusing can again appear. The characteristic features of the final stage of filamentation can be explained by the Bespalov—Talanov theory. By using the time-averaged equations and parallel methods, a program complex is developed for qualitative simulations of the propagation of high-power laser pulses over long distances.

Formula omitted]-pair production by a focused laser pulse in vacuum

The probability of electron–positron pair creation by a focused laser pulse is calculated. For description of the focused laser pulse we use a 3-dimensional model of the electromagnetic field which is based on an exact solution of Maxwell equations. There exists two types of focused waves: e- and h-polarized waves with only either electric, or magnetic vector being transverse respectively. It is shown that pair production is possible only in e-polarized electromagnetic wave. The dependence of the pair production probability on the intensity of the laser pulse is obtained. It is argued that there exists a natural physical limit for attainable focused laser pulse intensities. This limit is posed by the pulse energy loss due to the effect of pair creation.