Maintaining Constant Pulse-Duration in Highly Dispersive Media Using Nonlinear Potentials

A method is shown for preventing temporal broadening of ultrafast optical pulses in highly dispersive and fluctuating media for arbitrary signal-pulse profiles. Pulse pairs, consisting of a strong-field control-pulse and a weak-field signal-pulse, co-propagate, whereby the specific profile of the strong-field pulse precisely compensates for the dispersive phase in the weak pulse. A numerical example is presented in an optical system consisting of both resonant and gain dispersive effects. Here, we show signal-pulses that do not temporally broaden across a vast propagation distance, even in the presence of dispersion that fluctuates several orders of magnitude and in sign (for example, within a material resonance) across the pulse’s bandwidth. Another numerical example is presented in normal dispersion telecom fiber, where the length at which an ultrafast pulse does not have significant temporal broadening is extended by at least a factor of 10. Our approach can be used in the design of dispersion-less fiber links and navigating pulses in turbulent dispersive media. Furthermore, we illustrate the potential of using cross-phase modulation to compensate for dispersive effects on a signal-pulse and fill the gap in the current understanding of this nonlinear phenomenon.

Redshift estimates for fast radio bursts and implications on intergalactic magnetic fields

ABSTRACT Fast radio bursts are transient radio pulses from presumably compact stellar sources of extragalactic origin. With new telescopes detecting multiple events per day, statistical methods are required in order to interpret observations and make inferences regarding astrophysical and cosmological questions. We present a method that uses probability estimates of fast radio burst observables to obtain likelihood estimates for the underlying models. Considering models for all regions along the line of sight, including intervening galaxies, we perform Monte Carlo simulations to estimate the distribution of the dispersion measure (DM), rotation measure (RM), and temporal broadening. Using Bayesian statistics, we compare these predictions to observations of fast radio bursts. By applying Bayes theorem, we obtain lower limits on the redshift of fast radio bursts with extragalactic DM ≳ 400 pc cm−3. We find that intervening galaxies cannot account for all highly scattered fast radio bursts in FRBcat, thus requiring a denser and more turbulent environment than an SGR 1935+2154-like magnetar. We show that a sample of ≳103 unlocalized fast radio bursts with associated extragalactic RM ≥ 1 rad m−2 can improve current upper limits on the strength of intergalactic magnetic fields.

ASTIGMATISM COMPENSATION IN BLOCK OF TEMPORAL BROADENING OF PULSE FOR PUMP CHANNEL OF HIGH POWER LASER SYSTEM

A high peak and high average power femtosecond laser system based on media doped with Yb3+ ions is being developed at the Institute of Laser Physics of the SB RAS. For efficient laser amplification and to avoid optical damage is actually to compensate wave front distortion caused by grating astigmatism in pump channel. Based on theory of propagation of gaussian beam in space and through optical elements the calculation of optimal parameters of two lenses telescope and comparison with experimental data has been performed. The obtained results can be used for decrease of astigmatic effect on beam profile quality in design of laser systems with elements involving astigmatism.

Five new real-time detections of fast radio bursts with UTMOST

We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between 2017 June and 2018 December. By using a real-time FRB detection system, we captured raw voltages for five of the six events, which allowed for coherent dedispersion and very high time resolution (10.24 $\mu$s) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering time-scales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with three peaks each separated by 1 ms. We searched for phase-coherence between the leading and trailing peaks and found none, ruling out lensing scenarios. Based on this survey, we calculate an all-sky rate at 843 MHz of $98^{+59}_{-39}$ events sky−1 d−1 to a fluence limit of 8 Jy ms: a factor of 7 below the rates estimated from the Parkes and ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index α = −1.6 (Fν ∝ να). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio, and X-ray follow-up has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.

Can we see pulsars around Sgr A⋆? The latest searches with the Effelsberg telescope.

Radio pulsars in relativistic binary systems are unique tools to study the curved space-time around massive compact objects. The discovery of a pulsar closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr A⋆, would provide a superb test-bed for gravitational physics. To date, the absence of any radio pulsar discoveries within a few arc minutes of Sgr A⋆ has been explained by one principal factor: extreme scattering of radio waves caused by inhomogeneities in the ionized component of the interstellar medium in the central 100 pc around Sgr A⋆. Scattering, which causes temporal broadening of pulses, can only be mitigated by observing at higher frequencies. Here we describe recent searches of the Galactic centre region performed at a frequency of 18.95 GHz with the Effelsberg radio telescope.