Dual-Wavelength Fiber Lasers for the Optical Generation of Microwave and Terahertz Radiation

Dual-Wavelength Fiber Lasers (DWFLs), which provide a simple and cost-effective approach for the optical generation of Microwave (MHz) and Terahertz (THz) radiation. The emphasis of this review is to trace the early development of DWFLs, including the issues and limitations faced by the various gain media right to the latest advancements in this field as well as their roles in generating the desired output. This review covers both the simple approaches of narrow-band filters and comb filters for microwave radiation generation, as well as the use of DWFLs with diethylaminosulfurtetrafluoride or LiNbO3 crystals for generating THz radiation

On the photometric error calibration for the `differential light curves' of point-like Active Galactic Nuclei

It is important to quantify the underestimation of rms photometric errors returned by the commonly used \emph APPHOT algorithm in the \emph IRAF software, in the context of differential photometry of point-like AGN, because of the crucial role it plays in evaluating their variability properties. Published values of the underestimation factor, $\eta$, using several different telescopes, lie in the range 1.3 - 1.75. The present study aims to revisit this question by employing an exceptionally large data set of 262 differential light curves (DLCs) derived from 262 pairs of non-varying stars monitored under our ARIES AGN monitoring program for characterizing the intra-night optical variability (INOV) of prominent AGN classes. The bulk of these data were taken with the 1-m Sampurnanad Telescope (ST). We find $\eta$ = 1.54$\pm$0.05 which is close to our recently reported value of $\eta$ = 1.5. Moreover, this consistency holds at least up to a brightness mismatch of 1.5 mag between the paired stars. From this we infer that a magnitude difference of at least up to 1.5 mag between a point-like AGN and comparison star(s) monitored simultaneously is within the same CCD chip acceptable, as it should not lead to spurious claims of INOV.Comment: 20 pages, 3 figures and 1 table; accepted for publication in JAp

Extragalactic radio sources with sharply inverted spectrum at metre wavelengths

We present the first results of a systematic search for the rare extragalactic radio sources showing an inverted (integrated) spectrum, with spectral index $\alpha \ge +2.0$, a previously unexplored spectral domain. The search is expected to yield strong candidates for $\alpha \ge +2.5$, for which the standard synchrotron self-absorption (characterized by a single power-law energy distribution of relativistic electron population) would not be a plausible explanation, even in an ideal case of a perfectly homogeneous source of incoherent synchrotron radiation. Such sharply inverted spectra, if found, would require alternative explanations, e.g., free-free absorption, or non-standard energy distribution of relativistic electrons which differs from a power-law (e.g., Maxwellian). The search was carried out by comparing two sensitive low-frequency radio surveys made with sub-arcminute resolution, namely, the WISH survey at 352 MHz and TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains 7056 WISH sources classified as `single' and brighter than 100 mJy at 352 MHz. We focus here on the seven of these sources for which we find $\alpha > +2.0$. Two of these are undetected at 150 MHz and are particularly good candidates for $\alpha > +2.5$. Five of the seven sources exhibit a `Gigahertz-Peaked-Spectrum' (GPS).Comment: 8 pages, 2 figures, accepted for publication in MNRA

Novel Y2O3-codoped Yb/Tm-doped picosecond fiber laser

We demonstrate the novel picosecond mode-locked Y2O 3-codoped Yb/Tm-doped fiber lasers, operating at 1950 nm and producing pulses of up to 1 nJ energy, using a SESAM and an Er-doped pump fiber laser operating at the wavelength 1590 nm or a semiconductor pump laser operating at the wavelength of 1560 nm. We also report on the spectroscopic characterization of these new fibers with various compositions, identifying the optimum one for the maximum Yb/Tm energy transfer, the latter increasing with the increase of the Y concentration. The observed energy transfer between Yb and Tm makes this laser promising also for direct diode-pumping with most advanced and low cost 975 nm diodes, making this laser attractive for compact low cost picosecond Tm-doped fiber laser systems

Investigation of Q-Switched and Mode-Locked Pulses From a Yb3+-Doped Germano-Zirconia Silica Glass Based Fiber Laser

The Q-switched and mode-locked (QML) pulses of an all normal dispersion Yb3+-doped fiber laser (YDFL) using the nonlinear polarization rotation technique with symmetric and asymmetric Q-switched envelopes were investigated. For QML pulses with a symmetric shape, the width of envelope decreases as the pump power increases, achieving a shortest duration of ~1.4 μs. An alternate QML state of the YDFL exhibited an asymmetric envelope with a broad spectrum bandwidth, and whose short duration mode-locked pulses could be used as a light source for the cascaded Raman scattering generation through the injection of these high intensity QML pulses into an amplifier, using Yb3+-doped germano-zirconia silica (YD-GZS) glass based fiber as the gain fiber. In addition, the generation of a relatively broad spectrum near the IR range and the observation of some emission bands in the visible and UV range have been demonstrated at high pump power

Mass measurements of 60–63Ga reduce x-ray burst model uncertainties and extend the evaluated T=1 isobaric multiplet mass equation

We report precision mass measurements of neutron-deficient gallium isotopes approaching the proton drip line. The measurements of Ga60–63 performed with the TITAN multiple-reflection time-of-flight mass spectrometer provide a more than threefold improvement over the current literature mass uncertainty of Ga61 and mark the first direct mass measurement of Ga60. The improved precision of the Ga61 mass has important implications for the astrophysical rp process, as it constrains essential reaction Q values near the Zn60 waiting point. Based on calculations with a one-zone model, we demonstrate the impact of the improved mass data on prediction uncertainties of x-ray burst models. The first-time measurement of the Ga60 ground-state mass establishes the proton-bound nature of this nuclide, thus constraining the location of the proton drip line along this isotopic chain. Including the measured mass of Ga60 further enables us to extend the evaluated T=1 isobaric multiplet mass equation up to A=60