Linewidth Measurements Research Articles

Linewidth enhancement factor in a microcavity Brillouin laser

The linewidth of regenerative oscillators is enhanced by amplitude–phase coupling of the oscillator field [Phys. Rev. 160, 290 (1967)]. In laser oscillators, this effect is well known for its impact on semiconductor laser performance. Here, this coupling is studied in Brillouin lasers. Because their gain is parametric, the coupling and linewidth enhancement are shown to originate from phase mismatch. The theory is confirmed by measurement of linewidth in a microcavity Brillouin laser, and enhancements as large as 50 × are measured. The results show that pump wavelength and device temperature should be carefully selected and controlled to minimize linewidth. More generally, this work provides a new perspective on the linewidth enhancement effect.

Doppler narrowing, Zeeman and laser beam-shape effects in Λ-type electromagnetically induced transparency on the 85Rb D2 line in a vapor cell

We study Λ-type Electromagnetically Induced Transparency (EIT) on the Rb D2 transition in a buffer-gas-free thermal vapor cell without anti-relaxation coating. Experimental data show well-resolved features due to velocity-selective optical pumping and one EIT resonance. The Zeeman splitting of the EIT line in magnetic fields up to 12 Gauss is investigated. One Zeeman component is free of the first-order shift and its second-order shift agrees well with theory. The full width at half maximum (FWHM) of this magnetic-field-insensitive EIT resonance is reduced due to Doppler narrowing, scales linearly in Rabi frequency over the range studied, and reaches about 100 kHz at the lowest powers. These observations agree with an analytic model for a Doppler-broadened medium developed in (Javan et al 2002 Phys. Rev. A 66 013805; Lee et al 2003 Appl. Phys. B, Lasers Opt. (Germany) B 76, 33–9; Taichenachev et al 2000 JETP Lett. 72, 119). Numerical simulation using the Lindblad equation reveals that the transverse laser intensity distribution and two Λ-EIT systems must be included to fully account for the measured line width and line shape of the signals. Ground-state decoherence, caused by effects that include residual optical frequency fluctuations, atom-wall and trace-gas collisions, is discussed.

Understanding biases in measurements of molecular cloud kinematics using line emission

ABSTRACT Molecular line observations using a variety of tracers are often used to investigate the kinematic structure of molecular clouds. However, measurements of cloud velocity dispersions with different lines, even in the same region, often yield inconsistent results. The reasons for this disagreement are not entirely clear, since molecular line observations are subject to a number of biases. In this paper, we untangle and investigate various factors that drive linewidth measurement biases by constructing synthetic position–position–velocity cubes for a variety of tracers from a suite of self-gravitating magnetohydrodynamic simulations of molecular clouds. We compare linewidths derived from synthetic observations of these data cubes to the true values in the simulations. We find that differences in linewidth as measured by different tracers are driven by a combination of density-dependent excitation, whereby tracers that are sensitive to higher densities sample smaller regions with smaller velocity dispersions, opacity broadening, especially for highly optically thick tracers such as CO, and finite resolution and sensitivity, which suppress the wings of emission lines. We find that, at fixed signal-to-noise ratio, three commonly used tracers, the J = 4 → 3 line of CO, the J = 1 → 0 line of C18O, and the (1,1) inversion transition of NH3, generally offer the best compromise between these competing biases, and produce estimates of the velocity dispersion that reflect the true kinematics of a molecular cloud to an accuracy of $\approx 10{{\ \rm per\ cent}}$ regardless of the cloud magnetic field strengths, evolutionary state, or orientations of the line of sight relative to the magnetic field. Tracers excited primarily in gas denser than that traced by NH3 tend to underestimate the true velocity dispersion by $\approx 20{{\ \rm per\ cent}}$ on average, while low-density tracers that are highly optically thick tend to have biases of comparable size in the opposite direction.

In vivo characterization of downfield peaks at 9.4 T: T2 relaxation times, quantification, pH estimation, and assignments.

Relaxation times are a valuable asset when determining spectral assignments. In this study, apparent T2 relaxation times ( ) of downfield peaks are reported in the human brain at 9.4 T and are used to guide spectral assignments of some downfield metabolite peaks. Echo time series of downfield metabolite spectra were acquired at 9.4 T using a metabolite-cycled semi-LASER sequence. Metabolite spectral fitting was performed using LCModel V6.3-1L while fitting a pH sweep to estimate the pH of the homocarnosine (hCs) imidazole ring. were calculated by fitting the resulting relative amplitudes of the peaks to a mono-exponential decay across the TE series. Furthermore, estimated tissue concentrations of molecules were calculated using the relaxation times and internal water as a reference. of downfield metabolites are reported within a range from 16 to 32 ms except for homocarnosine with of 50 ms. Correcting for exchange rates ( ) resulted in relaxation times between 20 and 33 ms. The estimated pH values based on hCs imidazole range from 7.07 to 7.12 between subjects. Furthermore, analyzing the linewidths of the downfield peaks and their contribution led to possible peak assignments. relaxation times were longer for the assigned metabolite peaks compared to the unassigned peaks. Tissue pH estimation in vivo with proton MRS and simultaneous quantification of amide protons at 8.30 ± 0.15 ppm is likely possible. Based on concentration, linewidth, and exchange rates measurements, tentative peak assignments are discussed for adenosine triphosphate (ATP), N-acetylaspartylglutamate (NAAG), and urea.

Discovery of High-velocity Hα Emission in the Direction of the Fermi Bubble

Abstract Wisconsin H-Alpha Mapper observations reveal high-velocity Hα and [N ii]λ6584 emission lines in the same direction and velocity as ultraviolet absorption-line features that have been previously associated with the biconical gamma-ray lobes known as the Fermi Bubbles. We measure an extinction-corrected intensity of for emission with line center v LSR = −221 ± 3 km s−1, corresponding to an emission measure of . This emission arises at the same velocity as Hubble Space Telescope/Cosmic Origins Spectrograph observations of ultraviolet absorption features detected in the PDS 456 quasar sight line that passes through the northern bubble near l = 10.°4, b = +11.°2. We estimate the total column density of ionized gas in this velocity component to be . The comparison of ionized gas emission and absorption yields an estimate for the characteristic density of n e,c = 1.8 ± 0.6 cm−3 and a characteristic length of L c = 0.56 ± 0.21 pc assuming 30% solar metallicity. For a temperature of —consistent with the measured line widths and [N ii]/Hα line ratio—the gas has a thermal pressure of . Assuming the gas is ∼6.5 kpc distant, the derived density and pressure appear to be anomalously high for gas ∼1.3 kpc above the Galactic midplane. The large thermal pressure is comparable to both a hot halo or Fermi Bubble model, but suggest that the Hα arises in an overpressurized zone.

Ultranarrow-linewidth levitated nano-oscillator for testing dissipative wave-function collapse

Levitated nano-oscillators are seen as promising platforms for testing fundamental physics and testing quantum mechanics in a new high mass regime. Levitation allows extreme isolation from the environment, reducing the decoherence processes that are crucial for these sensitive experiments. A fundamental property of any oscillator is its line width and mechanical quality factor, Q. Narrow line widths in the microHertz regime and mechanical Q's as high as $10^{12}$ have been predicted for levitated systems, but to date, the poor stability of these oscillators over long periods have prevented direct measurement in high vacuum. Here we report on the measurement of an ultra-narrow line width levitated nano-oscillator, whose line width of $81\pm\,23\,\mu$Hz is only limited by residual gas pressure at high vacuum. This narrow line width allows us to put new experimental bounds on dissipative models of wavefunction collapse including continuous spontaneous localisation and Di\'{o}si-Penrose and illustrates its utility for future precision experiments that aim to test the macroscopic limits of quantum mechanics.

Measurement of Brillouin gain spectrum with dual‐frequency laser for optical fibre sensor

In this Letter, the authors present a first measurement of Brillouin gain spectrum with dual-frequency solid-state laser. This allows a coherent detection of the Brillouin backscattered spectrum in a frequency range below the hundreds of MHz by fine-tuning of the difference between the two cross-polarised fields emitted by the laser. The photodetection at low-frequency benefits from the balanced detection technique where high dynamic range measurements are largely available. The narrow line-width of the cross-polarised field permits also the direct measurement of the Brillouin spectrum line-width. The system is demonstrated by measuring the Brillouin frequency shift with temperature when standard single-mode fibre-28 (SMF-28) and large effective area fibre (LEAF) are fully heated. This gives temperature coefficients close to theoretical expectations with values of 1.16 MHz/°C for SMF-28 and 1.15 MHz/°C for LEAF.

Interplay of large two-magnon ferromagnetic resonance linewidths and low Gilbert damping in Heusler thin films

We report on broadband ferromagnetic resonance linewidth measurements performed on epitaxial Heusler thin films. A large and anisotropic two-magnon scattering linewidth broadening is observed for measurements with the magnetization lying in the film plane, while linewidth measurements with the magnetization saturated perpendicular to the sample plane reveal low Gilbert damping constants of $(1.5\pm0.1)\times 10^{-3}$, $(1.8\pm0.2)\times 10^{-3}$, and $<8\times 10^{-4}$ for Co$_2$MnSi/MgO, Co$_2$MnAl/MgO, and Co$_2$FeAl/MgO, respectively. The in-plane measurements are fit to a model combining Gilbert and two-magnon scattering contributions to the linewidth, revealing a characteristic disorder lengthscale of 10-100 nm.

Comparative Analysis of Functional and Structural Decline in Retinitis Pigmentosas.

Retinitis pigmentosa (RP) is a category of inherited retinal dystrophies that is best prognosticated using electroretinography (ERG). In this retrospective cohort study of 25 patients with RP, we evaluated the correlation between 30 Hz flicker ERG and structural parameters in the retina. Internationally standardized 30 Hz flicker ERG recordings, short-wavelength autofluorescence (SW-AF), and spectral domain–optical coherence tomography (SD-OCT) were acquired at two visits at least one year apart. Vertical and horizontal hyperautofluorescent ring diameter measurements with SW-AF, as well as ellipsoid zone (EZ) line width measurements with SD-OCT, were used as structural parameters of disease progression. The 30 Hz flicker ERG amplitude decreased by 2.2 ± 0.8 µV/year (p = 0.011), while implicit times remained unchanged. For SD-OCT, the EZ line decreased by 204.1 ± 34.7 µm/year (p < 0.001). Horizontal and vertical hyperautofluorescent ring diameters decreased by 161.9 ± 25.6 µm/year and 146.9 ± 34.6 µm/year, respectively (p = 0.001), with SW-AF. A correlation was found between the progression rates of the 30 Hz flicker amplitude recorded with Burian–Allen electrodes and both the horizontal ring diameter (p = 0.020) and EZ line (p = 0.044). SW-AF and SD-OCT, two readily available imaging techniques, may be used to prognosticate disease progression because of the reliability of their measurements and correlation with functional outcome.

Filtering magnetic relaxation mechanisms of YIG(001) thin films using ferromagnetic resonance

In this work we used the ferromagnetic resonance (FMR) technique to measure the contributions for the magnetic relaxation in Y3Fe5O12 textured films deposited onto (001) Gd3Ga5O12 by magnetron sputtering. Through the numerical fitting of the angular dependence of the in-plane FMR linewidth measurements we identified the contribution of the Gilbert, two-magnon and mosaicity relaxation mechanisms. Regarding to the line broadening due to two-magnon mechanism, we interpreted it as a result of rectangular defects distributed over the film plane. The overall defects were decomposed into three crystallography direction chosen according to the in-plane FMR resonance field experimental results. We observed the two-magnon scattering rate being relevant for YIG thickness below 150 nm. Although it is also present for thicker samples, the Gilbert mechanism dominates with mosaicity being very important to corroborate the shape of the angular dependence, but with small amplitude. We point out the relevance of our finding regarding two-magnon scattering effect in YIG thin films, directly impacting in an overestimation of the spin pumping contribution to the FMR line broadening in YIG/ heavy-metal systems.

Ultra-narrow-linewidth measurement utilizing dual-parameter acquisition through a partially coherent light interference.

Laser linewidths of the order of 100 Hz are challenging to measure with existing technology. We propose a simple, efficient method to measure ultra-narrow linewidths using dual-parameter acquisition based on partially coherent light interference. The linewidth is obtained using two parameters that are easily extracted from the power spectrum. This method reduces the influence of 1/f noise by utilizing a kilometer-order-length delay fiber and is independent of the fiber-length error for a general situation. Simulation results show that, for a length error less than 10%, the total linewidth measurement error is less than 0.3%. Experimental results confirm the feasibility and superior performance of this method.

Unveiling delay-time-resolved phase noise statistics of narrow-linewidth laser via coherent optical time domain reflectometry.

Laser with high spectral purity plays a crucial role in high-precision optical metrology and coherent communication. Thanks to the rapid development of laser frequency stabilization, the laser phase noise can be remarkably compensated, allowing its ultra-narrow linewidth subject to mostly quantum limit. Nevertheless, the accurate characterization of phase noise statistics and its linewidth of a highly coherent laser remains ambiguous and challenging. Here, we present an approach capable of revealing delay-time-resolved phase noise statistics of a coherent laser based on coherent optical time domain reflectometry (COTDR), in which distributed Rayleigh scattering along a delay fiber essentially allows a time-of-flight mapping of a heterodyne beating signal associated with delay-time-dependent phase information from a single laser source. Ultimately, this novel technique facilitates precise measurement of ultra-narrow laser linewidth by exploiting its delay-time-resolved phase jitter statistics of random fiber laser with pump lasers of various linewidths, confirmed with the analytical modeling and numerical simulations.

Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H Iself absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the13CO emission. The peak velocities of the HISA and13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020to 1021cm−2. The column density of molecular hydrogen, traced by13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H Iemission (the warm and cold neutral medium), and13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.

Extreme Quasars as Distance Indicators in Cosmology

Quasars accreting matter at very high rates (known as extreme Population A [xA] quasars, possibly associated with super-Eddington accreting massive black holes) may provide a new class of distance indicators covering cosmic epochs from present day up to less than 1 Gyr from the Big Bang. At a more fundamental level, xA quasars are of special interest in studies of the physics of AGNs and host galaxy evolution. However, their observational properties are largely unknown. xA quasars can be identified in relatively large numbers from major optical surveys over a broad range of redshifts, thanks to selection criteria defined from the systematic changes along the quasars main sequence. It has been possible to build a sample of about 250 quasars at low and intermediate redshift, and larger samples can be easily selected from the latest data releases of the Sloan Digital Sky Survey. A large sample can clarify the main properties of xA quasars which are expected - unlike the general population of quasars - to radiate at an extreme, well defined Eddington ratio with small scatter. As a result of the small scatter in Eddington ratio shown by xA quasars, we propose a method to derive the main cosmological parameters based on redshift-independent "virial luminosity" estimates from measurements of emission line widths, roughly equivalent to the luminosity estimates based from line width in early and late type galaxies. A major issue related to the cosmological application of the xA quasar luminosity estimates from line widths is the identification of proper emission lines whose broadening is predominantly virial over a wide range of redshift and luminosity. We report on preliminary developments using the AlIII 1860 intermediate ionization line and the Hydrogen Balmer line H-beta as virial broadening estimators, and we briefly discuss the perspective of the method based on xA quasars.

Tunable passively Q-switched ultranarrow linewidth erbium-doped fiber laser

A tunable passively Q-switched erbium-doped fiber laser with ultranarrow linewidth is proposed and demonstrated. With the single wall carbon nanotubes (SWCNTs) as the passive Q-switch, the ultranarrow linewidth laser operation is realized based on spectral filtering effect.using an ultranarrow tunable bandpass filter (UNTBF). The Q-switched laser spectrum is tunable from 1525 to 1561 nm, covering a wavelength range of 36 nm. At 1545 nm, the highest repetition rate of ~38 kHz and the lowest pulse width of 1.15 μs are obtained, with the corresponding pulse energy of approximately 0.18nJ. The linewidth measurement using self-heterodyne technique yields a linewidth value of 17.5 kHz, which may be the narrowest linewidth for a Q-switched fiber laser to the best of our knowledge.

Quasar emission lines as virial luminosity estimators

Quasars accreting matter at very high rates (known as extreme Population A [xA]) may provide a new class of distance indicators covering cosmic epochs from present day up to less than 1 Gyr from the Big Bang. We report on the developments of a method that is based on "virial luminosity" estimates from measurements of emission line widths of xA quasars. The approach is conceptually equivalent to the virial estimates based on early and late type galaxies. The main issues related to the cosmological application of luminosity estimates from xA quasar line widths are the identification of proper emission lines whose broadening is predominantly virial over a wide range of luminosity, and the assessment of the effect of the emitting region orientation with respect to the line of sight. We report on recent developments concerning the use of the AlIII 1860 intermediate ionisation line and of the Hydrogen Balmer line H$\beta$ as "virial broadening estimators."

Electrodynamic theory of ferromagnetic resonance and its applications in precise measurements of ferromagnetic linewidth, permeability tensor and saturation magnetization

This review paper describes the state of the art in measurements of ferromagnetic linewidth, permeability tensor and saturation magnetization employing electrodynamic theory of ferromagnetic resonance. It is shown that the electrodynamic theory allows significant improvements of measurement accuracy of these parameters with respect to the commonly used perturbation and magnetostatics theories. Contrary to the perturbation method the electrodynamic theory is not limited to small samples. It allows determination of the resonance frequencies and Q-factors for arbitrary size spherical and cylindrical gyromagnetic samples in suitably chosen metal enclosures. Results obtained with the electrodynamic theory for very small samples are identical to those obtained with perturbation and magnetostatics theories. Results of measurements of the ferromagnetic linewidth, permeability tensor and saturation magnetization at microwave frequencies are presented.

Monte Carlo study for correcting the broadened line-scan profile in scanning electron microscopy.

Line-scan profile is always broadened due to the probe shape of the primary electron (PE) beam in scanning electron microscopy (SEM), which leads to an inaccurate dimension metrology. Currently, the effective electron beam shape (EEBS) is suggested as the broadening function to overcome this issue for theoreticalanalysis, rather than the widely used Gaussian profile. However, EEBS is almost impossible to be acquired due to it strongly depends on both the sample topography and the electron beam focusing condition, which makes it is impossible to be applied in practical analysis. Taking the case of gate linewidth measurement, an approach is proposed to find a best-fit traditional Gaussian profile, which can optimally replace the EEBS in the case of the same sample structure and experimental condition for construction of a database of the parameter in traditional Gaussian profile. This approach is based on the use of the ideal and broadened line-scan profiles which are both obtained from Monte Carlo (MC) simulation, but respectively by an ideal and a focusing incident electron beam model. The expected value of parameter can be obtained through deconvoluting (here using a maximum-entropy algorithm) the broadened line-scan profile then fitting it to the ideal profile. Experimenters can benefit from this database to obtain true line-scan profiles for accurate gate linewidth measurement. This work should prove useful for samples of other structures and be an extension of the database in the future.

Influence of exterior acoustic noise on narrow linewidth laser measurements using self-homodyne optical fiber interferometer

Spectral linewidth measurements of a HeNe laser were obtained by using a self-homodyne optical fiber Mach-Zehnder interferometer located in a vacuum controlled chamber. The data showed an apparent broadening of the highly-coherent laser linewidth from few hertz up to kilohertz with the change in the air chamber pressure, evincing an influence of external acousto-optic effects on the interferometer optical fiber. To support the results, a theoretical harmonic acoustic analysis was developed and then related to the correspondent interferometer phase noise spectrum measurements. Lorentzian approximations of the experimental laser apparent linewidth suggest that the interferometer was working in a quasi-coherent regime for all tested chamber air pressure.

Comparison of the UV and Optical Fe ii Emission in Type 1 AGNs

Abstract We present the kinematical properties of the UV and optical Fe ii emission gas based on the velocity shift and line width measurements of a sample of 223 Type 1 active galactic nuclei (AGNs) at 0.4 &lt; z &lt; 0.8. We find a strong correlation between the line widths of the UV and optical Fe ii emission lines, indicating that both Fe ii emission features arise from similar distances in the broad-line region (BLR). However, in detail we find differing trends, depending on the width of Fe ii. While the velocity shifts and dispersions of the UV Fe ii (Fe ii uv) and optical Fe ii (Fe ii opt) emission lines are comparable to each other for AGNs with relatively narrow Fe ii opt line widths (i.e., FWHM &lt; 3200 km s−1; Group A), Fe ii opt is broader than Fe ii uv for AGNs with relatively broad Fe ii opt (i.e., FWHM &gt; 3200 km s−1; Group B). Fe ii emission lines are on average narrower than Hβ and Mg ii for Group A, indicating the Fe ii emission region is further out in the BLR, while for Group B AGNs Fe ii opt is comparable to Hβ and broader than Mg ii. While Fe ii emission lines are on average redshifted (40 ± 141 km s−1 and 182 ± 95 respectively for Fe ii uv and Fe ii opt), indicating inflow, the sample as a whole shows a large range of velocity shifts, suggesting the complex nature of the gas kinematics.