MHz Accuracy Research Articles

Deep Learning Enhanced Long-Range Fast BOTDA for Vibration Measurement

In this paper, we propose and experimentally demonstrate a scheme of deep learning enhanced long-range fast Brillouin optical time-domain analysis (BOTDA). The volumetric data from fast BOTDA is denoised and demodulated by using a deep video denoising network and a deep neural network, respectively. Benefitting from the advanced deep learning algorithms, the sensing range of fast BOTDA is extended to 10 km successfully. In experiment, vibration signal is measured with a sampling rate of 23 Hz, 2 m spatial resolution, and 1.19 MHz accuracy over 10 km single-mode fiber with only 4 averages. Due to the low computational complexity and GPU acceleration, the network takes less than 0.04 s to process 100 × 21800 data, which is much faster than the conventional algorithms. This method provides the potential for real-time vibration measurement in fast BOTDA with long sensing range.

Midinfrared Tunable Laser with Noncritical Frequency Matching in Box Resonator Geometry.

Monolithic optical parametric oscillators extend laser frequencies in compact architectures, but normally guide and circulate all pump, signal, and idler beams. Critical frequency matching is raised among these resonances, limiting operation stability and continuous tuning. Here, we develop a box resonator geometry that guides all beams but only resonates for signal. Such noncritical frequency matching enables 227GHz continuous tuning, with sub-10kHz linewidth and 0.43W power at 3310nm. Our results confirm that monolithic resonator can be effectively used as a tunable laser including midinfrared wavelength, as further harnessed with methane fine spectral measurement at MHz accuracy.

Ba4Al2Ti10O27 based glass ceramic as dielectric in high frequency applications

Glass ceramics obtained via a true glassy phase are homogeneous and pore free, which makes them interesting for electronic applications, especially in the GHz frequency range. We discuss the glass-ceramic system with Ba4Al2Ti10O27 as the main crystalline phase and show material properties, including dielectric properties measured up to 110 GHz. Application examples are shown for filters and antennas in the GHz frequency range. The extremely high material homogeneity makes very precise manufacturing possible and enables MHz accuracy and reproducibility in GHz applications. The high linearity of the material properties is shown as well and makes high power applications, e.g., in cell phone base stations, possible. Also, applications in the MHz range are discussed.

Direct Observation of Terahertz Frequency Comb Generation in Difference-Frequency Quantum Cascade Lasers

Terahertz quantum cascade laser sources based on intra-cavity difference frequency generation from mid-IR devices are an important asset for applications in rotational molecular spectroscopy and sensing, being the only electrically pumped device able to operate in the 0.6–6 THz range without the need of bulky and expensive liquid helium cooling. Here we present comb operation obtained by intra-cavity mixing of a distributed feedback laser at λ = 6.5 μm and a Fabry–Pérot device at around λ = 6.9 μm. The resulting ultra-broadband THz emission extends from 1.8 to 3.3 THz, with a total output power of 8 μW at 78 K. The THz emission has been characterized by multi-heterodyne detection with a primary frequency standard referenced THz comb, obtained by optical rectification of near infrared pulses. The down-converted beatnotes, simultaneously acquired, confirm an equally spaced THz emission down to 1 MHz accuracy. In the future, this setup can be used for Fourier transform based evaluation of the phase relation among the emitted THz modes, paving the way to room-temperature, compact, and field-deployable metrological grade THz frequency combs.

Transition frequencies in the (2-0) band of D2 with MHz accuracy

Series of spectra of the S(0) and Q(1)-Q(4) transitions of the (2-0) band of D2 near 5860 cm−1 are recorded by comb-referenced cavity ring down spectroscopy, at four pressures up to 200 Torr. After averaging, high resolution (30 MHz step) and low noise (noise equivalent absorption, αmin≈ 1.5 × 10−11 cm−1) spectra are obtained for each pressure value. This leads to signal to noise ratios of several hundred for the considered very weak electric quadrupole transitions with intensities ranging between 4 × 10−29 and 4 × 10−28 cm/molecule. The Q(3) and Q(4) transitions are newly detected.A multi-spectrum treatment of the D2 line profiles at different pressures performed using a quadratic speed-dependent Nelkin-Ghatak profile leads to residuals close to the noise level. Considering statistical fit errors and possible biases, total uncertainties on the frequencies extrapolated at zero pressure are estimated to be between 0.30 MHz and 1.83 MHz. The derived experimental frequencies and intensities of (2-0) transitions are gathered with literature values and compared to ab initio values reported in J. Komasa, M. Puchalski, P. Czachorowski, G. Łach, K. Pachucki, Phys. Rev. A 100, 032519 (2019). An agreement within the combined uncertainties is achieved for the S(0), Q(1), Q(2) and Q(3) transitions. Line intensities are also discussed in relation with literature values.

A Precision High Voltage Pulse Divider

Abstract—A the high-voltage divider for pulsed signals was developed for multichannel DAQ-system of the LIA-20 linear induction accelerator. The divider satisfies a range of requirements: a range of operating voltages up to 30 kV, a 30 MHz bandwidth, accuracy and stability characteristics that are better than 1%, and repeatability of its parameters during mass production. An analysis of various variants of high-voltage dividers is given: passive capacitive dividers and capacitive dividers with active components, as well as resistive, resistive-capacitive, and combined dividers. The configuration selection of the divider circuit with the necessary properties is substantiated based on the analysis.

Fabrication and characterization of short acetylene-filled photonic microcells.

We have developed short (6-10 cm), connectorized acetylene-filled photonic microcells (PMCs) from photonic bandgap fibers that may replace near-IR frequency references for certain applications based on gas-filled glass cells. By using a tapering technique to seal the microcells, we were able to achieve a high transmission efficiency of 80% and moderate line center accuracy of 10 MHz (1σ). This approaches the National Institute of Standard Technology Standard Reference Material 2517a 10 MHz (2σ) accuracy. Using an earlier Q-tipping technique, 37% off-resonant transmission and 5 MHz accuracy were achieved in finding the line center, but a large 13% etalon-like effect appears on the wings of the optical depth. The etalon-like effect is reduced to less than 1% by using the tapering method. In both cases, the microcells could be connectorized, albeit with a reduction in off-resonant transmission efficiency, for integration into multimode fibers or free-space optical systems. Although contamination is introduced during both fabrication techniques, the P13 PMC line center shifts are small with respect to the sub-Doppler line center. This shows that the PMC can be used for moderate-accuracy frequency measurements. Finally, repeatable measurements show that PMCs are stable in terms of total pressure over approximately one year.

Isotopologue consistency of semi-empirically computed infrared line lists and further improvement for rare isotopologues: CO2 and SO2 case studies

The semi-empirical molecular rovibrational IR line lists, such as ExoMol, TheoReTs, and Ames, combine the experimental accuracy and theoretical power to reach better than 0.1 cm−1 accuracy for line positions and better than 80–90% agreement for line intensities. The quality of these existing semi-empirical IR lists allows further improvements of intensity and line positions for those unobserved minor isotopologues. This paper presents our new BTRHE (Best Theory + Reliable High-resolution Experiment) strategy implementation. For line intensity, the isotopologue consistency and the patterns of mass dependence in the Ames-296 K SO2 and CO2 IR lists are quantitatively presented along the mass-inverse coordinates. The consistency and patterns are better than those in existing experimental data. The methodology proposed here can be used to identify inconsistencies, outliers, and mistakes in intensities, and help improve Effective Dipole Model (EDM) and molecular IR databases. We call for an experimental study on the 50006 and 60007 bands of CO2 628. For line position predictions, a simple approach combining the variational IR line lists with Effective Hamiltonian (EH) model may refine the effective rotational constants A0/B0/C0 and quartic centrifugal distortion constants of minor isotopologues. The prediction accuracy may be improved by two orders of magnitude, i.e. reaching 0–5 MHz prediction accuracy in the range of J < 20–30, Ka < 10–20, and 0.01–0.02 MHz accuracy for A0/B0/C0. Several important factors have been systematically investigated and discussed, e.g. convergence, uncertainties, higher order terms, fixing EH parameters, mass coordinates, etc. A microwave (MW) line set consisting of 644,636 strong transitions (at 296K) for all 30 isotopologues and corresponding refined EH(Ames) parameters are reported in the supplementary material. This approach may be easily extended to rovibrational bands, hot bands, and other molecular systems.

All-solid-state VUV frequency comb at 160 nm using high-harmonic generation in nonlinear femtosecond enhancement cavity.

We realized a solid-state-based vacuum ultraviolet frequency comb by harmonics generation in an external enhancement cavity. Optical conversions were so far reported by only using gaseous media. We present a theory that allows the most suited solid generation medium to be selected for specific target harmonics by adapting the material's bandgap. We experimentally use a thin AlN film grown on a sapphire substrate to realize a compact frequency comb high-harmonic source in the Deep Ultraviolet (DUV) / Vacuum Ultraviolet (VUV) spectral range. By extending our earlier VUV source [Opt. Express26, 21900 (2018)] with the enhancement cavity, a sub-Watt level Ti:sapphire femtosecond frequency comb is enhanced to 24 W stored average power, its 3rd, 5th, and 7th harmonics are generated, and the targeted 5th harmonic's power at 160 nm increased by two orders of magnitude. The emerging nonlinear effects in the solid medium, together with suitable intra-cavity dispersion management, support optimal enhancement and stable locking. To demonstrate the realized frequency comb's spectroscopic ability, we report on the beat measurement between the 3rd harmonic beam and a 266 nm CW laser reaching about 1 MHz accuracy.

Single-chip source-free terahertz spectroscope across 0.04-0.99 THz: combining sub-wavelength near-field sensing and regression analysis.

This paper demonstrates a methodology to miniaturize THz spectroscopes into a single silicon chip by eliminating traditional solid-state architectural components such as complex tunable THz and optical sources, nonlinear mixing and amplifiers. The proposed method achieves this by extracting incident THz spectral signatures from the surface of an on-chip antenna itself. The information is sensed through the spectrally-sensitive 2D distribution of the impressed current surface under the THz incident field. By converting the antenna from a single-port to a massively multi-port architecture with integrated electronics and deep subwavelength sensing, THz spectral estimation is converted into a linear estimation problem. We employ rigorous regression techniques and analysis to demonstrate a single silicon chip system operating at room temperature across 0.04-0.99 THz with 10 MHz accuracy in spectrum estimation of THz tones across the entire spectrum.

High-resolution Brillouin optical correlation domain analysis with no spectral scanning.

Distributed Brillouin fiber sensors typically rely on the reconstruction of the steady-state Brillouin gain spectrum (BGS), through spectral scanning of the frequency offset between the pump and signal waves. In this work, we propose and demonstrate an alternative approach, in which the local Brillouin frequency shift (BFS) is extracted from temporal transient analysis of the step response of the amplified signal wave. Measurements are taken at only two arbitrary frequency offsets between pump and signal. No spectral scanning and no prior knowledge of a reference BGS are necessary. The principle is supported by analytic and numeric solutions of the differential equations of stimulated Brillouin scattering. The BFS of a 2 meters-long fiber under test was measured with 1 MHz accuracy and a dynamic range of 200 MHz. Transient measurements were also performed in a Brillouin optical correlation domain analysis (B-OCDA) experiment with 4 cm resolution, standard deviation of 2.4 MHz and 100 MHz dynamic range. A 4 cm-wide hot-spot was properly identified in the measurements. Multiple correlation peaks could be addressed in a single flight of a pump pulse. The results represent the first B-OCDA that is free of spectral scanning. This new measurement concept may be applicable to random-access distributed and dynamic monitoring of sound and vibration.

Highly Sensitive Power Independent Frequency Measurement System

A broadband frequency measurement system capable of measuring both frequency and amplitude of an RF signal with $-$ 31 dBm sensitivity and 100 MHz accuracy over 40 MHz to 40 GHz range is practically demosntrated. The amplitude independence was achieved via employing two orthogonal measurements and the high sensitivity was obtained by utilization of the lock-in amplification technique.

Determination of transition frequencies in a singleBa+138ion

Transition frequencies between low-lying energy levels in a single trapped $^{138}$Ba$^{+}$ ion have been measured with laser spectroscopy referenced to an optical frequency comb. By extracting the frequencies of one-photon and two-photon components of the line shape using an eight-level optical Bloch model, we achieved 0.1 MHz accuracy for the 5d $^{2}$D$_{3/2}$ - 6p $^{2}$P$_{1/2}$ and 6s $^{2}$S$_{1/2}$ - 5d $^{2}$D$_{3/2}$ transition frequencies, and 0.2 MHz for the 6s $^{2}$S$_{1/2}$ - 6p $^{2}$P$_{1/2}$ transition frequency.

An all-optical locking of a semiconductor laser to the atomic resonance line with 1 MHz accuracy

An all-optical locking technique without extra electrical feedback control system for a semiconductor laser has been used in stabilizing the laser frequency to a hyperfine crossover transition of 87Rb 5(2)S(1/2), F = 2 → 5(2)P(3/2), F' = 2, 3 with 1 MHz level accuracy. The optical feedback signal is generated from the narrow-band Faraday anomalous dispersion optical filter (FADOF) with nonlinear saturation effect. The peak transmission of the narrow-band FADOF corresponding to 5(2)S(1/2), F = 2 → 5(2)P(3/2), F' = 2, 3 crossover transition is 18.6 %. The bandwidth is as wide as 38.9 MHz as the laser frequency changes. After locking, the laser frequency fluctuation is reduced to 1.7 MHz. The all-optical laser locking technique can be improved to much higher accuracy with increased external cavity length. The laser we have realized can provide light exactly resonant with atomic transitions used for other atom-light interaction experiments.

High-Resolution Spectroscopy of CH2D+ in a Cold 22-Pole Ion Trap

The method of laser-induced reaction (LIR) is used to obtain high-resolution IR spectra of CH2D(+) in collision with n-H2 at a nominal temperature of 14 K. For this purpose, a home-built optical parametric oscillator (OPO), tunable in the range of 2500-4000 cm(-1), has been coupled to a 22-pole ion trap apparatus. In total, 112 lines of the ν1 and ν4 bands have been recorded. A line list is inferred from a careful analysis of the shape of the LIR signal. Line positions have been determined to an accuracy of 1 × 10(-4) cm(-1), allowing for the prediction of pure rotational transitions with MHz accuracy. In addition, an IR-THz double-resonance LIR depletion technique is applied to H2D(+) to demonstrate the feasibility for pure rotational spectroscopy with LIR.

Microwave spectroscopy of singlet Mg I in L = 0–4 Rydberg states

Microwave resonances among high-lying spin-singlet Rydberg levels in neutral Mg have been measured with approximately 1 MHz accuracy for principal quantum numbers n = 25–40 and orbital angular momentum L = 0–4. P, D and F levels were excited by multistep laser excitation in an atomic beam, and one- or two-quantum microwave transitions to nearby levels having Δn = 0, 1 or 2 at frequencies 79–234 GHz were detected by pulsed field ionization. Quantum defects and Ritz expansion coefficients were obtained for S, P, D, F and G levels from a global analysis in which it was unnecessary, within experimental accuracy, to make any allowance for channel perturbation from the Mg I 3p2 1D2 doubly excited configuration. The present results allow prediction and identification of Rydberg transition frequencies in Mg I, for instance in radio recombination lines from the interstellar medium, with accuracy greater by two or three orders of magnitude than previously possible.

Rotational transitions of CH2D+determined by high-resolution IR spectroscopy

Context. Deuterated forms of CH + are responsible for deuterium fractionation in warmer environments. Current searches for CH2D + are hampered by a lack of accurate laboratory data. Aims. We demonstrate that IR spectroscopy at very high resolution can make accurate rotational predictions. Methods. By combining a low-temperature ion trap with a narrow-bandwidth IR light source, we are able to measure vibrational transitions with high accuracy. A subsequent fit using an asymmetric rotor model allows predictions of MHz accuracy or even better. Results. We predict rotational transitions up to 1.5 THz.

Microwave spectroscopy of Ag I atoms in Rydberg states: S, P and D terms

Microwave resonance transitions of Ag I atoms laser excited to Rydberg S, P and D states of the configuration 4d10nℓ, for n = 28–39, were measured with MHz accuracy at frequencies from 76 to 363 GHz. Quantum defects and Rydberg–Ritz coefficients were determined based on 47 new resonances, together with doublet fine-structure splittings of P and D terms. An efficient single-laser two-photon excitation path 52S1/2 → 282D5/2 was discovered based on a coincidence with twice the term difference of the transition 52S1/2 → 52P3/2.

Laboratory Detection of a Linear Carbon Chain Alcohol: HC 4 OH and Its Deuterated Species

The rotational spectra of HC4OH and its deuterated species have been detected in the 17-39 GHz region by microwave spectroscopy. The spectra were generated after pyrolysis of 2-butynol at 830°C; a deuteration reaction to produce DC4OH and HC4OD was caused by mixing the 2-butynol with deuterated water vapor. The rotational constants B and C were precisely determined for HC4OH and the rotational constant (B + C)/2 was determined for DC4OH and for HC4OD. With these rotational constants, the entire radio spectrum of HC4OH can be accurately estimated to better than 0.42 MHz accuracy at frequencies up to 51.1 GHz. These transition frequencies should be of great use for astronomical identification of this molecule.

Cancellation of Stark Shifts in Optical Lattice Clocks by Use of Pulsed Raman and Electromagnetically Induced Transparency Techniques

We propose a combination of electromagnetically induced transparency-Raman and pulsed spectroscopy techniques to accurately cancel frequency shifts arising from electromagnetically induced transparency fields in forbidden optical clock transitions of alkaline earth atoms. At appropriate detunings, time-separated laser pulses are designed to trap atoms in coherent superpositions while eliminating off-resonance ac Stark contributions, achieving efficient population transfer up to 60% with inaccuracy <10(-17). Results from the wave-function formalism are confirmed by the density matrix approach.