Frequency Modulation Spectroscopy Research Articles

Multiplexing of frequency-modulation spectroscopy by spread-spectrum codes, demonstrated in continuous-wave LIDAR.

Frequency-modulation spectroscopy (FMS) is generally suited to code-division multiplexing, and we demonstrate that capacity in a form of continuous-wave LIDAR, utilizing a sharp CO2 absorption transition at 1.6 µm in simple ranging setups. The approach retains the advantages of FMS, including coherent detection and good rejection of broad absorption backgrounds. Extensions of this multiplexed approach to the continuous, simultaneous detection of several transitions would come by transmitting an encoded combination of frequency-modulated carriers, each tuned to detect a unique absorption transition. Signal analysis at the receiver involves a simple process of de-multiplexing that, in a general application, reveals targets at various distances and the absorption-related FMS signals in between.

Recycled Photons Traveling Several Millimeters in Waveguides Based on CsPbBr3 Perovskite Nanocrystals

AbstractReabsorption and reemission of photons, or photon recycling (PR) effect, represents an outstanding mechanism to enhance the carrier and photon densities in semiconductor thin films. This work demonstrates the propagation of recycled photons over several mm by integrating a thin film of CsPbBr3 nanocrystals into a planar waveguide. An experimental set‐up based on a frequency modulation spectroscopy allows to characterize the PR effect and the determination of the effective decay time of outcoupled photons. A correlation between the observed photoluminescence redshift and the increase of the effective decay time is demonstrated, which grows from 3.5 to near 9 ns in the best device. A stochastic Monte Carlo model reproduces these experimental results and allows the extraction of the physical mechanisms involved. In the waveguide under study recycled photons follow a drift (directional enhancement) velocity ≈5.7 × 105 m s−1, dominating over the diffusive regime observed in a standard thin film (D ≈ 420 m2 s−1). This means that recycled photons propagate mm‐distances in shorter traveling times in the waveguide (≈5 ns) as compared to the film (>20 ns). These results are expected to pave the road for exploiting the PR effect in future optoelectronic and photonic devices.

Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4\u2009THz bandwidth

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

Study the effect of expired tenoxicam on the inhibition of carbon steel corrosion in a solution of hydrochloric acid

The effect of expired tenoxicam drug as an inhibitor on the corrosion inhibition of carbon steel in 0.5 M HCl was examined at different concentrations. Besides, the effect of potassium iodide additives on inhibition was investigated. All of these are applied by three methods; electrochemical studies (electrochemical frequency modulation, potentiodynamic and electrochemical impedance spectroscopy, superficial studies (XPS, FTIR, and UV-spectroscopy) and theoretical studies (quantum chemistry calculations and molecular dynamic calculations). The presence of tenoxicam as an inhibitor in the solution gives an increase in charge transfer resistance and a decrease in the capacity of the double layer. Potentiodynamic polarization studies have demonstrated that tenoxicam is a mixed inhibitor. In this study, we examined the influence of temperature on the inhibition of corrosion of carbon steel, and we also computed the thermodynamic parameters. The potentiodynamic method designated that the inhibitor was used showed the best protection productivity of up to 81.0% for C-steel at 4 × 10−4 M from tenoxicam in 0.5 M HCl solutions at 30 °C. On the other hand, the inhibition efficiency (I%) increased synergistically on the addition of potassium iodide to 90.3%. Tenoxicam adsorption was found on the C-steel surface for Temkin's isothermal obedience. The mechanism of the inhibition process was discussed in light of the chemical composition and theoretical study of the inhibitor probe. Expired Tenoxicam was first studied as a carbon steel corrosion inhibitor in a solution of hydrochloric acid. It provides a way to deal with expired medications, thus reducing environmental pollution. The study explored the inhibition mechanism affecting expired tenoxicam, and provided theoretical support for the results obtained from experimental studies. The expired tenoxicam drug can be used as a cheap and safe corrosion inhibitor for the studied system.

Laser stabilization to neutral Yb in a discharge with polarization-enhanced frequency modulation spectroscopy

Isotope selective optical excitation of atoms is important for experiments with neutral atoms, metrology, and work with trapped ions, including quantum information processing. Polarization-enhanced absorption spectroscopy is used to frequency stabilize a tunable external cavity laser diode system at 398.9 nm for isotope selective photoionization of neutral Yb atoms. This spectroscopy technique is used to measure isotope resolved dispersive features from transitions within a see-through configuration ytterbium hollow-cathode discharge lamp. This Doppler-free dichroic polarization spectroscopy is realized by retro-reflecting a laser beam through the discharge and analyzing the polarization dependent absorption with balanced detection. The spectroscopy signal is recovered using lock-in detection of frequency modulation induced by current modulation of the external cavity laser diode. Here, we show an order of magnitude improvement in the long-term stability using polarization-enhanced absorption spectroscopy of Yb compared to polarization spectroscopy.

Container Closure Integrity Test Using Frequency Modulation Spectroscopy Headspace Analysis with Carbon Dioxide as a Tracer Gas.

As described in USP <1207>, the container closure integrity (CCI) of a pharmaceutical package must be maintained throughout the product lifecycle to ensure sterility and stability. Current CCI test methods can be time-consuming, destructive, and lack the required sensitivity. This study presents a novel, fast, and nondestructive method for CCI testing that uses carbon dioxide as a tracer gas under effusive pressure conditions. Two types of defects were tested: laser-drilled defects located in the glass body (2, 5, and 10 μm nominal diameter) and tungsten wires inserted between the stopper and the landing seal of the vial (41, 64, and 80 μm outer diameter). During each test session, vials were placed in a pressure vessel, isolated from ambient conditions, and pressure-cycled by first pulling a vacuum and then applying an overpressure of pure carbon dioxide gas. After being exposed to 20 psig (34.7 psia) of carbon dioxide for 30 min, the overpressure was released and the vials were measured on an FMS-Carbon Dioxide Headspace Analyzer. This headspace gas analyzer utilizes a tunable diode laser absorption spectroscopy technique that employs frequency modulation to enhance measurement sensitivity. An increase of ≥1 torr in the headspace carbon dioxide content after completion of the pressure cycling procedure was intended to serve as confirmation of leak detection. All empty vials with either a 2 µm laser-drilled defect or 41 µm wire (effective defect size ∼2 µm), or greater, at the stopper-seal interface were detected by this method. Furthermore, vials filled with 1 mg/mL bovine serum albumin in phosphate-buffered saline containing a 5 μm laser-drilled defect below the liquid level or a 64 µm wire (effective defect size ∼6.1 µm), or greater, at the stopper-seal interface (defect above the liquid level) were detected. This test can be used for a wide variety of vial types and headspace compositions.

Dispersive microwave electrometry using Zeeman frequency modulation spectroscopy of electromagnetically induced transparency in Rydberg atoms

We herein developed and demonstrated a Zeeman frequency modulation scheme for improving the signal-to-noise ratio of microwave electric field measurement using Rydberg atoms. The spectra of the electromagnetically induced transparency (EIT) and Autler-Townes splitting of Rydberg atoms is frequency modulated by an alternating current magnetic field. The signal-to-noise ratio of the corresponding dispersive error signal is enhanced more than 10 times than that of the original spectrum. Furthermore, we show that the slope of the dispersive error signal near the resonance of the Rydberg EIT can be used to characterize the weak microwave electric field amplitudes. The more intuitive and simpler structure compared with other existing frequency modulation technologies greatly reduces the difficulties of experiments and experimental data analysis.

Inelastic Scattering of CN Radicals at the Gas–Liquid Interface Probed by Frequency-Modulated Absorption Spectroscopy

We describe the results of the first application of frequency-modulated (FM) absorption spectroscopy to molecular scattering dynamics at a gas–liquid interface. A pulsed direct-current (dc) electri...

Dispersive detection of atomic ensembles in the presence of strong lensing

We experimentally and theoretically investigate in-medium propagation effects of off-resonant light in dense, spatially inhomogeneous ultracold atomic gases. Focussing on frequency modulation spectroscopy as the dispersive detection tool of atoms, we observe that the refractive gradient-index lenses presented by localised atomic ensembles can significantly modify the interpretation of the dispersive signal even for large probe detuning, owing to the collective response of the atoms. We identify criteria for distinguishing between thin and thick atomic lenses, leading to either diffraction-dominated and lensing dominated regimes for the outgoing probe beams. Our findings are consistent with experimental data and solutions of paraxial wave equation for light propagation. Our study provides important practical insights for dispersive, minimally intrusive optical detection and imaging schemes of ultracold atoms and will be valuable for choosing optimal parameter regimes in numerous applications.

Adsorption and Anticorrosion Behavior of Expired Meloxicam on Mild Steel in Hydrochloric Acid Solution

The inhibiting effect of the expired meloxicam on the corrosion of mild steel (MS) in 1 M HCl was tested by the weight loss, electrochemical frequency modulation, potentiodynamic polarization (PP) and electrochemical impedance spectroscopy techniques. The PP curves showed that this drug acts as mixed type inhibitor. This drug was adsorbed chemically on the MS surface following the Temkin adsorption isotherm. Some thermodynamic parameters were computed and discussed. The results indicated that the protection efficiency increases by raising both the doses of the inhibitor and the temperature of the medium. The morphology of the MS surface was analyzed by scanning electron microscopy, atomic force microscopy, and Fourier transform infrared spectroscopy. All of the obtained results from different techniques are similar.

Heat flux sensor to create a design space for freeze-drying development

Freeze-drying methodology requires an in-depth understanding and characterization for optimal processing of biopharmaceuticals. Particularly the primary drying phase, the longest and most expensive stage of the process, is of interest for optimization. The currently used process analytical technology (PAT) tools give highly valuable insights but come with limitations. Our study describes, for the first time, the application of a heat flux sensor (HFS) to build a primary drying design space and predict the process evolution. First, the heat transfer coefficient (Kv) generated by HFS and by the most accurate, but time-consuming and invasive, gravimetric method were compared. Second, the applicability to generate a design space was tested and verified. Obtained results revealed a good agreement of the values generated from this new and fast HFS compared to the gravimetric determination. Additionally, residual moisture assessed by Karl-Fischer titration and frequency modulated spectroscopy (FMS) support the quality of the obtained predictions. Thus, the HFS approach can substantially accelerate evaluation, development and transfer of a freeze-drying cycle.

Raman-Laser System for Absolute Gravimeter Based On 87Rb Atom Interferometer

The paper describes a Raman-laser system with high performance for an absolute gravimeter that was based on 87Rb atom interferometer. As our gravimeter is a part of the standard acceleration of gravity of China, the Raman lasers’ characteristics should be considered. This laser system includes two diode lasers. The master laser is frequency locked through the frequency-modulation (FM) spectroscopy technology. Its maximum frequency drift is better than 50 kHz in 11 h, which is measured by home-made optical frequency comb. The slave laser is phase locked to the master laser with a frequency difference of 6.8346 GHz while using an optical phase lock loop (OPLL). The phase noise is lower than −105 dBc/Hz at the Fourier frequency from 200 Hz to 42 kHz. It is limited by the measurement sensitivity of the signal source analyzer in low Fourier frequency. Furthermore, the power fluctuation of Raman lasers’ pulses is also suppressed by a fast power servo system. While using this servo system, Raman lasers’ pulses could be fast re-locked while its fast turning on again in the pulse sequence. The peak value fluctuation of the laser power pulses is decreased from 25% to 0.7%, which is improved over 35 times. This Raman-laser system can stably operate over 500 h, which is suited for long-term highly precise and accurate gravity measurements.

Simulation technique enabling calibration-free frequency-modulation spectroscopy measurements of gas conditions and lineshapes with modulation frequencies spanning kHz to GHz.

A simulation technique enabling calibration-free measurements of gas properties (e.g., temperature, mole fraction) and lineshapes via wavelength- or frequency-modulation spectroscopy (WMS or FMS) is presented. Unlike previously developed models, this simulation technique accurately accounts for (1) absorption and dispersion physics and (2) variations in the WMS/FMS harmonic signals, which can result from intensity tuning induced by scanning the laser's carrier frequency [e.g., via injection-current tuning of tunable diode lasers (TDLs)]. As a result, this approach is applicable to both WMS and FMS experiments employing a wide variety of light sources and any modulation frequency [typically kilohertz (kHz) to gigahertz (GHz)]. The accuracy of the simulation technique is validated via comparison with (1) simulated signals produced by established WMS and FMS models under conditions where they are accurate and (2) experimental data acquired under conditions where existing models are inaccurate. Under conditions where existing WMS and FMS models are accurate, this simulation technique yields nearly identical (within 0.1%) results. For experimental validation, the wavelength of a TDL emitting near 1392 nm was scanned across a single absorption line of H2O with a half-width at half-maximum of 350 MHz while frequency modulation was performed at 100 MHz. The best-fit first-harmonic (1f) signal produced by this simulation technique agrees within 1.6% of the measured 1f signal, and the H2O mole fraction and transition collisional width corresponding to the best-fit 1f spectrum agree within 1% of expected values.

One-colour (∼220 nm) resonance-enhanced (S1 − S0) multi-photon dissociation of acetylene: probe of the C2 A 1 Π u − X 1 Σ+ g band by frequency-modulation spectroscopy

In a recent paper, we demonstrated that one-colour (∼220 nm), resonance-enhanced (SS), photodissociation of acetylene generates strong Swan band () and Deslandres-d'Azambuja band () fluorescence, and long-lived (>3 µs) fluorescence from an electronically-excited H species. It was not known whether the and states are also directly populated in this process. In this work, multiple vibration-rotation transitions between the -state v = 2 and the X-state v = 0 level are examined by time-resolved frequency-modulation (FM) spectroscopy. The photolysis laser wavelength is tuned into resonance at the one-photon level with SS transitions that populate individual rotational levels of the S-conformer , , and vibrational states. By comparing the phase of the FM signals from the transitions with that from the Rb D-line absorption transition, we determine that, for all of the probed A−X transitions, the X-state level is more populated than the A-state level. We propose that the acetylene S level is excited by the second photon to an acetylene dissociation precursor state, which undergoes sequential C-H bond-breaking to produce the state. The dissociation precursor is assigned as the valence state, which correlates to a doubly-excited configuration, , at linear geometry. Based on the rotational distributions of the -state fragments, we believe that at least one of the transition states involved in the photolysis via S has a larger CC-H bend-angle for the departing H-atom than that involved in the S and photolysis.

Large optical depth frequency modulation spectroscopy.

Band-resolved frequency modulation spectroscopy is a common method to measure weak signals of radiative ensembles. When the optical depth of the medium is large, the signal drops exponentially and the technique becomes ineffective. In this situation, we show that a signal can be recovered when a larger modulation index is applied. Noticeably, this signal can be dominated by the natural linewidth of the resonance, regardless of the presence of inhomogeneous line broadening. We implement this technique on a cesium vapor, and then explore its main spectroscopic features. This work opens the road towards measurement of cooperative emission effects in bulk atomic ensemble.

Self-corrected frequency modulation spectroscopy immune to phase random and light intensity fluctuation.

Light intensity fluctuations and phase randomness in quadrature demodulation disturb the accuracy of frequency modulation spectroscopy. The proposed self-corrected method eliminated these effects: the profile of correctly demodulated signals identified whether a demodulation was phase matched and corrected a phase-mismatched demodulated signal; we extracted the measured signal's direct current component and corrected the light intensity fluctuation. We conducted theoretical analysis and experimental verification to reduce light-intensity errors by 16.8% under different intensity conditions and obtained spectral features by phase difference corrections under the same measurement conditions. We reduced the effect of light intensity fluctuation and demodulated signals were freed from phase stability limitations.

Single-tone mid-infrared frequency modulation spectroscopy for sensitive detection of transient species.

A single-tone mid-infrared frequency modulation (MIR-FM) spectrometer consisting of a cw-OPO-based laser system, a 500 MHz LiTaO 3 electro-optical modulator (EOM), and a high-bandwidth GaAs mid-infrared detector has been developed. In order to assess the instrument's sensitivity and time resolution, FM spectra of selected CH 4 transitions around 3070 cm -1 were measured and the reaction Cl + CH 4 following the 193 nm excimer laser photolysis of oxalyl chloride was investigated by recording concentration-time profiles of HCl at 2925.90 cm -1 in a low-pressure slow-flow reactor. Furthermore, OH radicals were generated by UV photolysis of H 2O 2 and its transients were recorded at 3447.27 cm -1. The minimal detectable absorption of the spectrometer was determined to be A min=4⋅10-4 (Δ f BW=1 MHz, ν~=3447 cm -1) by using the Allan approach. Mainly due to thermal noise contributions of the easy-to-saturate photodetector, the detection limit is about a factor of 4 above the shot-noise limit. To the best of our knowledge, this work reports the first implementation of a single-tone MIR-FM spectrometer based on an external EOM modulation scheme and its use for the detection of transient molecular species.

Determination of the sign of the population difference in a two-level system by frequency-modulation spectroscopy

ABSTRACTPopulation inversions can occur in two-level systems as a result of chemical kinetic processes that involve pulsed excitation of a system with a large amount of excess energy, for example, multi-photon dissociation. In this work, we report the use of frequency-modulation (FM) spectroscopy to determine, from the phase of the FM signal, the sign of the population difference in a two-level system – specifically, whether the two-level system is inverted. As a proof of principle demonstration of this technique, we show that FM signals from an inverted I transition, prepared by a pulsed laser, have opposite phase with respect to signals from a non-inverted system, the Rb D-line absorption transition.

Recent Developments in Modulation Spectroscopy for Methane Detection Based on Tunable Diode Laser

In this review, methane absorption characteristics mainly in the near-infrared region and typical types of currently available semiconductor lasers are described. Wavelength modulation spectroscopy (WMS), frequency modulation spectroscopy (FMS), and two-tone frequency modulation spectroscopy (TTFMS), as major techniques in modulation spectroscopy, are presented in combination with the application of methane detection.

Cinnamomum zeylanicum Extract as Green Corrosion Inhibitor for Carbon Steel in Hydrochloric Acid Solutions

The extract of cinnamomum have been evaluated as green inhibitor for the corrosion of Carbon steel in 1M HCl solution was investigated using weight loss, potentiodynamic polarization, ac electrochemical impedance spectroscopy (EIS) , electrochemical frequency modulation (FEM) and energy dispersion spectroscopy (EDS) and scanning electron microscopy (SEM) methods of monitoring corrosion . The inhibitive property of the extract is attributed to the presence of cinnamic aldehyde as major constituent in the extract. measurements showed that this extract act as mixed-type inhibitor. The inhibition efficiency was found to increase with inhibitor concentration. Results obtained by various techniques are close to each other and maximum efficiency of 81.1 % is acknowledged at the inhibitor concentration of 600 ppm. Langmuir isotherm model is found most suitable to explain adsorption behavior of inhibitor for C-steel surface. Molecular adsorption of inhibitor over C- steel surface is found responsible for corrosion inhibition of C-steel in acid.