Recovery Of Line Shapes Research Articles

Practical high-resolution spectroscopy with a spatial heterodyne spectrometer: Determination of instrumental function for lineshape recovery

The spatial heterodyne spectrometer (SHS) is a well-recognized platform for its high resolving power in various use cases of spectroscopy. Same as other spectrometer topologies, the SHS, unfortunately, also suffers from classical challenges such as distorted lineshape due to the instrumental function. The goal of this work is to tackle this persisting issue through a simple numerical approach. With the inherent characteristics of an SHS interferogram, we report the direct extraction and determination of the instrumental function in its numerical representation from an SHS interferogram; this instrumental function was further used for spectral data processing that enables significant improvements in spectral resolution through deconvolution algorithms.Here, we systematically discuss the recognition of the embedded instrumental function among various ingredients within an interferogram. To verify the numerical approach, lithium was chosen as the model sample, resembling the use of SHS in an isotopic analysis application. Specifically, the resonance transition of lithium D-lines (2P1/2,3/2 ← 2S1/2) was selected to assess the performance of the spectral processing. With the spectral deconvolution, the spectral features that represent the 6Li and 7Li were nearly baseline-separated, allowing for the accurate measure of the isotopic abundance without external references or algorithm adjustments (e.g., curve fitting).

Recovery of Absorption Line Shapes With Correction for the Wavelength Modulation Characteristics of DFB Lasers

Tunable diode laser spectroscopy combined with wavelength modulation spectroscopy (WMS) is an important technique for noninvasive measurements of gas parameters such as pressure, concentration, and temperature in high-noise harsh environments. A variety of laser types are used for these applications, and the modulation characteristics can have significant effects on line shape recovery. Here, we identify important characteristics of distributed feedback (DFB) lasers that need to be taken into account in the context of WMS and illustrate the effects with a 2- $\mu\textrm{m}$ wavelength multiquantum-well DFB laser used for CO2 detection. The modulation response of the laser is measured, and we demonstrate how the phasor decomposition method (PDM) may be used to obtain accurate line shapes from the first harmonic WMS signals by correcting for phase variation across the laser's low-frequency current sweep. We also demonstrate how the PDM approach can be improved by removing the need to preset the orientation of the lock-in axis to isolate the residual amplitude modulation component, making it more suitable for field applications.

Recovery of Absolute Absorption Line Shapes in Tunable Diode Laser Spectroscopy Using External Amplitude Modulation With Balanced Detection

Accurate recovery of an absorption lineshape is important in many industrial applications for simultaneous measurement of gas concentration and pressure or temperature. Here, we demonstrate a method, based on a modification to the Hobbs balanced receiver circuit, for background signal nulling when external amplitude modulation of the laser output is used. Compared with direct or non-nulled detection techniques, we demonstrate that the method significantly improves the signal-to-noise ratio to a level comparable with that of the conventional second harmonic wavelength modulation spectroscopy. Most importantly, normalisation and recovery of the lineshape is straightforward and immune to the difficulties that afflict lineshape recovery with the conventional wavelength modulation spectroscopy.

Excitation intensity dependence of plasmonic enhancement of energy transfer between quantum dots

We studied the impact excitation power on the plasmonic enhancement of Forster resonance energy transfer (FRET) between CdSe/ZnS colloidal dots. We showed that when such quantum dots (QDs) were in the vicinity of metallic nanoparticles (MNPs), with the increase of the excitation laser intensity the spectral width of their emission was initially increased and then decreased, reaching its original value when the laser intensity was low. Such a spectral lineshape recovery happened while the emission peak wavelength of the quantum dots underwent a significant amount of red shift. By tuning the exciton–plasmon coupling strength and including the effects of the heat generated by plasmonic absorption, we showed that these results could be explained in terms of transition from a donor-dominated spectrum at low laser intensity to an acceptor-dominated one at high laser intensity. This suggests that excitation intensity can increase the rate of FRET between colloidal QDs when they are in close proximity to MNPs.

Recovery of Absolute Gas Absorption Line Shapes Using Tuneable Diode Laser Spectroscopy with Wavelength Modulation ¿ Part I: Theoretical Analysis

Tunable diode laser spectroscopy is extremely important for gas detection in a wide variety of industrial, safety and environmental monitoring applications. Of particular interest is the development of calibration-free, stand-alone systems and instrumentation which can operate in high-temperature or high-pressure environments (such as in fuel cells, or gas turbine engines) where continuous and simultaneous monitoring of pressure, temperature and concentration of gases may be required. Here, in Part 1 of this two-part paper, we present the full theoretical basis and range of techniques for calibration-free line-shape recovery to allow simultaneous measurements of concentration and pressure/temperature for a wide range of potential applications. Firstly, on the basis of Fourier analysis, we present the general signal components that arise with both intensity and frequency modulation of diode lasers and identify the issues and difficulties associated with accurate line-shape recovery in conventional wavelength modulation spectroscopy (WMS). We then show how line-shape recovery may be effectively performed using first harmonic signals and, by use of a general correction function from Fourier coefficients, we extend the techniques previously reported to include arbitrary large modulation indices, different line-shape profiles and high gas concentration with non-linear absorption. Previous approximate techniques based on Taylor series expansions are included as a special case of the Fourier analysis for low modulation indices. We also show that the signal amplitudes obtained in this way can be comparable to, or even exceed, that of conventional WMS by appropriate choice of the modulation index and frequency.

Absorption line profile recovery based on residual amplitude modulation and first harmonic integration methods in photoacoustic gas sensing

Two methods for recovery of gas absorption line profiles are presented in this paper using photoacoustic spectroscopy and tunable diode laser spectroscopy (TDLS) with wavelength modulation (WM). A theoretical analysis based on Fourier coefficients is given in order to describe the various components that arise under simultaneous intensity and frequency modulation. The first method makes use of the residual amplitude modulation (RAM) signal which is always present in current modulation of distributed feedback (DFB) tunable diode lasers. The second method involves integration of a near-pure first harmonic derivative signal, separated from other distorting components by appropriate choice of the lock-in detection phase in the case of low modulation index. Good agreement is obtained with both methods between the experimental results and the theoretical simulation for the P17 absorption line of acetylene at 1535.39 nm but the second method gives a much improved accuracy and signal-to-noise ratio in lineshape recovery with photoacoustic spectroscopy.

Laser line shape recovery system based on a double pixelated axilens

We describe a scanning system for shape recovery that employs a pair of one-dimensional interlaced axilenses encoded onto a reflective liquid-crystal spatial light modulator. The design of these axilenses with opposite depth presets allows the generation of a high-quality laser line pattern with extended depth of focus, which is applied in the shape recovery. Other attributes of the system allow the scanning of objects whose lateral dimensions are larger that those of the spatial light modulator itself. The high performance of the scanning system is demonstrated by its application to the shape recovery of ceramic objects.

Rapid-scan EPR with triangular scans and fourier deconvolution to recover the slow-scan spectrum

Direct-detected rapid-scan EPR signals were recorded using triangular field scan rates between 1.7 and 150 kG/s for deoxygenated samples of lithium phthalocyanine (LiPc) and Nycomed trityl-CD 3. These scan rates are rapid relative to the reciprocals of the electron spin relaxation times and cause characteristic oscillations in the signals. Fourier deconvolution with an analytical function permitted recovery of lineshapes that are in good agreement with experimental slow-scan spectra. Unlike slow-scan EPR, direct detection rapid-scan EPR does not use phase sensitive detection and records the absorption signal directly instead of the first derivative of the absorption signal. The amplitude of the signal decreases approximately linearly with applied magnetic field gradient. Images of phantoms constructed from samples of LiPc and trityl-CD 3 were reconstructed by filtered back-projection from data sets with a missing angle. The lineshapes in spectral slices from the image are in good agreement with slow-scan spectra and the spacing between sample tubes matches well with the known sample geometry.

Asymmetric spectral lineshape recovery using higher order photon-correlation spectroscopy

The feasibility of recovering the spectral lineshape of a scattered light field using a higher-order photon-correlation spectroscopy technique is investigated. The proposed analysis procedure is outlined and a description is given of a laboratory experiment, in which an estimate is made of the lineshape of light scattered by an arrangement designed to produce a well-defined and asymmetric lineshape. The results indicate that it is feasible to recover an asymmetric lineshape, although with limited resolution. Potential applications associated with particle-sizing using light scattering techniques are discussed.