Maximum Optical Path Difference Research Articles

Molecular-beam spectroscopy with an infinite interferometer: spectroscopic resolution and accuracy

An interferometer with effectively infinite maximum optical path difference removes the dominant resolution limit for interferometric spectroscopy. We present mass-correlated rotational Raman spectra that represent the world’s highest resolution scanned interferometric data and discuss the current and expected future limitations in achievable spectroscopic performance.

Wide-spectrum optical synthetic aperture imaging via spatial intensity interferometry

High resolution imaging is achieved using increasingly larger apertures and successively shorter wavelengths. Optical aperture synthesis is an important high-resolution imaging technology used in astronomy. Conventional long baseline amplitude interferometry is susceptible to uncontrollable phase fluctuations, and the technical difficulty increases rapidly as the wavelength decreases. The intensity interferometry inspired by HBT experiment is essentially insensitive to phase fluctuations, but suffers from a narrow spectral bandwidth which results in a lack of effective photons. In this study, we propose optical synthetic aperture imaging based on spatial intensity interferometry. This not only realizes diffraction-limited optical aperture synthesis in a single shot, but also enables imaging with a wide spectral bandwidth, which greatly improves the optical energy efficiency of intensity interferometry. And this method is insensitive to the optical path difference between the sub-apertures. Simulations and experiments present optical aperture synthesis diffraction-limited imaging through spatial intensity interferometry in a 100 nm spectral width of visible light, whose maximum optical path difference between the sub-apertures reaches 69<italic>λ</italic>. This technique is expected to provide a solution for optical aperture synthesis over kilometer-long baselines at optical wavelengths.

A wavelength calibration method for photoelastic-modulated Fourier transform spectrometers

PurposeAn interferogram is produced by modulating the difference between the extraordinary refractive index and the ordinary refractive index for photoelastic crystals in photoelastic-modulated Fourier transform spectrometers (PEM-FTs). Due to the influence of the refractive index dispersion characteristics on the maximum optical path difference of the interferogram, it is necessary to study wavelength calibration methods.Design/methodology/approachA wavelength calibration method for PEM-FTs was proposed based on the modulation principle of the photoelastic-modulated interferometer and the relationship between the maximum optical path difference and the refractive index difference. A 632.8 nm narrow-pulse laser was used as a reference source to measure the maximum optical path difference () of the interferogram, and the parameter was used to calculate the discrete frequency points in the frequency domain. To account for the influence of refractive index dispersion on the maximum optical path difference, the refractive index curve for the photoelastic crystal was used to adjust the discrete frequency coordinates.FindingsThe error in the reconstructed spectral frequency coordinates clearly decreased. The maximum relative error was 2.5%. A good solar absorption spectrum was obtained with a PEM-FT experimental platform and the wavelength calibration method.Originality/valueThe interferogram is produced by adjusting the difference between extraordinary refractive index and ordinary refractive index for the photoelastic crystal in the PEM-FTs. Given the wavelength dependence on the refractive indices, in view of the modulation principle of the photoelastic modulated interferometer, the relationship between the maximum optical path difference and the refractive index difference, the variation law of the refractive index of the photoelastic crystal and the process of spectral reconstruction is presented in this paper.

Learning a Fully Connected U-Net for Spectrum Reconstruction of Fourier Transform Imaging Spectrometers

Fourier transform imaging spectrometers (FTISs) are widely used in global hyperspectral remote sensing due to the advantages of high stability, high throughput, and high spectral resolution. Spectrum reconstruction (SpecR) is a classic problem of FTISs determining the acquired data quality and application potential. However, the state-of-the-art SpecR algorithms were restricted by the length of maximum optical path difference (MOPD) of FTISs and apodization processing, resulting in a decrease in spectral resolution; thus, the applications of FTISs were limited. In this study, a deep learning SpecR method, which directly learned an end-to-end mapping between the interference/spectrum information with limited MOPD and without apodization processing, was proposed. The mapping was represented as a fully connected U-Net (FCUN) that takes the interference fringes as the input and outputs the highly precise spectral curves. We trained the proposed FCUN model using the real spectra and simulated pulse spectra, as well as the corresponding simulated interference curves, and achieved good results. Additionally, the performance of the proposed FCUN on real interference and spectral datasets was explored. The FCUN could obtain similar spectral values compared with the state-of-the-art fast Fourier transform (FFT)-based method with only 150 and 200 points in the interferograms. The proposed method could be able to enhance the resolution of the reconstructed spectra in the case of insufficient MOPD. Moreover, the FCUN performed well in visual quality using noisy interferograms and gained nearly 70% to 80% relative improvement over FFT for the coefficient of mean relative error (MRE). All the results based on simulated and real satellite datasets showed that the reconstructed spectra of the FCUN were more consistent with the ideal spectrum compared with that of the traditional method, with higher PSNR and lower values of spectral angle (SA) and relative spectral quadratic error (RQE).

Design and laboratory performance of a fiber-fed Fourier transform spectrograph based on off-the-shelf components for astronomical medium and high-resolution spectroscopy

We present the design and performance verification of a fiber-fed Fourier transform spectrograph (FTS) for spectroscopy in the optical band with the ability to reach a maximum optical path difference of 15 mm and allowing for an adjustable spectral resolution (λ / Δλ) between 1 and 15,000. The designed FTS system was successfully constructed using only off-the-shelf optical components. The technique for correction of the phase distortion in the FTS using a metrology interferogram and cubic spline interpolation was investigated and discussed. The contrast performance and the instrument line shape of the FTS were measured and analyzed. To further verify the performance of the developed system, the absorption spectrum of the sunlight was measured and compared with a synthetic model with identified telluric and absorption lines. The result shows that the developed FTS can detect the absorption lines with a spectral resolution close to 15,000.

Spatial High Precision Scanning Control Method Based on Feedforward Closed-Loop Model Reference Adaptive

The time-modulated Fourier transform spectrometer realizes spectrum detection by scanning the optical path of the corner mirror. During the scanning process, the servo system is required to have high-precision and low-speed characteristics. Aiming at the fluctuation of scanning speed caused by spatial micro-vibration during scanning, a closed-loop model reference adaptive control algorithm based on feedforward is studied. The permanent magnet synchronous linear motor is used to drive the angle mirror to move back and forth along the guide rail to achieve large optical path and high-precision scanning with the maximum optical path difference of ± 34cm, the speed stability ≥ 99%.

Dependent factors for improving the spectral resolution and signal-to-noise ratio in a spatially modulated Fourier transform spectrometer comprised of a Wollaston prism

We studied the dependent factors for improving spectral resolution and signal-to-noise ratio (SNR) in a spatially modulated Fourier transform spectrometer comprised of a Wollaston prism. The obtainable spectral resolution was investigated for the factors such as the ratio of overlapped to nonoverlapped sectors in a focal plane, SNR, and the maximum optical path difference. The accurate application of signal padding for the interferogram was effective in obtaining better spectral resolution and SNR in the spatially modulated Fourier transform spectrometer based on the Wollaston prism, for which it is difficult to have adequate optical path difference. We investigated the change of spectral resolution and shape for the measured and signal padded interferograms of the 6382 cm − 1 center wavenumber light source. The best obtainable spectral resolution was drastically improved to 27 cm − 1, whereas the SNR was improved up to 12 times compared to that of the not signal padded.

New infrared absorption cross sections of difluoromethane (HFC-32) for atmospheric remote sensing

Difluoromethane (CH2F2; HFC-32), a hydrofluorocarbon (HFC) used as a refrigerant in air conditioning and heat pump systems, is currently being phased out under the terms of the Montreal Protocol (Kigali Amendment). In order to monitor its concentration profiles using infrared-sounding instruments, for example the Atmospheric Chemistry Experiment – Fourier transform spectrometer (ACE-FTS), accurate laboratory spectroscopic data are required. This work describes new high-resolution infrared absorption cross sections of difluoromethane / dry synthetic air over the spectral range 850–1335 cm−1, derived from spectra recorded using a high-resolution Fourier transform spectrometer (Bruker IFS 125HR) and a 26-cm-pathlength cell. Spectra were recorded at resolutions between 0.009 and 0.03 cm−1 (calculated as 0.9/MOPD; MOPD = maximum optical path difference) over a range of temperatures and pressures (7.6–760 Torr and 188–297 K). The new absorption cross sections in this work improve upon those currently available in HITRAN for remote sensing.

New infrared absorption cross sections for the infrared limb sounding of carbon tetrafluoride (CF4)

Carbon tetrafluoride (CF4), also known as tetrafluoromethane and sometimes CFC-14, has a natural lithospheric source, however the enhanced modern-day concentrations relative to this natural source have arisen from leaks into the atmosphere by industry, most significantly related to the production of aluminium and semiconductors. A potent greenhouse gas, with one of the longest atmospheric lifetimes of 50,000 years, the abundance of carbon tetrafluoride is steadily increasing in the atmosphere. In order to monitor its concentration profiles using infrared sounders, accurate laboratory spectroscopic data are required. This work describes new high-resolution infrared absorption cross sections of pure and air-broadened carbon tetrafluoride over the spectral range 1190–1336 cm−1, derived from spectra recorded using a high-resolution Fourier transform spectrometer (Bruker IFS 125HR) and a 26-cm-pathlength cell. Spectra were recorded at resolutions between 0.0018 and 0.03 cm−1 (calculated as 0.9/MOPD; MOPD = maximum optical path difference) over a range of atmospherically relevant temperatures (190 – 296 K) and pressures (up to 760 Torr). These new absorption cross sections improve upon those previously used for remote sensing, and will provide a more accurate basis for retrievals in the future.

New infrared absorption cross sections for the infrared limb sounding of sulfur hexafluoride (SF6)

Sulfur hexafluoride (SF6), widely used in industry for electrical insulation, is a potent greenhouse gas with a steadily increasing atmospheric abundance. In order to monitor its concentration profiles using infrared sounders, accurate laboratory spectroscopic data are required. This work describes new high-resolution infrared absorption cross sections of pure and air-broadened sulfur hexafluoride over the spectral range 780 – 1100 cm−1, derived from spectra recorded using a high-resolution Fourier transform spectrometer (Bruker IFS 125HR) and a 26-cm-pathlength cell. Spectra were recorded at resolutions between 0.002 and 0.03 cm−1 (calculated as 0.9/MOPD; MOPD = maximum optical path difference) over a range of atmospherically relevant temperatures (189 – 294 K) and pressures (up to 751 Torr). These new absorption cross sections improve upon those previously used for remote sensing, and will provide a more accurate basis for retrievals in the future.

Improvement of spectral resolution by signal padding method in the spatially modulated Fourier transform spectrometer based on a Sagnac interferometer.

We propose the signal padding method to solve the problem of low spectral resolution in the spatially modulated Fourier transform spectrometer. The required number of sampling data for the given optical path difference was calculated and applied to the experimental data. The spatially modulated Fourier transform spectrometer based on the Sagnac interferometer was fabricated and its maximum optical path difference was 8mm. A one-dimensional detector of 512pixels was used to measure the spectrum of a LED light source of 6451 cm-1 center wavenumber. The obtained spectral resolution in an experiment for 1mm optical path difference was 271 cm-1; however, it could be drastically improved to 31 cm-1 when the signal padding method was applied to the sidelobes of the measured interferogram.

Time-Domain Characterization of the Radiation Pattern of the Terahertz Photoconductive Antennas

We present a novel method to characterize the radiation pattern of the terahertz photoconductive antennas in time-domain. The method uses a modified version of the terahertz time-domain spectroscopy system, where we use the basics of physical optics and the optical path compensation principle using the delay line in the system in order to detect the spatial distribution of the terahertz radiation of the photoconductive antennas in two major planes. Using this method, we investigate two terahertz photoconductive antennas, namely dipole and spiral antennas, and compare the measurement results with simulations in time-domain. The comparison of the calculation and measurement results shows good agreement, where we measure an average and maximum optical path difference error of 4.7 and 8%, which results in an average and maximum half-power beam width error of 1.87 and 3.2°, respectively. We believe that the proposed method will be useful for predicting the system level performance of the terahertz time-domain spectroscopy systems in real-life applications.

Correction of the optical path difference on the tempo-spatially mixed modulated polarization imaging interference spectrometer

According to the technical requirements of the tempo-spatially mixed modulated polarization imaging interference spectrometer (TSMMPIIS), we designed and verified the maximum optical path difference (OPD) through two different schemes, and analyzed the influences of the field of view and calcite dispersion on the OPD. When the angles between the incident plane and the principal section of the Savart left plate were 0°, 45° and 90°, we simulated the variations of the OPDs changing with different parameters. We found that the variations were proportional to the thickness t and the incident angle i. In order to reduce the influences of the Savart polariscope dispersion and field of view on the OPD, the corrected models were established, and the correctness was validated by the experiment. This work would provide theoretical and practical guidance for the design and engineering of the TSMMPIIS.

Infrared absorption cross sections for air-broadened 1,1-dichloro-1-fluoroethane (HCFC-141b)

1,1-dichloro-1-fluoroethane (CC12FCH3; HCFC-141b), a hydrochlorofluorocarbon (HCFC) used as a foam blowing agent, a solvent in electronics, and for precision cleaning applications, is currently being phased out under the terms of the Montreal Protocol because it depletes stratospheric ozone. In order to monitor its concentration profiles using infrared-sounding instruments, for example the Atmospheric Chemistry Experiment – Fourier transform spectrometer (ACE-FTS), accurate laboratory spectroscopic data are required. This work describes new high-resolution infrared absorption cross sections of 1,1-dichloro-1-fluoroethane / dry synthetic air over the spectral range 705–1280 cm−1, derived from spectra recorded using a high-resolution Fourier transform spectrometer (Bruker IFS 125HR) and a 26 cm-pathlength cell. Spectra were recorded at resolutions between 0.01 and 0.03 cm−1 (calculated as 0.9/MOPD; MOPD = maximum optical path difference) over a range of temperatures and pressures (7.5–760 Torr and 188–295 K). This new cross-section dataset is the first for air-broadened HCFC-141b over a range of pressures and temperatures appropriate for atmospheric conditions.

Residual amplitude modulation and its mitigation in wedged electro-optic modulator.

We theoretically analyze and experimentally investigate the dependence of residual amplitude modulation (RAM) on the beam radius within the electro-optic crystal (EOC), the wedge angle of the EOC and the overlap efficiency between the extraordinary and ordinary beams, and the overlap efficiency is determined by the distance from the wedge facet to the downstream polarizer. The results show that the RAM with the maximum optical path difference Δ at the edge of light spot presents a sinc-like curve,and the magnitude of Δ is directly proportional to the beam radius and the wedge angle. As a scaling factor, with the decrease of the overlap efficiency between the ordinary and extraordinary beams, the RAM can be further reduced.

New and improved infrared absorption cross sections for trichlorofluoromethane (CFC-11)

Abstract. Trichlorofluoromethane (CFC-11), a widely used refrigerant throughout much of the twentieth century and a very potent (stratospheric) ozone-depleting substance (ODS), is now banned under the Montreal Protocol. With a long atmospheric lifetime, it will only slowly degrade in the atmosphere, so monitoring its vertical concentration profile using infrared-sounding instruments, and thereby validating stratospheric loss rates in atmospheric models, is of great importance; this in turn requires high-quality laboratory spectroscopic data. This work describes new high-resolution infrared absorption cross sections of trichlorofluoromethane/dry synthetic air over the spectral range 710–1290 cm−1, determined from spectra recorded using a high-resolution Fourier transform spectrometer (Bruker IFS 125HR) and a 26 cm pathlength cell. Spectra were recorded at resolutions between 0.01 and 0.03 cm−1 (calculated as 0.9/MOPD; MOPD: maximum optical path difference) over a range of temperatures and pressures (7.5–760 Torr and 192–293 K) appropriate for atmospheric conditions. This new cross-section dataset improves upon the one currently available in the HITRAN (HIgh-resolution TRANsmission) and GEISA (Gestion et Étude des Informations Spectroscopiques Atmosphériques) databases through an extension to the range of pressures and temperatures, improved signal-to-noise and wavenumber calibrations, the lack of channel fringing, the better consistency in integrated band intensities, and additionally the coverage of the weak combination band ν2+ν5.

Probing multipulse laser ablation by means of self-mixing interferometry.

In this work, self-mixing interferometry (SMI) is implemented inline to a laser microdrilling system to monitor the machining process by probing the ablation-induced plume. An analytical model based on the Sedov-Taylor blast wave equation is developed for the expansion of the process plume under multiple-pulse laser percussion drilling conditions. Signals were acquired during laser microdrilling of blind holes on stainless steel, copper alloy, pure titanium, and titanium nitride ceramic coating. The maximum optical path difference was measured from the signals to estimate the refractive index changes. An amplitude coefficient was derived by fitting the analytical model to the measured optical path differences. The morphology of the drilled holes was investigated in terms of maximum hole depth and dross height. The results indicate that the SMI signal rises when the ablation process is dominated by vaporization, changing the refractive index of the processing zone significantly. Such ablation conditions correspond to limited formation of dross. The results imply that SMI can be used as a nonintrusive tool in laser micromachining applications for monitoring the process quality in an indirect way.

Resolution improvement and general study in moving-optical-wedge Fourier transform spectrometer

We propose two modifications to the ordinary moving-optical-wedge interferometer as it works as Fourier transform spectrometer. First we propose new direction of motion of the wedge and the second is of structure of that moving wedge by attaching a mirror to it. We increase the maximum OPD by maximizing the displacement. On the other side we obtain factor (OPD/Displacement) less than two for immunity against motion error fluctuations. Upon using both of the two modifications at the same time we get very high resolution. Typically obtained values are fractions in (1/cm). Actually, we increase the maximum optical path difference to improve resolution. Further, we present a general study of the interferometer regarding: reflection, transmission, and phase difference in the interferometer. We discuss how these parameters affect performance of the system. These parameters degrade the interferometer performance.

Certain properties of the spread function of a Fourier spectrometer

The dependences of the spread function of a Fourier spectrometer on the wavenumber, maximum optical path difference, size of the field of view, and angle between the axes of the field of view and an interferometer are considered in this article.

Image quality and spectral performance evaluations of a polarization imaging spectrometer based on a Savart polariscope.

The modulation transfer function (MTF) and signal-to-noise ratio (SNR) are the key parameters to evaluate quantitatively the image quality and spectral performance in a polarization imaging spectrometer based on a Savart polariscope. In order to evaluate the image quality and reflect the detecting ability of the imaging spectrometer, calibration experiments on the MTF, SNR, and spectral resolution were carried out and some important conclusions were obtained. For incident radiance values 4.464, 3.119, and 0.523 w/m2·sr, the average SNRs of the interferogram were 500, 400, and 200 dB, respectively, and the MTF is 0.24. During the spectral resolution calibration, the maximum optical path difference was set as 57.08 µm, and the measured value is greater than the theoretical value, which is mainly caused by the structural design of the polarization imaging spectrometer. For the wavelength range of [500 nm, 600 nm], the SNR of the spectrum is lower and about 50 dB, while the SNR is obviously higher in a range of λ∈[600 nm, 960 nm]. This study provides a theoretical and practical guidance for improving the image quality and judging the spectral performance of the polarization imaging spectrometer.