Optical Sampling Research Articles

Rapid Magnetic Susceptibility Characterization of Coastal Morphosedimentary Units at Two Insular Strandplains in Estonia

Coastal archives of changing hydrometeorological conditions include mineralogical anomalies, such as heavy-mineral concentrations (HMCs) of variable thickness and intensity, which contain varying ferrimagnetic (e.g., magnetite) fractions. As an effective alternative to laborious mineralogical and granulometric analysis, we present the first set of bulk-volume low-field magnetic susceptibility (MS) databases from beach and dune lithosomes in the Western Estonian archipelago: Harilaid cuspate foreland (westernmost Saaremaa Island) and Tahkuna strandplain (northernmost Hiiumaa Island). Readings were conducted both in situ from trench walls and on core subsamples. At the Tahkuna site, late Holocene beach ridges reveal substantially lower values: quartz-dominated dune sequences grade from 5–20 μSI downward to diamagnetically dominated (−1–7 μSI) beach facies. Values are higher (20–140 μSI) in historically reactivated parabolic dunes that are encroaching southward over the strandplain. At the Harilaid site, four beach dune ridges (height: 2–3 m) that span the past 250–300 years show a general increase in mean MS from 320–850 μSI with decreasing age, with peaks of 1000–2000 μSI below the dune crests (depth: ~0.3–0.6 m) likely related to contemporary wind acceleration during ridge aggradation. The highest mineralogical anomalies range from 2000–5500 μSI in the historic dune sections and exceed 8000 μSI along the actively eroding upper-berm segments, typical of HMCs generated by moderate storms. MS anomalies are likely correlated with high-amplitude electromagnetic signal responses in georadar records and provide useful information for optical luminescence sampling strategies. Our study demonstrates that magnetic susceptibility trends provide a useful means of rapidly assessing relative temporal changes in overall wave/wind climates, help identify and correlate discrete anomalies related to extreme events, serve as local beach/dune boundary indicators, and represent potentially quantifiable paleo-energy indices.

First insight into seasonal variability of urban air quality of northern Pakistan: An emerging issue associated with health risks in Karakoram-Hindukush-Himalaya region

There is a dire need of air quality monitoring in the high-mountain areas of Karakoram-Hindu Kush-Himalaya (HKH) region, particularly related to the recent activities undergoing the China-Pakistan Economic Corridor (CPEC). This study presents the first baseline monitoring and evaluation findings from Gilgit city, Gilgit-Baltistan. Hourly data collection for air quality parameters (PM2.5, NO, NO2, SO2, O3 and CO) were measured using air-pointer (recordum, Austria) from 1 Jan 2018 to 31 Mar 2018 (winter) and 1 Jun 2018 to 31 Aug 2018 (summer). Our findings depict PM2.5 health limits were crossed in the winter season, while NO, NO2 and SO2 remained below their health limits. O3 and CO showed a rising trend in summer months, crossing the 8-h health limits during the season. Seasonal correlation in meteorology found an inverse relationship between most parameters and temperatures; reverse was true for O3 and CO. In parallel, thermal optical carbon analysis filter-based sampling characterized air quality into mass concentrations of PM2.5, organic carbon (OC), elemental carbon (EC) and various heavy metals. Filter-based PM2.5 correlated well with analyzer-based PM2.5 for all months that were studied, except February and March 2018. PM2.5, OC and EC were higher in summer as compared to winter, whereas higher heavy metal contributions were measured predominantly during summer. Health impacts were found to be above health limits for Ni in children only. Furthermore, principal component analysis-multiple linear regression (PCA-MLR) technique was applied to determine source apportionment, confirming the role of biomass burning in winters, and vehicular emissions in summers, highlighting the need for flexible monitoring of technologies/approaches, and communications among the various public, private agencies, and all relevant stakeholders.

High-Speed THz Time-of-Flight Imaging with Reflective Optics

In this study, we develop a 3D THz time-of-flight (TOF) imaging technique by using reflective optics to preserve the high-frequency components from a THz antenna. We use an Fe:InGaAs/InAlAs emitter containing relatively high-frequency components. THz-TOF imaging with asynchronous optical sampling (ASOPS) enables the rapid scanning of 100 Hz/scan with a time delay span of 100 ps. We characterize the transverse resolution using knife edge tests for a focal length of 5; the Rayleigh resolution has been measured at 1.0 mm at the focal plane. Conversely, the longitudinal resolution is determined by the temporal pulse width, confirmed with various gap structures enclosed by a quartz substrate. The phase analysis reveals that reflected waves from the top interface exhibit a phase shift when the gap is filled by high-indexed materials such as water but shows in-phase behavior when it is filled with air and low-indexed material. Our imaging tool was effective for inspecting the packaged chip with high lateral and longitudinal resolution. Importantly, the phase information in 2D and 3D images is shown to be a powerful tool in identifying the defect-in particular, delamination in the chip-which tends to be detrimental to the packaged chip's stability.

Imaging scatterometer for observing in situ changes to optical coatings during air annealing.

Annealing of amorphous optical coatings has been shown to generally reduce optical absorption, optical scattering, and mechanical loss, with higher temperature annealing giving better results. The achievable maximum temperatures are limited to the levels at which coating damage, such as crystallization, cracking, or bubbling, will occur. Coating damage caused by heating is typically only observed statically after annealing. An experimental method to dynamically observe how and over what temperature range such damage occurs during annealing is desirable as its results could inform manufacturing and annealing processes to ultimately achieve better coating performance. We developed a new, to the best of our knowledge, instrument that features an industrial annealing oven with holes cut into its sides for viewports to illuminate optical samples and observe their coating scatter and eventual damage mechanisms in situ and in real time during annealing. We present results that demonstrate in situ observation of changes to titania-doped tantala coatings on fused silica substrates. We obtain a spatial image (mapping) of the evolution of these changes during annealing, an advantage over x ray diffraction, electron beam, or Raman methods. We infer, based on other experiments in the literature, these changes to be due to crystallization. We further discuss the utility of this apparatus for observing other forms of coating damage such as cracking and blisters.

Terahertz Time-of-Flight Ranging with Adaptive Clock Asynchronous Optical Sampling.

We propose and implement a terahertz time-of-flight ranging system based on adaptive clock asynchronous optical sampling, where the timing jitter is corrected in real time to recover the depth information in the acquired interferograms after compensating for laser instabilities using electronic signal processing. Consequently, the involved measurement uncertainties caused by the timing jitter during the terahertz sampling process and the noise intensity of the terahertz electric field have been reduced by the utilization of the adaptive clock. The achieved uncertainty range is about 2.5 μm at a 5 cm distance after averaging the acquisition time of 1876 ms 5000 times, showing a significant improvement compared with the asynchronous optical sampling using a constant clock. The implemented terahertz ranging system only uses free-running mode-locked lasers without any phase-locked electronics, and this favors simple and robust operations for subsequent applications that extend beyond the laboratory conditions.

Single-cavity dual-comb fiber lasers and their applications

Single-cavity, dual-comb lasers are those specially designed mode-locked lasers that can emit more than one, asynchronous ultrashort pulse trains with stable repetition frequency difference between them. Unlike the long-studied, widely-used femtosecond lasers generating one stable pulse train, systematic investigation on them and their potential dual-comb applications only began, based on the fiber laser platform, around a decade ago, despite sporadic and limited reports of similar lasing phenomena since the beginning of the mode-locked laser studies. From a historic perspective, the birth of this novel technology is the lucky outcome of the timely collision of perpetual search for novel pulsing laser dynamics and concerted pursuit of open-minded solutions for out-of-lab dual-comb systems in the 2010s. In this review article, first, the current schemes to implement single-cavity dual optical frequency comb fiber lasers and their applications are summarized, based on the concept of multiplexed mode-locked lasers. The characteristics of reported single-cavity, dual-comb fiber lasers are discussed as well as their applications in spectroscopy, ranging, Terahertz (THz) spectroscopy, and asynchronous optical sampling (ASOPS). Finally, the more recent development of single-cavity, multi-comb lasers is presented.

Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers.

High-repetition-rate (up to GHz) femtosecond mode-locked lasers have attracted significant attention in many applications, such as broadband spectroscopy, high-speed optical sampling, and so on. In this paper, the characteristics of dispersion-managed, polarization-maintaining (PM) 1-GHz mode-locked fiber lasers were investigated both experimentally and numerically. Three compact and robust 1-GHz fiber lasers operating at anomalous, normal, and near-zero dispersion regimes were demonstrated, respectively. The net dispersion of the linear cavity is adjusted by changing types of PM erbium-doped fibers (EDFs) and semiconductor saturable absorber mirrors (SESAMs) in the cavity. Moreover, the long-term stability of the three mode-locked fiber lasers is proved without external control. In order to better understand the mode-locking dynamics of lasers, a numerical model was constructed for analysis of the 1-GHz fiber laser. Pulse evolution simulations have been carried out for soliton, dissipative-soliton, and stretched-pulse mode-locking regimes under different net dispersion conditions. Experimental results are basically in agreement with the numerical simulations.

Characterization of Gangliosides from Mouse Optic Nerve Samples Using Mass Spectrometry.

This chapter focuses on identifying gangliosides in the optic nerve of the mouse using mass spectrometry techniques. The described protocol will also permit the characterization of the sample's lipidome. Two deuterium-labeled ganglioside standards and a general lipid class standard will be utilized for extraction efficiency and quantification. Using reversed-phase high-performance liquid chromatography (HPLC) coupled to a Q Exactive mass spectrometer, the samples will be analyzed. The method will consist of both an untargeted approach and a targeted approach with a ganglioside-specific inclusion list.

A Field Trial of Multi-Homodyne Coherent Detection over Multi-Core Fiber for Encryption and Steganography

The concept of multi-homodyne coherent detection is used for stealth and secured optical communications, demonstrating a transmission rate of 20 Gbps under negative (in dB) optical signal-to-noise ratio (OSNR), in a deployed multi-core fiber (MCF) network. A 10 GBd QPSK signal is assigned with a bandwidth of multi-hundreds GHz, optically encoded with spectral phase mask, and transmitted below ASE noise in the MCF channel. To enable such transmission scheme, a semiconductor based gain-switched laser (GSL) frequency comb, used as both the signal carrier and local oscillator (LO), are then mixed in intradyne coherent receiver. The ultra-short time-domain overlapping between the pulsed signal and the pulsed LO allows the authorized user to collect the information, while discriminating most of the noise. Consequently, the electrical SNR is dramatically improved compared to the optical SNR, resulting in optical processing gain. Nevertheless, ultra-short pulses mixing is rather susceptible to a chromatic dispersion (CD), which imposes a major challenge in this spread-spectrum transmission technique. In this contribution, the effect of CD in multi-homodyne coherent detection is analyzed in an analytic model and in experiments. The model suggests a modified CD transfer function, comprised of the optical sampling of a conventional CD transfer function. In addition, the SNR penalty associated with the CD effect is quantified, as well as its dependency in propagation length and overall GSL's bandwidth. To circumvent the devastating effect of the CD, multi-core fibers are proposed. Two different weakly-coupled cores of a homogeneous MCF allow the signal and the LO to co-propagate through a very similar dispersive channel. A field trial in a deployed 6.29 km MCF network has validated an encrypted and stealthy 20 Gbps coherent transmission, at -2 dB OSNR.

A Gas-Solid Two-Phase Optical Sample Cell System for Detecting Gas Absorption Spectra at High Temperature in Particulate Environment

A Gas-Solid Two-Phase Optical Sample Cell System for Detecting Gas Absorption Spectra at High Temperature in Particulate Environment

Femtosecond Laser Distance Measurement Based on Electronically Controlled Optical Sampling

Femtosecond Laser Distance Measurement Based on Electronically Controlled Optical Sampling

Tunable compact asynchronous optical sampling system using Er-doped fiber laser

Abstract We report a compact, tunable, self-starting, all-fiber laser-based asynchronous optical sampling (ASOPS) system. Two Er-doped fiber oscillators were used as the pulsed-laser source, whose repetition rate could be set at 100 MHz with a tuning range of 1.25 MHz through a fiber delay line. By employing phase-locked and temperature control loops, the repetition rate offset of the two lasers was stabilized with 7.13 × 10−11 fractional instability at an average time of 1 s. Its capabilities in the terahertz regime were demonstrated by terahertz time-domain spectroscopy, achieving a spectral bandwidth of 3 THz with a dynamic range of 30 dB. The large range of repetition rate adjustment in our ASOPS system has the potential to be a powerful tool in the terahertz regime.

Optical recognition of constructs using hyperspectral imaging and detection (ORCHID)

Challenges to deep sample imaging have necessitated the development of special techniques such as spatially offset optical spectroscopy to collect signals that have travelled through several layers of tissue. However, these techniques provide only spectral information in one dimension (i.e., depth). Here, we describe a general and practical method, referred to as Optical Recognition of Constructs Using Hyperspectral Imaging and Detection (ORCHID). The sensing strategy integrates (1) the spatial offset detection concept by computationally binning 2D optical data associated with digital offsets based on selected radial pixel distances from the excitation source; (2) hyperspectral imaging using tunable filter; and (3) digital image binding and collation. ORCHID is a versatile modality that is designed to collect optical signals deep inside samples across three spatial (X, Y, Z) as well as spectral dimensions. The ORCHID method is applicable to various optical techniques that exhibit narrow-band structures, from Raman scattering to quantum dot luminescence. Samples containing surface-enhanced Raman scattering (SERS)-active gold nanostar probes and quantum dots embedded in gel were used to show a proof of principle for the ORCHID concept. The resulting hyperspectral data cube is shown to spatially locate target emitting nanoparticle volumes and provide spectral information for in-depth 3D imaging.

Prospects of Genetically Encoded Flim Indicators for the Quantitative Assessment of Intracellular Parameters

A significant number of genetically encoded indicators based on fluorescent proteins that allow detecting changes in various parameters: membrane potential shift, pH, concentrations of hydrogen peroxide, lactate, pyruvate, NAD+/NADH, ATP, calcium cations, etc. have been created. Some of them (for example, indicators of calcium cations and hydrogen peroxide) are successfully used by numerous groups of researchers in experiments in vivo. The fundamental limitation of most fluorescent indicators is their ability to only qualitatively assess the change in the parameter of interest in the cell. The nature of this limitation is that it is difficult to compare the absolute value of the measured parameter and the detected fluorescent signal for various reasons. Quantitative measurements impose an additional requirement on the indicator signal: it should not depend on the technical features of the experiment (such as the level of protein expression, the type and power of the excitation source, a set of fluorescent filters, etc.). The fulfillment of this requirement may be associated with the detection of the indicator signal in the time domain, where the main characteristic of the signal is the fluorescence lifetime. The fluorescence lifetime is an absolute value, insensitive to changes in the intensity of the excitation source, detection settings, losses in the optical path or sample, changes in the concentration of the fluorophore in the sample, photobleaching. Thus, when calibrating an indicator in the time domain, a specific value of the measured parameter is associated with a specific value of the fluorescence lifetime. The time-resolved imaging mode imposes its own limitations on the properties of the indicator's fluorescent core. Thus, the lifetime of its fluorescence should change in proportion to the value of the measured parameter, which is not characteristic of any fluorophore. In this work, we make a brief review of the principles of detection of the fluorescent signal of genetically encoded indicators, focusing on current approaches related to the analysis of time-resolved data.

Conditional H i Mass Functions and the H i-to-halo Mass Relation in the Local Universe

We present a new H i mass estimator that relates (M HI/M *) to a linear combination of four galaxy properties: stellar surface mass density, color index u − r, stellar mass, and concentration index, with the scatter of individual galaxies around the mean H i mass modeled with a Gaussian distribution function. We calibrate the estimator using the xGASS sample, including both H i detection and nondetection, and constrain the model parameters through Bayesian inferences. Tests with mock catalogs demonstrate that our estimator provides unbiased H i masses for optical samples like SDSS. We apply our estimator to the SDSS spectroscopic sample to estimate the H i mass function (HIMF) of local galaxies, as well as the conditional H i mass function in galaxy groups and the H i–halo mass relation. Our HIMF agrees with the ALFALFA measurements at M HI ≳ 5 × 109 M ⊙, but with higher amplitude and a steeper slope at lower masses. We show that this discrepancy is caused primarily by the cosmic variance, which is corrected for the SDSS sample but not for ALFALFA. The total CHIMFs for all halo masses can be described by a single Schechter function, while those of central galaxies show a double-Gaussian profile. The total H i mass in a group increases monotonically with halo mass, but for central galaxies, the H i mass shows weak dependence on halo mass when M h ≳ 1012 M ☉. The observed H i–halo mass relation is not reproduced by current hydrodynamic simulations and semianalytic models of galaxy formation.

Masers in evolved stars; the Bulge Asymmetries and Dynamical Evolution (BAaDE) Survey

AbstractThe Bulge Asymmetries and Dynamic Evolution (BAaDE) project attempts to improve our knowledge about the structure and dynamics of the inner Milky Way galaxy by sampling tens of thousands of infrared color-selected evolved stars with circumstellar envelopes (CSEs). The SiO masers in these CSEs yield the object’s line-of-sight velocity instantly and accurately, and together provide a sample of point-mass particles that are complementary to high-mass star formation masers typically found in the Galactic Disk as well as to optical samples that cannot reach into the Galactic Plane and Bulge due to extremely high visual extinction. This presentation introduces the BAaDE survey and highlights current results.

Fast THz-TDS Reflection Imaging with ECOPS-Point-by-Point versus Line-by-Line Scanning.

We built a high-speed TDS setup with the use of electronically controlled optical sampling (ECOPS), which can measure up to 1600 terahertz pulses per second. The movement of the sample was provided by two fast-speed motorized linear stages constituting the gantry. We developed a flat-bar-based metal marker approach for the synchronization of continuous line-by-line scans. We carefully compared the performance of the terahertz reflection time-domain spectroscopy (TDS) scanner operating in a slow point-by-point and a one-hundred-times faster line-by-line imaging scheme. We analyzed images obtained for both schemes for a uniform metallic breadboard with holes, as well as a glass composite sample with defects. Although the measurement time was reduced by 100 times in terms of the line-by-line scheme, the overall performance in both schemes was almost identical in terms of the defects' sizes, shapes and locations. The results proved that the proposed ECOPS TDS system can provide uniform and extremely fast scanning without any deterioration in image quality.

Determination of a High-Power THz Detector for EA-FEL Radiation Using Optical Sampling of GaAs and ZnTe Crystals

This paper presents a diagnostic method for THz pulses produced by EA-FEL. Experimental results present a comparison between ZnTe and GaAs crystals for the detection of a single terahertz pulse using electro-optic sampling. In order to match an electro-optic detector for the EA-FEL radiation, the THz pulse from a source was simultaneously detected by ZnTe and GaAs electro-optic detectors. The GaAs detection system was found to have a shorter response time but low signal-to-noise ratio (SNR) compared to the ZnTe system. The GaAs crystal is suitable for the detection of short THz pulses due to fiber coupling, and the SNR of the GaAs system can be improved using a faster and more sensitive photodetector.

Terahertz PHASR Scanner with 2 kHz, 100 picosecond Time-Domain Trace Acquisition Rate and an Extended Field-of-View Based on a Heliostat Design.

Recently, we introduced a Portable HAndheld Spectral Reflection (PHASR) Scanner to allow THz Time-Domain Spectroscopic (THz-TDS) imaging in clinical and industrial settings using a fiber-coupled and alignment-free telecentric beam steering design. The key limitations of the version 1.0 of the PHASR Scanner were its field-of-view and speed of time-domain trace acquisition. In this paper, we address these limitations by introducing a heliostat geometry for beam scanning to achieve an extended field-of-view, and by reconfiguring the Asynchronous OPtical Sampling (ASOPS) system to perform Electronically Controlled OPtical Sampling (ECOPS) measurements. The former change improved the deflection range of the beam, while also drastically reducing the coupling of the two scanning axes, the combination of which resulted in a larger than four-fold increase in the FOV area. The latter change significantly improves the acquisition speed and frequency domain performance simultaneously by improving measurement efficiency. To accomplish this, we characterized the non-linear time-axis sampling behavior of the electro-mechanical system in the ECOPS mode. We proposed methods to model and correct the non-linear time-axis distortions and tested the performance of the high-speed ECOPS trace acquisition. Therefore, here we introduce the PHASR Scanner version 2.0, which is capable of imaging a 40×27 mm2 FOV with 2000 traces per second over a 100 picosecond TDS range. This new scanner represents a significant leap towards translating the THz-TDS technology from the lab bench to the bedside for real-time clinical imaging applications.

Systematic Study of LED Stimulated Recovery of Radiation Damage in Optical Materials

The radiation damage in optical materials mostly manifests itself as the loss of optical transmission. The optical materials recover from radiation damage to some extent in the presence of natural light, and at a faster rate in the presence of stimulating light. On the other hand, the systematic study of the dynamics of the recovery as a function of the stimulating light parameters such as its wavelength, intensity and exposure duration and method has not been performed in detail. We established an LED recovery station which provides pulsed and continuous light at various wavelengths at custom geometries. The study starts with the irradiation of optical samples at various gamma doses at a rate of 87.5 Gy/min. The optical transmittance of the samples are then measured in 200 nm - 1500 nm range for an extended period of time. Here we report on the details of the irradiation and recovery setups, and the results of recovery from radiation damage under different light exposure mechanisms.