Analysis of multi-component fluorescence emission by phase-sensitive detection using one modulation frequency

Substantial progress in the field of light spectroscopy and imaging of tissues was achieved when the group of Chance, Patterson and Wilson showed that the optical parameters of a turbid medium can be obtained from time resolved measurements of short light pulses propagating in the medium (Patterson et al, 1991a). Essentially, a fit of the intensity as a function of time, measured at some distance from the source, can provide separately the values of the absorption and of the reduced scattering coefficients. This demonstration was important because the focus was shifted from attempts to separate the scattering from absorption, using empirical corrections to the Beer-Lambert law, to a rigorous application of a physical model. During the same period, our lab proposed employing the Fourier transform equivalent concept using an intensity modulated light source (Gratton et al, 1990). Since frequency domain methods have better resolution and sensitivity and are much faster than the time domain methods, our proposal was followed by many others including Chance, Patterson and Wilson (Boas et al, 1993, 1994; Cui and Ostrander, 1993; Duncan et al, 1993; Kaltenbach and Kaschke, 1993; O’Leary et al, 1992; Patterson et al, 1991b; Tromberg et al, 1993). It is well known that time domain and frequency domain measurements are mathematically equivalent, when the frequency domain measurement is carried out using a wide range of modulation frequencies (Gratton et al, 1983; Alcala et al, 1984). However, frequency domain measurements can be made at a single modulation frequency, thereby sacrificing some of the information. The advantage of measurements at a single frequency is that they can be very fast, accurate and have an excellent signal-to-noise ratio.

We present a novel phase unwrapping framework for the Time-of-Flight sensor that can match the performance of systems using two modulation frequencies, within a single shot. Our framework is based on an interleaved pixel arrangement, where a pixel measures phase at a different modulation frequency from its neighboring pixels. We demonstrate that: (1) it is practical to capture ToF images that contain phases from two frequencies in a single shot, with no loss in signal fidelity, (2) phase unwrapping can be effectively performed on such an interleaved phase image, and (3) our method preserves the original spatial resolution. We find that the output of our framework is comparable to results using two shots under separate modulation frequencies, and is significantly better than using a single modulation frequency.

The paper derives the theoretical expression of the cavity pulling factor in Galileo space passive hydrogen Maser, based on the 'single frequency modulation'. The sensitivity of the cavity pulling vs. the phase selection is deduced in order to account for the experimental errors in the phase adjustments. The theoretical calculation shows the new phase setting exists to achieve a zero cavity pulling, and the preliminary experimental results comparing with the two phase selection are presented.

The single frequency dithering technique for coherent beam combination with one single modulation frequency and without a reference beam is presented for the first time to our knowledge. Coherent beam combination of four fiber amplifiers is successfully demonstrated by using the single-dithering technique. When the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved by more than 85% from 8% in open loop, and the residual phase error is less than lambda/20. Experimental results reveal that the single-dithering technique has great potential in scaling to a large number of beamlets with a simple phase control system and low cost.

We present a distributed optical fiber sensor based on a hybrid Brillouin optical frequency/correlation-domain analysis (BOFDA/BOCDA) configuration for both static and dynamic strain measurements. Distributed static strain (or temperature) measurements are realized using the conventional BOFDA method, i.e., acquiring the baseband transfer function of the fiber through a vector network analyzer (VNA). With little modifications, the same setup can perform dynamic, position-selective measurements synthesizing a correlation peak through a frequency modulation of the laser source while operating the VNA at a single modulation frequency. Experimental tests, carried out at a sampling frequency up to 40 Hz and a spatial resolution of ≈5 cm, demonstrate the validity of the proposed approach.

Recently it has become possible to measure fluorescence phase-shift and modulation data over a wide range of modulation frequencies. In this paper we describe the analysis of these data by the method of nonlinear least squares to determine the values of the lifetimes and fractional intensities for a mixture of exponentially decaying fluorophores. Analyzing simulated data allowed us to determine those experimental factors that are most critical for successfully resolving the emissions from mixtures of fluorophores. The most critical factors are the accuracy of the experimental data, the relative difference of the individual decay times, and the inclusion of data measured at multiple emission wavelengths. After measuring at eight widely spaced modulation frequencies, additional measurements yielded only a modest increase in resolution. In particular, the uncertainty in the parameters decreased approximately as the reciprocal of the square root of the number of modulation frequencies. Our simulations showed that with presently available precision and data for one emission bandpass, two decay times could be accurately determined if their ratio were greater than or equal to 1.4. Three exponential decays could also be resolved, but only if the range of the lifetimes were fivefold or greater. To reliably determine closely-spaced decay times, the data were measured at multiple emission wavelengths so that the fractional intensities of the components could be varied. Also, independent knowledge of any of the parameters substantially increased the accuracy with which the remaining parameters could be determined. In the subsequent paper we present experimental results that broadly confirm the predicted resolving potential of variable-frequency phase-modulation fluorometry.

A tritiated water vapour monitor that utilizes modulation of the input tritium concentration to allow phase sensitive detection is described. This measurement technique is useful in those measurement situations where there are slowly varying interferences from other radiations or species; or where it is necessary to measure quite low concentrations. The increased immunity to interferences is because, in general, the frequency spectrum of the interferences is peaked at zero frequency and will not have major components at the modulation frequency. The increased sensitivity of the monitor is because it is not limited by input leakage currents nor slow drifts in zero.

Validation of wavelength modulation spectroscopy techniques for oxygen concentration measurement

We designed and built a measuring system to detect a very small optical rotation. The total measuring system consists of a laser, a Faraday modulator, a photo-detector, and a lock-in amplifier. To test the sensitivity of the system, we built another system to generate the small polarization rotation angle. The rotation-generating system includes a dense coil around a magneto-optic crystal, a power amplifier working at modulation frequency about 1 kHz. The rotationgenerating system can drive probe beam polarization by 2°due to the Faraday effect under 0-2 kHz modulation frequency. We monitor the frequency and amplitude of the modulating signal by observing the second harmonic signal received by the photo-detector. Some factors which influence the detection system have been analyzed experimentally to found the best working setup of the system. Recently we can detect an angle of magnitude at 10 -6 rad and the SNR of the output of the lock-in amplifier is above 331.The resolution of the angle detection is better than 10 -7 rad.

A new approach to electrode kinetics and dynamics by potential modulated Fourier transform infrared spectroscopy

In this article, recent research progress of laser induced fluorescence (LIF) diagnostics in ASIPP is reviewed. The fundamental goal of ASIPP's effort on the LIF diagnostics is to develop a working velocity distribution function (VDF) diagnostic to monitor helium ash removal in burning plasmas. Real-time monitoring of helium ash VDF requires consideration in radiation safety, measurement response time, and compatibility of tokamak operation, as well as ion/neutral VDFs with a temperature at possibly >20 eV. These considerations, almost unique to large-scale instruments or specifically to tokamak plasmas, require renewed research and development (R&D) efforts in optical design, automation, and expansion of laser functionality for the LIF diagnostics that were often not considered in conventional applications of the LIF diagnostics. To this end, ASIPP invested a series of research effort into the development of LIF techniques. Signal evaluation has been performed on a diagnostics-test device (LTS), and the obtained signal strength has been extrapolated to tokamak edge and divertor relevant environments with promising results. The extrapolations compared the available solid angle and plasma density between the test device and the design scenario where the LIF diagnostics will potentially be implemented in a tokamak edge and divertor plasma. The estimation neglects the higher density of energetic electrons in the edge and divertor regions, making it a conservative assessment. Automated post-DAQ processing of LIF signals using both “conventional” methods and an AI-augmented method were also explored. Both approaches yielded usable results, but the AI-trained method showed a faster response, which is advantageous for real-time plasma diagnostics. Basic mechanisms of LIF diagnostics were also explored. Specifically, the limitations of lock-in modulation was demonstrated to determine the limitation of time-resolved measurements using such methods. De-modulation and degradation of the LIF signal occurred as the modulation frequency exceeded 1/10th of the fluorescence frequency. This finding sets a practical limit of the modulation frequency up to which we can benefit from lock-in amplification. These published works along with other unpublished progress will be thoroughly discussed in this article to illustrate ASIPP's effort towards the implementation of LIF diagnostics in future tokamak devices and to promote the application of LIF diagnostics in other fields of research.

The Air Force Wright Research and Development Center and the Arnold Engineering Development Center are continuing a program for measuring optical effects of satellite material outgassing products on cryo-optic surfaces. This paper presents infrared (4000 to 700 cm-1) transmittance data for contaminant films condensed on a 77 K geranium window. From the transmittance data, the contaminant film refractive and absorptive indices (n, k) were derived using an analytical thin-film interference model with a nonlinear least-squares algorithm. To date 19 materials have been studied with the optical contants determined for 13 of those. The materials include adhesives, paints, composites, films, and lubricants. This program is continuing and properties for other materials will be available in the future.

A method is presented to measure the magnetic field vector in prominences by means of the polarimetric observations in the D3 line of He obtained with the High Altitude Observatory Stokes polarimeter. The characteristics of the observed Stokes profiles are discussed. The theory of the Hanle effect is reformulated in the representation of the irreducible tensors of the density matrix, and is generalized to derive the circular polarization profiles across the spectral line in terms of the intensity and direction of the prominence magnetic field. The circular polarization profile so deduced can be employed to obtain useful information which adds to that carried by the linear polarization observations. A non-linear least-squares algorithm is proposed to derive the measurement of the magnetic field from the observations, and a consistency check is suggested to test the adequacy of the theoretical model to describe the physics of the He I atomic excitation in prominences.

Summary New technologies that allow geothermal energy production in colder conditions result in interest for geothermal exploration in low heat flux regions. The Province of Québec, eastern Canada, is such a case. Mapping the Curie point depth (CPD) is appealing as an exploration tool due to the scarcity of the direct data. We have revisited a methodology to estimate the CPD using a fractal source distribution model and aeromagnetic data. Our methodology relies on a statistical model of crustal magnetization having a constant magnetization direction and random magnetization amplitude. The shape of the radial average of the logarithm of the power spectrum of magnetic anomalies is predicted using this model. The model parameters (thickness and depth to the top of the magnetic layer, the fractal exponent β and the constant C’) are obtained by calculating the best fit between the theoretical and observed radial power spectra using a non-linear least-square algorithm. Rather than using a constant value for the fractal exponent β, we propose a new calibration workflow based on heat flux measurements and lithology. This workflow includes the use of normal kriging of heat flux data. Mapping the CPD will help identifying potential areas for further detailed exploration programs.

Oriented patterns in an image often carry important information about the scene represented. Rao and Jain developed a technique to analyze images with oriented texture using phase portraits, where the parameters of a planar first-order phase portrait are locally estimated using a nonlinear least-squares algorithm. The method gives accurate results, but is computationally expensive. Shu and Jain proposed a faster linear method for the estimation of the parameters of the phase portrait. However, their formulation leads to the minimization of a different error measure, which is not as robust as the nonlinear least-squares procedure in the presence of noise, and also makes the implicit assumption that the orientation field was truly generated by a phase portrait model (with an extra weighting factor to compensate for noise sensitivity). We propose a new derivation of Shu and Jain's linear estimator that leads to similar estimation equations, while making explicit the nature of the error measure. Our procedure includes an iterative scheme, of which Shu and Jain's linear estimator is a particular case. We show that our estimator is more robust to noise than Shu and Jain's linear estimator.