Sensitive Optical Detection Research Articles

Laser-based refractive-index detection for capillary electrophoresis: ray-tracing interference theory

The fringe pattern observed in a far field after a laser beam illuminates a fused silica capillary immersed in a refractive-index matching material and filled with an analyte fluid is exploited to develop a sensitive optical detector for capillary chemical analysis. The inner capillary interface splits the laser beam into a reflected beam fan and a refracted beam fan, which, on overlapping in the far field, lead to interferences. The intensity and the position of the fringes for capillaries with 250 microm >/= i.d. (inner diameter) >/= 25 microm are well reproduced by the presented model. The calculation predicts the fringe pattern for various beam/i.d. geometric configurations and is used to optimize the performance of the nanoliter-picoliter refractive-index on-column detection studied. It is found that the best contrast corresponds to a capillary that is illuminated with a beam waist of omega(0) ~ i.d./12, which is off-center focused with an offset of s ~ i.d./2. For a given interference pattern, the fringes that are found to be more sensitive to Deltan are those that appear near the optical axis but still retain high intensity and contrast. The sensitivity increases approximately linearly with the fringe number, and the maximal fringe number increases proportionally with the i.d.

Influence of DC Electric Field on Optical Detection by Short-Gap Electrode Glow Discharge in DC Magnetic Field

In optical detection of a short-gap electrode glow discharge tube (PIG type), the influence of DC electric field on the optical detection sensitivity is discussed. The sensitivity decreases with increasing DC degree of ionization. This tendency is marked in the range from normal glow to abnormal glow.

Optical detection of partial discharges using fluorescent fiber

An optoelectronic system for detecting partial discharges in a sheltered HV device is described. The system consists of a fluorescent plastic optical fiber, with photomultipliers or photodiodes measuring the light intensity at the end of the fiber. The result depends on various factors that might influence the optical detection sensitivity. Information is given on commercially available fluorescent fibers to facilitate the best choice of fiber/photodetector pair for a given application. >

Optical discharge detection in SF/sub 6/-insulated systems

A description is given of an optoelectronic system for detecting and locating corona discharges in SF/sub 6/-filled gas-insulated systems (GIS). The system consists of optical fiber links, photomultiplier light detectors, an operational amplifier, and a computer-based data collection system. Various factors that can influence the optical discharge detection sensitivity are investigated. It is shown that an optical system can detect discharges that are located even in the hidden areas in GIS. The detector can sense corona discharges of >

Highly sensitive NO 2 optical detector with squarylium dye Langmuir-Blodgett film containing J aggregate

A highly sensitive optical gas detector based on the fluorescence quenching of a squarylium dye Langmuir-Blodgett film is reported. A squarylium dye Langmuir-Blodgett film containing J aggregate deposited on a substrate emits fluorescence from the J aggregate by excitations of He-Ne laser (632.8 nm) and laser diode (750 nm). This fluorescence was observed to be quenched quickly and reversibly by the presence of NO 2 in an air atmosphere. The change in fluorescence intensity can be easily detected by an NO 2 level of only parts per billion. High sensitivity was discussed in relation to the high mobility of excited states in the J aggregate.

光ファイバ・レーザを用いた磁性流体の流速測定法

A new method to measure the velocity distribution of magnetic fluid flows using optical fibers and laser beam was developed. In this method, the local velocity of the magnetic fluid was obtained by measuring the time taken for a tracer particle to pass between two sets of fibers. After this method was adjusted with the rotational flow in a rotating channel, it was applied to the flow of a magnetic fluid in a circulation channel excited by a travelling magnetic field made by a stator of a single-sided linear induction motor. From the measurements, the induced velocity was found to be proportional to the frequency and the strength of the applied current to the inductor. It was important to choose the size of tracer particles, the wavelength of the light source and the sensitive optical detector for better results.

Infrared optical detector using superconducting oxide thin film

A BaPb 0.7Bi 0.3O 3 (BPB) polycrystalline thin film makes a highly sensitive optical detector. Optical signals create quasiparticles and induce changes in the superconducting energy gap. These changes are measured by Josephson tunneling junctions which are formed at the grain boundaries in the polycrystalline film. The detector sensitivity is about 10 4 V/W in the wavelength range of above 1 μm and the detector is capable of responding up to 1 GHz at the wavelength of 1 μm. New high T c superconducting oxide thin films also can detect optical signals. The response is three orders of magnitude slower than that of a BPB film. The present detector works as a bolometer.

High sensitivity optical detection methods in hydroxide eluent suppressed anion chromatography via postsuppression ion exchange

Detection by post suppression ion exchange, as an adjunct to suppressed hydroxide eluent ion chromatography, involves exchange of eluting sample anion or concurrently present H/sup +/ for a fluorescent/optically absorbing anion or cation such as anthranilate or Ce/sup 3 +/. Membrane-based post suppression devices permit reliable continuous operation, and detection limits can be better than those attainable by conductivity detection. The anion exchange membrane-anthranilate system also yields excellent detection limits by UV absorption except that membrane degradation is a problem. The ion exchange at the membrane interface is not an equilibrium process. The dependence of the membrane response on an external high frequency electrical field suggests that the transport of the ions through the membrane is not a rate-determining factor.

Precision spectroscopy on stored atomic ions

Examples of recent experiments are presented which show characteristic features associated with the technique of ion trapping: Ultrahigh resolution in hyperfine spectroscopy due to long coherence time, lifetime measurements for very long living states and rare isotope investigation due to the high sensitivity of optical detection of stored ions. The possibility of experiments of fundamental quantum mechanical interest will be pointed out. In addition a few open questions associated with the behaviour of large ion clouds, which might influence the precision of future experiments, will be discussed.

Extremely Sensitive Optical Detection System for Stopped‐flow Measurements of Reactions with Very High Rate Constants or Small Absorbance Changes

AbstractStopped‐flow experiments are usually performed with optical detection. In case of low absorptivities or low concentrations of the measured substances a very high sensitivity is necessary. A special optical detection system with this feature is described. By a differential configuration of the detectors and the use of high light intensities fluctuations of the light source and detector shot noise were greatly reduced in their effects. The detection system is capable of performing extremely sensitive kinetic measurements from 290 to 620 nm within the time‐scale of stopped‐flow experiments. Kinetic traces with a full scale absorbance down to 5 · 10−4 were recorded. In this extreme case, the signal to noise ratio was still as good as 25:1. With typical values for absorptivities, optical path length, and dead time, even diffusion controlled electron transfer reactions could thus be observed in a stopped‐flow apparatus.

Development of a coherent OTDR instrument

Optical time domain reflectometry (OTDR) is a commonly used technique for characterization and fault location of optical fiber transmission systems. It involves measuring the fraction of a probe pulse that is scattered back (by Rayleigh scattering) from a silica fiber. Because of the very small levels of backscatter in single-mode fiber at long wavelengths, very sensitive optical detection is necessary to achieve adequate range performance. This paper gives the principles of operation and performance of a coherent OTDR system designed for use at 1.3 μm and its realization as a robust field-portable instrument with a specified performance of better than 24-dB one-way range, and 0.2-dB fault loss resolution. Coherent detection gives ultimate receiver sensitivity and also reduces the dynamic range requirements of the electronic signal processing. To realize these advantages, a novel technique is used to provide the necessary highly coherent source from a semiconductor laser. Sophisticated Signal processing techniques are employed both pre- and post-detection to integrate the signal out of the noise and also to reduce coherent fading effects. The various components of the system and their development into a robust instrument are described, and the performance of the system over long lengths of fiber both in the laboratory and more importantly on installed transmission links is presented in detail, showing achieved performance considerably better than this specified minimum.

Optical detector using superconducting BaPb0.7Bi0.3O3 thin films

This paper describes a highly sensitive optical detector using a perovskite-type superconductor, BaPb0.7Bi0.3O3 (BPB) thin film. Optical signals create quasiparticles and induce changes in the superconducting energy gap. These changes are measured by tunnel junctions which are formed along the grain boundaries in the BPB polycrystalline thin film. The detector sensitivity is about 104 V/W in the 1–8-μm wavelength range, and the detector is found to be capable of responding up to 1.3 GHz at the 1.3-μm wavelength. Consequently, this BPB detector is well suited in infrared spectrometer and optical-communications system applications.

Fluorescence detected circular dichroism as a detection principle in high-performance liquid chromatography

A highly sensitive and selective laser-based detector for high-performance liquid chromatography is presented. The detector combines the selectivity of fluorescence and the selectivity of optical activity to provide specificity. A HeCd laser operating at 325 nm with about 8 mW power is used with electro-optic modulation at 150 kHz. As a highly sensitive optical activity detector, the detection limit is 170 pg injected riboflavin using conventional liquid chromatography.

Optical Detector Using Superconducting BaPb0.7Bi0.3O3 (BPB) Thin Films

Highly sensitive optical detectors have been fabricated by using perovskite type superconductor BaPb0.7Bi0.3O3 (BPB) thin films. Optical signals create quasi-particles and induce changes in the superconducting order parameter. These changes are measured by tunneling junctions along grain boundaries in the BPB polycrystalline thin films. The sensitivity is about 103 V/W in the 1.0∼10 µm wavelength range and they can respond up to 600 MHz at the wavelength of 3.2 µm. The observed results suggest that the BPB detectors are suitable for application in optical communication systems and infrared spectrometers.

Current work at the Freie Universität Berlin

Measurements of the Stark-effect in 1Σ-state diatomics have been extended to molecules with two strongly coupling nuclei. Rotational spectra of the 2Σ-radicals BaF, BaCl, SrF, SrCl, CaCl and CaBr have been obtained in the mm-wavelength region. Observation of transitions at lower frequencies requires the use of sensitive optical detection schemes. The new technique of “Microwave-Optical Polarization Spectroscopy” (MOPS) exceeds previous MODR-experiments by more than one order of magnitude both in sensitivity and resolution. Double resonance experiments in a molecular beam are the most promising tool for the measurement of dipole moments in these unstable molecules.

Raman spectroscopy with intensified vidicon detectors: A study of intact bovine lens proteins

AbstractRaman spectra of intact ocular lenses contain important structural information related to aging and cataract formation. However, the direct application of Raman spectroscopy to human eyes for cataract diagnosis has been considered unsafe because of the high laser power (∼100 mW) required to generate Raman spectra using the photomultiplier‐photon counting (PMT‐PC) detection method. The clinical application of Raman spectroscopy has become more likely with the development of sensitive multichannel optical detectors such as the ‘silicon intensified target’ (SIT) coupled to an optical multichannel analyzer (OMA). We have incorporated the SIT in a novel mismatched subtractive dispersion double monochromator that has optimum stray light (<5×10−9), throughput, and resolution for these experiments. It is demonstrated that the new SIT–OMA detection method is capable of obtaining high‐quality Raman spectra of an intact bovine lens with a laser power (∼1 mW) significantly below the threshold for retinal damage. The multichannel and photon counting detection systems have been carefully compared under high and low light level conditions so that the advantages of the SIT‐OMA system for other Raman experiments may be easily evaluated.

Fluorescence Limitations to Combustion Studies Using Raman Spectroscopy

Raman spectroscopy has the potential of providing detailed temperature and composition measurements at discrete points within a mixture of gases. Although cross sections for Raman scattering are characteristically very small, recent developments in high power lasers and sensitive optical detectors have resulted in the application of Raman spectroscopy to a variety of new experimental studies. Current applications include the remote measurement of atmospheric constituents and temperatures, concentration measurements in heterogeneous, nonreacting gasdynamic processes, remote detection of pollutants in gas turbine exhaust, and temperature and composition measurements in steady flame gases.

Synchronous Detection vs Pulse Counting for Sensitive Photomultiplier Detection Systems

Synchronous detection is compared experimentally with pulse counting, as techniques for use with photomultipliers in very sensitive optical detection systems. The criterion for comparison is the signalto-noise ratio, defined as the ratio of the value of a measurement to its standard deviation. Care is taken to obtain optimum S/N with each method. An EMI 9558B tube is used. Some observations are as follows. (1) For light levels below the dark current equivalent level, pulse counting gives somewhat better S/N than synchronous detection. The improvement in S/N is by a factor of three at 6 x 10(-17) W, the lowest illumination tested. The advantage of pulse counting increases as illumination decreases. On the other hand, for light levels of the order of the dark current equivalent level or larger, synchronous detection, pulse counting, and simple de current measurement give substantially equal results. (2) With this tube, signal-to-noise ratios are only slightly improved by cooling. The improvement in S/N is of the order of a factor of 1.5 for pulse counting and of the order of 3 for synchronous detection.