Source Intensity Fluctuations Research Articles

Self-Referencing Fiber-Optic Intensity Sensors Using Ring Resonators and Fiber Bragg Gratings

An improved ring resonator self-referencing technique in a new reflection configuration for remote fiber-optic intensity sensors is demonstrated using fiber Bragg gratings. Sensor sensitivity doubles and a single fiber lead is used. The sensor is interrogated at two subcarrier frequencies having a high rejection of interference from laser source intensity fluctuations and loss in the fiber lead. We experimentally demonstrate the efficiency of the new reflection configuration, the usefulness of the theoretical model proposed, and discuss design parameters for optimum insertion losses

Effects of measurement errors on both the amplitude and the phase reconstruction in phase-shifting interferometry: a systematic analysis

In many applications of phase-shifting interferometry (PSI) the actually recorded object wave is the diffraction field of an object surface. In this case not only the phase error but also the amplitude error in the retrieval process have effects on the object wave reconstruction in its original plane. In this paper, for the first time that we know, the amplitude errors in wave reconstruction in PSI are discussed systematically. Some general relations between the amplitude errors and the phase errors for three commonly encountered error sources, namely the phase shift error, the light source intensity fluctuation and the detector nonlinearity, are revealed. The analytical expressions of the amplitude and phase errors for six frequently used algorithms in PSI are derived. The statistical variances of the two kinds of error are introduced to describe the overall performance of these algorithms and are quantitatively calculated. A series of computer simulations are also given as a verification of our analyses. These results can be used for evaluating the complete wave errors in PSI and developing new methods for their correction.

Measurement of low-level gain in a visible chemical laser amplifier

High precision zero power gain measurements are used to demonstrate that the energy transfer sequence SiO*(b 3 ∏) + Na(3 s 2 S) →SiO(X 1 Σ + ) + Na*(4 d 2 D); Na*(4 d 2 D) → Na(3 p 2 P) + hv(569 nm); Na*(4 d 2 D) + hv(569 nm)→Na(3p 2 P) + 2hv(569 nm) represents a viable visible chemical laser amplifier candidate. Dual beam radiometry is used to advantage in a sensitive and stable measurement system, which provides correction for: 1. source intensity fluctuations, 2. variations in source spectral density, 3. variations in detector spectral response, and 4. gain variations in detector and amplification stages. Experiments are performed to determine the amplification of a probe beam at 569 nm by an extended path length (nominal gain length ∼5 cm) reaction energy transfer zone. For these quantitative gain measurements, the stability of the probe signal sets a limit on the sensitivity of the measurement. Considerable effort was expended to insure equivalent path lengths for probe and reference beams to compensate for source output variations so that the reference signal possesses the same temporal behavior as the probe, and the probe and reference optical systems image the same portion of the source on the detector. A gain coefficient, conservatively estimated as 0.8 to 1.5×10 - 3 cm - 1 , was measured. Based on this result, a Rigrod analysis indicates an expected full laser cavity output power between one and ten watts.

Cavity ring-down spectroscopy: Experimental schemes and applications

Cavity ring-down (CRD) spectroscopy is a direct absorption technique, which can be performed with pulsed or continuous light sources and has a significantly higher sensitivity than obtainable in conventional absorption spectroscopy. The CRD technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined in a closed optical cavity with a high Q factor. The advantage over normal absorption spectroscopy results from, firstly, the intrinsic insensitivity to light source intensity fluctuations and, secondly, the extremely long effective path lengths (many kilometres) that can be realized in stable optical cavities. In the last decade, it has been shown that the CRD technique is especially powerful in gas-phase spectroscopy for measurements of either strong absorptions of species present in trace amounts or weak absorptions of abundant species. In this review, we emphasize the various experimental schemes of CRD spectroscopy, and we show how these schemes can be used to obtain spectroscopic information on atoms, molecules, ions and clusters in many environments such as open air, static gas cells, supersonic expansions, flames and discharges.

A Charge Coupled Device Array Detector for Single-Wavelength and Multiwavelength Ultraviolet Absorbance in Capillary Electrophoresis

A fundamental limitation to the use of single-point absorbance detection for capillary electrophoresis is irradiance, since it is not possible to create an image at the detection point on capillary that is brighter than the light source. This limitation may be overcome by illuminating a length of the capillary using a fiber-optic bundle and using a charge coupled device (CCD) camera that can image the full length of the illuminated zone. The present paper describes design and development of a CCD detector for UV absorbance that can be used in both multiwavelength and single-wavelength modes. The CCD camera images analyte peaks in the capillary dimension, together with wavelength-resolved absorbance in the dimension perpendicular to the capillary. Successive snapshots of the peaks are added together, after appropriate correction for time-dependent peak displacement, without sacrificing spatial resolution. Measured baseline rms noise values at 200 nm are 34 μAU using a holographic grating in multiwavelength mode and 8 μAU with the addition of a band-pass filter. Both values are in excellent agreement with calculations of limiting shot noise. Performance in multiwavelength mode is constrained by the 470-ms readout time of the CCD used, which sets a maximum duty cycle of 2.3%. Noise contributions from source intensity fluctuations are reduced by using a portion of the CCD image to provide a baseline reference signal. With 4-hydroxybenzoate as test analyte, the linear dynamic range in multiwavelength mode is shown to be between 3 and 4 orders of magnitude. High-quality spectra of 2-, 3-, and 4-methylbenzoates are obtained on capillary and used in deconvolution of closely migrating peaks of the 2- and 3-isomers.

Phase sensitivity normalization in time‐division multiplexing of polarization‐insensitive interferometric sensors using phase‐generated carrier demodulation

The time-division multiplexing of a polarization-insensitive fi- ber optic Michelson interferometric sensor (TDM-PIFOMI) using phase- generated carrier (PGC) demodulation is presented. A new automatic gain control (AGC) process for the phase sensitivity normalization of each sensor in the TDM-PIFOMI system using the PGC demodulation is described and demonstrated. In the TDM-PIFOMI system, this process can eliminate effects including the fluctuation of the laser source intensity and the variation of the light pulse intensity distribution in all fibers. The advantage of the AGC process is to implement the process under the optimum condition of the PGC demodulation. © 1996 Society of Photo-Optical Instrumentation Engineers.

Radio wave scattering in the outer heliosphere: Preliminary calculations

Detailed first estimates are presented of angular broadening in the outer heliosphere due to scattering of radio waves by density irregularities. The application is to the 2–3 kHz radiation observed by Voyager. Two plausible turbulence models, which account very well for scattering within 1 AU, are extrapolated beyond 10 AU. Both models predict significant angular broadening in the outer heliosphere, accounting semi‐quantitatively alone for the source sizes inferred from roll modulation data. Predictions are presented for radial variations in the apparent source size if scattering is important. Comparisons with available data argue that scattering is important (and indeed is the dominant contributor to the apparent source size) and that the radiation source is located in the outer heliosphere. Other evidence that scattering is important, such as the fluctuations in apparent source direction and intensity, are also identified. The effects of scattering should be included in future analyses of the 2–3 kHz emissions.

New fluorescence tools for investigating enzyme activity

Novel fluorescence-based enzyme-substrate probes have been fabricated which incorporate a unique utilization of chemically modified micron-sized particles in conjunction with a single-excitation dual-emission wavelength ratio technique. By chemically modifying micron-sized particles with both an enzyme-specific substrate and a reference fluorophore the effects of source intensity fluctuations, fluorophore diffusion, and variances in substrate loading inherent in in situ biological fluorescence assays can be reduced. Thus these probes have the potential to provide more sensitive and less invasive fluorescence detection of enzyme activity in solution, in microorganisms and in single cells. In addition, proper selection of particle size facilitates selective targeting of microorganisms through natural ingestion processes. Examples of source fluctuation and substrate loading corrections are provided for in in vitro experiments with a common esterase species. The in situ application of these probes in individual microorganisms which are used as biosensors is also discussed.

Propagation of small-amplitude displacement waves on a vortex in the center of a circular tube

The propagation of three-dimensional small-amplitude long waves on a vortex filament running down the center of a circular tube is investigated analytically. The vortex flow is idealized by an infinite concentrated potential vortex filament immersed in an axial uniform flow that is displaced from the axis of the tube by a small three-dimensional perturbation. The rigid walls of the tube are represented by a potential surface source distribution that also has the nature of a small perturbation. A linearization of the flow-field equations, with respect to the basic state of a straight filament along the tube axis, results in an eigenvalue problem. The analytical solution shows that the vortex in the center of the tube is stable and the disturbances propagate as constant amplitude displacement waves along the vortex line and as source-intensity fluctuations along the inner surface of the tube wall. Two different basic modes of either a helical or rotating sinusoidal wave can propagate along the vortex filament. A ‘‘supercritical’’ state is defined by the characteristics of the flow in which waves can propagate only downstream. It is shown from available experimental data that vortex flows in tubes that experienced vortex breakdown farther downstream, were ‘‘supercritical’’ at the tube inlet.

Source-induced noise in optical systems driven by low-coherence sources

High-power quasimonochromatic light sources may deliver enough power into optical and fiber-optic systems to make the source noise dominant at the system output. With an RMS value proportional to light intensity, this noise limits the system sensitivity and dynamic range to levels which cannot be improved by injecting more light into the system. Source-related noise may originate not only from the source intensity fluctuations, but also from a phase-to-intensity conversion process which is characteristic of many single-spatial-mode multiple-path optical systems. The shape of the power spectral density of the source-induced noise, being critically dependent on the physical structure of the system, is analyzed for a self-homodyne Mach-Zehnder structure and for a recirculating delay line. For single-path communication systems, it is shown that source-originated noise exceeds both shot and thermal noise for a received optical power of only a few tens of microwatts.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Loss-compensated photoelastic fiber optic pressure sensor

A fiber optic pressure sensor based on the photoelastic effect has been set up in a four-port polariscope. The down lead insensitivity is achieved by application of the balanced bridge referencing technique. The sensor has an excellent insensitivity to source intensity fluctuations as well as mode dependent and mode independent changes in the damping of the down leads. The sensor exhibits a linearity of +/-0.1% and a temperature stability of +/-1% within a range of 0-60 degrees C with respeat to a span of 6 bar.

Noise reduction in laser photolysis studies of photolabile samples using an optical multichannel analyzer

An optical multichannel analyzer system for obtaining high-quality spectra of intermediates formed by nanosecond laser photolysis is described. The system is useful for samples which undergo irreversible photochemistry and which are available in limited amounts. Sample volumes as small as 25 μl are possible and a syringe pump driven by a linear stepper motor is used to deliver fresh sample after each photolysis pulse. The microchannel-plate intensified detector used here in gated mode can collect an entire spectrum in 5 ns, but proper synchronization of the detector scans is required to avoid severe distortion which can appear as random noise in signal averaging. Noise from various sources such as phosphor lag in the detector, time jitter, source intensity fluctuations, and the photon character of light are discussed. Experimental design for noise reduction is described including diagnostic procedures and light source filtering to produce more uniform signal-to-noise ratio throughout the spectrum. Finally, a comparison is made between the noise observed to occur between detector channels and the noise predicted to occur based on the photoelectron gain of the intensifier. The disagreement between these is discussed in terms of the spatial resolution of the detector.

A digital fiber-optic liquid level sensor

A fiber-optic liquid level sensor is presented which directly outputs digitally encoded liquid level information. The sensor operates on selective coupling at the liquid surface between a source waveguide and an array of digitally masked receiving waveguides. The receiving waveguides carry optical "high/low" signals to a remote detector and discriminator circuit. The sensor is capable of liquid level resolution of ± 2 mm over a range of up to several meters. A 40-cm-long prototype sensor has demonstrated an optical signal-to-noise ratio of 10-20 dB for several different transparent and semi-transparent liquids. The digital nature of the sensor provides good isolation from drift and the effect of tilt, source intensity fluctuations, and variations in liquid characteristics. Optical fibers are the only connections to the liquid tank, providing complete electrical isolation. The associated electronics are relatively simple and inexpensive.

Analysis of optical phase noise in fiber-optic systems employing a laser source with arbitrary coherence time

Performance of many optical circuits and systems, such as signal processing and sensing devices, is influenced by random fluctuations of the optical source emission field. This paper outlines a formalism for the analysis of laser phase noise effects on a general linear time-invariant optical system. Theoretical expressions are presented for the autocovariance function of the instantaneous output intensity which are valid for any source coherence time as long as the source intensity fluctuations are assumed to be negligible. Applications of the results to some fiber-optic systems such as Mach-Zehnder and recirculating delay line devices also are demonstrated, both in the coherent and incoherent regimes. Although the analysis is not limited just to fiber-optical systems, our attention is focused on fiber optics.

An Experimental Study of the Precision of Fluorescence Measurements

The precision of fluorescence measurements by the SLM 22 fluorometer was calculated with the use of fluorescein solutions of varying concentrations. Relative standard deviations for 10 replicate fluorescence measurements, each integrated over five seconds, are not observably affected by the changing of bandpasses from 1 to 8 nm, by the changing of cell orientation, or by fluorescein concentration at levels of 1.0 × 10−9M and above. Fluctuations in source intensity are shown to be the primary source of noise. Statistical fluctuations in photon and photoelectron flux do not seem to be a significant source of variability. Because of this, the precision of single-channel intensity measurements is poorer than for intensity measurements ratioed directly to source intensity. However, the precisions of the ratio of fluorescence intensities measured by dual detectors on opposite sides of the cell were often found to be better than for intensity measurements ratioed to the source. It is hypothesized that this is observed because the measurements of source intensity and fluorescence are not identically affected by fluctuations in the source. At fluorescein concentrations in the 10−9M range and above, percent RSDs for single channel measurements typically fall in the range of 0.2% to 0.5%, although occasionally larger values are observed. The percent RSDs for the ratio of simultaneous fluorescence measurements are typically in the range of 0.1 to 0.2% at higher concentrations.

Determination of Phenols in Water Using Raman Spectroscopy

The potential of Raman spectroscopy for quantitative analysis of phenols in water solution has been investigated. A group of six compounds—phenol, o-chlorophenol, 2, 4-dichlorophenol, 2, 4, 6-trichlorophenol, 2-chloro-S-methylphenol and 2-chloro-4-nitrophenol—was studied with the use of the 514.5, 488.0, and 457.9 nanometer lines of an argon ion laser. Attention was given to the effects of source intensity, optical alignment, and background fluctuations on quantitative results. It was found that the use of an internal standard with each measurement made a significant improvement in the accuracy and precision of results. Two methods of quantitation, peak area measurement and cross-correlation, were used. Results were somewhat better for cross-correlation, presumably because of more effective exclusion of background interference. Limits of detection were calculated based upon the slope and the standard deviation of the intercept of the standard curve. These varied from the range of 100 ppm to 0.3 ppm, depending on the compound. The most important factor controlling sensitivity is occurrence of resonance enhancement.

Dual beam interferometry and double chopping for scintillation free measurements

A dual beam interferometer with two inputs and two outputs is described. The instrument allows measurements of difference reflectivities in the far infrared region (10...500 cm−1). In the high vacuum compartment cryogenic equioment, like a superconducting magnet or a cryoflow cryostat can be installed. However the step scan version of the interferometer is sensitive to any fluctuation of source intensity, base-line shift in the interferogram and thermal distortions, effects causing scintillation noise in the spectra. A method is described which suppresses scintillation noise by additional chopping at the output of the interferometer and rationing digitally difference and sum interferograms. These are acquired by phase sensitive detection with two lock ins.

Multistation multiparameter flow cytometry: some influences of instrumental factors on system performance.

AbstractIn an effort aimed at design of a flow cytometer/sorter system in which light scattering, fluorescence, and extinction measurements could be made in as many as five spatially separated illuminating beams, the performance of different mechanical components, light sources, illumination and collection optics, and flow chambers was compared by changing individual components while maintaining the same basic instrument configuration. A protocol was developed, and is here described, allowing rapid setup of a flow cytometer using a mercury arc lamp and two laser beams for illumination. When two or more illuminating beams are derived from a single laser, it is generally necessary to incorporate automatic gain control in the detector electronics to compensate for source intensity fluctuations and so to achieve acceptable measurement precision. It is, however, possible to use a single laser beam as a source for several flow cytometers without the need for such compensation. Mercury arc lamps provide adequate excitation in the near ultraviolet (uv) for DNA fluorochromes, such as Hoechst 33342, and may be preferable to ion lasers for this purpose. Reported differences in precision and sensitivity between stream‐in‐air and enclosed flow systems appear owing more to differences in the illumination and collection optics used in different instruments; five flow chamber designs gave equivalent performance when tested in the same instrument. Relatively inexpensive achromatic microscope objectives of numerical aperture (NA) approximately 0.5 may offer the best combination of depth of focus and light gathering power when used as fluorescence collection lenses for flow cytometers; image plane apertures improve system performance in fluorescence measurement.

Simple differential lock-in amplifier for optical measurements

A lock-in amplifier for chopped optical signals is described. It is simple but effective. Its special features are: (a) The system reduces the effect on the experimental results of fluctuations in the light source intensity. (b) The locking in can be carried out with a chopper operating with a variable frequency. (c) A series of odd harmonics synchronous with the chopper can be chosen to be highly rejected. In particular, a signal due to pickup of the 3rd harmonics can be made small.

Double-beam fourier spectroscopy with two inputs and two outputs

A theoretical treatment of double-beam Fourier spectroscopy, with two inputs and two outputs measured by a single detector, is presented. One of the novel features is the use of the radiation that returns to the source in a conventional Fourier spectrometer, i.e., the anti-interferogram beam. The double beam feature allows simultaneous measurement of the difference in the spectra of the sample and a reference standard with optical cancellation of common features. Several versions of spectrometers suited for specific applications are discussed as special cases. Applications include the ability to measure small absorptivity or reflectivity against strong and varying background, to compare the emission spectra of two nearly equal sources, or the sensitivity of two different detectors. Moreover, by suitably chopping the input or output beams, the noise due to fluctuations in source intensity may be strongly reduced.