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Geiger mode single-photon detectors have been used in pulse ranging system and three-dimensional imaging systems due to high sensitivity and easy integration. The ranging accuracy and precision is influenced by many factors. Based on statistical theory, five main factors are discussed in this article, namely, echo signal intensity, pulse width, detector quantum efficiency, target position, and background noise. An analytical relationship among the ranging accuracy, precision, and these factors is obtained for a Q-switched laser pulse. Through this relationship, it is shown that the echo signal intensity and pulse width are more important than other factors and higher echo signal intensity and narrower pulse width can result in better accuracy and precision. [c] 2010 Optical Society of America OCIS codes: 040.1345, 100.6890.

We describe the development of a new suite of aerosol models for the retrieval of atmospheric and oceanic optical properties from the SeaWiFS and MODIS sensors, including aerosol optical thickness ([tau]), angstrom coefficient ([alpha]), and water-leaving radiance ([L.sub.w]). The new aerosol models are derived from Aerosol Robotic Network (AERONET) observations and have bimodal lognormal distributions that are narrower than previous models used by the Ocean Biology Processing Group. We analyzed AERONET data over open ocean and coastal regions and found that the seasonal variability in the modal radii, particularly in the coastal region, was related to the relative humidity. These findings were incorporated into the models by making the modal radii, as well as the refractive indices, explicitly dependent on relative humidity. From these findings, we constructed a new suite of aerosol models. We considered eight relative humidity values (30%, 50%, 70%, 75%, 80%, 85%, 90%, and 95%) and, for each relative humidity value, we constructed ten distributions by varying the fine-mode fraction from zero to 1. In all, 80 distributions (8Rh x 10 fine-mode fractions) were created to process the satellite data. We also assumed that the coarse-mode particles were nonabsorbing (sea salt) and that all observed absorptions were entirely due to fine-mode particles. The composition of the fine mode was varied to ensure that the new models exhibited the same spectral dependence of single scattering albedo as observed in the AERONET data. The reprocessing of the SeaWiFS data show that, over deep ocean, the average [[tau].sub.865] values retrieved from the new aerosol models was 0.100 [+ or -] 0.004, which was closer to the average AERONET value of 0.086 [+ or -] 0.066 for [[tau].sub.870] for the eight open-ocean sites used in this study. The average [[tau].sub.865] value from the old models was 0.131 [+ or -] 0.005. The comparison of monthly mean aerosol optical thickness retrieved from the SeaWiFS sensor with AERONET data over Bermuda and Wallops Island show very good agreement with one another. In fact, 81% of the data points over Bermuda and 78% of the data points over Wallops Island fall within an uncertainty of [+ or -] 0.02 in optical thickness. As a part of the reprocessing effort of the SeaWiFS data, we also revised the vicarious calibration gain factors, which resulted in significant improvement in angstrom coefficient ([alpha]) retrievals. The average value of a from the new models over Bermuda is 0.841 [+ or -] 0.171, which is in good agreement with the AERONET value of 0.891 [+ or -] 0.211. The average value of a retrieved using old models is 0.394 [+ or -] 0.087, which is significantly lower than the AERONET value. [c] 2010 Optical Society of America OCIS codes: 070.4550, 070.6110, 120.5050, 120.6650.

We report the modal analysis of optical fiber single-mode--multimode--single-mode intrinsic Fabry--Perot interferometer sensors. The multimode nature of the Fabry--Perot cavity gives rise to an additional phase term in the spectrogram due to intermodal dispersion-induced wavefront distortion, which could significantly affect the cavity length demodulation accuracy. By using an exact model to analyze the modal behavior, this phase term is explained by employing a rotating vector approach. Comparison of the theoretical analysis with experimental results is presented. [c] 2010 Optical Society of America OCIS codes: 060.2310, 060.2370, 120.2230, 120.3180, 030.4070.

The Zernike phase contrast method is a novel technique to phase the primary mirrors of segmented telescopes. It has been tested on-sky on a unit telescope of the Very Large Telescope with a segmented mirror conjugated to the primary mirror to emulate a segmented telescope. The theoretical background of this sensor and the algorithm used to retrieve the piston, tip, and tilt information are described. The performance of the sensor as a function of parameters such as star magnitude, seeing, and integration time is discussed. The phasing accuracy has always been below 15 nm root mean square wavefront error under normal conditions of operation and the limiting star magnitude achieved on-sky with this sensor is 15.7 in the red, which would be sufficient to phase segmented telescopes in closed-loop during observations. [c] 2010 Optical Society of America OCIS codes: 010.7350, 120.5050, 350.1260.

The indoor detection of the human body's thermal trace plays an important role in the fields of infrared detecting, scouting, infrared camouflage, and infrared rescuing and tracking. Currently, quantitative description and analysis for this technology are lacking due to the absence of human infrared radiation analysis. To solve this problem, we study the heating and cooling process by observing body contact and removal on an object, respectively. Through finite-element simulation and carefully designed experiments, an analytical model of the infrared trace of body contact is developed based on infrared physics and heat transfer theory. Using this model, the impact of body temperature on material thermal parameters is investigated. The sensitivity of material thermal parameters, the thermal distribution, and the changes of the thermograph's contrast are then found and analyzed. Excellent matching results achieved between the simulation and the experiments demonstrate the strong impact of temperature on material thermal parameters. Conclusively, the new model, simulation, and experimental results are beneficial to the future development and implementation of infrared trace technology. OCIS codes: 040.1880, 040.3060, 110.6820, 120.6810.

For a competitive sensor, both the sensitivity enhancement and calibration convenience are important. However, for surface-mounted fiber grating strain sensors, researchers in previous studies have seldom addressed these two capabilities simultaneously. Using the coupled-mode theory, we calculated the voltage signals from the filtered spectral-power interrogation system for a fiber Bragg grating with a glued grating or with a glue-free grating subject to different strain fields. Under a grating zone (the targeted surface), a one-dimensional linearly varied strain field, described by both the parameters of the average strain and the strain gradient, was considered. Finally, a simple and easy formula is provided (the improved-bonding linearly chirped fiber grating) to achieve both the sensitivity enhancement and calibration convenience at the same time for the strain sensor. [c] 2010 Optical Society of America OCIS codes: 060.2310, 060.2330, 060.2370, 060.3735, 060.4510.

We have developed a high-resolution imaging detector with five microchannel plates (MCPs) in a set of V and Z stacks and a resistive anode encoder (RAE) for future space applications. In a position-sensitive system with a RAE, the spatial resolution depends on the signal-to-noise ratios at the anode terminals. Therefore, a high and stable electron gain of MCPs allows the position determination of each photoelectron event with a high spatial resolution. We investigated the effect of the potentials applied to the detector on the pulse height distribution (PHD) and the spatial resolution by means of calculations and experiments. The calculations showed that the negative interstack potential reduced the size of the electron cloud at the Z-stack input by ~80% The result suggests that, under such a condition, the Z-stack MCP is operated in the completely saturated mode and exhibits a narrow PHD. On the other hand, in the measurements, applying the negative interstack potential reduced the width of the PHD by ~60% As a result, the spatial resolution of 45 [micro]m, corresponding to 480 x 480 pixels, was achieved. The results enable us to optimize and apply the technique to future missions. OCIS codes: 040.7480, 040.7190, 110.2970, 040.5250.

Shack--Hartmann wavefront sensors have found widespread application for testing ocular aberrations. These sensors provide an accurate map of the wavefront emerging from an eye in most cases. However, there is a growing class of patients with diffractive intraocular lenses that will potentially be measured incorrectly with Shack-Hartmann devices. We explore the pitfalls of measuring diffractive lenses with this technology. OCIS codes: 330.7327, 050.1970, 120.5050.

We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interferometer for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of down-hole application. A theoretical study of its loss and birefringence changes with different coiling diameters has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and 3.24nm/ MPa at ~1320 and ~1550 nm, respectively. High pressure measurement up to 20 MPa was achieved with our experiment. It shows both good linearity in response to applied pressure and good repeatability within the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensitivity and high temperature sustainability. It functions well without any measurable degradation effects on sensitivity or linearity at a temperature as high as 293 [degrees]C. These characteristics make it a potentially ideal candidate for downhole pressure sensing. OCIS codes: 060.2370, 280.5475, 060.5295, 120.5790, 060.2420.

We fabricate low-noise visible-blind ultraviolet photodetectors of indium tin oxide/SrTi[O.sub.3]/Ag (ITO/STO/ Ag) based on the properties of STO bandgap excitation and the conductance of ITO thin film. The ITO films are epitaxially grown on STO wafers as electrodes and windows of the photodetectors, simultaneously. The photodetectors have low noise and very good electromagnetic shielding. The dark current is as low as 270 pA even at a 200 V bias. The peak responsivity reaches to 30 mA/W at the wavelength of 360 nm. From the experimental results, the same ideas can be generalized to develop visible-blind and solar-blind UV photodetectors based on wide bandgap materials, such as LaA1[O.sub.3], LiNb[O.sub.3], LiTa[O.sub.3], BaTi[O.sub.3], ZnO, MgO, and Zr[O.sub.2]. OCIS codes: 040.5160, 040.5150, 040.7190, 310.7005.

A modified Hartmann test based on the interference produced by a four-hole mask can be used to measure an unknown wavefront. To scan the wavefront, the interference pattern is measured for different positions of the mask. The position of the central fringe of the diamond-shaped interference pattern gives a measure of the local wavefront slopes. Using a set of four-hole apertures located behind an array of lenslets in such a way that each four-hole window is inside one lenslet area, a set of four-hole interference patterns can be obtained in the back focal plane of the lenslets without having to scan the wavefront. The central fringe area of each interference pattern is narrower than the area of the central maximum of the diffraction pattern of the lenslet, increasing the accuracy in the estimate of the lobe position as compared with the Shack--Hartmann wavefront sensor. OCIS codes: 010.7350, 120.3180, 050.1220, 100.2960.

By synthesizing large effective apertures through the translation of a smaller imaging sensor and the subsequent proper phasing and correlation of detected signals in postprocessing, holographic aperture ladar (HAL) systems seek to increase the resolution of remotely imaged targets. The stripmap HAL process was demonstrated in the laboratory, for the first time to our knowledge. Our results show that the stripmap HAL transformation can precisely account for off-axis transmitter induced phase migrations. This in turn allows multiple pupil plane field segments, sequentially collected across a synthetic aperture, to be coherently mosaiced together. As a direct consequence, we have been able to confirm the capability of the HAL method to potentially provide substantial increases in longitudinal cross-range resolution. The measurement and sampling of complex pupil plane field segments, as well as target related issues arising from short laboratory ranges, have also been addressed. [c] 2010 Optical Society of America OCIS codes: 090.1995, 120.0280, 280.3640, 280.4788, 280.6730.

Conventional two-mode fiber-optic strain sensors measure strain by inducing a path difference between the two propagating modes and spatially interfering the modal output pattern. At high strain values, the output mode pattern changes (rotates), limiting the range of measured strain. We have applied a mode separation/recombination technique and demonstrated it with a two-mode strain sensor, resulting in a rotationally invariant/stable output mode pattern and extended range of measured strain. The sensor was designed to measure strain, but with very little modification, it can measure temperature, pressure, electric and magnetic fields, etc. The improved rotationally invariant two-mode fiber-optic strain sensor performs to within 2% of standard electrical strain gauges. [c] 2010 Optical Society of America OCIS codes: 060.2370, 060.2430.

A significant advance in sensitivity of liquid-crystal (LC)-based chemical and biological sensors can be achieved by actively monitoring anchoring energy change. We simulate the deformation of a LC director with different anchoring energies using the finite element method and the optical properties of the LC film using the finite-difference time-domain method. Polarizing micrographs are collected and compared with simulated textures. Measurement of optical transmission is used to monitor the anchoring change. Experimental and simulation results both demonstrate the optical method can effectively monitor the surface anchoring change due to the presence of targeted analytes. OCIS codes: 230.3720, 230.2090, 280.4788, 280.1415.

Knowledge of the water concentration of plants can be helpful in several environmental and agricultural domains. There are many methods for the determination of water content in plant leaves; however, most of them give a relative moisture level or an analytical measure after a previous calibration procedure. Even for other biochemical compounds such as dry matter or chlorophyll, the measurement techniques could be destructive. For this reason, a nondestructive method has been developed to measure the biochemical compounds of a plant leaf, using an infrared spectroscopy technique. One important advantage is the simplicity of the device (RAdiometre portatif de Mesure In Situ, RAMIS) and its capability to perform measurements in situ. The prototype is a leaf-clip configuration and is made of LEDs at five wavelengths (656, 721, 843, 937, and 1550 nm), and a silicon/germanium photosensor. To compute the water content of vegetative leaves, the radiative transfer model PROSPECT was implemented. This model can accurately predict spectral transmittances in the 400 nm to 2500 nm spectral region as a function of the principal leaf biochemical contents: water, dry matter, and chlorophyll. Using the transmittance measured by RAMIS into an inversion procedure of PROSPECT: A Model of Leaf Optical Properties Spectra, we are able to compute the values of water contents that show an agreement with the water contents measured directly using dry weight procedures. This method is presented as a possibility to estimate other leaf biochemical compounds using appropriate wavelengths. OCIS codes: 280.1415, 120.0120.

We explore the possibilities of obtaining compression in video through modified sampling strategies using multichannel imaging systems. The redundancies in video streams are exploited through compressive sampling schemes to achieve low power and low complexity video sensors. The sampling strategies as well as the associated reconstruction algorithms are discussed. These compressive sampling schemes could be implemented in the focal plane readout hardware resulting in drastic reduction in data bandwidth and computational complexity. OCIS codes: 110.1758, 100.6640.

Simulations on Kolmogorov phase screens are employed to compare the relative performance of an astigmatic hybrid wavefront sensor (AHS) to that of a Shack--Hartmann sensor (SHS). The AHS is shown to improve phase reconstruction accuracy when the subaperture phase contains significant energy in curvature modes and a moderate to high number of photons are collected. Dual use of the AHS and SHS may extend enhanced reconstruction to low signal levels. The AHS is also shown to have a small benefit for tilt-only reconstruction when the beam has sufficient power. [c] 2009 Optical Society of America OCIS codes: 010.7350, 110.1080, 350.1260.

We report a differential phase sensor capable of measuring Faraday rotation in reflection mode with a single measurement and a small field-depth factor. Based on a polarization-maintaining-fiber low-coherence interferometer, the sensor measures phase difference between two decorrelated and oppositely polarized circular states incident on a sample. Sensitivity of the sensor for the Faraday rotation is 0.31 arcmin, allowing applications on small volumes of liquids. The Verdet constants of various liquids, including clear and turbid samples, are measured at 857 nm. OCIS codes: 260.2110, 230.2240, 120.5050.

Microsphere-based ion optodes represent a promising and versatile tool to measure ionic actvities in confined samples. The reported methods of micro- and nanosphere optode fabrication, however, suffer from various degrees of complexity. We propose a large-scale fabrication of polymeric ion-selective optodes using a solvent displacement method. Plasticized poly(vinyl chloride) along with the optode components was dissolved in a solvent miscible with water. Injection of a polymer solution into a stirred aqueous phase containing a surfactant causes spontaneous emulsification. This technique does not require additional preparation steps and allows one to control the composition of the sensor matrix precisely. Several factors affecting the particle size distribution are examined such as composition of continuous and disperse phases. The concentration of the polymer in the organic solvent and the choice of the solvent nature allowed us to control the particle size distribution within 200 nm-30 [micro]m. The concentration and the nature of the surfactant had a little influence on the particle size distribution. We fabricated three different batches of ion-selective optodes using chromoionophore I, lipophilic ion-exchanger and sodium ionophore X, BME-44, and ETH 5234 for sodium, potassium, and calcium optodes, respectively. The sensors were fully functional with excellent selectivity to interfering ions.

An electro-optic sensor capable of detecting electric fields with a high degree of sensitivity and linearity is fabricated using optical D-fiber. The slab coupled optical sensor utilizes weak coupling and long evanescent interaction with a lithium niobate waveguide. Transmission dips from mode resonances have a line-width of 0.12 nm and a Q factor of ~13,000. These sharp resonances improve device sensitivity and are achieved due to the unique fabrication process possible with D-shaped fibers. The sensor deviates OCIS codes: 060.2370, 280.4788.

A refractive index sensor based on the thinned and microstructure fiber Bragg grating (ThMs-FBG) was proposed and realized as a chemical sensing. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated and the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length, and position. The sandwich-solution etching method was utilized to realize the microstructure of the ThMs-FBG, and the photographs of the microstructure were obtained. Experimental results demonstrated that the reflectance spectrum, phase shift down-peak wavelength, and reflected optical intensity of the ThMs-FBG all depended on the surrounding refractive index. However, only the down-peak wavelength of the ThMs-FBG changed with the surrounding temperature. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800 and 14 [micro]m, respectively, and the position of the microstructure of the ThMs-FBG is in the middle of grating region, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/refractive index unit with the wide range of 1.33-1.457 and a high resolution of 1.2 x 10 3. The temperature sensitivity was 0.0103 nm/[degrees]C, which was approximately equal to that of common FBG. America OCIS codes: 280.0280, 280.4788, 280.5715, 120.0120, 120.0280.

Spectral surface plasmon resonance (SPR) sensors with absorbance measurement were prepared. Resonant wavelengths ([[lambda].sub.R]) versus effective refractive indexes of the SPR mode were measured with different media in contact with the gold layer. An investigation into the refractive-index sensitivity of the sensor at a fixed angle reveals a linear dependence of [[lambda].sub.R] on the refractive index of the solution ([n.sub.c]), with [DELTA][[lambda].sub.R]/[DELTA][n.sub.c] 3553.6 nm in a small range of 1.333 [less than or equal to] [n.sub.c] [less than or equal to] 1.347. It was observed that the effective refractive index slowly decreases with increasing [n.sub.c], attributable to wavelength-induced modulation of optical dielectric constant for the gold layer. Adsorption of bromothymol blue (BTB) on the gold layer leads to a redshift of [DELTA][[lambda].sub.R] = 3.7 nm, larger than [DELTA][[lambda].sub.R] 2.5 nm induced by myoglobin (Mb) adsorption. On the basis of Fresnel equations, calculations with d ~ 1 nm and n 1.69 for BTB and d ~ 3 nm and n = 1.40 for Mb also demonstrate that the SPR band shift induced by full-monolayer adsorption of BTB is larger than that for full-monolayer Mb adsorption. The combination of both measured and calculated results suggests that the contribution of the adlayer index of refraction to the sensitivity of the sensor is greater than that of the adlayer thickness. OCIS codes: 300.6490, 240.6680, 280.1415.

In general applications, the output signals of the fiber-optic interferometric sensor (FOIS) need some demodulation techniques to linearly demodulate the sensing phase signal. The common signal demodulation circuit of the FOIS is passive homodyne demodulation using phase-generated carrier [(PGC) demodulation]. Preliminary analysis of the research demonstrates that a variation in the output demodulated signal is related to the length of the fiber, and the output demodulated signal will approach zero at some certain lengths of the fiber. To improve the performance of the FOIS, it is necessary to develop the modified PGC demodulation to compensate for the propagation delay of the fiber. OCIS codes: 060.2370, 060.2430.

Electro-optic (EO) modulation devices, which utilize an external electric field to modulate a beam of optical radiation, are strongly affected by parasitic effects, which change the polarization state of the optical beam. As a result, very small changes in the birefringence or optical path length within the EO material can result in very large fluctuations of the amplitude and phase of the optical modulation signal. A method of actively analyzing the modulated beam is described and demonstrated, which eliminates these fluctuations and keeps the modulation device stably operating at its peak responsivity. Applications to electric field detection and measurements are discussed. OCIS codes: 230.2090, 190.4400, 230.4110.

Atmospheric pressure dielectric barrier discharge (DBD) plasma can be used to split low molecular weight organic compounds, and the DBD-split/excited species can be swept into luminol solution to induce chemiluminescence (CL) emission. Based on this observation, a novel optical detector was proposed and preliminarily tested as a potential gas chromatographic (GC) detector in this work. The advantages of this new type of detector include the following: direct detection, fast response, high sensitivity, versatility (sensitive to a broad range of volatile organic compounds), simple and easy instrumentation, compactness (3.0 mm i.d. x 4.0 mm o.d. x 20 mm length of the DBD device), and low power (less than 5 W). Twelve volatile organic compounds, including methanol, ethanol, propanol, formaldehyde, acetaldehyde, benzene, dichloromethane, trichloromethane, tetrachloromethane, tetrahydrofuran, carbon bisulfide, and ethyl ether, were tested with this detector, and each of them produced a large signal. It was found that the CL signal was proportional to the analyte concentration and affected by the DBD parameters. Under the optimized experimental conditions, the limits of detection down to the tens of nanogram level were achieved for methanol, ethanol, propanol, formaldehyde, and acetaldehyde. It was then preliminarily tested as a GC detector for the separation of formaldehyde, ethanol, and propanol. This is the new application of DBD in analytical chemistry, and CL was for the first time generated in this way. The new detector can be a potential GC detector suitable for a wide range of volatile organic compounds.

Optical biosensors are described based on the measurement of absorbance, reflectance or fluorescence emission occurring in the ultraviolet (UV), visible or near-infrared (NIR). Specificity is achieved for optical biosensors by using specific biorecognition elements such as enzymes, antibodies, oligonucleotides and whole cells and tissues.

The design and implementation of optical chemical sensors, which have employed the basic sensor transduction principles, are described. The examples of optical chemical sensor applications are illustrated.