Showing papers in "Applied Optics in 1998"

Optical properties of metallic films for vertical-cavity optoelectronic devices.

Abstract: We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.

Mechanisms of light scattering from biological cells relevant to noninvasive optical-tissue diagnostics

Abstract: We have studied the optical properties of mammalian cell suspensions to provide a mechanistic basis for interpreting the optical properties of tissues in vivo. Measurements of the wavelength dependence of the reduced scattering coefficient and measurements of the phase function demonstrated that there is a distribution of scatterer sizes. The volumes of the scatterers are equivalent to those of spheres with diameters in the range between ~0.4 and 2.0 mum. Measurements of isolated organelles indicate that mitochondria and other similarly sized organelles are responsible for scattering at large angles, whereas nuclei are responsible for small-angle scattering. Therefore optical diagnostics are expected to be sensitive to organelle morphology but not directly to the size and shape of the cells.

Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm

Abstract: We report on two-photon fluorescence excitation (TPE) action cross sections for five widely used molecular fluorophores. Measurements were performed by use of ultrashort (∼100-fs) Ti:sapphire pulsed excitation over the range 690–960 nm. TPE spectra were obtained by comparison with a fluorescein calibration standard. Large cross sections were found for the cyanine reagent Cy 3 (∼140 GM) and for Rhodamine 6G (∼150 GM), both at 700 nm [1 GM = 10-50 (cm4 s)/photon]. Several fluorophores show interesting and desirable blue shifts with respect to twice the one-photon absorption wavelength. Fluorophore fluorescence intensities showed no significant departure (±4%) from quadratic illumination power dependence, indicating genuine two-photon processes. Implications of these measurements for two-photon laser-scanning microscopy are discussed.

High-spatial-resolution distributed strain measurement in optical fiber with rayleigh scatter.

Abstract: A method of measuring strain over 30-cm intervals to an accuracy of10 microstrain in unaltered low-loss communications-grade single-modeoptical fiber is presented. The method uses a tunable external cavity diode laser to measure the reflected intensity of a reflector-fiber system as a function of wavelength. This measurement is performed with no strain applied to the fiber to produce a reference and then again after a strain has been induced. Cross correlation of the Rayleigh scatter spectra from a selected section of fiber in the strained and unstrained states determines the spectral shift resulting from the applied strain.

Hyperspectral remote sensing for shallow waters. I. A semianalytical model.

Abstract: For analytical or semianalytical retrieval of shallow-water bathymetry andyor optical properties of the water column from remote sensing, the contribution to the remotely sensed signal from the water column has to be separated from that of the bottom. The mathematical separation involves three diffuse attenuation coefficients: one for the downwelling irradiance ~Kd!, one for the upwelling radiance of the water column ~Ku C !, and one for the upwelling radiance from bottom reflection ~Ku B !. Because of the differences in photon origination and path lengths, these three coefficients in general are not equal, although their equality has been assumed in many previous studies. By use of the Hydrolight radiativetransfer numerical model with a particle phase function typical of coastal waters, the remote-sensing reflectance above ~Rrs! and below ~rrs! the surface is calculated for various combinations of optical properties, bottom albedos, bottom depths, and solar zenith angles. A semianalytical ~SA! model for rrs of shallow waters is then developed, in which the diffuse attenuation coefficients are explicitly expressed as functions of in-water absorption ~a! and backscattering ~bb!. For remote-sensing inversion, parameters connecting Rrs and rrs are also derived. It is found that rrs values determined by the SA model agree well with the exact values computed by Hydrolight ~;3% error!, even for Hydrolight rrs values calculated with different particle phase functions. The Hydrolight calculations included bbya values as high as 1.5 to simulate high-turbidity situations that are occasionally found in coastal regions. © 1998 Optical Society of America OCIS codes: 010.0010, 280.0280.

Optical Scattering Properties of Soft Tissue: A Discrete Particle Model

Abstract: We introduce a micro-optical model of soft biological tissue that permits numerical computation of the absolute magnitudes of its scattering coefficients. A key assumption of the model is that the refractive-index variations caused by microscopic tissue elements can be treated as particles with sizes distributed according to a skewed log-normal distribution function. In the limit of an infinitely large variance in the particle size, this function has the same power-law dependence as the volume fractions of the subunits of an ideal fractal object. To compute a complete set of optical coefficients of a prototypical soft tissue (single-scattering coefficient, transport scattering coefficient, backscattering coefficient, phase function, and asymmetry parameter), we apply Mie theory to a volume of spheres with sizes distributed according to the theoretical distribution. A packing factor is included in the calculation of the optical cross sections to account for correlated scattering among tightly packed particles. The results suggest that the skewed log-normal distribution function, with a shape specified by a limiting fractal dimension of 3.7, is a valid approximation of the size distribution of scatterers in tissue. In the wavelength range 600 ≤ λ ≤ 1400 nm, the diameters of the scatterers that contribute most to backscattering were found to be significantly smaller (λ/4–λ/2) than the diameters of the scatterers that cause the greatest extinction of forward-scattered light (3–4λ).

Frequency-stabilized diode laser with the Zeeman shift in an atomic vapor

Abstract: We demonstrate a robust method of stabilizing a diode laser frequency to an atomic transition. This technique employs the Zeeman shift to generate an antisymmetric signal about a Doppler-broadened atomic resonance, and therefore offers a large recapture range as well as high stability. The frequency of a 780-nm diode laser, stabilized to such a signal in Rb, drifted less than 0.5 MHz peak–peak (1 part in 109) in 38 h. This tunable frequency lock can be constructed inexpensively, requires little laser power, rarely loses lock, and can be extended to other wavelengths by use of different atomic species.

Review and assessment of measured values of the nonlinear refractive-index coefficient of fused silica.

Abstract: The literature describes more than 30 measurements, at wavelengths between 249 and 1550 nm, of the absolute value of the nonlinear refractive-index coefficient of fused silica. Results of these experiments were assessed and best currently available values were selected for the wavelengths of 351, 527, and 1053 nm. The best values are (3.6 ± 0.64) × 10-16 cm2/W at 351 nm, (3.0 ± 0.35) × 10-16 cm2/W at 527 nm, and (2.74 ± 0.17) × 10-16 cm2/W at 1053 nm.

Imaging of hard- and soft-tissue structure in the oral cavity by optical coherence tomography.

Abstract: We have developed a prototype optical coherence tomography (OCT) system for the imaging of hard and soft tissue in the oral cavity. High-resolution images of in vitro porcine periodontal tissues have been obtained with this system. The images clearly show the enamel-cementum and the gingiva-tooth interfaces, indicating OCT is a potentially useful technique for diagnosis of periodontal diseases. To our knowledge, this is the first application of OCT for imaging biologic hard tissue.

Noninvasive determination of the optical properties of two-layered turbid media

Abstract: Light propagation in two-layered turbid media having an infinitely thick second layer is investigated in the steady-state, frequency, and time domains. A solution of the diffusion approximation to the transport equation is derived by employing the extrapolated boundary condition. We compare the reflectance calculated from this solution with that computed with Monte Carlo simulations and show good agreement. To investigate if it is possible to determine the optical coefficients of the two layers and the thickness of the first layer, the solution of the diffusion equation is fitted to reflectance data obtained from both the diffusion equation and the Monte Carlo simulations. Although it is found that it is, in principle, possible to derive the optical coefficients of the two layers and the thickness of the first layer, we concentrate on the determination of the optical coefficients, knowing the thickness of the first layer. In the frequency domain, for example, it is shown that it is sufficient to make relative measurements of the phase and the steady-state reflectance at three distances from the illumination point to obtain useful estimates of the optical coefficients. Measurements of the absolute steady-state spatially resolved reflectance performed on two-layered solid phantoms confirm the theoretical results.

Assessment of the Size, Position, and Optical Properties of Breast Tumors in Vivo by Noninvasive Optical Methods

Abstract: We present a method for the noninvasive determination of the size, position, and optical properties ~absorption and reduced scattering coefficients! of tumors in the human breast. The tumor is first detected by frequency-domain optical mammography. It is then sized, located, and optically characterized by use of diffusion theory as a model for the propagation of near-infrared light in breast tissue. Our method assumes that the tumor is a spherical inhomogeneity embedded in an otherwise homogeneous tissue. We report the results obtained on a 55-year-old patient with a papillary cancer in the right breast. We found that the tumor absorbs and scatters near-infrared light more strongly than the surrounding healthy tissue. Our method has yielded a tumor diameter of 2.1 6 0.2 cm, which is comparable with the actual size of 1.6 cm, determined after surgery. From the tumor absorption coefficients at two wavelengths ~690 and 825 nm!, we calculated the total hemoglobin concentration ~40 6 10 mM! and saturation ~71 6 9%! of the tumor. These results can provide the clinical examiner with more detailed information about breast lesions detected by frequency-domain optical mammography, thereby enhancing its potential for specificity. © 1998 Optical Society of America OCIS codes: 170.3830, 300.0300, 300.1030, 290.0290.

Measurements and modeling of acetone laser-induced fluorescence with implications for temperature-imaging diagnostics

Abstract: Recent determinations of the temperature dependence of acetone fluorescence have permitted the application of acetone planar laser-induced fluorescence imaging, which was already popular for mapping concentration, to the measurement of temperature. With a view toward developing temperature-imaging diagnostics, we present atmospheric-pressure fluorescence and absorption results acquired with excitation at eight wavelengths across the absorption feature of acetone and at temperatures from 300 to 1000 K. Modeling of the fluorescence yield of acetone is shown to be useful in explaining both these results and the variation of acetone fluorescence with pressure and composition that was observed in several studies. The model results in conjunction with the photophysics data provide guidance for the application of temperature diagnostics over a range of conditions while also suggesting useful multiparameter imaging approaches.

Gradient-index lens-array method based on real-time integral photography for three-dimensional images

Abstract: Because a three-dimensional (3-D) autostereoscopic image can be seen from a desired viewpoint without the aid of special viewing glasses, integral photography (IP) is an ideal way to create 3-D autostereoscopic images. We have already proposed a real-time IP method that offers 3-D autostereoscopic images of moving objects in real time by use of a microlens array and a high-definition television camera. But there are two problems yet to be resolved: One is pseudoscopic images that show a reversed depth representation. The other is interference between the element images that constitute a 3-D autostereoscopic image. We describe a new gradient-index lense-array method based on real-time IP to overcome these two problems. Experimental results indicating the advantages of this method are shown. These results suggest the possibility of using a gradient-index lens array for real-time IP.

Correction of satellite imagery over mountainous terrain

Abstract: A method for the radiometric correction of satellite imagery over mountainous terrain has been developed to remove atmospheric and topographic effects. The algorithm accounts for horizontally varying atmospheric conditions and also includes the height dependence of the atmospheric radiance and transmittance functions to simulate the simplified properties of a three-dimensional atmosphere. A database has been compiled that contains the results of radiative transfer calculations (atmospheric transmittance, path radiance, direct and diffuse solar flux) for a wide range of weather conditions. A digital elevation model is used to obtain information about surface elevation, slope, and orientation. Based on the Lambertian assumption the surface reflectance in rugged terrain is calculated for the specified atmospheric conditions. Regions with extreme illumination geometries sensitive to BRDF effects can be optionally processed separately. The method is restricted to high spatial resolution satellite sensors with a small swath angle such as the Landsat thematic mapper and Systeme pour l'Observation de la Terre high resolution visible, since some simplifying assumptions were made to reduce the required image processing time.

Speckle Reduction in Laser Projection Systems by Diffractive Optical Elements

Abstract: In laser projection systems the observer in the far field of the image points on the screen will recognize serious speckle noise There are many methods to reduce or eliminate speckles in the near field by reducing or eliminating temporal or spatial coherence of the laser But for the far field it is hardly possible to change the coherence properties of laser sources so that speckles will disappear We propose a new method for eliminating speckles in the far field by using a diffractive optical element The intensity modulation depth in the far-field speckle pattern can be reduced to a few percent while good beam quality is preserved

Turn-key Raman lidar for profiling atmospheric water vapor, clouds, and aerosols

Abstract: We describe an operational, self-contained, fully autonomous Raman lidar system that has been developed for unattended, around-the-clock atmospheric profiling of water vapor, aerosols, and clouds During a 1996 three-week intensive observational period, the system operated during all periods of good weather (339 out of 504 h), including one continuous five-day period The system is based on a dual-field-of-view design that provides excellent daytime capability without sacrificing nighttime performance It is fully computer automated and runs unattended following a simple, brief (~5-min) start-up period We discuss the theory and design of the system and present detailed analyses of the derivation of water-vapor profiles from the lidar measurements

Phasing the Mirror Segments of the Keck Telescopes: The Broadband Phasing Algorithm

Abstract: In a previous paper, we described a successful technique, the broadband algorithm, for phasing the primary mirror segments of the Keck telescopes to an accuracy of 30 nm. Here we describe a complementary narrow-band algorithm. Although it has a limited dynamic range, it is much faster than the broadband algorithm and can achieve an unprecedented phasing accuracy of approximately 6 nm. Cross checks between these two independent techniques validate both methods to a high degree of confidence. Both algorithms converge to the edge-minimizing configuration of the segmented primary mirror, which is not the same as the overall wave-front-error-minimizing configuration, but we demonstrate that this distinction disappears as the segment aberrations are reduced to zero.

Empirical equations for the principal refractive indices and column angle of obliquely deposited films of tantalum oxide, titanium oxide, and zirconium oxide.

Abstract: Values of the transmittance Ts and the phase retardation Δ were recorded in situ at two angles during the growth of thin films of tantalum oxide, titanium oxide, and zirconium oxide for deposition angles θν in the range 40°–70°. Column angles for the same films were determined ex situ from scanning electron microscopy photographs of deposition-plane fractures. We show that the experimental column angles are smaller than the corresponding values predicted by the tangent-rule equation ψ = tan-1(0.5 tan θν) and that the experimental values fit a modified form of the equation ψ = tan-1 (E1 tan θν) where E1 is less than 0.5. We also show that the principal refractive indices are represented well by quadratic functions of the deposition angle, for example, n1(θν) = A0 + A2 θν2.

Fluid jet polishing of optical surfaces.

Abstract: We present a new finishing process that is capable of locally shaping and polishing optical surfaces of complex shapes. A fluid jet system is used to guide a premixed slurry at pressures less than 6 bars to the optical surface. We used a slurry comprising water and 10% #800 SiC abrasives (21.8 μm to reduce the surface roughness of a BK7 sample from 350 to 25 nm rms and to vary the shape of a polished sample BK7, maintaining its surface roughness of 1.6 m rms, thereby proving both the shaping and polishing possibilities of the presented method. © 1998 Optical Society of America.

Mapping of birefringence and thermal damage in tissue by use of polarization-sensitive optical coherence tomography

Abstract: We demonstrate cross-sectional birefringence- and polarization-independent backscatter imaging of laser-induced thermal damage in porcine myocardium in vitro, using a polarization-sensitive optical coherence tomography system. We compare the generated images with histological sections of the tissue and demonstrate that birefringence is a more sensitive indicator of thermal damage than is backscattered light. Loss of birefringence in thermally damaged regions is quantified and shown to have significant contrast with undamaged sections of the tissue. A detailed theoretical analysis of the birefringence measurements is provided, including a calculation of the systematic errors associated with background noise, system imperfections, and tissue dichroism.

Single scattering by red blood cells

Abstract: A highly diluted suspension of red blood cells (hematocrit 0.01) was illuminated with an Ar or a dye laser in the wavelength range of 458-660 nm. The extinction and the angle-resolved intensity of scattered light were measured and compared with the predictions of Mie theory, the Rayleigh-Gans approximation, and the anomalous diffraction approximation. Furthermore, empirical phase functions were fitted to the measurements. The measurements were in satisfactory agreement with the predictions of Mie theory. However, better agreement was found with the anomalous diffraction model. In the Rayleigh-Gans approximation, only small-angle scattering is described appropriately. The scattering phase function of erythrocytes may be represented by the Gegenbauer kernel phase function.

Anisotropy in the absorption and scattering spectra of chicken breast tissue.

Abstract: Oblique incidence reflectometry is a simple and accurate method for measuring the absorption and the reduced-scattering coefficients of turbid media. We used this technique to deduce absorption and reduced-scattering spectra from wavelength-resolved measurements of the relative diffuse reflectance profile of white light as a function of source–detector distance. In this study, we measured the absorption and the reduced-scattering coefficients of chicken breast tissue in the visible range (400–800 nm) with the oblique incidence probe oriented at 0° and 90° relative to the muscle fibers. We found that the deduced optical properties varied with the probe orientation. Measurements on homogenized chicken breast tissue yielded an absorption spectrum comparable with the average of the absorption spectra for 0° and 90° probe orientations measured on the unhomogenized tissue. The reduced-scattering spectrum for homogeneous tissue was greater than that acquired for unhomogenized tissue taken at either probe orientation. This experiment demonstrated the application of oblique-incidence, fiber-optic reflectometry to measurements on biological tissues and the effect of tissue structural anisotropy on optical properties.

Performance of InGaAs/InP avalanche photodiodes as gated-mode photon counters

Abstract: We investigate the performance of separate absorption multiplication InGaAs/InP avalanche photodiodes as single-photon detectors for 1.3- and 1.55-mum wavelengths. First we study afterpulses and choose experimental conditions to limit this effect. Then we compare the InGaAs/InP detector with a germanium avalanche photodiode; the former shows a lower dark-count rate. The effect of operating temperature is studied for both wavelengths. At 173 K and with a dark-count probability per gate of 10(-4), detection efficiencies of 16% for 1.3 mum and 7% for 1.55 mum are obtained. Finally, a timing resolution of less than200 ps is demonstrated.

Changes in spectral shape of tissue optical properties in conjunction with laser-induced thermotherapy

Abstract: We measured the optical properties on samples of rat liver tissue before and after laser-induced thermotherapy performed in vivo with Nd:YAG laser irradiation This made it possible to monitor not only the influence of coagulation on the scattering properties but also the influence of damages to vessels and heat-induced damage to blood on the absorption properties An experimental integrating-sphere arrangement was modified to allow the determination of the g factor and the absorption and scattering coefficients versus the wavelength in the 600 ‐1050-nm spectral region, with the use of a spectrometer and a CCD camera The results show a relative decrease in the g factor of on average 21 6 7% over the entire spectral range following thermotherapy, and a corresponding relative increase in the scattering and absorption coefficients of 23 6 8% and 200 6 100%, respectively An increase of on average 200 6 80% was consequently found for the reduced scattering coefficient The cause of these changes in terms of the Mie-equivalent average radius of tissue scatterers as well as of the distribution and biochemistry of tissue absorbers was analyzed, utilizing the information yielded by the g factor and the spectral shapes of the reduced scattering and absorption coefficients These results were correlated with the alterations in the ultrastructure found in the histological evaluation The average radius of tissue scattering centers, determined by using either the g factors calculated on the basis of Mie theory or the spectral shape of reduced scattering coefficients calculated on the Mie theory, was estimated to be 21‐32% lower in treated than in untreated liver samples The Mie-equivalent average radii of scattering centers in untreated liver tissue deduced by the two methods corresponded well and were found to be 031 and 029 mm, respectively, yielding particle sizes in the same range as the size of a mitochondrion © 1998 Optical Society of America OCIS codes: 1403460, 1701420, 1703890, 1706930

Artefact reduction in photoplethysmography

Abstract: In order to remove motion artefact prior to digital processing, the invention provides a method of monitoring living tissue comprising the steps of emitting electromagnetic radiation at said tissue at at least first and second different wavelengths, receiving the radiation at the different wavelengths after it has been transmitted through or reflected within said tissue, providing at least first and second signals which are a logarithmic measure of the received first and second radiation wavelengths and subtracting the second signal from the first signal, removing a DC component of the result of the subtraction and providing an AC component to digital sampling means, and processing the digital samples in order to provide a desired value representing a property of the tissue.

Optical encryption system that uses phase conjugation in a photorefractive crystal.

Abstract: We implement an optical encryption system based on double-random phase encoding of the data at the input and the Fourier planes. In our method we decrypt the image by generating a conjugate of the encrypted image through phase conjugation in a photorefractive crystal. The use of phase conjugation results in near-diffraction-limited imaging. Also, the key that is used during encryption can also be used for decrypting the data, thereby alleviating the need for using a conjugate of the key. The effect of a finite space–bandwidth product of the random phase mask on the encryption system’s performance is discussed. A theoretical analysis is given of the sensitivity of the system to misalignment errors of a Fourier plane random phase mask.

Diffuse and Specular Reflectance from Rough Surfaces

Abstract: We present a reflection model for isotropic rough surfaces that have both specular and diffuse components. The surface is assumed to have a normal distribution of heights. Parameters of the model are the surface roughness given by the rms slope, the albedo, and the balance between diffuse and specular reflection. The effect of roughness on diffuse reflection is taken into account, instead of our modeling this component as a constant Lambertian term. The model includes geometrical effects such as masking and shadowing. The model is compared with experimental data obtained from goniophotometric measurements on samples of tiles and bricks. The model fits well to samples with very different reflection properties. Measurements of the sample profiles performed with a laser profilometer to determine the rms slope show that the assumed surface model is realistic. The model could therefore be used in machine vision and computer graphics to approximate reflection characteristics of surfaces. It could also be used to predict the texture of surfaces as a function of illumination and viewing angles.

Influence of a superficial layer in the quantitative spectroscopic study of strongly scattering media.

Abstract: We have experimentally investigated the meaning of the effective optical absorption @ma ~eff ! # and the reduced scattering @ms9 ~eff ! # coefficients measured on the surfaces of two-layered turbid media, using the diffusion equation for homogeneous, semi-infinite media. We performed frequency-domain spectroscopy in a reflectance geometry, using source‐detector distances in the range 1.5‐ 4.5 cm. We measured 100 samples, each made of one layer ~thickness in the range 0.08 ‐1.6 cm! on top of one semi-infinite block. The optical properties of the samples were similar to those of soft tissues in the near infrared. We found that the measured effective optical coefficients are representative of the underlying block if the superficial layer is less than ;0.4 cm thick, whereas they are representative of the superficial layer if it is more than ;1.3 cm thick. © 1998 Optical Society of America OCIS codes: 170.5280, 170.6510, 170.7050, 300.0300, 300.6340.

Fiber-optic bundle design for quantitative fluorescence measurement from tissue

Abstract: We demonstrate a new design for a fiber-optic bundle to measure fluorescence signals from tissue. In this design, the intensity of the signal is not significantly affected by the medium’s absorption and scattering coefficients and hence depends only on the fluorophore’s properties. Monte Carlo simulations of light scattering were used for designing and verifying the results obtained. The fiber-optic bundle was tested on tissue-simulating phantoms and compared with a standard nonimaging fiber-optic bundle. The new bundle was composed of 30 individual 100-μm fibers. Fibers on the end of the bundle that touch the tissue surface were separated from one another by approximately 1 mm. This design permits integration of the signal over several locations while maintaining localized sampling from regions smaller than the average mean free scattering path of the tissue. The bundle was tested by measurement of the fluorescence signals from tissue-simulating solutions containing fluorescent compounds. These studies demonstrate that the new bundle reduces the effect of the intrinsic absorption in the medium, permitting detection of fluorescence that is linearly proportional to the fluorophore concentration.

Practical Model for the Calculation of Multiply Scattered Lidar Returns

Abstract: An equation to predict the intensity of the multiply scattered lidar return is presented. Both the scattering cross section and the scattering phase function can be specified as a function of range. This equation applies when the cloud particles are larger than the lidar wavelength. This approximation considers photon trajectories with multiple small-angle forward-scattering events and one large-angle scattering that directs the photon back toward the receiver. Comparisons with Monte Carlo simulations, exact double-scatter calculations, and lidar data demonstrate that this model provides accurate results.