Showing papers in "Applied Optics in 2002"

Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters

Abstract: For open ocean and coastal waters, a multiband quasi-analytical algorithm is developed to retrieve absorption and backscattering coefficients, as well as absorption coefficients of phytoplankton pigments and gelbstoff. This algorithm is based on remote-sensing reflectance models derived from the radiative transfer equation, and values of total absorption and backscattering coefficients are analytically calculated from values of remote-sensing reflectance. In the calculation of total absorption coefficient, no spectral models for pigment and gelbstoff absorption coefficients are used. Actually those absorption coefficients are spectrally decomposed from the derived total absorption coefficient in a separate calculation. The algorithm is easy to understand and simple to implement. It can be applied to data from past and current satellite sensors, as well as to data from hyperspectral sensors. There are only limited empirical relationships involved in the algorithm, and they are for less important properties, which implies that the concept and details of the algorithm could be applied to many data for oceanic observations. The algorithm is applied to simulated data and field data, both non-case1, to test its performance, and the results are quite promising. More independent tests with field-measured data are desired to validate and improve this algorithm.

Optimization of a semianalytical ocean color model for global-scale applications

Abstract: Semianalytical (SA) ocean color models have advantages over conventional band ratio algorithms in that multiple ocean properties can be retrieved simultaneously from a single water-leaving radiance spectrum. However, the complexity of SA models has stalled their development, and operational implementation as optimal SA parameter values are hard to determine because of limitations in development data sets and the lack of robust tuning procedures. We present a procedure for optimizing SA ocean color models for global applications. The SA model to be optimized retrieves simultaneous estimates for chlorophyll (Chl) concentration, the absorption coefficient for dissolved and detrital materials [a(cdm)(443)], and the particulate backscatter coefficient [b(bp)(443)] from measurements of the normalized water-leaving radiance spectrum. Parameters for the model are tuned by simulated annealing as the global optimization protocol. We first evaluate the robustness of the tuning method using synthetic data sets, and we then apply the tuning procedure to an in situ data set. With the tuned SA parameters, the accuracy of retrievals found with the globally optimized model (the Garver-Siegel-Maritorena model version 1; hereafter GSM01) is excellent and results are comparable with the current Sea-viewing Wide Field-of-view sensor (SeaWiFS) algorithm for Chl. The advantage of the GSM01 model is that simultaneous retrievals of a(cdm)(443) and b(bp)(443) are made that greatly extend the nature of global applications that can be explored. Current limitations and further developments of the model are discussed.

Fast two-dimensional phase-unwrapping algorithm based on sorting by reliability following a noncontinuous path.

Abstract: We describe what is to our knowledge a novel technique for phase unwrapping. Several algorithms based on unwrapping the most-reliable pixels first have been proposed. These were restricted to continuous paths and were subject to difficulties in defining a starting pixel. The technique described here uses a different type of reliability function and does not follow a continuous path to perform the unwrapping operation. The technique is explained in detail and illustrated with a number of examples.

High-resolution full-field optical coherence tomography with a Linnik microscope

Abstract: We describe an original microscope for high-resolution optical coherence tomography applications. Our system is based on a Linnik interference microscope with high-numerical-aperture objectives. Lock-in detection of the interference signal is achieved in parallel on a CCD by use of a photoelastic birefringence modulator and full-field stroboscopic illumination with an infrared LED. Transverse cross-section (en-face, or XY) images can be obtained in real time with better than 1-microm axial (Z) resolution and 0.5-microm transverse (XY) resolution. A sensitivity of approximately 80 dB is reached at a 1-image/s acquisition rate, which allows tomography in scattering media such as biological tissues.

General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference

Abstract: The optical response of coherent thin-film multilayers is often represented with Fresnel coefficients in a 2 x 2 matrix configuration. Here the usual transfer matrix was modified to a generic form, with the ability to use the absolute squares of the Fresnel coefficients, so as to include incoherent (thick layers) and partially coherent (rough surface or interfaces) reflection and transmission. The method is integrated by use of models for refractive-index depth profiling. The utility of the method is illustrated with various multilayer structures formed by ion implantation into Si, including buried insulating and conducting layers, and multilayers with a thick incoherent layer in an arbitrary position.

New paradigm for imaging systems

Abstract: We describe a new paradigm for designing hybrid imaging systems. These imaging systems use optics with a special aspheric surface to code the image so that the point-spread function or the modulation transfer function has specified characteristics. Signal processing then decodes the detected image. The coding can be done so that the depth of focus can be extended. This allows the manufacturing tolerance to be reduced, focus-related aberrations to be controlled, and imaging systems to be constructed with only one optical element plus some signal processing. OCIS codes: 080.3620, 110.0110, 110.2990, 110.0180, 110.4850, 180.0180. 1. Introduction and Background The new paradigm that we describe for the design of imaging systems has been termed wave-front coding. These coded optical systems are arrived at by means of designing the coding optics and the signal processing as an integrated imaging system. The results are imaging systems with previously unobtainable imaging modalities and require a modification of the optics for coding the wave in the aperture stop or an image of the aperture stop. This coding produces an intermediate image formed by the optical portion of the system that gathers the image. Signal processing is then required for decoding the intermediate image to produce a final image. The coding can be designed to make the imaging system invariant to certain parameters or to optimize the imaging system’s sensitivity to those parameters. One example is the use of image coding to preserve misfocus and hence, range or distance information. Another example is the use of different types of codes to make the image invariant to misfocus. These new focusinvariant imaging systems can have more than an order of magnitude increase in the depth of field. Our emphasis in this paper is on the use of the increased depth of focus to design new types of imaging systems. An example of the new imaging systems that can be constructed is a single-element lens that has a small F#, wide field of view, and diffractionlimited imaging. It also can have greatly relaxed assembly tolerances, because of its invariance to focus-related aberrations. Coding of signals for optimally conveying particular information is not new. In radar the transmitted pulses are coded for optimally providing information concerning a target’s range, for example. The appropriate signal processing to extract the range information is designed in conjunction with the transmitted signal. The integrated design of the optical image-gathering portion along with the signal processing normally is not done in the design of imaging systems. There are exceptions such as tomography, coded aperture imaging, and sometimes, interferometric imaging. In 1984 a group that was investigating the limits of resolution pointed out the potential of increasing the performance of imaging systems by jointly designing the optics and the signal processing. 1

Surface characterization techniques for determining the root-mean-square roughness and power spectral densities of optical components

Abstract: Surface topography and light scattering were measured on 15 samples ranging from those having smooth surfaces to others with ground surfaces. The measurement techniques included an atomic force microscope, mechanical and optical profilers, confocal laser scanning microscope, angle-resolved scattering, and total scattering. The samples included polished and ground fused silica, silicon carbide, sapphire, electroplated gold, and diamond-turned brass. The measurement instruments and techniques had different surface spatial wavelength band limits, so the measured roughnesses were not directly comparable. Two-dimensional power spectral density (PSD) functions were calculated from the digitized measurement data, and we obtained rms roughnesses by integrating areas under the PSD curves between fixed upper and lower band limits. In this way, roughnesses measured with different instruments and techniques could be directly compared. Although smaller differences between measurement techniques remained in the calculated roughnesses, these could be explained mostly by surface topographical features such as isolated particles that affected the instruments in different ways.

Bidirectional reflectance of oceanic waters: accounting for Raman emission and varying particle scattering phase function

Abstract: The bidirectionality of the upward radiance field in oceanic case 1 waters has been reinvestigated by incorporation of revised parameterizations of inherent optical properties as a function of the chlorophyll concentration (Chl), considering Raman scattering and making the particle phase function shape (βp) continuously varying along with the Chl. Internal consistency is thus reached, as the decrease in backscattering probability (for increasing Chl) translates into a correlative change in βp. The single particle phase function (previously used) precluded a realistic assessment of bidirectionality for waters with Chl > 1 mg m-3. This limitation is now removed. For low Chl, Raman emissions significantly affect the radiance field. For moderate Chl (0.1–1 mg m-3), new and previous bidirectional parameters remain close. The ocean reflectance anisotropy has implications in ocean color remote-sensing problems, in derivation of coherent water-leaving radiances, in associated calibration–validation activities, and in the merging of data obtained under various geometrical configurations.

Phase function effects on oceanic light fields.

Abstract: Numerical simulations show that underwater radiances, irradiances, and reflectances are sensitive to the shape of the scattering phase function at intermediate and large scattering angles, although the exact shape of the phase function in the backscatter directions (for a given backscatter fraction) is not critical if errors of the order of 10% are acceptable. We present an algorithm for generating depth- and wavelength-dependent Fournier-Forand phase functions having any desired backscatter fraction. Modeling of a comprehensive data set of measured inherent optical properties and radiometric variables shows that use of phase functions with the correct backscatter fraction and overall shape is crucial to achieve model-data closure.

Design of optimal polarimeters: maximization of signal-to-noise ratio and minimization of systematic error.

Abstract: The relationship between system condition and signal-to-noise ratio (SNR) in reconstructed Stokes parameter images is investigated for rotating compensator, variable retardance, and rotating analyzer Stokes vector (SV) polarimeters. A variety of optimal configurations are presented for each class of systems. The operation of polarimeters is discussed in terms of a four-dimensional conical vector space; and the concept of nonorthogonal bases, frames, and tight frames is introduced to describe the operation of SV polarimeters. Although SNR is an important consideration, performance of a polarimeter in the presence of errors in the calibration and alignment of the optical components is also important. The relationship between system condition and error performance is investigated, and it is shown that an optimum system from the point of view of SNR is not always an optimum system with respect to error performance. A detailed theory of error performance is presented, and the error of a SV polarimeter is shown to be related to the stability and condition number of the polarization processing matrices. The rms error is found to fall off as the inverse of the number of measurements taken. Finally, the concepts used to optimize SV polarimeters are extended to be useful for full Mueller matrix polarimeters.

Review of the fundamentals of thin-film growth.

Abstract: The properties of a thin film of a given material depend on the film's real structure. The real structure is defined as the link between a thin film's deposition parameters and its properties. To facilitate engineering the properties of a thin film by manipulating its real structure, thin-film formation is reviewed as a process starting with nucleation followed by coalescence and subsequent thickness growth, all stages of which can be influenced by deposition parameters. The focus in this review is on dielectric and metallic films and their optical properties. In contrast to optoelectronics all these film growth possibilities for the engineering of novel optical films with extraordinary properties are just beginning to be used.

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser

Abstract: We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.

Broadband absorption spectroscopy in turbid media by combined frequency-domain and steady state methodologies

Abstract: A technique for measuring broadband near-infrared absorption spectra of turbid media (12) uses a combination of frequency-domain and steady-state reflectance methods Most of the wavelengths coverage is provided by steady-state measurement using a white light (21) and a spectrograph (22) The frequency-domain data are acquired at a few selected wavelengths using laser diodes (13) and an avalanche photodiode unit (16) Coefficients of absorption and reduced scattering derived from the frequency-domain data are used to calibrate the intensity of the steady-state measurements and to determine the reduced scattering coefficient at all wavelengths in the spectral window of interest, The absorption coefficient spectrum is determined by comparing the steady-state reflectance values with the predictions of diffusion theory, wavelength by wavelength Absorption spectra of a turbid phantom and of human breast tissue in vivo, derived with the combined frequency-domain and steady-state technique, agree well with expected reference values

Toward perfect antireflection coatings: numerical investigation.

Abstract: A perfect antireflection (AR) coating would remove completely the reflection from an interface between two media for all wavelengths, polarizations, and angles of incidence. The degree to which this can be achieved is investigated numerically. It is shown that wideband solutions can be found provided that layers can be deposited with refractive indices that are close to that of the low-index medium. Thus realistic solutions exist for interfaces between two solid media. Narrow-band high-angle AR solutions are also possible for polarized light and for unpolarized light in the vicinity of certain reststrahlen bands.

Inversion with regularization for the retrieval of tropospheric aerosol parameters from multiwavelength lidar sounding

Abstract: We present an inversion algorithm for the retrieval of particle size distribution parameters, i.e., mean (effective) radius, number, surface area, and volume concentration, and complex refractive index from multiwavelength lidar data. In contrast to the classical Tikhonov method, which accepts only that solution for which the discrepancy reaches its global minimum, in our algorithm we perform the averaging of solutions in the vicinity of this minimum. This averaging stabilizes the underlying ill-posed inverse problem, particularly with respect to the retrieval of number concentration. Results show that, for typical tropospheric particles and 10% error in the optical data, the mean radius could be retrieved to better than 20% from a lidar on the basis of a Nd:YAG laser, which provides a combination of backscatter coefficients at 355, 532, and 1064 nm and extinction coefficients at 355 and 532 nm. The accuracy is improved if the lidar is also equipped with a hydrogen Raman shifter. In this case two additional backscatter coefficients at 416 and 683 nm are available. The combination of two extinction coefficients and five backscatter coefficients then allows one to retrieve not only averaged aerosol parameters but also the size distribution function. There was acceptable agreement between physical particle properties obtained from the evaluation of multiwavelength lidar data taken during the Lindenberg Aerosol Characterization Experiment in 1998 (LACE 98) and in situ data, which were taken aboard aircraft.

Compression of digital holograms for three-dimensional object reconstruction and recognition

Abstract: We present the results of applying lossless and lossy data compression to a three-dimensional object reconstruction and recognition technique based on phase-shift digital holography. We find that the best lossless (Lempel-Ziv, Lempel-Ziv-Welch, Huffman, Burrows-Wheeler) compression rates can be expected when the digital hologram is stored in an intermediate coding of separate data streams for real and imaginary components. The lossy techniques are based on subsampling, quantization, and discrete Fourier transformation. For various degrees of speckle reduction, we quantify the number of Fourier coefficients that can be removed from the hologram domain, and the lowest level of quantization achievable, without incurring significant loss in correlation performance or significant error in the reconstructed object domain.

Experimental determination of the lidar overlap profile with Raman lidar.

Abstract: The range-dependent overlap between the laser beam and the receiver field of view of a lidar can be determined experimentally if a pure molecular backscatter signal is measured in addition to the usually observed elastic backscatter signal, which consists of a molecular component and a particle component. Two methods, the direct determination of the overlap profile and an iterative approach, are presented and applied to a lidar measurement. The measured overlap profile accounts for actual system alignment and for all system parameters that are not explicitly known, such as actual laser beam divergence and spatial intensity distribution of the laser light.

Real-time three-dimensional object reconstruction by use of a phase-encoded digital hologram.

Abstract: A three-dimensional (3D) object reconstruction technique that uses only phase information of a phase-shifting digital hologram and a phase-only spatial-light modulator is proposed. It is well known that a digital hologram can store both amplitude and phase information of an optical electric field and can reconstruct the original 3D object in a computer. We demonstrate that it is possible to reconstruct optically 3D objects using only phase information of the optical field calculated from phase-shifting digital holograms. The use of phase-only information enables us to reduce the amount of data in the digital hologram and reconstruct optically the 3D objects using a liquid-crystal spatial light modulator without optical power loss. Numerical evaluation of the reconstructed 3D object is presented.

Sellmeier and thermo-optic dispersion formulas for KTP

Abstract: KTP has been found to be phase matchable for type-2 second-harmonic generation of the third harmonics of the CO2 laser wavelengths at 9.2714 and 9.5525 microm at 20.0 degrees C. The resulting angle and temperature-tuning data combined with the literature values of the Nd:YAG laser-pumped optical parametric oscillator were used to improve the Sellmeier equations and the thermo-optic dispersion formula that reproduce well the temperature-tuned 90 degree phase-matching conditions of the flux-grown crystals. Applications to quasi phase matching are presented.

Determination of fringe order in white-light interference microscopy

Abstract: Combining phase and coherence information for improved precision in white-light interference microscopy requires a robust strategy for dealing with the inconsistencies between these two types of information. We correct for these inconsistencies on every measurement by direct analysis of the difference map between the coherence and the phase profiles. The algorithm adapts to surface texture and noise level and dynamically compensates for optical aberrations, distortions, diffraction, and dispersion that would otherwise lead to incorrect fringe order. The same analysis also provides the absolute height data that are essential to relational measurements between disconnected surfaces.

Optical properties of waveguides fabricated in fused silica by femtosecond laser pulses

Abstract: With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.

High-speed 1-frame/ms scanning confocal microscope with a microlens and Nipkow disks.

Abstract: We have developed a high-speed confocal laser microscope. A microlens-array disk set in front of a pinhole-array disk improved optical efficiency more than ten times compared with that of conventional Nipkow confocal microscopy. This new microscope achieves a high-speed measurement of 1 frame/ms. We expect that it will be used for measuring biological and industrial active samples.

Trapping forces, force constants, and potential depths for dielectric spheres in the presence of spherical aberrations

Abstract: We present and verify a theoretical model that predicts trapping forces (escape forces), force constants (trap stiffnesses), and trapping potential depths for dielectric spheres with diameters smaller than or equal to the wavelength of the trapping light. Optical forces can be calculated for arbitrary incident light distributions with a two-component approach that determines the gradient and the scattering force separately. We investigate the influence of spherical aberrations that are due to refractive-index mismatch on the maximum trapping force, the force constant, and the potential depth of a trap, which are important for optical tweezer applications. The relationships between the three parameters are explained and studied for different degrees of spherical aberration and various spheres (refractive indices ns = 1.39–1.57, radii a = 0.1–0.5 µm, λ0 = 1.064 µm). We find that all three parameters decrease when the distance to the coverslip increases. Effects that could make the interpretation of experimental results ambiguous are simulated and explained. Computational results are compared with the experimental data found in the literature. A good coincidence can be established.

Lidar ratio and depolarization ratio for cirrus clouds.

Abstract: We report on studies of the lidar and the depolarization ratios for cirrus clouds. The optical depth and effective lidar ratio are derived from the transmission of clouds, which is determined by comparing the backscattering signals at the cloud base and cloud top. The lidar signals were fitted to a background atmospheric density profile outside the cloud region to warrant the linear response of the return signals with the scattering media. An average lidar ratio, 29 +/- 12 sr, has been found for all clouds measured in 1999 and 2000. The height and temperature dependences ofthe lidar ratio, the optical depth, and the depolarization ratio were investigated and compared with results of LITE and PROBE. Cirrus clouds detected near the tropopause are usually optically thin and mostly subvisual. Clouds with the largest optical depths were found near 12 km with a temperature of approximately -55 degrees C. The multiple-scattering effect is considered for clouds with high optical depths, and this effect lowers the lidar ratios compared with a single-scattering condition. Lidar ratios are in the 20-40 range for clouds at heights of 12.5-15 km and are smaller than approximately 30 in height above 15 km. Clouds are usually optically thin for temperatures below approximately -65 degrees C, and in this region the optical depth tends to decrease with height. The depolarization ratio is found to increase with a height at 11-15 km and smaller than 0.3 above 16 km. The variation in the depolarization ratio with the lidar ratio was also reported. The lidar and depolarization ratios were discussed in terms of the types of hexagonal ice crystals.

Optoacoustic technique for noninvasive monitoring of blood oxygenation: a feasibility study.

Abstract: Replacement of invasive monitoring of cerebral venous oxygenation with noninvasive techniques offers great promise in the management of life-threatening neurologic illnesses including traumatic brain injury. We developed and built an optoacoustic system to noninvasively monitor cerebral venous oxygenation; the system includes a nanosecond Nd:YAG laser and a specially designed optoacoustic probe. We tested the system in vitro in sheep blood with experimentally varied oxygenation. Our results demonstrated that (1) the amplitude and temporal profile of the optoacoustic waves increase with blood oxygenation in the range from 24% to 92%, (2) optoacoustic signals can be detected despite optical and acoustic attenuation by thick bone, and (3) the system is capable of real-time and continuous measurements. These results suggest that the optoacoustic technique is technically feasible for continuous, noninvasive monitoring of cerebral venous oxygenation.

Digital in-line holography of microspheres.

Abstract: We have used digital in-line holography (DIH) with numerical reconstruction to image micrometer-sized latex spheres as well as ferrimagnetic beads suspended in gelatin. We have examined in detail theoretically and experimentally the conditions necessary to achieve submicrometer resolution of holographic reconstructions. We found that both transparent and opaque particles could be imaged with a resolution that was limited only by the wavelength of the light used. Simple inspection of intensity profiles through a particle allowed an estimate to be made of the particle’s three position coordinates within an accuracy of a few hundred nanometers. When the derivative of a second-order polynomial fitted to the intensity profiles was taken, the X, Y, Z position coordinates of particles could be determined within ±50 nm. More-accurate positional resolution should be possible with the help of more-advanced computer averaging techniques. Because a single hologram can give information about a large collection of distributed particles, DIH offers the prospect of a powerful new tool for three-dimensional tracking of particles.

Dynamic optical coherence tomography in studies of optical clearing, sedimentation, and aggregation of immersed blood.

Abstract: The concept of refractive-index matching to enhance the optical penetration depth of whole blood is discussed on the basis of in vitro studies that used the technique of near-infrared optical coherence tomography. It was found that optical clearing of blood is defined not only by refractive-index matching but also by changes in the size of red blood cells and in their aggregation ability when chemicals are added. For example, in whole blood diluted to twice its volume by saline with the addition of 6.5% glycerol, the total attenuation coefficient was reduced from 4.2 to 2.0 mm-1, and the optical penetration at 820 nm was correspondingly increased to 117%. For the other agents tested (glucose, dextrans, propylene glycol, and trazograph) the enhancement of penetration was 20–150.5%. In the blood sedimentation study, regular or irregular oscillations or jumps of the red-blood cell–plasma boundary were observed. The 1-min time period of regular oscillations correlated well with the kinetics of the aggregation process as described by the two subsequent stages of formation of linear and three-dimensional aggregates. The results also showed that optical clearing of blood by osmotic agents is potentially useful not only in blood sedimentation and aggregation studies but also in intravascular optical coherence tomography imaging techniques.

Information hiding technique with double phase encoding.

Abstract: We propose a technique for information hiding using double phase encoding. The proposed method uses a weighted double phase-encoded hidden image added to a host image referred to as the transmitted image. We develop an analytical presentation for the system performance using the statistical properties of double phase encoding. The peak signal-to-noise-ratio metric is used as a measure for the degradation in the quality of the host image and the recovered hidden image. We test, analytically, the distortion of the hidden image that is due to the host image and the effect of occlusion of the pixels of the transmitted image (that is, the host image containing the hidden image). Moreover, we discuss the effect of using only the real part of the transmitted image to recover the hidden image. Computer simulations are presented to test the system performance against these types of distortion. The simulations illustrate the system ability to recover the hidden image under distortions and the robustness of the hidden image against removal trials.

Optimization of a dual-rotating-retarder Mueller matrix polarimeter.

Abstract: The dual-rotating-retarder configuration is one of the most common forms of the Mueller matrix polarimeter. I perform an optimization of this polarimeter configuration by minimizing the condition number of the system data reduction matrix. I find the optimum retardance for the rotating retarders to be 127 degrees. If exactly 16 intensity measurements are used for a Mueller matrix calculation, a complex relationship exists between the condition number and the sizes of the angular increments of the two retarders. If many intensity measurements are made, thus overspecifying the calculation, I find broad optimal ranges of angular increments of the two retarders that yield essentially equal performance. Experimental results are given.

Multiscale displacement field measurements of compressed mineral-wool samples by digital image correlation

Abstract: We propose a multiscale approach to determine the displacement field by digital image correlation. The displacement field is first estimated on a coarse resolution image and progressively finer details are introduced in the analysis as the displacement is more and more securely and accurately determined. Such a scheme has been developed to increase the robustness, accuracy, and reliability of the image-matching algorithm. The procedure is used on two different types of examples. The first one deals with a representative image that is deformed precisely and purposefully to assess the intrinsic performances. In particular, the maximum measurable strain is determined. The second case deals with a series of pictures taken during compression experiments on mineral-wool samples. The different steps of the procedure are analyzed and their respective role is assessed. Both reflection and transmission images are tested.