Showing papers in "Applied Optics in 2004"

Windowed Fourier transform for fringe pattern analysis

Abstract: Fringe patterns in optical metrology systems need to be demodulated to yield the desired parameters. Time-frequency analysis is a useful concept for fringe demodulation, and a windowed Fourier transform is chosen for the determination of phase and phase derivative. Two approaches are developed: the first is based on the concept of filtering the fringe patterns, and the second is based on the best match between the fringe pattern and computer-generated windowed exponential elements. I focus on the extraction of phase and phase derivatives from either phase-shifted fringe patterns or a single carrier fringe pattern. Principles as well as examples are given to show the effectiveness of the proposed methods.

Ultrahigh-resolution full-field optical coherence tomography

Abstract: We have developed a white-light interference microscope for ultrahigh-resolution full-field optical coherence tomography of biological media. The experimental setup is based on a Linnik-type interferometer illuminated by a tungsten halogen lamp. En face tomographic images are calculated by a combination of interferometric images recorded by a high-speed CCD camera. Spatial resolution of 1.8 μm × 0.9 μm (transverse × axial) is achieved owing to the extremely short coherence length of the source, the compensation of dispersion mismatch in the interferometer arms, and the use of relatively high-numerical-aperture microscope objectives. A shot-noise-limited detection sensitivity of 90 dB is obtained in an acquisition time per image of 4 s. Subcellular-level images of plant, animal, and human tissues are presented.

Parameter-optimized digital holographic microscope for high-resolution living-cell analysis

Abstract: A parameter-optimized off-axis setup for digital holographic microscopy is presented for simultaneous, high-resolution, full-field quantitative amplitude and quantitative phase-contrast microscopy and the detection of changes in optical path length in transparent objects, such as undyed living cells. Numerical reconstruction with the described nondiffractive reconstruction method, which suppresses the zero order and the twin image, requires a mathematical model of the phase-difference distribution between the object wave and the reference wave in the hologram plane. Therefore an automated algorithm is explained that determines the parameters of the mathematical model by carrying out the discrete Fresnel transform. Furthermore the relationship between the axial position of the object and the reconstruction distance, which is required for optimization of the lateral resolution of the holographic images, is derived. The lateral and the axial resolutions of the system are discussed and quantified by application to technical objects and to living cells.

Annular Pupils, Radial Polarization, and Superresolution

Abstract: An annular pupil, which can be used to produce a Bessel beam, when combined with radially polarized illumination promises improvements in microscope resolution, increased packing density for optical storage, and finer optical lithography. When combined with a circular detection pupil in confocal microscopy a point-spread function 112 nm wide results (lambda = 488 nm). Radially polarized annular illumination of a solid-immersion lens can yield a focal spot smaller than 100 nm for lambda = 488 nm. Use of radially polarized illumination with pupil masks is discussed.

Production and characterization of spiral phase plates for optical wavelengths

Abstract: We describe the fabrication and characterization of a high-quality spiral phase plate as a device to generate optical vortices of low (3-5) specified charge at visible wavelengths. The manufacturing process is based on a molding technique and allows for the production of high-precision, smooth spiral phase plates as well as for their replication. An attractive feature of this process is that it permits the fabrication of nominally identical spiral phase plates made from different materials and thus yielding different vortex charges. When such a plate is inserted in the waist of a fundamental Gaussian beam, the resultant far-field intensity profile shows a rich vortex structure, in excellent agreement with diffraction calculations based on ideal spiral phase plates. Using a simple optical test, we show that the reproducibility of the manufacturing process is excellent.

Gamma correction for digital fringe projection profilometry.

Abstract: Digital fringe projection profilometry utilizes a digital video projector as a structured light source and thus gains great flexibility. However, the gamma nonlinearity of the video projector inevitably decreases the accuracy and resolution of the measurement. We propose a gamma-correction technique based on statistical analysis of the fringe images. The technique allows one to estimate the value of gamma from the normalized cumulative histogram of the fringe images. By iterating the two steps, gamma estimation and phase evaluation, the actual gamma value can be calculated. At the same time the phase distribution of the fringe pattern can be solved with higher accuracy. In so doing, neither photometric calibration nor knowledge of the device is required. Both computer simulation and experiment are carried out to demonstrate the validity of this technique.

Time-resolved multidistance near-infrared spectroscopy of the adult head: intracerebral and extracerebral absorption changes from moments of distribution of times of flight of photons

Abstract: We report on multidistance time-resolved diffuse reflectance spectroscopy of the head of a healthy adult after intravenous administration of a bolus of indocyanine green. Intracerebral and extracerebral changes in absorption are deduced from moments (integral, mean time of flight, and variance) of the distributions of times of flight of photons (DTOFs), recorded simultaneously at four different source-detector separations. We calculate the sensitivity factors converting depth-dependent changes in absorption into changes of moments of DTOFs by Monte Carlo simulations by using a layered model of the head. We validate our method by analyzing moments of DTOFs simulated for the assumed changes in absorption in different layers of the head model.

Optical fiber probe for biomedical Raman spectroscopy.

Abstract: In vitro experiments have demonstrated the ability of Raman spectroscopy to diagnose a wide variety of diseases. Recent in vivo investigations performed with optical fiber probes were promising but generally limited to easily accessible organs, often requiring relatively long collection times. We have implemented an optical design strategy to utilize system throughput fully by characterizing the Raman distribution from tissue. This scheme optimizes collection efficiency, minimizes noise, and has resulted in small-diameter, highly efficient Raman probes that are capable of collecting high-quality data in 1 s. Performance has been tested through simulations and experiments with tissue models and several in vitro tissue types, demonstrating that this new design can advance Raman spectroscopy as a clinically practical technique.

Scattering optics of snow

Abstract: Permanent snow and ice cover great portions of the Arctic and the Antarctic. It appears in winter months in northern parts of America, Asia, and Europe. Therefore snow is an important component of the hydrological cycle. Also, it is a main regulator of the seasonal variation of the planetary albedo. This seasonal change in albedo is determined largely by the snow cover. However, the presence of pollutants and the microstructure of snow (e.g., the size and shape of grains, which depend also on temperature and on the age of the snow) are also of importance in the variation of the snow's spectral albedo. The snow's spectral albedo and its bidirectional reflectance are studied theoretically. The albedo also determines the spectral absorptance of snow, which is of importance, e.g., in studies of the heating regime in snow. We investigate the influence of the nonspherical shape of grains and of close-packed effects on snow's reflectance in the visible and the near-infrared regions of the electromagnetic spectrum. The rate of the spectral transition from highly reflective snow in the visible to almost totally absorbing black snow in the infrared is governed largely by the snow's grain sizes and by the load of pollutants. Therefore both the characteristics of snow and its concentration of impurities can be monitored on a global scale by use of spectrometers and radiometers placed on orbiting satellites.

Aerosol lidar intercomparison in the framework of the EARLINET project. 3. Raman lidar algorithm for aerosol extinction, backscatter, and lidar ratio

Abstract: An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.

Terahertz imaging system based on a backward-wave oscillator

Abstract: We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 microm (nearly the diffraction limit), and the signal-to-noise ratio is 10,000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.

Artificial apposition compound eye fabricated by micro-optics technology

Abstract: By exploring micro-optical design principles and technology, we have developed an artificial apposition compound eye. The overall thickness of the imaging system is only 320 microm, the diagonal field of view is 21 degrees, and the f-number is 2.6. The monolithic device consists of an UV-replicated microlens array upon a thin silica substrate with a pinhole array in a metal layer on the back side. The pitch of the pinholes differs from that of the lens array to provide individual viewing angle for each channel. Theoretical limitations of resolution and sensitivity are discussed as well as fabrication issues and compared with experimental results. A method to generate nontransparent walls between optical channels to prevent cross talk is proposed.

Coherent differential absorption lidar measurements of CO2.

Abstract: A differential absorption lidar has been built to measure CO2 concentration in the atmosphere The transmitter is a pulsed single-frequency Ho:Tm:YLF laser at a 205-μm wavelength A coherent heterodyne receiver was used to achieve sensitive detection, with the additional capability for wind profiling by a Doppler technique Signal processing includes an algorithm for power measurement of a heterodyne signal Results show a precision of the CO2 concentration measurement of 1%–2% 1σ standard deviation over column lengths ranging from 12 to 28 km by an average of 1000 pulse pairs A preliminary assessment of instrument sensitivity was made with an 8-h-long measurement set, along with correlative measurements with an in situ sensor, to determine that a CO2 trend could be detected

Reconstruction of a high-resolution image on a compound-eye image-capturing system.

Abstract: The authors have proposed an architecture for a compact image-capturing system called TOMBO (thin observation module by bound optics), which uses compound-eye imaging for a compact hardware configuration [Appl. Opt. 40, 1806 (2001)]. The captured compound image is decomposed into a set of unit images, then the pixels in the unit images are processed with digital processing to retrieve the target image. A new method for high-resolution image reconstruction, called a pixel rearrange method, is proposed. The relation between the target object and the captured signals is estimated and utilized to rearrange the original pixel information. Experimental results show the effectiveness of the proposed method. In the experimental TOMBO system, the resolution obtained is four times higher than that of the unit image that did not undergo reconstruction processing.

Aerosol lidar intercomparison in the framework of the EARLINET project. 2.Aerosol backscatter algorithms

Abstract: An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET) The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals The exercise consisted of processing synthetic lidar signals of various degrees of difficulty One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar The intercomparison was performed in three stages with increasing knowledge on the input parameters First, only the lidar signals were distributed; this is the most realistic stage Afterward the lidar ratio profiles and the reference values at calibration height were provided The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well

Gated viewing and high-accuracy three-dimensional laser radar

Abstract: We have developed a fast and high-accuracy three-dimensional (3-D) imaging laser radar that can achieve better than 1-mm range accuracy for half a million pixels in less than 1 s. Our technique is based on range-gating segmentation. We combine the advantages of gated viewing with our new fast technique of 3-D imaging. The system uses a picosecond Q-switched Nd:Yag laser at 532 nm with a 32-kHz pulse repetition frequency (PRF), which triggers an ultrafast camera with a highly sensitive CCD with 582 x 752 pixels. The high range accuracy is achieved with narrow laser pulse widths of approximately 200 ps, a high PRF of 32 kHz, and a high-speed camera with gate times down to 200 ps and delay steps down to 100 ps. The electronics and the software also allow for gated viewing with automatic gain control versus range, whereby foreground backscatter can be suppressed. We describe our technique for the rapid production of high-accuracy 3-D images, derive performance characteristics, and outline future improvements.

Dualex: a new instrument for field measurements of epidermal ultraviolet absorbance by chlorophyll fluorescence

Abstract: Dualex (dual excitation) is a field-portable instrument, hereby described, for the assessment of polyphenolic compounds in leaves from the measurement of UV absorbance of the leaf epidermis by double excitation of chlorophyll fluorescence. The instrument takes advantage of a feedback loop that equalizes the fluorescence level induced by a reference red light to the UV-light-induced fluorescence level. This allows quick measurement from attached leaves even under field conditions. The use of light-emitting diodes and of a leaf-clip configuration makes Dualex a user-friendly instrument with potential applications in ecophysiological research, light climate analysis, agriculture, forestry, horticulture, pest management, selection of medicinal plants, and wherever accumulation of leaf polyphenolics is involved in plant responses to the environment.

Mid-infrared quantum cascade laser based off-axis integrated cavity output spectroscopy for biogenic nitric oxide detection

Abstract: Tunable-laser absorption spectroscopy in the mid-IR spectral region is a sensitive analytical technique for trace-gas quantification. The detection of nitric oxide (NO) in exhaled breath is of particular interest in the diagnosis of lower-airway inflammation associated with a number of lung diseases and illnesses. A gas analyzer based on a continuous-wave mid-IR quantum cascade laser operating at ∼5.2 μm and on off-axis integrated cavity output spectroscopy (ICOS) has been developed to measure NO concentrations in human breath. A compact sample cell, 5.3 cm in length and with a volume of <80 cm3, that is suitable for on-line and off-line measurements during a single breath cycle, has been designed and tested. A noise-equivalent (signal-to-noise ratio of 1) sensitivity of 10 parts in 109 by volume (ppbv) of NO was achieved. The combination of ICOS with wavelength modulation resulted in a 2-ppbv noise-equivalent sensitivity. The total data acquisition and averaging time was 15 s in both cases. The feasibility of detecting NO in expired human breath as a potential noninvasive medical diagnostic tool is discussed.

Total internal reflection ellipsometry: principles and applications

Abstract: A concept for a measurement technique based on ellipsometry in conditions of total internal reflection is presented. When combined with surface plasmon resonance (SPR) effects, this technique becomes powerful for monitoring and analyzing adsorption and desorption on thin semitransparent metal films as well as for analyzing the semitransparent films themselves. We call this technique total internal reflection ellipsometry (TIRE). The theory of ellipsometry under total internal reflection combined with SPR is discussed for some simple cases. For more advanced cases and to prove the concept, simulations are performed with the Fresnel formalism. The use of TIRE is exemplified by applications in protein adsorption, corrosion monitoring, and adsorption from opaque liquids on metal surfaces. Simulations and experiments show greatly enhanced thin-film sensitivity compared with ordinary ellipsometry.

Automated region detection based on the contrast-to-noise ratio in near-infrared tomography.

Abstract: The contrast-to-noise ratio (CNR) was used to determine the detectability of objects within reconstructed images from diffuse near-infrared tomography. It was concluded that there was a maximal value of CNR near the location of an object within the image and that the size of the true region could be estimated from the CNR. Experimental and simulation studies led to the conclusion that objects can be automatically detected with CNR analysis and that our current system has a spatial resolution limit near 4 mm and a contrast resolution limit near 1.4. A new linear convolution method of CNR calculation was developed for automated region of interest (ROI) detection.

Spatial carrier-fringe pattern analysis by means of wavelet transform: wavelet transform profilometry

Abstract: We present an analysis of a spatial carrier-fringe pattern in three-dimensional (3-D) shape measurement by using the wavelet transform, a tool excelling for its multiresolution in the time- and space-frequency domains. To overcome the limitation of the Fourier transform, we introduce the Gabor wavelet to analyze the phase distributions of the spatial carrier-fringe pattern. The theory of wavelet transform profilometry, an accuracy check by means of a simulation, and an example of 3-D shape measurement are shown.

Toward perfect antireflection coatings. 2. Theory.

Abstract: Recently we performed a numerical investigation of antireflection coatings that reduce significantly the reflection over a wide range of wavelengths and angles of incidence, and we proposed some experiments to demonstrate their feasibility. We provide a theoretical description of omnidirectional antireflection coatings that are effective over a wide range of wavelengths.

Liquid-crystal lens with a focal length that is variable in a wide range

Abstract: A liquid-crystal (LC) lens driven by two voltages is reported. The lens has a focal length that is electrically tunable. The range of the variable focusing power is very wide, covering approximately 0.8-10.7 D. In the entire focal range the LC lens maintains high optical quality. The LC lens can be driven in a simple way to prevent the occurrence of a disclination line. The use of the LC lens in image formation is demonstrated.

Partial spatial coherence effects in digital holographic microscopy with a laser source

Abstract: We investigate a digital holographic microscope that permits us to modify the spatial coherence state of the sample illumination by changing the spot size of a laser beam on a rotating ground glass. Out-of-focus planes are refocused by digital holographic reconstruction with numerical implementation of the Kirchhoff-Fresnel integral. The partial coherence nature of the illumination reduces the coherent artifact noise with respect to fully coherent illumination. The investigated configuration allows the spatial coherence state to be changed without modifying the illumination level of the sample. The effect of the coherence state on the digital holographic reconstruction is theoretically and experimentally evaluated. We also show how multiple reflection interferences are limited by the use of reduced spatial coherent illumination.

Modeling the optical properties of mineral particles suspended in seawater and their influence on ocean reflectance and chlorophyll estimation from remote sensing algorithms.

Abstract: The optical properties of mineral particles suspended in seawater were calculated from the Mie scattering theory for different size distributions and complex refractive indices of the particles. The ratio of the spectral backscattering coefficient to the sum of the spectral absorption and backscattering coefficients of seawater, b(b)(lambda)/[a(lambda) + b(b)(lambda)], was analyzed as a proxy for ocean reflectance for varying properties and concentrations of mineral particles. Given the plausible range of variability in the particle size distribution and the refractive index, the general parameterizations of the absorption and scattering properties of mineral particles and their effects on ocean reflectance in terms of particle mass concentration alone are inadequate. The variations in the particle size distribution and the refractive index must be taken into account. The errors in chlorophyll estimation obtained from the remote sensing algorithms that are due to the presence of mineral particles can be very large. For example, when the mineral concentration is 1 g m(-3) and the chlorophyll a concentration is low (0.05 mg m(-3)), current global algorithms based on a blue-to-green reflectance ratio can produce a chlorophyll overestimation ranging from approximately 50% to as much as 20-fold.

Aerosol lidar intercomparison in the framework of the EARLINET project. 1. Instruments.

Abstract: In the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET), 19 aerosol lidar systems from 11 European countries were compared. Aerosol extinction or backscatter coefficient profiles were measured by at least two systems for each comparison. Aerosol extinction coefficients were derived from Raman lidar measurements in the UV (351 or 355 nm), and aerosol backscatter profiles were calculated from pure elastic backscatter measurements at 351 or 355, 532, or 1064 nm. The results were compared for height ranges with high and low aerosol content. Some systems were additionally compared with sunphotometers and starphotometers. Predefined maximum deviations were used for quality control of the results. Lidar systems with results outside those limits could not meet the quality assurance criterion. The algorithms for deriving aerosol backscatter profiles from elastic lidar measurements were tested separately, and the results are described in Part 2 of this series of papers [Appl. Opt.43, 977–989 (2004)]. In the end, all systems were quality assured, although some had to be modified to improve their performance. Typical deviations between aerosol backscatter profiles were 10% in the planetary boundary layer and 0.1 × 10-6 m-1 sr-1 in the free troposphere.

Performance of the Keck Observatory adaptive-optics system

Abstract: The adaptive-optics (AO) system at the W. M. Keck Observatory is characterized. We calculate the error budget of the Keck AO system operating in natural guide star mode with a near-infrared imaging camera. The measurement noise and bandwidth errors are obtained by modeling the control loops and recording residual centroids. Results of sky performance tests are presented: The AO system is shown to deliver images with average Strehl ratios of as much as 0.37 at 1.58 μm when a bright guide star is used and of 0.19 for a magnitude 12 star. The images are consistent with the predicted wave-front error based on our error budget estimates.

Mueller polarimetric imaging system with liquid crystals

Abstract: We present a new polarimetric imaging system based on liquid-crystal modulators, a spectrally filtered white-light source, and a CCD camera. The whole Mueller matrix image of the sample is measured in approximately 5 s in the transmission mode. The instrument design, together with an original and easy-to-operate calibration procedure, provides high accuracy over a wide spectral range (500–700 nm). This accuracy has been assessed by measurement of a linear polarizer at different orientations and a thick wedged quartz plate as an example of a partially depolarized retarder. Polarimetric images of a stained hepatic biopsy with significant fibrosis have been taken at several wavelengths. The optical properties of Picrosirius Red stained collagen (diattenuation, retardance, and polarizance) have been measured independently from each other between 500 and 700 nm.

Silica-nanoparticle-dispersed methacrylate photopolymers with net diffraction efficiency near 100%.

Abstract: We demonstrate volume holographic recording in silica-nanoparticle-dispersed methacrylate photopolymers with reduced scattering loss as low as 2%. This is made possible by use of 13-nm silica nanoparticles. As a result a net diffraction efficiency near 100% is achieved for a transmission volume hologram of 45-μm thickness. Grating buildup dynamics are measured for various nanoparticle concentrations, and the effects of nanoparticle size on refractive-index modulation and polymerization shrinkage are also evaluated.

High-transfer-rate high-capacity holographic disk data-storage system.

Abstract: We describe the design and implementation of a high-data-rate high-capacity digital holographic storage disk system. Various system design trade-offs that affect density and data-rate performance are described and analyzed. In the demonstration system that we describe, high-density holographic recording is achieved by use of high-resolution short-focal-length optics and correlation shift multiplexing in photopolymer disk media. Holographic channel decoding at a 1-Gbit/s data rate is performed by custom-built electronic hardware. A benchmark sustained optical data-transfer rate of 10 Gbits/s has been successfully demonstrated.