This PDF file contains the editorial “Editorial: Thank You Reviewers” for OE Vol. 31 Issue 07

This PDF file contains the editorial “Guest Editorial: Special Section on Biomedical Optics” for OE Vol. 31 Issue 07

Silica optical fibers commonly are used for diagnostic, endoscopic, and surgical applications in medical industries. Optical fiber coatings play an important role in fiber strength, reliability, and biocompatibility. Three major coatings are commonly used on optical fibers: (1) polymeric coatings, such as UV-cured acrylate, silicone, and polyimide; (2) metallic coatings such as aluminum, indium, tin, and gold; and (3) inorganic coatings such as oxides, carbides, nitrides, and carbon. In the medical industry, polymeric coatings are used widely. Metallic and inorganic coatings on optical fibers can also be used. I describe the strength, fatigue, and biocompatibility of silica fibers with different coatings.

The development of sapphire tips, coupled to the distal end of silica fibers, introduced the use of a new surgical power delivery system known as contact tip surgery. An alternative medium to sapphire tips is the use of silica optical fibers with molded distal ends. We investigated a number of possible approaches for the fabrication of a variety of distal end configurations on silica fibers using an oxy-propane gas torch, arc fusion, Nd:YAG radiation, and CO₂ radiation. We fabricated seven silica contact tips that included a clear and frosted microlens, a clear and frosted ball lens, a clear and frosted conical lens, and a flat end lens. The most reliable method found to fabricate silica contact tips used a CO₂ laser because it offered complete control over the manufacturing process.

We analyze the optical properties of two commercial contact tips in air and saline: a silica conical tip manufactured by Heraeus Laser- Sonics and a sapphire rounded tip manufactured by SLT, together with a set of seven laboratory-fabricated silica tips. A statistical comparison (Dunnett's test) was used to determine the significance of the rounded sapphire contact tip compared to a set of five sculpted silica contact tips when used in a tissue mass ablating experiment using excised rat liver. A regression test was performed to determine a model for the relationship between mass ablation and energy. In addition, seven contact tips were assessed for an evaluation of laser-induced damage in excised rat liver. The tissue damage was assessed using histological sections, stained with hematoxylin and eosin, and examined using a light microscope. Dunnett's test showed that with a level of significance of P < 0.05, no mass ablating ability difference existed between the five silica contact tips and the sapphire contact tip. The regression test showed that with a level of significance of P < 0.05, a first-degree linear model applies to mass ablation and energy input. The results of the histological investigation indicate that no relationship exists between tissue damage and the geometrical profiles for the various contact tips.

A mathematical model has been developed for calculating the heating effects of a laser beam on biological tissue. The model may be used for calculating the temperature rise and the extent of damage in tissue exposed to laser irradiation. It can be applied in conjunction with laser coagulation, laser surgery, laser hyperthermia, etc. We briefly describe the model and discuss its use for calculating temperature fields in lased tissue. Experimentally, we measured the temperature profiles in soft and hard tissue under nonablating conditions. The experimental results are in good agreement with the theoretical ones.

A numerical model was developed for calculating the temperature distribution in material exposed to a laser beam. The model takes into account the different stages undergone by the tissue before material removal is achieved. The consecutive steps considered are: heating to the boiling temperature of water, boiling and evaporation of the water content, heating of the dehydrated tissue to its ablation temperature, removal of the relevant material, and adapting the new geometrical boundaries of the remaining tissue. The temperature field was computed by the model and measured experimentally in biological tissue exposed to pulses of highly absorbed laser radiation. Good agreement was obtained between the theoretical and the experimental results. This model may be applied to optimize the irradiation conditions for a specific application, to determine the temperature distribution during laser cutting, and to minimize the resulting thermal damage.

The recent use of fiber optic waveguides for biochemical analyses is based mainly on light intensity changes. Waveguide evanescent field ellipsometry (WEFE) is proposed for biosensor applications. The WEFE is based on the principle of ellipsometry. A laser beam coupled into the waveguide propagates in the waveguide with two fundamental modes with transverse electric fields parallel and perpendicular to the sensing surface. The evanescent waves interact with the adsorbed or conjugated layer of biomolecules, producing changes in the relative phase and amplitude upon interaction. The ellipsometric parameters change with biomolecule adsorption or conjugation on the sensing surface. From the ellipsometric parameter changes, we can estimate the surface concentration and thus the concentration of the biomolecules in solution. The principles of WEFE including light propagation in the waveguide, ellipsometry parameter analysis, the WEFE arrangement, materials and methods, and preliminary experimental results are presented.

Eleven autopsy specimens of healthy human abdominal aorta were irradiated under normal saline at room temperature (21.5±1.5°C) using an excimer laser (308 nm, 40 Hz, 115 ns, 57 mJ/mm², 900-μm fused silica fiber). After irradiation, transmission spectra from untreated areas and from the damage zone next to the ablation area were obtained by microspectrophotometry (250- to 800-nm spectral range, 25-μm cryosections, 6.3-μm-diam measured area). The optical density (OD) of the damage zone was significantly increased over that of untreated areas in the near ultraviolet and parts of the visible spectral range. The OD of the irradiated tissue was higher, up to a factor of 2.4 for intima, a factor of 3.7 for media, and a factor of 2.3 for adventitia. At 308 nm, OD increased from 0.094±0.019 to 0.237±0.041 for intima (p<0.001), from 0.165±0.053 to 0.501±0.034 for media (p<0.001), and from 0.119±0.017 to 0.276±0.026 for adventitia (p<0.001). We show that excimer laser irradiation using a high repetition rate in combination with a high energy density changes significantly the optical properties of a human vessel wall.

The first prerequisite for an optimum effect of photodynamic therapy with chlorophyll-derived photosensitizers is irradiation at the S₁ absorption maximum in the red spectral region. This absorption maximum changes its position due to molecular association by 20 to 100 nm depending on the subcellular environment, and must be determined by direct absorption spectrometry in the region of subcellular sensitizer localization. Fluorescence-intensifying video microscopy allows for localization of the sensitizer storage site at or near the Golgi apparatus of OAT 75 small-cell lung carcinoma cells. The absorption maximum at 760 nm taken from spectra of single cells and cell layers determines the postulated optimum condition for dye laser irradiation with bacteriopheophorbide a methyl ester as the sensitizer.

The intrinsic fluorescence of various cell cultures in the blue and green spectral range has been attributed mainly to hydrated nicotinamide adenine dinucleotide (NADH) and flavin molecules. Their fluorescence decay curves were measured with subnanosecond resolution. The reduced coenzymes NADH and hydrated nicotinamide adenine dinucleotide phosphate NADPH, both showed a biexponential decay pattern in solution with similar time constants, but different relative intensities of the two components. They could thus be distinguished from one another as well as from their oxidized forms. The NAD(P)H fluorescence of Saccharomyces cerevisiae was located within the cytoplasm and its organelles and was by about a factor 4 higher for respiratory-deficient than for intact yeast strains. Intracellular flavin fluorescence showed a triexponential behavior-probably due to a superposition of protein-bound and free flavin molecules. The lifetime of the shortest component varied within the time range of 0.20 to 0.50 ns between respiratory-deficient and intact yeast strains, and the relative intensity of this component as most pronounced for the intact strain DL1. Time-resolved fluorescence seems therefore to be an appropriate method to probe the function of the respiratory chain and-in the further step-to differentiate between various types of cells and tissues in medical diagnosis or environmental research.

Despite technical advances, the incidence of anastomotic leaks in elective colorectal surgery remains around 14%. Recent studies suggest that the use of low-energy lasers may enhance wound healing in different tissues in a selective, nondestructive manner. Based on these findings we have attempted to provide experimental background on the effects of a He-Ne laser during the early stages of healing in 155 colonic anastomoses performed on rats. The irradiation (external or endoscopical) ofthe anastomoses by repeated doses below 4 J/cm² suggest (1) the laser-induced enhancement of anastomotic healing can be obtained in both healthy colons or in situations in which anastomotic breakdown is more likely, such as colonic obstruction; (2) the photochemical effects observed can be transmitted through optic fibers; (3) energy densities above the range of 4 J/cm² show no benefit in wound-healing stimulation.

To develop an improved fiber optic biosensor both the optical component selection and the signal coupling efficiency were investigated. The emission filter and fiber connectors were carefully chosen to reduce their contribution to noise in the system. We used long, fused silica fibers that had several centimeters of cladding removed along the distal end. This exposed core is coated with the recognition molecules that bind analyte-fluorophore complexes from the sample solution. A fluorescent signal generated in the evanescent wave region of the unclad, immersed portion of the probe is lost as it enters the cladded portion of the fiber because of a V-number mismatch. To minimize the mismatch, the core radius is reduced along the uncladded region to form a continuous taper. An assay using the tapered fiber and the described optical configuration is presented that demonstrates instantaneous signal generation in response to nanogram amounts of a toxic material.

Laser fluorescence activated slit-scan flow cytometry offers an approach to a fast, quantitative characterization of chromosomes due to morphological features. It can be applied for screening of chromosomal abnormalities. We give a preliminary report on the development of the Heidelberg slit-scan flow cytometer. Time-resolved measurements of the fluorescence intensity along the chromosome axis can be registered simultaneously for two parameters when the chromosome passes perpendicularly through a narrowly focused laser beam combined by a detection slit in the image plane. So far automated data analysis has been performed off-line on a PC. In its final performance, the Heidelberg slit-scan flow cytometer will achieve on-line data analysis that allows an electroacoustical sorting of chromosomes of interest. Interest is high in the agriculture field to study chromosome aberrations that influence the size of litters in pig (Sus scrofa domestica) breeding. Slit-scan measurements have been performed to characterize chromosomes of pigs; we present results for chromosome 1 and a translocation chromosome 6/15.

Human skin shows a strong autofluorescence in the red spectral region with main peaks around 600, 620, and 640 nm caused by the porphyrin production of the gram positive lipophile skin bacterium Propionibacterium acnes. Irradiation of these bacteria reduces the integral fluorescence intensity and induces the formation of photoproducts with fluorescence bands around 670 nm and decay times of about 1 and 5 ns. The photoproduct formation is connected with an increased absorption in the red spectral region. The endogenous fluorescent porphyrins act as photosensitizers. Photodestruction of Propionibacteria acnes by visible light appears therefore to be a promising therapy. The photodynamic activity of the photoproducts is lower than that of protoporphyrin IX.

An apparatus to measure red cell deformability, the laser diffractoscope, is presented. As in ektacytometry, the apparatus generates diffraction patterns using a laser beam that passes through a red blood cell (RBC) suspension in a viscosimeter. Introducing a CCD camera to record the diffraction pattern and computer-aided image analysis has reduced the measurement error to below 1% (variation coefficient). Employing this apparatus we have studied the effects of mechanical stress on RBC deformability in vitro. We submitted the erythrocytes to different shear stresses of various magnitudes (260 to 2620/s, viscosity of suspending medium 24 cP) and variable duration 1 to 16 mm). We demonstrate that a high mechanical stress reduces deformability at low shear rates but does not influence elongation at high shear rates and that the rigidification is related to mechanical stress in a dose-dependent manner. The period of exposure as well as the degree of mechanical stress influences the extent of deformability loss. We also show that RBC rigidification accumulates if mechanical stress is applied repetitively, the cells could not recover from this stress, below a certain threshold (1100/s, 24 cP) shearing does not produce any loss of flexibility, and the decrease of deformability is not accompanied with detectable hemolysis. It is suggested that the shear-induced rigidification is due to rearrangements in the cytoskeleton of the erythrocyte.

Porphyrin monomers, dimers, and aggregates with various tumor- localizing and photosensitizing properties can be differentiated based on their fluorescence lifetimes. These individual components are shown to possess different degrees of polarization. This fact can be used both for their microscopic imaging and for investigation of the microenvironment of porphyrins within cells and tissues. The spectral bands around 635 and 675 nm were correlated with the emission of porphyrin monomers and aggregates, respectively. The formation of photoprotoporphyrin was not significant during the light-induced porphyrin reaction.

new methodology, developed for thinning fringes in photomechanics, splits fringe thinning into two distinct steps, namely, fringe edge detection and fringe skeletonization. A minimum intensity criterion is utilized to extract the fringe skeletons from broadbands. The algorithm is applicable to any orientations ofthe fringe. It is noniterative and requires less computations than the other reported methods. A novel scheme is developed to remove discontinuities and noise in the fringe skeleton. The various steps of the algorithm are explained by processing a test image free of noise. The applicability of the algorithm to actual experimental situations is verified by processing fringes obtained by photoelasticity and moire. Preprocessing of the fringes is discussed in detail. A method is also developed for determining the coordinates of any point chosen on the fringe contour. This step involves the use ofthe system characteristics of the PC-based image-processing system. Fringe ordering is kept interactive to utilize the experimentalist's expertise. A new approach to automating fringe ordering is proposed.

The three-dimensional coordinates of a cross grating fixed on a slightly curved surface are determined by stereo imaging using polynomial imaging functions. These functions transform each spatial point within a limited flat volume into adjoined points in the images of two stereo cameras. The same functions also solve the inverse problem: finding the spatial coordinates when the related image coordinates are given, provided the points were placed within the above mentioned volume when taking the photograph. The method is applied to crack tip propagation. The out-of-plane displacement of the surface and the plane strain tensor are determined for the 3-D coordinates of the grating in different load stages.

A model is presented for generation of synthetic images representing what an airborne or satellite thermal infrared imaging sensor would record. The scene and the atmosphere are modeled spectrally with final bandwidth determined by integration over the spectral bandwidth of the sensor (the model will function from 0.25 to 20 μm). The scene is created using a computer-aided-design package to create objects, assign attributes to facets, and assemble the scene. Object temperatures are computed using a thermodynamic model incorporating 24-h worth of meteorological history, as well as pixel specific solar load (i.e., self-shadowing is fully supported). The radiance reaching the sensor is computed using a ray tracer and atmospheric propagation models that vary with wavelength and slant range. Objects can be modeled as specular or diffuse with emissivities (reflectivities) dependent on look angle and wavelength. The resulting images mimic the phenomenology commonly observed by high-resolution thermal infrared sensors to a point where the model can be used as a research tool to evaluate the limitations in our understanding of the thermal infrared imaging process.

A whole-field in-plane displacement measurement method was developed for micromechanics studies. The method increased the sensitivity of conventional moiré interferometry by an order of magnitude. The increased sensitivity was achieved by a two-step process. Microscopic moiré interferometry, used for step 1 to map an original displacement field, provided a basic sensitivity of 4.8 fringes/μm displacement, which exceeds the previously conceived theoretical limit. Optical/digital fringe multiplication method (ODFMM) was implemented for step 2 to achieve further enhancement of sensitivity. The ODFMM consists of optical fringe shifting and a digital process to sharpen and combine the shifted fringes. The result is a map with p times as many fringe contours as the original map of step 1. A factor of β = 12 was achieved, providing a sensitivity of 57.6 fringes/μm displacement, which corresponds to that of moiré with 57,600 lines per mm (1,463,000 lines per in.). The optical, mechanical, and electronic systems implemented here are remarkably robust and quick. The method is demonstrated by three practical applications: fiber/matrix deformation of a metal/matrix composite, interface strains in a thick 0/90-deg graphite/epoxy composite, and thermal deformation around a solder joint in a microelectronic subassembly.

A radially polarized laser beam is useful for applications such as laser particle acceleration. The issues of transporting and focusing (with an axicon) a radially polarized beam that was generated in the laboratory are examined. Problems of preserving the polarization while directing the beam are solved by using a compound 90-deg-fold out-of-plane pair of mirrors. When focused by an axicon, the radially polarized beam produces a diffraction-free Bessel beam. The transverse intensity distribution agrees with theory.

Unstable resonator modeling has been carried out for an injection-seeded CO₂ transversely excited atmosphere (TEA) laser in the NOAA/ERL/Wave Propagation Laboratory (WPL) Doppler lidar to examine the effects of various cavity designs on the quality of the output beam. The results show the effects of an injection pinhole, electrode spacing, mirror tilt, and radial reflectivity function of the output coupler. The electrode spacing in our laser has negligible effect. The injection pinhole, however, produces complicated structures in the output patterns. If the pinhole is removed, the output pattern is much smoother, and the frequency jitter is smaller. Misalignment sensitivity is very closely related to the radial reflectivity function. The superparabolic function provides the highest coupling efficiency, largest beam size, and good collimation, but produces a slightly higher misalignment sensitivity compared with a parabolic function. The Gaussian function provides the lowest misalignment sensitivity, but it produces the smallest beam size and the largest beam divergence. Also, the coupling coefficient is 50% lower than the optimum value. Methods for using a flat diffraction grating in unstable resonators are also investigated. The best way is to use a flat grating/positive lens combination to replace the back concave mirror.

The definition of laser triangulation imaging is above all restricted by speckle noise corrupting the signals of the widely used CCD-line or matrix sensors. Speckle noise can be reduced by certain optical means or signal processing. In the field of signal processing for laser triangulation, a lot of different interpolation approaches are in common use. Regarding laser triangulation imaging as a stochastic process the variance of the corrupted data can be stabilized using principal component analysis. Subsequent nonlinear regressions yield a remarkable improvement in accuracy. The approach can be recommended if the data set is consistent and the required additional number crunching power can be provided in real time.

An interesting first-order property of some thick lenses and systems is described. Some points on the optical axis have analogous properties to those of the nodal and principal points. These points are examined and some applications for them are found.

The Hartmann test is frequently performed with a screen with many holes to test the quality of a mirror or the aberrations of a lens. An application of the simplest form of this test with only four holes is described. This test cannot detect spherical aberration, but it may be useful to align optical systems, to detect and measure focus errors, or to detect lens decenterings and coma.

A double relay lens for a commercial microscopic observation system based on a 10 x objective is presented. Using a pair of identical four-element single relay lenses placed back-to-back, the double relay lens was designed with the practical design method. A double relay lens with a focal length of -5.56 mm, a numerical aperture of 0.25, a long working distance of 22.58 mm, and a large total track of 100 mm has been achieved. By connecting the double relay lens and the commercial microscopic observation system together, the relayed microscopic observation system can be applied to observe in situ the laser micromachining process in air or in vacuum.

We discuss several concepts of connectivity for circuit graphs, including Rent's rule, line length distributions, and separators, all of which we argue are fractal concepts. We provide generalizations for systems for which the Rent exponent is not constant throughout the interconnection hierarchy.

Free-space optical interconnections are important in both massive digital optical computing and communication systems. The architectural features of three interconnection networks are analyzed and compared, and the optical butterfly interconnection is shown to have many advantages over other interconnections in implementing various basic logic functions such as addition, subtraction, multiplication, and fast Fourier transforms (FFTs). Starting with conventional Karnaugh maps and Boolean algebra, the characteristics of full addition and full subtraction are analyzed and compared. An n-bit parallel calculator that can implement both ripple carry full additions and ripple borrow full subtractions using multilayer butterfly interconnection networks is designed. Then the schematic and architecture of the full adder/subtractor, interconnection networks, and the patterns of key devices such as masks to implement AND and OR operations in this calculation are described in detail. The correct simulation results of several groups of multibit digits are provided. Finally, the development of the interconnections in implementing logic operations is discussed.

Device approaches are investigated for optically addressed SLMs based on molecular beam epitaxy (MBE) engineered III-V materials and structures. Strong photo-optic effects can be achieved in periodically δ-doped multiple quantum well structures, but are still insufficientfor high-contrast modulation with only single- or double-pass absorption through active layers of practical thickness. We use the asymmetric Fabry-Perot cavity approach that permits extinction of light due to interference of light reflected from the front and back surfaces of the cavity. Optically controlled modulation of the absorption in the active cavity layers unbalances the cavity and "turns on" the reflected output signal, thereby allowing large contrast ratios. This approach is realized with an all-MBE-grown structure consisting of GaAs/AlAs quarter-wave stack reflector grown over the GaAs substrate as the high reflectance mirror (≈ 0.98) and the GaAs surface as the low reflectance mirror (≈ 0.3). We use for our active cavity InGaAs/GaAs multiple quantum wells separated by periodically δ-doped GaAs barriers to achieve a sensitive photo-optic effect due to exciton quenching. High-contrast modulation (> 60:1) is achieved using a low-power (< 100 mW/cm²) InGaAs/GaAs quantum well laser for the control signal.

A highly sensitive method for measuring thermal expansion, mechanical strain, and creep rates has been developed. This technique is based on Yamaguchi's speckle strain gauge concept, but uses a novel data-processing approach that provides estimates of the time rate of in-plane strain. The approach is appropriate for assessing very small strain rates in hostile environments. It provides simultaneous global estimates of the strain at both small and large gauge sizes. This may be of importance in studying materials with different short- and long-range orders. General advantages of the technique are compact design, modest resolution requirements, insensitivity to surface microstructure changes (as seen with oxidation), and relative insensitivity to zero-mean noise processes such as turbulence and vibration.

This PDF file contains the editorial “Book Rvw: Nonlinear Optics: Basic Concepts," by D. L. Mills for OE Vol. 31 Issue 07

This PDF file contains the editorial “Book Rvw: Nonlinear Optics," by Robert W. Boyd for OE Vol. 31 Issue 07