This PDF file contains the editorial “The Bloom Is Off the Google” for OE Vol. 42 Issue 04

When a current passes through a metal-coated optical fiber, the Ohm's heat induces a phase change of the guided light within the fiber. This microampere current sensor is principally designed based on this thermal effect. A Michelson interferometer is used for detecting this phase change. By means of our sensor, the minimum detectable phase signal is 1.6443×10²⁵ rad/Hz.^1/2 The minimum DC current detected is 3.423×10²⁷ A. The dynamical range is 89 dB with ∓60.5-dB linearity. The minimum detectable AC current is 4.193×10²⁷ A with a dynamic range around 75 dB. The linearity is within ∓61 dB for AC current.

Low-loss optical waveguides for optoelectronic integration were realized by the combination of spin-on-glass (SOG) and plasma-enhanced chemical vapor deposition (PECVD). Undercladdings of 20-μm thickness together with cores of 6-μm height and 8-μm width were formed on Si substrates by PECVD and reactive ion etching. Overcladdings were formed by a combination of SOG and PECVD SiO₂ films. This method successfully filled narrow gaps between cores of optical circuits. The refractive index of overcladdings was adjusted by SOG curing and fluorine incorporation in PECVD SiO₂ films. A buried-type optical waveguide formed by the combination of SOG and PECVD showed single-mode propagation. Propagation losses and polarization dependent losses at the 1.3-μm wavelength were measured to be 0.3 dB/cm and below 0.15 dB, respectively.

A new multiview connection technique for 360-deg three-dimensional (3-D) measurement is presented. The 3-D shape of a 360-deg object is usually obtained by two steps, first measuring the object from different views and afterward connecting all the views together. We determine the relationship between two adjacent views from their overlapping areas by a least-squares algorithm. To promote computability, the coordinate transform formula is expanded into a Taylor series, and a linear equation system is deduced, from which the relationship parameters between two adjacent views can be easily calculated. According to the resulting parameters, the connection of different views is performed by coordinate transformation in 3-D space. The two steps, parameter calculation and coordinate transformation, are iterated until the algorithm converges or the remaining error decreases below a given threshold. The 360-deg 3-D shape of a plaster statue is measured and reconstructed, thus experimentally demonstrating the validity of suggested method.

A multispectral three-dimensional (3-D) digital infrared thermal imaging (DITI) system is demonstrated for medical thermographic applications. In this system, temperature distribution and the three-dimensional geometrical surface profile are measured with different wavelengths and digitally stored in a computer separately. A texture-mapping technique is used for the accurate digital reconstruction of a 3-D temperature distribution of the human body. 3-D human body surface thermograms are obtained with this system.

The performance of electro-optical systems can be substantially affected by aerosol particles that scatter and absorb electromagnetic radiation. The model that is most frequently used for the prediction of aerosols and their effect on extinction in the marine atmosphere is the US Navy Aerosol Model (NAM). However, NAM can be significantly less reliable in coastal areas than on the open ocean. Based on an extensive series of measurements conducted on the island of Inisheer (Irish West Coast), an empirical aerosol model for the coastal zone formulated as an extension of NAM, in which coastal effects are modeled as a function of fetch, has been developed. This work is extended to the Mediterranean using an aerosol dataset recorded on the island of Porquerolles in the Bay of Toulon (France) and has been coupled with the Mie theory to give a code for the extinction, the code MEDiterranean EXtinction (MEDEX).

The sensitivities of surface plasmon resonance spectroscopy are higher than reflection/transmission measurement in determining the anisotropic properties of an arbitrarily oriented anisotropic thin film. The corrected Airy formula for calculating the reflection coefficient is used to calculate the reflection of the Kretschmann configuration by considering the nonsymmetric reflection phenomenon in anisotropic film. The optical constants and the thickness of the anisotropic film are obtained from the attenuated-total-reflection (ATR) curve.

To produce stable beams used by pencil beam interferometers, a new equal optical path beamsplitter (EBS) is developed. The beamsplitter divides one incident pencil beam into two parallel exiting beams with a fixed separation. The new EBS is a monolithic unit comprised of two right-angle trapezoidal prisms with a thin film beamsplitting coating on the faces, which are cemented together. Adjusting the dimensions of each element according to a simple algebraic relation produces two identical, parallel beams separated by a fixed distance with a zero optical path difference between them. The monolithic EBS is very simple and stable. It has applications in interferometers that require the use of a pencil beam source, such as the long trace profiler (LTP) and the precise angle monitor. If a large-diameter wavefront is incident on the EBS, it can be used in shearing interferometer applications, such as the lateral translation shearing interferometer and the rotational shearing interferometer. Use of the EBS in all of these configurations significantly improves instrument stability and accuracy. Some application examples and test results are presented.

Wobbling limits the access speed and data storage density in hard disks. A method using optical triangulation of a projected linear array of light beams and time-delay integrated (TDI) imaging was explored to obtain the tilt distribution as the hard disk was rotated. This resulted in fringe image records. A fast-Fourier transform scheme was applied to process the fringe images for more accurate results using the technique. During experimental verification, the technique was shown to reveal the distribution of tilt of a typical hard disk rotating at 2000, 4000, and 6000 rpm.

We present a theoretical analysis of the noise behavior of a heterojunction bipolar transistor (HBT) used as a three terminal (3T) photodetector. The use of a HBT in the photodetector mode can greatly simplify the fabrication of HBT-based optical receivers in the monolithic form. The present model takes into account the effect of the received light on the intrinsic parameters and various noise components of the HBT when used as a detector. The model enables one to determine the signal-to-noise ratio at the output and also the noise equivalent power of the HBT. The model has been applied for characterization of an InP/InGaAs HBT used as a 3T photodetector in the 1.55-μm wavelength region.

Recent developments of digital cameras in terms of the size of charge-coupled device (CCD) arrays and reduced costs are leading to their applications in traditional as well as new photogrammetric, surveying, and mapping functions. Digital cameras, intended to replace conventional film-based mapping cameras, are becoming available along with many smaller formats capable of precise measurement applications. All such cameras require careful calibration to determine their metric characteristics, which are essential to carrying out photogrammetric activities. We introduce a new approach for incorporating straight lines in a bundle adjustment for calibrating off-the-shelf, low-cost digital cameras. The optimal configuration for successfully deriving the distortion parameters is considered when establishing the required test field. Moreover, a framework for automatic extraction of the straight lines in the images is presented and tested. The developed calibration procedure can be used as an efficient tool to investigate the most appropriate model that compensates for various distortions associated with the camera being calibrated. Experiments performed to compare line-based with traditional point-based self-calibration methods prove the feasibility of the suggested approach.

A fiber Bragg grating (FBG) sensor for detection of ultrasonic waves in liquids and solid structures is investigated. The sensor contains a fiber probe with a FBG, a tunable narrowband laser source, and a photodetector. When ultrasonic waves impinge on the fiber probe, the pitch of the grating is modulated through the elasto-optic effect. The corresponding changes in reflectivity of the grating are detected by a photodetector. The sensitivity of the sensor and its frequency and directional response in the 0.5 to 3.0 MHz range are measured. Some anomalous features of the FBG ultrasound sensor response are noted. It is shown that these features result from a combination of the sensor's response to the direct ultrasonic waves propagating through the medium and the signal generated by axially guided ultrasonic waves propagating through the fiber probe.

An etched long-period fiber grating with an outer liquid-crystal cladding capable of polarization-isolated electrical modulation, is described. A 5-dB transmission-dip modulation with 4-dB polarization isolation can be obtained at 225 V. The reduction in fiber diameter by a chemical etching process leads to an improvement in the long-range ordering of liquid-crystal molecules and a reduction in the controlling voltage. In addition, the experimental results are supported by numerical simulation.

The transient microbending loss of a double-coated optical fiber, which is subjected to a time-dependent thermal loading, is analyzed and it is found that the transient microbending loss is a function of the temperature drop, thickness, and material properties of the primary and secondary coatings. The individual effect of the thickness, Young's modulus, Poisson's ratio, and the thermal expansion coefficient of the coating layers on the transient microbending loss of the optical fiber is also investigated. According to the numerical results, the thermally induced transient lateral pressure, and thus the transient microbending loss, in a double-coated optical fiber can be minimized by a suitable selection of the coating layers.

We present the designs of a small residual dispersion fiber (SRDF) and a corresponding dispersion-compensating fiber (DCF), where the dispersion slopes of these fibers are so adjusted that a small length of the DCF will approximately compensate the accumulated dispersion in SRDF simultaneously at all wavelengths in the range of 1530 to 1565 nm [gain window (C-band) of erbium-doped fiber amplifiers]. The maximum values of the effective dispersion and the slope of the effective dispersion in the wavelength range of 1530 to 1565 nm are ~0.08 ps/km nm and ~0.01 ps/km nm², respectively.

Designing fibers with a certain modal profile may lead to improved performance. We demonstrate that it is possible to derive the index profile from the desired modal characteristics. Along with an efficient beam propagation code, the procedure may expedite the development of special fibers.

We present a Fourier transform spectrometer that associates a compact interferometer without any moving parts and various signal processing methods. The interferometer is composed of a polarizer, two identical spar Wollaston prisms, an analyzer, and a CCD sensor. Data and signal processing methods are introduced to improve the resolution. Due to the nonlinear behavior of the optical path difference versus the position along the sensor, a data preprocessing step is required before reconstructing the spectrum. This preprocessing method is based on key properties of the optical path difference that are identified. Then various spectrum analysis methods such as autoregressive, multiple signal classification (MUSIC), and Cauchy methods are illustrated, discussed, and compared with the periodogram by using the real data obtained from the spectrometer illuminated by a HgZnCd spectral lamp. From the optical system associated with the data processing and spectral analysis methods, it is possible to split the two closest spectral lines.

Although the grating optical low-pass filter (GOLF) has many advantages over the traditional birefringent low-pass filter, it is not adopted in commercial digital cameras because the shadow image of the grating is visible, especially when the F-number of a camera is large. We design and manufacture grating optical low-pass filters with multiple phases to compare their filtering characteristics and shadow images. From the simulation of the Fresnel diffraction pattern of a 1-D multiphase grating, we obtain the optimum width and phase of each grating step that minimizes the visibility of the Fresnel diffraction pattern. We design 2-D multiphase structures by extrapolating the 1-D structure. Four types of GOLF are manufactured with a semiconductor process and installed into a digital camera to compare grating images. We find that the visibility of grating images can be reduced by increasing number of phase steps.

The effectiveness of a coding method can be improved through a space-varying filterbank tree representation of the image, and this property can be conveniently exploited using appropriate bit allocation strategies among the spatial segments of the image. We examine the conditions for achieving a rational agreement on the distribution problem by stating axioms that its solution must obey in absence of a priori knowledge about regions of interest. First, a measure of benefit avoiding certain forms of behavioral inconsistency is to be assigned to each possible bit allocation in such a way that each region's preference may be inferred between any two bit allocations from their respective benefits. Second, individual regions are to agree on an allocation of bits, which is then to be brought about by a joint strategy. But under what conditions is their agreement rational? We propose a characterization of rational agreement whose solution is an application of a general procedure for cooperative action, where each may benefit only on terms that permit proportionately equal benefits to others. Experimental results are given to evaluate the performance of the strategy of collective rationality for the allocation of bits (CORAL), based on a validated predictor for visual distinctness from digital imagery.

This study proposes an interband version of the linear prediction approach for hyperspectral images. Linear prediction represents one of the best performing and most practical and general purpose lossless image compression techniques known today. The interband linear prediction method consists of two stages: predictive decorrelation producing residuals, and entropy coding of these residuals. Our method achieved an average compression ratio of 3.23 using 13 airborne visible/infrared imaging spectrometer (AVIRIS) images.

The Karhunen-Loeve expansion based on the calculation of the eigenvalues and eigenfunctions of the Karhunen-Loeve integral equation is known to have certain properties that make it optimal for many signal detection and filtering applications. We propose an analytical solution of the equation for a practical case when the covariance function of a stationary process is exponentially oscillating. Computer simulation results using a real aerial image are provided and discussed.

We apply the filter bank method, which extracts from a strongly defocused image the information on the amount of defocus, i.e., the information on the object distance, and simultaneously restores the object, to a defocused image of extremely low intensity, which is detectable only with a photon counting camera. Experimental results show that the information of the defocus amount can be extracted from such a blurred photon image, and the original object itself is simultaneously restored. The influence of the photon noise on the distance estimation is also quantitatively investigated. Then, the maximum likelihood-expectation maximization (ML-EM) method is adopted for image restoration, and the results are compared with those obtained by applying the conventional Wiener filter. It is experimentally shown that the ML-EM produces better image restoration than the Wiener filter in the extremely low intensity region where the maximal number of photons per pixel does not exceed about 450.

We report on numerical studies of phase front propagation for a large-baseline space interferometer. The main objective is to determine the sensitivity of the average phase of the metrology beam with respect to fluctuations in the pointing of the beam. For this purpose, the metrology beam is propagated numerically along the interferometric arm of the instrument. The effects of the obscurations from the secondary mirror and its supporting struts are studied in detail. Further, the effects of wavefront distortions that occur due to imperfections of the optical elements are estimated through a series of Monte Carlo simulations. The results of this study can be used to determine design requirements for the instrument.

The design and analysis of achromatic doublet prisms for use in laser beam steering is presented. The geometric relationships describing the maximum steering angle are given, as are discussions of first- and second-order dispersion reduction. Infrared (IR) material alternatives and optimum IR material characteristics for wide-angle achromatic prism beam steering are also investigated. Sixteen materials in 120 different combinations have been examined to date. For midwave IR applications it is shown that the minimum dispersion currently achievable across the full 2 to 5 μm spectrum is 1.7816 mrad at an average maximum steering angle of 45 deg. This is accomplished using LiF/ZnS doublet prisms. Several issues related to the azimuth and elevation angles into which light is steered as a function of prism rotation angles are also presented.

Beam distortion profiles are studied for scanning devices that have a single mirror with two rotational degrees of freedom (DOF), also named tip/tilt scanners. The case of a fast steering scanner used for high power material processing applications is studied. The scanner has a bandwidth of 700 Hz, a range of motion of ∓52 mrad (∓3 deg), and a resolution <5 μrad. The main dominant parameters that affect the distortion profile are identified. Furthermore, a vector analysis is derived to represent these distortions, and equations of the correction factors used to compensate for the systematic errors are proposed. A final accuracy better than 0.05% is obtained when these compensation factors are taken into account. The derivation proposed here can be extended to any scanning device with a single mirror with two-rotation DOF.

The factor of encircled energy (FEE) in the diffraction pattern formed by an optical system consisting of an aberration-free birefringent lens sandwiched between two linear polarizers is computed directly from the optical transfer function (OTF) of the system. FEE curves are given showing the variation of image quality of the proposed system at the Gaussian image plane and other observation planes with the birefringent lens parameters and the azimuthal settings of the transmission axes of the polarizers. The performance is compared to that of an ideal conventional glass lens.

We investigate convergence behaviors of the differential method for estimation of 2-D motion parameters. While the differential method has been widely researched for motion compensated predictive video coding, little attention has been paid to its convergence properties. Based on the nonseparable exponential covariance image model, we derive theoretical estimates of update terms for translational and affine motion models. The effects of noise, spatial correlation, choice of spatial gradient measures, and the size of a region, are quantitatively analyzed in relation to the convergence speed. Some empirical results are presented to verify the analysis.

We discuss a low-cost method for polishing laser rods of different dimensions. A technique is developed to design the holder/block required to hold the laser rods during the polishing operation. The design of the holder is based on considerations of the stability of the combined system (holder with laser rod) and load/unit area on the polishing tool during the polishing operation. For this purpose, the position of the combined system center of gravity is computed from the dimensional parameters of the holder and laser rod, using a computational scheme developed for this purpose. Dimensional parameters are adjusted to control the position of the center of gravity for good stability and the desired load/unit area on the polishing tool. Another important consideration to obtain a high-quality surface figure and surface finish on the polished end faces is the selection of a suitable matching material for use as a dummy in the holder. A ball-bearing-based polishing attachment is developed to carry out the polishing operation. Although the method is applied mostly to polish a large number of Nd-glass and Nd-YAG laser rods and occasionally for Nd-YLF rods, the procedures are general in nature and are applicable for other types of rods. They are also suitable to polish new laser rods as well as damaged ones.

We analyze the use of a film resonator in an angle-sensitive detector (ASD) for measuring very small deflections of a laser beam. A simple device consisting of a film resonator formed by a parallel air gap between two optical prisms is proposed. Quantitative modeling is done considering Gaussian laser beams. We present a detailed analysis of the sensitivity of the proposed ASD with respect to the beam's size, resonator's width, polarization, and prism refractive indices. An experimental verification of the proposed device is also presented.

InGaAs/InP multiple quantum well (MQW) PnpN depleted optical thyristors (DOTs) operating at 1.55 μm are proposed and fabricated. To analyze their switching characteristics, we simulate nonlinear s-shape current-voltage curves using the finite difference method (FDM) associated with the current-oriented method. Using the FDM, we calculate the effects of such parameters as doping concentration and the thicknesses of the outer and inner layers of the thyristor to determine an optimized structure in the view of fast and low-power-consuming operation. With these results, we fabricate waveguide- and vertical-type InGaAs/InP MQW PnpN DOTs. The waveguide-type DOT shows sufficient nonlinear s-shape I-V characteristics with a switching voltage of 4.03 V and a holding voltage of 1.77 V, and spontaneous emission along the waveguide. The current-voltage characteristics of the vertical-type DOT are shown very well under 150 μW of input power, while the s-shape disappears at 200 μW.

The design of some 1-D waveguiding photonic bandgap (PBG) devices has been carried out by utilizing a model based on the Bloch-Floquet theorem. A complete analysis of the propagation characteristics, including the modal propagation constants, electromagnetic field harmonics and total field distribution, transmission and reflection coefficients, total forward and backward power flow in the structure, guided and radiated power, and total losses, enable the optimization of the design in very short CPU time.

We report a new method to test the collimation of a laser beam using the diffraction efficiency of photorefractive Bi₁₂TiO₂₀ (bismuth titanium oxide) crystal and the temporal coherence of laser beam in a two-wave mixing geometry. The accuracy of the technique is comparable with that of the established Murty shear interferometric technique and can be improved considerably. Detailed experimental and theoretical analyses are presented.

Safety belts are specific fabrics manufactured to ensure the highest performance. Their manufacturing process not only has to assure its endurance to high tension strength, but also has to guarantee its correct visual appearance. Safety belts must not contain fiber breaks, knots, thickness variations, etc. Such defects imply the unfulfillment of rigorous safety standards. We describe the development of a computer vision inspection system, which control safety belts at a speed rate of 2 m/s. This inspection rate has been achieved by means of a parallel architecture and the use of optimized vision algorithms.

Both surfaces of fine ceramics used for electronic parts are covered with, for example, a resin, a ceramic alumina, or a solder. An automatic optical method for detecting cracks originating at the side faces of ceramic chips, the surfaces of which are covered with the mentioned materials, is developed and is reported. A laser beam is diffused into the specimen by illumination onto the side faces through an objective lens. When the specimen has a crack near the laser spot, the diffused laser light is reflected by the crack and the amount of radiant exitance near the crack is increased. The image of the side face is enlarged by the objective lens and detected by four photomultiplier tubes. It is clarified experimentally that it is possible to recognize the region surrounded by a close-ended crack wider than 20 μm and longer than 35 μm, or an open-ended crack longer than 50 μm, by detecting signals higher than the average level. Experimental results revel that the cracks in the ceramic parts can be detected automatically in a production line.

An optical system that can record the refractive index of a transparent liquid with an accuracy of 0.0005 and a temporal resolution of about 0.10 ms is developed. It employs a lateral photodetector to track the position of the refracted beam. The relationship between the refractive index of the liquid and the outputs of the photodetector is derived and confirmed by experiments. For a simple application the system is used to record the equilibration time of a binary mixture.

We propose a simple setup for measurement of a small 2-D rotation angle using a Twyman-Green laser diode interferometer. Sinusoidal phase-modulating interferometry is adopted to detect the phase of the interference signal with a high accuracy and to perform the phase lock to eliminate the phase fluctuation at the standard detection point. By detecting the phases at three points along an axis, the 1-D rotation angle is measured with an accuracy of 0.5 arcsec in the range of 282 arcsec and with a spatial resolution of 3 mm. The 2-D rotation angle measurement is easily made by adding two more detection points.

We examine the repeatability of the sol-gel production process. Methods based on statistical pattern recognition are used to evaluate microscopic images of sol-gel matrices. The applied classification is a two-steps scheme: dimensionality reduction of the data and cluster analysis. Two methods of dimensionality reduction and five methods of cluster analysis are used. We show that the repeatability varies between 82 and 93.77%. The obtained results demonstrate that the molar ratio R of the number of solvent moles to the number of substrate moles influences the stability of sol-gel materials, thus enabling us to reach a higher repeatability in the production process in the case of sol-gel produced with the addition of detergent.

The performance of an automatic target recognition (ATR) algorithm is not only influenced by the relevance of the prior (training) information, but also by the level of difficulty posed by the clutter that surrounds the target. Thus, an objective measure of target- (or signal-) to-clutter ratio (SCR) is important for the assessment of ATRs. We describe a new metric for SCR based on an eigen analysis of a two-class problem. It is believed that an SCR metric, along with a measure for the relevancy of the training data, are the key parameters for the characterization of ATR performance. Various examples are given to illustrate the application of eigen analysis in determining the difficulty level of finding a particular target in the presence of clutter, and consequently how this new SCR metric defines the potential for false alarms.

We present the in situ optical analysis of the active layers in a vertical-cavity surface-emitting laser structure with a GaInP/AlGaInP core at room temperature. To penetrate the high number of AlAs and AlGaAs Bragg mirrors, two-photon spectroscopy is employed to determine the characteristics of the cavity and specifically the emissive layer for a variety of excited electron-hole pair concentrations in the range of 5×10¹⁷ to 6×10¹⁹ cm^–3. It is found that many body effects due to optical doping lower the emission wavelengths of the devices. The shift is equivalent for several different structures and the effect is therefore assigned to the active zone. Furthermore, it is demonstrated that the emission wavelength of the lasers considerably depends on the detection angle.

Embedded signature-based authentication is a highly promising watermarking method for solving high-level authentication problems. This work proposes such a watermarking approach using channel statistics evaluated from a reference watermark. The basic concept is to consider the problem to be an example of communications with side information, which is the reference watermark embedded in the host with the signature at the sender site. At the receiver site, the reference watermark is first extracted to determine channel statistics, which are used to generate information about signature reliability. Three kinds of information are derived, including a reliability measure, an entropy measure, and an authenticated signature. Experimental results establish that the measures of reliability and uncertainty are meaningful, and that the embedded signature can survive high-quality JPEG compression and manipulations such as negation, cropping, replacement, and modification.