This PDF file contains the editorial “The Claw” for OE Vol. 41 Issue 04

The authors propose a novel technique to produce adjustable chirped gratings that allows spectral width tuning without center wavelength shift. The chirp rate can be precisely tuned by bending a uniform fiber Bragg grating (FBG) surface-mounted on one side of a simply supported beam. A maximum chirp tuning with a spectral width of 18.30 nm on a uniform FBG is obtained experimentally. Compared to the existing chirp tuning techniques, the proposed technique provides larger tuning range and smaller center wavelength shift.

High-density GaN:Er optical storage devices were fabricated with focused ion beam (FIB) milling techniques. In-situ Er-doped GaN films (1-1.5 ?m thick) were grown on Si substrates. To "write" a bit, the GaN:Er film was selectively milled with a 30-keV Ga+ FIB. Data retrieval is accomplished by upconversion emission at 535/556 nm upon 1-xm IR laser stimulation. Regions where the Er- doped GaN layer is completely removed (and do not emit) are defined as logic "0," while regions that are not milled (and do emit) are defined as logic "1." Data patterns with submicron bit size (or 100 Mb/cm2 density) have been fabricated by FIB milling. Data written by this approach has a theoretical storage capacity approaching 10 Gbits/cm2.

Confocal imaging by a parallel-mode confocal microscope is based on the real-time incoherent-holography technique. Besides the image amplitude, the image phase is inherently reconstructed. In this paper we prove that the phase image component can be converted into the height map, and, in this way, we measure the surface profile with the precision of several nanometers. Small height differences can be measured inside one optical section of the specimen surface, while the measurement of large differences needs to connect more optical sections. The two procedures are demonstrated experimentally even for the surface with large and steep height changes. The ambiguity in the height determination from the image phase component is overcome by means of the depth discrimination property of the microscope. The axial resolution is improved using broadband illumination.

There has been much interest in use of a tunable diode laser for absolute distance interferometry. We report on a novel method?using changeable synthetic wavelength chain (CsWC) to measure absolute length?based on a tunable diode laser. During the measurement, the desired synthetic wavelengths are calculated according to the distance to be measured and the measurement uncertainty. Then by tuning the wavelength of the laser, the desired synthetic wavelength chain is created. In addition, a differential measurement setup, which consists of two dual-beam interferometers, is proposed to test the feasibility of the CSWC method. The experiment results demonstrate that the CSWC method can work effectively. Although under present conditions a measurement uncertainty of only 3 x 10-5 is achieved, it is sufficient to design a differential optical probe.

A new method for weak birefringence inspection is proposed. Based on the models of Kubota and Inoue and Hansen and Conchello, the formulation of the diffracted pattern of a point source transmitting through a sample with weak birefringence is carried out. An optical polarizing microscope is used to measure the retardation and the optic axis of the sample. The condenser of microscope is replaced by a 100X/1.25 numerical aperture (NA) objective to increase the birefringence measurement sensitivity. This method is suitable for both visual and quantitative detection of weak birefringence. Tests on plastic films and CD substrates are conducted.

The Dempster-Shafer (DS) evidence theory is a new approach to the problem of segmenting multimodal images coming from different sources. The performance of such segmentation scheme is, however, largely conditioned by the appropriate determination of mass functions in DS evidence theory. We present a method of automatically determining the mass function for image segmentation problems. The idea is to link, at the image pixel level, the notion of mass functions to that of membership functions in fuzzy logic. The mass assigned to a pixel is obtained from both the membership degree of the current pixel and those of its neighboring pixels. The membership degree of each pixel is determined by applying fuzzy c-means (FCM) clustering to the gray levels of the image. A method is presented to determine the simple or composite classes in DS evidence theory from the obtained membership degree. Final segmentation is achieved using the DS combination rule and decision. The developed mass function determination method is illustrated with both simulations and examples of physical images. We demonstrate the value of introducing fuzzy clustering in evidence theory for image segmentation.

A coherent optical imaging system consisting of recording a digital hologram by a CCD array and numerical reconstruction of the complex wave field in a computer is subjected to a frequency analysis. This analysis recognizes Fresnel and lensless Fourier transform holography; collimated and divergent reference waves; as well as the real image, the virtual image, and the dc term. The influences of finite sampling and the fill factor of the CCD pixels are examined. The impulse response of the system is a shifted Fraunhofer diffraction pattern of the aperture defined by the CCD. A fill factor below unity leads to a contrast decrease, which is quantitatively described in the modulation transfer function.

We present the results of a theoretical numerical analysis of a novel fiber optic refractometric transducer. It consists of a hemispherical glass detection element and a pair of the multimode optical fibers attached symmetrically to the element's flat equatorial plane. The internal reflection of light from the element's spherical surface is sensitive to the refractive index of the surrounding medium. We exploit several internal reflections in series and the focusing of the beam by the element's spherical surface to achieve enhanced sensitivity to the refractive index of the surrounding medium and reduced intrinsic optical loss in the transducer. We present the data on the effect of transducer's parameters on its transmission function. We show that the transducer can operate in a relatively wide range of the refractive index of the surrounding medium (we show this for the interval n = 1.0 to n = 1.45). We also present experimental data confirming our theoretical results.

We describe a quasi-distributed optical fiber strain sensor, whose principle of operation is based on the coherent frequency- modulated continuous-wave (FMCW) reflectometry technique. The sensing system is basically composed of a laser diode, which is frequency swept by a triangle waveform injection current and an unbalanced two- beam interferometer. Its test arm consists of a number of single-mode fibers, which act as sensing fibers, with a mirror at the far end and mechanical splices as the connectors as well as the reflectors. Theoretical analysis shows that the measuring resolution of the strain is inversely proportional to the length of the sensing fiber. To measure the strain variation of each sensing fiber, the phase shifts of the beat signals are demodulated by a heterodyne signal processing system. The requirement of a high precision temperature control of light source can be eliminated in this way, since temperature effect is insignificant to the phase demodulation technique. To calibrate and verify the linearity, feasibility, and accuracy of this system, tensile and bending tests were carried out and strain gauge was employed as the benchmarking tool over the experimental results. It was observed that this system is feasible and its accuracy is excellent.

We present a new video sensor for multimeasurements in an aircraft cargo compartment called a video sensor unit (VSU). This sensor, based on CCD technology, operates in the near IR (NIR) spectral band to measure temperatures due to overheating and fire events. After a characterization of the measurement chain, a radiometric model is applied to the VSU, enabling the measurement of temperatures ranging from 350 to 900 °C with a noise equivalent temperature difference (NETD) lower than ±8 °C. In the same spectral band (NIR), the VSU can also detect smoke and 3-D load displacements. The geometric model of the VSU is described and the associated calibration procedure is presented.

The influence of an imaging system on the measurement error in coherent fringe projection methods is analyzed. The analyses are related to geometrical configurations and aberrations of optical systems. The problem is particularly important for full-field interferometry, where the aberrations are different for different field points. The analysis of the influence of geometrical and aberration parameters on measurement results for a chosen object is based on the harmonic approach to optical imaging. Theoretical considerations are supported by computer calculations.

A simple and fully automated technique for wavefront measurement is presented. This technique is based on the use of a computer display (for example, a CRT or a thin film transistor (TFT) monitor) to generate intensity-modulated patterns from which images are taken by a CCD camera. When a phase object is located between the display and the camera, the intensity patterns are distorted. By measuring this distortion, the gradients of the phase change caused by the object can be obtained. To simplify the data analysis it is practical to display on the monitor a grating with a sinusoidal intensity profile, which enables the use of standard fringe pattern analysis techniques. We use phase- shifting and temporal phase-unwrapping techniques. The use of a computer display for fringe (or grating) generation leads to the possibility of adjusting the sensibility of the measurement in function of the phase variation of the object to test and avoiding the problems of having a fixed grating period (as in the case of Ronchi rulings or printed gratings) or using mechanical parts to change them. Experimental measurements of two different ophthalmic lenses with local distributions of focal lengths prove the versatility of this method for optical testing. The method is simple, flexible, and low cost, yet it yields a remarkably high SNR. Compared with other techniques such as interferometry and moire deflecto- metry, the setup is cheaper and far easier to align.

Cholera is an acute intestinal infectious disease. It has claimed many lives throughout history, and it continues to be a global health threat. Cholera is considered one of the most important emergence diseases due its relation with global climate changes. Automated methods such as optical systems represent a new trend to make more accurate measurements of the presence and quantity of this microorganism in its natural environment. Automatic systems eliminate observer bias and reduce the analysis time. We evaluate the utility of coherent optical systems with invariant correlation for the recognition of Vibrio cholerae O1. Images of scenes are recorded with a CCD camera and decomposed in three RGB channels. A numeric simulation is developed to identify the bacteria in the different samples through an invariant correlation technique. There is no variation when we repeat the correlation and the variation between images correlation is minimum. The position-, scale-, and rotation-invariant recognition is made with a scale transform through the Mellin transform. The algorithm to recognize Vibrio cholerae O1 is the presence of correlation peaks in the green channel output and their absence in red and blue channels. The discrimination criterion is the presence of correlation peaks in red, green, and blue channels.

A new stereo image compression algorithm is described in which the residual image, extracted from the stereo image by the disparity-compensated prediction method, is compressed using the wavelet transform in consideration of the inter and intra correlation between subbands. Contrasting the conventional algorithms such as efficient pyramid image code (EPIC), embedded predictive wavelet image coder (EPWIC), and JPEG, compression performance of the suggested method is significantly improved through computer simulation. Finally, it is suggested that the stereo image having a good 3-D effect can be reconstructed from the compressed image data using a new algorithm described.

The fuzzy possibilistic c-means (FPCM) is embedded into a 2-D Hopfield neural network termed the fuzzy possibilistic Hopfield network (FPHN) to generate an optimal solution for vector quantization (VQ) in the discrete cosine transform (DCT) and the Hadamard transform (HT) domains. The information transformed by DCT or HT is separated into dc and ac coefficients. Then, the ac coefficients are trained using the proposed methods to generate a better codebook based on VQ. The energy function of the FPHN is defined as the fuzzy membership grades and possibilistic typicality degrees between training samples and codevectors. A near global-minimum codebook in the frequency domains can be obtained when the energy function converges to a stable state. Instead of one state in a neuron for the conventional Hopfield nets, each neuron occupies two states called the membership state and the typicality state in the proposed FPHN. The simulated results show that a valid and promising codebook can be generated in the DCT or HT domains using the FPHN.

We demonstrate a pulsed optical parametric oscillator (OPO) based upon periodically poled lithium niobate, and pumped by a Ti:sap- phire laser. Agile and continuous OPO tuning is achieved by tuning the pump laser, with no need for OPO crystal translation or temperature change. The Ti:sapphire laser uses a coupled cavity grazing incidence resonator for efficient, single-axial-mode operation within the OPO spectral acceptance bandwidth. The OPO tuning range is 1200 to 2400 nm, limited only by the available cavity mirrors.

The rearrangement of the optical axis in reversible photoanisotropic media under the action of a circularly polarized beam is demonstrated and quantitatively measured. These results are used to clarify the term ''optical erasure'' in reversible storage media. The detected residual anisotropy is taken into account to estimate the degradation of the signal to noise ratio in a storage device with a tilted readout design.

With a phase-shifting Fizeau interferometer, problems arise in making accurate measurements of nominally flat optical surfaces with high-reflectance coatings. We describe a simple technique that can be used to make accurate measurements of such surfaces.

A new measuring system is proposed that can measure the motions of arbitrary rigid bodies moving in 6 degrees of freedom. The measurement principle is based on the detection of laser beams reflected from a specially fabricated three-facet mirror that looks like a triangular pyramid having an equilateral cross-sectional shape. The mirror is mounted on the object to be measured. It reflects a laser beam, generated from a source, in three different directions, depending on the motion of the object of interest. The reflected beam is then detected by three position-sensitive detectors (PSDs). From the signals of the PSDs, we can calculate the 3-D position and orientation of the three-facet mirror, thus enabling us to determine the 3-D position and orientation of the object. We model the relationship between the 3-D position and orientation of an object in motion and the outputs of three PSDs. A series of experiments are performed to demonstrate the effectiveness and accuracy of the proposed method. The experimental results show that the proposed sensing system can be an effective means of obtaining 3-D position and orientation of arbitrary objects and provides reasonable measurement accuracy.

We introduce the concept of predetection based on an attention mechanism to improve the efficiency of small-target detection by limiting the image region of detection. According to the characteristics of small-target detection, local contrast is taken as the only feature in predetection and a nonlinear sampling model is adopted to make the predetection adaptive to detect small targets with different area sizes. To simplify the predetection itself and decrease the false alarm probability, neighboring nodes in the sampling grid are used to generate a saliency map, and a short-term memory is adopted to accelerate the "pop-out" of targets. We discuss the fact that the proposed approach is simple enough in computational complexity. In addition, even in a cluttered background, attention can be led to targets in a satisfying few iterations, which ensures that the detection efficiency will not be decreased due to false alarms. Experimental results are presented to demonstrate the applicability of the approach.

A theoretically sound and computationally inexpensive method is proposed for expressing in a canonical form not only quadratic, but any-degree polynomial surfaces, thus enabling the alignment of 3-D objects composed of such surfaces to a standard coordinate system. Furthermore, an analytical proof of the lower upper bound for convergence is presented, by which the efficiency of the proposed method is verified. Experiments are conducted with up to sixth-degree polynomial surfaces, thus demonstrating the method converges to the desired results within a few steps.

Free-space IR transmission provides high bandwidth and good security with small-sized and low-cost links, for instance, for high- bit-rate wireless LANs. Some robustness against shadowing is achieved using diffuse channels, but disadvantages are high path loss and multipath propagation. We use ray-trace simulation software to analyze IR channels in realistic office rooms. Simulations are performed to test the methods and to specify the bandwidth and power budget requirements of diffuse links. Both diffuse and specular reflections as well as shadowing effects due to furniture are considered. Based on the study, novel Monte Carlo ray-tracing software, such as ASAP (Advanced System Analyses Program), are suitable for analyzing the multipath dispersion and the optical power budget of infrared links in realistic indoor environments. The simulations also reveal the benefits of quasidiffuse link configurations, which are composed of multibeam transmitters with restricted beam divergences as well as of detectors with narrow fields of view. For implementation of multibeam transmitters we design an array- type diffractive element that modifies the beams of a 2-D vertical-cavity surface-emitting laser (VCSEL) array. A single element providing the largest 50-deg illumination angle is fabricated for demonstration. The measured diffraction efficiency is in fair agreement with the calculated one after considering the properties of the real VCSEL beam.