This PDF file contains the editorial “2003 in Review” for OE Vol. 43 Issue 02

A novel distributed fiber optic leak detection system is proposed. Its configuration is similar to an in-line Sagnac interferometer. The linear measurable range is from 3×10^–4 to 3×10^–2 rad/√Hz with a dynamic range around 40 dB.

This work describes a long-period grating filter with tunable attenuation. The device is ideally suitable for equalization of the output power spectrum of erbium-doped fiber amplified spontaneous emission sources. The tunability arises from incorporating the grating in a Mach-Zehnder configuration that includes a tunable coupler. Experimentally, we achieved a change in attenuation of approximately 14 dB in a wavelength band centered at 1531 nm. The peak-to-peak ripple in the power spectrum of the source improved from 12.2 to 3.8 dB in the wavelength range from 1524 to 1563 nm. Tunability assures that the spectral ripple can be controlled over a wide range of pump power.

Thresholding of video images is a great challenge because of their low spatial resolution and complex background. We investigate the issue of thresholding these images by reducing the number of colors to improve automated text detection and recognition. We develop an unsupervised approach to video images, which can be considered as an RGB color thresholding method. It applies a gray-level thresholding method to a video image in the (R, G, B) color space to produce a single threshold value for each domain. The three (R, G, B)-generated values will be subsequently processed by an effective unsupervised clustering algorithm that is based on a between-class/within-class criterion suggested by Otsu's method. Since thresholding methods designed for document images may not work effectively for video images in many applications, our proposed RGB color thresholding method has shown to be particularly effective in improvement on text detection and recognition, because it can reduce the background complexity while retaining the important text character pixels. Experiments also show that thresholding video images is far more difficult than thresholding document images, and the RGB color thresholding presented performs significantly better than simple histogram-based methods, which generally do not produce satisfactory results.

We fabricate ferroelectric liquid crystal display (FLCD) devices exhibiting a thresholdless V-shaped switching by adopting a mesogenic liquid crystal polymer stabilization and also FLCDs exhibiting half-V-shaped switching using an undoped FLC material. These FLCDs are respectively called polymer-stabilization (PS)-V-mode FLCDs and intrinsic H-V-mode FLCDs. By using especially developed polyimide materials such as RN-1199 and RN-1411 series (from Nissan Chem., Ind.) for the surface alignment and together with a well-controlled rubbing technique, photoalignment, and ion-beam alignment, we succeed in fabricating zigzag defect-free FLCDs. In particular, the PS-V-mode FLCD fabricated in this way exhibits continuous operation with a high contrast ratio of 700:1, and the intrinsic H-V-mode FLCD has good temperature stability by using the combined alignment technique. Particularly, a field-sequential full-color (FS-FC) LCD using PS-V-mode FLCD shows good long-term stability for a continual operation. Using these FLCDs, we also develop several prototype models of FS-FC active matrix LCDs with the specification of video graphics arrays (VGAs) and extended graphics arrays (XGAs), where we take advantage of their high-speed response (time constants 100 to 500 µs).

A fiber optic sensing system for simultaneous measurement of temperature and salinity based on fiber Bragg grating technology is presented. The fabrication process, which relies on chemical etching of the fibre, is described and its performance is evaluated. Theoretical and experimental results are given, which are summarized on the obtained resolutions of ±0.06°C/√Hz and ±0.2/√Hz for temperature and salinity, respectively.

We measure phase and group modal birefringence and also polarimetric strain sensitivity in a three-mode elliptical-core fiber for all spatial modes. Our results show a very high difference in fiber sensitivity to strain versus mode order, which is negative for the E₁₁ mode and positive for E₃₁ mode. We also model all measured parameters using Optiwave BPM-Cad and Cosmos/M software packages. The measured values of phase and group modal birefringence agree well with those obtained from calculations for all spatial modes. We do not observe as good an agreement between experimental and calculated values of strain sensitivity, which is most probably caused by the lack of reliable material constants for SiO₂/GeO₂ glasses.

A novel hybrid-integrated fiber optic accelerometer, based on Michelson interferometry, is developed. It makes use of a seismic mass suspended by a transduction fiber in a mass support. The Michelson interferometer is realized by a symmetrical double Y-branching waveguide and three polarizers, which are monolithically integrated on an x-cut, y-propagating LiNbO3 substrate with dimensions of X×Y×Z = 2×38×6 mm. On the distal endfaces of both interferometric arms, high-reflectance aluminum films are directly coated and serve as reflecting mirrors. An alternating current phase tracking (PTAC) homodyne scheme has been employed to process the signal. The performance of a prototype of the accelerometer is characterized. The measured frequency spectrum is in good agreement with the theoretical prediction.

The principal disadvantage of using free space optics (FSO) telecommunication systems is the disturbing role played by the atmosphere on light propagation and thus on the channel capacity, availability, and link reliability. The wavelength choice is currently a subject of disagreement among designers and users of FSO equipments. Generally this equipment operates in the visible and the near IR at 690, 780, 850, and 1550 nm. Several authors affirm that equipment working at 1550 nm presents less atmospheric attenuation in the presence of fog and thus better link availability. Others consider that for dense fogs (visibility<500 m), all wavelengths are attenuated in the same way (wavelength independence). Fog attenuation in the visible and IR regions is reviewed from an empirical and theoretical point of view. Laser system performance in the presence of fog (advection and convection) in the 0.4- to 15-µm spectral zone is investigated using FASCOD computation. A transmission gain of 42% for a lasercom system working at 780 nm is observed compared to the same system working at 1550 nm. This gain reaches 48% if the same system works at 690 nm. Finally, we propose a fast transmission relationship based on an exact Mie theory calculation valid in the 0.69- to 1.55-µm spectral bands. It enables us to predict fog attenuation according to visibility without using heavy computer codes.

Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen with Gaussian spectrum (Rytov theory and ABCD ray matrices are applied). The relation between the second- and fourth-order statistics for a beam with any degree of coherence in the atmosphere is introduced with the help of "effective" beam parameters, deduced from the free-space MCF. In particular, the scintillation (in weak and strong atmospheric conditions), based on these parameters, is studied as a function of the diffuser's strength and that of the atmosphere. The model is applied for the calculation of the SNR and bit error rates (OOK modulation) of the communication link with diffuser at the transmitter and slow detection system. The improvement of bit error rates is observed in weak and strong atmospheric turbulence. In the weak regime, the optimal diffuser can be found.

The gain-flattening filter of an erbium-doped fiber amplifier is experimentally demonstrated by using the technology of etching long-period gratings (LPGs). An error of gain flatness less than 1 dB is obtained by using three cascaded LPGs with different cladding diameters in the range from 1536 to 1565 nm.

A hybrid fiber amplifier (HFA) comprising an erbium-doped fiber amplifier (EDFA) and a discrete compensating Raman amplifier (DCRA) can compensate both the span loss and the chromatic dispersion. The gain saturation and nonlinear effect induced in the HFA are discussed. The experimental results show that although the EDFA-DCRA has lower noise figure than the DCRA-EDFA configuration, it induces gain saturation and the nonlinear effect. When designing an HFA, the setup of the DCRA-EDFA configuration is proved a better choice.

Hyperspectral imagery can provide very valuable information on land cover classes. However, it also presents many challenges in data analysis and interpretation as a result of the large amounts of data collected. For example, conventional methods for land use and land cover classifications may not be directly applicable. Such conventional methods typically require a preprocessing step to transform high dimensional data to a lower dimension, mostly by eliminating data redundancy. For decades, principal component analysis (PCA) has been widely used to decorrelate spectral bands for reducing dimensionality. It is a useful technique if the spectral class structure of the transformed data is distributed along the first few axes. Otherwise, the transformed data may be similar to the original data. In such cases, we have shown in an earlier work that the wavelet decomposition technique is a better approach. Wavelet decomposition can reduce hyperspectral data in the spectral domain for each pixel. By carefully combining PCA and wavelet techniques, we engender a new method that benefits from the strength of both techniques. The intent of the hybrid method is to provide a tradeoff between the accuracy and speed, as compared with PCA and wavelet methods. The effectiveness of this method is demonstrated by using hyperspectral data from the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) hyperspectral instrument. The experimental results show that, for high reduction rates, the hybrid method is superior to pure PCA and to pure wavelet-based techniques.

A new finite-state vector quantization scheme called jigsaw-puzzle vector quantization (JPVQ) is proposed to provide better image quality, especially in the low bit rate context. For low bit rate image coding with conventional finite-state vector quantization (FSVQ) techniques, image quality degrades due to error propagation from one state to the next. The proposed JPVQ algorithm exploits the four-step side-match prediction technique to optimize the spatial continuity of each encoded block to improve the coding performance and reduce the error propagation effect. In the proposed coding scheme, an input block can be encoded by the jigsaw-puzzle block, the dynamic codebook, or the supercodebook. It is demonstrated with experimental results that JPVQ performs significantly better than traditional FSVQ techniques.

Discrete cosine transform (DCT)- and wavelet-based schemes have been highly successful for image compression in the lossy mode. Further improvements in one or more stages of these schemes are frequently reported in literature. The input images to these methods are nearly always 8-bit/pixel images. We explore a bit-plane-based decomposition of images to reduce the input data amount for a compression system with the objectives that perceptual quality should remain acceptable and the compressibility of resulting data at the subsequent stage should be unaffected or improved. The proposed method shows favorable results and presents opportunities of further work in this area.

IR extinction cross sections and mass extinction coefficients of micrometer-sized particles of a variety of polar or partly polar insulators and semiconductors are calculated using several single-particle electromagnetic scattering models including the well-established Mie theory for homogeneous spheres and more modern techniques such as the T matrix method. The range of scattering models employed enable a detailed investigation of the impact of particle morphology on IR extinction. Aerosol chamber measurements of the mass extinction coefficients of a selection of particulate materials are also made and compared with the theoretical calculations. Distinct extinction characteristics in the mid- and far-IR corresponding to reststrahlen band dispersions in the optical constants were predicted and observed. The results indicate the potential limitations of this class of material as military IR obscurants.

We present shearing interferometry and Fourier analysis techniques to measure the temperature and temperature profile of a gaseous burner flame. The methodology is expected to give a better accuracy than techniques that use the location of fringe centers because of the complete separation of phase from the unwanted irradiance variations.

A simple polarized-light-based phase shifting method for analyzing fringe patterns formed in Burch's common-path scatterplate interferometer is proposed. The system employs circularly polarized beam at the input, a small quarter-wave plate acting on the reference beam only, and a rotating analyzer in two orthogonal circular polarization beams at the output of the interferometer. The simplicity of our approach is proved theoretically and experimentally. Problems related to nonideal light modulation at the output and the ways to overcome them are discussed.

We study the variation in the back-scattered light from a slab of silicate glass on a change of the glass temperature. Coherent illumination scattered from a ground surface of the glass creates a speckle pattern resulting from the sum of two different scattering events: one originating from the backward scattering from the rough surface, and the second from the forward-scattered light, which is Fresnel reflected from the flat back surface of the slab and rescattered (in the forward direction) by the rough surface. Experimental and theoretical studies show that on heating the glasses, the correlation between the initial and the final speckle patterns oscillates as a function of temperature with a frequency proportional to the thermally induced changes in the optical path in the glass. A measurement of the thermal expansion coefficient α is obtained from the oscillations with 0.01% accuracy, provided that the change in refractive index with temperature is known. The observed slow decrease in the amplitude of the temperature-induced oscillations is in agreement with the theoretical analysis of the effect of uniform thermal expansion of the scattering surface. The surface expansion also results in a damped, slow oscillation of the correlation due to the nonrandom motion of the expanding scatterers, which result in the partial rephasing of the scattered waves when the scatterers move a distance on the order of a wavelength. If the refractive index change with temperature is unknown, the decrease in the amplitude of oscillations can be used to determine α, the thermal expansion coefficient while the oscillations can be used to determine the refractive index change with temperature. The method presented is thus an alternative for measuring the thermal expansion coefficient and the change of index with temperature, and can be applied when dealing with strongly scattering surfaces, where one cannot use the normal fringe method. Different glass thicknesses and sample tilts with respect to the incident laser beam are shown to modify the correlation function.

The measurands of several reported laser-based measurement techniques are sensitive to both the propagation direction of the laser and the viewing direction from the region of interest to the detector. For such imaging techniques, the view vector must be determined uniquely for each pixel in the detector array. The bulk view vector is often physically measured and a simple model used to determine the view vector for each pixel. This, however, has limitations where access is limited, the distances involved are small, or the optical system employed introduces errors. We describe a procedure to determine the unique view vector from a planar region to the detector (CCD camera) for each element in a 2-D array based on a reference target aligned with the planar region of interest. Determination of the view vector is based on the spatial distribution of the mapping function used to dewarp the view. No physical measurement of the view vector is required. Good agreement is achieved when the procedure is compared to a simple pin-hole camera model of the view using a computed test target.

The fundamental conception of the detection range of a low-light-level imaging system is described. Based on the common apparent distance detecting equation, a more practicable and reliable equation is established by using factors such as atmospheric transmissivity, spectral matching factor, and so on. The relations of these parameters, which have an influence on performance of the system, are analyzed. Finally, some approaches are proposed for better estimates of the field performance and optimization of the design of night vision system.

Efficient one-step addition and subtraction algorithms are proposed. The arithmetic operations use the negabinary modified signed-digit number representation, which is an extension of the negabinary number system. Truth tables representing the required symbolic substitution computation rules are determined. Then a reduction of the number of the symbolic substitution computation rules is achieved by considering the digits in the lower positions of the negabinary modified signed digits in the computation. The proposed algorithms are very suitable for optical holographic and nonholographic implementations. An incoherent correlation based on the efficient shared content-addressable memory (SCAM) is suggested as an optoelectronics implementation.

The dynamics of all-optical switching in C₆₀ has been theoretically analyzed based on the principle of nonlinear intensity-induced excited-state absorption to achieve high contrast and fast switching. The transmission of a cw probe laser beam at 885 nm corresponding to the peak absorption of the S₁ state through C₆₀ in toluene is switched by a pulsed pump laser beam at 532 nm that excites molecules from the ground state. Switching characteristics are shown to be sensitive to the variation of pump pulse width, peak pumping intensity, thickness of the medium, intersystem crossing time between the S₁ and T₁ states and the absorption cross section of the S₁ state at a probe wavelength, and their effect are analyzed in detail. It is shown that the transmission of the probe beam can be completely switched off (100% modulation) by the pump beam at a pump intensity of 100 MW/cm² and a pulse width 1.5 ns, with switch on and off times of 2.5 and 7.5 ns, respectively. The results are used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses, which are the basic building blocks of computing circuits.

We present an implementation of a genetic algorithm in the structural design of cemented triplets according to a prespecified set of Gaussian characteristics and primary aberration targets. The approach is directed toward obtaining a suite of promising solutions allowing some floating of the primary aberration targets. This method obviates the need for any heuristic preselection of glasses for the three elements of the triplet. Searches for optimal solutions are conducted in the total configuration space of the degrees of freedom, consisting of continuous variables like shape factor and power distributions, and discrete variables like available glass types. The role of genetic diversity in the evolving population on convergence of the optimization runs has been investigated, and, accordingly suitable modifications are incorporated in the basic genetic algorithm to eliminate undue stagnation and premature convergence problems. Some illustrative examples are given.

A novel optical sampling scheme using an electroabsorption (EA) modulator as the sampling component is presented that will greatly reduce the complexity of presented schemes. The 10-GHz optical analog signal's waveform is measured, with a 100 MHz waveform observed on an oscilloscope. The relationship between the EA sampling width and sampling bandwidth is analyzed using a two-step model, which is presented for the first time. Calculation indicates that the Gaussian-shaped filter can perfectly simulate the effect of the EA sampling width. Detailed analysis also reveals that the optical sampling with an EA modulator is a promising scheme for measuring waveform signals in future optical networks.

We propose to image through an optical waveguide using wavelength diversity. We demonstrate the technique by imaging through a parallel plate waveguide. Wavelength diversity is an alternative to the spatial shift diversity first reported to image through the plate waveguide. Severe optical aberrations are caused by the dispersion between the waveguide modes. The resulting point spread function (PSF) is highly nonisoplanatic, causing images that are transmitted to be severely distorted. Wavelength diversity is used to compensate for the aberrations and correct the image transmission. Two images are collected, each at different wavelengths, and the phase diversity algorithms applied. Computer imaging simulations are shown for imaging through a parallel plate waveguide, with allowance made for aperture and length measurement error due to mechanical drift. The image sensor is assumed to have an additive Gaussian noise model with noise covariance R_Nand pixel quantization of eight bits.

Unexpectedly high efficiency stimulated Raman scattering are seen in micrometer-sized water jets. The pumping laser beam of a 100-W frequency-doubled Nd:YAG laser is coupled into the water jet similar to an optical fiber. The light is guided within the water jet, providing a high irradiance level throughout its stable portion (typically 60 to 80 mm in the case of a jet of 60 µm in diameter). This results in a long interaction length between the high intensity beam and water. At irradiance levels, one order of magnitude smaller than the breakdown threshold 26.7% of the pumping light was converted to Stokes emission. A single stimulated Raman scattering (SRS) peak at 653 nm (3489 cm^–1 in wavelength shift) is observed to be emitted from the jet in the upstream direction, which is different from the spectra reported in the case of bulk water or small water cylinders.

Discrete wavelet transforms (DWTs) have become a very powerful tool in signal processing. The first point to be considered if implemented in hardware is what wordlength to use. We detail a mathematical approach that enables the determination of the exact dynamic range at any filter output of a J-stage 1-D and 2-D forward or inverse DWT for any arbitrary input signal. Once applied on the forward transform, our approach leads to wordlength bit savings compared to the work published in the literature. These savings are relevant when implementing the transform in hardware since they lead to considerable area savings and speed improvement. In addition, we provide the first investigation in determining the exact dynamic range of the inverse transform stages. Although our focus is on the DWTs, the approach can be applied on any finite impulse response (FIR)-filter-based architecture.

A procedure is proposed to calibrate a generic structured light system, consisting of one camera and one projector. The proposed procedure is based on defining a unique coordinate system for both devices in the structured light system, and thus, a rigidity constraint is introduced into the transformation process. This constraint is used to derivate a simple function for the simultaneous estimation of the parameters, resulting in parameters that are more reliable. The performance of the proposed procedure is shown on examples of the calibration of two different structured light systems.

We investigate tracking filters in electro-optical target-tracking systems with bearing-only measurements and a stationary tracker. In passive tracking, for maintaining the target in the camera field of view, two tracking angles should be controlled. To extract the target position, there is at least one frame period latency resulting from time duration required for image processing. Three filtering methods, a simple Kalman filter, a novel filtering approach based on curve fitting on time series data, and an interactive multiple model filter, are studied. Since target range is neither available nor observable, in the all mentioned techniques, instead of applying filters to the target states (position and velocity in the space), each filter is directly applied to the tracking angles. The performance of each filter in this approach is evaluated by tracking angles error with two maneuvering targets.

A nonlinear measure for template matching is introduced to accommodate the signal amplitude variation of the targeting objects in noisy images. The pixels in the image area corresponding to the template are partitioned into two groups: the brighter pixels and the darker ones. The proposed measure is defined as the difference of the numbers of the brighter pixels in the object and the background. It is compared with the classical normalized covariance for the stability of the response and the success rate under the white Gaussian noise and the impulsive one, verifying its superiority. The proposed measure is tested also for a set of real images, where the target objects have disparate backgrounds. It is proved to have better performance than the normalized covariance, by comparing the correct match responses with the maximum of the mismatch responses, as well as the receiver operating characteristics.

We derive an analytical expression for the variance of the timing jitter of a soliton transmission system using super-Gaussian sliding-frequency filters. By theory and computer simulation, we demonstrate that super-Gaussian sliding-frequency filters are more effective to reduce timing jitter than conventional filters without sacrificing the SNR. We also show that the sliding action of the super-Gaussian filters enhances timing jitter.

The National Aeronautical and Space Administration (NASA) is planning to send two Mars Exploration Rovers (MER) to Mars in 2003. Onboard these rovers will be a number of scientific and engineering cameras. Mars is a dusty place, so dust will accumulate on the front surface of the camera optics. When the sun shines on the dusty front surface, light will be scattered to the detector. This increases glare and reduces contrast. The rover lenses must work, even when the sun shines on the front element. Therefore, the veiling glare has been evaluated by experiments. We discuss these experiments and the results.

The movement description of translating targets in time-varying images detected through the atmosphere is a relevant topic with application in many different fields. The technique consisting of the evaluation of the spatiotemporal Fourier transform of a sequence of frames is applied to images distorted by the atmospheric turbulence and partially compensated using an adaptive optics system. The technique accuracy for estimating the object velocity is checked using simulated low-light-level images. This analysis comprises the effect of the object light level and that of the correction degree provided by adaptive optics on the motion estimation.

We evaluate the degradation in a video sequence that suffers from increasing packet loss ratio (PLR), by using several kinds of quality measures, namely, mean square error (MSE); A_max (the root MSE, which is weighted by the maximum signal change between pairs of pixels—the current pixel and eight surrounding neighbors); the video perceptual distortion measure (VPDM); and a perceptual distortion metric for digital color video (VPDM2). Packet losses can occur at different positions in the compressed video sequence, for example, in headers, I frames, P frames, and B frames. Each packet loss results in a different amount of distortion in the received video signal. We demonstrate the differences between two packet loss models—IID (identical independent distribution) and burst (a sequence of packets lost together). We also examine the influence of packet loss in a communication network on MPEG2 compressed digital video quality. Three of these measures emulate features of the human visual system: the VPDM, which considers temporal masking of distortion (the human eye notices less degradation within a frame, if it appears during fast changes between consequent frames, such as scene cuts); the A_max, which encompasses spatial masking, resulting in small objects within high-frequency neighborhoods being less noticeable; and the VPDM2, which takes into account the human vision color channels and temporal and spatial masking. The results point out that loss due to burst degrades the quality of the compressed video stream significantly less than loss due to IID. The video quality can still be tolerable for a 0.1 PLR using the burst loss model, but for the IID loss model, even for lower PLRs (i.e., 0.01), the video is not perceptible to the human viewer.