This PDF file contains the editorial “Editorial: Copyright Problems” for OE Vol. 31 Issue 01

This PDF file contains the editorial “Guest Editorial: Optical Fiber Sensor Based Smart Materials and Structures” for OE Vol. 31 Issue 01

A closed-loop control experiment is described in which vibrations of a cantilevered beam are suppressed using measurements from a modal-domain fiber optic sensor. Modal-domain sensors use interference between the modes of a few-mode optical waveguide to detect strain. The fiber is bonded along the length of the beam and provides a measurement related to the strain distribution on the surface of the beam. A model for the fiber optic sensor is derived, and this model is integrated with the dynamic model of the beam. A piezoelectric actuator is also bonded to the beam and used to provide control forces. Control forces are obtained through dynamic compensation ofthe signal from the fiber optic sensor. The compensator is implemented with a real-time digital controller. Analytical models are verified by comparing simulations to experimental results for both open-loop and closed-loop configurations.

We introduce the use of fiber optic strain sensors embedded within a macromodel composite employed to study and validate micromechanical theories. Fabry-Perot (FP) fiber optic strain sensors (FOSSs) embedded within a macromodel composite are shown to be an accurate and precise means of making local internal strain measurements in and around damage events. The measurements made by the sensors compare closely to those obtained from resistance strain gauge data verified through presently accepted micromechanics. The optical strain sensors effectively measure both the signature of fiber fracture and the resulting strain concentration due to the damage event. The sensors add a new dimension to the validation and development of micromechanics. The approach assists in the formulation of new concepts for interpretation and prediction of actuator/sensor response in smart materials.

This paper presents the principle of fiber optic sensors for the detection of low-frequency magnetic fields. Magnetostrictive materials such as Tertenol-D and Metglas® are used as the sensing elements in a Mach-Zehnder interferometer configuration. Low-frequency fiber optic magnetometers are constructed and tested in both planar and cylindrical geometries. These magnetometers consist of an optical fiber bonded to a magnetostrictive metallic glass ribbon in either strip or cylindrical geometry. The characteristics of specific sensor designs are also discussed and suggestions to improve magnetic field sensor fabrication are given. From a number of experiments, the best sensitivity achieved is with sandwich geometries under the application of a dc biasing field. The performance of these sensors increases with improved mechanical design and thermal annealing of the metallic glass ribbon. Much of the reported work to date has been concerned with the response of the system in the frequency range of 0.1 to 10 kHz. The work presented here, however, deals with the response of an interferometric system using Metglas® in the lower frequency range of 10 to 50 Hz.

We present the first demonstration of active acoustic control using optical fiber sensors. The fiber sensors are oriented symmetrically about the vertical centerline of a thin, simply supported baffle plate and are configured for the most effective detection of the odd-odd plate modes. The output from the fiber sensor is used as an error signal, a least-mean-square algorithm is used for digital processing, and control is achieved using a single piezoelectric actuator. Preliminary results obtained from the use of elliptical-core, two-mode fiber sensors attached to the plate show effective acoustic attenuation of 25 dB in on-resonance cases and 10 dB in off-resonance cases.

An overview of our research toward the development of fiber optic-based smart structures is presented. This includes the development of the first full-scale fiber optic damage assessment test system for an aircraft composite leading edge; the use of a Michelson fiber optic strain sensor for monitoring acoustic emission from within composite specimens; the first real-time correlation of acoustic emission with the incremental growth of a delamination; the fabrication and characterization of an all-fiber intrinsic Fabry-Pérot fiber optic strain gauge embedded within composite materials; and the fabrication of the first practical fiber optic strain rosette and an evaluation of its ability to map the strain tensor within a composite structure.

We have measured the dark current (I_d) and noise equivalent power (NEP) of four large-area (~0.8 cm²) avalanche photodiodes (APD) to determine their sensitivity limit for detecting weak optical communication signals. We found that from room temperature down to about -10°C the I_d decreased by one-half for every 8-deg drop in temperature and the NEP decreased by one-half for every 16-deg drop in temperature. Below this temperature, the NEP leveled off at somewhat less than 5 pW in a 100-kHz bandwidth. We describe the measurement technique, present an analysis of the data, and discuss the statistical properties of these APDs according to the model developed by McIntyre et al.

The injection efficiency of an 8- to 14-μm hybrid IR/CCD FPA is calculated from a transcendental equation involving the detector, CCD input currents, and the detector impedance R_d at the operating point. The latter has been varied either by changing the composition of x in the Hg_1-x Cd_x Te detector or by changing the tunneling current while x is kept constant. Calculations were carried out for both weak and strong inversion cases of the input MOSFET of the CCD. It is argued that for the 8- to 14-μm region, since the detector currents are high, operation of the input of the CCD in the strong inversion regime seems more reasonable.

Off-axis particle field holography and imaging with two types of cylindrical test sections without surface modification or index matching are described. One type is the liquid tunnel with circular inner and flat outer surfaces. The other case is that of cylindrical tubes such as biological ampules. The analytical description assumes that if the astigmatism-related longitudinal image position uncertainty is smaller than the nominal aperture-limited uncertainty, then imaging is possible. A general mathematical formulation is presented with examples of water tunnel, glycerine tunnel, and a typical biological ampule.

Holographic interferometry and electronic speckle pattern interferometry (ESPI) have both been used to measure the radial expansion of a heated diesel engine piston. A simple "mirror" concept introduced enables simultaneous measurement of in-plane and out-of-plane components of displacements. This concept was verified by using a cantilever and the results agreed to within 5% of the predictions of deflection theory. ESPI and holographic fringes on two views of the piston were recorded on a single frame of a CCD camera. Phase-stepped ESPI fringes were analyzed to obtain two phase maps of different sensitivities on a single video frame. Comparison between holographic and ESPI results were made and possible reasons for the smaller values obtained using ESP1 are proposed. A critical comparison of the two techniques shows that holography has the advantage of enabling study of a 2000 rise in a single stage due to its high quality fringes. ESPI limits the temperature rise to 5°C because of its poorer fringe definition and hence displacement from four stages has to be added vectorially.

Photodeposition is an emerging new thin-film material deposition and optical image patterning technique with photographic recording characteristics. In the past, lasers and incoherent light sources were exploited for photodeposition of various materials in micropatterns. Here we report that photodeposition can also be used for recording holographic relief gratings of 2000 lines/mm and periodically modulated depths of 1 to 20 nm.

Digital high-pass filtering is used frequently to enhance details in scientific, industrial, and military images. High-pass filtered (HPF) images also are used both to illustrate and test models of visual perception. The visual system appears to interpret HPF images in the context of a multiplicative model of high-frequency reflectance and low-frequency illumination whenever possible. HPF images can be treated as a form of two-dimensional amplitude modulation signals. The low-frequency information, which is coded in the modulation envelope, disappears with the carrier if low-pass filtered. The envelope may be retrieved (demodulated) using one of many possible nonlinear operations followed by a low-pass filter. The compressive nonlinearity ofthe visual system is shown to suffice for demodulating such images. Simulations show that HPF images cannot be used to reject the hypothesis that illusions and grouping phenomena are due to low-frequency channels.

The Fourier-transforming properties of coherent imaging systems and their applications in spatial filtering techniques are well known and thoroughly developed. Most of these previous systems were static and unable to adapt for inputs exhibiting different sets of characteristics. I present an adaptive coherent imaging system utilizing simulated annealing in the frequency domain. The iterative process of simulated annealing enables the filter to adapt to a variety of inputs and automatically optimize even in the presence of random noise. A set of possible guidelines for parameter selection is presented to facilitate the implementation of this technique into similar applications.

A video rate laser range finder based on the autosynchronized scanning approach is presented. This range camera simultaneously provides range and intensity information of a scene in EIA-RS-170 format. A spatial resolution of 420 horizontal lines each having 512 pixels was achieved at a rate of 30 images per second. A wide dynamic range in intensity (16 bits) coupled with drift compensation allows this sensor to achieve a range resolution of 8 bits. The volume of view is 50 x 50 x 50 mm³.

Thin lens solutions and optimization programs are utilized to avoid thick meniscus elements in the design of diffraction-limited scan lenses. Reasons for the appearance of thick meniscus elements and the procedures for eliminating them are explained.

A generalized approach to modeling resist performance is introduced and applied toward characterizing a negative chemically amplified resist system. The Generalized Characteristic Model for lithography is used to extract parameters to evaluate easily development rates from characteristic curves. The model suggests that two lumped parameters, αn and E₀, dominate lithographic response for negative chemically amplified resists. Both αn and E₀ were regressed from characteristic curves over a postexposure bake temperature and time range from 110 to 150°C and 30 to 90 s and develop times from 30 to 150 s. The parameter E₀ showed the predicted postexposure bake temperature and time and develop time dependencies over the processing window, while αn did not. Possible explanations for this discrepancy are discussed. These parameters were used to simulate linewidths that were compared with experimental results. Linewidth predictions using parameters extracted with the generalized characteristic model agreed to within 15% of experimental results over the entire processing window.

To cope with the trend of high-speed access, a high-performance optical head with a sliding and rotary-type of lens actuator has been developed. An advantage of this type of lens actuator is that the moving part of the actuator is dynamically well balanced so that the objective lens is not shaken by acceleration or deceleration during seeking. As a result, the stability of the velocity control during seeking has been improved and the waiting time when the operation mode changes from the seek mode to the tracking mode has been reduced. To cope with the trend of high-speed transfer, we have increased driving efficiency and set the frequency of the parasitic resonance sufficiently high. Also, the rotational latency, which is an important factor of the access time, has been shortened. As a result, a data transfer rate of 7.4 Mbits/s and average access time of 48 ms have been attained in a 130-mm rewritable optical disk drive.

The radiation-induced optical transmittance loss in polycrystal Csl was investigated in the 2.5- to 40-μm infrared region. Exposure of polycrystal Csl to ionizing radiation doses in the 0.3- to 1-Mrad range using a research reactor causes a significant degradation of the optical transmittance. Losses of up to 50% over the infrared transmiffance spectrum are reported; however, changes saturate at ⪅ 1 Mrad. The loss is particularly evident at shorter infrared wavelengths, shifting the threshold wavelength for transmittance to higher wavelengths. Results also show that cerium-doped polycrystal Csl exhibits improved radiation resistance.

Rayleigh-Brillouin scattering measurements have been conducted on a thoriated fluoride and a fluorozirconate glass. Values of the elastic and photoelastic constants of these glasses pertaining to the longitudinal and transverse acoustic modes have been determined. In addition, corrections due to absorption have been explicitly included in the calculation of the photoelastic constants. We note that although fluoride glasses of similar compositions have been previously measured, little or no data on the transverse Brillouin components have been reported, while values of C₄₄ and P₄₄, have been determined for both fluoride glasses in our study. Additional results on the Rayleigh scattering losses have also been obtained and indicate smaller scattering losses from the fluorozirconate glass than from the thoriated fluoride glass.

Hybrid optical/electronic nonlinear optical devices are presented that have bistable and hysteresis properties. These devices are based on linear optical modulators and nonlinear electronic feedback that controls the modulators. Because the nonlinearity is contained entirely in the feedback electronics, the nonlinearity is electronically and optically tunable. An experimental implementation of a hybrid device using a linear acousto-optic modulator and a nonlinear electronic Schmitt trigger is discussed. A conceptual electro-optical device, which may be implemented in arrays, is described.

An optical crossbar switch is proposed that uses a semiconductor-addressed, deformable-mirror spatial light modulator to direct gigabit-per-second optical transmissions to several receivers. The switch performance is described in terms of size, number, and deflection range of micromechanical mirror elements, as well as emitter, detector, and lens dimensions. These parameters are incorporated in a link energy budget and estimates of system dimensions. Maximum-sized 128- to 1000-channel crossbars appear feasible based on current and projected performance limits of various lightwave components.

We demonstrate a technique that makes possible the robust packaging and interconnection of fiber pigtails to glass waveguide silicon devices. An integrated 19 x 19 star coupler, formed on a silicon substrate, is covered with a silica slab attached by means of a low-index epoxy. The slab both isolates the waveguides, which are just a few micrometers below the surface, from their surroundings and provides a means for attaching fiber connectors. Measurements of the performance of the star coupler before and after covering even show a slight (<0.5 dB) improvement for many of the waveguides due to the silica cover.

The digital photoelastic technique was used to obtain an experimental solution of the transient thermal stresses of two interacting semielliptic surface defects due to a step temperature change at the free surface. A series of computer programs were developed on the digital photoelastic system to extract the maximum transient thermal stress on the defect boundary. By using a statistical analysis package, the variation of the maximum transient thermal stresses and their locations were correlated with the time, temperature difference, and geometrical parameters of the defects.

Orbital measurements of the Earth's long-wave emitted radiation, and the Sun's radiation reflected by the Earth have been made by scanning radiometers on three spacecraft platforms in both high-altitude Sun-synchronous polar (833 km) and low-altitude (600 km) equatorial orbits. The Earth Radiation Budget Satellite (ERBE) instruments have been designed to measure the radiance from the Earth with an absolute radiometric error of less than 1%. The scanner instruments consist of three broadband radiometer channels: short wave, 0.25 to 3.5 μm; long wave, 5.0 to 50 μm; and total, 0.25 to >50 μm, and use thermistor bolometers as sensing elements. These radiometers repetitively scan the Earth from limb to limb every 4 s producing contiguous pixels on the Earth's surface. Each scan contains views of cold space and of an internal calibration source, to obtain a radiometric reference and a calibration check, respectively. The design and operation of the ERBE scanner are described, and an overview of the ground calibration approach and inflight calibration stability is given along with an analysis of on-orbit anomalous behavior.

An optical velocity sensor, based on the sheet-pair transit-time technology, was designed, built to flightworthy standards, and test flown on an F-16B to 50,000 ft and Mach 1.2, as part of an optoavionic air data system. Brief descriptions of the work leading to 24 flights in January and February 1990 are given, with examples of data and discussions of experiences. Compared with conventional pneumatic sensors the system offers advantages that include potential improvements in accuracy, latency, calibration, dynamic range of speed and attitude, robustness, and possibly life, cost, and range of application, without modification of the vehicle skin contour. Measurements corresponding well with the aircraft system were obtained under all conditions except heavy cloud, which demands small design changes for future systems. The importance of modeling for software and hardware design optimization is stressed and measurements are presented.

This PDF file contains the editorial “Book Rvw: CRC Handbook of Laser Science and Technology Supplement 1: Lasers," edited by Marvin J. Weber for OE Vol. 31 Issue 01

This PDF file contains the editorial “Book Rvw: Optical Components, Systems, and Measurement Techniques," by Rajpal S. Sirohi and Mahendra P. Kothiyal for OE Vol. 31 Issue 01