This PDF file contains the editorial “Editorial: Kingslake Medal and Prize” for OE Vol. 31 Issue 08

This PDF file contains the editorial “Guest Editorial: Special Section on Optical Engineering in U.K. Industry” for OE Vol. 31 Issue 08

The paper reports on the progress in gas sensing using real-time correlation spectroscopy, where a gas is used as a matched optical filter to "recognize" its own spectral absorption lines. The basic concept of correlation spectrometry involves the passage of light sequentially through two gas cells: a reference cell containing a known quantity of the gas to be detected, and a sampling cell where the presence of the gas is to be determined. An optical signal passing through both cells will suffer absorption due to the gas in each. If the absorption in the reference cell is periodically modulated, then the total absorption depends on whether the gas absorption lines in the sampling cell correlate with those in the reference cell gas. Two methods of modulating the reference cell absorption are reported, pressure and Stark modulation. Results are presented for methane detection employing pressure modulation. The pressure fluctuations are generated within a compact resonant acoustic cell driven by a piezoelectric transducer. Also given are results for cross-sensitivity measurements with ethane as the contaminant gas. The Stark technique is applied to ammonia detection here, but can be used with many gases that exhibit a strong dipole moment.

Methane detectors based on the absorption of infrared radiation show some promise as the basis of sensitive and accurate industrial instrumentation. Comb filter methods that utilize the vibration-rotation structure of the methane spectrum in an optically efficient manner have recently aroused particular interest in this context. We describe a mathematical model to simulate the behavior of scanning interferometric comb filters in methane detection systems and discuss the use of the model to investigate optimum design parameters for such systems. The behavior of prototype methane detectors based on scanning Fabry-Pérot and Mach-Zehnder interferometers is briefly reported. Possible techniques for obtaining further improvements in the design of comb filters for methane detection are suggested.

We describe three novel optical fiber probes, all based on the focused-beam reflective principle, to measure displacement, form, and surface topography, respectively. Each depends on deriving twindisplacement/optical output characteristics that may be resolved by difference/sum referencing. The displacement sensor adopts twowavelength operation using a zone plate to give opposing displacement/output characteristics. The device is noncontacting, and a resolution of better than 0.1% of span is anticipated. A 3-D optical touchprobe has been built for use with coordinate measuring machines. A mathematical model has been generated that relates output to stylus movement, and the model has been verified experimentally. A resolution of 0.1 μm in the x-y plane and 1.0 μm in the z direction is achievable. A further noncontacting probe has been developed for the measurement of surface topography whose output shows low dependency on surface reflectance. A mathematical model has shown good correspondence against a wide range of surface compositions and textures; a resolution of better than 1 μm is foreseen. Proposals are made for industrial implementation of all three probes.

An optical pressure sensor has been designed using silicon micromachining technology. A resonant silicon beam is mounted above a diaphragm and its resonant frequency changes with applied pressure. The sensor is temperature compensated by way of a second pressure-insensitive resonator. Both resonators are optically addressed via the same optical fiber. The sensor is designed to give an overall accuracy of 0.05% full-scale pressure, which is currently between 130 kPa or 3 MPa. Optical technology allows the optical pressure sensor to operate in a harsh aerospace environment where electronic pressure sensors cannot survive.

A light-in light-out absolute encoder is described based on a Manchester encoded pseudo-random binary sequence scale. Optical fiber connections of 100 μm are employed and 13-bit resolution is demonstrated.

We combine the operational advantages ofthe complementary techniques of electronic speckle pattern interferometry (ESPI) and laser Doppler velocimetry (LDV) for out-of-plane vibrational analysis. A cw laser was used in a fiber optic system to generate time-averaged ESPI fringes. By mutual phase-locking of the ESPI and LDV signals, automatic heterodyning of the ESPI signal was achieved (i.e., without operator intervention) together with compensation for unwanted nondeterministic out-of-plane whole-body motion of the object under study. Automatic heterodyning extends the operating range of time-averaged ESPI systems to higher vibrational amplitudes and reveals phase relationships in the object vibration. A practical demonstration of the technique is described.

A novel optical system designed as an illuminator for line-scan cameras is described. This system produces a linear illumination pattern on a planar surface and uses plastic Fresnel lenses to reduce the system cost and dimensions. A custom-made optical fiber array, which acts as a 1-D diffuser, forms part of a high numerical aperture projection system providing a half-angle cone of illumination in excess of 35 deg. This wide range of incident angles allows the imaging of surfaces ranging from near-specular to Lambertian diffusers.

An image recognition system is described that can distinguish between a number of different human faces and is capable of processing a field of view covering a typical room in ~ 2 s. The system uses a novel infrared illumination system which overcomes the variable effects of ambient illumination, allowing highly reliable recognition. The system is based on a neural network known as WISARD. Images obtained from the imaging system, and typical face recognition results are presented. The results clearly demonstrate the ability of the system to distinguish between individual faces, and between faces and arbitrary background scenes.

A novel method is reported by which the alignment response time of liquid crystals contained in a display may be measured by a relatively simple and inexpensive system. The method has the advantage that the output is in the frequency domain and is strongly intensity independent. The dependence of the resonance frequency of the system with temperature is measured, and it is envisaged that the system can be further miniaturized to serve as a compact optical temperature sensor.

A cooled Fabry-Pérot etalon has been constructed for which the parallelism and mirror spacing can be servo-controlled and tuned using an existing commercially available etalon control system. Capacitance sensors monitor the mirror spacing and parallelism, and error signals produced as a consequence of changes in these values are used to stabilize the cavity with piezoelectric actuators. The etalon operates at liquid nitrogen temperature, has a free aperture of 50 mm and a nominal mirror spacing of 60 μm, and is coated for the 3- to 5-μm spectral region. Under servo-control at operating temperature, the response time of the etalon is 30 ms and the cavity tuning range is 6 μm, corresponding to ~ 3 orders of interference at the midrange wavelength of 4 μm.

A tunable Fabry-Pérot filter is described in which the cavity length is continuously monitored by a capacitance sensor and servo-tabilized. The filter can be random access tuned over > 50 nm wavelength range at 1550 nm with a response time < 1 ms. Finesse values in the range 100 to 500 have been achieved with insertion losses of between 3 and 8 dB depending on finesse. For a filter with a cavity length of 15 μm electronic noise is equivalent to a wavelength jitter of ~ 15 pm (rms) and the system temperature coefficient is equivalent to a temperature sensitivity of 0.15 to 0.35 pm C^-1 in transmitted wavelength. The filter has been used as a spectral analyzer in laboratory test equipment, a demultiplexer, and in optical amplifiers for noise reduction and wavelength selection.

Recent advances in materials technology has led to the development of new and improved optically nonlinear materials that can be used for a wide variety of applications ranging from deep UV conversion for laser fusion experiments to fast optical switching devices in future telecommunication systems. The processibility that many of these new materials have to offer will allow devices to be designed for specific applications. Some recent developments in the search for new materials are described and the future role that some of these may play in the design of nonlinear optical devices is discussed.

A parametric method for the objective measurement of surface damage such as digs and scratches on optical components is described and is intended for use with the new British Standard 4301 (1991). By a comparison of image visibilities of defects with line and spot standards, scratches and digs can be quantified in terms of their line-equivalent widths and spot-equivalent diameters. The codes used on optical drawings are described and guidelines for surface damage thresholds are discussed. The preferred equipment to aid the various stages of inspection and measurement is described and aflow diagram illustrating atypical system for the quality control of transmitting and reflecting components is presented.

Holographic nondestructive testing (HNDT) is used to investigate the complex structures of bones of various shapes and sizes subjected to forces. Three antlered deer skulls of different species were investigated and significant species-specific differences were observed. The HNDT method was also used to verify the advanced healing of an osteosynthetized sheep jawbone. Radioulnar bones of a normal and an orphaned moose calf were subjected to a bending test. The undernourished calf showed torsio displacement combined with the bending of the bone, which was not seen in the normal calf. The effects ofthe masticatory forces on the moose skull surface were studied by simulating masseter muscle contractions with jawbones in occlusion. The fringe patterns showed fast-moving bone surfaces on the naso-maxillo-lacrimal region.

To compare the elasticity of bones covered with soft tissue and the elasticity of defleshed and dried bones, we used sampling screws to make the surface movements of the bones visible through the soft tissue. We compared fresh and dry European moose skulls before and after skinning. External forces were focused on the skull bones through the pedicles. A high correlation in fringe orientation was observed in the case of thick bone structures with rigid interdigited sutures. We also compared compression dynamics of fresh and dry moose antler cubes.

Because the dissociated bromine in CuBr lasers has serious effects on the lifetime of CuBr lasers and the stability of the discharged plasma, it is important in long-lived sealed-off CuBr lasers. The adsorption of bromine on the surface of the copper around the electrodes was measured by a transmission electron microscope and an x-ray energy dispersive spectrometer. It was found that the bromine dissociated from the CuBr molecule by collision in the laser discharge plasma is a negative bromine ion; no positive bromine ion existed. On the average, the atomic percentage of the adsorbed bromine is about 20%. The radial distribution of the atomic percentage of bromine around the copper anode is given and has been explained qualitatively.

A four-beam two-focus differential laser velocimetry system with an acousto-optic modulator, which is suitable for measuring solid deformation or movement at ultralow speed, is presented. The signal-to-noise ratio is good enough for phase discrimination, the frequency response range is 0 to 20 kHz, and the measurable velocity range is 0.001 m/s to 300 mm/s.

Texture analysis approaches for extracting ocean wavelengths, directions, and heights from ocean wave images are presented. In the frequency domain, we use Fourier power spectral features to construct the texture measures. In the spatial domain, we introduce three texture measures based on the distribution of local extrema, a gray-level co-occurrence matrix, and two-dimensional histograms of the absolute-relative peak heights to discriminate different kinds of ocean wavelengths, directions, and heights. The experimental results indicate that these approaches are powerful.

Temperature distributions inside multilayer filters are often required to assess performance. This subject is rarely treated in the literature. Guidelines were developed that can be used to calculate a thermal model for various applications. Data such as the optical and physical properties of glass and coatings are required to build the model. Calculations can be performed with available thermal analysis software packages. In this study the guidelines are applied to an optical filter. This filter, which is not actively thermally controlled, is the front element in a space experiment devoted to the observation of solar intensity fluctuations. Temperature distributions were calculated for different designs using ESA software (ESATAN). The final design described here minimizes temperature gradients, as well as degradation effects due to UV and particle radiation. The effect of space exposure on temperature distributions inside the filter is also addressed.

A method for measuring the dimensions of patterned metal features on a lithographic photomask during iterative-etch processing has been developed. Features are inspected using reflected light from the unpatterned side of the mask; a specially corrected 0.55-NA objective lens provides diffraction limited resolution at 488-nm wavelength for mask thicknesses up to 5 mm. The method provides more accurate process control than conventional transmitted or reflected light metrology: Experimental data show nearly three times smaller measurement error due to the presence of resist overhanging the metal edges. The method may also have benefit for process control in the manufacture of certain types of rim-shifting phase-shift photomasks.

A system has been developed and tested for optical noncontact measurement of displacement. The system consists of a light source, an objective lens, a quadrant pattern, and a quadrant photodiode. The principle is based on the detection of the contrast variation of the projected quadrant pattern as a function of defocus. This system has coaxial projection and observation axes and is hardly influenced by the color and inclination of the surface. The dynamic range ofthe measurement system is 150 μm with vertical resolution of 1 μm. Experimental tests verify the principle of the method and some tests of the system in applications are tried with expected results.

Lidar systems are widely used in remote-sensing measurements relating to the study of atmospheric physics and its application to environmental protection. Large optical depth values give rise to multiplescattering effects that should be corrected for many lidar applications- in atmospheric gaseous constituent concentration measurements using the differential absorption method and for optical communications. On the other hand, these effects can be used to extract information about the scatterer. In both cases, the single-scattering events need to be separated from those caused by multiple scattering. A lidar simulation program is explained. Experimental methods are described that separate the multiple-scattering effects and use it for the determination of cloud microphysical parameters.

The scattered light resulting from polystyrene spheres residing on mirrors was measured at λ = 0.6328 μm, and results are compared to that predicted by a modified Mie theory. The method for cleaning the samples, counting and measuring particles, the measurement procedure, and the theoretical model employed to predict the scatter from contaminants on mirrors are discussed. The comparisons between theory and experiment indicate that the theory predicts the forward scatter, but the backscatter predictions are not as successful. The indication is that the developed model can accurately predict the scatter from dust on mirrors.

The scattered light from dust-contaminated mirrors was measured at λ = 0.6328 μm, and the results are compared to those predicted by a modified Mie theory. Comparisons between theory and experiment indicate that the theory accurately predicts the forward-scattered and backward-scattered radiation.

The scattered light from dust-contaminated mirrors was measured at λ = 10.6 μm, and results are compared to that predicted by a modified Mie theory. The theory is in good agreement with experiment. The far-infrared measurements of the "clean" mirrors were, for angles beyond about 8 deg from specular, limited by the scatter from particulates and not the surface microroughness. Similar limitations can be experienced by other irregularities such as scratches, digs, and pinholes in coatings. It is shown that contaminant scatter dominance in the far-infrared requires only a few very small particles, so the necessity for good cleaning techniques and good clean room practice is quite evident.

It is shown that the far-infrared scatter from smooth mirrors can be dominated by the scatter from just a few very small particles or defects. This emphasizes the necessity for good cleaning techniques and good clean-room procedures. Several effects of this finding are discussed, as well as several other related topics: the cleanliness required for the scatter to be dominated by a mirror's surface microroughness; a proposed specification for low-scatter infrared mirrors; incident angle invariance of clean and contaminated mirrors; the shape ofthe bidirectional reflectance distribution function (BRDF) curves; and the relation between surface cleanliness level, clean-room cleanliness class, and BRDF.

A new speckle suppression filter using adaptively tailored windows to preserve edges when they are present is proposed. Results are presented for synthetic aperture radar images and comparison is made with existing speckle filtering methods that do not have the shape adaptivity property.

Various optical fibers were compared theoretically for application to remote radiation thermometry. The highest optical transmission efficiency is attained with a fluoride glass fiber for the temperature range of > 700°C, with a halide crystalline fiber for 120 to 700°C, and with a dielectric-coated metal hollow waveguide for < 120°C. To realize remote sensing of lower temperatures, i.e., from room temperature to ~300°C, a radiometric experiment was carried out using a Ge-coated Ag hollow waveguide. When ambient temperature changed from 22 to 50°C, a serious error arose in the measured temperature, which is attributed to the increase of thermal radiation from the heated waveguide. Since the effect of such noise radiation is inevitable in thermometry using infrared fibers, we propose a measurement method to compensate for the noise level. With the compensation, the effect of ambient temperature was reduced successfully and reliable thermometry was established.

This PDF file contains the editorial “Book Rvw: Fiber Optic Communications Handbook," edited by Federico Tosco for OE Vol. 31 Issue 08

This PDF file contains the editorial “Book Rvw: Theory of Dielectric Optical Waveguides: 2nd Edition," byDietrich Marcuse for OE Vol. 31 Issue 08