This PDF file contains the editorial “Editorial” for OE Vol. 40 Issue 01

This PDF file contains the editorial “Distance and Displacement Measurements by Laser Technique” for OE Vol. 40 Issue 01

We review some usual laser range finding techniques for industrial applications. After outlining the basic principles of triangulation and time of flight [pulsed, phase-shift and frequency modulated continuous wave (FMCW)], we discuss their respective fundamental limitations. Selected examples of traditional and new applications are also briefly presented.

We review laser diode feedback interferometry as a general tool for sensing applications. After outlining the basic principles and the theoretical approaches used to describe the phenomenon, we present a few selected examples of applications in interferometry, as developed by various groups in recent years, such as a displacement sensor, a veloci- meter or vibration sensor, and an absolute distance meter or range finder and angle sensor. Experimental results are also reported as an illustration.

I describe four unusual laser systems for absolute measures of linear distance. All employ the familiar techniques of multiple wavelengths, chirped wavelength, optical feedback and intensity modulation, but in somewhat unfamiliar architectures. These examples serve to illustrate the breadth of solutions to this important problem.

Frequency modulated laser radar is an interferometric device that offers for metrology the possibility to measure absolute distances of the order of several meters with an accuracy down to 10 ?m. This value corresponds to the roughness of many technical surfaces. Dynamical effects such as movement of the object of interest or scanning of the laser beam can cause large systematic measurement errors due to the Doppler effect or time-dependent phase variations. Especially in industrial environments, a sufficient isolation against vibrations cannot always be guaranteed. To overcome this problem we develop a setup with two laser diodes that are simultaneously tuned upward and downward in frequency. After compensation of the nonlinear tuning characteristics of the tunable laser diodes, the heterodyne signals from both interferometers are multiplied with each other. By appropriate filtering, the undisturbed intermediate frequency, which is proportional to the distance, can be evaluated. Experimental results prove the new concept.

A compact and noncontact sensor using the self-mixing interference inside a semiconductor laser is designed to classify moving surfaces. An artificial neural network is employed for the data processing. The results indicate more than 92% correct classification for eight different surfaces of different materials, different manufacturing methods and different surface roughnesses. The accuracy of the system is restricted by the localized irregularities on the surface and the mechanical instabilities of the carrying stage over which the surfaces are placed.

We introduce a new 3-D sensor based on a diffractive optical element (DOE). Structured light is obtained by the interference of two beams. The design of the projection unit has the following features: the system is very simple, small and cheap and the fringe spacing is easily adjustable. Fringes generated by the DOE interferometer are analyzed by Fast Fourier Transform (FFT) method, which produced a 3-D perspective plot. The technique and the experimental results of real surface profiles are given.

We describe a new laser rangefinder for measurement of the shapes of specular objects. In many industrial contexts, objects with both Lambertian surfaces and specular surfaces are often manipulated. However, ordinary laser rangefinders usually cannot be used to measure specular objects. We propose a laser rangefinder for the measurement of specular objects based on a new idea in which the incident angle of a light-stripe upon the image sensor is limited through the use of a special light device. The concept of this rangefinder involves the application of triangulation to specular objects, which enables measurement of the shape of specular objects as well as Lambertian objects. We designed and built a prototype light-stripe projection rangefinder based on these principles. Experiments using the rangefinder successfully demonstrate the rangefinder's accuracy. The proposed rangefinder is a promising tool for the performance of many common industrial tasks including measurement of the shapes of polished metal and the inspection of soldered joint surfaces.

Machine vision systems based on direct laser rangefinders are an interesting option for applications demanding high lateral and radial resolution, along with the capability to operate under varying illumination conditions. Furthermore, as outdoor applications such as collision avoidance or smart highways automated navigation appear, the interest of direct laser rangefinders for machine vision will rise, since they do not present the limitations commonly found in intensity based vision systems. We present a 3-D vision system designed around a direct laser rangefinder (which we also developed). Although initially conceived for short-range indoor applications, the system is designed so that it can be easily adapted to longer range outdoor operation. The main features of this system are its high-speed optomechanical scanner, based in a single scanning mirror, and its direct interface to the industry standard architecture (ISA) bus, which enables the system to operate in any PC- based application.

In the development of new products there are two parallel lines for the development process to follow: the traditional or "real" and the new computer aided or "virtual." The traditional line is to develop prototypes that can be used for testing strength, functionality, and visual appearance of the product. In the virtual line digital (CAD) models are developed that can be tested entirely in a computer by simulations, e.g., using finite element analysis (FEA) and other tools. For transformation from the real to the virtual world some kind of 3-D acquisition system is needed. The shape of the model should be measured together with further data concerning the visual appearance, material properties, etc. We develop electronic recording techniques for doing this based on the Light-in-Flight technique. This technology has all the advantages of holography, i.e., apart from the shape it is also possible to measure how much light is reflected from different parts of the object along with interferometric information, which can give mechanical data for the object. This can be used, for example, for visualization and to give tactile and haptic information to a virtual reality system about how the object would be perceived by a person touching it.

Abstract. Subnanometer distance stabilization and positioning of large optical structures is a key active control issue for future space telescopes needing picoradian precision. We present solutions and results of a leading experiment, called COSI (control optics structure interaction), aimed at investigating Fabry-Perot cavities as active optical links, capable of stiffening large and massive structures operating in a vacuum. The optical length of each cavity is actuated by a piezoelectric translator (PZT)- driven mechanism and the cavity ensemble is controlled by a model- based digital control unit. The first experiment stabilized the relative tip- tilt motion of two 6.9 kg annular plates, 0.5 m distant. A residual error, lower than 3?-1/2pm(1?), ?> 0.1 s, is achieved in the presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating the feasibility of the COSI concept and technology. © 2001 Society of Photo-Optical Instrumentation Engineers.

A phase shifting polarizing speckle interferometer is presented and applied to the analysis of the displacement field of composite sandwich materials submitted to indentation tests. The phase shift is obtained by a pockels cell and we present a calibration method based on a least squares fitting. The systematic error induced on the phase measurement by both the random fluctuations of the calibration parameter and the polarizing beamsplitter is investigated. It is shown that this systematic error depends sinusoidally on the random reference phase of the speckle field and on the phase to be measured and is closely related to both the statistics of the calibration parameter and those of the extinction ratio of the polarizing beamsplitter. The in-plane displacement field of a PVC foam core is measured over a total range of 1 mm. The experimental results obtained with the speckle technique are then compared to the model developed from a finite element analysis.

We present a novel noncontact measurement procedure to determine displacements of and engineering strains within specimens at very high measurement rates. Appropriate hybrid optical and digital signal processing can be employed to realize two dimensional (2D) correlation processing. Due to appropriate optical signal processing, the data acquisition can be performed in one dimension through CCD line-scan cameras. As a result, only 1D digital signal processing algorithms have to be executed and hence the processing power requirements decrease greatly. in this way, the measuring rates are substantially increased. it is shown that optimal Lloyd-Max quantizers implemented in the analog-to- digital converter (ADC) used in combination with a particular digital correlation algorithm are able to yield a further increase of the measurement rates compared with standard digital correlation techniques.

We propose a new technique to measure the angle of a remote flat surface with respect to the propagation direction of a laser beam, based on injection detection in a laser diode. The surface under test acts as the remote mirror of an external cavity laser, and causes the laser diode to operate in the coherent collapse regime. The power emitted by the laser depends on the alignment of the remote surface, and an ac technique enables us to measure angle with a sensitivity of 0.1 arcsec (i.e., 5 x 10-7 rad). The attained performances are comparable to those of existing autocollimators, with the advantage of simplicity and compactness that makes the new technique interesting for the development of a measuring instrument.

The hue saturation intensity (HSI) color model resembles the color sensing properties of human vision. Thus, it is more suitable to color image analysis than other color models. However, the hue (H) component is a periodic angular signal, to which the conventional signal processing algorithms cannot be applied without producing artifacts. We have developed an algorithm of generalized filtering operations on the angular signals, such as hue images. The developed method divides the input angular data into four groups according to the sign of filter coefficients and the range of angular signals. Each group is processed separately, and the results from the four groups are combined together. In order to verify our algorithm, we have applied it to the spatial smoothing and gradient processing of hue images.

Three variable-rate image-coding schemes are proposed in this paper. The goal of these schemes is to exploit variable-size segmentation of images using subsampling vector quantization. In the schemes, three simple block type classifiers are designed to decide the block activities of the image blocks. If two adjacent blocks are sufficiently similar in block activities, then they are encoded by subsampling vector quantization. Otherwise, the traditional vector quantization scheme is employed to encode each image block independently. According to the experimental results, the new schemes cannot only achieve significant reduction of bit rates, but also provide good subjective image quality. Therefore, the proposed schemes are suitable for the compression of digital images.

The purpose of the project WIND (wind infrared Doppler lidar) is the development of an airborne conical scanning CO2 Doppler lidar in French-German cooperation by CNRS/CNES and DLR. The instrument and its peculiarities are described. Measurements of the Doppler shift from a moving platform with an accuracy of 1 m/s require instantaneous access to the position data of the platform. Therefore this part of the instrument is described in detail. Ground-based tests, airborne tests and a validation flight were performed. The instrument can be used for mesoscalic meteorology to test models and contribute to a spaceborne Doppler lidar in the future.

Thermal recovery of the main amplifier of the SG-III prototype is numerically simulated. The calculations indicate that, with a shot period of 4 h, we can achieve an acceptable laser slab average temperature and thermal gradient across the slab aperture through optimized active cooling for both the flashlamp cassettes and the slab cavities. After 4 h of thermal recovery, the average temperature and the largest temperature drop across the aperture of the laser slab will be less than 0.13°C above ambient and 0.11°C respectively. The active cooling for all the flashlamp cassettes will need a total cleaned-air flow rate of 80 m3/min (10 ft3/min per lamp) and a total nitrogen flow rate of 16 m3/min for all the slab cavities. Additionally, the temperature of the cooling gas in the flashlamp cassette must be reduced to ?1.0°C (relative to ambient temperature) during the first 2.5 h of the thermal recovery cycle.

The method of the optimization for a binary phase hologram used in a security system is proposed. The hologram is made by a joint Fourier transform method and a simulated annealing technique is used for the optimization. A hologram of a fingerprint image with a random key reference is successfully optimized and almost the same image quality as the original one is obtained as a decoded image by the proposed method. The usefulness of the method in an optical security system is also discussed.

Passive optical limiters are significant for protecting sensors and human eyes from high intensity laser. The selection of materials is still the key point for optical power limiters. The optical limiting performances have been measured for six new pentaazadentate metal complexes with initial linear transmission of 0.90 in a 2 mm cell in chloroform at 532 nm with 8 ns pulses, in which the reverse saturable absorption from the triplet?triplet excited state is responsible for the optical limiting. The optical limiting thresholds at T/T0 = 0.5 are 139 mJ/cm2, 350 mJ/cm2 and 420 mJ/cm2 for [(PMC-NO2)Cd]Cl, [(PMC-Np)Cd]Cl and [(PMC)Sm]Cl, respectively. The lowest nonlinear transmittance for [(PMC-NO2)Cd]Cl is 10%, while the incident maximum fluence reaches to 600 mJ/cm2. The ratios of T0/Tlim are estimated at 2.3 to 9.0. The limiting thresholds are strongly influenced by the nature of the substituents on the benzene ring and the heavy atom effect. Electron withdrawing groups of the complexes can enhance the limiting capability, and the heavy atom effect facilitates the decreasing of the limiting threshold.

Several single-mode fiber optic applications benefit from the use of depolarized light. Examples are suppression of polarization hole burning (PHB) in erbium-doped fiber amplifiers (EDFA),1–3 reduction of polarization-dependent loss arising from fiber components (e.g., couplers and wave-division multiplexers),4 reduction of the signal fading in fiber optic sensors,5–7 and coherent detection systems.8,9 Depolarizers are also used for fiber optic component testing.

Substitution of electrical links by their optoelectronics (OE) counterparts has been regularly debated for more than a decade. The potential advantages of the optical over the electrical links, at all levels of electronic systems, have been examined by many researchers using generic approaches (see for instance Refs. 1–4 and references therein).