Digital Shearography, a laser interferometric technique in conjunction with the digital image processing, has the potential for vibration analysis due to its simple optical system and insensitivity against small rigid body motions. This paper will focus on its recent developments for vibration analysis and for nondestructive testing (NDT) by dynamic (harmonical) excitation. With the introduction of real time observation using automatically refreshing reference frame, both small and large rigid body motions are greatly suppressed. The development of a smaller and more mobile measuring device in conjunction with a user guided comfortable program Shearwin enables the digital shearography to be applied easily as an industrial online testing tool.

The density distribution in the air surrounding an airgun muzzle is visualized using pulsed TV holography. A ruby laser emitting short coherent light pulses (30 ns) is used as light source. Pulsed holograms are captured by a CCD-camera and their optical phase difference is evaluated by means of the Fourier transform method. A number of experiments are performed where the event is recorded at different instants of time as the lead bullet is about to leave the muzzle. Phase maps showing the integrated density distribution are presented. A jet with decreasing density ahead of the bullet can be observed. At the moment the bullet leaves the muzzle, a spherical sound pressure wave starts to propagate out into the air. The density in the jet in front of the traveling bullet can be calculated by means of full shock-wave theory. The theory confirms the decrease in density observed in the measurements.

Wavelength measurements with Michelson interferometers have major disadvantages of high costs and low measurement rates. The paper shows two alternative method for wavelength measurement: one based on interference filters, the other based on double photodiodes. Interference filters are made of several optically transparent layers with different refraction indices. Within a certain wavelength range transmittance and reflectivity of the interference filter change constantly with the laser wavelength. Therefore, after calibration measurements, the wavelength can be obtained within milliseconds with a resolution of approximately plus or minus 0.01 nm by measuring the intensity of both the transmitted and the reflected beam. Double photodiodes are a combination of two single photodiodes with different spectral sensitivities in one case. Within a specified wavelength range both sensitivities are linear but have different gradients. Hence, by measuring the respective currents of each photodiode the wavelength can be obtained with a resolution of approximately plus or minus 0.1 nm also within milliseconds. Furthermore, the experimental set-up that is used for each method is illustrated, measurement results of both methods are presented, discussed and compared.

In order to optimize the vibro-acoustic behavior of panel-like structures in a more systematic way, accurate structural models are needed. However, at the frequencies of relevance to the vibro-acoustic problem, the mode shapes are very complex, requiring a high spatial resolution in the measurement procedure. The large number of required transducers and their mass loading effects limit the applicability of accelerometer testing. In recent years, optical measurement methods have been proposed. Direct electronic (ESPI) imaging, using strobed laser illumination, or more recently, pulsed laser illumination, have lately created the possibility to bring the holographic testing approach to the level of industrial applicability for modal analysis procedures. Therefore an automated ESPI system has been developed for the measurement of frequency response functions using stepped sine testing. A conventional numerical modal analysis procedure is used to obtain the modal information. The present paper discusses the various critical elements of a holographic ESPI modal testing system. Next to the optical parts, the integration with the modal analysis procedures, including the integration of geometry and response measurement, are discussed. The paper furthermore discusses test results obtained on a car panel in a vibro-acoustic setup. The results show, that the quality of the frequency response functions is very good, when compared with acceleration sensor measurements. The measurement data are used to predict interior noise.

Digital recording of holograms followed by numerical reconstruction of the wave field offers new alternatives in particle diagnostics. A plane wave passes a particle stream and the hologram is recorded directly by a CCD-sensor without any focusing optics. The reconstruction of the particle distribution and velocity from the recorded hologram is done numerically. Multiple holograms of the particle stream are recorded at a single instant of time from different directions. By evaluating these holograms independently it is possible to determine the 3D distribution and velocity of particles with a very high accuracy.

Electronic Speckle-Pattern Interferometry (ESPI) is commonly used to measure down-to nanometers long deformations of optically rough surfaces. Topography measurement is also possible with ESPI by changing the illumination conditions, either by sliding the light sources (translation technique) or by changing the wavelength of the light source (multiple wavelength technique). Adapting both techniques to the approach of coded light sectioning, makes it possible to describe discontinuous surfaces with high steps. With adequate ESPI experimental set-ups, both shape and deformation measurements can be done and it becomes a strain and stress measuring technique able to compete to the wire resistance strain-gauges, one which measurement set-up is very time- consuming and returns just single point strain values, without lateral resolution. ESPI-methods extend this kind of metrology to a matrix of some thousands points even including sensitivity in the out-of-plane-direction. In this paper current work on the field of stress and strain measurements with ESPI stress sensor prototype is described. The experimental result of several technical applications is shown and compared to measurements with strain-gauges. Additionally experimental results using the multiple wavelength approach with tunable laser sources to perform the contouring measurements will be discussed.

An optoelectronic system based on digital holography is used to measure the three dimensional vector components and object shape of a vibrating object. Pulses from a ruby laser, with a separation in the range from 1 to 1000 microseconds, are used to record holograms on CCD sensors, which are later digitally reconstructed. Three different illumination directions are used to get the deformation along three different sensitivity vectors, that are afterwards combined into a 3D resultant deformation. To measure the shape of the object the two- wavelength method is used. The wavelength change is produced by changing the distance between the plates of the laser output etalon, thus obtaining the shape by subtracting the phases of the wavefronts recorded at those wavelengths. The data sets for the shape and 3D-deformation are combined and graphically shown. Finally, by using a non linear crystal (BBO) it was possible to double the frequency of the radiation emitted by the ruby laser allowing to get two wavelengths (694 nm and 347 nm) simultaneously and thus to record digital holograms with different sensitivities.

The cultural heritage of many nations consist of a great variety of structures of high intrinsic value, which are often composed of natural building stones (NBS), as granite, limestone, marble and sandstone. The use of accurate inspection devices, such as laser interferometry, allows us to acquire information regarding the mechanical properties and damage (tensile cracks) of NBS, which represents the first step in the restoration process. In this paper, the potential application of an electronic speckle pattern interferometry (ESPI) is shown, with particular attention to the observed displacement field and the crack penetration during laboratory testing. In ESPI, by superimposing a reflected light to a reference digitized image, an interference phenomenon is produced. By comparing two recorded interference patterns (before and after loading), the corresponding deformation can be evaluated. The application of ESPI in several laboratory tests on NBS is presented in this paper. In particular, during bending tests performed on geometrically similar NBS specimens, it was observed that the size and shape of the localized damage zone do not depend on the specimen size. These results allow for an interpretation of the 'size- effect,' which consists of a reduction of nominal strength as the specimen size increases.

We present two fast methods for the evaluation of object- adapted fringe images. One method, a fast and simple skeleton method, is purely digital. The method is stable against varying object reflectivities and illumination conditions. We also demonstrate the optical evaluation of object-adapted fringe images by means of a moire technique. This second method uses a Ronchi grating for the optical processing of the images in order to detect topographic defects with high speed. Additionally, we present a new method for the generation of object-adapted fringe masks where only a few images have to be recorded in order to obtain the mask.

This paper describes a new phase unwrapping algorithm based on morphological image processing. A region of interest corresponding to the areas where interferences occur is first extracted. This is achieved by processing the input interferograms with a combination of morphological closing and opening operators. The boundaries of the wrapped fringes are then enhanced by two gradient operators: morphological gradient by erosion for the fringe boundaries with high values and morphological gradient by dilation for the fringe boundaries with low values. Gaps along the boundaries which would lead to errors in the unwrapped phase map are filled in by the watershed transformation. Finally, the wrapped phase map is unwrapped by means of a region growing procedure based on priority queue data structures.

Some material testing procedures do require the determination of surface element displacements and displacement gradients through non-contacting preferably optical techniques able to operate at relatively high measuring rates. They are essential in particular load-dynamic testing of mechanical properties of so called new materials such as ceramics matrix compounds (CMC) which show creep-effects at their usual operating temperature beyond 1000 degrees Celsius but also testing of thin films down to thicknesses in the range of microns where contacting methods are clearly prohibitive. In this paper we present a displacement sensor system and the signal processing necessary to determine engineering strain within specimen which employs both optical and digital signal processing to form -- in a hybrid way -- a two-dimensional cross-correlation function used to estimate feature displacements on speckled images drawn from surface inspection. It is further 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 measuring rates compared with standard digital correlation techniques.

Laser-induced breakdown spectrometry (LIBS) is a non-contact in-situ method for the chemical analysis of various materials. Our R&D activities concentrate on the improvement of the analytical features of LIBS for the quantitative multi- elemental analysis of low-alloyed steel grades. The analytical sensitivity for the crucial elements C, S, and P, but also for Mn, Si, Cr and Ni was enhanced significantly. The limit of detection achieved for these elements is below 10 (mu) g/g. For low alloyed steel, the analytical performance of LIBS has now achieved for most elements the level of conventional laboratory based methods such as spark-discharge optical emission spectrometry (SD-OES).

In the paper we report some factors which determine the quality of stroboscopic holography. A one-side fixed ruler is considered as a model for theoretical investigation. It is shown that by compensation of one frequency of mode-shape the viewed interferometric picture depends not only on mode-shape of a second frequency (harmonic, for example), but also from the duration of strobe-pulses and accuracy of phases of stroboscopic illumination. A parasitic system of interferometric fringes and mode-shape distortion may be observed. Computer calculations have shown that the displacement of position of interferometric fringes leads to errors in the analysis of holographic information. Experimental investigation by using the special stroboholographic system confirmed the theoretical investigation. A parasitic system of interferometric fringes is distinctly observed on the hologram when phases of strobe- pulses are set up incorrectly. A new stroboholographic arrangement where parameters of stability of strobe-pulse duration and accurate phase improved the results.

Sinusoidal modulation method is utilized in optical distance meters. In this method, the phase delay of the returned beam is measured with high resolution. One of the dominant error factors in the phase measurement is cyclic error. We have already reported on a cyclic error compensation of an optical waveguide distance meter using a preliminary phase measurement with mechanical displacement. In this paper, a new technique of the cyclic error compensation is described, which is based on the modulation frequency scanning of the waveguide distance meter around 14 GHz. We have compared two types of cyclic error curves; one was obtained from the data during the displacement of the target and the other was obtained from the data during the scanning of the modulation frequency. The curve fitting technique was used to determine the amplitude and the phase of the cyclic error component for the two curves. We have obtained almost the same values for both parameters of the amplitude and the phase for the two cyclic error curves. Therefore, the cyclic error curve obtained from the data of frequency scanning can be utilized for the cyclic error compensation instead of the displacement of the target.

Many sensors of magnetic field and the electric current sensors are based on the magneto-optical (MO) effects. MO Kerr effects in reflection are widely used for thin film magnetization study. A magneto-optical hysteresis loop measurement and an observation of the magnetic domain (MO microscopy) have an important advantage for the study of the magnetism of thin and ultrathin films. The observation and careful separation of three magnetization components (sometimes called vectorial magnetometry) are the tasks for a precise MO measurement. The paper is devoted to the influences of various magnetization components on the ellipsometric observables. The component separation is discussed including both linear and quadratic terms in magnetization. Presented theory is based on a solution of the wave equation in MO medium described by the permittivity tensor. The eigenmode (characteristic) equation is solved for a general magnetization direction in the linear and quadratic approximations using both numeric and symbolic methods. The boundary conditions for the electric and magnetic fields at interfaces are written in compact form based on 4 X 4 Yeh's matrix formalism. Three basic magnetization configurations are usually distinguished -- polar, longitudinal and transversal. The component perpendicular to the plane of incidence (transversal) affects only the r_pp reflection coefficient in the linear approximation. The components lying in the plane of incidence (polar component is normal and longitudinal one is parallel to the interface) affect the conversion reflection coefficients r_sp, r_ps and the ellipsometric angles (Kerr rotation and ellipticity). The quadratic effects are observed when both the transversal magnetization component (perpendicular to the plane of incidence) and the longitudinal one (parallel to the plane of incidence) are present simultaneously. These quadratic effects are observed as a product of the magnetization components in the conversion reflection coefficients.

The efficiency of high-average-power frequency doubling is always limited by thermal effects inside the nonlinear crystal. Usually it is assumed that the limitation is caused by a change of the index of refraction due to the temperature variation inside the crystal. This effect is characterized by the temperature acceptance of the nonlinear crystal. Additionally, the temperature distribution inside the crystal produces thermally induced mechanical stress that leads to phase mismatch due to the photoelastic effect. Depending on the sign, this effect will decrease or increase the conversion efficiency. In order to investigate the influence of stress- induced phase mismatch independently of phase mismatch caused by a temperature variation on the conversion efficiency, KTP (Type II) and LBO (Type I) crystals for second-harmonic generation (SHG) of 1064 nm radiation were temperature- stabilized and mechanical stress was applied along different crystal axes. The conversion efficiency of a weak probe beam was measured as a function of the stress.

A high speed shape measurement system has been developed based on the method of projected fringes. An optimized sequence of fringe pitches is used in which the spatial frequency is reduced exponentially from the maximum value. Temporal unwrapping of the phase values at each fringe pitch provides an independent co-ordinate from each camera pixel. A commercial data projector produces the fringe patterns at a rate of 30 s^-1; the images are analyzed in real time by means of a pipeline image processor. A total acquisition and analysis time of 0.87 s is required for 250,000 coordinates. The main results of an experimental study to optimize the measurement precision of the system (from the original value of one part in 2,000) are also described. Issues that have been investigated include (1) the relative performance of different phase-shifting algorithms (4-frame, 7-frame and 15-frame); (2) the relative performance of different temporal phase unwrapping algorithms; and (3) the effect of projector defocus. At the optimum focal position, a measurement precision of better than one part in 20,000 of the field of view was achieved.

Increasing demands for accuracy in manufacturing and international standards of quality control require faster and more precise measurement techniques. Surface inspection and shape control of technical workpieces is commonly done by tactile profilometers. Interferometric testing of optically rough surfaces is faster, but the interference pattern is usually disturbed by high contrast speckle noise. Grazing incidence interferometry is an appropriate method to increase the effective illuminating wavelength. This leads to dramatically reduced speckle noise. Increasing of the wavelength from the visible to the infrared region is another opportunity to diminish speckle. An interferometric set-up combining both methods is presented. Well collimated laser light is splitted into several diffraction orders by a computer generated hologram (CGH). The zero diffraction order passes through to a second CGH and is used as the reference wavefront. The first diffraction order hits the object and is reflected to the second CGH where it is recombined with the reference wave. In the ideal case only uniform intensity is observed. Deviations from the ideal shape and misalignments of the object in the set-up lead to interference fringes after the second CGH. The fringe pattern is evaluated by using phase shifting interferometry. Further data processing eliminates the misalignment errors and reconstructs the shape of the object. The sensitivity of the interferometer depends on the design of the CGHs and can be adapted in a wide range to technical needs. The use of infrared light expands the measurement range. Rough surfaces can be tested with a convenient resolution in the direction of the optical axis. The capability of the IR-interferometer will be shown with some measurements of cylindrical workpieces.

For the testing of flats, spheres or aspheres, the optical scanning technique has become a competitor to interferometric techniques. The set-up implemented at PTB for the ultra- precise optical scanning of optical flats will be presented and the underlying principles explained, with a special view to the avoidance of error influences in the optical set-up and to the elimination of the remaining errors. The basic principle of the scanning set-up is that an autocollimation telescope combined with a movable pentagon prism is used to register the surface slope angles. To avoid higher-order errors originating in the pentagon prism, its angular position is kept constant in space by active stabilization. Another advantageous principle is that difference measurements for slopes are performed. This has become possible thanks to a new error-free difference method recently developed by PTB. The combination of these principles eliminates, theoretically, the influences of all possible errors of the scanning facility. An experiment using a high-quality flat ((lambda) /60) 140 mm in diameter showed a reproducibility of 0,2 nm over one week.

In this paper an all-optical combination of coherence conversion and spatial frequency self-filtering using two optically addressed liquid crystal (OALC) is presented. A periodic object to be inspected is illuminated by a white light source, imaged onto a OALC and modulates its polarization properties. The OALC then modulates a coherent read-out beam, which can be Fourier transformed to enable frequency self-filtering. Self-filtering uses an additional OALC and is able to detect defects in periodic images. It is independent of the position or rotational orientation of the object and therefore a promising approach for a real-time assembly line inspection. We present experimental results for this cameraless processor and compare it to an alternative approach using a CCD camera and a Matrix-LCD for coherence conversion. In addition adaptive brightness regulation by an OALC in an intermediate image plane is a promising tool for image pre-processing. This adaption process reduces shiny local spots and brightens dark areas. Theory on brightness adaption using an OALC is given and some experimental results are discussed.

A technique named synthesized optical coherence tomography (SOCT) has been developed as an alternative method of the well-known optical coherence tomography (OCT) for cross- sectional imaging of scattering objects. The SOCT is based on the principle of the synthesis of optical coherence function. Instead of the low coherence light source in the OCT, the SOCT uses a super structure grating (SSG) distributed Bragg reflector tunable laser diode as the light source. By stepwise optical frequency modulation, a comb-shaped power spectrum is obtained; thus the optical coherence function is synthesized into a delta-function-like peak at an arbitrary location. When the injection currents to the SSG sections and the phase control section are modulated synchronically and in a proper relation, an equally spaced frequency tuning of a range of near 40 nm is obtained, corresponding to a spatial resolution of several tens of microns theoretically. The location of the coherence peak can be adjusted by the spacing in the frequency modulation and scanned by the simultaneous phase modulation in the reference wave. The longitudinal scattering distribution of the object under test is thus obtained without mechanically driven reference. Two-dimensional tomography was demonstrated in a basic experiment with lateral scanning mechanism.

We developed a novel sensing equipment for the measurement of roughness and waviness of rolls during grinding. The sensors can operate on different materials, in harsh conditions, and measurements can be performed on-line, in a non-contact fashion. The sensing equipment makes use of a commercial triangulation sensor, suitably integrated with an additional optical head. The triangulator produces two signals, respectively proportional to the distance from the target surface and to the intensity of the scattered light. The optical head consists of two photodiodes placed in the proximity of the excitation beam and produces a signal proportional to the intensity of the light scattered at smaller angle. The distance signal is used to extract the waviness information, whereas its combination with the intensity of the light scattered both at large and small angles is necessary to derive the roughness information. The resulting sensor is able to measure waviness and roughness in the spatial wavelength ranges of 1 mm divided by 100 mm and 0.1 mm divided by 1 mm respectively; with height range of the defects equal to 200 nm divided by 5 micrometer. The measurement uncertainty is less than 2.5% and the linearity is 1% of the measuring range.

In this paper a novel low-coherence sensor based on a self- mixing super-luminescent diode (SM-SLD) is described. A commercial low-cost super-luminescent diodes (SLD) driven at constant current is used as low-coherence source. In the same case of the SLD there is a photodiode generally used to monitor the optical power at the emitting junction back-face. In the SM-SLD technique, this photodiode is exploited to detect the interference signal. Hence, the interference signal is optically amplified by the high-gain active medium. The sensor has been properly designed to work in industrial environments and is suitable for measurement of semitransparent slabs thickness, i.e. glass, Plexiglas, polyethylene, etc. Measurements carried out on glass slabs show a measuring range of 15 mm and a linearity error and stability of 1.3 micrometer and 1.6 micrometer respectively.

A telecentric flying spot scanner consisting of a rotating multifaceted mirror and a 500 mm diameter parabolic mirror for the inspection of large flat and embossed surfaces up to 400 mm wide is described. Real time high resolution 2-D images at three different angles are obtained providing various types of intensity profiles of the surface. These channels are linearly combined to form pseudo-color images. The image size per channel is limited to about 4 Mbytes by the maximum direct memory access buffer area used by the image grabber card. Images with variable resolutions (for example, 2048 X 2048 pixel or 10000 X 400 pixel) are therefore feasible. Photomultiplier tubes are used for light detection to offer good signal-to-noise ratio and to cover a wide dynamic signal detection range varying from scattered to specular light. Composite images for ceramic tiles with different textures, colors and surface features are presented. These images show some interesting features of the surface profile and texture which otherwise could not be observed using a CCD camera or a single channel scanner. Subtle flaws such as fine scratches and finger marks on the surface and the surface texture variations can be easily identified.

A complete range camera system, working with the time-of- flight principle, is introduced. This ranging system uses modulated LEDs as active illumination source and a new lock-in CCD sensor as demodulator and detector. It requires no mechanically scanning parts because every pixel of the sensor contains a lock-in amplifier, enabling both intensity and range measurement for all pixels in parallel. Two such lock-in imagers are realized in 2.0 micrometer CMOS/CCD technology, (1) a line sensor with 108 pixels and an optical fill factor of 100% and (2) a 64 X 25 pixel image sensor with 20% fill factor. The basics of time-of-flight ranging are introduced with a detailed description of the elements necessary. Shot noise limitation to ranging resolution is deduced and confirmed by simulation. An optical power budget is offered giving the relation between the number of photons in a pixel depending on the light source, the observed object, and several camera parameters. With the described lab setup, non- cooperative targets can be measured over a distance of several meters with a resolution of some centimeters.

A new type of highly transparent (95%) two dimensional position sensor has been developed which allows the accurate positioning (below 10 micrometers r.m.s.) of successive elements to which each sensor is attached, transversely to a laser beam used as a reference straight line. The present useful area of the sensor is about 15 X 15 mm², and can be further increased.

An optical amplitude modulated laser radar has been developed for periodic in-vessel inspection in large fusion machines and its overall optical aiming is developed taking into account the extremely high radiation levels and operating temperatures foreseen in the large European fusion machines (JET and ITER). In this paper an in vessel viewing system based on a transceiving optical radar using an RF modulated single mode 840 nm wavelength laser beam is illustrated. The sounding beam is transmitted through a coherent optical fiber and a focusing collimator to the inner part of the vessel by a stainless steel probe on the tip of which a suitable scanning silica prism steers the laser beam along a linear raster spanning a -90 degree to +90 degree in elevation and 360 degrees in azimuth for a complete mapping of the vessel itself. All the electronics, including laser source, avalanche photodiode and all the active components are located outside the bioshield, while passive components (receiving optics, transmitting collimator, fiber optics), located in the torus hall, are in fused silica so that the overall vision system is radiation resistant. The Active and passive components are contained in separated stainless steel boxes connected through two silica fiber optics. The laser radiation backscattered by the resolved surface element of the vessel is received by a collecting silica optics and remotely transmitted through a multimode fiber on the surface of an avalanche photodiode detector located in the active module at 120 m distance. The received signal is then acquired, the raster lines being synchronized with the aid of optical encoders linked to the scanning prism, to give a TV like image. The scanning accuracy expected in scanning process is less than 1 mm at 10 m of distance: this is a suitable resolution to yield a high quality image showing all the damages due to plasma disruptions. Preliminary results have been obtained scanning large sceneries including several real targets having different light backscattering properties, colors and surfaces reflectivity ranging over several decades to simulate the expected dynamic range of the video signals incoming from the vessel.

We have developed an optical system which permits the absolute positioning of an element with respect to a reference laser beam. The resolution is of the order of 10 micrometer in translation and 50 (mu) rad in rotation. It is highly transparent (greater than 90%) permitting several elements to be aligned. A calibration procedure has also been studied in order to be independent of internal deformations.

The diffractive optical element (DOE) is a revolutionary technology for sophisticated optical systems. It has recently been launched within the optical industry, which is constantly seeking improvements over conventional optics. We have designed and fabricated three types of binary-phase DOE for array generation. The surface relief of a ZnSe substrate was patterned and etched with each intended phase distribution by using photolithography and reactive ion etching (RIE) technologies. The optical properties of anti-reflection coated samples were then examined by measuring the intensity distribution of their converging diffractive beams and the results compared with the calculated beam propagation.

In plants of the chemical, nuclear and off-shore industry, application specific high-alloyed steels are used for pipe fittings. Mixing of different steel grades can lead to corrosion with severe consequential damages. Growing quality requirements and environmental responsibilities demand a 100% material control in the production of the pipe fittings. Therefore, LIFT, an automatic inspection machine, was developed to insure against any mix of material grades. LIFT is able to identify more than 30 different steel grades. The inspection method is based on Laser-Induced Breakdown Spectrometry (LIBS). An expert system, which can be easily trained and recalibrated, was developed for the data evaluation. The result of the material inspection is transferred to an external handling system via a PLC interface. The duration of the inspection process is 2 seconds. The graphical user interface was developed with respect to the requirements of an unskilled operator. The software is based on a realtime operating system and provides a safe and reliable operation. An interface for the remote maintenance by modem enables a fast operational support. Logged data are retrieved and evaluated. This is the basis for an adaptive improvement of the configuration of LIFT with respect to changing requirements in the production line. Within the first six months of routine operation, about 50000 pipe fittings were inspected.

In this paper, time-domain analysis of a tandem configuration of a nondispersive Michelson interferometer and a two-mode optical fiber, and spectral-domain analysis of intermodal interference at the output of the optical fiber alone are presented when a low-coherence exciting source is used and effects of first-order and second-order intermodal dispersion are included. Time-domain analysis shows that the visibility dependence of the output interferogram consists of two side peaks corresponding to positive and negative imbalance of the interferometer that compensate for the group optical path difference (OPD) of two modes in the optical fiber. It is shown that the effect of second-order dispersion gives rise to a flattening and a symmetrical broadening of these two side peaks. Spectral-domain analysis shows that the wavelength- dependent modulation of the source spectrum is obtained at the output of the two-mode optical fiber alone. By processing the spectral modulation, the intermodal dispersion parameters can be obtained. Theoretical results are verified experimentally: the visibility dependences measured in the setup comprising a bulk-optic Michelson interferometer and a two-mode optical fiber give the intermodal group OPDs, and the wavelength- dependent modulations of the source spectrum measured at the outputs of the two-mode optical fibers alone give the wavelength dependences of the group OPDs between two modes of the optical fibers. Moreover, good agreements between the experimental results of both measuring techniques are confirmed.