A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Using a metasurface comprising an array of nanorods with different orientations and a backreflector, a hologram image can be obtained in the visible and near-infrared with limited loss of light intensity.

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Metasurface holograms reaching 80% efficiency

Sensitive optical biosensors for unlabeled targets: a review.

There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

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Optical coherence tomography - principles and applications

This tutorial/survey paper: (1) provides a concise point of departure for researchers and practitioners alike wishing to assess the current state of the art in the control and monitoring of civil engineering structures; and (2) provides a link between structural control and other fields of control theory, pointing out both differences and similarities, and points out where future research and application efforts are likely to prove fruitful. The paper consists of the following sections: section 1 is an introduction; section 2 deals with passive energy dissipation; section 3 deals with active control; section 4 deals with hybrid and semiactive control systems; section 5 discusses sensors for structural control; section 6 deals with smart material systems; section 7 deals with health monitoring and damage detection; and section 8 deals with research needs. An extensive list of references is provided in the references section.

Structural control: past, present, and future

The numerical calculation of the Rayleigh-Sommerfeld diffraction integral is investigated. The implementation of a fast-Fourier-transform (FFT) based direct integration (FFT-DI) method is presented, and Simpson's rule is used to improve the calculation accuracy. The sampling interval, the size of the computation window, and their influence on numerical accuracy and on computational complexity are discussed for the FFT-DI and the FFT-based angular spectrum (FFT-AS) methods. The performance of the FFT-DI method is verified by numerical simulation and compared with that of the FFT-AS method.

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Fast-Fourier-transform based numerical integration method for the Rayleigh-Sommerfeld diffraction formula

An all-fused-silica pressure sensor fabricated directly onto a fiber tip of 125 μm diameter is described. Simple fabrication steps include only cleaving and fusion splicing. Because no chemical processes are involved, the fabrication is easy, safe, and cost effective. Issues in sensor design and loss analysis are discussed. The sensor has been tested for static pressure response, showing a sensitivity of 2.2 nm/psi, a resolution of 0.01 psi (68.9 Pa), a hysteresis of 0.025%, and capability of operation at temperatures up to 600°C. This miniature sensor may be suitable for medical diagnostics, environmental monitoring, and other industrial applications.

All-fused-silica miniature optical fiber tip pressure sensor

A diaphragm-based interferometric fiber optic sensor that uses a low-coherence light source was designed and tested for on-line detection of the acoustic waves generated by partial discharges inside high-voltage power transformers The sensor uses a fused-silica diaphragm and a single-mode optical fiber encapsulated in a fused-silica glass tube to form an extrinsic Fabry-Perot interferometer, which is interrogated by low-coherence light Test results indicate that these fiber optic acoustic sensors are capable of faithfully detecting acoustic signals propagating inside transformer oil with high sensitivity and wide bandwidth

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Fiber Fabry-Perot Sensors For Detection Of Partial Discharges in Power Transformers

This letter presents a miniature fiber-optic pressure sensor fabricated directly on the tip of a fiber. The sensor measures only 125 /spl mu/m in diameter. A Fabry-Pe/spl acute/rot cavity and a thin silica diaphragm are fabricated by simple techniques involving only fusion splicing, cleaving, and wet chemical etching. Interference pattern of the sensor is analyzed and issues in sensor design are discussed. The sensor has been tested for static pressure response, showing excellent repeatability and no hysteresis. Its all-fused-silica structure lends it great potential for high-temperature pressure measurements. Pressure testing results at 530/spl deg/C are given. The sensor may also find uses in medical applications.

Miniature fiber-optic pressure sensor

This paper presents self-calibrated interferometric-intensity-based optical fiber sensors, which combine for the first time fiber interferometry and intensity-based devices into a single sensor system. The sensor involves an extrinsic Fabry-Perot (FP) interferometric cavity. The broadband light returned from the FP cavity is split into two channels in such a way that one channel has a coherence length much longer than the doubled air-gap separation in the sensor so the FP generates effective interference, while the coherence length in the other channel is so short that no effective interference takes place. As a result, the optical signal in the channel with a long coherence length yields information about the FP cavity length while the signal in the other channel is proportional only to the source power, fiber attenuation, and other optical loss factors in the optical path. To eliminate fringe direction ambiguity and relative measurement limitations associated with interferometric sensors, the sensor is designed such that it is operated over the linear range between a valley and a peak of one interference fringe in the first channel. Moreover, the ratiometric signal-processing method is applied for the signals in the two channels to obtain self-calibrating measurement to compensate for all unwanted factors, including source power variations and fiber bending losses. Various pressure and temperature sensors based on the self-calibrated interferometric/intensity-based scheme are designed, fabricated, and tested. Experimental results show that a resolution as high as 0.02% of full scale can be obtained for both the pressure and temperature measurements.

Anbo Wang

Papers

Fast-Fourier-transform based numerical integration method for the Rayleigh-Sommerfeld diffraction formula

All-fused-silica miniature optical fiber tip pressure sensor

Fiber Fabry-Perot Sensors For Detection Of Partial Discharges in Power Transformers

Miniature fiber-optic pressure sensor

Self-calibrated interferometric-intensity-based optical fiber sensors