Optical Point Sensor Research Articles

Interferometric sphere surface metrology with cylindrical reference for precision topography.

We demonstrate a method for measuring a surface map of a spherical body with interferometric optical point sensors while rotating the test subject. The setup takes advantage of the excellent performance of heterodyne interferometry at nanometer levels and suppression of common-mode errors, as a cylindrical mirror mounted adjacent to the sphere is used as a reference. Future space based missions for gravitational wave research demand an improved inertial reference sensor with reduced acceleration noise levels. Spherical test masses can enable increased performance by suspension-free operation, contrary to cuboid solutions suffering from cross-coupling of attitude control noise into test mass position. However, interferometric readout is affected by surface irregularities and test mass attitude. An accurate surface map for compensation of the center of gravity readout should be established, by characterization either a priori or in-flight, when optical path length changes due to the surface occur in the measurement bandwidth.

Effect of a Misidentified Centre of a Type ASG Material Measure on the Determined Topographic Spatial Resolution of an Optical Point Sensor

The article presents the determination of the topographic spatial resolution of an optical point sensor. It is quantified by the lateral period limit DLIM measured on a type ASG material measure, also called (topographic) Siemens star, with a confocal sensor following both a radial measurement and evaluation, as proposed by ISO 25178-70, and the measurement and subsequent evaluation of two line scans, proposed by the NPL Good Practice Guide. As will be shown, for the latter, an only slightly misidentified target centre of the Siemens star leads to quite significant errors of the determined DLIM. Remarkably, a misidentified target centre does not necessarily result in an overestimation of DLIM, but lower values might also be obtained. Therefore, a modified Good Practice Guide is proposed to determine DLIM more accurately, as it includes a thorough determination of the centre of the Siemens star as well. While the measurement and evaluation effort is increased slightly compared to the NPL Good Practice Guide, it is still much faster than a complete radial measurement and evaluation.

Material classification through distance aware multispectral data fusion

Safety applications require fast, precise and highly reliable sensors at low costs. This paper presents signal processing methods for an active multispectral optical point sensor instrumentation for which a first technical implementation exists. Due to the very demanding requirements for safeguarding equipment, these processing methods are targeted to run on a small embedded system with a guaranteed reaction time T < 2 ms and a sufficiently low failure rate according to applicable safety standards, e.g., ISO-13849. The proposed data processing concept includes a novel technique for distance-aided fusion of multispectral data in order to compensate for displacement-related alteration of the measured signal. The distance measuring is based on triangulation with precise results even for low-resolution detectors, thus strengthening the practical applicability. Furthermore, standard components, such as support vector machines (SVMs), are used for reliable material classification. All methods have been evaluated for variants of the underlying sensor principle. Therefore, the results of the evaluation are independent of any specific hardware.

Nondestructive Evaluation Technique Utilizing Embedded Thermal Fiber Optic Sensors

We investigated a thermal nondestructive evaluation (NDE) technique based on thermography that uses optical fiber thermal sensors to detect damage within a laminated graphite epoxy composite specimen. Two sets of composite samples were used for testing, one set had fiber optic sensors embedded between the layers while the other relied on thermocouple sensors attached to the exterior. A simulated impact system was used to induce damage of varying degrees into the samples. The damage was confirmed either with X-ray or C-scan imaging. The test apparatus in addition to the test data obtained from both sets of samples are presented. The data confirms that the fiber optic thermal sensors can not only detect the presence of damage, but can measure the severity of damage as well. This is the first known attempt to use in situ fiber optic point sensors with thermography to detect damage.

Use of the original silicone cladding of an optical fiber as a reagent-immobilization medium for intrinsic chemical sensors

It is demonstrated that extended-length sensors can be fabricated by the direct immobilization of suitable reagents into the original cladding of a plastic-clad silica (PCS) optical fiber. This cladding, a copolymer of vinyl-terminated poly(dimethylsiloxane) and poly(dimethylmethylhydrosiloxane), is an attractive immobilization matrix for a wide variety of reagents and opens up new avenues of sensor design. Unlike fibers with custom-drawn cladding, the new approach offers greater photo- and thermal stability and permits immobilization of several reagents in adjacent sections of a single fiber. Further, compared to room-termperature vulcanizable (RTV) silicone films used often in optical point sensors, the silicone cladding of a PCS optical fiber offers a number of advantages, including a dynamic fluorescence quenching constant for an immobilized fluorophore that is up to 3.4 times higher, tolerance to aggressive environments (e.g. highly alkaline solutions), lower rates of indicator leaching, high uniformity, and applicability to extended-length sensing. The homogeneity of the microenvironment of the fiber cladding, its resistance to aggressive alkaline solutions, and its ability to transport water vapor were probed by introducing a variety of reagents into the cladding, including a fluorescent ruthenium complex and acid-base and solvatochromic indicators. The new sensor-fabrication approach should find wide application, including detection of neutral species in gases and dissolved in water, and for spatial analyte mapping over extended, remote areas.

Wavelength division multiplexed optical point-sensor networks using injection laser diode sources

Wavelength division multiplexing (WDM) offers an elegant solution to the problem of increasing sensor numbers on an optical point-sensor network. In this paper the provision of WDM optical point-sensor networks is examined by determining the optical power budgets required for their implementation. A substantial increase in the number of distributed optical point-senors is demonstrated when using injection laser diode sources in conjunction with two new power-efficient WDM sensor network topologies.

Improved performance of power‐efficient topologies for wdm optical sensor networks using an extended spectral‐width broadband led

AbstractThe provision of WDM multiple optical point‐sensor networks is examined by modeling the power budgets required for their implementation. A larger number of distributed optical point sensors are indicated than previously obtained by using an extended spectral width broadband LED together with two power‐efficient WDM sensor network topologies. © 1993 John Wiley &amp; Sons, Inc.

A simple frequency domain multiplexing system for optical point sensors

A new technique for the multiplexing of intensity modulated optical fiber point sensors using a frequency domain approach is described. A fully dedicated network, consisting of three sensors, has been built. The system performance was evaluated and results are presented.