An Introduction to Optoelectronic Sensors

The following sections are included:

• PREFACE

• CONTENTS

The chapter summarizes the fundamentals of light propagation in fiber and integrated optics and explains the basic working principles of optical sensors making use of these waveguides. Outstanding applications where these sensors have been used are also presented.

Over the last few years, optical fiber sensors have seen increased acceptance and widespread use for a variety of applications. Among the large number of fiber optic sensing configurations, Fiber Bragg Grating (FBG) based sensors, more than any other particular sensor type, have become widely known and popular within and out the photonics community and seen a rise in their utilization and commercial growth. The capability of FBGs to measure a multitude of parameters such as strain, temperature and pressure and many others coupled with their flexibility of design to be used as single point or multi-point sensing arrays and their relative low cost, make them ideal devices to be adopted for a multitude of different sensing applications and implemented in different fields and industries. This work, involving the present and next chapter, reports on recent FBG sensing applications in several industrial fields. In particular, we first summarize the FBG major milestones of their technological evolution in thirty years from the discovery of Kenneth Hill in 1978 and then focus the attention on FBG recent application in civil engineering. We also report on FBG applications in aerospace, energy, oil and gas, transportation and underwater industrial fields. In particular relevant works ranging from structural sensing and health monitoring of composites and structures in aeronautic areas, to pressure and temperature sensors for oil and gas reservoir monitoring, to acoustic sensors for underwater applications, to high voltage and high current sensing systems for the power industry to name just a few, proposed by research groups and industries in last years are discussed.

Optical fibers offer the unique advantage of allowing spatially distributed sensing of several quantities. This is especially important for the monitoring of large, critical structures. In this chapter we review the main techniques for distributed sensing using optical fibers.

A review of Fiber Bragg Grating (FBG) features and the different schemes of fiber sensors interrogation are reported. The interrogation system represents the key element of monitoring systems in terms of both performance and cost as it has to measure relatively small shifts in Bragg wavelength of FBG elements.

An innovative interrogation system prototype for structural sensing based on a high-performance electro-optic edge filter on glass is also presented here. It provides a wavelength-dependent transmittance with a linear relationship between the Bragg wavelength shift and the output intensity change of the filter. The resulting device is the demonstration of a simple and inexpensive technology to implement low cost FBG sensors monitoring system based on innovative integrated optic functional component on glass.

Surface Plasmon Resonance (SPR) is an optical technique that uses evanescent waves as a valuable tool to investigate chemical and biological interactions taking place at the surface of a thin sensing layer. SPR offers a real time analysis of dynamic adsorption and desorption events for a wide range of surface interactions. After a brief theoretical introduction, examples of a wide range of applications of SPR are presented. Main application areas involve the detection of biological analytes and study of biomolecular interactions in liquid phase. Applications in chemical sensors will be also illustrated by using different classes of organic and inorganic material as sensing layers.

Nowadays sensing represents a very active area of research due to many possible applications. A particular need exists for miniature sensors for the detection of several biochemical species and tracking mechanical changes. Several optical techniques have proven to be quite effective. Here we provide a quick overview of the recent progresses in the development of optical biosensors based on resonant cavities, where light propagation occurs through whispering-gallery modes (WGMs). The effect of any perturbation to the optical resonance structure of a WGM resonator is such that a very high sensitivity can be achieved.

A new class of materials, called Photonic Crystals (PhCs), affects a photon's properties in much the same way that a semiconductor affects an electron's properties. PhCs, in fact, possess a photonic bandgap, which means that light of certain wavelengths cannot propagate through them. These structures have very interesting properties of light confinement and localization together with the strong reduction of the device size, orders of magnitude less than the conventional photonic devices, allowing a potential very high scale of integration. Due to their unique features, these structures can possess unique characteristics that enable the structures to behave like optical waveguides, high Q resonators, selective filters, lens or superprism, just to name a few. The ability to mold and guide light leads naturally to novel applications in several fields including optoelectronics, telecommunications. The authors present in this chapter an introductory survey of the basic concepts of this new technology with particular emphasis on their applications for chemical and biological sensing.

In this contribution we review some of the more conventional micromachining technologies, underlining their advantages and disadvantages. Then, new developments in some of the existing technologies as well as new technologies will be illustrated.

Finally, some examples of integrated micro-machined sensors and devices are presented to underline the expectations of these technologies.

The aim of this chapter is to give some basic elements concerning the utilization of absorption, luminescence, Raman and Brillouin spectroscopies in the field of optical sensors. Particular attention is paid to the diagnostics of thin films and to the applications of activated optical waveguides.

In this chapter the potentials of Laser Doppler Vibrometry (LDV) technique for the measure of vibration velocity on solid surfaces are presented. LDV allows to perform vibration measurements without contact, eliminating the intrusivity of traditional devices. This makes LDV suitable for many applications, from noise&vibration analysis in automotive domains up to works of art diagnostics. Different LDVs configurations are discussed (i.e. single-point, differential in fibre, scanning system, in-plane and rotational). A description of the optical schemes and signal processing strategies is given.

A short historical survey of laser Doppler velocimetry is presented, followed by a description of the measurement principle, basic configurations and main components used in existing instrumentation. The measurement volume is described. Aspects and problems related to optical access and fluid seeding are discussed. Some hints are given on signal processes.

The basics of photoacoustic spectroscopy in solid and gaseous samples are summarized. A survey of the applications of near-infrared diode lasers and mid-infrared quantum cascade lasers in photoacoustic spectroscopy is reported.

This chapter describes the possibility to use Digital Holography as a tool to carry out a non-contact and non-destructive characterization and inspection of micro-electro-mechanical systems (MEMS). The technique allows to evaluate quantitatively, with high accuracy, different features of a typical MEMS: the profile; the deformations induced by external influences; the behavior when actuated under operation conditions. Digital holography provides two main advantages consisting in the possibilities to perform a dynamic characterization of the MEMS and to reconstruct the in-focus image of the object for 3D structures. The evaluation of the MEMS performance is particularly useful when studying and understanding the effectiveness of the design and of the fabrication process. Several examples of MEMS inspection are illustrated to demonstrate the reliability of the technique.

Infrared technologies (materials, devices and systems) generally have been confined within a selected scientific community till in the 80's, when the Focal Plane Arrays development was a real breakthrough which could supply smart solutions for new product manufacturing and for opening new wide markets. These developments integrated with the advanced signal processing, thanks to the elimination of cryogenic cooling in the new microbolometers, allow to foresee that the integrated structures of “Smart Sensors” will be strategic components for important areas like transports, environment, territory control and security. Theory and design of the most important IR Sensors, their main historical developments and their trends in future developments are described.

This chapter is an overview on terahertz technologies and applications for sensing. The most advanced imaging and spectroscopy techniques are described, considering current opportunities and limitations in comparison to probes in the adjacent regions of the e.m. spectrum. Potential applications are highlighted, with a specific focus on security for detection of illicit substances and revealing of hidden objects. The technological status and current bottlenecks on sources and detectors are reviewed and future trends discussed.

Squeezed states of light represent the most famous type of non-classical radiation states. They are characterized by a reduction of the quantum noise in one of the field observables, with respect to the noise affecting a coherent beam of the same amplitude (the standard quantum limit). This peculiarity in the noise property has suggested the use of these states in particular measurement schemes to beat the limit imposed by standard quantum noise. This contribution aims to briefly review three applications of squeezed states: (i) quantum interferometry; (ii) absorption measurements; (iii) high resolution imaging.

A great demand exists nowadays to special systems that permit the monitoring and controlling in real time, and under diverse range of operating conditions, the performance of structural and mechanical elements or structures with minimum cost and effort. The objective of this chapter is to build a base of knowledge containing a brief demonstration of a candidate system and highlighting the most important fields of its applications. The practical experience of famous authors, researchers, companies and great industries, rather in presenting or/and in utilizing this technology is going to be discussed through the following lines.

The purpose of this chapter is to present the gyroscope technologies available on the market or still under development. Besides mechanical and electro-optical systems, a new class of miniaturized devices based on the vibrating structure is described. In particular, the technological solution investigated at ST Microelectronics to realize a silicon micromachined device is described. A new interferometric optical method is then applied for the characterization of the vibration modes of the fabricated prototypes.

Recent years have witnessed remarkable interest in the study of optical sensors applied in medicine, mainly for the detection of chemical and biochemical parameters. Health-care is surely the application field which seems to have the best future development perspectives for optical sensors, not only considering invasive applications (the high degree of miniaturization of optical fiber sensors, their considerable geometrical versatility, and extreme handiness make it possible to perform a continuous monitoring of numerous parameters, thus enabling performances which are often unique) but also taking into account the developments of optical multiarray biochips for the analysis of multiple parameters. The role of optical sensors in the European Integrated projects CLINICIP - Closed Loop Insulin Infusion in Critically Ill Patients - and CAREMAN - HealthCARE by Biosensor Measurements And Networking - is also described.

A scenario of capabilities of laser systems devoted to environmental monitoring control is given with particular emphasis to atmospheric and marine surveillance applications. Theoretical background of the different laser based techniques has been reported together with their limitations. Applications developed at the ENEA Laser Remote Sensing laboratory have been described and compared with complementary measurements obtained from passive remote information. Such systems are far from being considered mature and new perspectives are expected in the future with the application of new laser sources.

Strong concern about anthropogenic effects on environmental and global climate issues motivated a large amount of studies and triggered development of sensitive techniques for monitoring relevant parameters. Laser spectroscopy offers powerful tools for fast and accurate measurements of very low concentrations of a large number of chemical species. We present an overview of the laser-based spectroscopic techniques and their application to in-situ measurements of gas species for environmental monitoring. We describe the basic principles of absorption spectroscopy and detection strategies. Major features of coherent radiation sources are described and several examples of laser-based sensors are given.

Laser material processing today is widely used in industry. Especially laser welding became one of the key-technologies, e. g., for the automotive sector. This is due to the improvement and development of new laser sources and the increasing knowledge gained at countless scientific research projects. Nevertheless, it is still not possible to use the full potential of this technology. Therefore, the introduction and application of quality-assuring systems is required.

For a long time, the statement “the best sensor is no sensor” was often heard. Today, a change of paradigm can be observed. On the one hand, ISO 9000 and other by law enforced regulations have led to the understanding that quality monitoring is an essential tool in modern manufacturing and necessary in order to keep production results in deterministic boundaries. On the other hand, rising quality requirements not only set higher and higher requirements for the process technology but also demand qualityassurance measures which ensure the reliable recognition of process faults. As a result, there is a need for reliable online detection and correction of welding faults by means of an in-process monitoring.

The chapter describes an advanced signals analysis technique to extract information from signals detected, during the laser welding process, by optical sensors. The technique is based on the method of reassignment which was first applied to the spectrogram by Kodera, Gendrin and de Villedary^22,23 and later generalized to any bilinear time-frequency representation by Auger and Flandrin.²⁴

Key to the method is a nonlinear convolution where the value of the convolution is not placed at the center of the convolution kernel but rather reassigned to the center of mass of the function within the kernel. The resulting reassigned representation yields significantly improved components localization. We compare the proposed time-frequency distributions by analyzing signals detected during the laser welding of tailored blanks, demonstrating the advantages of the reassigned representation, giving practical applicability to the proposed method.

Visible light, scattered within an angle of few degrees, (Small Angle Light Scattering, SALS) yields information on the spatial correlations and dynamical properties on the scale of the micrometers. In this way a quick and non-invasive characterization of a variety of samples is feasible. Lately the SALS instruments have been built around multielement optical sensors (CCD, CMOS), allowing the simultaneous measurement of the complete structure factor even during fast kinetics. An assessment of some sensor matrices of different technology will be presented. The macromolecular assemblies produced by polysaccharides or proteins can be functional or dysfunctional, their properties being either desirable or detrimental. Anyhow, their morphology often depends, in a very delicate way, on the presence of cosolutes, on the thermal history, on the biopolymer concentration etc. We present some applications of low angle dynamic and static light scattering to the study of gelling systems (agarose, pectin, insulin).

Modern day electronic applications are increasingly demanding ultrahigh-speed electronic and optoelectronic devices as well as integrated circuits (ICs) that operate at frequencies over 10 GHz for a wide range of applications such as communications network systems and instruments. Because of this, a thorough design and a reliable production of these devices require a novel approach to contactless characterization. This indeed is a wide topic, and it is hard to be considered in full detail in a single chapter.

We have selected only some peculiar contactless techniques favoring those which can be easily employed for circuits and materials characterization; we include: scanning electron, photoexcitation and force microscope, electro-optical sampling techniques charge sensing probes and SNOM. As we will explain, such techniques are applied to signal survey, detection and measurement of microcracks, temperature, lifetime, surface recombination velocity, and diffusivity.

The following sections are included:

• INDEX