Roadmap on optical sensors

Mário F S Ferreira, Enrique Castro-Camus, David J Ottaway, José Miguel López-Higuera, Xian Feng, Wei Jin, Yoonchan Jeong, Nathalie Picqué, Limin Tong, Björn M Reinhard, Paul M Pellegrino, Alexis Méndez (+3 others)
<span title="2017-07-24">2017</span> <i title="IOP Publishing"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/ohjzwmpi25bm5hfuwnimdgk6fe" style="color: black;">Journal of Optics</a> </i> &nbsp;
Sensors are devices or systems able to detect, measure and convert magnitudes from any domain to an electrical one. Using light as a probe for optical sensing is one of the most efficient approaches for this purpose. The history of optical sensing using some methods based on absorbance, emissive and florescence properties date back to the 16th century. The field of optical sensors evolved during the following centuries, but it did not achieve maturity until the demonstration of the first laser
more &raquo; ... n 1960. The unique properties of laser light become particularly important in the case of laser-based sensors, whose operation is entirely based upon the direct detection of laser light itself, without relying on any additional mediating device. However, compared with freely propagating light beams, artificially engineered optical fields are in increasing demand for probing samples with very small sizes and/or weak light-matter interaction. Optical fiber sensors constitute a subarea of optical sensors in which fiber technologies are employed. Different types of specialty and photonic crystal fibers provide improved performance and novel sensing concepts. Actually, structurization with wavelength or subwavelength feature size appears as the most efficient way to enhance sensor sensitivity and its detection limit. This leads to the area of micro-and nano-engineered optical sensors. It is expected that the combination of better fabrication techniques and new physical effects may open new and fascinating opportunities in this area. This roadmap on optical sensors addresses different technologies and application areas of the field. Fourteen contributions authored by experts from both industry and academia provide insights into the current state-of-the-art and the challenges faced by researchers currently. Two sections of this paper provide an overview of laser-based and frequency combbased sensors. Three sections address the area of optical fiber sensors, encompassing both conventional, specialty and photonic crystal fibers. Several other sections are dedicated to microand nano-engineered sensors, including whispering-gallery mode and plasmonic sensors. The uses of optical sensors in chemical, biological and biomedical areas are described in other sections. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed. Advances in science and technology required to meet challenges faced in each of these areas are addressed, together with suggestions on how the field could evolve in the near future. Current and future challenges: While there has been enormous progress in the creation of terahertz devices and in the proof-of-concept of various applications, currently the main reason why terahertz technology is uncommon in 'real world' applications is its cost. Almost two decades ago the first commercial terahertz system was introduced. Currently the price of THz spectroscopy starts in the range of 50000 US dollars but can go up to several hundred thousand depending on their capabilities. Furthermore, although transportable, terahertz systems are still far from being hand-held devices. In addition, terahertz systems are still relatively unreliable, usually requiring regular minor 'tweaking' by qualified staff, and maintenance by specialized engineers every few months. An application that will probably develop strongly in the coming years is the use of terahertz frequencies for high speed telecommunications. In this particular case, the development of Ferreira et al. Status: The mid-infrared is commonly defined as the part of the electromagnetic spectrum that has wavelengths between 3-8 μm. It offers significant opportunities for trace gas sensing since many strong absorption lines occur in this region (see figure 2 ). The absorption lines in this part of the spectrum can be two orders of magnitude stronger than the overtone lines that occur in the shorter end of the infrared. A significant number of sensing techniques have been developed in this shorter wavelength region, which are suitable to deployment in the mid-infrared. Ferreira et al. Suspended core fiber sensing relies on the evanescent field to interact with the material under testing. The numerical aperture of an air-glass interface is very high which means that the cores need to be significantly less than 1 μm such that a significant fraction of the light propagates outside the core when operated in near IR/visible. The longer wavelength of the Ferreira et al. Status: Sensors are devices or systems used to capture, quantify and faithfully translate magnitudes from any domain to the electrical one. When light sciences and technologies are used, the sensing using light (SuL) area of photonics appears. Optical fiber sensors (OFS) can be understood as a subarea of SuL in which fiber technologies are employed. In an OFS the measurand modulates characteristics of light in some part of an optical fiber system (the transducer), faithfully reproducing it in the electric domain [29] . Any OFS can be considered to be integrated by three main parts: transducer, channel and optoelectronic or interrogation units ( figure 4) . It can be said that the development of OFS started in the latter half of the 1970s. As reported by Giallorenzy [30], relevant contributions were achieved on temperature, magnetic, acoustic and pressure point based sensors by 1982. Many techniques were explored such as those implementing interferometry (Michelson, Mach-Zehnder, Sagnac,..) for the development of current sensors (Faraday effect), hydrophones for the navy and also fiber optic gyroscopes (Sagnac effect) [31] . The possibility of multiplexing sensors in a given fiber (enabling OFS networks and reducing the cost-per-measurement-point) was demonstrated by 1986. Based on scattering and optical time-domain reflectometry (OTDR) techniques, distributed sensors appeared by the end of the 1980s. Linear (Rayleigh) and Ferreira et al. Status: Coherent oscillations of the conduction band electrons in noble metal nanoparticles (NPs), so-called plasmons, give rise to large resonant optical scattering and absorption crosssections. As a consequence, individual gold and silver NPs can be detected in a variety of optical imaging schemes. Noble metal NPs with diameters ⩾40 nm can be imaged in differential interference contrast microscopy or in scattering microscopy under darkfield or total internal reflection illumination [90] . NPs smaller than 40 nm can be visualized through photothermal imaging. Small metal NPs retain relatively large absorption cross-sections that allow for an efficient heating of the immediate ambient environment [91] . The resulting local changes in the refractive index can be detected optically even for NP diameters as Ferreira et al. Status: Whispering-gallery mode (WGM) sensors are emerging as a platform technology with applications as varied as sensing physical, chemical and biological entities [121] . The exploding interest in the WGM platform is driven by its versatility and its extreme sensitivity. These attributes duly enable precision measurements for various applications in sensing. This section presents a roadmap for the future of WGM sensors. By far the most accessible WGM sensor is a ~100 μm glass microsphere that recirculates coherent light by total internal reflection (figures 23(a), (b)). The prolonged total internal reflection on many thousands of roundtrips results in strong interference if the resonance condition is met. At resonance, an exact integer number of wavelengths fit the optical path at the circumference of the bead and a sharp (i.e. high quality factor) spectral response emerges. The resonance wavelength or frequency of the WGM can be determined with great precision by laser interferometry. Ferreira et al.
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