Sensors allow humans to feel and understand their world, and their development lays the foundation for the fulfillment of information society and has formed a huge industry. Photonic sensor is a device that senses light and converts to electricity.Photonics sensors can be defined as the set of techniques and scientific knowledge concerning the generation, propagation, control, amplification, detection, storage, and processing of the optical spectrum signals.
Among them, photonic sensors have been widely used in the areas of information, industry, energy, military, security, biology, medicine, and smart structures due to their unique advantages and have become one mainstream in the sensor society. They have wide applications including for smart barcodes, new sensing systems in the fight against crime and terrorism, advanced manufacturing and production techniques, environmental monitoring and in new medical diagnostic devices. Photonic sensors, including fiber optic sensors, have been the subject of intensive research over the last two decades for use in civil and military environments for detection of a wide variety of biological, chemical and nuclear agents.
Photonic technologies provide incomparable advantages such as high sensitivity, possibility of integration with electronic devices, compactness, metal-free operation, low-cost and electromagnetic immunity. It provides cheaper, smaller, faster and lighter components and devices with superior functionality and less energy consumption.Moreover, the integration of photonics, microelectronics and microfluidics into the same chip represents an intriguing technological platform for the realization of lab-on-a-chip systems, which are fundamental for fast, multiplexed and real-time measurements in various application fields.
Because of their immunity to electromagnetic interference, lightning and electrical noise, optical fiber sensors have been steadily gaining in popularity among wind turbine manufacturers as a practical, reliable and cost-effective online structural safety and fatigue monitoring tool integrated with the condition-based monitoring system of wind turbines. Fiber Bragg grating (FBG) strain sensor arrays – either surface-mounted or embedded – can monitor the mechanical behavior of rotor blades in different types of wind turbines. The results can be used during the design and qualification stages to corroborate the measured strain values against design models. In service, premounted FBG sensors help monitor online the condition of the blades while rotating or stationary, and under various wind conditions.
Over the past decade, optical fiber sensors have gained widespread acceptance within the oil and gas industry due to their reliability, flexibility and low operating costs, as well as the benefits brought by their multipoint and distributed sensing capabilities. More recently, extensive interest has been shown in the development and commercialization of fiber optic seismic and acoustic sensing arrays – land and underwater – for oil and gas exploration, pipeline surveillance, geophysical monitoring, reservoir monitoring and management, geothermal monitoring, and structural monitoring of offshore platforms and oil tankers. Distributed acoustic sensing (DAS) enables detection, discrimination and location of acoustic events on an optical fiber over tens of kilometers.
Photonic systems are becoming more significant in military platforms, especially in aircraft, where the increase in data rate, improved EMI hardness, improved safety, and reduced weight, are together a significant advantage. The optical fibre systems have been applied for the structural health monitoring of UAVs, and the revolutionary electro-optical Distributed Aperture System utilised on the Lockheed Martin F-35 Lightning II.
Photonic sensors can utilize different components of the optical signal such as intensity based, interferometric, polarization, spectroscopic, pulse shape or arrival time based, giving rise to a large number of different sensor designs. These differences may arise in the physical structures employed, in the optical source or detection systems, in the signal demodulation systems, or in new combinations of these.
“Emerging technologies like silicon- and InP-based photonics will allow the integration of most sensor reading unit functions at (the) chip level, strongly decreasing the fabrication cost and opening the way to mass-scale applications, such as smart structures embedding fiber Bragg gratings, as well as distributed fire-detection systems.
Progress in photonic sensor designs and applications continues at a fast pace with new types of optical fibers – photonic band gap fibers (PBG), microstructure optical fibers (MOF), random hole optical fibers (RHOF); and hybrid ordered random hole optical fibers (HORHOF); higher resolution, lower cost, and or expanded detection range capability for sources and detection schemes; and new signal demodulation algorithm designs
Laser Sensor Can Detect Damage to Military Assets
A distributed feedback fiber laser sensor has detected acoustic emission signatures associated with cracks in riveted lap joints, demonstrating that it has the potential to uncover structural damage in U.S. Navy assets before the damage reaches critical levels.
Developed by researchers at the U.S. Naval Research Laboratory (NRL), the laser sensor consists of a single fiber, similar in width to a human hair, which is integrated into a shallow groove formed in the lap joint. The sensor has a small system footprint and can be multiplexed.
To test the technology, researchers installed the laser sensors into a series of riveted aluminum lap joints and measured acoustic emission over a bandwidth of 0.5 megahertz (MHz) generated during a two-hour accelerated fatigue test. They took equivalent measurements with an electrical sensor.
The embedded laser sensors demonstrated acoustic sensitivity comparable to or greater than that achieved by existing electrical sensors. The laser sensors were able to detect low-level acoustic events generated by periodic fretting from the riveted joint, in addition to acoustic emissions from crack formation. Time-lapse imagery of the lap joint revealed that the observed fracture correlated with the signals measured. In addition to crack detection, the fiber laser sensor also showed the ability to measure compromising impacts.
“Our research team has demonstrated the ability of this fiber laser technology to detect acoustic emission at ultrasonic frequencies from cracks generated in a simulated fatigue environment,” said Dr. Geoffrey Cranch, research physicist. “The novel part of this work is the fiber laser technology and how it is being applied.
The fiber laser sensor system has now been expanded to multiplex many sensors onto a single fiber. Efforts are underway to interpret the acoustic emission data to calculate metrics such as probability of failure. Future enhancements may include implementing phased array beam forming techniques to facilitate crack location.
The fiber laser sensor also has the potential to integrate with existing fiber optic strain and temperature sensing systems. Integrating the sensor with these systems would provide a multiparameter sensing capability that could meet the operational safety requirements for an SHM system at significantly lower cost.
“An automated, in-situ structural health monitoring (SHM) system, capable of monitoring key structural parameters such as temperature, strain, impacts and cracks, and capable of reliably detecting damage well before reaching a critical level is needed to increase safety and readiness while lowering operational cost of Navy platforms” said Cranch. “At present, none of the services are using in-situ technologies to manage the structural health of their assets.”
“Our focus is on Navy platforms, such as aircraft, ships and submarines, but the technology could also be used on civilian aircraft,” he said. “Applications to bridges and buildings are also possible if there are critical parts prone to fatigue and failure that would benefit from continuous monitoring.”
New photonics sensor masts to improve submarine stealth and survivability
U.S. Navy submarine experts needed improved sensor photonics masts for Virginia-class fast attack submarines to improve stealthiness and survivability. Officials of the Naval Sea Systems Command announced an $111.8 million contract to L-3 KEO to develop deployable prototypes of the Low Profile Photonics Mast (LPPM).
Photonics masts operate in place of the traditional submarine periscope aboard Virginia-class attack submarines. The photonics mast uses various electro-optical sensors, and does not penetrate the submarine hull like a traditional periscope does. Photonics mast sensors connect to the submarine by optical fiber.The LPPM is a low-observable optical mast that reduces the submarine’s risk of detection by enemy submarines and surface warships while the system is in use, while improving the submarine’s sensor capability
Imagery from the LPPM is displayed on high-definition screens aboard the submarine. Features include short-wave infrared (SWIR) and high-definition imaging, laser rangefinding, special stealth features, and an antenna suite with broad spectral coverage and direction finding. Navy Chief of Naval Operations Adm. Jonathan Greenert has asked for all Pacific Fleet Virginia-class submarines to be equipped with the LPPM and spare parts for the system to be made available beginning this year to support sensitive missions vital to national security.
Photonics Sensor Market Report, published by Allied Market Research, forecasts that the global market is expected to garner $18 billion by 2021, registering a CAGR of 17.7% during the period 2016-2021.The photonic sensors market is segmented by product types and end user applications. The product type is sub-divided into laser-based sensors, fiber optic sensors and biophotonic sensors and the end user application is sub-segmented into energy industry, construction, military and aerospace, oil and gas, medical and industrial application.
Global demand for military electro-optics and infrared sensors should grow by 40 percent over the next six years, predict analysts at market research Markets and Markets in Vancouver, Wash. The worldwide military electro-optics and infrared systems market should grow from $10.15 billion this year to $14.2 billion in 2022, which represents a combined annual growth rate (CAGR) of 5.75 percent, analysts say.
Driving growth in the electro-optics and infrared market are two primary factors: increasing demand for battlespace awareness by defense forces; and technological advancements to improve efficiency, analysts say. Also fueling the electro-optics and infrared market is growing demand for border surveillance and situational awareness using unmanned aerial vehicles (UAVs), analysts say.
Based on sensor technology, the staring sensor segment should lead the military electro-optics and infrared systems market during the forecast period because these kinds of sensors gather more radiation and are able to handle complex moving parts of sensors and cameras on the battlefield, analysts say. Staring sensors are used widely in missile warning receivers and electronic warfare systems. They are preferred over scanning sensors, as they are cost efficient and have varied technological applications.
Some of the market players in this industry segment are Prime Photonics, Smart Fibers, Mitsubishi Electric Corporation, Bayspec Inc, Omron Corporation, Qorex Llc and Fiso Technologies Inc among others.Major players in the military electro-optics and infrared systems market are Lockheed Martin Corp., Raytheon Co., and Northrop Grumman Corp. in the U.S.; BAE Systems Plc. in the United Kingdom; and Thales Group in France.
Optical Sensing Principles
“Waveguide-based devices are becoming more and more attractive in the field of optical elaboration of signals for sensing applications in different areas, especially in chemical and bio-chemical detection, angular rate rotation estimation and electric field detection,” write Vittorio M. N. Passaro and others.
In case of chemical and biochemical sensing, the operating principle consists of the variation of the effective index of the optical mode propagating into the structure, as a consequence of the presence of a chemical substance to be detected close to the sensor surface.
Optical sensing of chemical species is typically based on the variation of waveguide optical properties in the presence of some target analyte near the sensor surface. In particular, the principal sensing mechanisms exploited for this specific application include the variation of waveguide effective index or waveguide absorption coefficient, as a function of the concentration of the chemical species to be sensed.
The integration of photonic waveguides in resonant microcavities, ensures a suitable optical readout at the sensor output. Ring resonators can be employed as resonant architectures for sensing applications, where the sensing principle consists in monitoring the resonance wavelength shift as a consequence of the variations of the waveguide refractive index in the presence of the substance to be detected.
Ring resonators (RR) based sensors can be also employed for the realization of integrated optical gyroscopes (IOGs) for estimating the angular velocity in inertial systems. In this case, the operating principle characterizing the photonic sensing mechanism is the Sagnac effect, which leads to a phase shift between two counter-propagating beams proportional to the angular velocity at which the device is rotating.
The Sagnac effect has been widely employed in fiber optic based sensor for pressure, temperature and torsion monitoring. Moreover, it is the most exploited operating principle in optical gyroscopes. The detection of the angular velocity relies upon different effective path lengths experienced by two counter-propagating beams of light in a closed path
The Raman effect is an inelastic scattering process which is becoming widely used in photonic devices, especially in those fabricated using optical fibers or SOI technology. It is important to distinguish among three different scattering effects in optical fibers: Rayleigh scattering, Brillouin scattering and Raman scattering.
Rayleigh scattering is an elastic process since the energies of incident and scattered photons are the same. On the contrary, Raman and Brillouin scattering are inelastic effects. In fact, incident and scattered photons have different energies and the energy lost by the incident field leads to creation of phonons, which modify the vibrational states of the medium. The major difference between the Brillouin and Raman scattering effects concerns the different frequencies of the scattered phonons, since the former involves low frequency phonons, called acoustic ones, and the latter involves phonons at high frequency, called optical phonons.
Photonic Field Sensor are works mainly gathering data from reflection, angles of reflection, by reading emission from different kind of metals and distinguish their properties and identify them.
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