Your very close and extremely far encounters with photonic measurements

Optical coherence tomography may also be used for other, non-medical, purposes. As nowadays official documents like passports or identity cards use subtle details hidden between layers of the paper, OCT can be used to check for presence and correctness of those security measures and to detect falsification. It is used for studying and identifying forgery of paintings, too.

Photonic measurements guard our security at multiple fronts and distributed fiber-optic sensors in their many incarnations are rather versatile at providing monitoring possibilities and alarming us about possible forthcoming catastrophic events like leaking gas explosions, collapses of bridges, or earthquakes. The principle of measurement for most types of distributed fiber-optic measurement methods is quite similar to the optical coherence tomography. In this case the measurement part of the signal is sent down the optical fiber and an external factor – temperature or strain – acting on the fiber modifies its internal backscattering. Measuring the temperature changes with distributed fiber-optic sensors permits detection of fire in highway and railway tunnels, revealing leaks in pipelines (as the gas rapidly expands, it creates a ‘cold spot’ at the location of the leak) and finding defects in operation of mechanical or electrical machinery which is prone to overheating when working under fault conditions, for example in industrial conveyor belts, power plants and transformer stations. Distributed fiber-optic strain sensors are excellent solution for structural health monitoring of buildings, vehicles, and strategic infrastructure. Due to small size and light weight of optical fiber they can be installed on planes to monitor in real time the deflection of wings. Other advantageous properties of optical fibers might be even more crucial. As the optical fibers are usually made of silica glass, which is a dielectric material, they are electrically passive and thus – unlike electronics which are prone to sparking – may be used in environments with an explosion hazard. This, together with immunity to electromagnetic interference, enables the distributed fiber-optic sensors to be used in mines for monitoring of structural integrity of tunnels. In distributed fiber-optic sensing, a single tens-of-kilometers-long strand of optical fiber measuring with a spatial resolution of about one meter replaces many thousands of point sensors (like thermocouples) and enables much more straightforward localization of an event like a fire, a pipeline leakage, or a cracking in a structure of a building.

A certain technology of distributed fiber-optic sensing, namely distributed acoustic sensing (DAS), may impress you even more with its abilities. Frequent measurements of strain changes enable the distributed acoustic sensor to detect sound waves. This is used by gas and oil industry for monitoring infrastructure for possible threats – like an excavator working near a pipeline. Optical fibers installed on the international borders and connected to DAS interrogators help border guards thwart attempts at smuggling or illegal border crossings. Distributed acoustic sensing can be also used for perimeter security at other places: around military bases, along railways, et cetera. Other applications of DAS are in seismology (for detection of earthquakes and underground fault zones) ­­and establishing “smart cities” (for monitoring of traffic). Especially in those two areas of application distributed sensors can take advantage of the so-called dark fibers – providers of telecommunication services tend to cut (quite significant) installation costs by putting in the ground more strands of optical fiber than they currently need. However, in principle, distributed sensor can be additionally set up even on the fibers that are actively transmitting data if only the sensing is established using different spectral region.

Other successfully commercialized photonic measurement methods include ring laser gyroscopes, fiber-optic gyroscopes, optical time domain reflectometry and LiDAR (light detection and ranging). Photonic-based gyroscopes (both technologies are based on a specific type of an interferometer) practically ousted mechanical gyros from the market, except maybe for some extremely low-cost and low-accuracy applications. Submarines rely entirely on those photonic gyroscopes for navigation as the GPS is unavailable underwater and aircrafts keep them as a part of their backup systems. Optical time domain reflectometers (OTDRs) are used mainly for testing and maintenance of optical fiber networks. Optical time domain reflectometry is also a method of distributed measurement, a simpler one but with a lower spatial resolution. It can detect and localize excessive loss that may be caused by aging of optical fiber, its excessive bending, some loose connection, or a total breakage – in case of optical fibers responsible for connection between continents deployed on ocean bed, for example, due to a shark biting the cable. OTDRs work by sending pulses of light down the optical fiber and measuring amount of reflected and backscattered light as well as the time delay between sending the pulse and detecting the returning signal, which enables calculating the distance from which the signal comes (using the foreknown speed of light in the optical fiber). LiDAR is simply a version of radar based on light instead of its longer-wave cousins on the electromagnetic spectrum, radio waves. Using shorter wavelengths enables the LiDAR to achieve better resolution and thus register smaller details. LiDAR can be implemented either as a time-of-flight device similar to OTDR (but without an optical fiber), or in even more precise version similar to OCT and more sophisticated types of distributed fiber-optic sensing technologies. Many climate research and meteorological satellites are equipped with LiDAR to measure wind speeds (in this case LiDAR enabled more accurate longer-term weather forecasts), track aerosols (including pollutants) and clouds, measure thickness and movement of glaciers and map vegetation (for example for more precise estimation of amount of carbon stored in biomass). Another use of LiDAR is to create a virtual model of an environment, be it for documentation of a crime scene or for integration into a scene of your favorite summer blockbuster. LiDAR is also considered for cruise control in autonomous cars.

It is rather impossible to list all photonic measurements used in the world around us. Some kind of photonic measurement is present at presumably most, if not all, factories and research laboratories. Interferometers are used for testing of optical components and systems, but also for metrology of many other components, especially when flatness or shape has to be measured with great precision. And there is a broad group of photonic-based chemical and biological sensors and measurements, which can detect trace amounts of molecules. Some examples of their use are in processes of contamination removal in semiconductor or pharmaceutical industries, or in measurement of methane concentration at landfills. And there are still many other methods: confocal laser microscopy, Raman imaging, fluorescence imaging, and so on, and so on…