Ultra-short light pulses for sensing in harsh environments

At the forefront of many environmental and economic debates is the depletion of the world’s natural oil supply. However, thanks to new technologies, we have been able to expand our horizons in the oil and gas sector where the search for alternative sources of fuel has led to the extraction of heavy crude oil and bitumen from the oil sands. Online analytics are increasingly desired to improve the efficiency of these practices by allowing us to measure pressure, temperature, and chemical composition as well as to address environmental issues.

In its natural state, the crude oil and bitumen contained in the oil sands is not easily recoverable. However, there are a few methods which have been developed to enhance oil retrieval, one of which is referred to as steam-assisted gravity drainage. Using this method, companies will drill a pair of horizontal wells into an oil reservoir, one a few meters above the other. High pressure steam is then continuously injected into the upper well to heat the oil and reduce its viscosity, therefore allowing it to drain easily into the lower well where it is pumped out.

Below ground, light is being used to help optimize this process by enabling oil companies to monitor and gauge the temperature and pressure of the steam inside the injector well and oil flow in the producer. Fibre Bragg grating (FBG) is a unique light-based technology which is being used for this precise application, among many others in various industrial sectors.

An optical system for writing fiber Bragg gratings used in sensors that operate in extreme conditions. Credit: National Research Council of Canada.

An optical system for writing fiber Bragg gratings used in sensors that operate in extreme conditions. Credit: National Research Council of Canada.

FBGs are optical filtering devices inscribed within the core of an optical fibre that reflect light of specific wavelengths or colours depending on the inscribed grating period, or to put it in simpler terms, the inscribed pattern. The technology, invented in Canada during the 1970s, was initiated by the Communications Research Centre (CRC). The expertise has since been transferred to the National Research Council of Canada (NRC) which acquired the technology in 2013. Advanced optical fibre based measurement and grating design are now one of the technical services offered by NRC’s Quantum Photonic Sensing and Security (QPSS) program. FBG is considered one of the four milestone developments in optical communications next to the invention of the laser, optical fibre and optical amplifier.

These gratings act as sensors when exposed to varying environmental conditions such as thermal fluctuations and pressure strains which cause the grating spacings to change, thereby reflecting different wavelengths of light. Currently, there are two methods of inscribing these grating periods – ultraviolet (UV) laser and femtosecond-infrared (fs-IR) laser-induced photosensitivity.

The first generation of FBG relied on the UV laser-induced method. Through this approach, when the photosensitive Germanium‐doped core of a silica optical fibre is exposed to UV laser light, it produces a chemical reaction resulting in changes to its refractive index in a periodic fashion along the fibre: the grating period. The disadvantage of this method, especially from the sensor perspective, is that the gratings are impermanent; once exposed to temperatures above 250 °C the gratings will be erased.

On the other hand, the fs-IR laser-induced method produces a physical reaction whereby the gratings inscribed are permanent. The key to this method is found in the femtosecond laser which produces an ultra-short light pulse with the femtosecond being a quadrillionths (or millionths of a billionth) of a second. Therefore, although the total energy of the light is small, once compressed in such a short period of time its intensity is more than the surface of the sun. Concentrations of these femtosecond laser pulses cause “micro explosions” along the optical fibre, resulting in material compaction or defect formations which make up the grating period. This second generation of FBG allows the sensors to be used in environments with extreme temperatures as the gratings themselves cannot be erased until the fibre itself reaches its melting point of about 1200 °C for traditional silica fibres.

Among the many advantages of both generations of FBG is their passive nature. Before FBG became a popular technology for sensing applications, electrical devices were generally relied upon. However, sensing based on electrical devices is problematic in harsh or explosive environments where there are high levels of electromagnetic interference known to compromise these technologies. FBG, on the other hand, relies on light which is immune to such interference.

The other advantage of FBG is that multiple grating periods reflecting varying wavelengths of light can be inscribed into a given optical fibre. In this way a single fibre filament can be used to measure effects at different locations along its length.  This multiplexing capability, whereby it is possible to transmit different information using a single wire, makes FBG perfectly suited to be integrated into existing optical fibre communications networks as a way to expand network capacity. In fact, optical networking can be considered as the original use of this technology. Now, there are countless other applications, and this is due in part to modern developments in the field which are continually challenging the limits of what this technology can do.

Steam-assisted gravity drainage, as discussed above, is just one example of these applications. Bragg grating sensors have also been used for high temperature combustion monitoring in gas turbine power generation facilities, as well as for structural health monitoring of composite structural components and turbine engines for aerospace. Making use of the multiplexing capability of this technology, the sensors are also an excellent resource for pinpointing and measuring temperatures or stresses within these structures.

Another usage of FBG is the integration of the sensors into coal gasification reactors which, much like our example of steam-assisted gravity drainage takes us back to the use and importance of light for energy and the environment. Coal gasification is seen as clean technology for electricity generation. It can be considered a process through which we can convert coal into a gaseous mixture of carbon dioxide, carbon monoxide, water vapour and molecular hydrogen to be used for various purposes including, but not limited to, powering our hydrogen economy. During coal gasification, oxygen and steam are used to blow through the coal while it is heated. Regulating and monitoring this process to ensure that there is not a complete oxidizing or combustion of the fuel can be completed with the aid of FBG.

However, among the challenges of FBG specifically in relation to the UV laser-induced method, and particularly for the oil and gas sector applications, is an effect known as hydrogen darkening. Fibre optic sensors are exposed to high pressure hydrogen gas in the downhole environment.  Hydrogen darkening is a physical degradation which blocks the passage of light through the glass of the optical fibres, thereby rendering the fiber optic sensing technologies inoperable. The introduction of fs-IR laser-induced FBG is one solution to this problem, whereby FBGs can be inscribed in fibres resistant to hydrogen darkening.  Among the many benefits of the second generation of FBG technology, alongside its ability to withstand extreme temperatures, is its ability to be written in almost any transparent material.

Akin to this capability, fs-IR laser-induced FBG can also be embedded into other, more “exotic” materials like sapphire to expand its use into even more extreme environments. Unlike traditional glass silica fibre, sapphire fibres can withstand temperatures of up to about 2000 °C.

So we can now see that although there are challenges to overcome, as with just about any technology, we are not bound by these limitations. It is about finding new ways of doing something. Whether used directly or indirectly as part of an overall process, light is one of the tools we constantly use to meet this end. I have no doubt that it will continue to amaze us with its seemingly limitless capabilities for years to come.

D214-16-0351-5957Stephen Mihailov is the group leader of Fibre Photonics at the National Research Council of Canada. Throughout his career Dr. Mihailov has dedicated his time to studying fibre optics and contributing to developments in fibre Bragg grating technology.


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