A tunable fiber Bragg grating (FBG) optical filters

FBGs in optical filtersIn order to make the fiber Bragg grating tunable (it means controlling the reflected wavelength), the Bragg grating period must be controllable. This is achieved by one of several methods. For example, the application of a stretching force elongates the fiber, which thus changes its period (mechanical tuning). Mechanical tuning of the FBG results in a faster and large response in terms of wavelength shift.  The application of heat elongates the fiber, thus changing its period (thermal tuning). Changing the temperature of the FBG results in a slow wavelength shift; hence, a small tuning range is achieved.  FBG final applications are in fiber dispersion compensation, in gain flattening of erbium-doped fiber amplifiers, and in add-drop multiplexers/demultiplexers. However, the fiber used to make an FBG should be free of imperfections as well as microscopic variations of the refractive index.

Tunable FBG is a valuable option for all applications that require flexibility in center wavelength (and/or frequency). The tuning setup consists of the mechanical assembly where the FBG is applied to. The mechanics induce strain to the FBG, shifting its center wavelength homogeneously and chirp-free.

Common FBGs have the flat top response, low cost, and low insertion loss that meets the requirement of add/drop FBG multiplexers in the optical WDM fiber transmission systems. However, a great deal of research interest in FBGs involves the property of being tunable in both the Bragg wavelength and bandwidth. Recent studies have shown large tuning ranges of 110 nm in the Bragg wavelength and more than 10 nm in the 3-dB bandwidth for uniform FBGs. Given such desirable tunability, FBG is becoming a very flexible and “smart” optical fiber component. Bandwidth-tunable FBG optical filters have been studied in many applications, such as tunable dispersion compensation, phased array antenna, and temperature-independent fiber grating sensing.

The important parameters of light sources for FBG interrogators are optical power, tuning range, tuning speed, and continuous tuning capabilities. Among the tunable laser sources are DFB lasers, multi-section distributed Bragg reflector lasers, and external cavity lasers.

Bragg mirror (reflector) features and technologies

FBGs in Bragg mirrorA Bragg mirror (also known as distributed Bragg reflector / dielectric mirror) is a type of mirror consist of composite thin layers of dielectric material, usually settled on a substrate of glass or other optical material. By careful choice of the type and thickness of the dielectric layers, one can project optical coating with specified reflectivity at various wavelengths. Bragg mirrors can be adapted to obtain any reflectivity between 0% and nearly 100% for a specific target wavelength.

Different technologies can be used to fabricate Bragg mirrors. First are the thin-film coating technology, fabricated, for example, by electron beam evaporation or with ion beam sputtering. These technologies are used as laser mirrors in solid-state bulk lasers. Second are Fiber Bragg gratings, including long-period fiber gratings, which are often used in fiber lasers and other fiber devices. They can be fabricated by irradiating a fiber with spatially patterned ultraviolet light. Next, are lithographic methods for semiconductor Bragg mirrors. They are used, for example, in surface-emitting semiconductor lasers and semiconductor saturable absorber mirrors, but also as separate optical components.

There are also different types of Bragg reflectors used in other waveguides, based on, for example, corrugated waveguide structures that can be fabricated via lithography. Such kind of gratings is used in some distributed Bragg reflector or distributed feedback laser diodes.

There are some advantages of using Bragg mirrors:

  1. Low-loss, if the dielectric material is transparent to the wavelength under study. That means it can exhibit ultra-high reflectance, useful for a high-Q cavity.
  2. Broadband. It can be as broad as metal, at a much higher reflectance.
  3. Foreseeable reflection phase.

Bragg mirrors found their applications in laser cavity end mirrors, hot and cold mirrors, thin-film beam splitters, and the coatings on new-day mirrorshades.


Pipeline Integrity, Safety, Structural health Monitoring and leak detection with the Fiber Optic Sensor system

Distributed fiber optic sensing is tested through practice technology for online monitoring of temperature, strain, vibration, and sound over long distances with the high local resolution that is apt to improve pipeline integrity, pipeline safety, and security considerations. Fiber optics distributed temperature sensing techniques have found applications in various domains such as civil engineering, oil, and gas, power plants, fire detection, etc.

Pipelines are the most modern, effective, and reliable global transport systems for oil, gas, and water. In order to guarantee the smooth transport of products the pipeline systems must be regularly maintained and monitoring. Optical sensing systems today facilitate pipeline monitoring and integrity as preventive means to continuously protect or monitor pipelines. The systems are based on interferometric sensing where ultra-stable, low noise lasers interrogate an optical fiber acting as one long continuous sensor embedded or attached to the pipeline. The monitoring of temperature profiles over long distances by means of optical fibers represents a highly efficient way to fiber optic leak detection along pipelines, in dams, dikes, or tanks. Different techniques have been developed taking advantage of the fiber geometry and of optical time-domain analysis for the localization of the information.

Distributed temperature sensing is used in all cases to improve the performance of computational monitoring systems. Although distributed temperature sensing is a well-proven technology that has shown to be able to detect very small leaks in a short time, it is very hard to calculate the minimum detectable leak size or to guarantee a maximum detection time which in many cases are necessary to receive pipeline operation licenses. Leak rates as low as 50 ml/min have been detected on a brine pipeline by temperature monitoring and identification of local temperature anomalies.

GeoHazards like earthquakes, landslides, and surface subsidence result in ground movement and thus put additional stress on the pipelines, tunnels, and other underground infrastructures. Structural health monitoring is a promising and challenging field of research in the 21st century. Civil structures are the most precious economic assets of any country, proper monitoring of these are necessary to prevent any hazardous situation and ensuring safety. Fiber Bragg Grating (FBG) sensors have emerged as a reliable, in situ, nondestructive device for monitoring, diagnostics, and control in civil structures. FBG sensors offer several key advantages over other technologies in the structural sensing field.

The transformer is the key equipment in a power system, to ensure its safe and stable operation is important. Transformers either raise a voltage to decrease losses or decreases the voltage to a safe level. The failures of transformers in service are broadly due to temperature rise, low oil levels, overload, poor quality of LT cables, and improper installation and maintenance. Out of these factors temperature rise, low oil levels and overload, need continuous monitoring to save transformer life. A DTS system increases the reliability of the distribution network, by monitoring critical information such as oil temperature, and the oil level of the transformer. Data are collected continuously. Monitoring the transformers for problems before they occur can prevent faults that are costly to fix and result in a loss of service life. With modern technology, it is possible to monitor a large number of parameters of a transformer monitoring system at a relatively high cost. At the present day, the challenge is to balance the functions of the monitoring system and its cost and reliability. 

Fiber Optic Sensor Solutions: today’s field experience of sensing system

Fiber optic sensor solutions have created new opportunities in general industry applications. The fiber optic sensors (temperature, pressure, deformation, and displacement) are designed to deliver precise measurements in harsh environments and in the case of electromagnetic interference, magnetic resonance imaging, radiofrequency, microwave, and high voltage applications.

The greatest advantages of the fiber optic sensors are its inert material. It is also an optimum transducer for use in harsh environments, like in geotechnical applications.

Fiber optic sensors more and more are attracting attraction in the aerospace and defense due to resistance to chemical corrosion, high pressure, high voltage, and lighting environments. Its lightweight and small size permit weight reduction on different monitoring areas and the capacity of long-range operation still conserving high sensitivity and large bandwidth ensuring a long term reliable monitoring.

Chemistry and food are both often related to microwave radiation. Gallium Arsenide (GaAs) semiconductor crystal is non-conductive, offers high temperature operating capability, they are suitable for microwave heated installations chemical digestion under pressure and temperature conditions. A fiber optic sensor can also be used in the food industry where microwaves are used in new product development to identify precise safe and effective cooking temperatures and time.

The inherent benefits of fiber optic point sensing technology for structural health monitoring applications. In addition, they are not affected by the external environment, large temperature variation, transversal strain, or lightning.

The renewable energy market is developing quite rapidly over the last years. With wind farms or hydrokinetic systems, there is a big need for low maintenance monitoring systems that are reliable and stable for a long period. With the property of being immune to EMI and insensitive to lightning strikes, it makes it a great fit for the growing renewable energy market.

The fiber optic sensing system is the ideal solution for monitoring well pressure and temperature in thermal recovery applications like steam-assisted gravity drainage (SAGD) or cyclic steam stimulation (CSS). The FBG sensors can be used in coiled or production tubing in production and injection wells giving accurate properly measurements for reservoir surveillance, process optimization, or pump control.

In-situ and continuous monitoring of the pressure in the producing for the better understanding of the reservoir condition FBG sensors rapidly detect steam breakthrough and efficiently diagnose changes in the reservoir.  Optimization of the production process forces the oil recovery rate and reduces the operating costs associated with steam injection and oil recovery.  A fiber optic sensing solution is offered in both single and multiple-point configurations. Hence, it gives the operators a large choice of real-time monitoring from monitoring well pumps to profiling a producing well. The application “downhole pressure and temperature measurement solution” is only one example of what is possible with fiber optic technology. The fiber optic measurement solutions are custom made according to the requirements.