Superstructure fiber Bragg gratings (sampled SFBGs)

superstructure FBGsThe FBG structure can change with the use of the refractive index, or the grating period. The grating period can be uniform or graded, and either localized or distributed in a superstructure. The refractive index has two main features, the refractive index profile, and the decline. The refractive index can be uniform or apodized, and the refractive index decline is positive or zero.

A superstructure fiber Bragg grating (SFBG), also called a sampled fiber Bragg grating, is a special FBG that consists of several small FBGs placed in close proximity to one another. SFBGs have attracted attention in recent years with the discovery of techniques allowing the creation of equivalent chirp or equivalent phase shifts. The biggest advantage of an SFBG with the equivalent chirp or equivalent phase shifts is the possibility to generate gratings with a greatly fluctuating phase and amplitude by adjusting the spatial profile of the superstructure. The realization of SFBGs with the equivalent chirp or equivalent phase shifts requires only sub-millimeter precision.

 

Superstructure (sampled) fiber Bragg gratings (SFBGs) are good optical filters for optical communication and optical sensor systems, because of their comb-like filter response. The length of SFBG is conventionally limited by that of the phase mask. However, the length of the high-quality phase mask fabricated by an interferometric technique is limited to ~ 5 cm.  The main fabrication technique for long SFBGs based on scanning phase masks and trimming relative phases between FBGs. This technique permits to fabricate of long SFBGs with short phase masks. Superstructure fiber Bragg grating is a novel fiber Bragg grating. Because it is flexible, passive, low insertion loss, and small polarization dependent. It causes great interest and enthusiasm for the people in many areas. Ambient temperature and strain can both affect the sampled grating reflection spectrum.

 

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.

Phase-shifted fiber Bragg gratings (πFBGs)

Phase-shifted FBGsNowadays, the special type of FBGs whose reflection spectrum has a notch arise from a π-phase discontinuity in the center of the grating (called π-phase-shifted FBGs) attracts ground interests among researchers. Because of their highly sensitive ultrasonic detection, πFBG may provide a solution to the sensitivity problems of the FBG. By introducing a π- phase shift into a refractive index modulation of the fiber Bragg grating during its fabrication, the spectral transmission has a narrow bandpass resonance appearing within the middle of the reflection lobe of the FBG. Such an element allows reaching a very narrow transmission band of few picometers.

Fabrication of π-phase-shifted FBGs is achieved by splitting the standard FBG into two identical sub-FBGs with a half-period phase difference between them. The two sub FBGs create an interfere with each other and generate an ultra-narrow transmission window at the center of the FBG spectrum.

Due to the phase discontinuity, a πFBG can be conceptually described as a Fabry–Perot cavity formed by two FBG mirrors. When the two FBGs are highly reflective, the quality factor of the Fabry–Perot cavity is increased, leading to an extremely narrow spectral notch for highly sensitive ultrasonic detection.

Using special structures, even multiple transmission bands are possible.

The primary method used for the fabrication of π-Phase-shifted fiber Bragg grating is based on the UV laser and phase mask method. The occurrence of two peaks/dips is attributed to the refractive index modulations along with the fiber core, with the periodicity of the π-phase mask that has been observed previously in images of gratings that cause destructive interference in a reflected wave at the Bragg condition owing to the phase difference between the grating phases. Thus, the standard phase mask technique produced an alternative type of pi-phase-shifted grating at twice the design Bragg wavelength.

The phase-shifted gratings have found application in distributed feedback lasers, wavelength division multiplexing, athermal setup, or temperature stabilization, as well as to a tuning setup. Also, the π-phase-shifted FBG can be used in highly accurate wavelength references, ASE filtering, spectroscopy, and optical CDMA.

 

Inclination measurement system: incline FBG sensors

Fiber Bragg Grating sensors one of the most requested fiber optic technologies have superior sensitivity and frequency specifications, making them well suited for many spheres of applications. The FBG inclinometers can be used to identify internal damage at a very early stage.  The FBG inclinometers are devices used to monitor subsurface movements through sensors designed to measure inclination with respect to vertical. When installing the FBG inclinometer casing, it is important to select the appropriate diameter. The large-diameter casing is better suited to shear zones, multiple shear zones, and slope failures. Moderate- to small-diameter casing can be used for short-term installations or slopes where smaller displacements distributed along the borehole are anticipated. Correct installation of the casing is important; and deep holes, particularly the influence of helical deformation must be considered. A conventional FBG inclinometer system consists of a plastic casing that is installed in a nearly vertical position in the ground, with a servo-accelerometer or electro-level sensor inserted into the casing to measure the local tilt of the casing in response to ground movement. The sensor element is lowered and raised, guided by grooves in the inner surface of the casing, with the tilt of the casing being recorded at fixed spatial intervals.

Incline FBG sensors have been widely used to monitor ground movements in various applications, for example, landslides, tunnels, and foundations, etc., where they provide vital ground movement information including magnitude, rate, and location. The produced information can be used for checking design assumptions and provide early warning of problems.

Another type of FBG sensors that can monitor inclination is a tiltmeter. Tiltmeters are devices used to monitor the change in the inclination of a ground surface point. The device consists of a gravity sensing transducer capable of measuring changes in inclination as small as one arc second. They are used to monitor slope movements where the landslide failure mode is expected to contain a rotational component. The advantages of using tiltmeters are their lightweight, simple operation, and relatively low cost. They may be combined with an incline FBG sensor and extensometers in what has been termed as integrated pit slope monitoring systems.

Chirped Fiber Bragg Gratings (CFBG) for high-speed fiber optic communications systems

A chirp is a linear variation in the grating period, that can be added to the refractive index profile of the grating. The reflected wavelength fluctuates with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the ability to add dispersion—especially, different wavelengths reflected from the grating will be subject to different delays.

A non-uniform resonance wavelength along the length of the grating in a CFBG can be accomplished by varying the period or by varying the average effective refractive index. The average refractive index can be changed using different methods, for example, changing the amplitude of the reflective index modulation profile or variation the fiber in the region of the grating length. The chirped FBG was manufactured with the usage of a chirped phase mask to generate a variation in the period of the refractive index.

Chirped fiber Bragg gratings have been widely used for dispersion compensations in high-speed fiber optic communications systems because they are able to retard pulsed light depending on its wavelength. Experience has proven that ideas in one field find applications in another. Actually, this type of optical device has been attracting significant attention in the fiber optic sensing community, in high sensitivity sensors or wavelength discriminators in interrogation systems.

There are two prevailing fields of application of chirped FBG: measurement of curvature based on chirped fiber Bragg gratings and new interrogation system, written in an Erbium-doped fiber. The increasing demand for measurement of curvatures has stimulated the appearance of few sensing systems that depend on the intrinsic characteristics of fiber Bragg gratings. A curvature measurement technique using a smart composite consists of two chirped fiber Bragg gratings. The two gratings are embedded on the opposite sides of the composite laminate and serve as curvature sensors and as wavelength discriminators enabling a temperature-independent intensity-based scheme for the measurement of the radius of curvature.

FBG interrogation relies on the usage of the edge filtering concept applied to a chirped fiber Bragg grating written in an erbium-doped fiber as the processing element. Through the combination of the photon amplification of the erbium-doped fiber and of the distributed wavelength reflection characteristics of the chirped FBG, it becomes possible to reach different reading sensitivities and amplification of the remote sensing signal. The ability of chirped FBG has also been employed successfully in the development of interrogation techniques. One of these techniques uses the group-delay in a Sagnac loop interferometer and another the spectra response of broadband chirped gratings.

Apodized Fiber Bragg Gratings (FBG)

Apodized FBGsFiber Bragg Gratings is one of the most meaningful developments in the areas of optical fiber technology, due to their flexibility and unique filtering performance. FBG is defined as the key component in dense wavelength division multiplexing on the basis of their low insertion loss, high wavelength selectivity, low polarization dependent loss, and low polarization modal dispersion.

When light propagates through the FBG in the narrow range of wavelength, the total internal reflection appears at Bragg wavelength and other wavelengths don`t have influence by the Bragg grating except some side lobes existing in the reflection spectrum. These side lobes are sometimes interfering, e.g. in some applications of fiber Bragg gratings as optical filters. They can be largely brought out with the technique of apodized FBG: the strength of the index modulation is smoothly ramped up and down along the grating.

 

The term apodization is concerned with the grading of the refractive index to approach zero at the end of the grating. Apodized gratings introduce the essential improvement in side-lobe suppression while maintaining reflectivity and narrow bandwidth. Gaussian and raised cosine methods are typically used to apodize an FBG. Each method has some special features and different methods of fabrication. The fabrication of apodized Gaussian Bragg gratings is using the two UV-pulse interfere with variable time delay, which strongly reduces the reflectivity at sidelobes and this method makes it possible to write off truly apodized gratings by the single exposure of a uniform phase mask.

The fabrication of Apodized Fiber Bragg Gratings has raised much interest because of its reduced reflectivity at sidelobes. It, therefore, increases the quality of optical filters and improves the dispersion compensation by simultaneously reducing the group delay ripples. Apodized FBG can be used to optimize the parameters of the introduced optical switch and may also prove to be useful as the optical sensing element in a range of other fiber sensor configurations, especially for grating-based chemical sensors, pressure sensors, and accelerometers.

Photonic Sensors: Technology and Applications for Safety Monitoring

Nowadays, photonics is an important part of the innovation-driven growth in an increasing number of fields. The application of photonics is distributed across several sectors: from optical data communications to imaging, lighting, and displays; from the manufacturing sector to life sciences, health care, security, and safety.

Photonic sensors have been designed for use in laboratories and industrial environments in order to detect a wide range of physical, biological, and chemical parameters. 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. In recent years, photonic sensors have been recognized as the technology that improves, extends across, and strengthens a wide variety of industrial sectors: healthcare, security, manufacturing, telecommunications, environment, aerospace, and biotechnology.

The photonic sensors market is classified generally by types and photonics applications. Type wise consist of fiber optic sensors, laser-based sensors, and biophotonic sensors. Application-wise, it is divided into construction, energy industry, oil and gas, military and aerospace, medical and industrial application. The photonic sensors market is diversified and fragmented. Photonic technology also has found a use for the industry, surgeries, and ordinary life besides its use in research laboratories.

The photonic sensors market is slowly moving from making general objectives to complicated tasks such as aircraft manufacturing applications that prescribed maximum precision. The usage of photonic sensing technology is increased in early detection and early-warning systems for biological hazards, structural flaws, and security threats. Sensors play a major role in the near future and, especially, in different photonic applications in the generation, distribution, and conservation of energy, as well as in the mitigation of the effects of energy production and consumption on the environment.

The photonic industry now is centering on the development of eco-efficient products that are projected to be developed and launched over the next few years. The need for enhanced safety and security solutions, better alternatives for conventional technology, and a rise in wireless sensing technology are some of the major factors that act as drivers for the photonic sensor market.