Fiber Bragg grating
Encyclopedia
A fiber Bragg grating is a type of distributed Bragg reflector
constructed in a short segment of optical fiber
that reflects particular wavelength
s of light and transmits all others. This is achieved by adding a periodic variation to the refractive index
of the fiber core, which generates a wavelength specific dielectric mirror
. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
(UV) source such as a UV laser
. Two main processes are used: interference and masking. The method that is preferable depends on the type of grating to be manufactured. Normally a germanium
-doped silica fiber is used in the manufacture of fiber Bragg gratings. The germanium-doped fiber is photosensitive
, in that the refractive index of the core changes with exposure to UV light, with the amount of the change a function of the intensity and duration of the exposure. To write a high reflectivity fiber Bragg grating directly in the fiber the level of doping with germanium needs to be high. However, standard fibers can be used if the photosensitivity is enhanced by soaking the fiber in hydrogen. More recently, fiber Bragg gratings have also been written in polymer fibers, this is described in the PHOSFOS
entry .
is split into two beams which interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light that it is exposed to. This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser
light.
having the intended grating features may also be used in the manufacture of fiber Bragg gratings. The photomask is placed between the UV light source and the photosensitive fiber. The shadow of the photomask then determines the grating structure based on the transmitted intensity of light striking the fiber. Photomasks are specifically used in the manufacture of chirped Fiber Bragg gratings, which cannot be manufactured using an interference pattern.
beam may also be used to 'write' the grating into the fiber point-by-point. Here, the laser
has a narrow beam that is equal to the grating period. This method is specifically applicable to the fabrication of long period fiber gratings
. Point-by-point is also used in the fabrication of tilted gratings.
The fundamental principle behind the operation of a FBG, is Fresnel reflection
. Where light traveling between media of different refractive indices may both reflect
and refract
at the interface.
The grating will typically have a sinusoidal
refractive index variation over a defined length. The reflected wavelength (
), called the Bragg wavelength, is defined by the relationship,
,
where
is the effective refractive index of the grating in the fiber core and 
is the grating period. The effective refractive index quantifies the velocity of propagating light as compared to its velocity in vacuum. 
depends not only on the wavelength but also (for multimode waveguides) on the mode in which the light propagates. For this reason, it is also called modal index.
The wavelength spacing between the first minima (nulls, see Fig. 2), or the bandwidth (
), is given by,
,
where
is the variation in the refractive index (
), and 
is the fraction of power in the core.
The peak reflection (
) is approximately given by,
,
where
is the number of periodic variations. The full equation for the reflected power (
), is given by,
,
where,
.
mechanism by which grating fringes are produced in the fiber. The different methods of creating these fringes have a significant effect on physical attributes of the produced grating, particularly the temperature response and ability to withstand elevated temperatures.
Thus far, five (or six) types of FBG have been reported with different underlying photosensitivity mechanisms. These are summarized below:
Type I gratings are usually known as standard gratings and are manufactured in fibers of all types under all hydrogenation conditions. Typically, the reflection spectra of a type I grating is equal to 1-T where T is the transmission spectra. This means that the reflection and transmission spectra are complementary and there is negligible loss of light by reflection into the cladding or by absorption. Type I gratings are the most commonly used of all grating types, and the only types of grating available off-the-shelf at the time of writing.
Type IA gratings were first published in 2001 during experiments designed to determine the effects of hydrogen loading on the formation of IIA gratings in germanosilicate fiber. In contrast to the anticipated blue shift of the peak Bragg wavelength, a large positive wavelength shift was measured. This type IA grating appeared once the conventional type I FBG had reached saturation followed by subsequent complete or partial erasure, and was therefore labeled as regenerated. It was also noted that the temperature coefficient of the regenerated grating was lower than a standard grating written under similar conditions.
There is a clear relationship between type IA and IIA gratings insomuch as their fabrication conditions are identical in all but one aspect: they both form in B/Ge co-doped fiber but IAs form only in hydrogenated fibers and IIAs form only in non-hydrogenated fibers.
Later research by Xie et al. showed the existence of another type of grating with similar thermal stability properties to the type II grating. This grating exhibited a negative change in the mean index of the fiber and was termed type IIA. The gratings were formed in germanosilicate fibers with pulses from a frequency doubled XeCl pumped dye laser. It was shown that initial exposure formed a standard (type I) grating within the fiber which underwent a small red shift before being erased. Further exposure showed that a grating reformed which underwent a steady blue shift whilst growing in strength.
Archambault et al. showed that it was possible to inscribe gratings of ~100% (>99.8%) reflectance with a single UV pulse in fibers on the draw tower. The resulting gratings were shown to be stable at temperatures as high as 800˚C (up to 1000C in some cases, and higher with femtosecond laser inscription). The gratings were inscribed using a single 40mJ pulse from an excimer laser
at 248 nm. It was further shown that a sharp threshold was evident at ~30mJ; above this level the index modulation increased by more than two orders of magnitude, whereas below 30mJ the index modulation grew linearly with pulse energy. For ease of identification, and in recognition of the distinct differences in thermal stability, they labeled gratings fabricated below the threshold as type I gratings and above the threshold as type II gratings. Microscopic examination of these gratings showed a periodic damage track at the grating’s site within the fiber [10]; hence type II gratings are also known as damage gratings. However, these cracks can be very localized so as to not play a major role in scattering loss if properly prepared
The structure of the FBG can vary via the refractive index, or the grating period. The grating period can be uniform or graded, and either localised or distributed in a superstructure. The refractive index has two primary characteristics, the refractive index profile, and the offset. Typically, the refractive index profile can be uniform or apodized, and the refractive index offset is positive or zero.
There are six common structures for FBGs;
The first complex grating was made by J. Canning in 1994. This supported the development of the first distributed feedback (DFB) fiber lasers, and also laid the groundwork for most complex gratings that followed, including the sampled gratings first made by Peter Hill and colleagues in Australia.
, given as
,
and the grating strength,
. There are, however, three properties that need to be controlled in a FBG. These are the reflectivity, the bandwidth, and the side-lobe strength. As shown above, the bandwidth depends on the grating strength, and not the grating length. This means the grating strength can be used to set the bandwidth. The grating length, effectively 
, can then be used to set the peak reflectivity, which depends on both the grating strength and the grating length. The result of this is that the side-lobe strength cannot be controlled, and this simple optimisation results in significant side-lobes. A third quantity can be varied to help with side-lobe suppression. This is apodization
of the refractive index change. The term apodization refers to the grading of the refractive index to approach zero at the end of the grating. Apodized gratings offer significant improvement in side-lobe suppression while maintaining reflectivity and a narrow bandwidth. The two functions typically used to apodize a FBG are Gaussian and raised-cosine.
. The reflected wavelength changes with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the property of adding dispersion
—namely, different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna systems and polarization mode dispersion compensation, as well.
. They typically have grating periods on the order of 100 micrometers, to a millimeter, and are therefore much easier to manufacture.
The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as notch filters. They are also used in optical multiplexers and demultiplexers with an optical circulator
, or Optical Add-Drop Multiplexer
(OADM). Figure 5 shows 4 channels, depicted as 4 colours, impinging onto a FBG via an optical circulator. The FBG is set to reflect one of the channels, here channel 4. The signal is reflected back to the circulator where it is directed down and dropped out of the system. Since the channel has been dropped, another signal on that channel can be added at the same point in the network.
A demultiplexer can be achieved by cascading multiple drop sections of the OADM, where each drop element uses a FBG set to the wavelength to be demultiplexed. Conversely, a multiplexer can be achieved by cascading multiple add sections of the OADM. FBG demultiplexers and OADMs can also be tunable. In a tunable demultiplexer or OADM, the Bragg wavelength of the FBG can be tuned by strain applied by a piezoelectric transducer. The sensitivity of a FBG to strain is discussed below in fiber Bragg grating sensors.
, the Bragg wavelength is also sensitive to temperature
. This means that fiber Bragg gratings can be used as sensing elements in optical fiber sensors. In a FBG sensor, the measurand causes a shift in the Bragg wavelength,
. The relative shift in the Bragg wavelength, 
, due to an applied strain (
) and a change in temperature (
) is approximately given by,
,
or,
.
Here,
is the coefficient of strain, which is related to the strain optic coefficient 
. Also, 
is the coefficient of temperature, which is made up of the thermal expansion coefficient of the optical fiber, 
, and the thermo-optic coefficient, 
.
Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating. Technically, the absorbent material is the sensing element, converting the amount of gas to a strain. The Bragg grating then transduces the strain to the change in wavelength.
Specifically, fiber Bragg gratings are finding uses in instrumentation applications such as seismology
, pressure sensor
s for extremely harsh environments, and as downhole sensors in oil and gas wells for measurement of the effects of external pressure, temperature, seismic vibrations and inline flow measurement. As such they offer a significant advantage over traditional electronic gauges used for these applications in that they are less sensitive to vibration or heat and consequently are far more reliable. In the 1990s, investigations were conducted for measuring strain and temperature in composite materials for aircraft
and helicopter
structures.
Development Platforms:
Other:
Distributed Bragg reflector
A distributed Bragg reflector is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation...
constructed in a short segment of optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
that reflects particular wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s of light and transmits all others. This is achieved by adding a periodic variation to the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of the fiber core, which generates a wavelength specific dielectric mirror
Dielectric mirror
A dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...
. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
History
The first in-fiber Bragg grating was demonstrated by Ken Hill in 1978. Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Gerald Meltz and colleagues demonstrated the much more flexible transverse holographic technique where the laser illumination came from the side of the fiber. This technique uses the interference pattern of ultraviolet laser light to create the periodic structure of the Bragg grating.Manufacture
Fiber Bragg gratings are created by "inscribing" or "writing" systematic (periodic or aperiodic) variation of refractive index into the core of a special type of optical fiber using an intense ultravioletUltraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) source such as a UV laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
. Two main processes are used: interference and masking. The method that is preferable depends on the type of grating to be manufactured. Normally a germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
-doped silica fiber is used in the manufacture of fiber Bragg gratings. The germanium-doped fiber is photosensitive
Photosensitivity
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...
, in that the refractive index of the core changes with exposure to UV light, with the amount of the change a function of the intensity and duration of the exposure. To write a high reflectivity fiber Bragg grating directly in the fiber the level of doping with germanium needs to be high. However, standard fibers can be used if the photosensitivity is enhanced by soaking the fiber in hydrogen. More recently, fiber Bragg gratings have also been written in polymer fibers, this is described in the PHOSFOS
PHOSFOS
PhosFOS is a research and technology development project co-funded by the European Commission.-Project Description:The PHOSFOS project is developing flexible and stretchable foils or skins that integrate optical sensing elements with optical and electrical devices as well as onboard signal...
entry .
Interference
The first manufacturing method, specifically used for uniform gratings, is the use of two-beam interference. Here the UV laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is split into two beams which interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light that it is exposed to. This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
light.
Photomask
A photomaskPhotomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
having the intended grating features may also be used in the manufacture of fiber Bragg gratings. The photomask is placed between the UV light source and the photosensitive fiber. The shadow of the photomask then determines the grating structure based on the transmitted intensity of light striking the fiber. Photomasks are specifically used in the manufacture of chirped Fiber Bragg gratings, which cannot be manufactured using an interference pattern.
Point-by-point
A single UV laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
beam may also be used to 'write' the grating into the fiber point-by-point. Here, the laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
has a narrow beam that is equal to the grating period. This method is specifically applicable to the fabrication of long period fiber gratings
Long-period fiber grating
A long-period fiber grating couples light from a guided mode into forward propagating cladding modes where it is lost due to absorption and scattering...
. Point-by-point is also used in the fabrication of tilted gratings.
Production
Originally, the manufacture of the photosensitive optical fiber and the 'writing' of the fiber Bragg grating were done separately. Today, production lines typically draw the fiber from the preform and 'write' the grating, all in a single stage. As well as reducing associated costs and time, this also enables the mass production of fiber Bragg gratings. Mass production is in particular facilitating applications in smart structures utilizing large numbers (3000) of embedded fiber Bragg gratings along a single length of fiber.Theory
Fresnel equations
The Fresnel equations , deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices...
. Where light traveling between media of different refractive indices may both reflect
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
and refract
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
at the interface.
The grating will typically have a sinusoidal
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
refractive index variation over a defined length. The reflected wavelength (
), called the Bragg wavelength, is defined by the relationship,,where
is the effective refractive index of the grating in the fiber core and is the grating period. The effective refractive index quantifies the velocity of propagating light as compared to its velocity in vacuum. depends not only on the wavelength but also (for multimode waveguides) on the mode in which the light propagates. For this reason, it is also called modal index.The wavelength spacing between the first minima (nulls, see Fig. 2), or the bandwidth (
), is given by,,where
is the variation in the refractive index (), and is the fraction of power in the core.The peak reflection (
) is approximately given by,,where
is the number of periodic variations. The full equation for the reflected power (), is given by,,where,
.Types of gratings
The term “type” in this context refers to the underlying photosensitivityPhotosensitivity
Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...
mechanism by which grating fringes are produced in the fiber. The different methods of creating these fringes have a significant effect on physical attributes of the produced grating, particularly the temperature response and ability to withstand elevated temperatures.
Thus far, five (or six) types of FBG have been reported with different underlying photosensitivity mechanisms. These are summarized below:
Standard gratings or Type I gratings
Written in both hydrogenated and non-hydrogenated fiber of all typesType I gratings are usually known as standard gratings and are manufactured in fibers of all types under all hydrogenation conditions. Typically, the reflection spectra of a type I grating is equal to 1-T where T is the transmission spectra. This means that the reflection and transmission spectra are complementary and there is negligible loss of light by reflection into the cladding or by absorption. Type I gratings are the most commonly used of all grating types, and the only types of grating available off-the-shelf at the time of writing.
Type IA gratings
- Regenerated grating written after erasure of a type I grating in hydrogenated germanosilicate fiber of all types
Type IA gratings were first published in 2001 during experiments designed to determine the effects of hydrogen loading on the formation of IIA gratings in germanosilicate fiber. In contrast to the anticipated blue shift of the peak Bragg wavelength, a large positive wavelength shift was measured. This type IA grating appeared once the conventional type I FBG had reached saturation followed by subsequent complete or partial erasure, and was therefore labeled as regenerated. It was also noted that the temperature coefficient of the regenerated grating was lower than a standard grating written under similar conditions.
There is a clear relationship between type IA and IIA gratings insomuch as their fabrication conditions are identical in all but one aspect: they both form in B/Ge co-doped fiber but IAs form only in hydrogenated fibers and IIAs form only in non-hydrogenated fibers.
Type IIA (Also known as Type In gratings)
- These are gratings that form as the negative part of the induced index change overtakes the positive part. It is usually associated with gradual relaxation of induced stress along the axis and/or at the interface. It has been proposed that these gratings could be relabeled Type In (for Type 1 gratings with a negative index change; Type II label could be reserved for those that are distinctly made above the damage threshold of the glass).
Later research by Xie et al. showed the existence of another type of grating with similar thermal stability properties to the type II grating. This grating exhibited a negative change in the mean index of the fiber and was termed type IIA. The gratings were formed in germanosilicate fibers with pulses from a frequency doubled XeCl pumped dye laser. It was shown that initial exposure formed a standard (type I) grating within the fiber which underwent a small red shift before being erased. Further exposure showed that a grating reformed which underwent a steady blue shift whilst growing in strength.
Regenerated gratings
These are gratings that are reborn at higher temperatures after erasure of gratings, usually Type I gratings and usually, though not always, in the presence of hydrogen. They have been interpreted in different ways including dopant diffusion (oxygen being the most popular current interpretation) and glass structural change. Recent work has shown that there exists a regeneration regime beyond diffusion where gratings can be made to operate at temperatures in excess of 1295C, outperforming even Type II femtosecond gratings. These are extremely attractive for ultra high temperature applications.Type II gratings
- Damage written gratings inscribed by multiphoton excitation with higher intensity lasers that exceed the damage threshold of the glass. Lasers employed are usually pulsed in order to reach these intensities. They include recent developments in multiphoton excitation using femtosecond pulses where the short timescales (commensurate on a timescale similar to local relaxation times) offer unprecedented spatial localization of the induced change. The amorphous network of the glass is usually transformed via a different ionization and melting pathway to give either higher index changes or create, through micro-explosions, voids surrounded by more dense glass.
Archambault et al. showed that it was possible to inscribe gratings of ~100% (>99.8%) reflectance with a single UV pulse in fibers on the draw tower. The resulting gratings were shown to be stable at temperatures as high as 800˚C (up to 1000C in some cases, and higher with femtosecond laser inscription). The gratings were inscribed using a single 40mJ pulse from an excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
at 248 nm. It was further shown that a sharp threshold was evident at ~30mJ; above this level the index modulation increased by more than two orders of magnitude, whereas below 30mJ the index modulation grew linearly with pulse energy. For ease of identification, and in recognition of the distinct differences in thermal stability, they labeled gratings fabricated below the threshold as type I gratings and above the threshold as type II gratings. Microscopic examination of these gratings showed a periodic damage track at the grating’s site within the fiber [10]; hence type II gratings are also known as damage gratings. However, these cracks can be very localized so as to not play a major role in scattering loss if properly prepared
Grating structure
There are six common structures for FBGs;
- uniform positive-only index change,
- Gaussian apodizedApodizationApodization literally means "removing the foot". It is the technical term for changing the shape of a mathematical function, an electrical signal, an optical transmission or a mechanical structure.- Apodization in signal processing :...
, - raised-cosine apodized,
- chirped,
- discrete phase shift, and
- superstructure.
The first complex grating was made by J. Canning in 1994. This supported the development of the first distributed feedback (DFB) fiber lasers, and also laid the groundwork for most complex gratings that followed, including the sampled gratings first made by Peter Hill and colleagues in Australia.
Apodized gratings
There are basically two quantities that control the properties of the FBG. These are the grating length,, given as,and the grating strength,
. There are, however, three properties that need to be controlled in a FBG. These are the reflectivity, the bandwidth, and the side-lobe strength. As shown above, the bandwidth depends on the grating strength, and not the grating length. This means the grating strength can be used to set the bandwidth. The grating length, effectively , can then be used to set the peak reflectivity, which depends on both the grating strength and the grating length. The result of this is that the side-lobe strength cannot be controlled, and this simple optimisation results in significant side-lobes. A third quantity can be varied to help with side-lobe suppression. This is apodizationApodization
Apodization literally means "removing the foot". It is the technical term for changing the shape of a mathematical function, an electrical signal, an optical transmission or a mechanical structure.- Apodization in signal processing :...
of the refractive index change. The term apodization refers to the grading of the refractive index to approach zero at the end of the grating. Apodized gratings offer significant improvement in side-lobe suppression while maintaining reflectivity and a narrow bandwidth. The two functions typically used to apodize a FBG are Gaussian and raised-cosine.
Chirped fiber Bragg gratings
The refractive index profile of the grating may be modified to add other features, such as a linear variation in the grating period, called a chirpChirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
. The reflected wavelength changes with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the property of adding dispersion
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
—namely, different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna systems and polarization mode dispersion compensation, as well.
Tilted fiber Bragg gratings
In standard FBGs, the grading or variation of the refractive index is along the length of the fiber (the optical axis), and is typically uniform across the width of the fiber. In a tilted FBG (TFBG), the variation of the refractive index is at an angle to the optical axis. The angle of tilt in a TFBG has an effect on the reflected wavelength, and bandwidth.Long-period gratings
Typically the grating period is the same size as the Bragg wavelength, as shown above. For a grating that reflects at 1500 nm, the grating period is 500 nm, using a refractive index of 1.5. Longer periods can be used to achieve much broader responses than are possible with a standard FBG. These gratings are called long-period fiber gratingLong-period fiber grating
A long-period fiber grating couples light from a guided mode into forward propagating cladding modes where it is lost due to absorption and scattering...
. They typically have grating periods on the order of 100 micrometers, to a millimeter, and are therefore much easier to manufacture.
Communications
Optical circulator
An optical circulator is a special fiber-optic component that can be used to separate optical signals that travel in opposite directions in an optical fiber, analogous to the operation of an electronic circulator. An optical circulator is a three-port device designed such that light entering any...
, or Optical Add-Drop Multiplexer
Optical add-drop multiplexer
An optical add-drop multiplexer is a device used in wavelength-division multiplexing systems for multiplexing and routing different channels of light into or out of a single mode fiber . This is a type of optical node, which is generally used for the construction of optical telecommunications...
(OADM). Figure 5 shows 4 channels, depicted as 4 colours, impinging onto a FBG via an optical circulator. The FBG is set to reflect one of the channels, here channel 4. The signal is reflected back to the circulator where it is directed down and dropped out of the system. Since the channel has been dropped, another signal on that channel can be added at the same point in the network.
A demultiplexer can be achieved by cascading multiple drop sections of the OADM, where each drop element uses a FBG set to the wavelength to be demultiplexed. Conversely, a multiplexer can be achieved by cascading multiple add sections of the OADM. FBG demultiplexers and OADMs can also be tunable. In a tunable demultiplexer or OADM, the Bragg wavelength of the FBG can be tuned by strain applied by a piezoelectric transducer. The sensitivity of a FBG to strain is discussed below in fiber Bragg grating sensors.
Fiber Bragg grating sensors
As well as being sensitive to strainStrain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
, the Bragg wavelength is also sensitive to temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. This means that fiber Bragg gratings can be used as sensing elements in optical fiber sensors. In a FBG sensor, the measurand causes a shift in the Bragg wavelength,
. The relative shift in the Bragg wavelength, , due to an applied strain () and a change in temperature () is approximately given by,,or,
.Here,
is the coefficient of strain, which is related to the strain optic coefficient . Also, is the coefficient of temperature, which is made up of the thermal expansion coefficient of the optical fiber, , and the thermo-optic coefficient, .Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating. Technically, the absorbent material is the sensing element, converting the amount of gas to a strain. The Bragg grating then transduces the strain to the change in wavelength.
Specifically, fiber Bragg gratings are finding uses in instrumentation applications such as seismology
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
, pressure sensor
Pressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
s for extremely harsh environments, and as downhole sensors in oil and gas wells for measurement of the effects of external pressure, temperature, seismic vibrations and inline flow measurement. As such they offer a significant advantage over traditional electronic gauges used for these applications in that they are less sensitive to vibration or heat and consequently are far more reliable. In the 1990s, investigations were conducted for measuring strain and temperature in composite materials for aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
and helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
structures.
See also
- Bragg's lawBragg's lawIn physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
- Bragg diffraction
- DiffractionDiffractionDiffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
- Diffraction gratingDiffraction gratingIn optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
- Diffraction grating
- Dielectric mirrorDielectric mirrorA dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...
- Hydrogen sensor
- Long-period fiber gratingLong-period fiber gratingA long-period fiber grating couples light from a guided mode into forward propagating cladding modes where it is lost due to absorption and scattering...
- Photonic crystal fiber
- Distributed temperature sensingDistributed temperature sensingDistributed temperature sensing systems are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature...
by fiber optics - PHOSFOSPHOSFOSPhosFOS is a research and technology development project co-funded by the European Commission.-Project Description:The PHOSFOS project is developing flexible and stretchable foils or skins that integrate optical sensing elements with optical and electrical devices as well as onboard signal...
project - embedding FBGs in flexible skins
External links
International Optical Sensor Societies:- FOSNE - Fibre Optic Sensing Network Europe
Development Platforms:
- TFT - Technobis Fibre Technologies
Other:
- Bragg grating as hydrogen detector
- http://www.patentgenius.com/patent/7133582.html - Fiber - Optic filter with tunable grating