|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| A '''fiber Bragg grating''' ('''FBG''') 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 creating a periodic variation in 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.
| | Ed is what individuals contact me and my wife doesn't like it at all. To perform lacross is some thing I truly appreciate doing. Credit authorising is how she tends to make a residing. Mississippi is exactly where her home is but her spouse desires them to transfer.<br><br>my blog post :: [http://www.herandkingscounty.com/content/information-and-facts-you-must-know-about-hobbies psychics online] |
| | |
| [[File:Fiber Bragg Grating-en.svg|thumbnail|right|400px|Figure 1: A Fiber Bragg Grating structure, with refractive index profile and spectral response]]
| |
| | |
| ==History==
| |
| The first in-fiber Bragg grating was demonstrated by Ken Hill in 1978.<ref>{{cite journal |last=Hill |first=K.O. |year=1978 |title=Photosensitivity in optical fiber waveguides: application to reflection fiber fabrication |journal=Appl. Phys. Lett. |volume=32 |issue=10 |pages=647 |doi=10.1063/1.89881 |last2=Fujii |first2=Y. |last3=Johnson |first3=D. C. |last4=Kawasaki |first4=B. S. |bibcode = 1978ApPhL..32..647H }}</ref> 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 inscription technique where the laser illumination came from the side of the fiber. This technique uses the [[#Interference|interference pattern of ultraviolet laser light]]<ref>{{cite journal |last=Meltz |first=G. |coauthors=''et al.'' |year=1989 |title=Formation of Bragg gratings in optical fibers by a transverse holographic method |journal=Opt. Lett. |volume=14 |pages=823 |doi=10.1364/OL.14.000823 |pmid=19752980 |issue=15 |bibcode = 1989OptL...14..823M }}</ref> to create the periodic structure of the fiber 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 [[ultraviolet]] (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 [[photosensitivity|photosensitive]], which means that the refractive index of the core changes with exposure to UV light. The amount of the change depends on the intensity and duration of the exposure as well as the photosensitivity of the fibre. 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 pre-soaking the fiber in hydrogen. More recently, fiber Bragg gratings have also been written in polymer fibers, this is described in the [[PHOSFOS]] entry.<ref>http://www.phosfos.eu/eng/Phosfos/Journals/Bragg-grating-in-polymer-optical-fibre-for-strain-bend-and-temperature-sensing</ref>
| |
| | |
| ===Interference===
| |
| This was the first method used widely for the fabrication of fiber Bragg gratings and uses two-beam [[Interference (wave propagation)|interference]]. Here the UV [[laser]] 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.
| |
| | |
| ===Sequential writing===
| |
| Complex grating profiles can be manufactured by exposing a large number of small, partially overlapping gratings in sequence. Advanced properties such as phase shifts and varying modulation depth can be introduced by adjusting the corresponding properties of the subgratings. <ref>R. Stubbe, B. Sahlgren, S. Sandgren, and A. Asseh, "Novel technique for writing long superstructured fiber Bragg gratings," in Postdeadlin Papers, Photosensitivity and Quadratic Nonlinearity in Glass Waveguides: Fundamentals and Applications, Vol. 22of 1995 Technical Digest Series (Optical Society of America, Washington, D.C., 1995), p. PD 1.</ref> In the first version of the method, subgratings were formed by exposure with UV pulses, but this approach had several drawbacks, such as large energy fluctuations in the pulses and low average power. A sequential writing method with continuous UV radiation that overcomes these problems has been demonstrated and is now used commercially. <ref>I. Petermann, B. Sahlgren, S. Helmfrid, and A.T. Friberg, "Fabrication of advanced fiber Bragg gratings by use of sequential writing with a continuous-wave ultraviolet laser source," Applied Optics, vol. 41, p. 1051-1056 (2002).</ref> The photosensitive fiber is translated by an interferometrically controlled airbearing borne carriage. The interfering UV beams are focussed onto the fiber, and as the fiber moves, the fringes move along the fiber by translating mirrors in an interferometer. As the mirrors have a limited range, they must be reset every period, and the fringes move in a sawtooth pattern. All grating parameters are accessible in the control software, and it is therefore possible to manufacture arbitrary gratings structures without any changes in the hardware.
| |
| | |
| ===Photomask===
| |
| A [[photomask]] 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|chirped Fiber Bragg gratings]], which cannot be manufactured using an interference pattern.
| |
| | |
| ===Point-by-point===
| |
| A single UV [[laser]] 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 grating|long period fiber gratings]]. 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 [[Optical fiber#Preform|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 structure]]s utilizing large numbers (3000) of embedded fiber Bragg gratings along a single length of fiber.
| |
| | |
| ==Theory==
| |
| {{expert-subject|Physics|section|talk=Bragg Bandwidth Theory|date=December 2011}}
| |
| [[File:Fbg2.GIF|thumbnail|right|400px|Figure 2: FBG reflected power as a function of wavelength]]
| |
| The fundamental principle behind the operation of a FBG is [[Fresnel equations|Fresnel reflection]],where light traveling between media of different refractive indices may both [[Reflection (physics)|reflect]] and [[Refraction|refract]] at the interface.
| |
| | |
| The refractive index will typically alternate over a defined length. The reflected wavelength (<math>\scriptstyle \lambda_B</math>), called the Bragg wavelength, is defined by the relationship,
| |
| | |
| :<math>\lambda_B = 2 n_e \Lambda\,</math>
| |
| | |
| where <math>\scriptstyle n_e</math> is the effective refractive index of the grating in the fiber core and <math>\scriptstyle \Lambda</math> is the grating period. The effective refractive index quantifies the velocity of propagating light as compared to its velocity in vacuum. <math>\scriptstyle n_e</math> 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 (<math>\scriptstyle \Delta\lambda</math>), is (in the strong grating limit) given by,
| |
| | |
| :<math>\Delta \lambda=\left[\frac{2 \delta n_0 \eta}{\pi}\right]\lambda_B</math>
| |
| | |
| where <math>\scriptstyle \delta n_0</math> is the variation in the refractive index (<math>\scriptstyle n_3 \,-\, n_2</math>), and <math>\scriptstyle \eta</math> is the fraction of power in the core. Note that this approximation does not apply to weak gratings where the grating length, <math>\scriptstyle L_g</math>, is not large compared to <math>\scriptstyle \lambda_B</math> \ <math>\scriptstyle \delta n_0</math>.
| |
| | |
| The peak reflection (<math>\scriptstyle P_B(\lambda_B)</math>) is approximately given by,
| |
| | |
| :<math>P_B(\lambda_B) \approx \tanh^2 \left[\frac{N \eta (V) \delta n_0}{n}\right]</math>
| |
| | |
| where <math>\scriptstyle N</math> is the number of periodic variations. The full equation for the reflected power (<math>\scriptstyle P_B(\lambda)</math>), is given by,
| |
| | |
| :<math>
| |
| P_B(\lambda) =
| |
| \frac
| |
| {\sinh^2\left[\eta (V) \delta n_0 \sqrt{1-\Gamma^2} \frac{N \Lambda}{\lambda}\right]}
| |
| {\cosh^2\left[\eta (V) \delta n_0 \sqrt{1-\Gamma^2} \frac{N \Lambda}{\lambda}\right] - \Gamma^2}
| |
| </math>
| |
| | |
| where,
| |
| | |
| :<math>\Gamma (\lambda) = \frac{1}{\eta (V) \delta n_0}\left[\frac{\lambda}{\lambda_B} - 1\right]</math>
| |
| | |
| ==Types of gratings==
| |
| The term ''type'' in this context refers to the underlying [[photosensitivity]] 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.<ref>J. Canning, Fiber Gratings and Devices for Sensors and Lasers, Lasers and Photonics Reviews, 2 (4), 275-289, Wiley, USA (2008)</ref> These are summarized below:
| |
| | |
| ===Standard, or type I, gratings===
| |
| Written in both hydrogenated and non-hydrogenated fiber of all types, 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===
| |
| * Regenerated grating written after erasure of a type I grating in hydrogenated germanosilicate fiber of all types
| |
| | |
| Type IA gratings were first observed in 2001<ref name="Liu, Y.">{{cite journal | author =Liu, Y. | title =Advanced fiber gratings and their application | publisher =Ph.D. Thesis, Aston University | year =2001 }}</ref> during experiments designed to determine the effects of hydrogen loading on the formation of IIA gratings in germanosilicate fiber. In contrast to the anticipated decrease (or 'blue shift') of the gratings' Bragg wavelength, a large increase (or 'red shift') was observed.
| |
| | |
| Later work showed that the increase in Bragg wavelength began once an initial type I grating had reached peak reflectivity and begun to weaken. For this reason, it was labeled as a regenerated grating.
| |
| | |
| Determination of the type IA gratings' temperature coefficient showed that it was lower than a standard grating written under similar conditions.
| |
| | |
| The key difference between the inscriprion of type IA and IIA gratings is that IA gratings are written in hydrogenated fibres, whereas type IIA gratings are written in non-hydrogenated fibres.<ref>{{cite journal
| |
| | author =Simpson, A. G.
| |
| | title =Optical Fibre Sensors and Their Interrogationdv
| |
| | publisher =Ph.D. Thesis, Aston University
| |
| | year =2005}}</ref><ref>{{cite conference
| |
| | first =A. G.
| |
| | last =Simpson
| |
| | coauthors =Kalli, K.; Zhou, K.; Zhang, L.; Bennion, I.
| |
| | title =A method for the fabrication of temperature compensating IA-I strain sensors
| |
| | booktitle =OFS16
| |
| | pages =postdeadline paper PD4
| |
| | publisher =
| |
| | year =2003
| |
| | location =Nara, Japan}}</ref>
| |
| | |
| ===Type IIA, or 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).<ref>For a contemporary review, see J. Canning, Fiber Gratings and Devices for Sensors and Lasers, Lasers and Photonics Reviews, 2 (4), 275-289, Wiley, USA (2008)</ref>
| |
| | |
| 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.<ref>{{cite journal
| |
| | last =Xie
| |
| | first =W. X.
| |
| | coauthors =Niay, P.; Bernage, P.; Douay, M.; Bayon, J. F.; Georges, T.; Monerie, M.; Poumellec, B.
| |
| | title =Experimental-Evidence of 2 Types of Photorefractive Effects Occurring During Photo inscriptions of Bragg Gratings Within Germanosilicate Fibres
| |
| | journal =Optics Communications
| |
| | volume =104
| |
| | issue =1-3
| |
| | pages =185–195
| |
| | year =1993
| |
| | doi =10.1016/0030-4018(93)90127-Q|bibcode = 1993OptCo.104..185X }}</ref><ref>{{cite journal
| |
| | last =Niay
| |
| | first =P.
| |
| | coauthors =Bernage, P.; Legoubin, S.; Douay, M.; Xie, W. X.; Bayon, J. F.; Georges, T.; Monerie, M.; Poumellec, B.
| |
| | title =Behaviour of Spectral Transmissions of Bragg Gratings Written in Germania-Doped Fibres - Writing and Erasing Experiments Using Pulsed or CW UV Exposure
| |
| | journal =Optics Communications
| |
| | volume =113
| |
| | issue =1-3
| |
| | pages =176–192
| |
| | year =1994
| |
| | doi =10.1016/0030-4018(94)90606-8|bibcode = 1994OptCo.113..176N }}</ref>
| |
| | |
| ===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 1,295 °C, outperforming even type II femtosecond gratings.<ref>J. Canning, M. Stevenson, S. Bandyopadhyay, K. Cook, “Extreme silica optical fibre gratings”, Sensors, 8, pp.1-5, (2008)</ref> 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 1,000 °C in some cases, and higher with femtosecond laser inscription). The gratings were inscribed using a single 40 mJ pulse from an [[excimer laser]] at 248 nm. It was further shown that a sharp threshold was evident at ~30 mJ; above this level the index modulation increased by more than two orders of magnitude, whereas below 30 mJ 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 <ref>{{cite journal
| |
| | last =Dong
| |
| | first =L.
| |
| | coauthors =Archambault, J. L.; Reekie, L.; Russell, P. S. J.; Payne, D. N.
| |
| | title =Single-Pulse Bragg Gratings Written During Fibre Drawing
| |
| | journal =Electronics Letters
| |
| | volume =29
| |
| | issue =17
| |
| | pages =1577–1578
| |
| | year =1993
| |
| | doi =10.1049/el:19931051}}</ref><ref>{{cite journal
| |
| | last =Archambault
| |
| | first =J. L.
| |
| | coauthors =Reekie, L.; Russell, P. S. J.
| |
| | title =100-Percent Reflectivity Bragg Reflectors Produced in Optical Fibres By Single Excimer-Laser Pulses
| |
| | journal =Electronics Letters
| |
| | volume =29
| |
| | issue =5
| |
| | pages =453–455
| |
| | year =1993
| |
| | doi =10.1049/el:19930303}}</ref>
| |
| | |
| ==Grating structure==
| |
| [[File:Fbg3.GIF|thumbnail|right|300px|Figure 3: Structure of the refractive index change in a uniform FBG (1), a chirped FBG (2), a tilted FBG (3), and a superstructure FBG (4).]]
| |
| [[File:Fbg4.GIF|thumbnail|right|300px|Figure 4: Refractive index profile in the core of, 1) a uniform positive-only FBG, 2) a Gaussian-apodized FBG, 3) a raised-cosine-apodized FBG with zero-dc change, and 4) a discrete phase shift FBG.]]
| |
| | |
| 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;<ref>{{cite journal | last=Erdogan | first=Turan | title=Fiber Grating Spectra | journal=Journal of Lightwave Technology | url=http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=618322&isnumber=13456 | volume=15| issue=8 | pages=1277–1294 |date=August 1997 | doi=10.1109/50.618322|bibcode = 1997JLwT...15.1277E }}</ref>
| |
| # uniform positive-only index change,
| |
| # [[Gaussian function|Gaussian]] [[apodization|apodized]],
| |
| # [[Raised cosine|raised-cosine]] apodized,
| |
| # [[#Chirped fiber Bragg gratings|chirped]],
| |
| # discrete phase shift, and
| |
| # superstructure.
| |
| The first complex grating was made by J. Canning in 1994.<ref>J. Canning, M. G. Sceats, "p-phase-shifted periodic distributed structures in germanosilicate fibre by UV post-processing", Electron. Lett., 30, (16), 1344-1345, (1994)</ref>{{Citation needed|date=May 2009}} 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.{{Citation needed|date=May 2009}}
| |
| | |
| ===Apodized gratings===
| |
| There are basically two quantities that control the properties of the FBG. These are the grating length, <math>\scriptstyle L_g</math>, given as
| |
| | |
| :<math>L_g = N\Lambda\,</math>
| |
| | |
| and the grating strength, <math>\scriptstyle \delta n_0 \eta</math>. 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 [[#Theory|above]], in the strong grating limit (i.e., for large <math>\scriptstyle \delta n_0</math>) 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 <math>\scriptstyle N</math>, 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.
| |
| | |
| ===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 [[chirp]]. The reflected wavelength changes with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the property of adding [[Dispersion (optics)|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.
| |
| | |
| ===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.{{clarify|date=September 2012}}
| |
| | |
| ===Long-period gratings===
| |
| Typically the grating period is the same size as the Bragg wavelength, as shown [[#Theory|above]]. For a grating that reflects at 1,500 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 grating]]. They typically have grating periods on the order of 100 micrometers, to a millimeter, and are therefore much easier to manufacture.
| |
| | |
| ==Applications==
| |
| | |
| ===Communications===
| |
| [[File:Fbg oadm.GIF|thumbnail|right|400px|Figure 5: Optical add-drop multiplexer.]]
| |
| The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as [[notch filter]]s. They are also used in optical [[multiplexers]] and [[demultiplexer]]s 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 an 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]].
| |
| | |
| ===Fiber Bragg grating sensors===
| |
| As well as being sensitive to [[Strain (materials science)|strain]], the Bragg wavelength is also sensitive to [[temperature]]. This means that fiber Bragg gratings can be used as sensing elements in [[optical fiber sensor]]s. In a FBG sensor, the measurand causes a shift in the Bragg wavelength, <math>\scriptstyle \Delta \lambda_B</math>. The relative shift in the Bragg wavelength, <math>\scriptstyle \Delta \lambda_B/\lambda_B</math>, due to an applied strain (<math>\scriptstyle \epsilon</math>) and a change in temperature (<math>\scriptstyle \Delta T</math>) is approximately given by,
| |
| | |
| :<math>\left[\frac{\Delta \lambda_B}{\lambda_B}\right] = C_S\epsilon + C_T\Delta T</math>
| |
| | |
| or,
| |
| | |
| :<math>\left[\frac{\Delta \lambda_B}{\lambda_B}\right] = (1-p_e)\epsilon + (\alpha_\Lambda + \alpha_n)\Delta T</math> | |
| | |
| Here, <math>\scriptstyle C_S</math> is the ''coefficient of strain'', which is related to the ''strain optic coefficient'' <math>\scriptstyle p_e</math>. Also, <math>\scriptstyle C_T</math> is the ''coefficient of temperature'', which is made up of the ''thermal expansion coefficient'' of the optical fiber, <math>\scriptstyle \alpha_\Lambda</math>, and the ''thermo-optic coefficient'', <math>\scriptstyle \alpha_n</math>.<ref>{{cite book |last=Othonos |first=Andreas |coauthors=Kalli, Kyriacos |year=1999 |title=Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing |publisher=Artech House |isbn=0-89006-344-3 }}</ref>
| |
| | |
| 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]],<ref>{{cite journal |title=On the possible use of optical fiber Bragg gratings as strain sensors for geodynamical monitoring |author=P. Ferraro |coauthors=G. De Natale |journal=Optics and Lasers in Engineering |volume=37 |issue=2-3 |pages=115–130 |year=2002 |doi=10.1016/S0143-8166(01)00141-5|bibcode = 2002OptLE..37..115F }}</ref> [[pressure sensor]]s for extremely harsh environments, and as [[downhole sensor]]s 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.<ref>{{Ref patent
| |
| |country=US
| |
| |number=5493390
| |
| |status=patent
| |
| |title=Integrated optical instrumentation for the diagnostics of parts by embedded or surface attached optical sensors
| |
| |gdate=Feb. 20, 1996
| |
| }}</ref><ref>{{Ref patent
| |
| |country=US
| |
| |number=5399854
| |
| |status=patent
| |
| |title=Embedded optical sensor capable of strain and temperature measurement using a single diffraction grating
| |
| |invent1=J.R. Dunphy
| |
| |invent2=''et al.''
| |
| |gdate=March 21, 1995
| |
| }}</ref>
| |
| | |
| ===Fiber bragg gratings used in fiber lasers===
| |
| Recently the development of high power fiber lasers has generated a new set of applications for fiber Bragg gratings (FBG’s), operating at power levels that were previously thought impossible. In the case of a simple fiber laser, the FBG’s can be used as the high reflector (HR) and output coupler (OC) to form the laser cavity. The gain for the laser is provided by a length of rare earth doped optical fiber, with the most common form using Yb3+ ions as the active lasing ion in the silica fiber. These Yb-doped fiber lasers first operated at the 1 kW CW power level in 2004 <ref>Jeong, Y., Sahu, J.K., Payne,D.N. and Nilsson., J., “Ytterbium-doped large-core fiber laser with 1kW continuous-wave output power”, Electronics Letters, 40: 470-472, 2004</ref> based on free space cavities but were not shown to operate with fiber Bragg grating cavities until much later.<ref>2. Xiao, Y., Brunet, F., Kanskar, M., Faucher, M., Wetter, A., and Holehouse, N., “1-kilowatt CW all-fiber laser oscillator pumped with wavelength-beam-combined diode stacks”, Optics Express, 20: 3296-3301, 2012.</ref>
| |
| | |
| Such monolithic, all-fiber devices are produced by many companies worldwide and at power levels exceeding 1 kW. The major advantage of these all fiber systems, where the free space mirrors are replaced with a pair of fiber Bragg gratings (FBG’s), is the elimination of realignment during the life of the system, since the FBG is spliced directly to the doped fiber and never needs adjusting. The challenge is to operate these monolithic cavities at the kW CW power level in large mode area (LMA) fibers such as 20/400 (20 um diameter core and 400 um diameter inner cladding) without premature failures at the intra-cavity splice points and the gratings. Once optimized, these monolithic cavities do not need realignment during the life of the device, removing any cleaning and degradation of fiber surface from the maintenance schedule of the laser. However, the packaging and optimization of the splices and FBGs themselves are non-trivial at these power levels as are the matching of the various fibers, since the composition of the Yb-doped fiber and various passive and photosensitive fibers needs to be carefully matched across the entire fiber laser chain. Although the power handling capability of the fiber itself far exceeds this level, and is possibly as high as >30 kW CW, the practical limit is much lower due to component reliability and splice losses.<ref>3. Dawson, J.W., Messerly, M.J., Beach, R.J., Shverdin, M.Y., Stappaerts, E.A., Sridharan, A.K., Pax, P.H., Heebner, J.E., Siders, C.W. and Barty, C.J.P.,“Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power” Optics Express, 16: 13240-13260, 2008</ref>
| |
| | |
| ===Process of matching active and passive fibers===
| |
| In a double-clad fiber there are two waveguides – the Yb-doped core that forms the signal waveguide and the inner cladding waveguide for the pump light. The inner cladding of the active fiber is often shaped to scramble the cladding modes and increase pump overlap with the doped core. The matching of active and passive fibers for improved signal integrity requires optimization of the core/clad concentricity, and the MFD through the core diameter and NA, which reduces splice loss. This is principally achieved by tightening all of the pertinent fiber specifications.<ref>Oulundsen, G., Farley, K. , Abramczyk, J. and Wei, K. “Fiber for fiber lasers: Matching active and passive fibers improves fiber laser performance”, Laser Focus World, Vol 48 Jan 2012. http://www.nufern.com/library/item/id/391/</ref>
| |
| | |
| Matching fibers for improved pump coupling requires optimization of the clad diameter for both the passive and the active fiber. To maximize the amount of pump power coupled into the active fiber, the active fiber is designed with a slightly larger clad diameter than the passive fibers delivering the pump power. As an example, passive fibers with clad diameters of 395-um spliced to active octagon shaped fiber with clad diameters of 400-um improve the coupling of the pump power into the active fiber. An image of such a splice is shown, showing the shaped cladding of the doped double-clad fiber.<ref name="rare-earth-fibres">B. Samson, A. Carter and K. Tankala, “Rare-earth fibres power up”, Nature Photonics, Vol 5, 8, (2011). http://www.nufern.com/library/item/id/383/</ref>
| |
| | |
| The matching of active and passive fibers can be optimized in several ways. The easiest method for matching the signal carrying light is to have identical NA and core diameters for each fiber. This however does not account for all the refractive index profile features. Matching of the MFD is also a method used to create matched signal carrying fibers. It has been shown that matching all of these components provides the best set of fibers to build high power amplifiers and lasers. Essentially, the MFD is modeled and the resulting target NA and core diameter are developed. The core-rod is made and before being drawn into fiber its core diameter and NA are checked. Based on the refractive index measurements, the final core/clad ratio is determined and adjusted to the target MFD. This approach accounts for details of the refractive index profile which can be measured easily and with high accuracy on the preform, before it is drawn into fiber.<ref name="rare-earth-fibres" />
| |
| | |
| ==See also==
| |
| * [[Bragg's law]]
| |
| * [[Diffraction]]
| |
| ** [[Diffraction grating]]
| |
| * [[Dielectric mirror]]
| |
| * [[Hydrogen sensor]]
| |
| * [[Long-period fiber grating]]
| |
| * [[Photonic crystal fiber]]
| |
| * [[Distributed temperature sensing]] by fiber optics
| |
| * [[PHOSFOS]] project - embedding FBGs in flexible skins
| |
| | |
| ==References==
| |
| <references/>
| |
| | |
| ==External links==
| |
| ;Commercial Software:
| |
| * [http://optiwave.com/category/products/component-design/optigrating/ Optiwave Systems Inc.]
| |
| ;International Optical Sensor Societies:
| |
| * [http://www.photonicscluster-nl.org/index.php?option=com_content&view=category&id=11&Itemid=14 FOSNE] - Fibre Optic Sensing Network Europe
| |
| ;Development Platforms:
| |
| * [http://www.tft-fos.nl TFT] - Technobis Fibre Technologies
| |
| ;FBG Manufacturing:
| |
| * [http://proximion.com/Technology/Interferometric_Writing Interferometric FBG manufacturing]
| |
| ;Other:
| |
| * [http://bit.or.at/irca/bbsshow8.php?ref1=OO/INTA/36&vQuelle=&bcc= Bragg grating as hydrogen detector]
| |
| * http://www.patentgenius.com/patent/7133582.html - Fiber - Optic filter with tunable grating
| |
| | |
| {{DEFAULTSORT:Fiber Bragg Grating}}
| |
| [[Category:Optical fiber]]
| |
| [[Category:Diffraction]]
| |