The present invention relates to optical fibers, and more particularly to fiber grating sensors.
Multiplexing capability, small footprint and measurement robustness have made fiber Bragg grating based sensors very attractive to many measurement applications. In fiber Bragg grating-based sensors, parameters to be sensed arc applied to the gratings either directly or indirectly through some arrangements. In most cases, change in the Bragg grating wavelength is the measurement, which correlates to the sensing parameters.
To measure the sensing parameters accurately, other factors are needed to be isolated. However, prior art isolation methods need some improvement.
The present invention provides a multi-core fiber grating sensor. A length of an optical fiber according to the present invention comprises: first and second doped cores that extend along the length of the fiber; each of the cores having a different dopant regime; each of the cores including a grating having substantially the same grating period; and a cladding that surrounds the first and second cores.
A method of making an optical fiber according to the present invention comprises the steps of: doping a first core with a dopant regime; doping a second core with a different dopant regime from the first core; providing a cladding layer to cover the first and second cores; and provide a grating in each of the cores, which has substantially the same grating period.
A temperature sensor according to the present invention comprises: an optical fiber having at least first and second cores; each of the cores being doped and having a different dopant regime; each of the cores having a grating with substantially the same grating period, the gratings being adapted to reflect an incoming optical signal differently and the difference in reflected wavelengths from each of the gratings varies in accordance with a temperature in a surrounding environment; and a cladding layer to cover each of the cores.
An optical fiber according to the present invention comprises: N cores parallel to each of the cores; each of the cores being doped and having a different dopant regime; and each of the cores having a grating with substantially the same grating period, wherein the gratings arc configured to measure N parameters.
Referring now to the drawings,
Multiplexing capability, small footprint and measurement robustness have made fiber Bragg grating based sensors very attractive in many measurement applications. In most of the fiber Bragg grating based sensors, a change in the Bragg grating wavelength correlates to a particular sensing parameter.
Both strain and temperature applied to a grating are known to cause a relatively large change in grating wavelength. Other factors that cause a change in the grating wavelength are the amount of hydrogen, deuterium, or any substance which can get into a core of an optical fiber and change the effective refractive index of its core; or an acoustic wave impinged on a grating in the fiber. To measure any particular sensing parameter accurately, it needs to be decoupled or isolated from other factors. Especially, if the sensing parameter is temperature or strain, decoupling of the strain and the temperature (i.e. isolate effect of temperature from strain or vise versa) is important.
Reference is now made in detail to the description of the embodiments as illustrated in the drawings. While several embodiments are described in connection with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents.
Referring now to
Four different fibers are tested: antimony (Sb)-doped fiber; SMF-28 like fiber 101; 040298 fiber 102; and 111495 fiber 103. The SMF-28 like fiber 101 is a low germanium (Ge) doped fiber similar to Coming SMF-28 fiber. The 040298 fiber 102 and the 111495 fiber 103 are OFS fibers with high Ge concentration and other doping. Each of the fibers also has a grating, which is substantially the same grating period to each other. However, those fibers have different dopant regimes. A “dopant regime” refers to the particular dopant or dopant concentrations used to modify the refractive index of an optical fiber or portion thereof.
Referring now to
Referring now to
When an optical fiber with a grating is subject to an external disturbance, the effective index of the core may change, and thus, the wavelength of light reflected by the grating shifts from its original wavelength. Disturbances, such as strain and temperature, applied to the grating, are known to cause larger shifts in the reflected light wavelength. The respective shifts in the reflected light wavelength due to the change of strain, Δε, and due to the change of temperature, ΔT, can be expressed as the following mathematical equations, respectively:
Δλε=KεΔε (1)
ΔλT=KTΔT (2)
Where Δλε is the change in the reflected light wavelength due to the change of strain, where ΔλT is the change in the reflected light wavelength due to the change of temperature, and where Kε and KT are coefficients relating to the rate of change of the strain and the temperature respectively, in accordance with the shift in the wavelength of light reflected by the grating. In this specification, parameters, such as Δε and ΔT, which are chosen to be measured, are referred to as “predetermined target parameters.”
If a cumulative change in the reflected light wavelength from a grating is expressed as ΔλG, then equations 1 and 2 can be combined as follows:
ΔλG=KεΔε+KTΔT (3)
Besides the temperature and the strain, hydrogen and/or deuterium effects shift the reflected light wavelength as well.
The result shows that after those gratings are left at room temperature for ˜50 hours (see dot 401), the relative wavelength shifts between the gratings on the 111495 fiber 103 and the SMF-28 like fiber 101 are substantially zero, which is the same status as they were prior to deuterium loading. This means that there is substantially no wavelength separation change for these two gratings that is attributable to deuterium concentration in the fibers. Since deuterium and hydrogen have similar chemical characteristics. (deuterium has one extra neutron in its core), it is reasonable to predict that even if hydrogen is loaded onto such fiber (and/or its core), there would be substantially no wavelength separation change due to the hydrogen concentration in the fiber.
In order to decouple the values of predetermined target parameters or measure the value of a predetermined target parameter independent from the effects from other factors, in one of the embodiments of the present invention, a twin-core fiber configuration was chosen.
In the embodiments, the twin-core fiber is made such that the gratings in both cores experience the same strain and temperature changes. If equation (3) is applied to each grating, the following equations are obtained:
ΔλG1=Kε1Δε+KT1ΔT (4a)
ΔλG2=Kε2Δε+KT2ΔT (4b)
Where ΔλG1 and ΔλG2 are changes in the reflected light wavelength by the first core and the second core, respectively, and Kε1 and KT1 are coefficients relating to the rate of change of the strain and the temperature in the first core, respectively, and Kε2 and KT2 are coefficients relating to the rate of change of the strain and the temperature in the second core, respectively.
The two cores are chosen such that the matrix related to the grating coefficients,
is well conditioned. Thus, equation (5) can be performed to obtain the predetermined target parameters, Δε and ΔT
In this case, the two predetermined target parameters, Δε and ΔT, are decoupled and obtained.
If the change of temperature is the only interest, equations (4a) and (4b) can be combined to obtain:
ΔλG12=Kε1Δε+KT1ΔT−Kε2Δε−KT2ΔT (6)
or
ΔλG12=(Kε1−Kε2)Δε+(KT1−KT2)ΔT (7)
Where ΔλG12=ΔλG1−ΔλG2 is the difference in the wavelength change between these two gratings.
In embodiments of the present invention, these gratings are configured such that the strain coefficients of those gratins are substantially the same (i.e. Kε1−Kε2) Therefore, the equation (7) can be simplified as follows:
ΔλG12≈(KT1−KT2)ΔT (8)
or
ΔT≈ΔλG12/(KT1−KT2) (9)
Equation (9) states that the difference in the wavelength change between these two gratings due to the strain and temperature can directly result in the temperature change measurement. Therefore, it simplifies further data analysis.
Furthermore, in embodiments of the present invention, these two gratings are configured such that the wavelength changes due to the deuterium (or hydrogen) concentration change are substantially the same. Thus, equation (9) can still be used to obtain the temperature change measurement when such three parameters co-exist (e.g. in high humidity environment).
The above results show that by combining two different fibers in a twin-core fiber, a grating-based sensor capable of decoupling the strain and the temperature or isolating the temperature measurement from other factors can be constructed. As an embodiment of the present invention, a core of the SMF-28 like fiber and a core of the 111495 fiber are combined to create a twin-core fiber. According to
As for the dopant regimes, in one embodiment of the present invention, different concentrations of the same dopant are used. For example, germanium can be used as a dopant for each of the cores The first core can be doped such that the difference in the refractive indexes between the first core and the cladding is approximately between 0.004 and 0.008. The second core can be doped such that the difference in refractive indexes between the second core and the cladding is approximately between 0.035 and 0.05.
Alternatively, the two cores may comprise with different dopants from one another. For example, the first core may be doped with antimony in such a way that the difference in the refractive indexes between the first core and the cladding is approximately between 0.004 and 0.008. The second core can be doped with geranium in such a way that the difference in the refractive indexes between the second core and the cladding is approximately between 0.035 and 0.05.
Also, each of the cores is so placed that cores are far enough apart that light transmitted from a coupler to each of the cores does not exchange between the cores and, at the same time, are close enough that a coupling region can transmit light to each of the cores simultaneously, for example, using a fiber tapering technique. It is known that a low-index trench outside of the core reduces the spatial spreading of optical power. A low-index layer more than 1 μm thick with an index between −0.001 DN and −0.025 DN may be placed between the cores to reduce inter-core coupling. This barrier layer also permits more compact placement of the cores within the twin-core fiber.”
In the embodiments of the invention, the diameter of the second core is chosen to be approximately between 2 μm and 3 μm, the diameter of the first core is chosen to be approximately 8 μm, and the diameter of the cladding is chosen to be at least 20 μm.
In some of the embodiments, the grating can be inscribed into these two cores simultaneously, and the resulting preferred location of the gratings 701 and 702 is schematically shown in
In the embodiments of the invention, an incoming optical signal is a broadband source and the wavelength range of the incoming optical signal is approximately from 1520 to 1620 nm.
The twin-core configuration can be also applied to more than two cores (i.e. multi-core) configuration. For example, an optical fiber comprising N cores parallel to each of the cores can be constructed. The cores have different dopant regimes. A grating with substantially the same grating period applied to each of the cores. The fiber with N cores described above can be configured to measure up to N parameters. The N parameters can be temperature, strain, hydrogen concentration, or any predetermined target parameters. For example, if two cores are selected (i.e. N=2), then temperature and strain may be chosen as the predetermined target parameters. Or, if three cores are selected (i.e. N=3), then temperature, strain and hydrogen loading may be chosen as the predetermined target parameters. For example with the SMF-28 like, the I 1495 and the 40298 fibers, all three fibers have substantially the same strain coefficients. The SMF-28 like and the 111495 fibers have almost the same deuterium (or hydrogen) coefficients, but the 40298 fiber has a slightly different deuterium (or hydrogen) coefficient. The thermal coefficients of those fibers are all different. If a triple-core fiber is constructed, the fiber is capable of measuring the temperature, the strain, and the deuterium (or hydrogen) concentration.
Since each of the N cores has a unique coefficient for the N predetermined target parameters, the N parameters are measured from a well-constructed matrix.
Since each of the N cores, has unique coefficients for the (N-A) predetermined target parameters, the parameters are measured from a well-constructed matrix and the extra measurements value(s) A are used for data redundancy to improve measurement accuracy.
In step 802, a second core is doped with a different dopant regime from the first core. This step maybe performed simultaneously with the previous step. Each of the cores is doped with a dopant regime according to the predetermined target parameters for sensor applications. For those two steps, each core may be fabricated by using modified chemical vapor deposition (MCVD) method or other well-known optical fiber fabrication methods.
Then, in step 803, a cladding layer is provided to cover the first and the second cores. For example, the cladding layer can be provided by drilling two holes on a glass rod, placing these two cores in the holes, and then collapsing the whole assembly to make a preform for the twin-core fiber. The twin-core fiber can be drawn from the preform in a regular fiber draw tower.
Next, the fiber can be loaded with deuterium or hydrogen to increase photosensitivity for grating writing, and then a grating is inscribed in each of the cores in step 804 by using well-known interferometric or phase-mask techniques. The periods of the gratings in both of the cores are the same because the gratings are written with the same mask or interferometric pattern. The gratings in the two cores are then stabilized by annealing at a higher temperature than the maximum measurement temperature. In the preferred embodiment of the present invention, the grating in each of the cores is made simultaneously at the same longitudinal location as shown in
One of the applications to use the twin-core fibers (or any multi-core fiber) in the present invention is as a temperature sensor. For this application, a twin-core fiber with two gratings, which have substantially the same strain coefficients but large difference in thermal coefficients, was manufactured.
A similar analysis was done based on temperature.
ΔλG
ΔλG
ΔλG
Thus, the grating sensor can achieve the separation of the strain and the temperature with improved measurement accuracy.
T=106.9Δλ3−554.5Δλ2+899.2Δλ+22.69 (12)
Where T is the temperature in ° C., and Δλ is the difference in the wavelength shift in nm between the two gratings (i.e. ΔλG
The above twin-core fiber can be installed in a fiber grating sensor measurement apparatus.
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and are not intended to limit the applications of the present invention. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. Thus, the present invention should not be limited by any of the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents.
This application claims the benefit of U.S. provisional patent application Ser. No. 61/045,315, filed Apr. 16, 2008, having the title “Twin-Core Fiber Grating Sensor,” which is incorporated herein by reference in its entirety.
Number | Name | Date | Kind |
---|---|---|---|
5500031 | Atkins et al. | Mar 1996 | A |
5563967 | Haake | Oct 1996 | A |
6024488 | Wu et al. | Feb 2000 | A |
6334018 | Fokine | Dec 2001 | B1 |
6833541 | Shu et al. | Dec 2004 | B2 |
6853792 | Logvin et al. | Feb 2005 | B2 |
6865194 | Wright et al. | Mar 2005 | B1 |
7027699 | Tao et al. | Apr 2006 | B2 |
7324714 | Cranch et al. | Jan 2008 | B1 |
7373062 | Huber | May 2008 | B2 |
7379631 | Poland et al. | May 2008 | B2 |
7421905 | Zerwekh et al. | Sep 2008 | B2 |
7512292 | MacDougall et al. | Mar 2009 | B2 |
20050118064 | Berg | Jun 2005 | A1 |
Number | Date | Country | |
---|---|---|---|
20090262779 A1 | Oct 2009 | US |
Number | Date | Country | |
---|---|---|---|
61045315 | Apr 2008 | US |