Fiber Bragg grating temperature sensor;A
Abstract:In recent years, considerable progress has been made in optical fiber sensors using a fiber Bragg grating (FBG). This is because such FBG sensors have many advantages for practical applications: ease of multiplexed operation, compactness, quasi-point sensing capability and simple structure. Fiber Bragg
gratings (FBG)have generated much interest for use as sensors for strain, temperature, and other physical quantities because of their important properties such as immunity to electromagnetic noise, high sensitivity, compactness, and simplicity of fabrication.
OCIS codes: (060.2370) Fiber optics sensors; (230.1480) Bragg reflectors.
Fiber Bragg grating (FBG) has been intensively studied and developed as an optical sensor for various sensing applications, such as health monitoring of civil structure, non-destructive testing of composite materials, smart structure, and traditional stain, pressure, and temperature sensing . To realize practical sensor systems with multiplexing capability, various kinds of FBG array interrogation techniques have been suggested . Recently, the demand for warning the abnormal temperature increase at restricted spaces has been rapidly increasing. In particular, due to different tolerable thresholds required at different locations, it is necessary to construct the low cost and easy interrogation configuration. In this letter, a novel multi-point temperature warning sensor using a multi-channel matched FBG-based multi-wavelength pulsed laser is proposed and demonstrated. The sensor has several advantages, including flexible setting of the tolerable temperature, simple structure, high signal-to-noise ratio, low system cost and quick response.
;1； Intracore fiber Bragg gratings for strain measurement in embedded composite structures
The Bragg wavelength is given by
, (1) ~，2n，Beff
nwhere is the effective refractive index of the fiber (modal index) and L is the Bragg eff
！~grating period. We obtain the Bragg wavelength shift in the temperature sensor by B
differentiating Eq. (1) with respect to temperature:
！~，2！n，？2n, (2) Beffeff
For a bare FBG, with temperature variation, the second term on the right-hand side of Eq. (2)
！，is negligible because the thermally induced fiber elongation effect () is much smaller than
the effect of the refractive index change. Attaching a metal strip to the FBG can make the
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fiber elongation much larger than that of bare FBG.
;2；Measuring Thermal and Mechanical Stresses on Optical Fiber in a DC Module Using Fiber Bragg Gratings
FBGs can be used as sensors to monitor stress and temperature during fiber processing, handling, installation, and in service events. When an FBG is subjected to a combination of mechanical and thermal loading, the return Bragg wavelength will shift proportionately to the magnitude of the load. It is important to be able to decouple the mechanical and thermal response of the reflected Bragg wavelength, in order for this sensor to achieve its intended usefulness of obtaining an accurate measure of mechanical stress when temperature varies. The shift in the return Bragg wavelength as a function of temperature and stress is given by 
λm = (3) ~k?！?？~kT！T？~BBB
measured Bragg wavelength; ~m
Bragg wavelength at a reference condition (usually at room temperature and stress ~B
change in stress, where it is assumed that stress is linearly related to strain; ！?
ΔT change in temperature;
stress coefficient; k?
temperature coefficient; kT
=, grating stress sensitivity (calibration constant); ~kmBT?
,grating temperature sensitivity or (calibrationconstant). ~k，mBTT
According to (3), the temperature coefficient is independent of wavelength. Therefore, kT
one only needs to measure the change in temperature to account for the thermal contribution to the shifted measured wavelength λm from the initial reference wavelength . The ~B
thermal and stress parameters and k, respectively, in (1) have been reported in a kT?
variety of published studies and are summarized in Table I. In most of these studies, kT
and kσare not given explicitly but can be calculated by dividing the published temperature and stress sensitivities (calibration constants for the grating) by the unperturbed Bragg wavelengthλB of the grating. The thermal and stress coefficients were converted to SI units when appropriate. Note that Table I is not an exhaustive review on the subject, but should provide enough data for comparison.
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The dependence of the index of refraction of glass on temperature is reported to cause an equivalent nonlinear error of 2 ?C over a temperature range of ?30 to 80 ?C . However,
this thermal nonlinearity effect in the glass fiber is not sufficient to be detected by the wavelength measurement system used in most studies and will be assumed to be negligible in this study. Several researchers ,  who have measured linear temperature ranges for Bragg-grating responses have also assumed the thermal nonlinearity effect in glass fiber to be negligible. Whereas there is experiment-to-experiment variability in the reported temperature response of uncoated FBGs in Table 1, their behavior is essentially linear with temperature between ?40 and 100 ?C.
One study  found the protective polymer coating to have a pronounced effect on the measured thermal coefficient at low temperatures. The thermal behavior of the coated grating becomes nonlinear for temperatures below 0 ?C.
From the statistical portion of Table I, The variation in the stress coefficients is
significantly greater than the variation in the thermal coefficients. This is a surprising result, since the maximum wavelength shift for a temperature differential of 100 ?C is close to 1 nm,
while a change in stress of 0.2 GPa will produce a wavelength shift close to 4 nm. The data from Echevarria et al.  indicates that the large variation in the stress coefficient is real and significant. Echevarria used the first- and second-order diffraction peaks from a single Bragg grating to monitor wavelength changes and found that the two resulting thermal coefficients were essentially identical and the stress coefficients varied significantly. The work by Shu et al.  suggests that the method for making an FBG affects the
measured thermal and stress coefficients. The effect of drawing tension  has also been shown to affect the photosensitivity of codoped core fibers and the formation of Bragg gratings. These differences in the manufacturing process of the FBGs may help explain the variations in the measurements of the thermal and mechanical coefficients within a particular study and between studies .
For FBGs manufactured using similar techniques and precisely controlled processes, (3) will generate a wavelength in dependent thermal coefficient , . Echevarria et al. 
monitored the first- and second-order diffraction wavelengths from a single grating to obtain thermal coefficients within 0.7% for a wavelength difference of 768 nm between the two diffraction orders. In the study by Flockhart et al. , three FBGs written at separate
wavelengths were thermally characterized. The fiber gratings were written into hydrogen loaded fiber by a two-beam holographic exposure using a frequency-doubled argon-ion laser. The two-beam holographic writing process with the frequency-doubled argon-ion laser is a more stable method than the more common process of using phase masks and an eximer UV source . The three gratings produced thermal coefficients within 0.2% over a wavelength range of 53 nm. It should be noted that nominal values for the thermal coefficients found in  and  differ by 1.4%. The orientation of the Bragg grating within the optical fiber does not appear to have a significant impact on the thermal coefficient for a tilt angle of 5.5? .
Table 1, PUBLISHED THERMAL AND STRESS COEFFICIENTS FOR FBGS OF
VARIOUS WAVELENGTHS, RECOAT STATUS, AND GRATING
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Initial Thermal Stress Recoat Status ？1？3o k;；GPa?10Kr() C?Bragg Range Range and Grating
WL(nm) (GPa) Trait
1557.70 7.06 20-80 5.38* 0-0.46 ?
1533.30 6.72 20-60 Na Na ?
1535.85 6.36 0-100 10.1 0=0.13 ?
* Indicates that value was not used in the statistical calculations.
;3；A high sensitive fiber Bragg grating cryogenic temperature sensor The reflected wavelength shift of FBG induced by temperature change is
, (4) ;；；，！~/！T，1？P；？(~B0B
where Po(? 0.22) is the photo-elastic constant, is the thermo-optic coefficient, and is (；
the strain change of the FBG.
Fig. 1. Basic structure of bimetal FBG sensor.
In Fig. 1, when the temperature increases 1 K, the distance of the gap between the two metals
, (5) ;；！d，：L？d？：L12
where L is the length of metal B, and are the CTEs of metal A and metal B, ：：12
respectively.When the FBG is stretched, its strain change in 1-K temperature variation is
;；, (6) ；，！；，：L？d)？：L/d12
The temperature sensitivity of bimetal device is thus obtained as
??;；;；；，！~/！T，1？P：L？d？：L/d？(~, (7) B012B
By changing the difference between and, or the ratio of L/d, the temperature ：：12
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sensitivity can be controlled .
3. Recent progress
According to the materials we researched, we mainly focus on the following aspects: Fiber
Bragg grating temperature sensor with controllable sensitivity; Intracore fiber Bragg gratings for strain measurement in embedded composite structures; Fully Distributed Chirped FBG Sensor and Application in Laser-induced Interstitial Thermotherapy; Measuring Thermal and Mechanical Stresses on Optical Fiber in a DC Module Using Fiber Bragg Gratings; High-Resolution Strain/Temperature Sensing System Based on a High-Finesse Fiber Cavity and Time-Domain Wavelength Demodulation; Temperature-Independent Inclination Management with Fiber Bragg Grating sensor; A high sensitive fiber Bragg grating cryogenic temperature sensor; Cantilever-based FBG sensor for temperature-independent acceleration measurement; Simultaneous measurement of underwater acoustic field and temperature using fiber Bragg grating sensor array with feedback control circuit; Simultaneous measurement of underwater acoustic field and temperature using fiber Bragg grating sensor array with feedback control circuit; Active temperature compensation design of sensor with fiber gratings; Measuring vibration by using fiber Bragg grating and demodulating it by blazed grating; Multi-Point Temperature Warning Sensor Using a Multi-channel Matched Fiber Bragg Grating; Multiplexing of temperature=compensated FBG magnetostrictive sensors with a self-seeded laser diode.
A practical process to decouple the effects of stress and temperature on fiber Bragg gratings (FBGs) was developed. The resulting stress and thermal coefficients are consistent with the range of coefficients found in literature. If the intended application can tolerate an average error of 7%, then (1) is applicable, and the grating coefficients can be considered to be universal to all gratings. Thus, no calibration is required and the effects of the coating can be ignored. If the intended application requires a fidelity better than an average of 7% error, each grating should be calibrated individually.
An intracore fiber Bragg grating has been used along with an associated interrogating system to monitor strain variation in a composite specimen under loading conditions. The method has been found to be accurate and to have high repeatability. It has been suggested that these technical advances can be extended to nondestructive evaluation and nondestructive testing or inspection because of the increasing use of fail-safe and damage-tolerant designs in composite structural engineering. This method can be used for precise on-line measurement or assessment of damage. We have shown the applicability of intracore FBG for monitoring smart composite materials. Also, a detailed analysis of strain sensing and the effects of temperature has been made.
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