&lt;title&gt;Strain monitoring by evanascent wave spectroscopy&lt;/title&gt;

Kapila, Vivek; Kjerengtroen, Lidvin; Cross, William; Johnson, Francis; Kellar, Jon J.

doi:10.1117/12.352789

Cited by 3 publications

(1 citation statement)

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“…Fiber Bragg grating sensors make use of many physical phenomena including evanescent wave coupling [4] and Bragg reflectors [5]. Fiber Bragg gratings are formed by imprinting periodic refractive index modulations within the core of a short section (typically a few millimeters) of an optical fiber, and were first discovered in 1978 by Hill et al [6].…”

Section: Introductionmentioning

confidence: 99%

Evanescent field fiber Bragg grating sensors for index of refraction sensing with applications to structural health monitoring

et al. 2006

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Fiber Bragg grating sensing is a relatively mature fiber optic sensor technology currently being used in structural health monitoring systems. Therefore, there are significant benefits to using this technology as a platform for other sensing modalities. In this work, a side polished fiber Bragg sensor is described for sensing refractive index changes. The effective refractive index of a fiber Bragg grating is a function of the refractive index of the media surrounding it, and its sensitivity may be optimized with appropriate design. As the external refractive index changes, the wavelength at which incident light experiences a maximum reflection from the grating will shift. The sensitivity of a fiber Bragg grating to external refractive index changes increases when the grating is polished on one side. This side-polishing technique enables the Bragg grating to preserve a greater portion of its mechanical strength compared with other techniques such as chemical etching. This work utilizes side-polished fiber Bragg grating sensors centered at a 1542.9 nm wavelength with cladding thickness values of approximately 1-2 µm. The response of these sensors to small refractive index changes was studied. Previous work on fiber Bragg grating sensors has shown that the peak wavelengths can be measured with 3 pm repeatability. With this repeatability, this study demonstrated that a 0.001 refractive index change can be observed. By using materials that change index with moisture or pH, this technique can be used to construct both pH and moisture sensors.

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Section: Introductionmentioning

confidence: 99%

Evanescent field fiber Bragg grating sensors for index of refraction sensing with applications to structural health monitoring

et al. 2006

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Accurate strain measurements with fiber Bragg sensors and wavelength references

Rivera¹,

Thomson²

2006

Smart Mater. Struct.

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Fiber Bragg grating (FBG) sensors are one of many fiber optic sensor technologies that are currently being used in structural health monitoring systems. The sensors operate by detecting a shift in the wavelength of the reflected maxima due to applied strain. This paper studies a new fiber Bragg interrogation method that combines a swept wavelength laser in combination with wavelength references. These include a gas cell which is used as the long term wavelength standard and an etalon used for accurate interpolation of peak wavelengths. An etalon is essentially a filter that has a periodic response over a broad wavelength range. Since its wavelength response spacing is smaller than the gas cell, it can be used to determine the intermediate wavelengths between two gas cell absorption lines. Peak location is a key element of this interrogation method and several detection algorithms are investigated. It was determined that polynomial peak fitting is the most computationally efficient method and yields a resolution of better than 0.5 pm with signal to noise ratios of 30:1 or better. With higher signal to noise ratios, polynomial peak fitting can yield a resolution of better than 0.25 pm. Using a tunable laser, a hydrogen cyanide (HCN) gas cell and an etalon with maxima every 140 pm, static load tests have demonstrated a resolution of 1 pm and an accuracy of less than 5 pm. Also, this accuracy will be maintained over a long period of time as it is based on absorption lines in the gas cell. The results of this study demonstrate that absolute accurate strain measurements can be obtained with the use of wavelength references in conjunction with a suitable peak location algorithm.

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