Bend sensor using an embedded etched fiber Bragg grating

Abstract

ABSTRACT:We present a new bend sensor which can measure the curvature using an etched fiber Bragg grating (FBG) INTRODUCTIONFiber Bragg gratings (FBGs) have been received much attention in various fiber-optic sensor implementations [1,2]. These devices have some advantages, such as electrically passive operation, MI immunity, high sensitivity, and low cost. However, one of the most obvious limitations is their crossing sensitivity to temperature and strain. To solve this problem, several different techniques have been reported, such as the reference grating technique [3], the dual-wavelength technique [4], the hybrid-grating technique [5], and the double Bragg grating with different temperature sensitivities technique [6, 7]. Wavelength changes are usually detected by an optical spectrum analyzer or wavelength meter in all these techniques.There have been many reports about the technique of using long-period gratings (LPGs) for curvature measurement, based on either cladding-mode resonance increasing with bending curvature [8], or resonant mode splitting [9]. Both techniques have the same problem of the grating being very sensitive to the variety of temperature. Bending sensors based on FBGs have also been proposed in [10, 11]. All of them adopt double FBGs on the same substance at the same temperature, using the difference in Bragg wavelength between the two gratings to determine the curvature independently of temperature. However, in order to compensate for the temperature across-infection, this technique must maintain both gratings at the same temperature, thus increasing the fabrication difficulty and cost, and decreasing the multiplexing capability.In this paper, we present a practical sensor for the curvature measurement. The sensor is based on a linearly etched FBG embedded in a beam structure. The reflection band of the grating is linearly broadened with an increase of the curvature of the beam. The curvature can be measured with the changes of reflection power in all reflection bandwidth, as the reflection power is directly proportional to the variation in the bandwidth. Reflection power can be detected by the optical-power meter or opto-electronic diodes, which enormously decreases the sensor cost and increases the practical value. The variation of reflection power under the same curvature is dependent upon the etch depth of the grating, thus a highly sensitive bend sensor can be fabricated using this technique. SENSING PRINCIPLEThe Bragg wavelength B at the axial position z of an FBG is expressed as [12]:where n eff ( z) and ⌳( z) are the effective refractive index and period at position z, respectively. We know that when the FBG is held under tension, its central wavelength at position z becomeswhere is the strain-dependence factor for the effective index, n eff 0 and ⌳ 0 correspond to ϭ 0, and ( z) is the axial strain at position z formed by the applied tension, and can be expressed aswhere F is the tension applied to the grating, E is the Yong's modulus, and A( z) is the cross-section area of the...

Development of FBG probes for dimensional metrology with micro parts of high aspect ratio

“…It can be seen through the above analysis that the axial sensitivity of the FBG probe with single core is as high as those of existing FBG sensors which have a theoretical resolution of hundreds of nanometers [12]. So, a nano-stage P-622.1CD was utilized to produce the axial displacement of tens of nanometers, but the radial sensitivity is not high as expected.…”

“…The axial resolution of a FBG probe with single core can usually reach several hundred nanometers or even higher, and it can be as high as that of a conventional FBG sensor [12]. The axial resolution of FBG probe with single core can be used for dimensional metrology with micro parts, but its radial sensitivity is not good enough to accomplish the measurement of micro parts of high aspect ratio.…”

Study on high‐temperature and high‐pressure measurement by using metal‐coated FBG

“…The basic principle of operation commonly used in a FBG based sensor system is to monitor the shift in wavelength of the returned "Bragg" signal with the changes in the measurand (e.g., strain, temperature). The Bragg wavelength, or resonance condition of a grating, is given by the expression [10] B ϭ 2n eff ⌳…”

Bend Sensor Using Fiber Bragg Gratings to Determinate the Distance from the Sensor to the Neutral Axis in Compression and Extension

“…Al momento de obtener las relaciones necesarias para determinar la distancia entre el sensor y el eje neutro se deben tener presentes las siguientes ecuaciones [20]: La deformación que sufre la fibra está dada por (4), donde ∆L indica la variación de la longitud de la fibra, mientras que la relación entre la variación de longitud de onda, ∆λ, y la variación de la deformación de la fibra, ∆ǫ, viene dada por (5), donde la lámina en posición horizontal tiene un ∆ǫ = 0.…”