Highly sensitive bend sensor with hybrid long-period and tilted fiber Bragg grating

doi:10.1016/j.optcom.2010.03.013

Optics Communications

Volume 283, Issue 13, 1 July 2010, Pages 2690-2694

https://doi.org/10.1016/j.optcom.2010.03.013 Get rights and content

Abstract

We demonstrate a new type of fiber optic bend sensor with a hybrid structure made up of a long period grating (LPG) and a tilted fiber Bragg grating (TFBG). The sensing mechanism is based on the spectrum of power transfers between the core and cladding modes from a TFBG located downstream from a LPG. We show that the curvature of a beam can be determined by the reflected power difference between the core mode and the recoupled cladding modes. We further provide design rules for the LPG and TFBG to optimize and linearize the sensor response. In addition, the temperature cross-sensitivities of this configuration are also investigated for two different types of fiber.

Introduction

Fiber Bragg gratings (FBG) have been used in many configurations for applications in structural health monitoring. For instance, bend sensors have been proposed using two FBGs bonded to opposite sides of a beam [1]. The difference between the two Bragg wavelengths provides a measure of curvature of the beam. Other FBG bend sensors are also demonstrated based on bandwidth broadening due to a chirp in the period of FBGs with curvature [2], [3]. It is also well known that the sensitivity of LPGs to bending is superior to that of FBGs and a number of LPG based bend sensors have been proposed recently. The typical mechanisms utilized for LPG bend sensors are based on a bend-induced wavelength shift, on the depth change of the attenuation band, or on the splitting of some attenuation bands [4], [5], [6]. Some researchers have also investigated the embedding of LPG sensors in some materials to discriminate the bending from some other measurands [7]. Some disadvantages of LPG sensors are that they operate in transmission and that they have a relatively wide attenuation band which causes difficulty in reading the exact wavelength of the loss dip. Alternatively, Baek et al proposed another kind of bend sensor based on a TFBG [8]. This sensor is based on the transmission loss change of low order cladding modes in a TFBG. Jin et al also used a TFBG for the same purpose but made it work in reflection by inserting a short section of multimode fiber (MMF) upstream of the TFBG written on a single mode fiber (SMF) [9]. The MMF has a larger core diameter compared to SMF and the mismatched interface between the fibers allows some low order cladding modes to recouple back to core mode of the SMF after going through the MMF. In that configuration, the bending changes the amount of power of the recoupled cladding modes but has no effect on the core mode reflection of (the Bragg reflection) of the TFBG. Therefore, the amount of bending can be measured by monitoring the reflected power differences between the core and the cladding modes. The disadvantage of this scheme is that the mode field mismatch at the two splices between MMF and SMF introduces a large insertion loss in the device. The wide bandwidth of reflected cladding mode spectrum is also a problem in signal processing.

In this paper, we demonstrate a hybrid structure consisting of an LPG and a TFBG for bending/curvature measurements. An LPG located upstream from the TFBG only recouples one of the cladding modes excited by the TFBG. Similarly to the above mentioned previous work on TFBG bend sensors, the reflections of core Bragg mode and of the recoupled cladding mode vary in opposite directions when the bending is applied. The differential power change of these two modes provides an efficient measure of bending with enhanced sensitivity. The advantage of the current configuration is that the combined reflection spectrum is relatively narrowband, thereby facilitating the interrogation of the two dominant reflection peaks, and that the introduction of the LPG does not compromises the mechanical integrity of the fiber (by opposition to splices or tapers). Our results further indicate how to properly design the TFBG and LPG spectra to optimize the combined response for the bending application. Finally, the temperature sensitivities of these devices are also studied for two different types of fiber.

Section snippets

Operation principle

An LPG with a typical period of several hundred micrometers can induce co-directional coupling of the core mode and cladding modes in optical fibres. Contrary to the LPG, the TFBG with a much shorter period of several hundred of nanometres couples the incoming core light power into backward propagated core and cladding modes. But normally, the backward-propagating cladding modes in a TFBG are rapidly attenuated by the fibre jacket and cannot be seen in the reflected spectrum. In our proposed

Fabrication and experimental setup

TFBGs and LPGs were fabricated by using the phase mask technique and the amplitude mask technique respectively. In order to study the effect of the transmission loss of the LPG on the sensor's curvature sensitivity, two LPGs were made in two segments of photosensitive fiber (PS1250/1500 from Fibercore, Ltd) with different transmission losses of 5.2 dB and 18.0 dB, respectively. They have the same length of 20 mm and different periods of 375 μm and 377 μm. Their resonance wavelengths (for coupling to

Conclusions

In conclusion, hybrid LPG-TFBG bend sensors have been implemented and packaged by inserting the fibre into a polymer capillary that was bonded to a steel beam. One of the backward propagating cladding modes excited by the TFBG is recoupled into the core with the assistance of the upstream LPG. The differential reflected power changes of the Bragg mode and recoupled cladding mode are employed to determine the curvature of the sensor. A relatively high curvature sensitivity is achieved up to 2.36

Acknowledgement

This work is supported by the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chairs program and LxDATA.

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Highly sensitive bend sensor with hybrid long-period and tilted fiber Bragg grating

Abstract

Introduction

Section snippets

Operation principle

Fabrication and experimental setup

Conclusions

Acknowledgement

Opt. Comm.

Opt. Comm.

Opt. Comm.

Int. J. of Optoelect.

Smart Mater. Struct.

Microwave Opt. Technol. Lett.

Appl. Opt.

IEEE Photon. Technol. Lett.