In-fibre directional transverse loading sensor based on excessively tilted fibre Bragg gratings

Abstract

We report a distinctive polarization mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with a tilted angle exceeding 45°. The ex-45° TFBGs exhibit pronounced polarization mode splitting resulted from the birefringence induced by the grating structure asymmetry. We have fabricated TFBGs with a tilted structure at 81° and studied their properties under transverse load applied to their equivalent fast and slow axes. The results show that the light coupling to the orthogonally polarized modes of the 81°-TFBGs changes only when the load is applied to their slow axis, giving a prominent directional loading response. For the view of real applications, we further investigated the possibility of interrogating such a TFBG-based load sensor using low-cost and compact-size single wavelength source and power detector. The experimental results clearly show that the 81°-TFBGs plus the proposed power-measurement interrogation scheme may be developed to an optical fibre vector sensor system capable of not just measuring the magnitude but also recognizing the direction of the applied transverse load. Using such an 81°-TFBG based load sensor, a load change as small as 1.6 × 10−2 g may be detected by employing a standard photodiode detector.

Optical Fibre Sensors Based on UV Inscribed Excessively Tilted Fibre Grating

“…We can therefore identify an equivalent fast-axis and slowaxis similar to the conventional polarisation maintaining (PM) fibre structure as shown in Figure 5. It is this distinctive polarisation mode splitting mechanism makes ETFGs as ideal loading [17] and twisting sensors [16] based on their polarisation property and as refractive index sensors utilising intrinsic sensitivity of the high order modes to surrounding medium. …”

“…This chapter will review the recent development of ETFGs as various optical sensors. The chapter will be organised in three main parts: first part (sections 2) gives a general introduction and fundamental background on fibre gratings with a particular emphasis on the ETFG; second part (section 3) describes the inscription and characterisation of ETFG; third part (sections 4-9) discusses ETFG based sensors and fibre laser sensing systems including strain, twist [15,16], loading [17], refractive index (RI) and liquid level sensing [18].…”

All-fiber loading sensor based on 45° and 81° tilted fiber gratings

“…In order to eliminate measurement errors from axial-strain and bending effects, the 81°-TFG was fixed on the plate with a small axial tension to maintain it straight. The previously reported results [8] show that only fast-axis is sensitive to the loading effect, thus the transverse load was only applied to the equivalent fast-axis of 81°-TFG from 0 to 1.6 kg in a step of 0.1 kg by putting the weights on the top aluminum plat, as shown in Figure 2 (b). The transmission spectrum for each applied load is plotted in Figure 3.…”

“…Detailed experimental investigation and theory analysis on large angle TFGs have also been reported [7] by Zhou et al . Due to the inherent polarization mode splitting effect of such gratings resulting from their tilted structures, a novel in-fiber directional transverse loading sensor based on 81°-TFG [8] has been experimentally demonstrated. However, such a sensor system requires extra polarizer and polarization controller which represents a more complex, bulky and high cost system.…”

All-Fiber Loading Sensor Based on a Hybrid 45° and 81° Tilted Fiber Grating Structure

“…The large angle TFGs have been proposed as fiber sensors for the detection of strain [10], twist [11], refractive index (RI) and liquid level [12,13]. Due to its inherent polarization mode splitting effect caused by the asymmetric structure induced by the excessively tilted index fringes in the fiber core, a large angle TFG has been implemented as a novel in-fiber directional transverse loading sensor [14]. However, the interrogation of such a sensor requires an extra polarizer and polarization controller, imposing the disadvantages of complexity, bulk and cost.…”