Fiber-Optic Sensors for Measurements of Torsion, Twist and Rotation: A Review

Abstract

Optical measurement of mechanical parameters is gaining significant commercial interest in different industry sectors. Torsion, twist and rotation are among the very frequently measured mechanical parameters. Recently, twist/torsion/rotation sensors have become a topic of intense fiber-optic sensor research. Various sensing concepts have been reported. Many of those have different properties and performances, and many of them still need to be proven in out-of-the laboratory use. This paper provides an overview of basic approaches and a review of current state-of-the-art in fiber optic sensors for measurements of torsion, twist and/or rotation.Invited Paper.

Keywords: FBG; circular birefringence; fiber optic sensors; linear birefringence; long period FBG; optical fibers; polarization in optical fibers; rotation sensors; tilted FBG; twist sensors.

Figure 1

(a) Polarization plane and fiber direction when fiber is not twisted and (b) When fiber is twisted through 100°, the polarization plane is rotated through 7° in the opposite direction.

Figure 2

Fiber loop sensor system for angular measurements.

Figure 3

Finite element method simulation of a total stress distribution in a circularly symmetric fiber and circular non-symmetric fiber (containing two hollow regions): (a,c) show stress distribution when no twist is applied; (b,d) show the case when fibers are exposed to the twist. In case of the twisted fiber with side holes, a non-circularly symmetric stress build-up around the core can be observed, leading to linear birefringence modulation.

Figure 4

Twist/rotation sensor employing a Sagnac interferometer.

Figure 5

Twist sensor employing a loop mirror configuration with an output port probe.

Figure 6

Modal MZI twist sensor experimental setup and illustration of the fabricated SMF Helix.

Figure 7

Shear stress distribution along the twisted rod/fiber.

Figure 8

Mechanically induced LPFBG (M-LPFBG) twist/torsion test setup.

Figure 9

Corrugated LPFBG twist/torsion test setup.

Figure 10

Schematic diagram of asymmetric index profile within a cross-section of a CO₂-laser-induced LPFG.

Figure 11

Typical LPFBG twist/torsion test setup.

Figure 12

Fiber Mach-Zehnder interferometer for twist/rotation sensing.

Figure 13

Torsion sensor employing Mach-Zehnder LPFBG in fiber ring laser configuration.

Figure 14

(a) Helical twist/rotation sensor setup; and (b) Multicore twist/rotation sensor setup.

Figure 15

(a) E-field vector orientation remains unchanged during propagation through isotropic medium; (b) Rotation of the receiver rotates E-field vector in receiver’s local coordinate system.

Figure 16

Typical E-field vector displacement rotation sensors’ arrangements.

Figure 17

E-field vector displacement rotation sensors using single lead fiber solutions with E-field rotation encoding employing (a) An FBG inscribed in the Hi-Bi fiber; and (b) An all-fiber wave-plate.

Figure 18

(a) A quasi-distributed sensor array for twist/rotation setup; and (b) ILP with a semi-reflective mirror setup.

Figure 19

Tilted FBG formation.

Figure 20

Feasible arrangements for forward and backward coupling of cladding modes in TFBGs. (a) Back cladding modes coupling; (b) Forward cladding modes coupling; (c) Radiation modes coupling.

Figure 21

Each polarization state (E_y and E_x) yielding to different resonant solution due to its corresponding orientation.

Figure 22

An example of forward propagating spectrum in excessively tilted FBG and its spectral response; ratio of dips P1 and P2 changes with fiber rotation (CW/CCW).

Figure 23

TFBG setup for rotation measurement.

Figure 24

TFBG setup for twist/rotation measurements employing FBG’s inscribed in multi-mode fiber.

Figure 25

Rotation sensing system using a VCSEL light source with optical feedback.

Figure 26

Schematic setup of differential configuration of FBG sensor at 45° angle relative to the shaft axis.