High sensitivity fiber optic angular displacement sensor and its application for detection of ultrasound

Abstract

In this paper, we report on the development of an intensity-modulated fiber-optic sensor for angular displacement measurement. This sensor was designed to present high sensitivity, linear response, and wide bandwidth and, furthermore, to be simple and low cost. The sensor comprises two optical fibers, a positive lens, a reflective surface, an optical source, and a photodetector. A mathematical model was developed to determine and simulate the static characteristic curve of the sensor and to compare different sensor configurations regarding the core radii of the optical fibers. The simulation results showed that the sensor configurations tested are highly sensitive to small angle variation (in the range of microradians) with nonlinearity less than or equal to 1%. The normalized sensitivity ranges from 0.25 × V max to 2.40 × V max mV ∕ μrad (where V max is the peak voltage of the static characteristic curve), and the linear range is from 194 to 1840 μrad. The unnormalized sensitivity for a reflective surface with reflectivity of 100% was measured as 7.7 mV ∕ μrad. The simulations were compared with experimental results to validate the mathematical model and to define the most suitable configuration for ultrasonic detection. The sensor was tested on the characterization of a piezoelectric transducer and as part of a laser ultrasonics setup. The velocities of the longitudinal, shear, and surface waves were measured on aluminum samples as 6.43, 3.17, and 2.96 mm ∕ μs, respectively, with an error smaller than 1.3%. The sensor, an alternative to piezoelectric or interferometric detectors, proved to be suitable for detection of ultrasonic waves and to perform time-of-flight measurements and nondestructive inspection.

From Acoustic Waves to Microwaves

“…The velocities of longitudinal, shear, and surface acoustic waves were measured in aluminum samples yielding the values of 6.43, 3.17, and 2.96 mm/µs, respectively, with an error smaller than 1.3% [14]. Our sensor proved to be a suitable alternative to piezoelectric or interferometric detectors for detection of ultrasonic waves, timeof-ight measurements, and nondestructive inspection.…”

Geometrical parameter analysis of a high-sensitivity fiber optic angular displacement sensor

“…Receiving fiber The principle of operation of the sensor is based on the modulation of the intensity of light by the angular displacement (reflective surface angle), θ. The power transfer coefficient, defined as the ratio between the optical output power and input power [η(θ) = P o /P i ], was derived as a function of θ in a previous work [2], and is given by:…”

“…On reference [2], the static characteristic curves were simulated for nine different values of emitting/receiving core radii (a/a R in µm) : 4/4, 4/25, 4/52.5, 25/4, 25/25, 25/52.5, 52.5/4, 52.5/25, 52.5/52.5, showing that the normalized sensitivity of the sensor increases with a decrease on the fibers core radii (for both emitting and receiving fibers) and the normalized sensitivity is dominated by the emitting fiber core radius, a, while the receiving fiber core radius, a R , plays a secondary role. These simulations were corroborated by experiment [2], showing that the mathematical model is suitable to describe the sensor behavior. Therefore, the variation on a and a R in this work were only simulated using the same model.…”

“…Fiber optic angular displacement sensors have been designed for applications that require the measurement of very small angular displacements (in the order of 1 µrad or less). By measuring the angular displacement, other physical quantities can be indirectly measured or detected, as pressure, velocity, acceleration, linear displacement, and ultrasound for example [1,2]. Therefore, this type of sensor can be used as the optical detector of an atomic fier), arranged as shown in Fig.…”

“…The mathemati- cal model (theory) of the aforementioned sensor was also depicted and validated by the comparison of the sensor's characteristic curve obtained by computational simulation and by experiment. In that case, we regarded nine different sensor configurations by varying both emitting and receiving fibers core radii [2].…”