  In this paper, we report the development of an intensity modulated fiber
optic sensor for angular displacement measurement. This sensor was designed to
present high sensitivity, linear response, 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 $(0.25\times
V_{\rm max})$ to $(2.40\times V_{\rm max})$ mV/$\mu$rad (where $V_{\rm max}$ is
the peak voltage of the static characteristic curve) and the linear range, from
194 to 1840 $\mu$rad. The unnormalized sensitivity for a reflective surface
with reflectivity of 100\% was measured as 7.7 mV/$\mu$rad. 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 velocity of the longitudinal, shear, and surface
waves were measured on aluminum samples as 6.43 mm/$\mu$s, 3.17 mm/$\mu$s, 2.96
mm/$\mu$s, respectively, with an error smaller than 1.3\%. The sensor proved to
be suitable to detect ultrasonic waves and to perform time-of-flight
measurements and nondestructive inspection, being an alternative to the
piezoelectric or the interferometric detectors.
