Wideband robust optical detector of ultrasound for intravascular applications

Optical sensors of ultrasound are a promising alternative to piezoelectric
techniques. Specifically, fiber-based sensors are attractive for
intravascular optoacoustic imaging because of their miniature size and
optical transparency. One of the major limitations of optical sensing
technology is its susceptibility to environmental conditions, e.g. changes
in pressure and temperature, which may saturate the detection.
Additionally, the acoustic impendence mismatch between the fiber and
water may lead to acoustic resonances, which limit imaging capabilities.
In this work a fiber-based optical sensor of ultrasound is demonstrated.
The sensor is based on a pi-phase-shifted fiber Bragg grating with an
effective sensing length of approximately 350 µm. A novel technique
for optical readout of the sensor is developed which is based on pulse
Conference 8581:
Photons Plus Ultrasound: Imaging and Sensing 2013
278A Return to Contents
interferometry. In contrast to standard coherent continuous-wave
interrogation techniques, no locking of the laser’s wavelength to the
sensor’s wavelength is required: The pulses possess a wide spectrum
which covers the grating’s notch under all practical conditions, enabling
stable performance under strong external perturbations.
The new technique enables robust ultrasound detection under external
perturbations and exhibits sensitivity equivalent to that of standard
coherent continuous-wave techniques. A detection bandwidth of 30
MHz was demonstrated, comparable with intravascular piezoelectric
transducers. Additionally, despite acoustic impendence mismatches, the
sensor exhibited an orderly, resonance-free spatio-temporal response in
the frequency band 6 MHz – 30 MHz, facilitating imaging applications.