A Chinese team of researchers has created a new twist on fiber optic sensors that promote the development of a smart flexible photoacoustic imaging technique, which is regarded as a way to enlarge new applications in portable devices, instrumentation, and medical diagnostics.
The operation of the new ultrasound sensor is based on fiber lasers that rely on fiber optic technology to manufacture fiber sensors for photoacoustic imaging. To be more precise, these fiber laser systems apply fiber optic ultrasound determination, utilizing the acoustic effects on laser beam pulses via the thermoelastic effect (temperature changes).
The thing is that traditional fiber optic sensors trace very weak signals due to their high sensitivity via phase measurement. Herewith, this type of fiber sensors can be employed in military purposes to find low-frequency (kilohertz) acoustic waves. Nevertheless, they provide weak operation for “ultrasound waves at the megahertz frequencies used for medical applications because ultrasound waves usually propagate as spherical waves and have a very limited interaction length with optical fibers”.
It should be noted that the main application of the novel fiber sensors is medical imaging, herein, they offer better sensitivity than the piezoelectric transducers in use today. Components of a special ultrasound fiber optic sensor include a portable fiber laser set in the 8-micron-diameter core of a single-mode optical fiber. A standard length of the fiber optic system is only 8 millimeters.
The team uses two highly reflective fiber Bragg grating mirrors written into the fiber core to achieve optical feedback to manufacture the fiber laser. Then they dope the optical fiber with ytterbium and erbium to result in efficient optical gain at 1,530 nanometers. Additionally, a 980-nanometer semiconductor laser operates as the pump laser.
The applications of such fiber laser systems with a kilohertz-order linewidth (the width of the optical spectrum) can include their use as fiber sensors, so they provide a high signal-to-noise ratio. Also, ultrasound detection takes advantage of the combined methods because side-incident ultrasound waves spoil the optical fiber, modulating the laser beam frequency.
The fiber optic sensors based on fiber lasers are highly promising for application in photoacoustic microscopy. The team applies a 532-nm nanosecond pulsed laser to illuminate a sample and excite ultrasound signals leading to a photoacoustic image of the vessels and capillaries. Moreover, it is possible to use the fiber optic system for structural imaging of other tissues and functional imaging of oxygen distribution by employing other excitation wavelengths.
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