Researchers at Jinan University in China have tinkered with the design of optical fibers to create a novel new type of sensor for photoacoustic imaging. Because the technology uses flexible fibers, it may have applications for implantable, wearable, and diagnostic medical devices.

Photoacoustics involves using laser light to create ultrasound waves within tissue. This is possible because light will heat up the tissue and the thermal expansion that occurs generates waves through nearby tissues. The new imaging technique was applied to create an in vivo photoacoustic microscope, imaging from which is already revealing amazing details that would otherwise be only possible with a scalpel involved in the process.

A photoacoustic microscope image of blood vessels and capillaries in a mouse’s ear (size: 2.7×2.7mm2). Credit: Long Jin.

Some details about the technology according to The Optical Society, that is hosting the researchers at its OSA Frontiers in Optics + Laser Science APS/DLS event:

“Conventional fiber optic sensors detect extremely weak signals by taking advantage of their high sensitivity via phase measurement,” said [Long] Jin [of the Institute of Photonics Technology at Jinan University]. These same sorts of sensors are used in military applications to detect low-frequency (kilohertz) acoustic waves. But it turns out that they don’t work so well for ultrasound waves at the megahertz frequencies used for medical purposes because ultrasound waves typically propagate as spherical waves and have a very limited interaction length with optical fibers. The new sensors were specifically developed for medical imaging, Jin said, and can provide better sensitivity than the piezoelectric transducers in use today.

The group designed a special ultrasound sensor that’s essentially a compact laser built within the 8-micron-diameter core of a single-mode optical fiber. “It has a typical length of only 8 millimeters,” Jin said. “To build up the laser, two highly reflective grating mirrors are UV-written into the fiber core to provide optical feedback.”

This fiber then gets doped with ytterbium and erbium to provide sufficient optical gain at 1,530 nanometers. They use a 980-nanometer semiconductor laser as the pump laser.

The ultrasound detection benefits from the combined technique because side-incident ultrasound waves deform the fiber, modulating the lasing frequency.

“By detecting the frequency shift, we can reconstruct the acoustic waveform,” [said Yizhi Liang, an assistant professor at the Institute of Photonics Technology].

The team does not demodulate the ultrasound signal, extracting the original information, using conventional interferometry-based methods or any additive frequency locking. Rather, they use another method, called “self-heterodyning,” where the result of mixing two frequencies is detected. Here, they measure the radio frequency-domain beat note given by two orthogonal polarization modes of the fiber cavity. This demodulation also intrinsically guarantees a stable signal output.

The fiber laser-based ultrasound sensors offer opportunities for use in photoacoustic microscopy. The researchers used a focused 532-nanometer nanosecond pulse laser to illuminate a sample and excite ultrasound signals. They place a sensor in a stationary position near the biological sample to detect optically induced ultrasound waves.

Via: The Optical Society…

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