Researchers from the Institute of Photonic Technology at Jinan University in Guangzhou, China, have created new fiber optic sensors that can improve medical technology, wearable technologies and more. The new fiber, laser-based ultrasound sensor creates smart, flexible photoacoustic imaging. It uses fiber-optic ultrasound detection and acoustic effects on laser pulses with a thermoelastic effect.
Schematic of photoacoustic microscopy using a fiber-laser-based ultrasound sensor. (Source: Long Jin)
"Conventional fiber optic sensors detect extremely weak signals by taking advantage of their high sensitivity via phase measurement," said lead researcher Long Jin.
These types of sensors are currently used in military applications to detect low-frequency acoustic waves. But when used to detect megahertz frequencies in medical applications, the sensors fail. Typically, ultrasound waves are spherical waves that have limited interaction length with optic fibers.
But the new sensors have been specifically developed to overcome these issues and have better sensitivity and compatibility for medical uses.
The new ultrasound sensor is a compact laser that is built into an 8-micron diameter core with single-mode optical fibers.
"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."
Fiber dopes with ytterbium and erbium have an optical gain of 1,530 nanometers. The team used a 980-nanometer semiconductor laser as the pump laser in the new sensors.
"Such fiber lasers with a kilohertz-order linewidth -- the width of the optical spectrum -- can be exploited as sensors because they offer a high signal-to-noise ratio," said research team member Yizhi Liang, an assistant professor at the Institute of Photonics Technology.
The combined technique is better for ultrasound detection because side-incident ultrasound waves deform the fiber and modulate the lasing frequency.
"By detecting the frequency shift, we can reconstruct the acoustic waveform," Liang said.
The team used a new “self-heterodyning” method. This method mixes the two detected frequencies. It measures the radio frequency domain beat note that is provided by two orthogonal polarization modes of fiber cavity.
Researchers say there are potential applications for the fiber laser-based ultrasound sensors to be used in photoacoustic microscopy. A focused 532-nanometer, nanosecond pulse laser illuminated the sample and excites ultrasound signals. The sensor is stationed near the sample to detect the ultrasound waves.
"By raster scanning the laser spot, we can obtain a photoacoustic image of the vessels and capillaries of a mouse's ear," Jin said. "This method can also be used to structurally image other tissues and functionally image oxygen distribution by using other excitation wavelengths -- which takes advantage of the characteristic absorption spectra of different target tissues."
"The development of our laser sensor is very encouraging because of its potential for endoscopes and wearable applications," Jin said. "But current commercial endoscopic products are typically millimeters in dimension, which can cause pain, and they don't work well within hollow organs with limited space."
The team will present this technology at OSA Frontiers in Optics + Laser Science APS/DLS conference, being held Sept. 16-20 in Washington, D.C.
