New Fiber-Laser Sensor Will Improve Diagnostic Accuracy

While working on the creation of new optical fiber sensors, Chinese researchers invented a "smart" and flexible photoacoustic imaging technology that could potentially be used to create portable electronic devices, medical instruments, and diagnostic tools. The new fiber sensor, developed based on laser and ultrasound, was presented at an international conference dedicated to optics and laser technologies. The presentation of the innovation was conducted by Long Jin, a leading researcher at the Institute of Photonic Technologies at Jinan University in Guangzhou, who used a laser-acoustic microscope for this purpose.

The foundation of the new technology developed by Chinese scientists is fiber laser modulation, which utilizes the thermoelastic effect: when tissues are exposed to a laser, their temperature increases, leading to elastic deformation. "Conventional optical fiber sensors detect extremely weak signals due to their high sensitivity in phase measurement," says Jin. Similar sensors are used by the military to detect low-frequency (in the kilohertz range) acoustic waves.

However, it turns out that they are not as effective at capturing ultrasound waves at megahertz frequencies, which are used for medical purposes, because they typically propagate spherically and have a very limited interaction length with optical fibers. The new sensors were specifically designed for use in medical imaging and can provide higher sensitivity compared to the piezoelectric sensors currently in use.

The team of scientists developed a special ultrasound sensor, which is a compact laser with an 8-micron core made of single-mode optical fiber. "It has a standard length of 8 mm," says Jin. "In creating the laser, we embedded two highly reflective grating mirrors into the core of the fiber to ensure optical feedback." These optical fibers were then doped with ytterbium and erbium to guarantee sufficient optical amplification. As a pump laser, the researchers used a 980-nanometer semiconductor laser. Such lasers can be used as sensors because they have a fairly high signal-to-noise ratio, the researchers note.

Capturing ultrasound waves becomes more effective because they deform the fiber, modulating the frequency of laser generation. "By detecting the frequency change, we can recreate the acoustic wave pattern," says one of the researchers. They do not detect the ultrasound signal by extracting the original information using standard interferometry or any other frequency capture method. Instead, the scientists use a method called "self-heterodyning," which detects the mixing of two frequencies.

The laser-fiber ultrasound sensor can be applied in photoacoustic microscopy. Using a focused pulsed laser with a wavelength of 532 nanometers, the scientists illuminated a sample of biological tissues, inducing ultrasound pulses within them. They placed a stationary sensor next to the sample being studied to detect optically induced ultrasound waves.

"Through raster scanning of the laser spot, we were able to obtain a photoacoustic image of the blood vessels and capillaries of a mouse ear," says Dr. Jin. "This method can also be used for imaging other tissues." He adds that optical fibers have many advantages, including their miniaturization, light weight, and natural flexibility.