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

By creating a new twist on fiber optic sensors, researchers have developed a smart, flexible photoacoustic imaging technique that may have potential applications in wearable devices, instrumentation, and medical diagnostics.

Their new technique relies on optical fiber technology to provide new sensors for photoacoustic imaging. It uses fiber-optic ultrasound detection, exploiting the acoustic effects on laser pulses via the thermoelastic effect — temperature changes that occur as a result of the elastic strain.

The group designed a special ultrasound sensor that’s essentially a compact laser built within the 8-µm-diameter core of a single-mode optical fiber. This fiber then gets doped with ytterbium and erbium to provide sufficient optical gain at 1,530 nanometers. They use a 980-nm 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.

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-nm 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.