Ultrasound is one the most widely used non-invasive methods to examine internal and external structures. It has been implemented in many scientific fields, for various uses, and generally involves the use of an acoustic impedance matching layer due to a high acoustic coupling loss at the sample/air interface. Now, a team of researchers from China has developed a new method for the detection of ultrasound wave using capillary-based optical ring resonators, which will allow compact and sensitive ultrasound detection in many applications, such as air-coupled non-destructive ultrasound testing, photoacoustic imaging, and remote sensing.
The requirement for an acoustic impedance in matching layer in ultrasound sensing is due to high acoustic coupling loss at the sample/air interface and the large acoustic absorption of air at ultrasonic frequencies. Both factors are known to significantly reduce the intensity of the wave received by ultrasonic detectors. These acoustic losses also make the imaging process difficult when the detector and the sample are contactless, i.e. when imaging sensitive wounds or dangerous materials, and require highly sensitive acoustic detectors with low noise equivalent pressures (NEPs).
Optics-based ultrasound detection techniques have become an attractive alternative to traditional ultrasound detectors. Optical-based detectors are not only immune to electromagnetic interference, they do not suffer from geometry-dependent electrical noise. There have currently been two approaches developed when using optics-bases detectors. One involves using a remote optical detector to detect the ultrasound waves by interferometry and/or beam deflection, whereas the second option is using an optical resonator-based detector.
Optical resonators are the general method of choice employed by the researchers in their new detection system. Optical resonators directly measure the ultrasonic waves through the photoelastic effect and/or physical deformation of the resonator. There are many advantages for using resonators, from the ability to build a detection array, high pressure sensitivity and small form factors. They are also able to detect ultrasound waves directly, negating the problems of surface roughness- which is a common problem when using interferometric detectors. Various resonators have been tested as ultrasound detectors but always use a coupling media such as water. The Chinese researchers have managed to develop an air-coupled ultrasound detector using ring resonators based on fused silica capillaries.
In a ring resonator, the light is coupled into a whispering gallery mode (WGM) where is circulates around the resonators circumference. In the presence of ultrasonic (or any pressure) waves, the WGM undergoes a spectral shift caused by changes in the refractive index and shape of the resonator. The spectral change can easily be monitored and analysed, directly or indirectly, by the spectral domain and light transmission intensity, respectively.
The ring resonator was fabricated by stretching a fused silica capillary under laser illumination to generate curvature along the capillary. The researchers secured the capillary to the resonator by a U-shaped glass holder and glue, with an 800 KHz transducer (Japan Probe 0.9K14x20N-RX) above. The transmission of the device was measured by DC-coupled (New Focus 1811, DC – 125MHz bandwidth) and AC-coupled (New Focus 1611, 30 kHz – 1GHz bandwidth) photodetectors and analysed by an oscilloscope (Tektronix DPO 3014) and a spectrum analyser (Agilent N9010A).
The detector was found to have a high Q factor (>107), with noise equivalent pressures of 215 mPa/√Hz and 41 mPa/√Hz at 50 kHz and 800 kHz, respectively. The detector also shows a comparable detection limit to commercially available sensors and possess a resonance frequency of around 1.65 MHz.
There are future plans to broaden the frequency response, where materials such as low-refractive polymers could be used for mechanical damping. The researchers also aim to increase the sensitivity by using thinner capillaries. It is expected that this could increase the sensitivity five-fold whilst still retaining high optical Q-factors. It will also be possible in the future to improve the performance of the resonator by using a balanced detector and by using flow-through liquids that exploit a larger photoelastic coefficient. It is the first detector that relies on air rather than a liquid at the coupling interface and is a big development for ultrasonic detectors as a whole.
The development of this novel method using optical ring resonators has the potential to be used in many applications, such as in compact and sensitive air-coupled non-destructive ultrasound testing, photoacoustic imaging, and remote sensing. The new method will also provide a model system to fundamentally study the mechanical modes in high Q ring resonators, as well as using photoacoustic pulses to detect bio-analytes.
Kim K. H., Luo W., Zhang C., Tian C., Guo J. L., Wang X., Fan X., Air-coupled ultrasound detection using capillary-based optical ring resonators, Scientific Reports, 2017, 7, 109