Fiber-optical sensors
Optical measuring methods use fibre-optic sensors as sensing probes to determine the pressure variation of ultrasonic signals in space and time. Section 1.62 uses two different types (in the following referred to as displacement and pressure sensors), both based on the principle of the fibre tip acting as the sensitive element (fibre tip sensors). To carry out measurements, the fibre tip is, therefore, introduced into the sound field, and the fibre axis is aligned parallel to the direction of sound propagation.
Displacement sensors
The tip of a monomode optical fiber is coated with a titanium layer 200 nm in thickness, which increases the reflection factor. A laser beam is coupled into the fibre and reflected by the fibre end. If a sound wave hits the tip, it follows the displacement of the sound field. As a result, the optical path of the light inside the glass fibre is changed, and this results in a change of the phase of the optical field. This change can be detected with the aid of an interferometer (optical measuring method). Section 1.62 uses both homodyne and heterodyne techniques with a large bandwidth.
Use of the displacement sensors
At present, section 1.62 uses displacement sensors above all for the measurement of shock waves and pulses in the MHz-range. To establish a connection with the common standard of ultrasound measurement, the sensors were first calibrated. As components of the shock wave spectrum are at frequencies of more than 20 MHz, the frequency range of the calibration had to be extended. This has been achieved by a novel interferometric calibration method which can be used up to 50 MHz. The fibre-optic sensor system allows shock waves to be reliably measured and all parameters to be determined which are required to make a statement concerning the patients' safety or the effectivity of lithotrites.
Fibre-optical sensors are also suited to characterize high-power ultrasound in the frequency range from 40 to 100 kHz as used in technical applications. As the sensor dimensions are small, they hardly disturb the field and can be used for both the characterization of the ultrasound field and the detection of cavitation events.
Pressure sensors
Two or more hard dielectric layers are applied to the end faces of the optical fiber (cf. figure). The light field coupled into the fibre and reflected by the backside of the layer interferes with the light wave reflected by the front side (and with the multiply reflected portions). The reflection factor thus depends on the optical layer thickness (refractive index multiplied by the physical thickness). If the fibre is introduced into an ultrasound field, the layer is elastically deformed by the sound pressure, resulting in a change of the optical layer thickness and thus in a change of the complex reflection factor. The thickness - or the design - of the layer system is to be chosen such that as great a steepness as possible of the reflection factor's dependence on the optical thickness or the pressure amplitude is achieved. The sound pressure can be determined by measuring the intensity of the light field reflected at the fibre end.
The thickness and the index of the layers depend also on the temperature in the sound propagation medium. Thus temperature and sound pressure may simultaneously be measured by one sensor. Because of the different time constants of sound and heat propagation the measurement signals can be distinguished in the frequency domain.
Use of the pressure sensors
Fibre sensors with dielectric coating can be exploited in the entire ultrasonic region. At present section 1.62 uses the sensors to measure focused ultrasound fields as used, for example, in diagnostic units. The feature of the simultaneous measurement of sound pressure and temperature is suitable for an estimation of the potential risk during medical treatment and, for example, the emission of shock waves and the temperature increase during laser vitrectomy treatment could be investigated.