Application Cases

Experiment Name: Research on Ultrasonic Sensing Performance

Test Purpose:

Fiber optic Mach-Zehnder interferometers (MZIs) are highly sensitive and structurally flexible sensing structures. When an ultrasonic signal is applied to the MZI, it affects the optical path difference between the involved interfering lights, leading to a drift in the interference spectrum. Since mode coupling is a key element of fiber optic sensors based on MZI, various fiber coupling techniques have been developed, such as fiber tapers, multimode fibers, and misaligned splices. The single-mode-multimode-single-mode (SMS) fiber structure is one of the most widely used types of fiber Mach-Zehnder interferometers with strong mode coupling mechanisms and high environmental parameter perception. Various SMS fiber structures have been developed, such as direct splicing, misaligned splicing, tapered splicing, and cascading with fiber Bragg gratings. SMS fiber structures have been successfully applied in fiber optic filters, refractive index, and humidity sensing, but their application in ultrasonic sensing has rarely been reported. This chapter proposes an ultrasonic sensor based on the SMF-taper (Bitaper)-MMF-Bitaper-SMF fiber structure. By splicing two Bitapers at the connection between SMF and MMF, the sensor structure is robust, effectively utilizing the high sensitivity and high tolerance of the SMS fiber structure to achieve multidirectional ultrasonic detection.

Testing Equipment: ATA-2021B high-voltage amplifier, signal generator, PZT, digital oscilloscope, sensor, etc.

Experiment Process:

Figure 1: Schematic diagram of the sensor under ultrasonic waves from different directions

Figure 1 shows the sensor under ultrasonic waves from different directions. When ultrasonic waves act on the sensor from direction A or F, location ① contracts, and location ② stretches; when ultrasonic waves act on the sensor from direction C or D, location ① stretches, and location ② contracts; when ultrasonic waves act on the sensor from direction B, the upper side of locations ① and ② contracts, and the lower side stretches; when ultrasonic waves act on the sensor from direction E, the upper side of locations ① and ② stretches, and the lower side contracts. Finite element software was used to perform acoustic simulation on the sensor to simulate the performance of the ultrasonic sensor, with the corresponding simulation results shown in Figure 2(a), (b), and (c). Figure 2(a) shows the XZ cross-section of the spherical total sound pressure field, which clearly indicates the direction of sound wave incidence. Figure 2(b) shows the stress corresponding to the ultrasonic signal for this sensor structure. Since the two single modes are fixed, the sensing fiber in the middle is under certain stress, around 1 Pa, corresponding to the pressure amplitude of the incident ultrasonic signal. Figure 2(c) shows the sound pressure level and displacement of the spherical sound field and the sensor, respectively. The results show that the displacement at the Bitaper is larger. The Bitaper also has concentrated sound pressure levels, with a maximum sound pressure level reaching 120 dB, indicating that the Bitaper is more likely to detect ultrasonic signals. When the sensor structure receives ultrasonic signals, the deformation of the Bitaper affects the change in the sensor length L and the effective refractive index of the transmission mode, thereby detecting changes in the output spectrum.

Figure 2: Ultrasonic field simulation of the sensor: (a) Total sound pressure field; (b) Stress; (c) Sound pressure level and displacement

The underwater fiber optic ultrasonic detection system established in the experiment is shown in Figure 3. The system mainly includes two parts: ultrasonic generation and ultrasonic detection. The ultrasonic generation system consists of a signal generator, a high-voltage amplifier, and a PZT. The PZT is driven by a signal generator with a peak voltage of 20V and a high-voltage amplifier (ATA-2021B) with a maximum voltage of 200Vp-p, providing sinusoidal pulse ultrasonic waves. The ultrasonic detection system consists of a tunable laser, a sensor, a photodetector, and a digital oscilloscope. The wavelength bandwidth of the TSL-710 is 160nm, with a linewidth of 100kHz and a resolution of 0.1pm. An underwater environment method was used to detect the ultrasonic signal to reduce the propagation loss of the ultrasonic signal and enhance the coupling between the ultrasonic signal and the sensor. In the experiment, the PZT and the sensor were immersed in a water tank, spaced 2cm apart and fixed horizontally. The dimensions of the water tank are 45cm (length) * 15cm (width) * 10cm (height). When the ultrasonic wave acts on the sensor, it causes a drift in the reflection spectrum. Using the sideband filtering demodulation technique, the spectral shift is converted into a change in optical intensity and then converted into a voltage signal by the photodetector and output to the oscilloscope for display. To obtain the best ultrasonic response, the detection wavelength of the output light beam was set at the 3dB point of the interference spectrum.

Figure 3: Schematic diagram of the fiber optic ultrasonic detection system

Experimental Results:

Figure 4: (a) Time-domain response of the sensor to a 110kHz pulse ultrasonic signal; (b) Spectrum after FFT transformation

The experiment investigated the response of the sensor with an MMF length of 2cm to a 110kHz pulse sine signal, with the results shown in Figure 4(a). The sensor successfully detected the ultrasonic signal. The real-time response of the sensor is smooth, with an absolute voltage of approximately 0.013V and a signal-to-noise ratio of 21dB. Spatial frequency is the number of times a light field signal periodically repeats in a unit of time. The FFT transformation of Figure 4(a) yields the sensor's spectrum, as shown in Figure 4(b), with the center frequency of the spectrum at 110kHz, which matches well with the ultrasonic signal emission frequency.

High-Voltage Amplifier Recommendation: ATA-2021B

Figure: Specification Parameters of the ATA-2021B High-Voltage Amplifier

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