ULTRASOUND DETECTION BY APPLYING FIBER BRAGG GRATINGS

Subject of Research. This paper presents the results of an experimental study on the possibility of applying single fiber Bragg gratings as sensitive elements for detecting ultrasonic impacts in gaseous or liquid media and at placing fiber Bragg gratings into various materials and structures for their status monitoring. Method. During the experiment, an ultrasonic impact with a fundamental frequency of 65 kHz was alternately turned on two sensitive elements based on two fiber Bragg gratings with different parameters: the physical grating length, reflection coefficient, and slope of the linear part of spectral characteristics. Comparative analysis of the obtained data with data from the reference piezoelectric ultrasonic sensor was performed. The results were evaluated in the frequency domain, at the range up to 200 kHz. The three first harmonics of the signal were studied: 65, 130 and 195 kHz. The signal-to-noise ratio for each sensor element and the ratio of signal values obtained from various sensor elements were evaluated. Main Results. The measurement setup was created on the basis of a small-sized tunable VCSEL and FPGA. It is shown that single fiber Bragg gratings are suitable for creation of sensitive elements for ultrasonic sensors and have values of sensitivity and dynamic range comparable to piezoelectric sensors. The range of detected frequencies was theoretically estimated and the assessment of the FBG parameters effect on the sensor sensitivity to ultrasonic action was performed. The ratios of the signals measured by the Bragg grating with a slope of the spectral characteristics equal to 142 1/nm and a reflection coefficient equal to 100%, to the signals from the grating with a slope of 44 1/nm and a reflection of 40% are equal to 5.8, 3.8, 7.1 for 65, 130 and 195 kHz, respectively. The ratios of the signals measured by the reference piezoelectric sensor to the signals measured by Bragg grating with a slope of the spectral characteristics equal to 142 1/nm and a reflection coefficient of 100% are 3.8, 6.2, 7.7 for 65, 130 and 195 kHz, respectively. Practical Relevance. The results of this study show the possibility of applying fiber Bragg gratings as the sensitive elements of threshold and measuring ultrasonic sensors for the placement in the volume and on the surface of the materials under study. The features and advantages of fiber-optic measuring systems provide the ease of installation for the arrays of sensors in the material or structure under research during production, insensitivity to external electromagnetic interference and the possibility of multiplexing a large number of sensitive elements on a single optical fiber.