Fiber-Optic Sensor Based on Core-Offset Fused Unequal-Length Fiber Segments to Improve Ultrasound Detection Sensitivity

Fiber-optic sensors fabricated by core-offset splicing method with 2 to 12 unequal-length single-mode fiber (Corning SMF-28) segments are demonstrated for the ultrasound detection. High-order modes are created in the air and silica cladding to enhance multi-mode interference, leading to large spectrum range comparing to that case with the same number of equal fiber segments. This leads to larger wavelength tuning range which overcomes the free-spectrum-range limitation in Fabry-Perot interferometer with multiple equal fiber segments. Furthermore, wavelength shift associated with power change, and characterized as high spectrum slope defined as 1st derivative of the reflection spectrum, is realized based on these unequal-length and asymmetric waveguides, which enhances the sensitivity of ultrasound detection. Using piezoelectric transducer as ultrasound generator at 260 kHz with 10 V peak-to-peak driving voltage, we detect ultrasound response based on the fiber devices with 2 to 12 segments as sensing unit or as feedback/sensor in a fiber laser. In the former sensor case, the ultrasound response via photo-detector could be increased from 15.5 mV for the core-offset fiber with 2 unequal-length segments to 115.3 mV for that with 12 segments, corresponding to a factor of 7.4 times improvement due to narrow linewidth of reflection spectrum in the fiber-optic sensor with more unequal-length fiber segments. The fiber laser sensor for the ultrasound detection brings improvement from 31.6 mV to 207.1 mV with the enhancement of 6.6 times, thanks to higher quality factor (Q) due to the optical filter formed by unequal-length core-offset fiber.