DEMODULATION OF FIBER OPTIC SENSORS FOR ULTRASOUND DETECTION AND STRAIN MEASUREMENT

Optical fiber sensors are employed to study and investigate several physical-related parameters e,g, pressure, stress, vibration, rotation, current, bending, displacement etc. Fiber optic sensor (FOS) technology is associated with optical related accessories e,g, light processing (filters), optical source (laser, LED), optical detector (spectrometer. photodiode), light guiding (lenses) etc. In addition to the use of FOS technology, laser Diode (LD) has great deal of importance due to its easy integration, small size and moderate price. Semiconductor lasers are complex nonlinear systems where relatively small optical feedback can have a profound impact on the spectral and temporal behavior of the laser output. Under appropriate conditions, optical feedback provides a straightforward and highly effective way for laser linewidth reduction. These conditions can be met in the so-called self-injection locking [1, 2] or filtered optical feedback [3, 5] configurations where part of output light, after passing through an optical resonator, is injected back to the laser to interfere coherently with the light inside the laser internal cavity. Due to the excellent noise performance and straightforward implementation, fiber-pigtailed lasers under self-injection locking have been studied as light sources for fiber-optic sensor systems whose performance is sensitive to laser frequency noises such as phase-sensitive optical time-domain reflectometry systems and fiber-optic gyroscopes [6, 7, 8].We present a method to suppress the wavelength drift of a semiconductor laser with filtered optical feedback from a long fiber-optic loop. The laser wavelength is stabilized to the filter peak through actively controlling the phase delay of the feedback light. A detailed steady-state analysis of the laser wavelength is performed to illustrate the method. Experimentally, the wavelength drift was reduced by 75% compared to the case without phase delay control. The long optical feedback length makes the lasers prone to mode-hopping. There have been reported attempts at suppressing mode-hopping by light polarization control and using more compact resonator [6, 7] but no detailed characterization of mode-hopping and the associated laser instability has been reported. We studied the mode-hopping and laser instability of the self-injection locked laser and found that a mode hopping event causes an abrupt change in the laser intensity after the resonator inside the feedback loop. Experiment shows that the frequency of locked lasers could oscillate during unstable operations. We demonstrate the use of a self-injection locked distributed feedback (DFB) diode laser for high-sensitivity detection of acoustic emission (AE) using a fiber-coil Fabry-Perot interferometer (FPI) sensor. The FPI AE sensor is formed by two weak fiber Bragg gratings on the ends of a long span of coiled fiber, resulting in dense sinusoidal fringes in its reflection spectrum that allows the use of a modified phase-generated carrier demodulation method. The demodulation method does not require agile tuning capability of the laser, which makes the self-injection locked laser particularly attractive for the application. Little work has been reported on using self-injection locked lasers in fiber-optic AE or ultrasonic sensor systems due to the challenges induced by the lack of the agile wavelength tuning capability of a self-injection locked laser. Experimental results indicate that the self-injection locked laser increases the signal-to-noise ratio by ~33 dB compared with the free-running DFB laser. Furthermore, we have developed a low-cost fiber-optic sensor system that can measure absolute strain at multiple positions along a fiber using fiber-bragg grating sensors. In this system, we form a “rf interferometer” with low order fringes to ensure that the strain-induced rf spectral shift does not exceed half of the fringe period, rendering the possibility of absolute strain measurement.