High sensitivity, real time distributed and cost effective sensor system is in great need for structure healthy monitoring in civil engineering. In our lab, we are developing a distributed, Stimulated Brillouin Scattering based, fiber optic sensing system at 1550nm wavelength. Our current SBS-based fiber optic sensor system works at 1310nm wavelength. Two expensive Nd: YAG Lasers (US$40,000 each) are being used, which leads to a soaring high cost to the entire system and eventually limits its application. Distributed Feedback (DFB) lasers have large tenability, compact size and low cost (less than US$1000 each). But they are not stable enough for the sensing system. In this project, we use the frequency offset locking technique with optical delay line and electrical feedback circuit to optimizing the stability of DFB lasers so that the lasers of 1310 nm in the sensor system can be substituted by the lasers of 1550 nm that is the most often used band in modern fiber optic telecommunication system. Less than 100 kHz stability of the beat frequency is required to achieve temperature accuracy of 0.1°C and strain accuracy of 2me. In our system we have realized 20 kHz stability of beat frequency of two DFB lasers. Greater than 800MHz turning range is necessary for the detection of temperature range of 600 °C and strain range of 10,000 me. In our system we have achieved 925 MHz in 18.75 seconds. In the sensing part, we can vary the pulse width from 120ns to 5ns that means we can realize the spatial resolution of 50cm at least. Because the total optical loss in the setup is comparably smaller, the measurable fiber length is mainly determined by the optical power launched to the fiber, normally it is in tens kilometers.
A new generalized Poincare sphere method is proposed to measure the complex principal states of polarization vector for a system with polarization dependent loss or gain. The spectral resolved measurements with our propsoed test-set agree with the well known Jones eigenvalue analysis method.
A novel method of measuring the polarization dependent loss (PDL) and the polarization mode dispersion (PMD) of fiber Bragg gratings (FBGs) is proposed and demonstrated experimentally. This method eliminates the influence of components (e.g. circulator) on the PDL and PMD characterization of FBGs. The experimental results are compared with those measured using other methods.