Distributed Fiber Sensors

Structural health monitoring (SHM) has become an essential part of capital-intensive structures such as bridges, dams, oil/gas pipelines and aircraft for early detection of abnormalities thereby reducing maintenance cost and extending their lifetime. Monitoring of strain and temperature are quite helpful for SHM of these structures. Distributed sensors are advantageous to monitor these large structures. Optical fiber based sensors are capable of distributed sensing where a single strand of optical fiber is used as a sensor. We use Brillouin scattering which is a non-linear phenomenon for distributed measurement of strain and temperature.

Long range static measurements:
Brillouin-based distributed optical fiber sensing systems have been one of the most widely attracted, particularly the Brillouin optical time-domain analysis (BOTDA) measurement scheme as they provide high precision distributed temperature and strain measurements along hundreds of thousands of measurement positions over large structures. Multitudes of features that we are focusing are as follows.

• We have studied the acousto-optic interaction that leads to the Brillouin process numerically and explored the multiple features in the Brillouin gain spectrum for temperature and strain discrimination.
• We have implemented Golay complementary pairs for high spatial resolution in long-range applications. We are working on pulse On/OFF time of the Golay sequence for improving the performance.

Short range dynamic measurements:
The objective is to detect dynamic strain variations at frequencies of kHz order over few tens of meters with cm-order spatial resolution and to demonstrate simultaneous multi-point dynamic strain sensing. The application related to this is vibration monitoring on an aircraft. We use the Brillouin Optical Correlation Domain Analysis (BOCDA) technique for dynamic strain measurements which has proved to provide measurements with spatial resolution at the sub-meter levels. The salient results are as follows.

• We have mapped the spatial profile of the correlation peak which is Brillouin gain distribution in BOCDA by employing pulsed pump.
• We have proposed and demonstrated proof of concept of external phase modulation based BOCDA for simultaneous multi-point dynamic sensing.
• The ongoing work is on dynamic strain sensing.