Fiber Under Test Research Articles

Low Coherent Optical Frequency Domain Reflectometry Interrogates Fiber Bragg Grating Sensors

A novel fiber optical reflectometry is proposed for long distance measurements and sensing applications. The reflectrometric information is included in the phase difference of two interfering light beams reflected at the same scattering point on the fiber under test (FUT). The reflection position and local information can be determined by modulating the frequency difference and analyzing the reflecting signal in frequency domain. Theoretical analysis and experimental demonstration are presented. Two adjacent reflection points 50 km away from the reflectometry with a gap of 11 meters between each other is distinguished. It is estimated that the operating range could be over 500 km theoretically by employing a light source with 0.3 nm bandwidth. This novel reflectometry is applied as a fiber Bragg grating interrogation solution as an example.

Simple distributed optical fiber sensor based on Brillouin amplification of microwave photonic signals

A simple distributed fiber sensor is proposed by Brillouin amplification and depletion of microwave photonic signals. This technique utilizes optical carrier suppression (OCS) modulation to generate two sidebands acting as the pump waves. After Fresnel reflection at the cleaved end of the fiber under test (FUT), the pump waves are converted to probe waves. By sweeping the modulated microwave frequency, the stimulated Brillouin scattering (SBS) spectra and Brillouin frequency shift (BFS) can be precisely characterized. As a result, the accurate measurement of the Brillouin frequency shift makes good determination of the distributed temperature or stain along the FUT, and the position along the FUT can be resolved and controlled by the delay of the microwave pulses. Furthermore, our proposed sensing system is implemented with a single-ended configuration which has shown many advantages such as immunity of the laser frequency fluctuation and compact setup without additional pump laser source and filters.