Because of the phase modulation (PM) and amplitude modulation (AM) of the probe signal from the electric optical modulator (EOM), the Brillouin gain/loss spectrum becomes asymmetric. The central Brillouin frequency is shifted from that of AM pulse. The maximum extinction ratio of the EOM is limited to ~29 cB for power splitting ratio of 51% to 49%. The PM also induces sub-peaks in the Brillouin spectrum due to the interaction of phonon field and AM/PM based probe beam. The sub-peaks is enhanced when the beat frequency of pump and probe beam is off from the Brillouin frequency.
Proc. SPIE. 5855, 17th International Conference on Optical Fibre Sensors
KEYWORDS: Signal to noise ratio, Continuous wave operation, Sensors, Signal attenuation, Numerical simulations, Single mode fibers, Wave propagation, Temperature sensors, Spatial resolution, Pulsed laser operation
We present a sensing principle of the distributed fiber Brillouin strain and temperature sensor by coherent probe-pump technique that offers a new method to achieve centimeter spatial resolution with high frequency resolution. A combination of continuous wave (cw) and pulse source as the probe (Stokes) beam and cw laser as the pump beam have resulted in stronger Brillouin interaction of Stokes and pump inside the pulse-length in the form of cw-pump and pulse-pump interactions. We find that the coherent portion inside the pulse-length of these two interactions due to the same phase has a very high Brillouin amplification. The Brillouin profile originating from the coherent interaction of pulse-pump with cw-pump results in high temperature and strain accuracy with centimeter resolution, which has been verified by successfully detecting 1.5 cm out-layer crack on an optical ground wire (OPGW) cable.
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.
Optical fiber sensor technology has progressed at a rapid pace over the last decade. Many different sensing techniques have been developed to monitor specific parameters. In particular, distributed Brillouin scattering-based sensor systems provide an excellent opportunity for structural health monitoring of civil structures by allowing measurements to be taken along the entire length of the fiber, rather than at discrete points, by using fiber itself as the sensing medium. One class of Brillouin-based sensors is based on the Brillouin loss technique, whereby two counter-propagating laser beams, a pulse and a CW, exchange energy through an induced acoustic field.
This type of sensing has tremendous potential for structural health monitoring since the spatial resolution can be adjusted for different applications simply by altering the pulse duration, even after the fiber is installed. Although the spatial resolution can be improved using short pulse, the loss spectrum broadens as the pulse width decreases below the phonon lifetime t. Hence, it was generally believe that sub-meter resolutions were unachievable due to rapid linewidth increases when pulse width W < t = 10 ns provided a 1 m spatial resolution limitation.
In this paper, we will report the development of distributed optical fiber sensor with centimeter spatial resolution. The sensing principle will be presented. We will also report the test results of pipeline buckling and corrosion fatigue monitoring and small damages/cracks of 1.5 cm in an optical ground wire (OPGW) cable with centimeter spatial resolution.
We solved the three coupled partial differential equations in transient regime for the probe-pump Brillouin sensor to explain the sub-peaks in Brillouin loss spectra, which have been experimentally observed. We discovered that the Fourier spectrum of the pulsed signal and the off-resonance oscillation attributed to sub-peaks. The off-resonance oscillation at frequency [v - v<sub>B</sub>] is the oscillation in the Brillouin time domain when the beat frequency v of the two counter-propagating laser beams does not match the local Brillouin frequency v<sub>B</sub>. This is important in differentiating the sub-peaks from strain/temperature peaks.