Weiwen Zou

All-optical dynamic grating generation based on Brillouin scattering in polarization-maintaining fiber

We report a novel kind of all-optical dynamic grating based on Brillouin scattering in a polarization maintaining fiber (PMF). A moving acoustic grating is generated by stimulated Brillouin scattering between writing beams in one polarization and used to reflect an orthogonally polarized reading beam at different wavelengths. The center wavelength of the grating is controllable by detuning the writing beams, and the 3 dB bandwidth of approximately 80 MHz is observed with the tunable reflectance of up to 4% in a 30 m PMF.

Proposal of Brillouin optical correlation-domain reflectometry (BOCDR)

We propose a Brillouin optical correlation-domain reflectometry (BOCDR), which can measure the distribution of strain and/or temperature along an optical fiber from a single end, by detecting spontaneous Brillouin scattering with controlling the interference of continuous lightwaves. In a pulse-based conventional Brillouin optical time-domain reflectometry (BOTDR), it is difficult in principle to achieve a spatial resolution less than 1 m, and the measurement time is as long as 5-10 minutes. On the contrary, the continuous-wave-based BOCDR can exceed the limit of 1-m resolution, and realize much faster measurement and random access to measuring positions. Spatial resolution of 40 cm was experimentally demonstrated with sampling rate of 50 Hz.

Complete discrimination of strain and temperature using Brillouin frequency shift and birefringence in a polarization-maintaining fiber.

This paper presents a novel method that realizes simultaneous and completely discriminative measurement of strain and temperature using one piece of Panda-type polarization-maintaining fibre. Two independent optical parameters in the fiber, the Brillouin frequency shift and the birefringence, are measured by evaluating the spectrum of stimulated Brillouin scattering (SBS) and that of the dynamic acoustic grating generated in SBS to get two independent responses to strain and temperature. We found that the Brillouin frequency shift and the birefringence have the same signs for strain-dependence but opposite signs for temperature-dependence. In experiment, the birefringence in the PMF is characterized with a precision of approximately 10(-8) by detecting the diffraction spectrum of the dynamic acoustic grating. A reproducible accuracy of discriminating strain and temperature as fine as 3 micro-strains and 0.08 degrees Celsius is demonstrated.

Optical time-domain measurement of Brillouin dynamic grating spectrum in a polarization-maintaining fiber

We demonstrate the distributed measurement of Brillouin dynamic grating spectra in a polarization-maintaining fiber based on time-domain analysis. Local reflection spectra of the Brillouin dynamic grating are acquired by synchronized propagation of the pump and the probe pulses based on the map of the Brillouin frequency distribution. Large temperature sensitivity as high as −50.9 MHz/°C is observed with 2 m spatial resolution in 100 m range.

Demonstration of Brillouin Distributed Discrimination of Strain and Temperature Using a Polarization-Maintaining Optical Fiber

Distributed discrimination of strain and temperature is demonstrated by localizing both the stimulated Brillouin scattering (SBS) and the dynamic acoustic grating generated in the SBS process in a polarization-maintaining fiber with a correlation-domain continuous-wave technique. A 12- ¿¿ strain resolution and 0.3°C temperature resolution together with a 10-cm spatial resolution are experimentally validated.

Stimulated brillouin scattering and its dependences on strain and temperature in a high-delta optical fiber with F-doped depressed inner cladding

Stimulated Brillouin scattering (SBS) in a high-delta fiber with F-doped depressed inner cladding is studied through considering the interaction of acoustic and optical modes in the fiber. It is found that the number of acoustic modes in the fiber is reduced and the frequency spacing between neighboring modes is enlarged because of the F doping. The dependences of SBS on strain and temperature are measured and compared for each acoustic mode to investigate the feasibility of discriminative sensing of strain and temperature by use of the fiber.

Correlation-based distributed measurement of a dynamic grating spectrum generated in stimulated Brillouin scattering in a polarization-maintaining optical fiber.

We study the principle of correlation-based distributed generation and readout of dynamic grating spectrum generated in stimulated Brillouin scattering in a polarization-maintaining optical fiber. The experimental validation is demonstrated by applying synchronous sinusoidal frequency modulations to two orthogonally polarized laser sources that serve as the Brillouin pump-probe light and readout light of the dynamic grating, respectively. Temperature-induced opposite changes in the Brillouin frequency shift and in the birefringence-determined optical frequency deviation are clearly observed with 1.2 m spatial resolution and a 110 m measurement range.

Two-Dimensional Finite-Element Modal Analysis of Brillouin Gain Spectra in Optical Fibers

A modal analysis based on two-dimensional (2-D) finite-element method for the Brillouin gain spectra (BGS) in optical fibers is demonstrated. This method enables the evaluation of optical fibers with 2-D complicated geometry and/or refractive-index profile. The BGS of two fiber samples, a single-mode fiber and a PANDA polarization-maintaining fiber (PMF) of the same design except the stress-applying elements in the PMF, are calculated using this analysis and compared to each other. The BGS are also tested experimentally for confirmation

Operation of Brillouin Optical Correlation-Domain Reflectometry: Theoretical Analysis and Experimental Validation

We theoretically and experimentally analyze the operation of Brillouin optical correlation-domain reflectometry (BOCDR). First, we experimentally confirm that BOCDR is not based on stimulated Brillouin scattering but on spontaneous Brillouin scattering. Then, we theoretically prove that the spatial resolution of BOCDR is given well by the same expression as that of Brillouin optical correlation-domain analysis (BOCDA). Finally, we demonstrate that the modulation amplitude of the laser frequency, which has been conventionally limited to a half of the Brillouin frequency shift, can be enhanced further by employing a sensing fiber shorter than a half of the measurement range.

One-laser-based generation/detection of Brillouin dynamic grating and its application to distributed discrimination of strain and temperature

This paper presents a novel scheme to generate and detect Brillouin dynamic grating in a polarization-maintaining optical fiber based on one laser source. Precise measurement of Brillouin dynamic grating spectrum is achieved benefiting from that the pump, probe and readout waves are coherently originated from the same laser source. Distributed discrimination of strain and temperature is also achieved with high accuracy.