Pang Fufei

Brillouin Optical Time Domain Reflectometry with Sub-Meter Spatial Resolution Based on Double-Pulse Detection

In conventional Brillouin optical time domain reflectometry (BOTDR),mutual restraint between the spatial resolution and the width of spontaneous Brillouin scattering spectrum can not be avoided. To solve this problem, we ameliorated the approach using double ultra-short pulses as probe light. By detecting spectral envelope, the spatial resolution of BOTDR can be extracted. This method improves the spatial resolution of the BOTDR system and meanwhile avoids the influence of Brillouin gain spectrum broadening caused by narrowing pulse on measurement accuracy. The experimental results show that with this novel BOTDR system, the temperature measurement can be realized at a 0.5 m spatial resolution, and the large broadening of the spontaneous Brillouin scattering spectrum can be avoided.

The measurement system of Faraday effect in an optical fiber based on Stokes parameter analysis

Magneto-optical fiber plays an important role in magneto-optical devices. The Faraday effect has broad applications such as Faraday rotators, isolators and current sensors. This paper investigates the magneto-optical properties by measuring the polarization states in the fiber. A new magneto-optic system based on the Stokes parameters method is designed, which could analyze the Faraday effect in an intuitive way. The evolution of polarization states in an optical fiber is described by a series of Jones matrices and Stokes parameters. The relationship between the Jones matrix and Stokes parameters of a same polarization state is given. The corresponding Faraday rotation angle can be measured in the external magnetic field with different magnetic strength by using this measurement system. The magnetic field is generated in a customized solenoid driven by a current source. The optical fiber under test is put into a glass tube in the center of the solenoid. The light beam is coupled into the fiber by a positioning stage with lens and transferred all in free space in order to decrease the influence of intrinsic birefringence in the fiber. The output beam is received by the external receiver of the polarization analyzing system which extracts the Stokes parameters of the polarized light. The Faraday rotation angle equals the difference of two azimuths drawn from the Stokes parameters of the input and output polarizations. The final output polarization state described by Stokes parameters can also be viewed directly in the Poincare sphere. The Verdet constant of a commercial SMF is measured to be -2.36±0.01 rad/(T·m) at 660nm and -0.53±0.01 rad/(T·m) at 1550nm, respectively, which validate the good performance of this measurement system.

Fiber Optic Coupler as Sensing Probe of Temperature Sensor

The temperature sensing probe is obtained by coating the waist region of the fiber optic coupler with some temperature sensitive material. The coupler used here is made by the fusion and etching method from two standard communication single mode(SM) fibers. The influence of couplers initial coupling ratio on the sensing probe is studied. The goal of our research is to fabricate desired couplers for the fiber optic temperature sensor. The system for the temperature sensor is simply introduced, and the temperatures are tested with different initial coupling ratio in our experiment. Finally, the experimental results show that the temperature sensor has good characteristics of the temperature when the initial coupling ratios are between 0.2 and 0.3.

A method of PLL detection applied in fiber optic evanescent wave temperature sensor

A method of phase-lock loop (PLL) detection applied in fiber optic evanescent wave temperature sensor is presented in this paper. The fiber optic evanescent wave sensor is fabricated by a fiber optic coupler whose cladding of waist is replaced by temperature sensitive material. In order to demonstrate its sensitivity to temperature with high stability, the PLL detection circuit to measure the temperature variety with high accuracy is be described in detail. Compared with conventional DC signal detection, the PLL detection technique has great improvement on denoise, which is able to detect weak signal buried in noise. Experiments demonstrated that the resolution to temperature could reach 0.091degC with such temperature sensor system.