Wencai Jing

Wavelength Sweep of Intracavity Fiber Laser for Low Concentration Gas Detection

Wavelength sweep technique (WST) is introduced into intracavity fiber laser (ICFL) for low concentration gas detection. The limitation induced by noise can be eliminated using this method, and the performance of the system is improved. The sensitivity of the system is reduced to less than 200 ppm. With WST, the sweeping characteristic of the ICFL can be described according to known gas absorption spectra.

Distributed measurement of birefringence dispersion in polarization-maintaining fibers

A new method to measure the birefringence dispersion in high-birefringence polarization-maintaining fibers is presented using white-light interferometry. By analyzing broadening of low-coherence interferograms obtained in a scanning Michelson interferometer, the birefringence dispersion and its variation along different fiber sections are acquired with high sensitivity and accuracy. Birefringence dispersions of two PANDA fibers at their operation wavelength are measured to be 0.011 ps/(km nm) and 0.018 ps/(km nm), respectively. Distributed measurement capability of the method is also verified experimentally.

Characterization of birefringence dispersion in polarization-maintaining fibers by use of white-light interferometry

A new method for measuring the birefringence dispersion in polarization-maintaining fibers (PMFs) with high sensitivity and accuracy is presented. The method employs white-light interferences between two orthogonally polarized modes of PMFs. The group birefringence of the fiber is calibrated first. Then the birefringence dispersion and its variation along different fiber sections are acquired by analyzing the broadening of interferograms at different fiber lengths. The main sources of error are investigated. Birefringence dispersions of two PANDA fibers at their operation wavelength are measured to be 0.011 ps/(km nm) and 0.018 ps/(km nm). A measurement repeatability of 0.001 ps/(km nm) is achieved.

Design and implementation of a broadband optical rotary joint using C-lenses.

A broadband optical rotary joint (BORJ) was designed using C-lenses. Its insertion loss was less than 2 dB at both the 1300 nm and 1550 nm wavelength windows. Wavelength division multiplexing (WDM) technique was adopted to increase the number of data transmission channels. Hundreds of wavelength channels can be accommodated for data transmission through relatively rotating interfaces using this BORJ. The total data transmission rate through this BORJ can be more than 200 Gbit/s. By using Dove prism, both space division multiplexing (SDM) and WDM techniques can be implemented simultaneously in the design of BORJ with C-lenses. This structure of BORJ has a low cost. It can be used for optical data transmission and optical sensing.

Investigation of Wavelength Modulation and Wavelength Sweep Techniques in Intracavity Fiber Laser for Gas Detection

Wavelength modulation technique (WMT) and wavelength sweep technique (WST) are introduced into intracavity fiber laser for both gas concentration sensing and absorption wavelength detection in this paper. The principle of gas sensing and spectral analysis using WMT and WST was studied. Polynomial fit was adopted to model the system nonlinear characteristic, based on which absorption wavelength can be detected. The system optimization and acetylene gas sensing were both realized, and the absolute detected error can be increased less than 75 ppm. The absorption wavelengths of the detected gas were calculated based on the polynomial fitting results of the system nonlinear. The absorption wavelengths of acetylene were detected using this method, with absolute error no more than 0.445 nm. The system has the ability of realizing both concentration sensing and gas-type recognition.

Rotation angle optimization of the polarization eigenmodes for detection of weak mode coupling in birefringent waveguides

White light interferometry has been adopted to measure distributed polarization coupling in high-birefringence waveguides. Since the coupling mode is weak compared to the exciting mode, the contrast ratio of the interferogram is very low. This will increase the difficulty of direct detection of the polarization coupling intensity. By rotating the angle between the polarization eigenmodes and the principal axis of the linear polarizer from 45 degrees to 85 degrees , the contrast ratio of the interferogram can be improved more than 10 times. As a result, the measurement sensitivity can be improved more than 100 times.

Measurement accuracy improvement with PZT scanning for detection of DPC in Hi-Bi fibers

Piezo-electronic transducer is used in combination with mechanical scanning devices to improve measurement accuracy in a distributed polarization coupling detection system. For mechanical scanning range of 150 mm with 200-nm resolution, the measurement error of polarization-coupling intensity can be improved from 38% to 2% in combination with a 20-nm resolution piezo-electronic transducer.

Fiber Ring Laser Intra-cavity Absorption Spectroscopy for Gas Sensing: Analysis and Experiment

Fiber ring laser based intra-cavity absorption spectroscopic sensor has great potential for high sensitivity gas detection. Using the rate equations and propagation equations, we investigated theoretically factors that affect the sensitivity of such fiber ring laser sensors and determined the optimal design parameters and conditions for significant enhancement of the system sensitivity. Experiments have been conducted to determine the sensitivity enhancement performance. The results showed a factor of 25 ~ 30 in sensitivity enhancement in the experimental system, agreeing well with the theoretical expectations. Experiments on acetylene detection have also been carried out and the results showed that the ring cavity significantly increases the signal absorption and that high sensitivity can be obtained for gas detection.

Effects of angular misalignment in interferometric detection of distributed polarization coupling

White light interferometry is used to measure the distributed polarization coupling (DPC) in polarization-maintaining fibers (PMFs). By using a scanning Michelson interferometer to compensate the optical path difference (OPD) induced by the modal birefringence of PMFs, both the coupling strength and positions of the coupling points can be acquired. In ideal DPC measurement, the two reflective mirrors on the fixed and scanning arms of the Michelson interferometer are normal to each other. But in practice, the movable reflective mirror cannot be aligned normally to the fixed mirror exactly, which leads to an angular misalignment. The angular misalignment causes a variation of the OPD, which will reduce the fringe visibility. The theoretical simulation is investigated correspondingly. Consequently, the angular error leads to the miscalculation of the polarization coupling intensity. Based on the experimental results, a revised coupling strength calculation equation is proposed to minimize the influence of angular misalignment.

Design of single-channel optical rotary joint using bulk optical detector

The insertion loss of a fiber optical rotary joint depends on the mechanical misalignments of the collimating lenses. To achieve 3-dB or less insertion loss, the lateral misalignment of the graded-index (GRIN) lenses should be less than 100 μm, and the angular tilt of these lenses is limited to 0.1 deg. These requirements cause a high mechanical fabrication and assembly cost. To solve this problem, we design a single-channel optical rotary joint using a bulk optical detector. The requirement on lateral misalignment is relaxed to 500 μm, and the angular tilting is relaxed to 2 deg. Using this method, the cost of the optical rotary joint can be dramatically reduced.