Dagong Jia

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.

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.

A Quantitative Robustness Evaluation Model for Optical Fiber Sensor Networks

Optical fiber sensor networks (OFSNs) are facing the problem of a lack of systematic evaluation criteria to assess network performance. In this paper, a universal quantitative robustness evaluation model for OFSNs is proposed. The model defines robustness as the mathematical expectation of the monitoring coverage ratio, which has taken into account the performance under all possible network states and the probability of each state. This model is applied to four basic network topologies including line, ring, star and bus topologies, and their mathematical expressions of robustness are derived by analyzing all possible states in detail. Further simulation gives a quantitative comparison among these topologies, proving that the ring and star topologies are optimal for the monitoring of strip-shaped and square regions, respectively. Finally, two influencing factors, the attenuation coefficient and the threshold, are investigated for their impact on the robustness of the network.

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.

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.

A Self-Healing Passive Fiber Bragg Grating Sensor Network

A self-healing passive fiber Bragg grating (FBG) sensor network (SN) is presented in this paper. The self-healing network consists of a star subnet and a ring subnet. The self-healing ability with single and multiple fiber link failures is theoretically analyzed. In addition, the self-healing function under different network statuses is experimentally demonstrated. A quantitative method for determining the status of FBG sensing signals is presented based on experimental observations, and a threshold of intensity for the FBG sensors in the FBG SN is given.

Mitigation of the birefringence dispersion on the polarization coupling measurement in a long-distance high-birefringence fiber

Due to the birefringence dispersion, the polarization coupling parameter measurement in high-birefringence fiber decreases obviously with the fiber length, especially for long-distance fibers. In this paper, two methods for mitigating the birefringence dispersion in a long-distance fiber are proposed. The first method is a spectral-domain measurement method. The experimental setup and results are described in detail. The other method is a time-domain numerical dispersion compensation algorithm to amend the coupling strength calculation equation. It is based on the fact that the interferogram envelope area is constant even with the existence of birefringence dispersion. The experimental result shows that the time-domain algorithm has high accuracy, and the absolute deviation is less than 1%. The two methods are validated to mitigate the birefringence dispersion in the long-distance high-birefringence fiber effectively.

Spectral-domain measurement of chromatic dispersion difference of polarization modes in polarization-maintaining fibers

A spectral white-light interferometric (WLI) technique for measuring the chromatic dispersion difference of the two polarization eigenmodes in polarization-maintaining fibers (PMFs) is presented. The approach is based on the application of a transverse point-like force on the fiber that causes coupling between the two modes and utilizes their spectral interferogram. A Fourier-transform spectral interferogram (FTSI) and a polynomial curve fitting are applied to obtain the phase function. The chromatic dispersion difference of the long PMFs can be obtained by taking the second derivative of the phase function. The experimental results for two kinds of PMFs of 110 and 750 m length are presented, respectively.

Influence of vibration disturbance during polarization coupling measurement of polarization-maintaining fiber

The principle of the mode cross coupling in polarization-maintaining fiber based on white-light interferometry was analyzed. The method of measuring the polarization mode coupling with a spatial Michelson interferometer was presented. Analysis and emulation were carried out for the vibration disturbance signal caused by the mechanical scanning and the influence the vibration imposed on the judgment of coupling intensity. The interference signal envelope is extracted by a Hilbert transform and fitted by a Gaussian least-squares method under the different scanning speed. It is indicated that the detection accuracy varies with the vibration amplitude, which varies with scanning speed. The best scanning speed of the system should be from 0.7 mm/s to 0.9 mm/s to achieve the minimum detection error.

Theoretical Analysis and Experimental Validation of Sampling Volume in Tilted Imaging System

A method for accurately calculating the sampling volume in a tilted imaging system is proposed. Two types of tilted imaging systems are investigated, i.e., Type I (only the object plane is inclined to the optical axis) and Type II (both the object plane and the image plane are inclined to the optical axis). Based on geometrical optics, the sampling volumes of these two types of tilted imaging systems have been simulated. Compared with the central magnification of an imaging system and the focal length and F-number of the imaging lens, the effect of the tilted angle of the object plane on the sampling volume is determined to be more important. A tilted imaging experimental setup has been established, calibration plate images at various tilted angles and positions have been acquired, and the sampling volume has been obtained by image processing. The experimental results are in very good agreement with the theoretical predictions. As the tilted angle increases, for the Type-I system, the sampling volume increases, whereas for the Type-II system, the sampling volume decreases. In addition, the sampling volume of the Type-II system is larger than that of the Type-I system. Knowledge of the sampling volume is necessary in many quantitative applications of tilted imaging systems.