Liu Qingwen

Bio: Liu Qingwen is an academic researcher. The author has contributed to research in topics: Graded-index fiber & Luminescence. The author has an hindex of 2, co-authored 5 publications receiving 12 citations.

A mode-locked Yb 3+ -doped double-clad large-mode-area fiber laser

Abstract: A compact and highly stable high pulse energy passively mode-locked fiber laser is presented. A segment of Yb 3+ -doped double-clad large-mode-area fiber with extremely low nonlinearity is developed as the active medium. The self-starting mode-locked operation is achieved by the cooperation of nonlinear polarization evolution and semiconductor saturable-absorber mirror. The fiber laser generates laser pulses with the average power of 160mW at the repetition rate of 55.9MHz (corresponding to pulse energy of 3nJ), and the pulse duration is 10.6ps.

Advances of key technologies on optical reflectometry with ultra-high spatial resolution

Abstract: As the core technology of distributed fiber-optic sensing, optical reflectometry may realize the non-destructive measurement at a remote position It can be used to retrieve the distributed information such as reflectance, refractive index, polarization state along the optical fiber, and to diagnose the irregular “event” on fiber-optic links For some high-end fields, such as the fault diagnosis on the fiber-to-the-home (FTTH) access network, the deformation monitoring on large generating units and large transformers, and the security monitoring on structures of airplane wings, the requirements on spatial resolution and measurement range of the sensing tech-nologies are very high In this paper, we summarized the research status on state-of-art optical reflectometry technologies, and reviewed the advances of key technologies on optical reflectometry with ultra-high spatial resolution and long measurement range We proposed three different methods to improve the performance, and tried to promote their applications on distributed fiber-optic sensing systems

Luminescence properties of Yb 3+ /Na + codoped CaF 2 crystal and laser operation with low threshold

Abstract: Basing on measuring the luminescence spectra at room temperature pumped by 975nm light, the action mechanism of Na + in the novel Yb 3+ ,Na + :CaF 2 crystal was systemically investigated. The relation between the concentration of doping Na + and the laser operation threshold was analyzed. The optimum match concentration for Yb 3+ ,Na + :CaF 2 with low laser threshold was obtained. In the experiment, the lowest laser operation threshold at only 70mW of absorbing pumped power for 2%Yb 3+ , 3%Na + :CaF 2 crystal is realized, which is in good agreement with the analysis of luminescence properties.

A novel LD pumped Yb:GSO laser operating at 1090 nm with low threshold

Abstract: We report what we believe to be the first demonstration of laser operation with a novel laser material of Yb 3+ -doped Gd 2 SiO 2 (Yb:GSO)pumped by a laser diode at 940 nm. We obtained a low lasing threshold of 1.27 kW/cm 2 with the center wavelength of 1090 nm, which is lower than the value of 1.53 kW/cm 2 predicted for Yb:YAG. The maximal output power of 360 mW was obtained with a 2% output, which corresponds to a slope efficiency up to 19%.

Numerical and experimental investigation of dissipative solitons in passively mode-locked fiber lasers with large net-normal-dispersion and high nonlinearity

Abstract: Dissipative soliton evolution in passively mode-locked fiber lasers with large net-normal-dispersion and high nonlinearity is investigated numerically and confirmed experimentally. I have proposed a theoretical model including the nonlinear polarization evolution and spectral filtering effect. This model successfully predicts the pulse behaviors of the proposed laser, such as the multi-soliton evolution, quasi-rectangle-spectrum profile, trapezoid-spectrum profile, and unstable state. Numerical results show that, in contrast to the typical net- or all-normal-dispersion fiber lasers with the slight variation of the pulse breathing, the breathing ratios of the pulse duration and spectral width of our laser are more than three and two during the intra-cavity propagation, respectively. The nonlinear polarization rotation mechanism together with spectral filtering effect plays the key roles on the pulse evolution. The experimental observations confirm the theoretical predictions.

A Contextual GMM-HMM Smart Fiber Optic Surveillance System for Pipeline Integrity Threat Detection

Abstract: This paper presents a novel pipeline integrity surveillance system aimed to the detection and classification of threats in the vicinity of a long gas pipeline. The sensing system is based on phase-sensitive optical time domain reflectometry ( $\phi$ -OTDR) technology for signal acquisition and pattern recognition strategies for threat identification. The proposal incorporates contextual information at the feature level in a Gaussian Mixture Model and Hidden Markov Model (GMM-HMM) based pattern classification system and applies a system combination strategy for acoustic trace decision. System combination relies on majority voting of the decisions given by the individual contextual information sources and the number of states used for HMM modeling. The system runs in two different modes: first, machine+activity identification, which recognizes the activity being carried out by a certain machine, second, threat detection, aimed to detect threats no matter what the real activity being conducted is. In comparison with the previous systems based on the same rigorous experimental setup, the results show that the system combination from the contextual feature information and the GMM-HMM approach improves the results for both machine+activity identification (7.6% of relative improvement with respect to the best published result in the literature on this task) and threat detection (26.6% of relative improvement in the false alarm rate with 2.1% relative reduction in the threat detection rate).

Yb3+ doped (Ca,Sr,Ba)F2 for high power laser applications

Abstract: Yb3+ doped CaF2 has been proved in the last years to be one of the most attractive Yb3+ doped materials for broadly tunable, high power and short-pulse laser operation as well as for bi-frequency laser and THz wave generation. Interesting results were also obtained by using Yb3+:CaF2 as regenerative amplifier of femtosecond pulses in the mJ level with very high damage threshold and, even more recently, prove was made that cryogenically cooled materials could lead to even more promising performance at high power levels. The purpose of the present paper consists first in a clarification of a number of luminescence, thermal and laser properties as well as in the description of the most recent laser results, obtained in particular at low temperature.

A Multi-Position Approach in a Smart Fiber-Optic Surveillance System for Pipeline Integrity Threat Detection

Abstract: We present a new pipeline integrity surveillance system for long gas pipeline threat detection and classification. The system is based on distributed acoustic sensing with phase-sensitive optical time domain reflectometry (ϕ-OTDR) and pattern recognition for event classification. The proposal incorporates a multi-position approach in a Gaussian Mixture Model (GMM)-based pattern classification system which operates in a real-field scenario with a thorough experimental procedure. The objective is exploiting the availability of vibration-related data at positions nearby the one actually producing the main disturbance to improve the robustness of the trained models. The system integrates two classification tasks: (1) machine + activity identification, which identifies the machine that is working over the pipeline along with the activity being carried out, and (2) threat detection, which aims to detect suspicious threats for the pipeline integrity (independently of the activity being carried out). For the machine + activity identification mode, the multi-position approach for model training obtains better performance than the previously presented single-position approach for activities that show consistent behavior and high energy (between 6% and 11% absolute) with an overall increase of 3% absolute in the classification accuracy. For the threat detection mode, the proposed approach gets an 8% absolute reduction in the false alarm rate with an overall increase of 4.5% absolute in the classification accuracy.