Haifeng Qi

In this paper, an experimental ytterbium-doped distributed feedback fiber laser (Yb DFB-FL) was built. The main research focused on the effects of two pump configurations (forward and backward) and pump power on the performance of the laser, aiming to optimize the performance of the Yb DFB-FL. The experimental results show that compared with the forward pump configuration, the backward pump configuration has more complete absorption of the pump laser, higher output power and better unidirectionality. However, as the pump power increased, the output linewidth (LW) of the laser broadened significantly. To address this issue, we also designed an amplification configuration to minimize the broadening of the laser LW caused by the high pump power. Ultimately, an output laser power of 30 mW was achieved under a pump power of 150 mW, with a LW of 12 kHz.

The strain measurement range of an optical frequency domain reflectometry (OFDR) system is constrained by spatial resolution, which is affected by shifts in the distance domain. In this paper, a novel correlation spectrum self-compensation scheme based the reference signal is introduced, which improves the alignment between the reference and measurement signals along the sensing fiber. This contrasts with traditional cross-correlation by directly analyzing the spectrum shifts between the reference and measurement signals. The spatial mismatch caused by overstretching of the fiber can be suppressed with the spatial moving of the reference signal so that the trade-off between spatial resolution and wide strain measurement range can be overcame. Experimental results demonstrate a strain gradient range from 1000 μϵ to 10000 μϵ free of outliers with a spatial resolution of 2 mm. The proposed method offers a new approach to simultaneously achieve wide strain measurement and high spatial resolution in OFDR systems. This marks a significant step forward in high-performance distributed strain sensing system, particularly for applications requiring both broad strain measurement and enhanced spatial resolution.

The linear demodulation range and background noise of the Michelson interferometer system are investigated with a laser phase noise measurement system. We have theoretically and experimentally analyzed the performance of the interferometer system by changing the frequency modulation amplitude of the laser and the optical path difference (OPD) of the interferometer, respectively. It is shown that the linear demodulation range of the Michelson interferometer system is finite, which depends on the parameters of the system, such as the sample frequency, the delay time between two interferometer arms, and the system bandwidth. Furthermore, the experimental results indicate that the background noise of the interferometer system can be reduced by using a sufficiently long OPD so that the smaller true phase information can be detected with the demodulation system. The parameters of the measurement system could be optimized to satisfy the demand of the phase demodulation with different levels, which is of great significance for the phase monitoring interrogator, such as fiber-optical interferometer sensors and distributed acoustic sensors.

The temperature dependency of erbium doped distributed feedback fiber laser output performance is experimentally and theoretically investigated with two major pump schemes. The output power declined linearly as temperature increased from 30°C to 120°C for 980nm and 1480nm pumped test laser while the later exhibited stronger heat susceptibility. Further experiment and analysis identified the temperature induced absorption and emission cross section variation at pump and signal wavelength together with population inversion rate difference are the originations of the thermal sensitivity discrepancy between the two pump setup. Associated theoretical simulation concludes the erbium doped distributed feedback fiber laser thermal output sensitivity is dependent on its gain dynamics and grating structure simultaneously.

In this paper, a temperature self-compensated refractive index sensor based on fiber Bragg grating (FBG) and the ellipsoid structure is demonstrated. The ellipsoid can excite the cladding modes and recouple them into the fiber core. Two well-defined wavelength bands are observed in the reflection spectrum of the proposed sensor, i.e., the Bragg resonant peak and the cladding resonant peaks. By measuring the wavelength shift of the cladding resonant peak, the surrounding refractive index (SRI) can be determined, and the wavelength shift of the Bragg resonant peak can be used as a reliable reference to self-compensate the temperature variation (temperature sensitivity of 10.76 pm/°C). When the SRI changes from 1.3352 to 1.3722, the cladding resonant peak redshifts linearly with an average sensitivity of 352.6 pm/RIU (refractive index unit). When the SRI changes from 1.3722 to 1.4426, an exponential redshift is observed with a maximum sensitivity of 4182.2 pm/RIU. Especially, the sensing performance is not very reliant on the distance between the FBG and the ellipsoid, greatly improving the ease of the fabrication.

Type I fiber Bragg grating (FBG) regeneration is studied in terms of refractive index modulation (RIM) and characterized using optical low-coherent reflectometry (OLCR) for the first time. Highly resolved spatial OLCR results are used to demonstrate a uniform RIM regeneration ratio along an apodized type I the FBG during the regeneration process. RIM regeneration ratios are hence proved to be similar for FBGs with different RIM amplitudes. A clearer understanding of FBG regeneration results allows the establishment of a novel RIM regeneration ratio method to quantify and accurately evaluate FBG regeneration.

A regenerated fiber Bragg grating (RFBG) in silica fiber was used to observe the viscous relaxation process of the host silica fiber at high temperatures of around 1000 °C. Two factors, preannealing time and loaded tension, which affect viscous relaxation, were observed. When an RFBG is stretched after a longer preannealing, the measured viscosity of the optical fiber was observed to reach equilibrium faster, which means that preannealing accelerates viscous relaxation. A similar acceleration phenomenon was also observed when a larger load was applied to stretch the optical fiber, although the acceleration effect of loaded tension was not as strong as in the preannealing case. The results play an active role in establishing effective optical-fiber devices for application in high-temperature environments.