Resonator fiber-optic gyro with intensity error suppression using a bias-sampling compensation technique.

Resonator fiber-optic gyros (RFOGs) are one of the most promising candidates for next-generation inertial rotation sensors. The frequency servo loop is used to lock the laser frequency to the resonance frequency of fiber ring resonator via adjusting the laser diode current, which increases accompanying intensity variation and induces RFOG drift. RFOG output compensation using a bias-sampling technique is proposed to suppress intensity error. The linear relationship between the RFOG output and bias is verified by theory and experimental results, and the bias-sampling compensation technique by monitoring the bias signal is analyzed in detail. With the compensation technique, intensity error is significantly suppressed, and the RFOG bias stability is effectively improved from 127.2 to 7.4 deg/h, which demonstrates tactical-grade performance.

Frequency tracking out-of-lock control in a resonator fiber-optic gyro.

A resonator fiber-optic gyro (RFOG) is being pursued because of its theoretical potential to meet navigation-grade performance with small size, high precision, and lower cost. The stability of the RFOG operation is based on the synchronization of laser frequency to the fiber ring resonator (FRR) resonance frequency. Frequency tracking out-of-lock will lead to peak pulse and zero-bias change at the output of the RFOG, which seriously degrades the performance. First, the influence mechanism of frequency tracking out-of-lock is analyzed. The change of current and temperature in frequency tracking and the symmetry change caused by backscatter and polarization are the main reasons for the peak pulse and zero-bias error. Second, a scheme of out-of-lock control of the RFOG based on temperature closed-loop operation using digital signal processing is proposed. The improved scheme, signal processing, and implementation method are investigated in detail. Finally, a RFOG prototype is assembled and tested, and 10 min tracking of the laser frequency to the FRR's single-resonance frequency is realized by temperature closed-loop operation. The static performance of the RFOG over 1 h shows that the RFOG output errors caused by frequency tracking out-of-lock are successfully eliminated. The output peak pulse is reduced from 3000 to 200 deg/h, the zero bias is eliminated from 50 to 600 deg/h to 0, and the bias stability of the RFOG is improved from 15.2 to 1.85 deg/h, which indicates a remarkable advance in the performance of the RFOG to satisfy civil navigation application requirements.

Frequency tuning ratio testing of a laser via a hollow photonic crystal fiber resonator.

In order to satisfy the requirements of laser frequency tuning ratio (FTR) measurement, experimental equipment based on a hollow photonic crystal fiber resonator (HPCFR) is proposed in this paper. First, the principle scheme of the equipment consisting of HPCFR is designed, and the resonance curves of the HPCFR are theoretically analyzed, calculated, and simulated; second, the transmissive HPCFR sample is fabricated and the resonance curve is obtained; eventually, the experimental results from the established laser FTR experimental setup demonstrate that the FTRs of a narrow-linewidth fiber laser and semiconductor laser are 17.6 MHz/V and 30.9 MHz/mA, respectively, which are basically in accordance with the factory parameters of the lasers. This work shows that the FTR experimental equipment via HPCFR has the advantages of high precision and good long-term stability.

Embedded reservoir and constructed wetland for drinking water source protection: Effects on nutrient removal and phytoplankton succession.

An embedded reservoir that provides an efficient nutrient removal system protects drinking water. However, embedded reservoirs are rarely used in eutrophic shallow lakes because of their undetermined nutrient retention efficiency and unknown effects by the phytoplankton community. In this study, we aim to investigate the nutrient retention and algae succession in an embedded reservoir and adjacent wetland from April 2017 to September 2018 in the eastern part of Lake Taihu, China. More than 40% of total phosphorus (TP) and 45% of particulate phosphorous entering the reservoir were retained semi-annually, and the highest TP removal efficiency was achieved in the reservoir during autumn with an average value of 53.3% ±â€¯9.9%. The overall nitrogen retention efficiency (21.7% ±â€¯37.8%) was lower than that of TP (41.8% ±â€¯27.8%). Similar trends were obtained in the wetland area. An important pathway for phosphorus removal is through particulate matter retention. Our study revealed that nutrient retention mechanisms in the reservoir were primarily via macrophyte absorption, particulate substance sedimentation, and prolonged water residence time. Consequently, the phytoplankton biomass (Chl-a) in the reservoir decreased (from 48.0 to 25.2 µg/L) and water transparency improved, due to the decreased P level and transformation of the phytoplankton group into simple structures with good ecological status. Therefore, the combination of embedded reservoir and constructed wetland ecosystem can be used successfully to protect surface water. The results will be advantageous to groups seeking to preserve drinking water sources.