Shuidong Xiong

Phase noise characteristics of a diode-pumped Nd:YAG laser in an unbalanced fiber-optic interferometer

The measurement sensitivity of interferometric fiber sensors can be limited by the laser phase noise. We investigate the phase noise characteristics of a diode-pumped Nd:YAG laser in an unbalanced fiber-optic interferometer. Measurements were made of the phase-induced intensity noise of an interferometer with varying optical path differences. The experimental results of the relation between the noise and the optical path difference are given, and the application of the results in the design of a fiber-optic accelerometer is discussed.

A Magnetic Field Sensor Based on a Magnetic Fluid-Filled FP-FBG Structure

Based on the characteristic magnetic-controlled refractive index property, in this paper, a magnetic fluid is used as a sensitive medium to detect the magnetic field in the fiber optic Fabry-Perot (FP) cavity. The temperature compensation in fiber Fabry-Perot magnetic sensor is demonstrated and achieved. The refractive index of the magnetic fluid varies with the applied magnetic field and external temperature, and a cross-sensitivity effect of the temperature and magnetic field occurs in the Fabry-Perot magnetic sensor and the accuracy of magnetic field measurements is affected by the thermal effect. In order to overcome this problem, we propose a modified sensor structure. With a fiber Bragg grating (FBG) written in the insert fiber end of the Fabry-Perot cavity, the FBG acts as a temperature compensation unit for the magnetic field measurement and it provides an effective solution to the cross-sensitivity effect. The experimental results show that the sensitivity of magnetic field detection improves from 0.23 nm/mT to 0.53 nm/mT, and the magnetic field measurement resolution finally reaches 37.7 T. The temperature-compensated FP-FBG magnetic sensor has obvious advantages of small volume and high sensitivity, and it has a good prospect in applications in the power industry and national defense technology areas.

Intensity noise and relaxation oscillation of a fiber-laser sensor array integrated in a single fiber

We modeled the intensity noise of a distributed-feedback fiber laser (DFB-FL) with external laser injection. The transfer function for injected power perturbation was obtained. Simulation indicates that the laser relaxation oscillation frequency is not affected by external laser injection and is determined by the laser pump power, which provides a promising way to investigate the actual pump power budget over the whole wavelength-division multiplexing (WDM) fiber-laser sensor array integrated in a single fiber. The intensity noise enhancement for each sensing unit thus can be evaluated exactly, and we observe an increase of 9dB in the intensity noise level for two DFB-FLs incorporated in a WDM fiber-laser array.

Development of a 32-element fiber optic hydrophone system

The structure and performance of an all polarization maintaining optical fibre hydrophone element is described and a sensitivity of -158±1.5dB is achieved. A 32-element, spatially multiplexed system is constructed with a noise-equivalent sound pressure of ~3.58x10-4 Pa per square root Hz at 1kHz and the major results of sea trials indicate that the system is useful for research and industrial applications.

Heterodyne demodulation scheme for fiber-optic hydrophone arrays

A signal demodulation scheme based on heterodyne technique is demonstrated, which can help fiber-optic hydrophone system meet the demand of large dynamic range and large scale multiplexing. Optical system based on the heterodyne demodulation is given. Reference signal in the demodulation procedure is obtained from the optical heterodyne output, which is quite different from other reported heterodyne techniques. This method not only simplifies RF electronics, but also eliminates the effects of frequency shift vibration generated by the acoustic-optic modulators. Demodulation algorithm is presented and the maximum signal processing capability is analyze. Experiments are carried out and the results show that a signal with a frequency of 1kHz and amplitude of 40rad could be demodulated without distortion when the heterodyne frequency is 64kHz. The same signal is demodulated using PGC scheme with a modulating frequency equal to the heterodyne frequency, and the result shows distortion. Comparing the two demodulation methods verifies that the system we have designed works well and is more suitable for the detection of large signal. Besides, this architecture of system offering advantage of easy to be time-division multiplexed. Together with wavelength-division multiplexing, it demonstrates the potential for the fiber-optic hydrophone to achieve large-scale arrays with high dynamic range.

Fast-tuning narrow-linewidth all polarization-maintaining fiber ring laser

This paper describes the structure and operation of a stable, fast-tuning, narrow-linewidth, all polarization-maintaining fibre ring laser using erbium-doped fibre as a saturable absorber. The optimum pump power for single-mode operation in the laser is identified. Laser output power is ~4.0mW at 1536nm for a pump power of 80mW, the polarization extinction ratio is 25.0dB, the SNR is larger than 60dB, the relative intensity noise is below -118dB/Hz at frequencies above 90kHz. The phase noise achieves −107dB at 1kHz while the modulation frequency of lasing optical frequency is 12.5kHz.

Interrogation of miniature extrinsic Fabry-Pérot sensor using path matched differential interferometer and phase generated carrier scheme

Study of fiber optic extrinsic Fabry-Pérot sensors utilizing state-of-the-art MEMS technology mostly focus on sensor fabrication for various applications, while the signal interrogation is still insatiable to current application. In this paper, we propose a white light path matched differential interferometer dynamic sensing system utilizing phase generated carrier demodulation scheme. A step motor with a movable mirror and a fiber-wound piezoelectric transducer string are used to act path matching and phase modulation respectively. Experimental results show that the sensing signal could be correctly recovered with low distortion and the phase noise spectrum level is less than -100 dB re. rad/√Hz above 2.5 kHz.

Transmission-link-induced intensity and phase noise in a 400-km interrogated fiber-optics hydrophone system using a phase-generated carrier scheme

Abstract. Transmission-link induced-intensity noise, as well as the conversion of intensity fluctuation to demodulated phase noise in a 400-km interrogated hydrophone array with a phase-generated carrier (PGC) scheme are investigated. Theoretical and experimental results all show that a PGC scheme can increase the erbium-doped fiber amplifiers-induced phase noise level by ∼3.7  dB in intensity to phase noise conversion. However, it can significantly reduce the coherent double Rayleigh scattering (DRS)-induced phase noise by ∼11.7  dB when compared with the unmodulated DRS intensity noise. Considering the effects of these two kinds of noises, a PGC scheme still has significant advantages for noise suppression in the remote system, and the total phase noise of the 400-km hydrophone system is shown to be only 11 to 14  μrad/sqrt (Hz). Moreover, it is demonstrated that the spectra of DRS-induced relative intensity noise and phase noise all have Lorentzian line shapes and the full widths at half maximum are twice the width of the laser source. This result is helpful for us to choose a suitable laser source in long-distance hydrophone systems.

Investigation on upper limit of dynamic range of fiber optic interferometric sensors base on the digital heterodyne demodulation scheme

The upper limit of dynamic range (ULDR) of the fiber optic interferometric sensors (FOIS) based on the digital heterodyne demodulation scheme is investigated in this paper. According to the analysis of the demodulation process, the ULDR is restricted by two factors: the heterodyne frequency and the digital orthogonal demodulation algorithm. We discuss the influence of both the factors on the ULDR respectively in detail and give the total ULDR of the FOIS system. Experiments with the different delayed heterodyne scheme are conducted to verify the theoretical analysis and the results are consistent with the analysis conclusion.

Recent progress toward fiber optic hydrophone research, application and commercialization in China

The fiber optic hydrophone research in China has a history of over 20 years. This talk will summarize some remarkable millstones in this history. Recent progress toward the fiber optic hydrophone research and application will be discussed. Some commercialization explorations will also be introduced.