Zhiyong Dai

Optical fiber sensors for landslide monitoring

We present a landslide monitoring approach using a high-resolution distributed fiber stress sensor based on polarizationsensitive optical frequency reflectometry (P-OFDR) technology. The sensing system consists of a polarizationmaintaining (PM) fiber and an OFDR with a high spatial resolution. The PM fiber is used as a distributed sensing element. The OFDR is used to measure the polarization mode coupling loss in PM fiber causing by the external pressure along the PM fiber. With the advantages of frequency domain technique and coherent detection, the sensing system can achieve high spatial resolution, high sensitivity and large dynamic range. By monitoring the mechanical property distribution and variations in the landslide body, the occurrence of the landslides can be predicted accurately. We demonstrate an early landslide warning system based on polarization-sensitive distributed fiber stress sensor, which has a spatial resolution of 5cm, dynamic range of about 70dB and theoretical measuring range of 10km. The warning system is also investigated experimentally in the field trial.

Narrow Linewidth Fiber Grating F-P Cavity Laser and Application

Narrow line-width fiber grating 1535 nm erbium laser was demonstrated by introducing fiber grating F-P cavity. It was composed by high Er3+ doped fiber, 980/1550 nm WDM and fiber grating F-P cavity. Linear and ring cavity experiments were demonstrated respectively. In the experiment, pumped by 976 nm diode laser, the associated fiber lasers with frequency selecting by fiber grating F-P cavity exhibited about 11 mW threshold. 15% and 30% slope efficiency were obtained respectively in the experiments. The 3 dB line-width of fiber lasers were both less than 0.01 nm, and SNR were both about 50 dB. Narrow linewidth fiber laser can be used to build distributed fiber optical sensor systems. Frequency modulated continuous wave (FMCW) technique and Brillouin optical fiber time domain reflectometry (BOTDR) technique were described in this paper

Study of spectrum flattening of ASE fiber source based on long period fiber grating

Flattened ASE fiber sources with C+L bandwidth(1520-1620nm) are attractive. Long period fiber gratings (LPFG) have the characteristics of band-stopped, which can be used to flatten the spectrum of amplified spontaneous emission (ASE) light source. In this paper, spectrum flattening of ASE light source covering C-band and L-band based on a long period fiber grating is studied. As a flattening filter, LPFG is often placed in the end of the output port. The results in this paper show that the placed position of LPFG has great influence on the spectrum flattening of ASE light source. Output spectrum measured of filtered ASE source with the LPFG at different position is given, and the detailed theoretical analysis to explain the experimental results is followed. In the dual-stage double-pass structure, based on a long period fiber grating and two 980 nm high power semiconductor lasers, high power and ultra bandwidth ASE fiber source has been gained. The source has a bandwidth of 77 nm (1525.5 ~1602.5 nm) with ±1.6 dB ripple and 42 mw of output power.

A high precision metrology method and system for thin film's parameters based on reflectance spectrum

How to determine the parameters of thin film precisely and rapidly becomes more and more important. In this paper, a new method based on reflectance spectrum combining complex-shape method to determine multi parameters of thin film simultaneously by dwindling the polyhedron is first presented. On the one hand, this new method fix out the problem of multiple solutions well. On the other hand, for there is no dependence on interferential spectrum and transparent area, the method can be applied to diverse sorts of reflectance spectrum with a fast speed, nice convergence, and high precision. In the meantime, the metrology system based on this method is built up and measured experiment for standard thin film sample is implemented also. Comparing the measured results to the standard values of the standard thin film sample, the results show that the error degree is about ± 3%. The experimental results prove this method has a nice application value.

Study of the polarization maintaining optical fiber beat length variation with external force

The polarization maintaining fiber has been playing an important role in the fields of optical fiber sensing, communication, and so on. The beat length is one of the main parameters of polarization maintaining fiber, and it usually represents its polarization maintaining performance. In this paper, the beat length variation of Panda fiber with external force is investigated. The simulation results indicate that the beat length variation was determined both by the external force value F and the angle θ between the external force direction and the slow axis of Panda fiber. When F is a constant, the beat length of polarization maintaining fiber is changed in sinusoidal form whose various cycle is π with the variation of θ. Meanwhile, the minimum and maximum values of beat length will be obtained when the angles are even multiple of π/2 or odd multiple of π/2, respectively. When θ is a constant, the beat length is changed in linear form with the increasing of external force value. Finally, the experimental system of beat length measurement based on Sagnac interferometer loop is illustrated, and the result shows an excellent agreement with the theoretical analysis.

Distributed Fiber Optic Stress Sensor System Based on P-OFDR

We first proposed and demonstrated a distributed fiber sensor with high spatial resolution for detecting and locating distributed stress. The sensor system consists of a section polarization-maintaining fiber (PMF) and an optical frequency domain reflectometry (OFDR). The PMF used as sensing element and the polarization mode coupling loss in PM fiber resulting from the external stress would produce. Through monitoring the backscattered light with the distributed polarization mode coupling loss along the PMF under test, the distributed fiber optic stress sensor based on polarizationsensitive OFDR (P-OFDR) was obtained. In our experiment, a tunable single frequency fiber ring laser with narrow linewidth was developed to act as scanning frequency light source, and then the P-OFDR system was structured. The fiber laser and the distributed fiber sensor system have been investigated experimentally in detail. The experimental results show that its spatial resolution is about 5cm, the sensitivity is less than -80dBm and the maximum dynamic range are about 70dB. With advantages of high spatial resolution, high sensitivity and large dynamic range due to extremely narrow linewidth and heterodyne detection, this distributed fiber optic stress sensor has a number of potential applications for smart structure, structural health monitoring, defense and security monitoring.

Passive Q-switching dynamics in all-fiber lasers with ring interferometer

The cooperation of Rayleigh scattering (RS) in fiber interference loop and stimulated Brillouin scattering (SBS) in fiber can change Q factor in resonant cavity, based on which a all-fiber Er-doped Q-switched fiber laser with interference loop is designed. During experiment, the attributes of output laser exploiting interference loops with different coupling ratios and lengths is investigated. A train of pulse laser with pulse duration of 7.2ns, pulse period of 4.709μs, average output power of 5.4mW at pump power of 37mW is obtained which has a good waveform and a peak power fluctuation of : 30%. The results show that interference loop with too high coupling ratio has no contribution to the generation of pulse laser and the coupling ratio of 10% is suit for it. Even if the coupling ratio of : 10% is chosen, the length of the loop has influence to the attribute of the output laser pulse and the length around 2m is conducive to obtaining good output.

Measurement of spontaneous Brillouin scattering in optical fiber with a fiber Bragg grating Sagnac loop

A novel method for direct optical detection of spontaneous Brillouin scattering in optical fiber by using a fiber Bragg ngrating (FBG) Sagnac loop is introduced. The transmission character as an optical filter of FBG Sagnac loop is ninvestigated theoretically. The filter which is based on an asymmetric grating Sagnac loop is manufactured and used in nthe measurement of spontaneous Brillouin scattering sensing system, and the separation of backscattered spontaneous nBrillouin from Rayleigh is achieved effectively. It is demonstrated that the fiber grating Sagnac loop filter can be applied nin the distributed sensing system based on spontaneous Brillouin scattering.

Stable Narrow Linewidth Ring Fiber Laser with a Passive Fiber Bragg Grating Fabry-Perot Etalon and a Fiber Saturable Absorber

Narrow linewidth fiber lasers have become more and more necessary in field of coherent communication, fiber optic sensor, and high resolutions optical spectrum analysis, especially long-range distributed fiber sensing system. In this paper, a simple and stable narrow linewidth Er3+ -doped ring fiber laser is first proposed and experimentally demonstrated. In the cavity, a passive fiber Bragg grating Fabry-Perot (FBG F-P) etalon acts as mode-selecting device to greatly reduce the longitudinal-mode density, while a section un-pumped Er3+ -doped fiber serves as a saturable absorber to ensure the single longitudinal-mode (SLM) operation. Meanwhile the stability of lasing frequency is further improved by controlling the temperature of the FBG F-P, and then through varying the temperature the lasing wavelength tuning is achieved also. Finally stable SLM laser at 1550nm with linewidth about 7.5 kHz, maximum output power 39 mW and the corresponding slope efficiency 30% is acquired.

Research of Narrow Line-width Er3+-doped Fiber Ring Laser with FBG F-P Etalon and FBG Sagnac Loop

A novel method of narrow line-width Er3+-doped fiber ring laser based on FBG F-P etalon and FBG Sagnac loop is presented in this paper. The all-fiber single frequency and narrow line-width Er3+-doped fiber ring laser has been designed in which two 976 nm laser diodes are used as the pump sources, the high concentration Er3+-doped fiber as the gain medium, the fiber Faraday rotator is adopted to eliminate the spatial hole burning effect, the FBG F-P etalon and FBG Sagnac loop filter can discriminate and select laser longitudinal modes efficiently. The experiment system using 3m long Er3+-doped fibers is presented, when the maximum pump power of two 976nm laser diodes is 146mW, the fiber laser exhibits 16mW threshold and stable single frequency 1550nm laser with the output powers of 45mW is acquired, and the slope efficiency is about 34.6%. The 3dB line-width is less than 9.3 kHz, measured by the delayed selfheterodyne method with 15km single-mode fiber, and no mode hopping is observed. The fiber laser has the advantages of simple structure, high efficiency and high reliability and it has great potential applications in the fields of optical fiber sensing system.