Wen Xiao

Truly Distributed Fiber Vibration Sensor Using Pulse Base BOTDA With Wide Dynamic Range

We report on what we believe to be the first truly distributed fiber vibration sensor based on pulse base Brillouin optical time-domain analysis (BOTDA). The pulse base approach is used to realize one-end one-laser simplification of the BOTDA system, in which the modulated pulse creates the probe wave and interacts with the pulse pump. Short pulse ( 10 ns) is used to enhance the dynamic range for the sensed object. A signal acquisition system with a 1-GHz sampling rate and 2-kHz trigger rate is employed to measure the truly distributed vibrations along the sensing fiber. With 6.25-ns pulsewidth, 160-MHz dynamic range is verified and different vibrations with 0-, 1.4-, and 2.4-Hz frequency components are distinguished in a 168-m sensing range. The experimental results demonstrate that this proposed sensor may be applicable for distributed dynamic measurements of large structural components.

Distributed Temperature Sensing System Based on Rayleigh Scattering BOTDA

A distributed temperature sensor based on Rayleigh scattering Brillouin optical time domain analysis (Rayleigh-BOTDA) is proposed in this paper. The sensor uses Rayleigh backscattering effect of microwave modulated pulse base sidebands as probe wave and a high sensitive photon counting detector for Brillouin signal intensity detection. Compared with a conventional BOTDA system, the Rayleigh-BOTDA effectively suppresses polarization-induced signal fluctuation resulting in improved signal intensity. The experimental scheme presented is simplified by using a single laser with one-end access. The temperature accuracy of the new sensing system was demonstrated as 1°C on spatial resolution of 3 m.

Distributed fiber sensor based on modulated pulse base reflection and Brillouin gain spectrum analysis

In recent years, several distributed sensor systems based on stimulated Brillouin scattering in optical fibers have been proposed [J. Intell. Mater. Syst. Struct.10, 340 (1999); Proc. SPIE5855, 555 (2005)]. We propose a simpler scheme based on fiber-end reflection and Brillouin gain spectrum analysis. In this setup, only one optical source is necessary to provide both the pump and the probe wave; the latter is provided by the modulated pulse base. First, the physical mechanisms for two different Brillouin scattering processes in our sensor system are analyzed and an approximate theory model is proposed. In addition, it is demonstrated that the simple system configuration allows simultaneous acquisition of the time-domain and the frequency-domain information. It is experimentally demonstrated that this configuration is effective for strain measurements and could as well be applied to temperature monitoring.

Long-term quantitative phase-contrast imaging of living cells by digital holographic microscopy

The dynamic analysis of biological living samples is one of the particular interests in life sciences. An improved digital holographic microscope for long-term quantitative phase-contrast imaging of living cells is presented in this paper. The optical configuration is optimized in the form of a free-space-fiber hybrid system which promotes the flexibility of imaging in complex or semi-enclosed experimental environment. Aberrations compensation is implemented taking into account the additional phase aberration induced by liquid culture medium in long-term observation. The proposed approach is applied to investigate living samples of MC3T3-E1 and MLO-Y4 cells. The experimental results demonstrate its availability in the analysis of cellular changes.

Influence of membrane surface shape change on the performance characteristics of a fiber optic microphone.

At a reflective intensity modulated fiber optic microphone (RIM-FOM), the acoustic signal makes a membrane vibrate and modulate the reflected intensity. In the existing models of the RIM-FOM, the offset of all points of the membrane, due to the vibration, is assumed to be equal. However, this assumption does not represent the actual vibration of the membrane, which follows a continuous surface shape change caused by the acoustic signal. We establish a revised theoretical model in which the influence of the actual membrane surface shape change on the reflective intensity modulation is considered. Experiments show that there is a discrepancy between the experimental optimum operating distance and the analytical result from the existing model, while our new model gives a better agreement with the experimental results. In particular, our analysis shows that, in using the existing model, the other microphone performance characteristics are misestimated, while our revised model can provide a closer solution.

Research on the influence of membrane on the performance of vibration sensor

Intensity modulated fiber optic sensor is one of the fastest growing and most widely used fiber optic sensors. A double optical-fiber vibration sensing system with a new kind of membrane is introduced in this article. The system is based on optical intensity modulation and is characterized by the membrane that is used to sense vibration. The mechanism of film sensing is described and the model showing the relationship between film parameters (reflection coefficient and radius of curvature) and the sensor performance is established. With the simulated responses, a membrane structure with high sensitivity is developed. The experimental setup used to test the performance of the sensor is presented. The sensor can detect the minimum vibration amplitude of 0.02μm. The frequency response is from 300Hzto1.8kHz.

Optical tomographic reconstruction based on multi-slice wave propagation method

In optical tomography, it is challenging to obtain high-quality results for complex-structured objects which induce multiple scattering. Nonlinear reconstruction methods outperform linear ones in these situations. A promising nonlinear method is the approach based on beam propagation method, but its accuracy may decrease for complicated structures. In this paper, we describe a novel tomographic reconstruction method using multi-slice wave propagation method (WPM) as the forward model, which simulates the scattering process more precisely but has not been introduced in tomographic reconstruction before. The computational model of WPM is presented. To tackle the computational complexity, we propose an efficient scheme to compute the transmitted field and its derivative. We then use an iterative optimization method to recover the quantitative refraction index distribution. We also discuss the influences of the parameters in the method and how to determine their values. The experimental results demonstrate that this method can address multiple scattering problems and provide high accuracy for complex-structured objects.

Phase retrieval from double axially displaced holograms for dual-wavelength in-line holography

A phase retrieval method for dual-wavelength in-line digital holography is presented with double axially displaced holograms. A synthetic wavelength is used during iterative propagations to retrieve wrap-free phase distributions with a much extended measurement range. The simulation and experimental results demonstrate a better elimination of the twin image, a faster rate of convergence of the iterative routine and less number of wavelengths are compared with previously reported multiple-wavelength in-line holography.

Reduction of speckle noise in digital holography by multiple holograms

In this paper, we introduce and demonstrate a new method of reducing speckle noise by multiply holograms. It can utilize both the phase and amplitude information of the object wave-front. In the superposition process, the phase surfaces with high signal to noise ratio in the different phase images have been appropriately weighted according to reconstructed intensity image. We show a theoretical justification of the procedure and experimentally-obtained results of a USAF-1951 resolution test target, both of which demonstrate that the proposed method could reduce the speckle noise effectively, and improve the resolution of reconstructed image.

Real-time Micro-vibration Measurement Based on Digital Holographic Interferometry

An experimental system based on digital holographic interferometry is proposed for real-time high-precision micro-vibration measurement. Vibration information can be extracted directly and instantaneously from the holograms by using the fast micro-vibration testing algorithm. As the received light of the proposed system is scattered beam, no assumption is made on the object surface. Unlike other optical detection approaches, the proposed method can detect plane vibration rather than point vibration without any assumptions on object surface feature and receiving angle. The feasibility of the proposed system is demonstrated by using a vibrating object driven by standard sinusoidal signal.