Browsing by Author "Chen, Xiaopei"

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    Fiber Optic Pressure Sensor Fabrication Using MEMS Technology
    Chen, Xiaopei (Virginia Tech, 2003-05-08)
    A technology for fabricating fiber optic pressure sensors is described. This technology is based on intermediate-layer bonding of a fused silica ferrule to a patterned, micro-machined fused silica diaphragm, providing low temperature fabrication of optical pressure sensor heads that can operate at high temperature. Fused silica ferrules and fused silica diaphragms are chosen to reduce the temperature dependence. The fused silica diaphragms have been micro-machined using wet chemical etching in order to form extrinsic Fabry-Perot (FP) interferometric cavities. Sol-gel is used as an intermediate-layer for both fiber-ferrule bonding and ferrule-diaphragm bonding at relatively low temperature (250 °C). The pressure sensors fabricated in the manner can operate at temperatures as high as 600 °C. The self-calibrated interferometric-intensity-based (SCIIB) technology, which combines fiber interferometry and intensity-based sensing method into a single sensor system, is used to test and monitor the pressure sensor signal. The light returned from the FP cavity is split into two channels. One channel with longer coherence length can test the effective interference generated by the FP cavity, while the other channel with shorter coherence length can get signal proportional only to the source power, fiber attenuation, and other optical losses. The ratio of the signals from the two channels can compensate for all unwanted factors, including source power variations and fiber bending losses. [11]
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    Ultra-Narrow Laser Linewidth Measurement
    Chen, Xiaopei (Virginia Tech, 2003-09-08)
    In this report, we give a deeper investigation of the loss-compensated recirculating delayed self-heterodyne interferometer (LC-RDSHI) for ultra-narrow linewidth measurement, including the theoretical analysis, experimental implementation, further modification on the system and more applications. Recently, less than 1kHz linewidth fiber lasers have been commercialized. But even the manufacturers face a challenge on accurately measuring the linewidth of such lasers. There is a need to develop more accurate methods to characterize ultra-narrow laser linewidth and frequency noises. Compared with other currently available linewidth measurement techniques, the loss-compensated recirculating delayed-heterodyne interferometer (LC-RDSHI) technique is the most promising one. It overcomes the bottle-neck of the high resolution requirement on the delayed self-heterodyne interferometer (DSHI) by using a short length of fiber delay line. This method does not need another narrower and more stable laser as the reference which is the necessary component in heterodyne detection. The laser spectral lineshape can be observed directly instead of complicated interpretation in frequency discriminator techniques. The theoretical analysis of a LC-RDSHI gives us a guidance on choosing the optimal parameters of the system and assists us to interpret the recorded spectral lineshape. Laser linewidth as narrow as 700Hz has been proved to be measurable by using the LC-RDSHI method. The non-linear curve fitting of Voigt lineshape to separate Lorentzian and Gaussian components was investigated. Voigt curve fitting results give us a clear view on laser frequency noises and laser linewidth nature. It is also shown that for a ultra-narrow linewidth laser, simply taking 20dB down from the maximum value of the beat spectrum and dividing by $2\sqrt{99}$ will over estimate the laser linewidth and coherent length. Besides laser linewidth measurement in the frequency domain, we also implemented time-domain frequency noise measurement by using a LC-RDSHI. The long fiber delay obtained by a fiber recirculating loop provides a higher resolution of frequency noise measurement. However, spectral width broadening due to fiber nonlinearity, environmental perturbations and laser intrinsic 1/f frequency noises are still potential problems in the LC-RDSHI method. A new method by adding a transmitter switch and a loop switch is proposed to minimize the Kerr effect caused by multiple recirculation.