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Title: Advanced measurement techniques in optical fiber sensor and communication systems
Authors: HU JUNHAO
Keywords: Optical communication, fiber optics sensor, pulse measurement techniques, FBG sensors, CD monitoring, fiber laser
Issue Date: 15-Aug-2011
Citation: HU JUNHAO (2011-08-15). Advanced measurement techniques in optical fiber sensor and communication systems. ScholarBank@NUS Repository.
Abstract: In this thesis, several advanced measurement techniques in optical fiber communication and fiber optic sensor systems are introduced. Firstly, pulse train generation and measurement techniques are introduced. Q-switched single wall nanotubes (SWNTs) fiber laser with low insertion loss is firstly demonstrated in this thesis. As we know, a saturable absorber is the key component of passive Q-switched laser to generate the pulse trains. These saturable absorbers are normally semiconductor saturable absorber and crystal saturable absorber, which is not friendly using to fabricate all-fiber lasers. Nowadays, SWNTs are widely used to generate ultra-short pulse width mode-locked lasers. But Q-switched SWNTs all-fiber lasers have never been demonstrated before. In this thesis, we firstly reduce the SWNTs insertion loss from 3 dB to 0.7 dB. Then we introduce three different SWNTs based Q-switched fiber lasers. Secondly, a low power autocorrelator is proposed based on degree of polarization (DOP) measurement. The chirp factor and mismatching angle is studied in simulation. It is found that the pulse widths almost have the linear relationship with the chirp factors, which means our method can be used to measure the chirp factor if the original pulse width is known. And the small effects of mismatching angle on pulse width measurements prove the high misalignment tolerance of the system. Compared with the traditional second harmonic generation (SHG) autocorrelator, which requires very rigid alignment and high laser power. Thirdly, a novel 150-km multi-point long distance FBG temperature and vibration fiber sensing system is demonstrated based on Raman amplification. In addition to a Raman laser at 1395 nm and a laser at 1480 nm, the 150-km long distance system is constructed only by passive optical components, such as the coupler, SMF and EDF. It is an all fiber long distance temperature and vibration sensor system without any electrical components along the 150-km fiber. The accuracy of this temperature sensor is about 1 oC; and the vibration measurement range is from 1 Hz to 1000 Hz. Finaly, a low bandwidth receiver delay-tap sampling method is demonstrated and proved to have better performance than the high cost high bandwidth receiver delay-tap sampling method. It is proved that the low cost low bandwidth balanced receiver has increased the CD measurement range and the sensitivity in small CD range. Then we also find that one single low bandwidth photo-detector can achieve the same performance as balanced receiver. It is obvious that a single low bandwidth photo-detector is more welcomed for its low cost and simplicity.
Appears in Collections:Ph.D Theses (Open)

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