Investigation Of Novel High-resolution Optical Sensing System And Its Key Devices

Optical sensors have been played a critical role in the development of the Internet of Things(IOT)thanks to the key advantages such as high capability,superior security,long transmission distance after combining with optical fiber.In addition,the develop?ment of the IOT also requires more sensors with higher performances.In this thesis,four sensing systems with high-speed and high-resolution interrogation system have been proposed.Based on the modal interference structure,phase-shift fiber Bragg grat-ing(PS-FBG),micro-disk resonator(MDR)and random gratin,four novel interrogation methods involving the optical field and the microwave photonics filed have been demonstrated to improve the sensing sensitivities,interrogation speed and resolution.Every chapter in the thesis introduces a kind of sensing system with novel interrogation system and analyzes the basic theory,the realization of the key elements and the system performance.The main achievements and conclusions are showed as following:1.A new laser sensing system combining the novel optical fiber sensor based on the mode interference and fiber ring laser has been proposed and experimentally demonstrated.A special kind of few mode fiber with concentric ring core area(CRCF)has been designed and fabricated by MCVD technique.Based on the home-made CRCF and model interference theory,a kind of few mode-single mode-few mode structure,named as CSC structure,has been fabricated.According to the model interference theo-ry,the relationship between the length of CSC structure and the transmission spectrum has been analyzed.The CSC structure with best performance has been fabricated.As the sensing element and band-pass filter,the CSC structure has been combined with the fi-ber ring laser which transforms the wavelength change of the interference dip to the wavelength change of the laser.The optical signal-to-noise ratio of the sensing wave-length has been improved by 30dB,and the 3-dB bandwidth has been reduced to 3%of the original sensor based on CSC structure,which manifests a high measurement reso-lution.2.A sensing system combining the PS-FBG inscribed in a few-mode fiber and the optoelectronic oscillator(OEO)for simultaneous measurements of the strain and tem-perature has been proposed and experimentally demonstrated.For the first time to our knowledge,a PS-FBG inscribed in a home-made few-mode fiber,named as FM-PS-FBG,has been presented.Owing to the self-coupling between same modes,it exhibits two main grating peaks with notches in the reflection spectrum.According to the phase modulation-intensity modulation(PM-IM)theory,a frequency-switchable microwave photonics filter(MPF)has been realized which transforms the wavelength change of the FM-PS-FBG to the central frequency change of the MPF.By incorporat-ing the FM-PS-FBG into an OEO loop,a novel frequency-switchable OEO has been proposed whose oscillating frequency is determined by the central frequency change of the MPF.The sensing information has been encoded in the oscillating frequency of the OEO which can be detected by the electrical spectrum analyzer with high response speed and high resolution.Two oscillation frequencies demonstrate different strain sen?sitivities(54.3 MHz/?? and 58.5 MHz/??)and different temperature sensitivities(1.5189 GHz/? and 1.3412 GHz/?)respectively.Due to the characteristics of modes,two OEO-generated microwave signals exhibit different strain and temperature sensitiv-ities.Therefore,though monitoring the frequency shifts of microwave signals,the sim-ultaneous strain and temperature sensing can be realized by the proposed OEO sensor with high-speed and high-resolution interrogation system.3.A silicon photonic integrated micro-disk resonator(MDR)to realize simultane-ous measurements with high-speed and high-resolution has been proposed and experi-mentally demonstrated.The performance of the MDR has been analyzed from the basic theory,main factors and testing results.According to the two notches in the transmis-sion spectrum of the MDR and the PM-IM theory,a dual-passband MPF has been ob-tained.By applying a broadband linearly chirped microwave waveform to the MPF?two filtered microwave waveforms with their central frequencies corresponding to the wavelength shifts of the notches have been generated.To increase the signal-to-noise ratio of the filtered microwave waveforms,a noise reduction algorithm based on phase-only filter has been proposed and employed.The measurements of the central frequencies have been performed at high speed and high resolution using a digital signal processor.The on-chip optical sensing system can provide a high interrogation speed of 1?s,and high temperature and refractive index interrogation resolutions of 2.4×10-5?and 9.1×10-8 RIU.The resolution has been improved which are four orders of magni-tude better than a conventional optical sensing system using an optical spectrum ana-lyzer.4.A real-time random grating sensor array for quasi-distributed sensing has been proposed and experimentally demonstrated.The manufacture method of random grating,the basic theory of random grating and the comparison between random grating and FBG have been presented.Many random gratings inscribed in a section of single mode fiber can realize the quasi-distributed sensing system'based on the time division multi-plexing.According to the spectral-shaping and wavelength-to-time(SS-WTT)mapping,the central wavelength change of the reflection spectrum of the random grating has been converted to the time domain as a time shift of the electrical waveform.After detected by the oscilloscope and compressed by correlation in the digital signal processor,the interrogation time has been shorten to 50 ns.The sensing resolutions for temperature and RI are 0.23 ? and 2.5?? respectively,and the accuracies for temperature and strain are 0.11 ? and 1.2 ?? respectively.