Characterization And Sensing Application Of Long Period Fiber Gratings

With the continuous development of optoelectronic device fabricationtechnologies, fiber grating has become one of the most promising opticalpassive devices. From the1990s, long period fiber grating ‘s oversized perioddetermines it has a completely different coupling mechanisms compared withtraditional Bragg grating. Long period fiber grating periods are usually tens tohundreds of microns, it can coupled the energy of the guided mode of the coreto the transmission of the cladding modes of the same direction. Addition tolow cost, simple preparation, ease of integration same as traditional fibergrating, long period fiber grating has some special advantages such as lowinsertion loss, and sensitive to a number of environmental parameters, and sohave a very broad development in the field of communications, sensingprospects for the past two decades, and has attracted the concern of peoplefrom all walks of life, such as fiber optic and optoelectronic devices. Longperiod fiber grating have a practical effect in more areas, its structure,characteristics, materials and processing methods in continuous research andits modification and application have also done a lot of exploration, long periodfiber grating due to its high sensitivity in the sensing field has been widelyrecognized.Long period fiber grating has a large period. To satisfy the phase matchingconditions the coupling occurs between guided modes of the core andcladding mode with the same transmission direction, so long period fiber grating’s coupling wavelength and coupling strength have close relevanceswith the external environment changes. The resonance peak of the long periodfiber grating is very sensitive to the changes of the external environment,temperature, strain, refractive index, so long period fiber grating sensors has astrong advantage and a wide range of applications.We studied the characteristics of long period fiber grating and derivation ofthe long period fiber grating couple mode equations and analyzed severalfactors that can affect the spectral characteristics of long period fiber grating;studied the preparation of the long period fiber grating mainstream methodsand compared the spectral characteristics of long period fiber grating UVwritten method and with infrared femtosecond laser writing prepared; designedand prepared two long period fiber grating-based sensors: one for the smallmolecules based on porous film modificated long period fiber grating sensor;second is based on a small period long period fiber grating thickness sensor.The main research work:1. Based on three-step fiber model and theory of weakly guiding fiberapproximation, we derived the mode field distribution equation of LPFG andcalculated the effective refractive index of the core-guided mode and claddingmodes; derived the phase matching equations through mathematicalsimplification; analyzed the impact of a series of preparation parameters, suchas cycle, the gate length and the thickness of the cladding to the long-periodfiber grating spectral characteristics, which guides the later work of preparationof long period fiber grating and sensing applications research.2. We have studied a variety of technical methods for the preparation oflong period fiber grating: UV-write method, the ion implantation method,frequency CO2laser pulses writing, arc discharge method, mechanicalmicrobending, the corrosion groove method and femto-second laser writing.Using UV writting and femtosecond laser writing method we prepared period460 m long period fiber grating and analyzed the impact of processingparameters such as laser energy, the length of the grating, grating period on the long period fiber grating spectral characteristics. In addition, some specialprocessing can also affect the grating prepared, such as hydrogen loadingprocess, the annealing process, the final corrosion method could furtheradjusted the transmission spectrum of the long period fiber grating.3. We designed and prepared a small molecule sensor based on the theporous high refractive index film coated long period fiber grating. First for theproblem that the long-period fiber grating refractive index sensor is notsensitive to low concentrations of small molecule solution, a porous highrefractive index sensitive layer was deposited to improve the sensitivity of thefiber grating to small molecules, the fabrication of the film was with the methodof the electrostatic layer-by-layer self-assembly technology, this film has arefractive index higher than conventional polyelectrolyte films, and moreimportantly, it has a special nanoporous structure, which allows the entry andenrichment of small molecules. This film was coated onto the surface of longperiod fiber grating and then the resonance spectroscopy changed with theoutside small molecule medium very sensitively. Glucose molecules, forexample, was detected with TiO2/Polyion hybrid film modified long period fibergrating refractive index sensor, and the limit can reach10-7M. The sensitivityhas been a great improvement compared with the traditional long-period fibergrating refractive index.4. For the film growth monitoring, we designed and preparated a devicethat can realize in situ, real-time monitoring of the thickness variation of theelectrostatic assembly of thin film growth. First we simulate the differentparameters of the resonant wavelength of long period fiber grating with thesurface of the film thickness variation by numerical analysis method, and wefound that when a long period fiber grating period is shorter (less than100m), the resonant wavelength rapidly changes with the thickness of the filmwhile coating, and the change was the form of a third-order curve. Wepreparated a long period fiber grating with a period of100m by femtosecondlaser direct writing and detected the growth of three different films, the results showed that for different films, the resonance wavelength of the long-periodfiber grating variated with a film thickness very fast, the thickness sensitivity isup to0.24nm/nm, and the resolution us up to0.2nm for the0.05nmresolution of optical spectrum analyzer, the long period fiber grating detectionthickness resolution could fully meet the needs of the majority of electrostaticassembled film thickness monitoring.In summary, the long period fiber grating is a promising optical sensors, lowprices and excellent detection sensitivity promote its developmemt vigorously.Since the fiber itself is a standardized device, the fiber-based devices areeasily integrated in the future. We modified long period fiber grating refractiveindex sensor by electrostatic assembled films and the modifivation effectivelyimprove the detection range; through carefully designment and improvementof the long period fiber grating structure, the LPFG expressed some promisingresults while applied to the film thickness monitoring, this promoted long periodfiber grating sensing to a new level.