All-optical Switch And Fiber Cladding Grating Based On Azobenzene-doped Mixed Liquid Crystals

Azobenzene molecule can undergoes isomerization between trans form and cis formphotochemically or thermally, which draws widely attention. Currently, many plane gratingsbased on azo polymer have been reported. However, the tuning fiber cladding grating devicesbased on azo polymer and fiber has not been investigated. As the present fiber gratings areusually fixed, the application of the grating devices will be limited. In order to meet therequirements of different application of fiber grating, designing a tunable fiber grating is needed.The impact of nematic liquid crystal(NLC) cell with surface orientation on the performanceof all-optical switch, and the characteristic and application of the fiber cladding grating havebeen researched depending on photosensitive properties of azo-doped LCs.Firstly, The impact of nematic liquid crystal(NLC) cell with surface orientation in threedifferent assembly, on the performance of all-optical switch has been experimentally researchedby using liquid crystal mixtures. Here the thickness of NLC cell is equal to diameter of standardsingle mode fiber,125m, and these three kinds of assembly are non-oriented, parallel-alignedand twisted alignment, respectively. The all-optical switch is made of these NLC cells filled withphotosensitive liquid crystal mixtures(15wt%BMAB,85wt%NLCs). The experimental resultsshow that the transmission power of the probe light in the case of parallel-aligned NLC cell hasgood stability and repeatability. The surface orientation of the twisted NLC cell can improve notonly the stability and repeatability of all-optical switch, but also its modulation depth andresponse rate. This research confirms and points out the way for improving the performance ofthe all-optical controlled all-fiber devices by using azo-doped photosensitive LCs and fiber.Secondly，Fiber cladding grating has been built by using phase mask method and amplitudemask method, respectively, which the fiber consists of a side polished fiber (SPF) and theazo-doped LCs (15wt%BMAB,20%Chiral,65wt%NLCs) covering on it. The characteristicand application of the fiber cladding grating have been experimently researched. The resultsshow,1. A loss peak with25dB depth appears in the Transmission spectrum of SPF for the coating azo-doped LCs film on SPF;2. The loss peak blue shifts as the405nm light powerincreases, and the wavelength shift can reach67.5nm;3. There is a good linear relationshipbetween the wavelength shift of the loss peak and the pump light power, which linear correlationcoefficient is0.995;4. The effect of the fiber cladding grating can be erasured through thermalrecover of the azo;5. Putting SPF into an NLC cell can improve the stability of fiber claddinggrating devices.Finally, using the tunable characteristics of pump light power acting on the transmissionspectrum of fiber cladding grating, two applications of optical-controllable wavelength filter anda violet sensor were demonstrated. Extinction ratio of23.6dB and FWHM of6.8nm for thefilter can be achieved under some pump light power. The sensitivity of the violet sensor ismeasured to be1.154nm/mW.The innovations of this research are as follows:1. Fiber cladding grating has been built by using phase mask method and amplitude maskmethod, respectively, which the fiber consists of a SPF and the azo-doped LCs covering on it.2. Using the tunable characteristics of transmission spectrum of fiber cladding grating pump lightpower acting on the transmission spectrum of fiber cladding grating, an application ofoptical-controllable wavelength filter was demonstrated.3. Using the tunable characteristics of pump light power acting on the transmission spectrum offiber cladding grating, an application of violet sensor was demonstrated.