| With the swift and violent development of optical fiber communication technology, the technology of Dense Wavelength Division Multiplexing (DWDM) has already become a main way of high speed optical fiber communication network. Although, in DWDM system, because the signals transmitting in each channel cover different distance or EDFA, optical wave filter and optical switch are different in response to optical signal of every wavelength, which makes the channel performance worsen. In order to guarantee communication quality, it is necessary to carry on the optical power of each wavelength in balance on the node of the optical fiber network, the power balance of each channel is the important basis of the optical network performance quality in such optical network. At present, a lot of schemes to balance the optical power have already put forward. The variable optical attenuator is a kind of important solutions among them. It can attenuate the energy of the optical signal at the request of the user, compensate the optical power in each channel, complete the balance between each channel, improve the overall performance of optical network. Variable optical attenuator (VOA ) has already become the important power management device in the communication network, and it is used in optical fiber communication, the spaceflight field, the cable TV and other industry field widely.In this paper, polymer dispersed liquid crystal technology and body silicon process technology in MEMS are combined together to fabricate the variable optical attenuator using polymer dispersed liquid crystal. The shortcomings of traditional machinery type optical attenuator, such as large volume, high cost and responding slowly, are overcome. Such advantages as miniaturization withlow cost, integration, easily controlling, will help it to play an important role in the optical fiber network.Polymer dispersed liquid crystal (PDLC) is formed by dispersing the particles of the liquid crystal into polymer through a certain method. The particles of the liquid crystal is formed after liquid crystal separate from a solution or a solution, the particles of the liquid crystal disperse evenly in the polymer, and they do not link to each other in the polymer. The particles of the liquid crystal separate from each other, and the polymer forms the continuous looks, so the film with this kind of structure is called PDLC film. When there is no electric field on PDLC film, the small electric anisotropy particles of liquid crystal lie in the polymer in freedom orientation state, because the index of refraction of liquid crystal mismatch with the index of refraction of the polymer at this moment, so the particles have stronger scattering function on the light, the opaque or semitransparent milky white form appears on PDLC appearance, which is called"of state". When the electric field is added on PDLC, if the index of refraction of liquid crystal match with the index of refraction of the polymer, light can through PDLC film, and PDLC film will appear transparent form, so it is called"on state"at this moment. If the electric field is removed, then the film is replied to"of state"again. PDLC film scatters light differently when different electric field is added to it. PDLC film can attenuate light continuously through adding different electric field to it.The polymer dispersed liquid crystal film is the key material to fabricate the variable optical attenuator. Its performance plays an important role to the whole device. Based on the principle of polymer dispersed liquid crystal film, we choose liquid crystal TEB50, pair of phenol A surrounds oxygen acrylic acid ester, methyl acrylic acid first ester MMA, I - the base benzene baseketone(Irgacure-184 ) of hydroxyl ring to prepare polymer dispersed liquid crystal film by Ultra-violet Induced Phase Separation. Through changing a series of process conditions, such as content of liquid crystal, polymer to dispersant, content of optical initiator, solidification temperature, we have made a large number of contrast test and found out the best process conditions of preparing polymer dispersed liquid crystal film. The best process conditions: content of liquid crystal is 70%, polymer to dispersant is 1:1, content of Irgacure-184 is 3%, the density of ultraviolet for solidification is 0.4mW/cm2,the solidification temperature is 4℃, solidification time is 8 minutes. Under this condition, the minimum transmittance of polymer dispersed liquid crystal film is 5%, the maximum transmittance can be up to 83%, and its characteristic is good. In this way, we prepare good material to fabricate variable optical attenuator.After preparing good polymer dispersed liquid crystal film, we use body silicon process technology to etch optical fiber fixed trench on (110) silicon chip, then make two single mode optical fibers with transparent conductive ITO coupled in the optical fiber fixed trench. After the whole structure of the variable optical attenuator is fabricated, we test its performance. The measuring results show the threshold voltage of the fabricated VOA is near 6Vrms, the rang of operating voltage is 6Vrms to 35Vrms, the maximum attenuation range is about 5.98dB to 17.4 dB at 1550nm, and the minimum insertion loss is 5.98dB. |
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