The Study And Design Of LiNbO₃ Fiber Type Traveling-wave Optical Modulator Based On FEM

External optical modulators is one of the fields put most work into in the highspeed optical telecommunication system, they can overcome the bottleneck effectcaused by the direct modulating of internal modulators. Among the externalmodulators, LiNbO₃ waveguide type traveling wave optical modulator is consideredto be the longest in researching time, most mature in technology, and most stable inperformance. However, due to the high dielectric constant of LiNbO₃ material withwhich the optical waveguides and substrate of LiNbO₃ waveguide type traveling waveoptical modulator are made, the phase velocities of microwave and optical wave arehard to implement perfectly match, the modulating bandwidth is limited as a result.So, we put forward a novel method for designing optical modulators named asLiNbO₃ fiber type traveling-wave optical modulator allowing of the drawbacks ofLiNbO₃ waveguide type traveling-wave optical modulator. In this method, a SiO₂material with stable performance and low dielectric constant is applied as thesubstrate instead of the LiNbO₃ material with high dielectric constant, and in the sametime, the LiNbO₃ optical fiber with square cross section, is used as the opticaltransmitter taking the place of LiNbO₃ waveguide. Due to the fact that the dielectricconstant of the substrate is deeply reduced, the effective dielectric constant of thewhole modulator is apparently decreased, so it can achieve much higher modulatingbandwidth compared with the LiNbO₃ waveguide type traveling wave opticalmodulator with the same pattern and size of electrodes. As the LiNbO₃ optical fibersare also constructed to M-Z interferential apparatus, the successful experience inbuilding optical waveguide type M-Z interferential apparatus of LiNbO₃ waveguidetraveling wave optical modulator can be applied to analyze, calculate and design theLiNbO₃ optical fiber traveling wave optical modulator.This paper adopted finite element method to analyze two LiNbO₃ optical fibertraveling wave optical modulators with different type of electrodes, one with andielectric overlayer above, and the other with no dielectric overlayer above. Theinfinite element was introduced to deal with the open boundary problem for the firsttime, and it is proved to be more accurate. After meshing carefully and solving thetwo modulators respectfully, the corresponding effective dielectric constant,characteristic impedance, modulating bandwidth, microwave transmitting attenuationand half wave voltage were calculated and constructed respectively with the variationof electrode parameters. The modulator with dielectric overlayer is compared to beeasier to implement impedance matching at the cost of decreasing the modulatingbandwidth slightly.This paper applied the optimizing method based on finite element method, whichis simple in operation and fast in speed, to optimize the modulators. In the end, theoptimized electrode dimensions were obtained respectfully, with the theoreticalmodulating bandwidth as high as 500GHz and the characteristic impedance of 59?for the modulator without dielectric layer, and the theoretical modulating bandwidthas high as 240GHz and the characteristic impedance of 50.56? for the modulatorwith dielectric overlayer, at the same time, the half wave voltage and microwaveattenuation were proved to be no higher than the waveguide modulators.After analyzing the microwave performance, the optical performance were alsostudied, the variation of optical refractive index along the vertical and horizontalmidline of the cross section of LiNbO₃ optical fiber were given, thereout, the outputoptical intensity at the output port of M-Z interferential apparatus were educed.At last, the manufacturing arts and crafts of the LiNbO3 optical fiber typetraveling wave optical modulator were discussed, especially the craftwork of LiNbO3optical fiber type M-Z interferential apparatus was proposed for the first time. Itprovided theoretical guidance for the future manufacture and test of the LiNbO3optical fiber type traveling wave optical modulator.