Strain-introduced MZI Interleaver On LiNbO₃Substrate

The tunable interleaver has a great value in dense wavelength divisionmultiplexing （DWDM） system. In terms of functionality, it not only can separate onenarrow channel spacing light signal into two double channel spacing signal, which cangreatly eased the requirement of DWDM system, it also can be used as a wavelengthselective switch due to its center wavelength tunable characteristic. Furthermore, in theproduction process, its tunable feature also reduces the center wavelength alignmentrequirements in the fabrication process and the production cost of the device. Atpresent, both thermo-optic and electro-optic effect can be utilized to realize the tuningfunction. Taking into account the slower tuning speed of the thermo-optic tuning,electro-optic tuning is the best option in the case of some high-speed applications.Obviously, in order to achieve electro-optic tuning, interleaver need to be fabricated byelectro-optical material which having a large electro-optical coefficients, among them,Lithium Niobate （LN） material is the best choice. However, because of small refractiveindex difference of the usual LN waveguide fabricated by proton exchange orTi-diffusion, conventional interleaver based on LN waveguide fabricated by the twomain methods will be impossibly long, and fabricating such an interleaver is verydifficult. In this dissertation, we propose Strain-induced method to fabricate a tunableLN waveguide-based interleaver. Because of the use of residual thermal stress in thebend waveguide region to increase the index contrast of the core and clad, thecorresponding bending radius of the proposed interleaver can be reduced, thereby theentire size of the device can be reduced, which is conducive to the process of thedevice.At first, based on the basic theory of elasticity, strain distribution within thewaveguide is solved through theoretical analysis and Finite Element Method （FEM） byANSYS software and the results obtained by the two methods are compared. On thisbasis, the refractive index distribution of strain introduced waveguide is analyzedthrough theory of elasto-optical effect. Then, considering the sensitivities of the edgeforce to the deposition temperature and the thickness of the SiO₂film, we analyzed the effective index changes forE pqmode by Finite Difference Method （FDM） underdifferent circumstances. Comparison found that the introduction of the strain increasesthe effective refractive index of the waveguide up to7×10^-4. Eventually, based on theresults of the above analysis,100GHz strain-introduced tunable interleaver on LNwaveguide is designed.Then, transmission characteristics of the designed strain-introduced Interleaver isanalyzed. We discussed the influence of some factors which include the couplingcoefficient of the3dB coupler, the difference transmission loss of two non-symmetricalarms, refractive index difference of the asymmetric arms and tuning voltage on outputspectrum and dispersion characteristics of the device. Thus the feasibility of the devicedesign is proved.Finally, asymmetric arms and3dB coupler of the strain-introduced interleaver isdesigned in detail for the device production. Compared to the conventional MZIinterleaver, the length of the bending waveguide in this interleaver is just half of it in theconventional interleaver due to high refractive index difference. According to the designresults Lithography mask is made and strain-introduced tunable interleaver on LNwaveguide is produced successfully. Then the transmission, tunable and temperaturecharacteristics of the device is tested, the results prove that the isolation ofstrain-introduced interleaver is up to more than20dB, much higher than the traditionaldevice in the same structure size, this also indicates the length of the interleaver can bereduced by strained-introduced method. In addition, the tunable feasibility of the deviceis also tested, measured each volts causes about0.2nm wavelength shift, and whentuning voltage of the device Ⅰand Ⅱis to3.6V and5.4V respectively, outputconversion can be realized from the two output ports. And for the temperaturecharacteristics of the device, one degree temperature increasing introduces0.092nmwavelength shift, this indicates that the device is sensitive to temperature. Therefore,reducing the impact of the external environment to the output performance of the deviceby tuning function is essential.