Optimal Design And Preparation Of Er³⁺-Yb³⁺ Co-doped Polymeric Planar Optical Waveguide Amplifiers

It has been widely acknowledged that all-optical communication would be theprime method in optical communication systems. The integrated optical amplificationtechnology which can amplify the optical signal without any conversion from light toelectricity is the most effective method for the loss compensation. In recent years,much attention has been devoted to the development of erbium-doped waveguideamplifiers (EDWAs). Compared with inorganic host, polymer has excellent propertiessuch as: simple processing steps, high bandwidth, low cost, and compatibility withsilicon substrates. However, few successful EDWAs have been demonstrated usingerbium-doped polymers or organic/inorganic hybrid materials (OIHMs). Onebottleneck is that the insolubility of Er³⁺ions in organic materials cannot meet therequirement of EDWAs' small size and high Er³⁺-doping concentration. Anotherdifficulty is the relatively low photoluminescence quantum efficiency due to thevibration quenching caused by O-H and C-H groups and concentration quenchingcaused by the high Er³⁺-doping concentration or local non-uniform doping.In this thesis, we have made efforts to improve the polymeric planar opticalwaveguide amplifiers, particularly on the major technical difficulties mentioned aboveand the design of the device configuration. The main contents and innovations are asfollowing:1. Based on optical amplifying theory of excited emission and a six-level systemmodel for Er³⁺-Yb³⁺co-doped waveguide amplifier, the rate equations and propagationequations of optical power were established. Using the method of overlap integral, thegain characteristics of devices were numerical simulated. The luminescenceperformance parameters of materials were computed by the Judd-Ofelt theory. Toanalyze the mode characteristics and calculate the mode field distribution of an opticalwaveguide with jumps in the refractive index,the semi-vectorial finite-differencemethod (FDM) based on the Lagrange interpolation and the Effective Index Method(EIM) were implemented by programming with MATLAB. The results werecompared with the commercial software COMSOL Multiphysics and the both resultscould be confirmed.2. The Er element in the organic matrix often has lower quantum efficiency and a short fluorescence lifetime. To solve this problem, oleic acid (OA)-LaF₃:Er,Ybnanoparticles and OIHM were synthesized by chemical coprecipitation and sol-gelmethod respectively. Their properties were characterized by the method such asinfrared spectroscopic analysis, elementary analysis, etc. The diameter of the particleswas about8nm and the fluorescence lifetime was about100μs (for La:Er:Yb=85:3:12).The5%weight-loss temperatures of nanoparticles and OIHM were both above300degree. The OA-LaF₃:Er,Yb nanoparticles were dissolved in OIHM as content of33wt.%and a film with low surface roughness was obtained. The Judd-Ofelttheoretical calculation value of the metastable level lifetime was13.34ms. TheOA-LaF₃:Er,Yb nanoparticles doped OIHM waveguides with embedded structurewere tested using the fiber-waveguide coupling test system and gain test system. Awell near-field optical spot was showed. The relative gains of3.69dB (P_s=1mW),3.89dB (P_s=0.5mW) and4.11dB (P_s=0.2mW) were obtained in a2.9cm long devicewhen the pumping power was121mW, which showed that a smaller input signalcorresponds to a bigger gain. The relative gain of a1.9cm long device waveguideincreased from2.2dB to3.2dB corresponding before and after polishing, whichshowed that polishing was help to improve the pumping efficiency. The gaincharacteristics of the erbium-ytterbium co-doped waveguide amplifier (EYCDWA)device were simulated. The effects on the gain of the Er³⁺-doping concentration, theoverlap integral factor, the signal and pumping power, waveguide length and the Er³⁺ion metastable level lifetime were discussed.3. In order to optimize the device structure, the gain characteristics of a kind ofCore-cladding doped waveguide amplifier were analyzed and compared with the Coredoped waveguide amplifier. Within a certain thickness of the core, the Core-claddingdoped type could increase the gain, but it had a lower pumping efficiency. The peakconcentration of the Core-cladding doped type declined obviously. SU-8which hadthe UV thermo-optical bleaching effect was selected as a substrate material.OA-NaYF₄:Er,Yb nanoparticles which had a stronger luminescence property weresynthesized by high-temperature thermal decomposition method. The diameter of theparticles was about15nm. OA-NaYF₄:Er,Yb nanoparticles doped SU-8waveguidesboth of the Core doped type and the Core-cladding doped type was fabricated and hadwell optical performance. The relative gain of2.08dB (P_s=1mW, P_p=170mW) wasobtained in a1.5cm long Core doped type device waveguide.4. The dispersion ability and uniformity of the host-guest doped material werealways limited. To solve this problem, a film-formable solution-processable erbium-ytterbium co-doped complex was synthesized and characterized which had ahigh erbium concentration of10.8wt.%(In contrast,1.1wt.%corresponding that inOA-LaF₃:Er,Yb doped OIHM). The absorption spectrum of the complex film was insharp contrast with common host-guest doped type film, which almost had nobackground absorption of matrix materials. The full width at half maximum (FWHM)was about80nm (centered around1535nm). The relative gain of5.20dB (Ps=0.3mW,P_p=140mW) and2.52dB (P_s=0.2mW, P_p=170mW) were obtained in a1.5cm longdevice with a SU-8cladding layer and a2.2cm long device with a PMMA groove,respectively. Four different structures which were the most closely combined with theexperiment and the side-etching were analyzed to provide theoretical guidance for thedevice fabrication.5. Since there was a competition between gain and pumping efficiency in thehigh core region refractive index waveguides, a low core region refractive indexwaveguide (slot waveguide) was introduced. Firstly, the basic characteristics of slotwaveguides were analyzed. The average intensity coefficient γ in a slot waveguidewas two orders of magnitude higher than that in a high refractive index waveguide sothat it was not necessary to be considered. Slot waveguides presented low wavelengthsensitivity. The affect relationships between the overlap integral factor Γ and slotwidth, Si-sidewall width, Si-sidewall height were analyzed and the slot waveguidesize was determined. A four-level system model for Er³⁺doped waveguide amplifierwas established. The effects on the gain in a common high refractive index waveguideof the Er³⁺-doping concentration, the excitation state absorption, the cooperationup-conversion, the signal light and pump light transmission loss, signal and pumppower, the waveguide effective cross area and the overlap integral factor werediscussed. We found that there was optimal pump intensity (I_p-sat) in a certainwaveguide structure. So that for a slot waveguide, different waveguide dimensionscorresponded to different optimal pumping power and the optimal gains could becalculated using the optimal pumping powers. Multi-layer slot could greatly increasethe Γ and the most suitable slot number was3or4layers.This thesis presented two kinds of materials for fabricating polymeric planaroptical waveguide amplifiers: nanoparticles (OA-LaF₃:Er,Yb and OA-NaYF₄:Er,Yb)and film-formable solution-processable erbium-ytterbium co-doped complex. Theformer possesses a stronger luminescence property, and the latter exhibits higherbium-doping concentration and film uniformity. Taking account of these advantages,novel active materials would be prepared, which gives most promising application for enhancing the net gain of polymeric planar optical waveguide amplifiers.