An Experimental And Theoretical Study On Erbium-Doped Polymer Waveguide Amplifier

Erbium-doped waveguide amplifiers (EDWA) have received increasing attention in the past few years due to the small size and potential applications to integrate with other optical devices, such as splitters, couplers, and switches. Usually, inorganic host materials, such as glasses and crystals, are used to fabricate EDWA. The pertinent fabrication technologies have been well developed, and erbium-doped inorganic waveguide amplifiers have a typical gain value of 7 dB/cm. Compared to inorganic hosts, polymers or organic-inorganic hybrid materials (OIHMs) can also be used to fabricate EDWA. They have many advantages, such as simple processing methods, low cost, and good thermal stability. However, few successful EDWAs have been demonstrated using erbium-doped polymers or OIHMs. A bottleneck is the insolubility of Er3+ ions in polymers and OIHMs. One of the solutions is to synthesize erbium organic compounds. Another choice is to synthesize lanthanide ion doped LaF₃ nanoparticles with organic ligands.In this thesis, two kinds of EDWAs were demonstrated. One is the Erbium complex -doped polymer EDWA, the other is LaF₃:Er,Yb nanoparticle- doped OIHM waveguide amplifier. The main contents and innovations are as follow- ing:1. LaF₃: Er, Yb nanoparticle-doped organic-inorganic hybrid material waveguides were demonstrated for the first time. A relative optical gain of about 6.8dB was obtained at 1550 nm in a 20-mm-long waveguide.We have successfully synthesized Er3+, Yb3+-doped LaF₃ nanoparticles with organic ligands. The concentration of nanoparticles in OIHM was up to 50 wt%, which was reported for the first time. Transmission electron micro- scope (TEM) and X-ray powder diffractometertion (XRD) were used to characterize the LaF₃ :Er,Yb nanoparticles. The absorption and photo lumines- cence spectra were observed. The absorption spectrum consists of four absorption bands, corresponding to the transitions from the ground state 4I15/2 to the excited states: 2H11/2, 4F9/2, 4I11/2, and 4I13/2. A peak of the emission spectrum at 1535 nm was assigned to the 4I13/2→4I15/2 transition of Er3+. The FWHM of the peak was about 83 nm. The radiative lifetime of excited 4I13/2 state of Er3+ was 72μs.Using X-ray photoelectron spectroscopy (XPS) analysis, we found that the existence of LaF₃ was an obstacle when we fabricate waveguides using chemical dry etching technique. The AFM images of etched surface morphology at different concentrations of nanoparticles also testified this conclusion. So the waveguides with embedded structure were designed and the optical field propagation was simulated using beam propagation method. LaF₃: Er, Yb nanoparticle-doped organic-inorganic hybrid material waveguides with embedded structure were demonstrated utilizing reactive ion etching technology . The photolithography and reactive ion etching process were described in detail. The conformations of waveguides were characterized by SEM.The experimental setup for the optical gain measurement is established. The output near-field profile, optical gain, waveguides'propagation loss and system loss were measured. The relative gain as functions of waveguides'dimension, pump and signal powers were discussed. When the pump power is 120 mW and the signal power is 0.05 mW, a maximum gain of approximately 4.7 dB was observed at 1535nm in a 22-mm-long waveguide. When the waveguide dimension increased to 12μm×8μm, a relative gain of 6.8 dB can be obtained at 1550 nm in a 20-mm-long waveguide.The propagation loss was 3.2 dB/cm by cutback method.These research work mentioned above was published in the journal of Applied Physics Letters (2007.91. (161109), the first author, IF: 3.977) and the patent application is ongoing.2. Er (DBM)3Phen (DBM : dibenzoylmethane; Phen:1,10-phenanthroline) -doped PMMA-GMA (poly-methyl-methacrylate -glyciclylmethacrylate) copolymer waveguide amplifier with Rectangular structure was demonstrated for the first time. A relative optical gain of about 0.4 dB was obtained at 1535 nm in a 10-mm-long waveguide.The absorption, photoluminescence spectra and quantum efficiency were observed. The spectra consists of 5 absorption bands, corresponding to the transitions from the ground state 4I15/2 to the excited states4I13/2, 4I11/2,4 I9/2,4F9/2 and 4H11/2. The full width at half maximum (FWHM) was about 85 nm centred around 1535 nm. The radiative lifetime of excited 4I13/2 state of Er3+ was measured to be 17.9μs.Using the Judd-Ofelt theory, we calculated the spontaneous transition probability, radiative lifetime, absorption cross-sections, and the emission cross-sections of this polymer material. According to the material's refractive index , single-mode waveguides were designed to meet the 1535 nm and 980 nm laser propagation. The fundamental rate equations and power propagation equations for erbium-doped waveguide amplifiers were introduced. The relationship between the gain and these parameters were demonstrated. With the waveguides'cross-section dimensions and radiative lifetime we calculated, the gain of EDWA was simulated to be 0.9dB/cm.Considering the propagation loss in the waveguide, this conclusion could agree with the experimental result in substance.In experiments, the photolithography and reactive ion etching process were used to fabricate Er (DBM)3Phen-doped PMMA-GMA polymer waveguide amplifier. The waveguides'conformation were characterized by Scanning electron microscopy (SEM). The output near-field profile and a relative optical gain of about 0.4dB were measured at 1535 nm in a 10-mm-long waveguide.These research work mentioned above was published in the journal of Optics Communications (2007.278. (90-93), the first author, IF: 1.480). 3. The influence of photoluminescent quantum yield (PLQY) on the gain for EDWA was discussed for the first time.The relationships between the fluorescence lifetime and pump power, optical gain were analyzed. The calculated results show that in order to get the optical gain , the PLQY should be up to 0.6%. The sequential experiments testified the veracity of this conclusion.These research work mentioned above was published in the journal of Nanoscience & Nanotechnology (2007.8. (1-5), the first author, IF: 2.194). 4. The upconversion luminescence of Er3+ was observed in the LaF₃: Er, Yb nanoparticles-doped OIHM channel waveguides for the first time.Because of the special structure of the waveguide, the same phenomenon was also observed in planar waveguides. The possible upconversion mechanisms were discussed and the emission bands at 405nm, 520nm, 544nm and 650nm were mainly caused by energy transfer and excited state absorption processes between Er3+ and Yb3+ ions. The dependence of upconversion emission intensity on excitation power under 976nm excitation confirms a three-photon process contributes to the upconversion of the emission band 405nm and two-photon processes for the green and red emission bands. The influence of upconversion luminescence on the optical gain for the EDWA was discussed.These research work mentioned above was accepted by the Chinese Journal of Physics (the first author, IF: 1.242).Many factors, such as depletion of some relevant levels of Er3+, upconv- ersion effects, the waveguide length, and the propagation loss, could influence the gain of the waveguide amplifier. A higher gain can be expected by optimi- zing the waveguide length or by improving a relatively homogeneous distribu- tion of Er3+ in the polymer material.