Theories And Experiment Studies On Photorefractive-damage-resistant Locally Doped Ti:Mg:Er:LiNbO₃ Waveguide Amplifier

This dissertation was first at home and abroad to carry out the following work: 1) fabrication and characterization of Li-deficient MgO:LiNbO3 crystal, 2)fabrication of and experiment studies on photorefractive-damage-resistant locally Er/Mg doped near-stoichiometric (NS) Ti:Mg:Er:LiNbO3 waveguide amplifier, 3) dynamic simulation for amplified character of our waveguide amplifier. Thanks to the research work, practicality could be brought to LiNbO3-based positive devices, and it may open up some new applications such as 980-nm-pumped green upconversion and mid-infrared (2.7μm) waveguide lasers, and various quasi-phase-matching devices.The main work of the dissertation can be summarized as following three aspects:1. Li-deficient, off-congruent Z-cut MgO:LiNbO3 (Mg:LN) crystals were prepared by carrying out post-grown Li-poor vapor transport equilibration (VTE) treatments on 5 mol% MgO-doped congruent LiNbO3 plates (0.47 mm thick). The VTE duration dependence of the Li2O content reduction was determined by gravimetric method. The results show that the Li2O content decreases with prolonged VTE and the saturation tendency takes place for VTE duration > 300 h, where the Li2O content reduction is as much as ~ 3 mol%. The optical absorption edge (OAE) and the OH absorption of the VTE-treated Mg:LN were also studied against the Li2O content. Powder X-ray diffraction results reveal all of the VTE crystals retain still the LN phase and it can be apply to integrated optics.2. Z-cut congruent LiNbO3 plates (0.5 mm thick) were chosen as the starting materials. RF magnetic sputtering, Photolithography and Lift-off, and termal diffusion were used to carry out:○1 locally Er doping○2 Ti/Mg pre-diffusion○3 Li-rich VTE treatments. Then, 1-cm-long NS Ti:Mg:Er:LiNbO3 strip waveguides were fabricated which initial Ti thickness of 4, 5, 6 and 7μm. Comprehensive characterizations were performed on fabricated waveguides: 1) Waveguiding mode was observed and only TM mode was guided. All waveguides are single-mode at 1.3 and 1.5μm. The waveguide loss was evaluated from the measured insertion loss at 1.3μm and the propagation loss is then determined to be 1.4 dB/cm for the 7μm guide. 2) Optical absorption spectroscopy and refractive index measurement were then used to characterize Li-compositions in these prepared samples. Optical absorption study shows that the post VTE resulted in the blueshift of the optical absorption edge (at the absorption coefficient of 20 cm-1) from 319.1±0.3 nm to 318.2±0.3 nm, qualitatively showing that the VTE has brought the crystal surface closer to the stoichiometric composition. The Li2O content at the undoped crystal surface is evaluated as 49.8±0.1 mol%; 3) The crystalline phase in the guiding layer was characterized via the 1.5μm amplified simultaneous emission spectrum pumped at 980 nm. Moreover, the spectrum is completely identical to the typicalπ-polarized spectrum of an Er-doped LN crystal, implying that the Er ion presence in the waveguide layer is as the LN phase; 4) The photorefractive effect of the NS Ti:Mg:Er: LiNbO3 waveguide was studied in comparison with that of the congruent waveguide by examining the stability of the 1531 nm small-signal output under high 980 nm pump level. Experiment results show stable 1531 nm small-signal enhancement under the 980 nm pump power of 216 mW, implying that the photorefractive effect is effectively suppressed. 5) Amplification characteristic results show under the pump power of 220 mW, an enhancement of 1.5 dB/cm is steadily obtained; 6) Secondary ion mass spectrometry (SIMS) was used to study diffusion characteristics of Ti, Er and Mg ions in waveguide layer, and detailed diffusion characteristics were collected.3. In the end, small signal's amplified characters of 1531nm wavelength in NS Ti:Mg:Er:LiNbO3 waveguide amplifier pumped by 980nm or 1480nm light were dynamically simulated in details. Simulation results show that transmission small signal could get a gain of 1.2±0.1dB/cm under 216 mW coupled 980 nm pump level. Taking 1.4 dB/cm propagation loss into account, it equals to 2.6±0.1dB/cm which is comparable to experiment result of amplification characteristic.Innovation point contained in this dissertation: 1) Taking the lead in fabrication and characterization of Li-deficient MgO:LiNbO3 crystal, which resolves the problem that both the solubility and the diffusivity of the Er ions into an Mg:LiNbO3 with a congruent composition are extremely low. Er ions concentration diffused into Li-deficient MgO:LiNbO3 crystal is expected to increase by ~2mol%. 2) Combination of locally Er doping, Ti/Mg pre-diffusion and Li-rich VTE treatments was first used to fabricate NS Ti:Mg:Er:LiNbO3 waveguide successfully. 3) The photorefractive effect in our fabricated waveguide is effectively suppressed, opening a new way towards active waveguide device.