A computer-controlled ellipsometer was used to obtain data on the birefringence changes induced in lithium niobate crystals by irradiation with an argon-ion laser. The instrument was set to take readings on a rectangular grid of points before and after irradiation. Irradiation was performed with circular and with narrow rectangular light beams. Results are compared with the prediction of the model of Chen in which he postulated the presence of an internal field Eo in the direction of the c-axis of the crystal. According to his theory, electrons, photoexcited from traps by the laser light, drift under the influence of Eo along the c-axis before being retrapped. The resulting space-charge field set between positive ionised centres and trapped electrons gives rise to the observed birefringence changes via the linear electro-optic effect of the crystal. Approximate mathematical models based on Chen's theory are used to solve the problem with narrow rectangular light beams. Results are compared with experiments. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Le niobate de lithium (LN), en raison de ses propriétés, électro-optiques, optiques non linéaires et photoréfractives (PR) , s'impose comme un matériau de choix pour des applications en modulation, filtrage, stockage holographique ou conversion de fréquence optique. L'effet PR est gouverné à la fois par les centres profonds extrinsèques (Fe2+ et Fe3+ le plus souvent) et par les antisites niobium en site (Nbli5+), qui constituent des pièges préférentiels pour les électrons arrachés aux donneurs profonds et forment ainsi des polarons liés Nbli4+, caractérisables par une large bande d'absorption photo-induite (API) s"étendant dans le rouge et le proche infrarouge, ainsi que par une bande de photoluminescence (PL) légèrement décalée en longueur d'onde par rapport à la précédente. Nous présentons ici une étude expérimentale de la PL polaron dans le LN congruent dopé fer, fonction de la température et de l'intensité lumineuse excitatrice, en régime continu et en régime pulsé, ainsi qu'un modèle phénoménologique à trois centres permettent d'interpréter toutes les caractéristiques observées. Nous montrons que la PL permet en principe de doser les donneurs profonds dans le LN congruent en qualité infime, que soit leur nature. Pour les ions Fe2+, la détectivité est typiquement de 0,25 ppm à l'ambiante, ce qui est bien meilleur que la spectroscopie d'absorption. La PL résolue spatialement permet en outre, contrairement aux autres techniques, de cartographier la concentration de donneurs profonds à l'échelle micrométrique, d'où son potentiel pour la caractérisation de guides d'onde, de composants optiques intégrés ou autres microstructures / Because of their electro-optical, non-linear optical and photorefractive effet, lithium niobate crystals (LN) are used in many applications such as modulation, filtering, holographic storage or frequency conversion. Its known that the photorefractive effect is influenced by both extrincic deep centers (Fe2+ and Fe3+ in most cases) and by niobium antisites (Nbli5+) which constitute preferential sites to trap an electron hence giving a small bound polaron (Nbli4+). This defect is characterized by a photo_indiced obsorption (API) broad band in the NIR range, as well as slightly Stokes shifted photoluminescence band (PL). We presnt nerein an experime,tal study of the polaron related PL in iron doped congruent lithium niobate as a function of temperature and incident intensity in CW and pulsed regime. We also propose a phenomenological threecenter model as an interpretation of all observed results. We show that the PL permits one to determine the concentration of deep centres in congruent LN in trace amounts, whatever their nature. For Fe 2+ ions, the sensitivity is typically around 0,25 ppm at RT, which is better than absorption spectroscopy. In addition, unlike other techniques, the PL is spatially resolved (micrometer scale) which allows to maps the concentration of deep donors, hence its potential for characterizating waveguides, integrated optical components and other microstructures

In iron-doped lithium niobate and other similar crystals, exposure to light of appropriate wavelength induces small changes in the refractive index. This phenomenon is called the photorefractive effect. It allows phase holograms to be stored in these crystals. The work to be described was undertaken to obtain a better understanding of the mechanisms of the photorefractive effect and to investigate possible engineering applications. The photorefractive effect is believed to involve the spatial redistribution of photoexcited electrons among traps. This causes a space charge field to develop-which modulates the refractive index via the linear electro-optic effect. A new bulk photovoltaic effect special to ferroelectric crystals, first recognized by Glass et al., is important in the photorefractive effect in these crystals. It is shown that the finite electron transport length in this effect makes the photovoltaic current distribution spatially shifted from the light intensity pattern that causes it. Moreover, it is shown that the spatially varying photovoltaic current component which is responsible for the hologram formation decreases as the spatial frequency of the light interference pattern increases. Hologram writing by the photorefractive effect is modelled for arbitrary electron transport length. The treatment allows for the feedback effect of the space charge field and for the dark conductivity of the crystal. The model applies to uniform illumination and constant applied voltage conditions. It is shown that except in crystals where diffusion dominates the .hologram is spatially shifted from the light intensity pattern that caused it because of the finite electron transport length associated with the bulk photovoltaic effect. Experimental results which bear upon the bulk photovoltaic effect and the associated electron transport length are reported. Hologram writing with an arbitrary one-dimensional light intensity distribution is modelled allowing for the feedback effect of the space charge field at all writing times, A large scale space charge field associated with the envelope of the light is shown to affect the writing process. It is found that for any type of intensity distribution an increase in the fraction of the crystal which is illuminated improves the efficiency of the hologram writing process. Also for partially illuminated crystals the storage efficiency improves as the photoconductivity approaches the dark conductivity value from above. For a fully illuminated crystal the storage efficiency improves as the ratio of the photoconductivity to dark conductivity increases. Experimental observations of the effect of the large scale field on hologram storage are reported. Beam distortion and optically induced scattering are two problems encountered while storing holograms in lithium hipbate. We report experimental observations and theoretical models for these phenomena. It is shown that beam distortion is due to the defocusing action of the large scale refractive index change due to the envelope of light. Light scattering is suggested to be due to the lens action of the index variations due to laser speckles inside the crystal. A theoretical treatment of the spatial filtering properties of volume holograms is presented. Practical applications of volume holograms in the fields of interferometric testing and optical. communications are also discussed. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Unknown

Integrated optical devices use light to perform circuit functions. One class of devices using Ti indiffused waveguides in LiNb0₃ are investigated in this thesis. The applications studied include series and multibranch interferometers, a comparatorless A/D converter, and a high voltage sensor. An Impedance transformation equation for an exponential line is derived and used to obtain relations for the propagation constant of an optical waveguide with linearly graded index. The results are used to study the effect of a Ta₂O₅ film on the propagation constant of a Ti diffused waveguide. Heating in O₂ of the TA₂0₅ film, loading one arm of the Mach-Zehnder modulator, is shown to tune the modulator. The fabrication of devices in Y-cut LiNb0₃ by Ti diffusion is described. Microscope objectives are used to couple 0.6328 μm light into 4 μm wide waveguides through polished edges. An application of the Mach-Zehnder modulator and a two mode BOA (Bifurcation optique active) modulator to high voltage measurement is discussed. The output Intensity of the series interferometric filter is calculated. Measured results on a two section filter are presented. Also, the output intensity of a multibranch interferometric filter is determined. Experimental results on a three branch filter are included. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Exposure of some insulating crystals such as lithium niobate to light of appropriate wavelength induces small changes in the refractive index. This effect has been named the photorefractive effect. It allows phase holograms to be stored in these crystals. The work to be described was undertaken in order to obtain a better understanding of the hologram storage process which is believed to involve the spatial redistribution of photoexcited electrons among traps. This causes a space charge field to develop which modulates the refractive index via the linear electro-optic effect. A new reliable criterion for deciding whether the Raman-Nath or the Bragg regime of diffraction will be observed with a given hologram was proposed. It is shown that the distinction between "thin" and "thick" holograms is invalid as a criterion for which regime operates. The new bulk photovoltaic effect proposed by Glass et al. is an important mechanism in the photorefractive effect in ferroelectric crystals. It is shown that as formulated by Glass et al. it is formally equivalent to a fictional "virtual field" acting on the photo-liberated electrons provided that their migration length is short compared to the grating spacing. Hologram writing by the photorefractive effect was modelled in progressive stages of complexity. All the models were based on the assumption that the transport length of the free electrons is short compared to the grating spacing. This appeared to be a generally accepted assumption. The first treatment allowed for the feedback effect of the space charge field and for the dark conductivity. It was for uniform illumination and constant applied voltage. The effects of the modulation ratio and the applied field were investigated. This treatment was then modified to allow for the effect of the absorption constant in reducing the intensity of the light as it propagates through the crystal. It was shown that the hologram becomes nonuniform through the crystal thickness as a result of this effect. Hologram writing with one-dimensional Gaussian beams was modelled allowing for the feedback effect of the space charge field. A large scale space charge field associated with the envelope of the light pattern was shown to affect the writing process. It was found that an increase in the fractional illumination of the crystal improves the writing process. The dark conductivity is shown to have an important effect on the process. The final model was again for uniform illumination and allowed not only for the feedback effect of the photoinduced field and the effect of the dark conductivity and absorption but also for the interaction between the hologram being written and the light pattern which is writing it. This causes energy transfer between the two writing beams, thus modifying the light pattern. Optical erasure of holograms with the light incident either on and off the Bragg angle was modelled. The treatment allows for the feedback effect of the space charge fields and for the effect of the absorption in reducing the light intensity. The model allowed for the interaction between the diffracted and the reading beams for the case of incidence at the Bragg angle. The resulting interference pattern writes a new hologram which may add to or subtract from the hologram to be erased. An experimental method is described in which a normally incident ancillary light beam of different wavelength than that used to write the hologram allows the diffraction efficiency to be determined without errors due to multiple internal reflections. A limited experimental investigation was made of hologram storage in LiNbO₃. Photocurrent and optical measurement were carried out on the same crystal. Almost 100% relative diffraction efficiency was observed. The value of the virtual field obtained from holographic measurement was found to agree within 10% with the value obtained from photocurrent measurements. During hologram writing, energy transfer of up to 70% between the two writing beams was observed. However, since the "virtual" field in these experiments was much larger than the diffusion equivalent field, the model predicted only about 5% energy transfer. It is, therefore, suggested that the transport length of the photoexcited electrons in the crystal used, was not short compared to the grating spacing. It is also shown that light induced scattering can cause serious error in measuring the diffraction efficiency expecially during optical erasure. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Annealed proton-exchanged (APE) MgO:PPLN ridge waveguides have been successfully developed, and its optical properties have been researched. Nonlinear wavelength conversion experiments have been successfully conducted using the developed waveguide. The APE MgO:PPLN ridge waveguide has been proved suitable for high efficiency nonlinear optical conversions across its transparency spectrum from visible to mid-IR light. With distinctive features including flexibility, high efficiency and low cost, this new type of waveguide can find potential applications in various application fields. / Thesis / Candidate in Philosophy

by Loi Kwok Kwong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1992. / Includes bibliographical references (leaves 197-222). / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- An Overview of Integrated Optics --- p.1 / Chapter 1.2 --- Application of Lithium Niobate Integrated Optical Circuit --- p.4 / Chapter 1.3 --- Summary --- p.5 / Chapter Chapter 2 --- Optical Waveguide Theory --- p.7 / Chapter 2.1 --- Introduction --- p.7 / Chapter 2.2 --- Ray Optics Treatment of Planar Waveguide --- p.7 / Chapter 2.2.1 --- Step-index Waveguide --- p.8 / Chapter 2.2.2 --- Graded-index Waveguide --- p.13 / Chapter 2.3 --- Optical Channel Waveguide --- p.20 / Chapter 2.3.1 --- Marcatili's Method --- p.22 / Chapter 2.3.2 --- Effective Index Method --- p.26 / Chapter 2.4 --- Summary --- p.30 / Chapter Chapter 3 --- Waveguide Fabrication Technology --- p.32 / Chapter 3.1 --- Properties of Substrate Materials --- p.32 / Chapter 3.1.1 --- Glass --- p.32 / Chapter 3.1.2 --- Semiconductor --- p.34 / Chapter 3.1.3 --- Ferroelectric Material --- p.35 / Chapter 3.2 --- Waveguide Fabrication Techniques --- p.40 / Chapter 3.2.1 --- Ion Implantation --- p.40 / Chapter 3.2.2 --- Titanium Indiffusion --- p.41 / Chapter 3.2.3 --- Proton Exchange --- p.44 / Chapter 3.3 --- Summary --- p.48 / Chapter Chapter 4 --- Fabrication and Measurement of Optical Waveguides --- p.49 / Chapter 4.1 --- Fabrication of Optical Waveguides --- p.49 / Chapter 4.1.1 --- Planar Waveguides --- p.49 / Chapter 4.1.1.1 --- Substrate Cutting --- p.49 / Chapter 4.1.1.2 --- Substrate Cleaning --- p.49 / Chapter 4.1.1.3 --- Proton Exchange --- p.50 / Chapter 4.1.1.4 --- Post-exchange Annealing --- p.51 / Chapter 4.1.2 --- Channel Waveguides --- p.51 / Chapter 4.1.2.1 --- Patterning Technique: Photolithography and Lift-off --- p.51 / Chapter 4.1.2.2 --- Proton Exchange and Annealing --- p.56 / Chapter 4.1.2.3 --- Lapping and Polishing --- p.56 / Chapter 4.2 --- Measurement of Waveguide Parameters --- p.57 / Chapter 4.2.1 --- Coupling of Light into Optical Waveguide --- p.57 / Chapter 4.2.1.1 --- Prism Coupling --- p.58 / Chapter 4.2.1.2 --- End-fire Coupling --- p.60 / Chapter 4.2.2 --- Effective Index --- p.63 / Chapter 4.2.3 --- Refractive Index Profile --- p.63 / Chapter 4.2.4 --- Waveguide Depth --- p.67 / Chapter 4.2.5 --- Propagation Loss --- p.67 / Chapter 4.2.6 --- Near-field Intensity Profile --- p.69 / Chapter 4.3 --- Summary --- p.74 / Chapter Chapter 5 --- Results and Discussions --- p.75 / Chapter 5.1 --- Proton-exchanged Waveguides Using Phosphoric Acid --- p.75 / Chapter 5.2 --- Proton-exchanged LiNb03 Waveguides Using Toluic Acid --- p.112 / Chapter 5.3 --- Proton-exchanged LiNb03 Waveguides Using Stearic Acid --- p.127 / Chapter 5.4 --- Proton-exchanged LiNb03 Waveguides Using Cinnamic Acid --- p.148 / Chapter 5.5 --- Structural Characteristics of Proton-exchanged Waveguides --- p.174 / Chapter 5.5.1 --- Thermogravimetric Analysis --- p.174 / Chapter 5.5.2 --- Raman Spectroscopy --- p.174 / Chapter 5.5.3 --- Infrared Spectrometry --- p.179 / Chapter 5.5.4 --- Double Crystal X-ray Diffractometry --- p.185 / Chapter 5.6 --- Summary --- p.190 / Chapter Chapter 6 --- Conclusions --- p.192 / References --- p.197 / Chapter Appendix 1 --- Error Estimations --- p.219 / Chapter Appendix 2 --- List of Publications --- p.221

Several pulsed optical oscillators (OPOs) based on periodically poled lithium niobate (PPLN) and pumped by single longitudinal mode Ti:sapphire lasers have been developed. These OPOs provide access to important spectroscopic regions in the 1 - 5.5 [mu]m region and can be rapidly turned by varying the pump wavelength. Previously many of the OPOs developed to take advantage of PPLN relied on a combination of period selection and temperature tuning and as a result were slow and cumbersome to tune. This problem my be avoided by using tunable pump sources or acoustically induced strain waves. Several candidate OPO pump sources were characterised. These pump sources with themselves pumped by lasers operating at repetition reates of either 1.5 kHz (high repetition rate) or 10Hz (low repetition rate). High repetition rate systems include: a Ti-sapphire laser, injection seeded by a single longitudinal mode diode laser, several coupled cavity Ti:sapphire lasers with bandwidths less than 100 Ghz and Cr:forsterite lasers narrowed by prisms and étalons. The low repetition rate systems were all coupled cavity Ti:sapphire lasers one of which was single and double pass amplified. Of these it was found that only the high repetition rate injection seeded laser and the low repetition Ti:sapphire lasers were suitable as OPO pump sources. OPOs were characterised at high and low repetition rates. The high repetition rate system exhibited a low threshold of oscillation (18.7 [mu]J) and a low overall efficiency (25%) which was thought to be due to the pulse to pulse variability of the Ti:sapphire bandwidth. The tuning range of the OPO was 932 to 1310 nm (signal) and 1.989 [mu]m to 5.281 [mu]m (idler) using multiple poling periods and only 15 nm of pump tuning. OPO oscillation on two separate signals simultaneously was observed. Two separate low repetition rate systems were investigated; the first was tuned from 1200 to 1600 nm (signal) and from 1600 to 2400 nm (idler) on a single poling period with a high absolute efficiency of 35% and a threshold of 180 [mu]J. The second OPO was tuned from 940 to 1220 nm (signal) and 2.2 to 4.3 [mu]m (idler) on a single poling period. The absolute efficiency of the system was 25% and the threshold was 200 [mu]J. OPO oscillation on two separate signals was investigated using an OPO based on grazing incidence configured cavity. It was found that the signals coupled together through Raman transitions present in lithium niobate and that coupling reduced the efficiency of the device as a whole. The affect of an acoustically induced strain field on the optical nonlinearity of tetragonal ferroelectric materials was investigated. It was found that the optical nonlinear coefficient varies linearly with the cell displacement and as the square root of the acoustic power. A crystal designed to implement a quasi phase matched interaction based on this variation is proposed.

All-optical wavelength conversion will be one of the most key technologies in all-optical network. Wavelngth converter using nonlinear effect can resolve wavelength contentions with its high transparency and subcarrier-multiplexed channels in a complex all-optical network model which is more beneficial than O/E/O method. Periodically poled LiNbO3 crystal fiber for wavelength conversion is successfully grown by LHPG method with high-electric-field bias. It does not require conducting the metallic electrode to define domain period. The pitch depends on the frequency of applied external electric field and the growth speed. Domain period of 18.9 £gm for C-band wavelength converter and domain period of 15.45 £gm for tunable blue/green laser are demonstrated in this thesis. Micro-swing during growth is managed to assist poling process. More than 40 nm wavelength range of blue/green laser was generated and 10% internal SHG conversion efficiency was achieved. A simulation shows that maximum tuning range using a graded-period qusai-phase-matching structure can be as large as 33 nm. With the improvement of uniformity, broadband design for cascaded processes, and waveguide structure, superior performance of periodically poled LiNbO3 crystal fiber can be achieved.

Blue/Green lasers can be applied in a wide range such as high-density optical storage, display, biomedical analysis and under water communications. C-band wavelength conversion is one of the most key technologies in DWDM system. Optical Wavelength converter using nonlinear effect can provide high transparency, subcarrier-multiplexed channels and can be fused easily and directly with optical fiber. These characteristics have more advantages than those of O/E/O methods. In this thesis, periodically poled LiNbO3 (PPLN) crystal fiber for wavelength conversion is grown by LHPG method with high-electric-field bias. The relationship between the polarization inversion and micro-swing is analyzed. For different applications, PPLN crystal fiber need appropriate pitches of polarization inversion to meet quasi phase matching. For example, domain period of 15.45 um is used for tunable blue/green lasers. By means of cascaded SHG/SFG effect, when fundamental power is 100 mW, the internal conversion efficiency of SHG and cascaded SHG/SFG were -9.2 dB and -31.9 dB respectively. The SHG 3-dB bandwidth is 9 nm. While the domain pitch varies from 16.79 um to 25.79 um with 30 nm periodic increment , the simulation shows that the cascaded SHG/SFG 3-dB bandwidth is 65 nm in the range of 1476-1672 nm for fundamental wavelength. Domain period of 18.9 um is used for C-band wavelength converter. By means of cascaded SHG/DFG effect, when the crystal length is 1.8 mm, the effective nonlinear coefficient is 18.2 pm/V, which is 83 % of theoretical value. Conversion efficiency is about -59.3 dB when fundamental power and signal power were 350 mW and 15 mW, respectively.