Fabrication And Measurement Of Buried Ion-exchanged Channel Waveguides

Integrated optical waveguide is one of the most important components in optical communication system. It plays the role of the connecting wires in the optical circuitry and in addition it is the essential element in any integrated optical circuits. Now the optical waveguide is an important and fundamental component for various optic and photonic devices such as switches, WDMs, couplers and so on. The fabrication and characterization of such a component is thus greatly required for the advancement of optical communication systems.To build efficient waveguides, proper substrates are needed. Glass waveguides are considered to be prime candidates for integrated-optic device applications. This is mainly due to their low propagation losses, low fabrication cost, and compatibility with optical fibers. In addition, they can be fabricated and integrated into the system with relative ease.For the fabrication of an integrated glass waveguide, the ion-exchange technology is used. This simple technology enables to create a region of higher refractive index in the glass substrate by a simple thermal diffusion. However, surface waveguide fabricated by a simple thermal diffusion have its limitation such as surface scattering losses, waveguide-to-fiber coupling losses and so on. In order to solve this problem, we used an improved two-step ion-exchange in glass to get the buried waveguide, which is already established to have low surface scattering losses. In addition, its refractive index profile can be made symmetric to match the modal field profile of an optical fiber and thus to reduce waveguide-to-fiber coupling losses.In this thesis, two-step silver and sodium ion-exchange was used to prepare the channel optical waveguide in the K9 glass substrates, and we built a set of coupling and testing system between the optical fiber and the optical waveguide to get waveguide's index profile. Lastly we designed a novel method for testing the propagation loss.Waveguide fabrication:Firstly, channel waveguides were fabricated in K9 glass using a conventional procedure that is summarized in Fig.1. To localize the diffusion in specific regions in the substrate, the photolithography technique is used to create an aluminum mask on the glass substrate to prevent the exchange in the masked region. Lastly, the diffusion in a mixture of AgNO3 and NaNO3 melt was carried out at T=3400C and for time between 1h-3h. The glass plate which contains movable ions b (Na+) is immersed in a molten salt of ions a (Ag+). An ion exchange occurs between ions Ag+ and Na+ at the glass/molten salt contact zone. In a second step, the mask is removed and ions Ag+ inside the glass are buried deeper, in order to obtain low loss and easier optical fibers connections, by application of a voltage between the two sides of the glass plate. The field-assisted diffusion process was carried out at a temperature of 3400C for applied voltages U at 600 V for about 1-3 hours. The voltage was monitored during the entire fabrication process.Waveguide measurement: Fig.2 shows the setup used for testing the waveguide. It is a set of high accuracy coupling system between the optical fiber and the channel optical waveguide. The laser beam emerging from the SM fiber is injected into the sample waveguide and a CCD camera is used to monitor the output light. Here we use the end coupling method to achieve the coupling between the channel waveguide and tapered optical fiber. The near-field at the guide output was imaged with a 100×microscope objective on a computer-assisted vidicon infrared camera. We calculated the index profile of the channel waveguides from the propagation mode near-field technology with an inverse algorithm method. The test results are shown in Fig.3-Fig.5. The profiles show the effects of applied electric field E. This is a clear indication that silver ions were indeed driven into the glass. The index profile calculated appeared to be a buried waveguide's index profile and is in agreement with that estimated. The results showed that we have succeeded in fabricating buried channel waveguides. The parameters of the ion-exchange process are all determined by the experiment.Also we present a novel method to measure the propagation loss of the waveguides. The measurement system involves two circulars, a charge coupled device camera, and a signal processing unit. The propagation loss measured is independent of coupling conditions. And this method is free from the mechanical operation of the system and can also be applied to the buried waveguides. Endface 2 using circular C 2. The direct light ourput power P1 from the circular 1 A1 is measured. ( )2P1 = P0η1 2η23T fp C fw C wf 1? Rf (1)Now the circular 2 is removed and light P0 is launched into waveguide Endface 1 using circular 1. the back-reflected light P2 is measured at the same point. P2 is given by Dividing (2) by (1) and solving, the propagation loss coefficient is obtained as Where ( )Using the present technique, we didn't achieve the coupling of the fibers and waveguide, so we have not attained the propagation loss coefficient of the waveguide fabricated.We report the procedure for fabrication of buried ion-exchanged waveguides in a K9 glass. Channel waveguides are fabricated in K9 glass by two-step silver ion exchange in AgNO3/KNO3/NaNO3 melts with 5 percent AgNO3. The ion exchange processes are typically performed at exchange temperatures T at 3400C, and for applied voltages U at 600 V for 50 minutes. The index profile is successfully buried by applying an electric field in the second step.The testing results indicate that our laboratory has already had the ability to fabricate the channel optical waveguide. It provides the important basis and the reference value for us to further optimize the design of the channel optical waveguide. At the same time it also makes the good foundation for the following work, such as the light branch and the optical waveguide amplifier.