| Optical waveguides are the base of the integrated optical circuits. They are carriers for rapidly transmitting optical signals in large quantities. Waveguides and fibers connect all parts of optical systems. As the elements for restricting and guiding the lights, their essential is that the refractive index of waveguides is higher than that of external medium. In the integrated optical devices, structure and transmission characteristics of channel waveguides are similar to ones of single-mode fiber. Compared with other waveguides, when coupled with fibers channel waveguides have lower coupling loss. Y branch waveguides are also important in integrated circuits. Conventional Y branch waveguides have many limitations. Although new type Y branch waveguides could have wide angle, low loss, the technics used here are very complicated. It is useful to study how to use simple technics to fabricate Y branch waveguides.Many methods have been proposed to analysis the propagation characteristics of the dielectric optical waveguides. The beam propagation method (BPM) is the most widely used propagation technique for modeling integrated and fiber-optic photonic devices. In addition to its relative simplicity, the BPM is generally an efficient method and can be applied to complex geometries without having to develop specialized versions of the method. Furthermore, the approach automatically includes the effects of both guided and radiating fields as well as mode coupling and conversion. There are several reasons for the popularity of BPM: perhaps the most significant being that it is conceptually straightforward, allowing rapid implementation of the basic technique. In this thesis, the FD-BPM is used to study the propagation characteristics of channel and Y branch waveguides.For the fabrication of an integrated glass waveguide, the ion-exchange technology is widely used. This simple technich is not only suitable for mass production, but also cheap. It enables us to create a region of higher refractive index in the glass substrate by a simple thermal diffusion. We use 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 order to test waveguides we make, we built a set of coupling and testing system between the optical fiber and the optical waveguide to get waveguide's index profile and propagation loss. It is precisely measuring instrument in the optical communication. Lastly we designed a novel method for testing the propagation loss. It is proved to be effective. The testing result indicated that our laboratory has already had the ability to manufacture channel and Y branch optical waveguides. It provides us a solid basis and great values to optimize the design of ion-exchange waveguides. It also makes a good foundation for our following work, such as the light branch and the optical waveguide amplifier.Index Profile Simulation of Ion exchange waveguide The increment of indexΔn (λ, x ,y)have direct ratio with the Ag+ ion concentration.Δn (λ, x , y ) = d e(λ) g ( x ) f ( y) (1) ( )d eλis the dispersion gene; g ( x ) and f ( y )are distribution function of index towards width and depth. Here are values used in simulation. Width of mask, w = 4 ~12um(it is 4 in Fig1(c)). Diffusion width, d x= 2.4um. Diffusion depth, d y= 3.0um. We can draw the change of index increment towards width and depth in Fig 1. Fig 1(b) index increment towards depth g ( x ), f ( y ) andΔn ( x , y) all represents the percentage of their increments. Simulation of ion exchange channel waveguideWhen we simulate the 3D waveguide model, considering the practical experiment, here are values used in simulation.Light wavelength is 1.55um. Index of the substrate, BK7 glass under 1.55um is 1.50. Index of air is 1. Increment of indexΔn = 0.03. Diffusion width, d x= 2.4um. Diffusion depth, d y= 2.4um. Computing length on Z direction is 1.5um.We have 10000um length and 4-12um width in Fig 2. We find that propagation loss of channel waveguides are 0.0048-0.012dB/cm. It proves that narrower waveguides doesn't always give us lower loss. When width is 7um, we have the lowest loss. It is necessary to choose suitable width. In our condition, 7um, 8um and 9um are the best.During the simulation we found that our waveguides keeps good single mode characteristic.Fixing the width up to 7um, change the length, we can obtain Fig 3.Fig 3 Change of propagation loss with waveguide length (width is 7 um) Propagation loss becomes higher when waveguide becomes longer. Although it increases slow in our simulation, it increases faster in practical research. Simulation of ion exchange channel waveguideWe use FD-BPM to simulate Symmetric Y branch waveguides with S bend of Cosine function. L is the branch length and h is the height of branch. considering the practical experiment, here are values used in simulation. Light wavelength is 1.55um. Index of the substrate, BK7 glass under 1.55um is 1.50. Index of air is 1. Increment of indexΔn = 0.03. Diffusion width, d x= 2.4um. Diffusion depth, d y= 2.4um. Computing length on Z direction is 1.5um.In Fig 4, branch length is 15000um, branch height is 100um. It shows that the propagation loss is the lowest when the waveguide width is 6um.In Fig 5, the width of waveguide is fixed up to 6 um.It shows that when the width is fixed, propagation loss becomes higher as the branch height becomes higher. When the branch height is fixed, the propagation loss becomes higher as the length becomes longer. Fabrication of channel and Y branch waveguide Firstly, channel waveguides were fabricated in prepared K9 glass using a conventional procedure. 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 melt mixture was carried out under settled high temperature and for 1 hour. Then light can pass through the waveguide zone. We use the same process to fabricate Y branch waveguides. Fig 1 shows the process of thermal ion-exchange waveguide.In the 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 hour. The voltage was monitored during the entire fabrication processed. Fig.6 The process of ion exchange making glass waveguide Measurement of waveguideWe built a set of coupling and testing system between the optical fiber and the optical waveguide to get waveguide's index profile and propagation loss. It is precisely measuring instrument in the optical communication. The laser beam emerging from the Single-Mode fiber shoots 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. Fig.2 shows the coupling and testing system between the optical fiber and the optical waveguide. By changing the infrared CCD to an optical circulator, the system can be used to measure propagation loss. The test results of index profile are shown in Fig.3 and Fig.4. width of 8μmarea, thermal ion exchange is in AgNO3/KNO3/NaNO3 melts with 5 percent AgNO3 at T1=340 for 1 hours, the voltage of applied field is 600V, the field was carried out for T2= 50 minutes.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.We present a novel method to measure the propagation loss of the waveguides. The measurement system involves two circulars, a charge coupled device camera (CCD), 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, branch and other waveguides. The experimental scheme for this technique is shown in Fig.6. Light from a semiconductor laser of wavelength 1.55μmis launched into the waveguide end-face 2 using circular C 2. The direct light output power P1 from the circular1 A1 is measured.Now the circular 2 is removed and light P0 is launched into waveguide end-face 1 using circular 1. The back-reflected light P2 is measured at the same point. P2 is given byUsing 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 simulation and procedure for fabrication of buried ion-exchanged channel and Y branch waveguides and 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 613K, applied voltages U at 600 V and time for about 1 hour. The index profile is successfully buried by applying an electric field in the second step. When the width is 7um and the length is 1.8cm, the loss is 1.6dB/cm. When the width is 7um and the length is 1.8cm, the loss is 2.1dB/cm.The testing result indicated that our laboratory has already had the ability to manufacture the channel and Y branch optical waveguide. It provides us important basis and great values to optimize the design of optical waveguides. We make a good foundation for the following work, such as the light branch and the optical waveguide amplifier. |
PDF Full Text Request