Self-written Waveguides in Bulk Photosensitive Materials

                   

Principal Investigator

Martijn de Sterke

Department of Theoretical Physics

Sydney University

Photosensitivity is a mechanism by which permanent refractive index changes can be induced in germano-silicate glass using light at specific wavelengths. We have previously shown that this mechanism can be used to self-write a channel waveguide inside an originally uniform planar slab of photosensitive glass. We call this a self-written waveguide because the same beam of light which writes the waveguide is then guided by it. This technique for fabricating waveguides is potentially very useful because the current fabrication methods are typically multi-step processes, in contrast with the method described here.

We expect that it should also be possible to form waveguides in bulk photosensitive materials (as opposed to planar waveguides). No previous work had been done on this because it is computationally much more demanding than the planar problem. In particular, it is necessary to store the values of the refractive index and the electric field on a three dimensional array which describes the bulk material.

The basic question we are addressing in this work is the following: what types of waveguide structures can we self-write in the material? Different incident beam shapes result in very different types of waveguides in the glass. Also, the maximum attainable refractive index change differs in different photosensitive materials, and this also affects the structures which can be self-written in the glass.

     

Co-Investigators

       

Tanya M. Monro

Department of Theoretical Physics

University of Sydney

Leon Poladian

Optical Fibre Technology Center

Australian Photonics CRC

       

Projects

g63 - VPP

       
             
                   

       
                   

What are the results to date and the future of the work?

So far we have shown that using a circularly symmetric Gaussian beam, we can self-write a channel waveguide in an initially uniform bulk photosensitive material. We have investigated this process using several different photosensitivity models in order to determine the impact of the model on the waveguide which forms in the glass.

Now that the introductory study using circular incident beams is complete, we have just begun to investigate the waveguides formed using elliptical input beams. We are conducting a two parameter search where we vary both the beam ellipticity and the saturation value of the

 
                   

- Appendix B

 

   
                   

     

refractive index in the glass. The results of this search will show the influence of the beam ellipticity on the characteristics of the resulting self-written waveguide for a range of photosensitive glasses. This knowledge should lead to a better understanding of how the beam shape can be tailored to design specific waveguides.

What computational techniques are used?

The calculation involves the solution of two coupled partial differential equations on a three dimensional grid within the glass slab. We solve these equations using a split-step FFT beam propagation method, which we combine with updates of the refractive index distribution at each time step.

The first of these two partial differential equations is the paraxial wave equation, which describes the way in which light propagates through the slab for a given refractive index distribution. The second equation (the photosensitivity equation) is a phenomenological model of the photosensitivity process, and is used to determine the refractive index change at each grid point due to the light distribution in the slab.