## 2 Fabrication of Graded Index Polymer Waveguides  
### Mosquito Fabrication Method (Keio University)  
An innovative method of fabricating polymer waveguides with circular cores was developed by Keio University [1]. According to the \"Mosquito method\", core monomer is dispensed directly into a layer of cladding monomer. The core and cladding monomers are then both cured simultaneously, which simplifies the fabrication process. The fabrication process steps are outlined in Figure 1: A cladding monomer liquid with a given refractive index is first coated onto the substrate. A core monomer with a higher refractive index than that of the cladding monomer is placed into a syringe, which is mounted on a robotic arm. The tip of the syringe needle is then inserted into the liquid cladding monomer layer to the desired depth at which the waveguide is to be deposited. The robotic arm then moves the syringe tip in the plane of the substrate, while injecting the core monomer out of it at a constant pressure along the entire path of the planned waveguide. Once expelled and drawn out by the moving needle tip, the liquid core monomer coalesces into a cylindrical form suspended within the bulk liquid cladding monomer. This form is maintained along the entire path of the planned waveguide, but as both core and cladding monomers are in a liquid state, they continue to diffuse slightly into each other, giving rise to a graduated concentration distribution of core monomer from the center of the cylindrical construct into the cladding monomer. This in turn is reflected by a corresponding graduated decrease in refractive index from that of the core monomer in the center of the cylindrical construct to that of the cladding monomer at the edges. The robotic arm then repeats the process for subsequent waveguides. Once all cylindrical structures have been dispensed in their liquid form, then the whole substrate is exposed to UV light and the graduated cylindrical forms of liquid core monomer are partially cured into place to form the waveguides. The substrate is then baked to complete the curing process.  
The refractive index profile is determined by the amount of time core and cladding monomers are allowed to diffuse into each other. Therefore, by careful choice of the time between dispensing the core monomer and the curing step, one can tune the refractive index profile of the resulting waveguides to achieve a parabolic graded index profile. Greater control of the refractive index profiles can be achieved by altering other configurable parameters including choice of core and cladding materials, core monomer dispensing pressure and amending the scanning programme. It has been demonstrated that the graded index circular waveguides produced through this method outperform step index waveguides of similar size [6].  
Various types of UV curable resins have been used to produce Mosquito waveguides reported in this paper including silicone resin FX-712 (core) and FX-712 (cladding) from ADEKA Corp. (Tokyo, Japan) [7] and an organic-inorganic hybrid resins from Nissan Chemical Ind., Ltd. (Tokyo, Japan), NP-005 (core) and NP-208 (clad) [8].  
Figure 2 shows various cross-sections of the Mosquito waveguides characterized in this paper. The scanning trails of the needle are clearly visible under the circular graded index core.  
Figure 1: Procedures in the Mosquito method.  
Figure 2: Cross-section of Mosquito GI waveguides.