I have designed a straight PDMS Waveguide by changing the refractive index of the PDMS slab at some regions by 0.1 .so the waveguide is inside the PDMS slab. How can I measure the attenuation loss of my waveguide and eliminate the substrate mode (because PDMS slab guide light by default n=1.4) ?
Your question is a bit confusing. Correct me if I have assumed wrong.
You are designing a slab waveguide with core (n = 1.5) and cladding+substrate (n = 1.4). The easiest way to measure using simulations is to draw a 2D geometry using COMSOL. Excite your TE or TM mode from one direction (can select input power) and estimate the received mode power at the other. This can be easily performed using COMSOL.
Secondly, you can fabricate and do the measurements practically.
In order to eliminate your substrate modes, you require better refractive index contrast between your waveguide core and cladding. Since you have quite small refractive index contrast, you can eliminate the substrate mode/loss by increasing the thickness of your core.
Also, your design would be frequency dependent as well. Usually people consider PDMS's refractive index around 1.4 for quite a wide range of frequencies, so you can be good from 630nm to 1550nm with same refractive index. The design may not have much variations.
Refractive index of PDMS is relatively low. If you realyze slab waveguide with core (n = 1.5) and cladding (n = 1.4) on a glass wafer (n = 1.45), the burried cladding thickness must be sufficient to overcome propagation losses induce by leakage through the substrate. The best for loss measurements is to use grating or prism coupler in order to selectively injected light in the TE or TM modes of your waveguide without coupling to radiative or substrate modes.
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