Hello Rajkumar,
The decision would depend on 2 related factors:
- Do you need to include the dispersive effects of the material? For example if you are using a CW source then simply using the correct refractive index for your source wavelength would be adequate. If you are modelling a pulse then you need to consider the significance to the wavelengths away from the central wavelength
- If dispersive effects are of concern you then need to considering the dispersion curve for this material, how much does the refractive index vary over your range of operation. If there is little difference perhaps use of a constant dielectric set at your central wavelength is an adequate approximation.
Scott