hi Does anyone know about lpg?????? please help me :-(

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my grating image attached. i have a waveguide. wveguide core:PMMA_DR1 &
waveguide clad:PVA
i have 3 mod in waveguide.
my RI diffrent is about of 0.005 to 0.02
i writing grating and my grating period : 20 , 25 , 30, 35, 40, 45, 50 , 60 , 70
, 80, 90,100, 150,200,250 micron
but but but :-( answer is same
and i do not see dipes
my light source is helium-dotriom and my spectrometr is ocean optic please help
click to open the full size version of the image

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