Hi Tim,
There are probably a lot of different ways to do this, I would use the X Coupler component found in Passives Library->Optical->Couplers.
Attach your first input to the optical signal you want to split and the other to an Optical Null component. From here, you can change the coupling coefficient, c, to get these splitting ratios. Take a look at the technical background for this component and square the expression to get the relation between input and output powers.
Regards