Yes you are right so for solving this problem that I found after one week is you have to find out till where your design for example will be single mode for my case 600 micron then I duplicate it and one more time, still it is single mode, the question is why when I draw in a single waveguie it was multi-mude but after I cut it in to three straight waveguide it changed to a very nice single mode? because in reality in my fabrication the machine will not understand I cut it to three waveguide, and it will fabricate as a straight waveguide, so how I can be sure my waveguide is single mode? I already simulate both of them, or we can say because of the single mode is 600 micron now I have 3 of them it will obey this rule? otherwise it would be multi mode? I need a strong answer many thanks 🙂