Hi Shahbaz,
For your simulation FILE2.osd (laser arrays) actually you can see four frequencies (see image file2-1 attached), but not spaced 2nm from one another, as you have defined your channel spacing as 100GHz which is equal to 0.8 nm. If you set channel spacing as 2nm, then you would have the spectrum as shown in image file2-2 attached. You can see the main peaks are spaced as determined.
In your FILE-2.osd simulation (with separate lasers), the channel spacing defined in the WDM mux doesn’t go with the frequency spacing of lasers which is 2 nm and that’s why you don’t get desired result.