and 37 c will be diffracted by the second grating at a different respective angle in the illustrated example of fig3 it is assumed that the second grating 42 diffracts the first diffracted optical signals 37 a 37 b 37 c into five orders i e m u00b1 2 u00b1 1 0 each however the fan out angles for example the second fan out angles u03b8 m2a u03b8 m2b u03b8 m2c the angles between the highest magnitude diffraction orders m 2 a m 2 b and m 2 c respectively emanating from grating 42 in this example where m 2 are not dependent on the incident angles of first diffracted optical signals 37 a 37 b 37 c accordingly as second grating 42 has a single grating period d 2 in this example the second fan out angle associated with each channel e g u03b8 m2a u03b8 m2b u03b8 m2c are the same when d 2 is 8 69 u03bcm for example the second fan out angle for each of u03b8 m2a u03b8 m2b u03b8 m2c is 25 u00b0 the full fan out angle for each channel u03b8 fulla u03b8 fullb u03b8 fullc however are different in this example for example u03b8 fulla 55 6 u00b0 u03b8 fullb 55 3 u00b0 u03b8 fullc 55 0 u00b0 where u03b8 fulla u03b8 m1a u03b8 m2a etc accordingly.