this occurs because of power transfer among the different channels. the same effect can be witnessed by just analyzing the power at different frequencys.this effect occurs because of four wave fixing problem (FWM), scattering. this effect can be decreased by changing the refractive index which will give different delays(which leads to different group velocity) to different channels.
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https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwj5ianNncDMAhWLVI4KHX-lClYQFgg2MAI&url=http%3A%2F%2Fwww.ifsc.usp.br%2F~dispoptic%2FAulas2013%2FAgrawal-internet%2FPhotonics%2520West%2520Tutorial%2520-SPIE.pdf&usg=AFQjCNGOcTCbt-P84iotkmbPeNX3ukDH1w&sig2=-KTFVYuttRXjSmehbUx0lQ