When I use multimode fiber to guide the quantum dot ensemble photoluminescence signal to my Horiba FHR100 spectrometer. I always get a spectrum with periodic peaks as pic.jpg .

It looks like the peaks has a exact period of 1.5 nm.

Only when I guide the PL signal with a single-mode fiber canI get the smooth signal I wanted to see.

Could someone please help explain what the source of this periodic peak is and How can I avoid them .

Thank you!