This picture is from thorlabs' website.
Continues light can be modulated by an optical chopper. According to the fourier transform theroy, I think this kind of modulation will bring new frequency compoments to the continues light. However, there seems no energy exchange happen between the new and initial frequency light, I cannot write a suitable equation to describe the energy and momentum conservation process. Therefore, can modulation of continuous light by an optical chopper generate new frequency photon?
In non-linear-optics, Self-Phase Modulation(SPM) and other non-linear effects seems another kind of modulation through light-matter interactions. It is clear to me that new frequency light has been generated by SPM since the spectrum is wider after SPM in my experiment. And I can take the new frequency light as another light source by a suitable optical band-pass filter.
Are there any differences between these two ways of modulation? It seems to me that optical choppers cannot generate photons at new frequencies because the principle of chopping is just periodically blocking the light source.