[extropy]

That sounds very wrong. Not a physicist, but light does not interact with magnets, or you need to get into the superconducting strength levels before it does.

https://en.m.wikipedia.org/wiki/Optical_rotation

It seems the optical rotation happens due to molecular structure of specific materials.

Edit, found this: https://en.wikipedia.org/wiki/Faraday_effect

So a specific material plus magnetic field is needed. But still not sure how strong of a field is sufficient.

[tomn]

For more details on this specific application these pages might be more relevant:

https://en.wikipedia.org/wiki/Optical_isolator

https://en.wikipedia.org/wiki/Faraday_rotator

[ycui1986]

Magnet there is probably used as an isolator. The laser can only go out,but not come back.( yes, there will be laser coming back from reflection, which also interrupt the operation of the source laser.