Modes competition in a dual modes HeNe laser with optical feedback

The optical feedback characteristics in a short cavity dual modes HeNe laser are studied under different modes strength condition. Modes competition can be obviously observed during optical feedback. The whole laser intensity curve has intensity branches and its leaning direction is related to the moving direction of the external feedback mirror. There are two cross points at different voltage levels between the two orthogonally polarized lights in a period of the optical feedback fringe and the order that the cross points come out is different when the feedback mirror moves at different direction which can be used for direction discrimination. Sometimes there are two peaks of the same size in a period of the optical feedback curve which can be used to improve the resolution of an optical feedback system.