I have a diode laser that emits multi-mode light. Two frequencies of this light are selected in an external cavity and reflected back into the laser diode. The laser diode's gain medium then resonate at these two frequencies and emits dual mode light. The laser operates in the 1550nm region. When the two back-reflected frequencies have a difference of around 2THz a third average mode is also emitted. That is, for proper power balance the laser diode will then emit three modes of light simultaneously, the two expected modes and an additional mode with a frequency at the average of the other two. I am looking for some sort of theory or explanation for this, likely a non-linear interaction but I have not idea what. It is not four-wave-mixing since the new light mode is at the average frequency and not 2*f1-f2 or 2*f2-f1 which would be outside of the other light modes. Does anyone have any thoughts on what could be the cause of this? Thanks in advance for any help!