Below-threshold polarization behaviors of a VCSEL under external optical feedback

In this paper, the behavior of the lasing and non-lasing polarization modes in a vertical-cavity surface-emitting laser (VCSEL) with polarization-rotated feedback has been studied experimentally with a focus on the below-threshold behavior. The results below the threshold demonstrate that: (i) the non-lasing mode shows enhancement of the power when the mode itself is reflected into the laser’s cavity (optical (self-) feedback, (OF)). The non-lasing mode in the free-running laser even becomes the dominant mode. (ii) Polarization switching (PS) occurs before the two modes pass through threshold current when the non-lasing mode has a strong enough OF. (iii) The two modes show polarization fluctuations and competition when they have the same level of OF at a polarization rotation angle of 450. (iv) When a PS between the polarization modes happens close to the threshold current, back-switching occurs above the threshold. Therefore, OF affects polarization behavior similarly above and below the threshold. OF can enhance or suppress the polarization fluctuations below the threshold, depending on the OF strength and polarization state of the feedback light. Understanding polarization behaviors below the threshold allows for optimizing VCSEL performance and controlling polarization mode fluctuations.