Enhancement in microwave modulation efficiency of vertical cavity surface-emitting laser by optical feedback

We investigate a versatile vertical cavity surface-emitting laser (VCSEL) with a slight optical feedback from a simple wedge positioned at a well determined distance from the VCSEL. We use such a laser in conjunction with a miniature Rb atomic cell to detect coherent population trapping (CPT) when the external cavity mode spacing frequency corresponds to half the hyperfine frequency of Rb. The feedback greatly enhances the rf modulation efficiency of the VCSEL and as a result we obtain high contrast CPT signals at an input of -16 dBm of rf modulation power compared to +1.25 dBm needed in the absence of the external cavity.