Spectral and temporal characteristics of a 635 nm laser diode subjected to optical feedback from an etalon

Abstract

We investigate the spectral and temporal characteristics of a 635 nm laser diode (LD) in an external cavity consisting of an etalon. The system utilizes optical feedback (OF) from the etalon which transforms the effective reflectivity of the LD cavity. For a fixed operating current Iop and varying feedback (reflectivity R = 0.2 to 0.6), the LD exhibits varying spectral profiles and number of modes. A significant linewidth reduction compared to a free-running laser was achieved for I_op = 65 mA at R=0.34 where the spectrum contains a single dominant peak having a reduced linewidth of 0.6 Å. At I_op = 60, 65 and 70 mA, the coherence lengths calculated for the free-running LD are z_c = 1.55, 1.053 and 0.91 µm respectively; with feedback, the maximum associated coherence lengths obtained are z_c = 7.19, 15.22 and 15.48 µm. These maximum values all fall within the reflectivity range 0.3 < R < 0.4. This suggests that a possible control of temporal coherence length is possible by controlling OF from etalon within this range.