Dynamic analysis on external optical-feedback-resistant characteristics in partially corrugated waveguide laser diodes

Abstract

This paper clarifies superior external optical feedback resistant characteristics in partially-corrugated-waveguide laser diodes (PC-LDs), compared to conventional distributed feedback laser diodes (DFB-LDs). Based on a novel large single dynamic analysis by using the van der Pol equations in single-mode laser diodes (LDs), it is found that the external optical feedback resistance in single-mode LDs is dominated by the transient fluctuation of mirror loss (total net threshold gain), and depends on the grating phases at the cleaved facets. Theoretical results predict that in the PC-LDs, mirror loss is insensitive to the grating facet phases due to a unique waveguide, which consists of a corrugated waveguide near the antireflection-coated front facet and an uncorrugated waveguide near the high- reflection-coated rear facet. Therefore the variation of phase conditions for oscillation caused by the external optical feedback gives rise to a relative low transient fluctuation of the mirror loss that suppresses the positive feedback effect of mirror loss, as well as the optical output fluctuations. Furthermore, optimum-grating length, i.e. 150 micrometers for 250 micrometers cavity length, was derived by the calculations. The relative intensity noise (RIN) caused by external optical feedback was measured for PC-LDs with different grating length over a wide feedback level range from -40 dB to -20 dB. Experimental results show that, for the cavity length of 250 micrometers , the PC-LDs with a grating length of 150 micrometers have the most excellent external optical feedback resistant characteristics. The increase of RIN was suppressed to as low as -126 dB/Hz with the external optical feedback of -20 dB. These results agreed well with the theoretical analysis.