Very low threshold current-complex-coupled laser diode using reduced duty rate InGaAs absorptive grating

Summary form only given. Compared to conventional index-coupled laser diodes, superior characteristics of complex-coupled (CC) distributed feedback (DFB) laser diodes have been studied both theoretically and experimentally. There are two types of CC laser diodes. One is the laser diode with a corrugated active layer to give an injected carrier-dependent-coupling, the other is the loss-coupled (LC) laser diode with an absorption layer over or below an active layer to give a periodic change in net gain. For a LC laser diode, it was theoretically predicted that the excess absorption into the embedded absorption layer causes the threshold current to increase and the external efficiency to decrease. In order to improve the above characteristics of LC laser diode, we designed and fabricated a LC laser diode with an embedded 200-/spl Aring/-thick InGaAs absorption layer on n-InP. The first-order InGaAs absorptive grating with the duty rate of 18% was obtained by our new grating fabrication method using CH/sub 4//H/sub 2/ reactive ion etching (RIE). The multiple quantum well consisted of 10 periods of 60 /spl Aring/ InGaAs and 100 /spl Aring/ InGaAsP (/spl lambda/=1.24 /spl mu/m). The threshold current and the slope efficiency measured at the optical power of 5 mW are 5.5 mA (threshold current density=1.5 kA/cm/sup 2/) and 20% for each uncoated facet, respectively. The optical spectrum shows that there is very little stop band as evidence of the CC laser diode. This is the lowest threshold current,to our knowledge, compared to other similar types of long-wavelength LC laser diodes.