Abstract

Compared to the conventional iadex-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 diode¹ with a corrugated active layer to give an injected carrier-dependent-coupling, the other is the loss-coupled (LC) laser diode² 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 predicted³ 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-A-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 method⁴ using CH₄/H₂ reactive ion etching (RIE) as shown in Fig. 1. The multiple quantum well consisted of 10 periods of 60 Å InGaAs and 100 Å InGaAsP (λ = 1.24 μm). Dry etching using CH₄/H₂ RIE was applied to confine the active layer to 1.2-μm-wide mesa. As shown in Fig. 2, our LC laser diode has a planar-buried heterostrucrure using semi-insulating InP as a current blocking layer. Figure 3 shows the current versus light power and the optical spectrum of the laser diode. 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/cm²) and 20% for each uncoated facet, respectively. Optical spectrum shows that there is very little stop band as an evidence of CC laser diode. This is the lowest threshold current, to our knowledge, compared to other similar types of long-wavelength LC laser diodes. Because our LC laser diode was detuned 15 nm from the gain peak, we believe that low threshold current results only from reduction in the duty rate of the lossy grating. Side-mode suppression ratio (SMSR) was 40 dB at 10 mW. Temperature-dependent lasing wavelength shift was 0.12 nm/C°. Further optimization of grating height and duty rate is expected to improve other performances such as SMSR and linewidth.

© 1996 Optical Society of America