1.

Introduction

Buried heterostructures (BHs) have been widely used in discrete optical devices^{1, 2, 3} and advanced integrated optical devices,^{4, 5, 6} mainly because of the attractive advantages over the ridge structure, such as a lower threshold current, a circular-like and stable output beam, and lower thermal resistance.^{7, 8} The conventional BH is realized by two steps. First, a mesa including a ${\mathrm{SiO}}_2$ dielectric mask and active region stripe is made by dry or wet etching. Then, the current blocking layers are formed by the selective regrowth method. The quality of these selectively grown blocking layers is heavily dependent on the mesa shape and the dielectric mask quality, which is inevitably worse than that of the blocking layers fabricated by the unselective regrowth method.

In this work, the unselective regrowth of buried heterostructures (BHs) is developed for $\mathrm{InGaAsP}$ multiple-quantum-well (MQW) distributed-feedback (DFB) lasers, which are independent of the mesa shape and the dielectric mask. The fabrication process of the lasers is first described amply in Sec. 2. Then, the characteristics of the $\mathrm{InGaAsP}$ MQW DFB BH lasers are demonstrated in Sec. 3.

2.

Device Structure and Fabrication

The schematic structure of the multiple-quantum-well (MQW) distributed-feedback (DFB) buried heterostructure (BH) laser is shown in Fig. 1. The wafer was fabricated by three epitaxial growths using low pressure MOVPE. First, the strained-compensated $\mathrm{InGaAsP}$ MQWs sandwiched by two $\mathrm{InGaAsP}$ separate confinement heterostructure (SCH) layers were grown on (100) $\mathrm{n}\text{-}\mathrm{InP}$ substrates. The photoluminescence (PL) spectrum of the active region is shown in Fig. 2. The peak wavelength of the PL spectrum is about $1560\mathrm{nm}$ with a small full width at half maximum (FWHM) of $53\mathrm{nm}$ . The grating was made on the upper $\mathrm{InGaAsP}$ SCH layer. To increase the differential gain of the DFB BH lasers, the negative detuning of the lasing wavelength from the material gain peak was adopted. After etching of a $1.2\text{-}\mathrm{\mu }\mathrm{m}\text{-}\mathrm{wide}$ mesa stripe along the $\left[110\right]$ direction, the $\mathrm{p}∕\mathrm{n}\text{-}\mathrm{InP}$ current blocking layers were then grown on the whole area by the unselective regrowth method. A current channel was successively dug by etching the n-type current blocking layer just over the active region, which can be seen in Fig. 3a. The alignment for this $\mathrm{n}\text{-}\mathrm{InP}$ etching is not difficult, because a clear pattern correlated with the active region can be seen from the common light microscope even after the current blocking layers have grown on the wafer. Finally, after the growth of a $1.8\text{-}\mathrm{\mu }\mathrm{m}\text{-}\mathrm{thick}$ $\mathrm{p}\text{-}\mathrm{InP}$ cladding layer and $0.2\text{-}\mathrm{\mu }\mathrm{m}$ $\mathrm{p}\text{-}\mathrm{InGaAs}$ contact layer, the electrode fabrication process was performed. The cross section of the fabricated DFB BH lasers is displayed in Fig. 3b.

Fig. 1

Schematic structure of the $\mathrm{InGaAsP}$ MQW DFB BH laser fabricated by the unselective regrowth method.

Fig. 2

PL spectrum of the strained-compensated $\mathrm{InGaAsP}$ MQW adopted as the active region of the laser.

Fig. 3

SEM pictures of (a) a current channel that was dug by etching the n-type current blocking layer just over the active region, and (b) cross section of the DFB BH laser fabricated by the unselective regrowth method.

3.

Results and Discussion

The fabricated wafer was cleaved into laser bars with different cavity lengths. The current dependence of the cw output power from one facet of the multiple-quantum-well (MQW) distributed-feedback (DFB) buried heterostructure (BH) lasers at room temperature is displayed in Fig. 4a. The threshold current increases gradually with the increase of the cavity length. The $200\text{-}\mathrm{\mu }\mathrm{m}\text{-}\mathrm{long}$ laser has a lowest threshold current of $8.5\mathrm{mA}$ and a largest slope efficiency of $0.55\mathrm{mW}∕\mathrm{mA}$ . Based on the data mentioned, the differential quantum efficiency $\left({\eta }_d\right)$ , internal differential quantum efficiency $\left({\eta }_i\right)$ , and internal loss $\left({\alpha }_i\right)$ were extrapolated. Figure 4b shows the feature of the differential quantum efficiency $\left({\eta }_d\right)$ dependence on the cavity length $\left(L\right)$ . A high internal differential quantum efficiency $\left({\eta }_i\right)$ of 78% and internal loss $\left({\alpha }_i\right)$ of $6.9{\mathrm{cm}}^{-1}$ were obtained by fitting the expression attached in Fig. 4b. The as-cleaved lasers with a cavity length of $300\mathrm{\mu }\mathrm{m}$ were then selected to test the far-field pattern and the linewidth of the lasers.

Fig. 4

(a) P-I characteristics of the DFB BH lasers with different cavity lengths and (b) differential quantum efficiency dependence on the cavity length.

Figure 5 shows the far-field patterns in the vertical and horizontal directions for the DFB BH lasers. As can be seen in this figure, the laser emits in single transverse mode, and the divergence angles are $32\times 34\mathrm{deg}$ in the horizontal and vertical directions. The beam shape of the DFB BH lasers is shown to be more circular than that of ridge structure lasers, and thus more efficient optical coupling into a single mode fiber will be achieved with the DFB BH lasers.

Fig. 5

Far-field patterns of the DFB BH laser with output power of $8\mathrm{mW}$ .

Finally, the dependence of the linewidth on the output power is investigated for the DFB BH lasers fabricated by the unselective regrowth method. Stable output power and high differential gain of the strained-compensated MQW DFB BH lasers are the main factors to obtain a narrow linewidth. Figure 6 shows the linewidth of the DFB BH lasers under different output powers. The linewidth decreases rapidly with the increase of the output power from one laser facet. When the output power increases to $23.6\mathrm{mW}$ , the linewidth becomes as narrow as $2.5\mathrm{MHz}$ . The lasing spectrum of the laser under three times the threshold current is also plotted in the insert of Fig. 6. The side mode suppression ratio is as high as $51\mathrm{dB}$ with a lasing wavelength of $1542\mathrm{nm}$ .

Fig. 6

Dependence of the spectrum linewidth on the output power for the DFB BH lasers. Inset shows the lasing spectrum of the device.

4.

Summary

A $1.5\text{-}\mathrm{\mu }\mathrm{m}$ $\mathrm{InGaAsP}$ MQW BH DFB laser formed by an unselective regrowth method is demonstrated. The current blocking layers are performed by the unselective regrowth method, which results in no deterioration of the quality of the BH layers. The P-I characteristics of the laser show superior features, such as a low threshold of $8.5\mathrm{mA}$ and a high slope efficiency of $0.55\mathrm{mW}∕\mathrm{mA}$ . The BH lasers provide circular-like far-field patterns with divergence angles of $32\times 34\mathrm{deg}$ in the horizontal and vertical directions. A narrow linewidth of $2.5\mathrm{MHz}$ is also obtained for the DFB BH lasers fabricated by the unselective regrowth method. The results suggest that the unselective regrowth method can be used to fabricate high performance DFB BH lasers.