Highly reliable 1.3-μm InGaAlAs MQW DFB lasers

Abstract

Uncooled 1.3-pm semiconductor lasers are key devices in access networks and datalink applications. The large conduction band offset relative to the valence band offset of InGaAlAs system allows laser operation at high temperatures; thus, it has been attracting increasing attention [l, 21. However, there have been few reports on the reliability of InGaAlAs-MQW lasers since the crystal growth technology is immature. Concerning DFB lasers in particular, difficulties arising from the regrowth of InGaAlAs-MQW active layer on a grating must be overcome. Accordingly, we have surmounted these difficulties and developed high-performance 1.3-pm InGaAlAs-MQW DFB lasers with high reliability. The fabricated laser is schematically shown in Fig. 1 and has a reverse-mesa ridgewaveguide structure. The whole laser structure was grown on a (100) n-type InP substrate in two steps by low-pressure metalorganic vapor-phase-epitaxy. The active layer consists of five InGaAlAs wells enclosed in InGaAlAs graded-index separate-confinement-heterostructure layers. In terms of easy crystal growth, an upper grating, which is placed in a p-InP cladding layer above an active layer, is preferable. In practice, however, n-type impurities that slip into the regrowth interface on the grating result in a high differential resistance. After optimizing growth conditions, we employed a lower grating in which periodic InGaAsP regions were embedded in the n-InP cladding layer. Typical continuous-wave light-current characteristics of an AR/HR coated 300-~mlong laser are shown in Fig. 2. The laser was mounted on a Sic substrate in a junction-up configuration. The threshold currents are 20 mA at 25°C and 37 mA at 85°C. This current increase corresponds to a very high characteristic temperature of 98 K. The temperature degradation of the slope efficiency is 68%. Figure 3 shows the dependence of the differential resistance of the upper and lower gratings on current. The differential resistance of the upper grating is 5 at the threshold current and slightly decreases to 3.5 !2 as current increases. This slight decrease implies the existence of a potential barrier at the regrowth interface. On the other hand, the differential resistance of the lower grating remains almost constant at 3.3 a. The lasing spectra measured at 25 and 85°C are shown in Fig. 4. These spectra show that the laser has stable single-longitudinal-mode operation with a side-mode-suppression ratio of more than 40 dB at all temperatures. The results of aging tests on the seven laser chips are shown in Fig. 5. The test condition was 10 mW at 85°C. These test confirm that a stable lasing operation was maintained up to 2000 h. In conclusion, we have successfully demonstrated high characteristic temperature, low differential resistance, and stable single-longitudinal-mode operation of 1.3-pm InGaAlAs MQW DFB lasers with high reliability. These excellent characteristics indicate that this laser is quite suitable for low-cost transmitters.