Narrow inewidth, High Speed, AIInGaAs, Strained L MQW, 1.55 pm, DFB Laser Diodes

The AlInGaAs material syste provides the opportunity to improve the gain characteristics needed high speed AlInGaAs Fabry Perot devikes' and InGaAsP DFB devices' have been reported, this is the first demonstration of an AlInGaAs DFB deyice with both narrow linewidth and wide bandwidth. The AlInGaAs MQW wafers lwere grown by MOCVD on an (100) n-type InP substrate. The active region had eight, 7 nm wide conbpressively strained wells with 10 nm tensile strained barriers. The optical confinement in the quantum we41 material was 9%. A rather strong grating of 120 cm-' was used to ensure single mode operation for shorq devices. The wafers were fabricated into ridge waveguide lasers with unpatterned p-metal contacts. Thd finished wafers were cleaved into 250 micron long lasers, mounted uncoated and p-side up for CW and RF kesting. The P-I-V curves for a typical aser are shown in figure 1. The threshold current was 20 mA, while the forward resistance was 6.9 ohms a 4 50 mA. Although the device is mounted epi up, there is minimal roll-over up to 150 mA. Figure 2 is the! optical spectrum at 100 mA with a center wavelength of 1556.lnm and SMSR of 45 dB. The uncoated dev'ce demonstrated an SMSR240 dB for drive currents from 30 mA to narrow linewidth (<200 kHz) was obse ved above Ith. With increasing drive current, the spectral linewidth rapidly drops to less than 100 kHz, he resolution limit of the fiber interferometer measuring scheme employed. The actual linewidth is U der further investigation. The small signal frequency response is shown in figure 4. With a laser bias pf 150 mA, the f.3dB is 20 GHz. Plotting the resonance frequency against the square root of the laser curr nt above threshold yielded a slope of 1.6 GHd(mA)ln. laser^.^ The AlInGaAs system's large1 conduction band offset relative to the valence band offset allows operation at high current densities and elevated local temperatures without electron overflow which enhances high frequency response. In addition, the lower valence band offsets with AlInGaAs allow more efficient and uniform hole injection wiih the greater number of quantum wells needed for high gain, short devices. One advantage of the compensated strain active region is to maintain the volume of strained gain material while reducing the device 1eng;th and capacitance. The compressive strain in the well increases the laser gain by decreasing the effective hole mass, while at the same time the tensile strained barriers further reduce the valence band offset, thbs enhancing the uniformity of hole injection and improving perf~rmance.~ The calculated conductipn and valence band offsets for the device under study were 197 and 95 meV respectively compared to 129 Cnd 163 meV calculated for lattice matched InGaAsP materials with comparable well and barrier band gaps., In summary, AlInGaAs straided MQW DFB lasers at 1.55 pm have been fabricated and tested which exhibit both the narrow linewidth and 20 GHz performance which are ideal for high speed direct modulation in DWDM systems.