Threshold current, differential gain, and relaxation resonance frequency of 1.55-μm bulk and MQW DFB laser diodes

We investigate threshold currents, differential gains, and relaxation resonance frequencies of 1.55-μm bulk and nine-well multiple-quantum-well (MQW) distributed feedback (DFB) laser diodes (LDs) experimentally and theoretically. The experimentally obtained threshold current (I_th), differential gain (dg/dN), and relaxation resonance frequency per square root of output power (fr/(P) ^1/2) of the 1.55-μm bulk DFB LD are 19 mA, 2.4×10 ^-16 cm², and 3.2 GHz/mW^1/2, which nearly agree with the calculated I_th of 21.6 mA, dg/dN of 3.1×10^-16 cm², and fr/(P)^1/2 of 3.6 GHz/mW^1/2, respectively. The experimentally obtained I _th, dg/dN, and fr/(P)^1/2 of the 1.55-μm InGaAs MQW DFB LD with barriers (λg=1.3 μm) are 9 mA, 6.0×10 ^-16 cm², and 4.6 GHz/mW^1/2 compared with the calculated I_th of 7.8 mA, dg/dN of 4.0×10^-16 cm², and fr/(P)^1/2 of 3.7 GHz/mW^1/2, respectively. However, the experimentally obtained I_th, dg/dN, and fr/(P)^1/2 of the 1.55-μm InGaAs MQW DFB LD with barriers (λg=1.15 μm) are 20 mA, 2.6×10^-16 cm², and 2.85 GHz/mW^1/2 , which greatly disagree with the calculated I_th, of 4.9 mA, dg/dN of 6.0×10^-16 cm², and fr/(P) ^1/2 of 4.3 GHz/mW^1/2, respectively. Taking into account the nonuniform hole distribution among nine wells, which is attributed to the valence band discontinuity, ΔEυ (the separation between the first quantized hole level in the well and the top energy of the valence band for the barrier), the corrected I_th and dg/dN of the 1.55-μm MQW DFB LD with λg of 1.15 μm are 18.3 mA and 2.2×10^-16 cm², which fairly agree with the above measured values. By p-type doping to 1.5×10¹⁸ cm^-3 into the wells to improve the nonuniform distribution of holes among nine wells, I_th, dg/dN, and fr/(P)^1/2 of the 1.55-μm MQW DFB LD with λg of 1.15 μm can be expected to be 3.7 mA, 8.8×10^-16 cm², and 5.4 GHz/mW^1/2, respectively