Threshold current, differential gain, and relaxation resonance frequency of 1.55-/spl mu/m bulk and MQW DFB laser diodes

We investigate threshold currents, differential gains, and relaxation resonance frequencies of 1.55-/spl mu/m bulk and nine-well multiple-quantum-well (MQW) distributed feedback (DFB) laser diodes (LDs) experimentally and theoretically. The experimentally obtained threshold current (I/sub th/), differential gain (dg/dN), and relaxation resonance frequency per square root of output power (fr/(P)/sup 1/2/) of the 1.55-/spl mu/m bulk DFB LD are 19 mA, 2.4/spl times/10/sup -16/ cm/sup 2/, and 3.2 GHz/mW/sup 1/2/, which nearly agree with the calculated I/sub th/ of 21.6 mA, dg/dN of 3.1/spl times/10/sup -16/ cm/sup 2/, and fr/(P)/sup 1/2/ of 3.6 GHz/mW/sup 1/2/, respectively. The experimentally obtained I/sub th/, dg/dN, and fr/(P)/sup 1/2/ of the 1.55-/spl mu/m InGaAs MQW DFB LD with barriers (/spl lambda/g=1.3 /spl mu/m) are 9 mA, 6.0/spl times/10/sup -16/ cm/sup 2/, and 4.6 GHz/mW/sup 1/2/ compared with the calculated I/sub th/ of 7.8 mA, dg/dN of 4.0/spl times/10/sup -16/ cm/sup 2/, and fr/(P)/sup 1/2/ of 3.7 GHz/mW/sup 1/2/, respectively. However, the experimentally obtained I/sub th/, dg/dN, and fr/(P)/sup 1/2/ of the 1.55-/spl mu/m InGaAs MQW DFB LD with barriers (/spl lambda/g=1.15 /spl mu/m) are 20 mA, 2.6/spl times/10/sup -16/ cm/sup 2/, and 2.85 GHz/mW/sup 1/2/, which greatly disagree with the calculated I/sub th/, of 4.9 mA, dg/dN of 6.0/spl times/10/sup -16/ cm/sup 2/, and fr/(P)/sup 1/2/ of 4.3 GHz/mW/sup 1/2/, respectively. Taking into account the nonuniform hole distribution among nine wells, which is attributed to the valence band discontinuity, /spl Delta/E/spl upsi/ (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/sub th/ and dg/dN of the 1.55-/spl mu/m MQW DFB LD with /spl lambda/g of 1.15 /spl mu/m are 18.3 mA and 2.2/spl times/10/sup -16/ cm/sup 2/, which fairly agree with the above measured values. By p-type doping to 1.5/spl times/10/sup 18/ cm/sup -3/ into the wells to improve the nonuniform distribution of holes among nine wells, I/sub th/, dg/dN, and fr/(P)/sup 1/2/ of the 1.55-/spl mu/m MQW DFB LD with /spl lambda/g of 1.15 /spl mu/m can be expected to be 3.7 mA, 8.8/spl times/10/sup -16/ cm/sup 2/, and 5.4 GHz/mW/sup 1/2/, respectively.