S. Kakimoto

High-power operation of broad-area laser diodes with GaAs and AlGaAs single quantum wells for Nd:YAG laser pumping

The dependencies of the main lasing characteristics on the facet reflectivity for GaAs and AlGaAs single-quantum-well (SQW) separate-confinement heterostructure (SCH) broad-area laser diodes (LDs) are analyzed. Conditions for the facet reflectivity to achieve optimum values are identified. Under these conditions, the authors obtained respective maximum output powers of 2.9 and 2.6 W for GaAs-SQW and AlGaAs-SQW single-stripe LDs for 150 mu m stripe width, lasing at about 808 nm under a continuous-wave (CW) condition. These LDs were stably operated for over 2000 h under the condition of 1 W constant output power with automatic power control circuits at 45 degrees C in CW operation. >

Wavelength dependence of characteristics of 1.2-1.55 mu m InGaAsP/InP p-substrate buried crescent laser diodes

Laser diodes with the p-substrate buried-crescent structure have been fabricated for the 1.2-1.55- mu m wavelength region. The dependence of laser characteristics on wavelength has been measured. Up to 70 degrees C, the increasing rates of the threshold current with temperature are similar, while, above 70 degrees C, a shorter-wavelength laser shows a larger increasing rate. At the same full width at half maximum of the far-field pattern perpendicular to the junction plane, the external differential quantum efficiency of the 1.55- mu m laser diode is only 10% smaller than that of the 1.3- mu m laser. The absorption loss coefficients in the active layer of the 1.2-, 1.3-, and 1.55- mu m laser are estimated to be 26, 34, and 73 cm/sup -1/, respectively. >

Characteristics of laser diodes with a partially intermixed GaAs-AlGaAs quantum well

Laser diodes (LDs) with a partially intermixed quantum-well (QW) active layer are fabricated by Zn out-diffusion from a p-cladding layer to the QW region. The dependencies of the degree of intermixing, measured by the photoluminescence (PL) shift, on Zn concentration of the p-cladding layer (P/sub clad/) and the Al content of the guiding layer (X/sub g/) in a separate-confinement-heterostructure (SCH) are investigated. P/sub clad/ changes in the range from 1/spl times/10 /sup 18/ cm/sup -3/ to 4/spl times/10/sup 18/ cm/sup -3/ and X/sub g/ changes in the range from 0.21-0.37. When P/sub clad/ is 2/spl times/10/sup 18/ cm/sup -3/ and X/sub g/ is 0.37, large bandgap energy shift of 96.1 meV is observed. The lasing wavelengths of the LDs, with the partially intermixed QW, are blue-shifted linearly with increasing P/sub clad/ and X/sub g/. For the bandgap energy shift of 66.8 meV by PL, the threshold current density is increased by 33% from that of the nonintermixed LD. Reliability of LDs with the partially intermixed QW is investigated for the first time. In spite of a large degree of intermixing the reliability of the LD with the partially intermixed QW of 66.8 meV energy shift by PL is the same as the nonintermixed one, which is confirmed by the aging test of 2500 hours at 45/spl deg/C with the output power of 1 W under CW operation. >

Achieving broad-area laser diodes with high output power and single-lobed far-field patterns in the lateral direction by loading a modal reflector

A modal reflector, which consists of a high-reflectivity region surrounded by low-reflectivity regions, is added to the front facet of two types of broad-area laser diodes (LDs) to control the lateral modes. One type of LD is the self-aligned laser with a bent active layer (SBA LD) that has a real index-guiding mechanism. The other is a planar-stripe LD that consists of a Zn-diffused region to confine the current flow and has a gain-guiding mechanism. For the SBA LDs with a modal reflector, stable single-lobed far-field patterns (FFPs) are obtained at up to 0.3 and 0.4 W output powers in continuous wave (CW) operation and pulsed operation, respectively. In addition, for planar-stripe LDs with a modal reflector, stable single-lobed FFPs are obtained at up to 0.4 W in CW operation. The lateral modes inside the cavity are analyzed by utilizing a slit model and FFPs are calculated. Good agreement is found between experimental and calculated FFPs for a large Fresnel number. >

Threshold currents of 1.3-/spl mu/m bulk, 1.55-/spl mu/m bulk, and 1.55-/spl mu/m MQW DFB P-substrate partially inverted buried heterostructure laser diodes

We investigate the threshold currents of 1.3-/spl mu/m bulk, 1.55-/spl mu/m bulk, and 1.55-/spl mu/m multi-quantum-well (MQW) distributed feedback (DFB) P-substrate partially inverted buried heterostructure (BH) laser diodes experimentally and theoretically. In spite of the larger internal loss of the 1.55-/spl mu/m bulk laser diodes, the threshold current of the 1.55-/spl mu/m bulk DFB P-substrate partially inverted BH laser diode is almost the same as that of the 1.3-/spl mu/m bulk DFB P-substrate partially inverted BH laser diode. The experimentally obtained average threshold current of the 1.3-/spl mu/m bulk DFB P-substrate partially inverted BH laser diodes is 17 mA and that of the 1.55 /spl mu/m bulk DFB P-substrate partially inverted BH laser diodes is 16 mA. The calculated threshold current of the 1.3-/spl mu/m bulk DFB laser diode is 15.3 mA and that of the 1.55-/spl mu/m bulk DFB laser diode is 18.3 mA, which nearly agree with the calculated values, respectively. We have fabricated two types of five-well 1.55-/spl mu/m InGaAs-InGaAsP MQW DFB P-substrate partially inverted BH laser diodes. One has barriers whose bandgap energy corresponds to 1.3 /spl mu/m, and the other has barriers of which bandgap energy corresponds to 1.15 /spl mu/m. The calculated threshold current of the MQW DFB laser diode with the barriers (/spl lambda//sub g/=1.3 /spl mu/m) is 8.5 mA, which nearly agrees with the experimentally obtained value of 10 mA. However, the calculated threshold current of the MQW DFB laser diode with the barriers (/spl lambda//sub g/=1.15 /spl mu/m) is 7.9 mA which greatly disagrees with the experimentally obtained value of 19 mA, which suggests that the valence band discontinuity between the well and the barrier severely prevents the uniform distribution of the injected holes among five wells.

Absorption loss coefficient of the active layer for 1.48-/spl mu/m bulk and MQW lasers

It is shown that the absorption loss coefficient of the active layer for 1.48-/spl mu/m bulk lasers is 66 cm/sup -1/ which is between 45 and 107 cm/sup -1/ for 1.3-/spl mu/m bulk lasers and for 1.55-/spl mu/m bulk lasers, respectively. It is also described that the absorption loss coefficient of the active layer for 1.48-/spl mu/m multiple-quantum-well (MQW) lasers is 28 cm/sup -1/ which is about two-fifths of that for 1.48-/spl mu/m bulk lasers. Therefore, the high slope efficiency of the 1.48-/spl mu/m MQW lasers is attributed not only to the small optical confinement factor but also to the small absorption loss coefficient of the active layer.

Extremely narrow spectral linewidth and low chirping of the MQW-DFB-PPIBH laser diode

The low threshold current of 9 mA, the high side-mode suppression ratio of more than 45 dB, the extremely narrow spectral linewidth of 1.1 MHz, and the low chirping of 2.8 AA at -20 dB at 2 Gb/s nonreturn to zero (NRZ) modulation have been achieved in the multiple quantum well (MQW) distributed feedback (DFB) p-substrate partially inverted buried heterostructure (PPIBH) laser diode. The spectral linewidth of 1.1 MHz is the narrowest value among 300- mu m-length solitary laser diodes. These results suggest that the MQW-DFB laser diodes are a promising light source for longer distance and higher bit-rate optical communication systems and coherent optical communication systems. >

High output power and high temperature operation of 1.5 mu m DFB-PPIBH laser diodes

Highly efficient 1.5- mu m distributed-feedback (DFB) p-substrate partially-inverted buried heterostructure laser diodes with a thin active layer developed using a metal-organic chemical vapor deposition technique are discussed. An average slope efficiency of 0.26 mW/mA (quantum efficiency 33%) and maximum slope efficiency of 0.39 mW MW/mA (49%) were achieved. The full width at half maximum in the direction perpendicular to the junction plane of 25 degrees was obtained. A high output power of 77 mW was obtained under CW conditions at room temperature. This laser diode lased up to 120 degrees C, and more than 10 mW was obtained, even at 90 degrees C. >

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.

Threshold currents of 1.2-1.55 mu m P-substrate buried crescent laser diodes

P-substrate buried crescent (PCB) laser diodes whose wavelength ranged from 1.2 to 1.55 mu m have been fabricated. The threshold currents as low as 10 mA have been obtained in this wavelength range experimentally. The calculated threshold currents of 13, 13, and 14 mA at 1.2, 1.3, and 1.55 mu m almost coincide with the measured values. >