The fundamental issues in designing LW-VCSELs fall into four main categories:
optical, gain, electrical and thermal considerations. First, in considerations of optical elements in LW-VCSEL, how to fabricate both mirrors with high reflectivity, large stop-band width and short penetration depth is important. The choices of materials for DBRs will influence the structure and the fabrication methods of VCSELs. In order to fabricate an efficient device, the optical confinement in transverse plane is necessary.
In GaAs-based VCSELs, the oxidized high Al-contained aperture has been successfully applied to provide not only the optical confinement in transverse plane but also the precision current path to the active region, which makes good overlapping between optical field and carrier distributions. Unless the LW-VCSELs are made by wafer-fusion, in which the AlGaAs DBRs can be integrated with InP-based active region, other kinds of mechanisms for optical confinement in transverse plane have to be figured out. Buried tunnel junction [25] and under-cut active layer [26] have been successfully demonstrated.
Second, due to the stringent requirement in fiber communication, the LW-VCSELs have to be operated at 85°C with high-modulated speed. To improve the high temperature characteristics, it is effective using the materials in active region with large conduction-band offset, which can reduce the probability of hot electrons jumping over the barrier potential. The strained materials discussed above are also effective for increasing the high temperature characteristics since the reduction of threshold current and probability of Auger recombination. Strained multiple quantum
wells can help to improve the high temperature characteristics and high-speed modulation due to the increase of the differential gain. However, the number of strained multiple quantum wells cannot be too large. The strained field can be built-up while the number of strained multiple quantum wells increases to produce defects. Some groups had proposed strain-compensating multiple quantum wells to lessen the defect showing up. In addition, too many quantum wells in VCSEL cavity will reduce the longitudinal enhancement factor. The extra quantum wells will not provide more gain in laser operation but will increase the chance of absorption loss.
In electrical considerations, it’s very important to efficiently transport the carriers into the effective active regions if there’s an accurate definition of carrier path.
Ion-implantation, oxidized aperture, buried-tunnel-junction [25] and under-cut [26]
aperture have been successfully applied in VCSELs. Low serial resistance is also crucial in LW-VCSEL continuous-wave operations while low capacitance structure leads to high modulation speed. The major challenge in early VCSEL development is the relatively high serial resistance in p-type DBR. The resolution is to use the parabolic graded interface and modulation doping at the interfaces where are nodes of the standing wave field in VCSEL cavity.
The last issue is the thermal consideration in LW-VCSELs. Since the gain materials for long-wavelength range are sensitive to the high temperature, how to design a efficient device with low thermal resistance is rather important. As far as the materials are concerned, the dielectric mirrors are usually not good thermal conductors. In alloy system, the thermal conductance for binary compound is better than ternary compound; and ternary is better than quaternary. Flip-chip bonding can provide more direct heat dissipation path to the heat sink.
In addition to the above concerns, the single mode and low chirping operations
are important issues in optical communications. The reliability assurance and the capability of mass-production are also critical factors when designing LW-VCSELs.
However, the one consideration in any category will influence or induce other considerations in other categories. As shown in Figure 2-22, the designing considerations influence with each other. For example, the DBRs made by InP-airgap demonstrate extremely high reflectivity, large stop-band width and short penetration depth in optical considerations. However, in electrical and thermal considerations, the InP-airgap DBRs are electrically and thermally insulating in longitudinal direction.
Other kinds of design have to be adopted. For another example, the ion-implantation is an easy and mature technique to define current aperture in VCSEL structure.
However, the ion-implanted aperture cannot provide good optical confinement in transverse plane. All in all, to successfully design and fabricate the LW-VCSELs, not only will every issue require optimization, all considerations related to each other require thorough optimizations.
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Table 2-1 Comparison of threshold conditions for EEL vs VCSEL.
EEL VCSEL
Active length, La 300 µm 0.01 µm
Confinement factor, Γ 0.03 1 (or 2)
1/ΓLa 1/9 µm-1 1/0.01 µm-1
Mirror loss, ln(1/R) 1.2 (R ~ 0.3) 0.001 (R ~ 0.998)
Threshold gain, gth 0.1 µm-1 0.1 µm-1
Table 2-2 Magnitude of |M|2 for various material systems.
Material systems 2|M|2/mo in eV Reference
GaAs 28.8 ± 0.15 [14, 15]
AlxGa1-xAs (x < 0.3) 29.83+2.85x [16]
InxGa1-xAs 28.8-6.6x [14, 15]
InP 19.7 ± 0.6 [14, 15]
In1-xGaxAsyP1-y (x=0.47y) 19.7+5.6y [15, 17]
Table 2-3 Magnitude of |MT|2/|M|2 for different transitions and polarizations.
C-HH represents the transition from electron band to heavy hole band. C-LH represents the transition from electron band to light hole band
Quantum-well
Bulk (kt ~ 0)
Polarization C-HH C-LH C-HH C-LH
TE 1/3 1/3 1/2 1/6
TM 1/3 1/3 0 2/3
.
Table 2-4 Various material combinations for making high reflectivity DBRs DBR materials Δn/no
Required pairs to reach R>99.9%
Penetration depth (LDBR)
InP/Air 1.038 4 0.11 μm
TiO2/SiO2 0.509 7 0.14 μm
GaAs/AlAs 0.153 27 0.79 μm
AlGaAsSb/AlAsSb 0.149 28 0.87 μm
InGaAlAs/InP 0.102 41 1.26 μm
InGaAlAs/InAlAs 0.090 47 1.45 μm
INGaAsP/InP 0.082 51 1.59 μm