In this thesis, we have studied the high power singlemode PC-VCSEL with SMSR > 40dB throughout the operation current range.
In chapter 4, we focused the study on single transverse mode with high output-power and high modulation speed VCSEL. We propose hybrid-guided structure by using oxygen implantation and selective oxide-confined for gain-guide and index-guide, respectively we have fabricated 850 nm GaAs VCSELs with large emission aperture of 8 μm exhibit good performance with threshold currents of 1.5 mA, a single transverse mode emission within the full operational range and a maximum output power of 3.8 mW. Moreover, the single mode VCSELs demonstrate superior high speed performance up to 10 Gb/s.
By cooperating photonic crystal on gain-guided proton-implanted VCSELs, we achieved high side mode suppression ratio (SMSR) single-mode VCSELs. In this hybrid-guided VCSEL, the 2D photonic crystal and proton-implanted structures are for gain-guide and index-guide, respectively. The superior optical field confinement of photonic crystal resulted in single-output transverse mode output of the VCSELs with high side-mode suppression ratio (SMSR) over 40 dB. The output aperture of 10 μm defined by proton-implantation is larger than traditional oxide-confined VCSELs, so the photonic-crystal VCSELs possess potential for higher output power than traditional VCSELs. These devices also exhibited ultra-low divergence angle about 6.5o.
In chapter 5, by cooperating photonic crystal on gain-guided oxidised VCSELs, we report a single-mode InGaAs SML QD PC-VCSEL with SMSR > 35dB
throughout the operation current range. Single fundamental mode CW output power of 3.8mW at 28mA has been achieved in the 990nm range, with the threshold current of 0.9 mA. The output aperture of 10 μm defined by photonic crystal is larger than traditional oxide-confined VCSELs, so the photonic-crystal VCSELs possess potential for higher output power than traditional VCSELs. The beam profile and near-field image study of the PC-VCSEL indicates that the laser beam is well confined by the photonic crystal structure of the device. These devices also exhibited ultra-low divergence angle about 6.8o.
In chapter 6, we also applied this technique on the long-wavelength 1.3 μm InAs quantum-dots VCSELs and the VCSELs also show the single-output transverse mode in whole operation range. The InAs quantum dot photonic crystal VCSEL for fiber optic applications is first demonstrated. Single fundamental mode CW output power of 0.2 mW has been achieved in the 1300nm range, with a threshold current of 4.75mA. We report the QD PC-VCSEL with SMSR > 40dB throughout the operation current range.
7.2 Future Work
Vertical-cavity surface-emitting lasers (VCSELs) have attracted much attention in recent years because of their potential for low-cost manufacturability, simple two-dimensional (2-D) array fabrication, and light source for fiber-optic data communication links. A drawback of VCSELs is that the polarization of the lasing output is not defined, because of the symmetry of the epitaxial layers along the growth axis. The polarization can change with increasing current [1]. The anisotropy in gain or losses is needed to stabilize the polarization [2,3]. This isotropy can be introduced by an asymmetry either in the mirror, cavity, or in the active area. Recently,
polarization switching in VCSELs by using asymmetric current injection has been realized [4]. Polarization switching VCSELs made with asymmetric device structure for high contrast switching is needed to be realized.
In this part, we design cross-shaped VCSELs for polarization switching in dual-channel fiber-optic communications. The combined oxide layer with proton implantation is used for better current confinement. The finished structure is shown in Fig. 7-1. Four p-type ohmic contacts were formed at the ends of the four arms of the cross-shaped aperture. The dimensions of the cross are specified by its length (L) and width (W). The n-type ohmic contact is formed at the bottom surface of the substrate.
The biasing currents and therefore the modulating signals can apply simultaneously to the two opposite ends of the contacts. The other two contacts of the cross-shaped aperture are unbiased (or remain floating). The polarization of the lasing output can therefore aligned alone the direction of the arms of the cross aperture with biasing current. By switching the biasing current between two pairs of contacts of the device, the polarization of the VCSEL can switch almost perpendicularly between two polarization directions. The epitaxial layers of the device were grown by metal organic chemical vapor deposition (MOCVD). The cross-shaped aperture of the device is defined by using reactive ion etching (RIE). The lasing wavelength is approximately 840 nm.
Fig. 7-2 shows CW light-current-voltage (L-I-V) output of our cross-shaped VCSELs, with biasing current applied to the opposite ends of the ohmic contacts. The dimensions of the device are L×W=20μm×8μm. The current applied either to the x-axis contacts (IX) or y-axis contacts (IY). The threshold currents (Ith) are both approximately 3.4 mA. The threshold voltages (Vth) are both 1.6 mA. Fig. 3 (a) and (b) show the polarization resolved L-I-V characteristics for IX and IY, respectively. The L-I-V curves were measured with a polarizer mounted on top of the tested device. The
polarizer is aligned with its polarization direction either parallel to x-axis (0°) or y-axis (90°) of the VCSEL. The characteristics in Fig. 7-3 clearly show switching of the polarization direction as the applied current changing from x-axis to y-axis. The switching contrast, L(0°)/L(90°) or L(90°)/L(0°) is greater than 5 for both biasing condtions. Fig. 7-4 (a) and (b) shows the photographs of the polarization-switching VCSEL with biasing current of 8 mA applied to x-axis and y-axis contact pads, respectively. The lasing areas of the device are near the ohmic contacts of the cross-shaped aperture. This lasing property also indicates that the polarization direction tends to align parallel to the x-axis or y-axis, is dependent on the direction of current flow, which is also related to the geometry of the device. The polarization of the VCSEL can therefore switch between two states (0° and 90°) alternatively.
In summary, we report cross-shaped polarization switching VCSEL for dual-channel communications. The polarization of the device is clearly dependent on the geometry and direction of current flow of the device. High switching contrast between two polarization states has been achieved.
.
Fig.7-1 Schematic of cross-shaped polarization switching VCSEL
Fig.7-2 L-I-V characteristics of a cross-shaped polarization VCSEL with biasing current apply to x-axis contacts (Ix, solid lines) and y-axis contacts (Iy, dash lines), respectively.
0 1 2 3 4 5 6 7 8
Fig.7-3 Polarization resolved L-I-V characteristics of a cross-shaped polarization VCSEL with (a) biasing current applied to two x-axis contacts and (b) biasing current applied to two y-axis contacts. The dimensions of the cross are L×W=20μm×8μm. The L-I-V were measured with the polarizer at 0° and 90°.
(a) (b)
Fig.7-4 Photographs of a cross-shaped polarization VCSEL with biasing current of 8 mA applied to (a) two x-axis contacts and (b) two y-axis contacts, respectively. The dimensions of the cross are L×W=20μm×8μm.
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