D. W. Xu

High-Temperature Continuous-Wave Single-Mode Operation of 1.3-

In this letter, we have demonstrated continuous-wave single-mode operation of 1.3-mum InAs-GaAs quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) with p-type modulation-doped QD active region from 20degC to 60degC. The highest output power of 0.435 mW and lowest threshold current of 1.2 mA under single-mode operation are achieved. The temperature-dependent output characteristics of QD-VCSELs are investigated. Single-mode operation with a sidemode suppression ratio of 34 dB is observed at room temperature. The critical size of oxide aperture for single-mode operation is discussed.

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Efficient room-temperature (RT) continuous-wave (CW) lasing operation of the 1.3 µm MBE (molecular-beam epitaxy) In(Ga)As/GaAs quantum-dot (QD) top-emitting oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) for the second-generation optical-fibre communication has been achieved. In their design, a concept of a QD inside a quantum well (QW) has been utilized. The proposed In(Ga)As/GaAs QD active region is composed of five groups of three 8 nm In0.15Ga0.85As QWs, each containing one InAs QD sheet layer. In each group located close to successive anti-node positions of the optical standing wave within the 3λ cavity, QWs are separated by 32 nm GaAs barriers. Besides, at both active-region edges, additional single InGaAs QWs are located containing single QD layers. For the 10 µm diameter QD VCSELs, the RT CW threshold current of only 6.2 mA (7.9 kA cm−2), differential efficiency of 0.11 W A−1 and the maximal output power of 0.85 mW have been recorded. The experimental characteristics are in excellent agreement with theoretical ones obtained using the optical-electrical-thermal-recombination self-consistent computer model. According to this, for the 10 µm devices, the fundamental linearly polarized LP01 mode remains the dominating one up to the current of 9.1 mA. The lowest RT CW lasing threshold below 5 mA is expected for 6 µm devices.

m p-Doped InAs–GaAs Quantum-Dot VCSELs

A self-consistent rate equation model is presented to investigate the influence of carrier relaxation on the modulation response of 1.3 mum InAs-GaAs quantum dot lasers. In this model, the carrier dynamics in GaAs barrier, relaxation pathways, and the phonon- and Auger-assisted relaxation are considered. The dependence of 3 dB bandwidth on the relaxation time and relaxation pathway is discussed. It is shown that carrier relaxation via less energy level has better carrier confinement and higher 3 dB bandwidth. The improvement of bandwidth by tunnelling injection QD structure is investigated from the point of view of relaxation pathway. The different effects of tunnelling into ground state and excited state on the 3 dB bandwidth are analyzed. The enhanced carrier relaxation by p-type modulation doping and its effect on the bandwidth are investigated. It is found that there exists a tradeoff on the improvement of bandwidth by p-doping, which is explained as the competition between the bandwidth limitation of K -factor and relaxation dynamics. Increase in the bandwidth of QD lasers by improving both the carrier relaxation dynamics and K-factor limitation is discussed.

Room-temperature continuous-wave operation of the In(Ga)As/GaAs quantum-dot VCSELs for the 1.3 µm optical-fibre communication

In this paper, we present results from room-temperature continuous-wave operation of 1.3-mum p-doped InAs-GaAs quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) with high T 0 of ~510 K and low threshold current density of ~65 A/cm2 per QD layer. The highest output power from the device is over 0.74 mW. The temperature characteristics of the devices are investigated. It is demonstrated that deterioration in QD VCSEL performance due to self-heating results from the temperature sensitivity of QD emission, instead of mismatch between the gain wavelength and cavity modes. The real temperature at the QD VCSEL active region above threshold is estimated from the shift in lasing wavelength, which is in good agreement with calculations based on a self-consistent rate equation and thermal conduction model. The analysis shows that enhancing the carrier confinement in the QD wetting layer contributes to improving the saturated output power of the QD VCSEL.

Carrier Relaxation and Modulation Response of 1.3-

The present work seeks to demonstrate the elegance and simplicity of monolithic integration via plasma-activated direct wafer bonding. Two-inch gallium arsenide and silicon wafers were directly bonded through argon plasma activation. The highest specific bond energy was found for plasma conditions of 30 s, 120 mTorr, and 200 W, followed by low temperature annealing at 140 °C, and was 478 mJ/m2. Through this process, a processed silicon integrated circuit could be integrated with optoelectronics gallium arsenide on a wafer scale.

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Microphotoluminescence (mu-PL) investigation has been performed at room temperature on InAs quantum dot (QD) vertical cavity surface emitting laser (VCSEL) structure in order to characterize the QD epitaxial structure which was designed for 1.3 mu m wave band emission. Actual and precise QD emission spectra including distinct ground state (GS) and excited state (ES) transition peaks are obtained by an edge-excitation and edge-emission (EEEE) mu-PL configuration. Conventional photoluminescence methods for QD-VCSELs structure analysis are compared and discussed, which indicate the EEEE mu-PL is a useful tool to determine the optical features of the QD active region in an as-grown VCSEL structure. Some experimental results have been compared with simulation results obtained with the aid of the plane-wave admittance method. After adjustment of epitaxial growth according to EEEE mu-PL measurement results, QD-VCSEL structure wafer with QD GS transition wavelength of 1300 nm and lasing wavelength of 1301 nm was obtained.

m InAs–GaAs Quantum Dot Lasers

We present the 1.3-μ m In(Ga)As quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) fabricated by the dielectric-free (DF) approach with the surface-relief (SR) process. Compared with the conventional dielectric-dependent (DD) method, the lower differential resistance and improved output power have been achieved by the DF approach. With the same oxide aperture area, the differential resistance is reduced by 36.47% and output power is improved by 78.32% under continuous-wave operation; it is up to 3.42 mW under pulsed operation with oxide aperture diameter ~15 μm. The surface-relief technique is also applied, for the first time, in 1.3- μm QD VCSELs, and it effectively enhances the emission of the fundamental mode. The characteristic of small signal modulation response is also analyzed.

Temperature Characteristics of 1.3-

We present the 1.3-μm InAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) with novel planar electrodes configuration. The lasing wavelength is around 1274 nm. The lowest threshold current of wafer level device is ~1 mA, which corresponds to a low threshold current density of ~1.3 kA/cm2 or 76 A/cm2 per QD layer. The maximum output power of 1 mW can be obtained at room temperature. High temperature stability can be seen in temperature dependence L-I characteristics of InAs QD VCSEL 3-dB modulation frequency response of 1.7 GHz can be obtained in the small signal response measurements.

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The influence of quantum dot (QD) density, uniformity and layer number on the 3dB bandwidth of 1.3μm InAs-InGaAs QD VCSELs is investigated by the small signal analysis of all-pathway rate equations. The dependence of bandwidth on the QD density is shown. Linearly dependence of bandwidth on the QD uniformity is demonstrated. High speed operation (> 10GHz) of QD VCSEL emitting at 1.3μm is predicated.

m p-Doped InAs–GaAs Quantum-Dot Vertical-Cavity Surface-Emitting Lasers

We present fabrication of 1.3-µm InAs QD-VCSELs and arrays. The output power of single VCSEL exceeds 1.2 mW. Modulation bandwidth of 2.65 GHz and 2.5 GHz are achieved for single-mode and multi-mode VCSELs. Maximum output power of 28 mW is demonstrated for VCSEL arrays with threshold current of 50 mA.