Yugang Zeng

Design and comparison of GaAs, GaAsP and InGaAlAs quantum-well active regions for 808-nm VCSELs

Vertical-cavity surface-emitting lasers emitting at 808 nm with unstrained GaAs/Al0.3Ga0.7As, tensilely strained GaAs(x)P(1-x)/Al0.3Ga0.7As and compressively strained In(1-x-y)Ga(x)Al(y)As/Al0.3Ga0.7As quantum-well active regions have been investigated. A comprehensive model is presented to determine the composition and width of these quantum wells. The numerical simulation shows that the gain peak wavelength is near 800 nm at room temperature for GaAs well with width of 4 nm, GaAs0.87P0.13 well with width of 13 nm and In0.14Ga0.74Al0.12As well with width of 6 nm. Furthermore, the output characteristics of the three designed quantum-well VCSELs are studied and compared. The results indicate that In0.14Ga0.74Al0.12As is the most appropriate candidate for the quantum well of 808-nm VCSELs.

High-Power Ultralow Divergence Edge-Emitting Diode Laser With Circular Beam

A recognized drawback of edge-emitting diode lasers is their high divergence and elliptical beam shape since the first diode laser was demonstrated. In this paper, we demonstrated the ultranarrow circular beam emission from the broad area diode laser based on a modified Bragg-like waveguide. The low vertical divergence of 9.8° with 95% power content and 4.91° with the full-width at half-maximum was realized in the devices with 150 μm stripe width. The maximum output power was 4.2 W under quasi-continuous-wave operation and presently limited by thermal rollover. The detailed design principle was presented and it was found that reducing the refractive index and thickness of the defect layer was able to improve the vertical divergence and achieve the stable circular beam emission by controlling the lateral current distribution using the deep stripe. The packaged device with 90 μm stripe width demonstrated a maximum continuous wave power of 4.6 W at 10 °C. A direct fiber coupling efficiency of 90.6% had been achieved with a common fiber of 105 μm core diameter.

High-Power-Density High-Efficiency Bottom-Emitting Vertical-Cavity Surface-Emitting Laser Array

We report on the lasing characteristics of a two-dimensional (2D) vertical-cavity surface-emitting laser (VCSEL) array with three In0.2GaAs/GaAs0.92P QWs emitting at 977 nm. The contribution of a large-bandgap barrier material, GaAsP, to improve the output power was investigated. More than 123 W of pulsed peak power at 110 A was achieved, corresponding to 24.6 kW/cm2 of power density and 1.11 W/A of slope efficiency. The thermal effect dependence of the characteristics of the array was illustrated. Moreover, the device performance was estimated by a functional method using a p-parameter.

Design and analysis of high-temperature operating 795?nm VCSELs for chip-scale atomic clocks

We propose a new structure design and analysis of high-temperature (> 350 K) operating 795 nm vertical-cavity surface-emitting lasers (VCSELs) for Rb-87 based chip-scale atomic clocks (CSACs). Compositions and thicknesses of the InAlGaAs/AlGaAs multiple quantum wells (MQWs) are optimized for high optical gain at elevated temperatures. The temperature sensitivity of the threshold current is estimated by calculating the temperature dependence of the cavity-mode gain. A self-consistent VCSEL model based on quasi-3D finite element analysis is employed to investigate the temperature distribution and output of the proposed structure. An output of 1 mW with a 3 dB bandwidth of 6 GHz is obtained from a 3 mu m aperture VCSEL under 2.4 mA current at temperature higher than 340 K. These findings indicate that high-gain QWs and gain-offset consideration are especially crucial to make high-performance VCSELs at elevated temperatures.

Power and spectra polarization of large-aperture rectangular-shaped vertical-cavity top-emitting lasers

Abstract980-nm large-aperture top-emitting vertical-cavity surface-emitting lasers (VCSELs) were investigated through introducing the rectangular post. Both H-polarization (horizontal) and V-polarization (vertical) coexisted in the rectangular-shaped large-aperture VCSELs. Under the entire range of operation current, H-polarization dominated over V-polarization which was parallel to the shorter side of the rectangular output aperture. It was found that spectrum blue-shift of H-polarization light occurred with respect to V-polarization light at three different aspect ratios, and this can be explained by the dependence of longitudinal propagation constant on the aspect length. The rectangular post structure was found to be effective for polarization stabilization in broad-area VCSELs without serious degradation of light output characteristics.

High-peak-power vertical-cavity surface-emitting laser quasi-array realized using optimized large-aperture single emitters

We report on a high-peak-power vertical-cavity surface-emitting laser (VCSEL) quasi-array emitting at approximately 980 nm. The quasi-array is composed of four tightly packaged single emitters connected in series. The external quantum efficiency of large-aperture VCSEL was improved by optimizing the current distribution, and single emitters with a 62 W peak power were developed as the basic building block of this quasi-array. More than 210 W peak power is demonstrated from a ~2.2 × 2.2 mm2 quasi-array at a current of 110 A under 30 ns and 5 kHz pulsed operation. This is the first report on a hundreds-of-watts VCSEL module consisting of single emitters.

980-nm High-Power Low-Divergence VCSELs Achieved by Optimization of Current Density Distribution

In this paper, we report the design and fabrication of 980-nm vertical-cavity surface-emitting lasers and arrays that provide high output power and low divergence angle without any additional collimating optics. The influence of the structure of the contact on the current density distribution inside the active region was analyzed using the 3-D finite-element method. Uniform current distribution is achieved by optimizing the diameter of the p-contact, and the consequent improvement in beam divergence and thermal behavior is shown. A low divergence angle of 5.4° is obtained for a single device with continuous-wave (CW) power of 1.46 W at room temperature. The 8 × 8 array shows a divergence angle of 10.2° at 4 A with a CW power of 1.95 W. In addition, the measured results were compared with the results from devices without optimization of current density distribution.

Bimodal-sized quantum dots for broad spectral bandwidth emitter

In this work, a high-power and broadband superluminescent diode (SLD) is achieved utilizing bimodal-sized quantum dots (QDs) as active materials. The device exhibits a 3 dB bandwidth of 178.8 nm with output power of 1.3 mW under continuous-wave (CW) conditions. Preliminary discussion attributes the spectra behavior of the device to carrier transfer between small dot ensemble and large dot ensemble. Our result provides a new possibility to further broadening the spectral bandwidth and improving the CW output power of QD-SLDs.

Bragg reflection waveguide twin-beam lasers

We report on an edge-emitting diode laser with an ultra-narrow twin-lobed far field based on a dual-sided Bragg reflection waveguide. Lasers with a stripe width of 10 m exhibit two stable near-circular beams symmetrically located at about 31 with full width at half maximum (FWHM) of 7.2 and 5.4 respectively in the vertical and lateral directions. The mechanism behind the achievement of the twin-beam output is presented, and the symmetry of these two beams is analyzed by comparing the beam FWHM and power distribution. Single longitudinal mode operation is achieved and periodic spectral modulation is observed. 2013 Astro Ltd.

Microscopic View of Defect Evolution in Thermal Treated AlGaInAs Quantum Well Revealed by Spatially Resolved Cathodoluminescence

An aluminum gallium indium arsenic (AlGaInAs) material system is indispensable as the active layer of diode lasers emitting at 1310 or 1550 nm, which are used in optical fiber communications. However, the course of the high-temperature instability of a quantum well structure, which is closely related to the diffusion of indium atoms, is still not clear due to the system’s complexity. The diffusion process of indium atoms was simulated by thermal treatment, and the changes in the optical and structural properties of an AlGaInAs quantum well are investigated in this paper. Compressive strained Al0.07Ga0.22In0.71As quantum wells were treated at 170 °C with different heat durations. A significant decrement of photoluminescence decay time was observed on the quantum well of a sample that was annealed after 4 h. The microscopic cathodoluminescent (CL) spectra of these quantum wells were measured by scanning electron microscope-cathodoluminescence (SEM-CL). The thermal treatment effect on quantum wells was characterized via CL emission peak wavelength and energy density distribution, which were obtained by spatially resolved cathodoluminescence. The defect area was clearly observed in the Al0.07Ga0.22In0.71As quantum wells layer after thermal treatment. CL emissions from the defect core have higher emission energy than those from the defect-free regions. The defect core distribution, which was associated with indium segregation gradient distribution, showed asymmetric character.