GaAs-based Distributed Bragg Reflectors Research Articles

Spectral Efficiency and Energy Efficiency of Pulse-Amplitude Modulation Using 1.3 μm Wafer-Fusion VCSELs for Optical Interconnects

We compare the modulation efficiency of on–off keying (OOK) and four-level pulse-amplitude modulation (PAM4) using 1.3 μm wafer-fusion vertical-cavity surface-emitting lasers (VCSELs) for single-channel short-reach optical interconnects. O-band wafer-fusion VCSELs combine high optical gain from InP-based active regions with high-reflectivity, low-absorption, and high-thermal-conductive GaAs-based distributed Bragg reflectors. VCSELs offer a low-cost and low-power-consuming light source for optical interconnects in data centers and fiber-based local-area networks, presenting an attractive alternative to common edge-emitting lasers. Our investigation focuses on the highest error-free bit rates and energy efficiency of OOK and PAM4 signaling. We achieved an error-free bit rate of 38 Gbit/s with PAM4 for devices having a small-signal bandwidth of 9.9 GHz, presenting a spectral efficiency of 3.8 bit/s/Hz. These PAM4 values offer an improved error-free bit rate of 26% and consume 32% less energy-per-bit when co...

Thermal Management in Long-Wavelength Flip-Chip Semiconductor Disk Lasers

We address the thermal management of flip-chip semiconductor disk lasers (SDLs) emitting at wavelengths 1.3–1.6 μm. The emphasis of the study is on fabricating thin SDL structures with high thermal conductance. An essential part of this task is to use GaAs-based materials in the distributed Bragg reflector (DBR), because they can provide a combination of high thermal conductivity and high refractive index contrast. Furthermore, the reflectivity of the GaAs-based DBR should preferably be enhanced using a thin dielectric layer and a highly reflecting metal layer. Such a configuration enables very thin mirror structures with a reduced number of DBR layer pairs without compromising the reflectivity. The concept is demonstrated experimentally with a 1.32-μm flip-chip SDL, where the GaAs-based DBR is finished with a thin Al2O3 layer and a highly reflective Al layer. In addition, the design principles, thermal management, and the development issues related to semiconductor–dielectric–metal mirrors in 1.3–1.6-μm flip-chip SDLs are discussed.

Multi-Watt Semiconductor Disk Laser by Low Temperature Wafer Bonding

We present wafer bonding techniques applied for the first time in integrating GaAs-based distributed Bragg reflectors (DBRs) with InP-based active regions in optically pumped semiconductor disk lasers. The bonding procedures are performed at a modest temperature of 200 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C and enable multi-watt output powers from 1.3 μm semiconductor disk lasers. These technologies are critical for vertical-cavity lasers emitting in the range 1.3-1.6 μm since monolithically grown lattice-matched InP structures suffer from DBRs with low refractive index contrast and poor thermal conductivity when compared with GaAs-based DBRs.

Continuously Tunable, Polarization Stable SWG MEMS VCSELs at 1.55 $\mu{\rm m}$

We present polarization stable, tunable long-wavelength vertical-cavity-surface-emitting lasers (VCSELs) at 1.55 μm based on InP. The tuning is accomplished by a two-chip membrane mounting technique denoted as bulk-micro-machining. The membrane consists of a GaAs-based distributed Bragg reflector and represents the top mirror. By its electro thermal actuation, cavity tuning occurs. This results in a continuous wavelength shift. Polarization stability is achieved by an implemented semiconductor/air subwavelength grating (SWG) written in the top-most layers of the membrane facing the VCSEL eye after mounting process. The SWG integration is realized by a combination of holographic lithography, barrel, and reactive ion etching. The manufactured micro-electro-mechanical-system SWG-VCSELs show a continuous, polarization stable wavelength tuning of ~ 25 nm with a peak optical power of 4 mW fiber-coupled in an MMF and thresholds of 3-3.5 mA.

Long-wavelength semiconductor saturable absorber mirrors using metamorphic InP grown on GaAs by molecular beam epitaxy

Metamorphic growth of InP on GaAs has been used to tailor the recovery time of 1.55μm semiconductor saturable absorber mirrors. By decreasing the thickness of the InP “lattice reformation layer” grown between a GaAs-based distributed Bragg reflector and the active region, we were able to purposely reduce the recovery time. For thickness of the reformation layer below 200nm, the semiconductor saturable absorber mirrors exhibited a short absorption recovery time enabling a robust, self-starting passive mode-locked operation of an erbium-doped fiber laser. We are also reporting preliminary results on the stability of the saturable absorbing mirrors parameters after rapid thermal annealing.

Long-wavelength fast semiconductor saturable absorber mirrors using metamorphic growth on GaAs substrates

Metamorphic growth of InP on GaAs has been used to decrease the absorption recovery time of 1.55μm semiconductor saturable absorber mirrors. We show that the recovery time can be reliably controlled by changing the thickness of an InP “lattice reformation layer” grown between the GaAs-based distributed Bragg reflector and the active region. Semiconductor saturable absorber mirrors with a thickness of the InP reformation layer around 200 nm or smaller exhibit a recovery time short enough to reliably mode-lock fiber lasers.

Stable polarization operation of 1.3-/spl mu/m wavelength vertical-cavity surface-emitting laser (VCSEL) fabricated by orientation-mismatched wafer bonding

We present stable polarization of a long-wavelength vertical-cavity surface-emitting laser (LW VCSEL). The polarization control was achieved through growing its active region on a (113)B InP substrate, which was integrated to [001] GaAs-based distributed Bragg reflectors by a wafer-bonding technique. Theoretical investigation showed that to achieve high polarization stability, a large dichroism such as an anisotropic gain is needed. It was also shown that the (113)B and other planes of the (11n) family have asymmetry, which results in asymmetric stress and anisotropic optical gain in a strained multiquantum well. An index-guiding mesa structure was fabricated in an asymmetric shape. The index guiding either enhanced or distracted the polarization stability originating from gain anisotropy, depending on its orientation of the asymmetry, as was confirmed by a statistical summary. Using a VCSEL with an appropriate index-guiding structure, we performed 1-Gb/s modulation and confirmed single polarization under large-signal modulation.

1.3 μm wavelength vertical cavity surface emitting laser fabricated by orientation-mismatched wafer bonding: A prospect for polarization control

We propose and demonstrate a long-wavelength vertical cavity surface emitting laser (VCSEL) which consists of a (311)B InP-based active region and (100) GaAs-based distributed Bragg reflectors (DBRs), with an aim to control the in-plane polarization of output power. Crystal growth on (311)B InP substrates was performed under low-migration conditions to achieve good crystalline quality. The VCSEL was fabricated by wafer bonding, which enables us to combine different materials regardless of their lattice and orientation mismatch without degrading their quality. The VCSEL was polarized with a power extinction ratio of 31 dB.

InP-based 1.5 [micro sign]m vertical cavity surface emitting laser with epitaxially grown defect-free GaAs-based distributed Bragg reflectors

Room temperature pulsed operation of a novel 1.5 µm vertical cavity surface emitting laser is reported. A GaAs/Al0.95Ga0.05As heterostructure is grown directly on the patterned InGaAsP/InP quantum well active region. Selective lateral oxidation of a lattice matched In0.52Al0.48As layer is used for current confinement and, to create the top AlxOy/GaAs distributed Bragg reflector. The minimum threshold current is 12 mA.

Long-wavelength resonant vertical-cavity LED/photodetector with a 75-nm tuning range

A design for a highly tunable long-wavelength LED/photodetector has been investigated. The device consists of a GaAs-based distributed Bragg reflector (DBR) that is wafer-bonded to InP-based active layers, with a surface-micromachined tunable top DBR mirror to produce the wavelength shift. A 1.5-/spl mu/m device has been fabricated with a continuous tuning range of 75 nm. An extinction ratio of greater than 20 dB existed across the entire tuning range.