InGaAs Vertical-cavity Surface-emitting Lasers Research Articles

Optical loss and interface morphology in AlGaAs∕GaAs distributed Bragg reflectors

It is shown that n-type doping of AlGaAs∕GaAs distributed Bragg reflectors (DBRs) grown by metal-organic vapor-phase epitaxy has a profound negative impact on the performance of vertical-cavity surface-emitting lasers (VCSELs) based on such mirrors. Using an intracavity contact scheme, 1.3-μm-range InGaAs VCSELs with and without doping in the bottom DBR are directly compared. Doped mirrors lead to lower slope efficiency, lower output power, and higher threshold current. From x-ray diffraction, high-accuracy reflectance measurements, and atomic force microscopy studies, it is suggested that this performance degradation is due to the doping-enhanced Al–Ga interdiffusion, leading to interface roughening and increased scattering loss.

Suppression of Higher Order Transverse and Oxide Modes in 1.3-$\mu$m InGaAs VCSELs by an Inverted Surface Relief

It is shown, by a systematic variation of design parameters, that the use of an inverted surface relief is very effective for suppressing higher order transverse modes in oxide confined 1.3-mum InGaAs vertical-cavity surface-emitting lasers (VCSELs). Single-mode emission is achieved for a large variety of oxide aperture and surface relief diameters, with optimum designs, having a surface relief with a diameter half of that of the oxide aperture, producing 1.1-1.3 mW of single-mode power. It is also shown that the anti-phase layer employed to enable the use of an inverted surface relief is effective for suppressing oxide modes that otherwise appear in oxide confined VCSELs with a large detuning between the gain peak and the cavity resonance

Low threshold current density, highly strained InGaAs laser grown by MOCVD

It is possible to delay the growth mode transition from a 2-D growth mode to 3-D growth mode (the so-called Stranski–Krastanov growth mode) by means of non-equilibrium growth process employing a low growth temperature and a high growth rate, and adopting a high V/III ratio simultaneously to maintain crystal quality. Record emission wavelengths in multiple InGaAs/GaAs quantum wells without a strain-compensated barrier are grown by metal organic chemical vapor deposition (MOCVD). An InGaAs vertical-cavity surface emitting laser (VCSEL) with emission spectrum up to 1.26 μm under CW operation has been realized. Optimizing the quality of InGaAs QWs leads to a low threshold current density. The qualities of the highly strained InGaAs QWs of this VCSEL structure, such as internal quantum efficiency, internal loss, and transparency current density, were investigated by standard broad area (BA) laser process. Combined with an extensive gain-cavity detuning and the high-quality InGaAs QWs, the InGaAs VCSEL, we believe, is a promising candidate for application to long wavelength, low-cost source fiber-optical communication systems.

Temperature-dependent characteristics and burn-in performance of GaAs-based long-wavelength vertical-cavity surface-emitting lasers emitting at 1.26 µm

Continuous-wave operation of highly strained double quantum well vertical-cavity surface-emitting lasers (VCSELs) with active regions of GaInNAs and InGaAs emitting at 1.26 µm at room temperature has been realized by metal organic chemical vapour deposition. The proposed InGaAs VCSEL can operate at high temperatures and exhibits superior temperature stability. The degradation of output power of the 1.26 µm InGaAs VCSEL is approximately 2.8% after a 1000 h burn-in life test.

1.3 μ m InGaAs vertical-cavity surface-emitting lasers with mode filter for single mode operation

We report on the performance and analysis of 1.3μm range InGaAs∕GaAs vertical-cavity surface-emitting lasers (VCSELs) with an integrated mode filter consisting of a patterned silicon layer on the top distributer Bragg reflector. In this way, 1mW of single mode power is obtained from a device with a wavelength of 1265nm and a threshold current of 2.6mA at room temperature. An effective index model is used to extract the internal and external losses of the VCSEL structure and to predict the modal losses with and without mode filter, thereby providing a useful design tool for single mode VCSELs.

Temperature sensitivity of the threshold current of long-wavelength InGaAs-GaAs VCSELs with large gain-cavity detuning

Record-long emission wavelengths up to 1.3 /spl mu/m have recently been demonstrated from highly strained InGaAs-GaAs double-quantum-well (DQW) vertical-cavity surface-emitting lasers (VCSELs). The operation of InGaAs VCSELs at such long wavelengths has relied on a large detuning between the spectral positions of QW gain maximum and cavity resonance. This detuning also affects the high-temperature performance and temperature sensitivity of such devices. In this paper, we present and evaluate the threshold current-temperature characteristic of such lasers in relation to the gain-cavity detuning at room temperature (RT). For a near-zero gain peak offset from the emission wavelength at RT, the minimum threshold current is found at the temperature where the gain peak wavelength and the cavity resonance are approximately aligned. This is well in line with a common design rule for GaAs-based VCSELs. However, we show that this design rule fails in the case of larger gain-cavity misalignment at RT. Instead, a minimum threshold current is obtained considerably below the temperature of zero gain offset. We propose a conceptual model that relates the gain-cavity detuning at RT to the temperature sensitivity of the active region performance, which qualitatively describes the threshold current-temperature characteristic typical of VCSELs. The results demonstrate the importance of improving the temperature characteristic of the active region in order to reduce the high temperature sensitivity of devices with large detuning.

High performance selectively oxidized VCSELs and arrays for parallel high-speed optical interconnects

High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode filter is employed. Fabricated VCSELs are applied in various interconnect schemes. InGaAs quantum-well based VCSELs at 935 nm emission wavelength are investigated for use in perfluorinated graded-index plastic-optical fiber (GI-POF) links. We obtain a 7 Gb/s pseudo random bit sequence (PRBS) nonreturn-to-zero (NRZ) data transmission over 80 m long 155 /spl mu/m diameter GI-POF. We investigate data transmission over standard 1300 nm, 9 /spl mu/m core diameter single-mode fiber with selectively oxidized single-mode GaAs and InGaAs VCSELs. We achieve biased 3 Gb/s and bias-free 1 Gb/s pseudo-random data transmission over 4.3 km at 830 nm emission wavelength where a simple fiber mode filter is used to suppress intermodal dispersion caused by the second order fiber mode. For the first time, we demonstrate 12.5 Gb/s data rate transmission of PRBS signals over 100 m graded-index multimode fiber or 1 km single-mode fiber using high performance single-mode GaAs VCSELs of 12.3 GHz modulation bandwidth emitting at /spl lambda/=850 nm.

Fabrication of InGaAs vertical-cavity surface-emitting lasers by molecular beam epitaxy on (4 1 1)A GaAs substrates and its room-temperature operation

We investigated the optimum growth conditions for fabrication of a high-reflective distributed Bragg reflector (DBR) with extremely flat heterointerfaces on (4 1 1)A GaAs substrates by molecular beam epitaxy (MBE). A high-reflective DBR consisting of AlAs GaAs was successfully fabricated under the investigated optimum conditions of 580°C with V III ( As 4 Ga ) ratio of ∼ 8. Using this high-reflective DBR, we succeeded for the first time in the optically pumped pulse operation of InGaAs vertical-cavity surface-emitting lasers (VCSELs) at room temperature on (4 1 1)A GaAs substrates with improved threshold characteristics compared to conventional (1 0 0) GaAs substrates.

Fabrication processes for low threshold InGaAs vertical-cavity surface-emitting lasers

In order to realize a low threshold vertical-cavity surface-emitting laser (VCSEL), we have fabricated a micro-cavity structure by a native oxidation technique. The strong optical and electrical confinements enabled to achieve low thresholds. For further threshold reduction, we have demonstrated an n-type modulation doped quantum well (QW) laser and a 30% reduction of threshold in n-type doped QW laser compared with undoped QW lasers is observed. These are the first experiments for InGaAs/AlGaAs materials.

Calculation of differential gain and linewidth enhancement factor in 980-nm InGaAs vertical cavity surface-emitting lasers

The optimization of differential gain and linewidth enhancement factor within VCSEL's is required to achieve improved performance under dynamic conditions. In this paper, the differential gain and linewidth enhancement factor in an InGaAs-GaAs VCSEL have been calculated over a range of carrier densities and at various wavelengths across the gain spectrum. The results show the importance of low threshold gain values in achieving high differential gain/low linewidth enhancement factor. By temperature tuning the lasing wavelength to shorter wavelengths than the gain peak, values of /spl alpha//sub H/ as low as 0.6 can be achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Precise nonselective chemically assisted ion-beam etching of AlGaAs/GaAs Bragg reflectors byin situ laser reflectometry

Precision etch-depth control is realized by a chemically assisted ion-beam etching system incorporated within situ laser reflectometry. By counting the number of interference fringes, etch-depth control better than a quarter-wave thickness is easily obtained. Optimized etching conditions for highly anisotropic etching of bulk GaAs and AlGaAs/GaAs distributed Bragg reflectors are obtained. With the ability to etch-stop just below the active region by thein situ monitoring, InGaAs vertical-cavity surface-emitting lasers with CW threshold current density as low as 380 A cm-2 with output power >11 mW are fabricated. Spatial uniformity is 5% over a 1-cm2 sample, which corresponds to one pair over 20 pairs of quarter-wave stacks of AlGaAs/GaAs distributed Bragg reflectors.

High-Reflectivity InGaP/GaAs Multilayer Reflector Grown by MOCVD for Highly Reliable 0.98-µm Vertical-Cavity Surface-Emitting Lasers

We report an InGaP/GaAs multilayer grown by low-pressure MOCVD for highly reliable 0.98-µ m InGaAs vertical-cavity surface-emitting lasers. High reflectivity of over 99% is obtained for the 45-period multilayer. The stability of the layer thickness over the entire multilayer is discussed from the viewpoint of the asymmetrical reflectivity spectrum. Our results indicate that the layer thickness slightly decreases during epitaxial growth.

HIGH-PERFORMANCE, PRODUCIBLE VERTICAL-CAVITY LASERS FOR OPTICAL INTERCONNECTS

In this paper we review the state-of-the-art performance of producible, 850 nm, current-guided GaAs/AlGaAs , top-emitting vertical cavity surface emitting lasers (VCSELs). We discuss the motivation and desired characteristics for pursuing VCSELs, particularly in the area of high speed optical data links. We demonstrate that this structure is indeed producible and reproducible using MOVPE, where exceptional uniformity across wafers and arrays is obtained from commercial chambers. Record performance is also reported using MOVPE-grown GaAs VCSELs. These records include submilliamp (0.68 mA) CW room temperature threshold currents, &lt;1.6 V threshold voltages, over 28% total wall-plug efficiency, over 59 mW of (unbonded) power, 200° C lasing, operation over a 100 nm wavelength regime, and other records that rival or exceed those obtained even from strained-layer InGaAs VCSELs of any structure. We also present novel extensions of this base VCSEL platform for lateral mode control, illustrating the flexibility and extendibility of this technology. Finally application of these arrays as 32-channel-wide Opto-Electronic Technology Consortium (OETC) parallel links are shown with error free operations up to 700 Mbits/s (Manchester coded) through 100 m of fiber.

High modulation efficiency of intracavity contacted vertical cavity lasers

A very high current modulation efficiency of 5.7 GHz/√(mA) has been demonstrated in a low threshold (0.85 mA) InGaAs vertical-cavity surface-emitting laser. These top surface emission devices have been fabricated with intracavity contacts on semi-insulating substrates to realize their high speed potential. Data is presented for various laser diameters. Modulation bandwidths up to 9.3 GHz at a bias of only 4.5 mA have been measured for 7 μm diameter lasers. Derived parameters indicate that considerably higher bandwidths should be possible with better heat dissipation and modified cavity designs.

Low threshold current density InGaAs surface-emittinglaserswith periodic gain active structure

The authors have obtained very low threshold current densities for InGaAs vertical-cavity surface-emitting lasers using a periodic gain active structure. For a 40 µm diameter device, the threshold current density for room-temperature CW operation was as low as 380 A/cm2 with light output power of more than 11 mW.

Integration of GaAs vertical-cavity surface emitting laser on Si by substrate removal

Substrate removal techniques are attractive for the integration of III-V compound semiconductor devices on Si for the integration of optical and electronic devices, and on thermally conducting substrates for heat sinking. Here we discuss the bonding of strained quantum well InGaAs vertical-cavity surface-emitting lasers on both Si and Cu substrates. The GaAs substrates are then removed by selective etching. Lasing was achieved with pulsed electrical pumping with Jth=2.5 kA/cm2. The performance characteristics of the Si and Cu bonded devices are compared.

InGaAs vertical-cavity surface-emitting lasers

The authors give theoretical and experimental results for vertical-cavity surface-emitting lasers (VCSELs). The modeling is applied to the design of InGaAs VCSELs. A simple method to calculate the reflectivity of semiconductor stack mirrors with graded interfaces and compound metal/semiconductor stack mirrors is introduced. The theoretical predictions are compared to results from actual device measurements. A novel technique is introduced to determine material parameters: fabrication of in-plane lasers from VCSEL material. The procedure used to determine the optical mode in such an in-plane laser is described. Using the insight gained from modeling, the external efficiency was increased to >30% with a threshold current density of 1 kA/cm/sup 2/. >