Avago’s 850nm VCSELs for applications requiring modulation at 25-28Gbps have been designed for -3dB bandwidths in excess of 19GHz over the extended temperature range of 0-85°C. The DBR mirrors have been optimized to minimize optical losses and thermal and electrical resistance. The active region is designed to provide superior differential gain for high optical bandwidth. In this paper we will describe the design for performance and manufacturability of Avago’s high speed 25-28Gbps VCSEL. Analysis of the high-speed modulation characteristics and results of wearout reliability studies will be presented. We will also discuss the manufacturability of this next generation of high performance, reliable lasers. The challenges of epitaxial growth and wafer fabrication along with the associated process control technologies will be described.
Applications of 850 nm VCSELs have bloomed in recent years arising from their low cost, and the ease of forming one- and two-dimensional arrays. In addition to the traditional measures of device lifetime, operation over a wide temperature range and link length, the figures of merit increasingly include power consumption (pJ/bit), footprint (bits/mm<sup>2</sup>) and cost ($/Gb/s). As 1 × 12 arrays of 10G VCSELs are widely adopted, there is a clear need for improvement along all these fronts. This is achieved through development of VCSELs operating at higher data rates, and modifications to the oxide VCSEL structure. In this paper, we discuss the development of VCSELs with electrostatic discharge protection, and high bandwidth for operation at 10 – 25 Gb/s.
Avago’s 850nm oxide VCSEL for applications requiring modulation at 25-28G has been designed for -3dB bandwidths in excess of 18GHz over an extended temperature range of 0-85C. The VCSEL has been optimized to minimize DBR mirror thermal resistivity, electrical resistance and optical losses from free carrier absorption. The active region is designed for superior differential gain to enable high optical bandwidths. The small-signal modulation response has been characterized and the large-signal eye diagrams show excellent high-speed performance. Characterization data on other link parameters such as relative intensity noise and spectral width will also be presented.
In this paper we will discuss 14 Gb/s 850 nm oxide VCSEL performance and reliability. The device is targeted for the
16G Fibre Channel standard. The 14 Gb/s 850 nm oxide VCSEL meets the standard's specifications over the extended
temperature range to support transceiver module operation from 0C to 85C.
Frequently quoted advantages of VCSELs over other optical sources include wafer scale fabrication and testing, low
cost, ease of fabricating arrays and ease of fiber coupling. To benefit from these advantages a robust manufacturing
process and product demand are needed. Avago Technologies produces a range of single channel and parallel optical
link products incorporating 850nm band VCSEL sources operating at up to 10Gb/s per channel. This paper will explore
some important factors which need to be controlled for manufacturability of VCSEL devices.
Using a least squares technique we fit the measured normal-incidence reflectivity spectra of resonant cavity light emitting diode (RCLED) structures, using six fitting parameters-the thicknesses of the AlAs and AlGaAs layers in the top and bottom Bragg stacks, the thicknesses of the cavity region, and the Al concentration in the AlGaAs components of both Bragg stacks. We find that the fitting procedure indicates growth errors in these thicknesses and in the Al concentrations, and, in particular, gives a best fit when the Al concentration in the AlGaAs component of the Bragg mirrors is typically 61±1% instead of the intended 50%. Furthermore, the fitting reveals that the spatial period of the upper Bragg stack is typically 4% less than that in the lower stack, in these growth runs, a finding which is confirmed by a detailed analysis of scanning electron microscopy images of cleaved pieces of the RCLED wafers. This fitting method provides a useful and non-destructive tool to determine as-grown thicknesses and compositions of complex multilayer heterostructures which are otherwise difficult to ascertain.
We have improved the design of our red emitting VCSELs to be less sensitive to leakage induced optical losses in the
output reflector. The current designs produce in excess of 0.2mW at 652nm and 50 degrees C. We also have devices emitting
6.5mW at 668nm at 20 degrees C. We use a simple model to predict the device performance improvements of minor
modifications to the device design. By reducing the bias voltage from the current high levels, we predict that c.w.
powers in excess of 0.5mW at 80 degrees C and up to 17mW at 20 degrees C should be possible without any further design or material
By measuring the spontaneous emission from normally operating ~1.3um GaInNAs/GaAs-based lasers grown by MBE and by MOVPE we have quantitatively determined the variation of monomolecular (defect-related ~An), radiative (~Bn2) and Auger recombination (~Cn3) as a function of temperature from 130K to 370K. We find that A, B and C are remarkably independent of the growth method. Theoretical calculations of the threshold carrier density as a function of temperature were also performed using a 10 band k·p Hamiltonian from which we could determine the temperature variation of A, B and C. At 300K, A=11x10-8 sec-1, B=8x10-11 cm3 sec-1 and C= 6x10-29 cm6 sec-1. These are compared with theoretical calculations of the coefficients and good agreement is obtained. Our results suggest that by eliminating defect-related currents and reducing optical losses, the threshold current density of these GaInNAs/GaAs-based edge-emitting devices would be more than halved at room temperature. The results from studies of temperature and pressure variation of ~1.3um VCSELs produced by similar MBE growth could also be explained using the same recombination coefficients. They showed a broad gain spectrum and were able to operate over a wide temperature range.
A comprehensive beam propagation method (BPM) for the modeling of oxide-confined visible emitting (665nm) vertical-cavity surface-emitting lasers (VCSELs) aimed at polymer optical fibre (POF) communications is presented. In this model all the major physical processes, including the current density distribution, the self-heating effect of the devices, the carrier lateral diffusion in quantum wells, and the optical field modes are considered self-consistently. Using the model, the current flow, carrier diffusion in the active quantum wells, and temperature distribution are calculated. The model indicates severe current crowding around the edge of the oxide aperture within the VCSEL. Using a simple function to describe the variation of optical gain with temperature and wavelength, the threshold properties, transverse modes and the optical output current-light characteristics are calculated. The simulation results are compare favorably with measurements.
This paper presents results that have emerged from the European funded ESPRIT Project, Bright Red Surface Emitting Lasers (BREDSELS-23455). The project's main objective has been to develop arrays of Vertical Cavity Surface Emitting Lasers (VCSEL's) emitting in the region of 650 nm. These VCSEL arrays, suitably coupled to plastic fiber ribbon, are potentially ideal sources for high-speed plastic optical fiber networks. Linear 1 X 8 VCSEL arrays have been fabricated from wafers grown in multi-wafer MOVPE reactors. Individual VCSELs are shown to generate a peak room temperature power of 2 mW at 674 nm and are capable of operating continuous wave to a temperature of 60 degrees Celsius. The use of selective oxidation in the fabrication process is found to be essential in terms of providing effective heat sinking to the active region, while free carrier absorption is found to be a significant loss mechanism. A detailed description of the device results including modal behavior is presented along with the initial results from the plastic fiber ribbon module.
Despite their complexity, vertical cavity surface emitting lasers (VCSELs) have become key devices for future low cost optical interconnections. The high quality dielectric distributed Bragg reflectors (DBRs) mirrors possible in the AlGaAs system have made GaAs based devices the most successful and most studied VCSELs. This paper reviews the work carried out at the University of Sheffield on devices which employ AlGaAs mirrors and emit at wavelengths across the range 640 to 1100 nm. The active layers in the different designs contain variously quantum wells of InGaAs, GaAs, AlGaAs, and AlGaInP. A major limitation of using Al based compounds, particularly AlGaAs, at shorter wavelengths has previously been the presence of oxygen and other impurities. But by improving the crystal quality and purity, respectable performance of arsenide compounds has been extended to the sub 700 nm wavelength region and further improvements are expected through structural optimization and the application of strained AlInGaAs layers. Issues regarding the growth, device resistance and reproducibility of emission wavelength are also discussed.
Vertical cavity surface emitting lasers (VCSELs) have been the subject of intense research in recent years. The compact nature of the devices means that heat generated within is not as readily dissipated as with more conventional stripe geometry lasers. Advances in the design of distributed Bragg reflector (DBR) cavity mirrors and intracavity contact schemes have reduced the threshold voltage from greater than 10 V to little more than the lasing photon potential, in some cases. However, thermal management is still a limiting factor for high power or high efficiency output from VCSELs By analyzing a variety of devices we have devised a simple but powerful model to explain the current-light response of VCSELs which is strongly dependant on the temperature rise in the active layer. Effects of the relative position of the cavity resonance and gain spectrum are also discussed.