TECHNOLOGY VCSELs
and cavity adjustments require selective etching of the InP-based material – this reduces the precision of wavelength setting. There are also 1310 nm VCSELs that are formed by growing a semiconductor active region and DBRs in a single run, and with type of design it is again very challenging to hit the wavelength specifications. Standard epitaxial growth techniques have a thickness tolerance of about 1 percent, and this leads to variations in emission wavelength that exceed the tolerance that is acceptable for components based on CDWM.
After the VCSEL wafers have been formed, they are processed with standard steps, such as dry and wet chemical etching, and deposition of dielectrics and metals for contacts and bond- pads. This creates about 15,000 VCSELs on a wafer, which are all characterised at room and elevated temperatures using automated probe stations. Only after performing all the necessary tests, including high-speed modulation characterization on selected devices, are dies scribed from the VCSEL wafer.
Coping with the heat These VCSELs can deliver 10 Gbit/s error-free transmission over 10 km of standard, single-mode fibre at ambient temperatures as high as 100°C (see Figures 4 and 5). These tests were performed without any cooling at a constant bias current of
Figure 3. Even though the wafer fusion process is performed at an elevated temperature of 600°C and the significant thermal expansion coefficient mismatch between GaAs- based and InP wafers, the current fabrication process allows production of fused wafers with a defect-free, InP-based active region of the VCSEL
8 mA, demonstrating that these wafer-fused VCSELs can perform excellently in the category of un-cooled 1310 nm communication lasers.
If these lasers are to be deployed in industry, their excellent performance must be combined with a level of reliability that conforms to industry standards. To determine if that is the case, we subjected these devices to a two-year reliability test programme: They passed all the assessments associated with the GR-468-CORE Telcordia Generic Reliability Assurance Requirements for Optoelectronic Devices. These assessments, including different mechanical tests like shocks, vibrations and die shear; temperature cycling and electrical tests; have shown that wafer-fused VCSELs behave in the same way as existing, commercially available lasers.
This set of tests included accelerated life tests on first-generation devices operating at 10 Gbit/s at a 9 mA driving current. The results of this assessment enabled us to predict that, at 25°C and 70°C ambient temperatures, times to 1 percent failure are 291 years and 19 years, respectively (see Figure 6).
Figure 4. VCSELs can be designed in a way that the threshold current does not change with temperature, while for DFBs the threshold current at elevated temperature is several times larger than that at room temperature
We have searched for defects in the active region of our device with various imaging techniques. This includes the use of scanning and transmission electron microscopy to scrutinise cross-sections and lamellas from degraded VCSEL material, which has been prepared for inspection via focused-ion-beam milling. The failure analysis study is ongoing and, if necessary, further optimization of fabrication will be implemented in new generations of the devices.
June 2013
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