TECHNOLOGY VCSELs


de Lausanne and BeamExpress. Our approach, which we have been developing for several years, has advanced to the stage where it is mature enough to challenge existing un-cooled 10 Gbit/s DFB lasers used for communication applications.


Figure 1. Similar to datacom VCSELs, wafer- fused 1310 nm VCSELs employ a GaAs substrate and can be fabricated in large volumes using standard processing steps in foundries that normally process AlGaAs/Ga(In)As-based devices


A further advantage of the VCSEL over its incumbent cousin is its substantially reduced sensitivity to changes in temperature. It is possible to design a VCSEL in a way that ensures that its threshold current does not change with temperature, but for DFBs, the threshold current at elevated temperature is always several times larger than that at room temperature. With currently developed VCSELs, 10 Gbit/s operation is achievable at a constant bias current of typically 7 mA across the full temperature range from 0°C to 85 °C, but with standard DFBs, the bias current must be constantly adjusted, depending on the ambient temperature. The operation of 10 Gbit/s VCSELs at a bias current at or below 7 mA enables the use of very low-power- consumption VCSEL driver arrays, which were developed for short-wavelength datacom VCSELs – these emit at 1µm or less.


Wafer fusion


Several approaches can be taken to fabricate 1310 nm VCSELs, including that pioneered by our team from Ecole Polytechnique Fédérale


Wafer-fused VCSELs are essentially a marriage of an InP-based active region that is used in well- established DFB lasers and the AlGaAs/GaAs distributed Bragg reflectors that are employed in short-wavelength datacom VCSELs (see Figures 1 and 2). In these hybrids, a tunnel junction provides carrier injection into the active region. Compared with a standard datacom VCSEL, this new element allows implementation of un-doped DBRs that deliver a considerable reduction in optical losses in the VCSEL cavity. This enhancement counters the reduction in material gain resulting from the switch from a GaAs-based active region to one revolving around InAlGaAs/InP, and ultimately allows the 1310 nm band VCSEL to deliver a performance that is comparable to its shorter- wavelength sibling used in datacom and optical interconnects.


Our wafer-fused VCSELs share other similarities with their datacomm cousins: They are formed on a GaAs substrate, and they can be fabricated in large volumes in foundries using standard processing steps that are normally employed for producing AlGaAs/Ga(In)As-based devices. Device fabrication begins by taking two, 2-inch wafers with AlGaAs/GaAs DBRs and fusing them to either side of an InP-based active cavity with standard wafer-bonding equipment.


Despite using elevated temperature of 600°C, and the substantial difference in thermal expansion coefficients of GaAs-based and InP-based wafers that have been grown by MOCVD, our fused wafers have a very low density of defects in the active region (see Figure 3). This great material quality, and an active region that is incredibly small – its typical diameter is only 7 µm – leads to devices that are nearly always defect free, and are produced with a very high fabrication yield.


Figure 2. Wafer-fused VCSELs employ the same InP-based active region material system as well-established DFB lasers and the same AlGaAs/GaAs distributed Bragg reflectors as short-wavelength datacom VCSELs. In these devices, carrier injection into the active region is performed by a tunnel junction. Compared with standard datacom VCSELs, this new element allows implementation of un-doped DBRs that result in considerable reduction of optical losses in the VCSEL cavity


42 www.compoundsemiconductor.net June 2013


A great strength of the wafer fusion technique is that it allows the precise wavelength of the VCSEL to be set. This is crucial for making products that are based on CWDM, because this application demands laser emission within ±2 nm of the target wavelength. Fulfilling this requirement is possible with our VCSEL design, because devices are assembled from three separately grown elements: one active region and two DBRs. As a result, before the first and second fusion steps are undertaken, the active cavity and the DBRs can be adjusted by a proper selection and/or selective chemical etching.


In sharp contrast, in VCSELs with dielectric DBRs, the mode is tightly confined in the active region


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