INDUSTRY VCSELs
Figure 3. Refinements of the VCSEL design have propelled speeds to 50 Gbit/s and beyond.
to lower resistance and optical losses. Completing this list of enhancements was the inclusion of multiple oxide layers, the introduction of an undoped substrate to lower capacitance, and the use of an AlAs binary compound in the bottom mirror to reduce the thermal impedance.
These changes to the VCSEL structures have impacted the epitaxial deposition process. Growth of the new device structures demands exceptionally tight control over compositional uniformity, maintenance of strain, and highly accurate placement of layers for forming an oxide aperture for current and optical confinement.
Changes to the VCSEL designs have wrought improvements in performance. For VCSELs emitting at 850 nm the modifications to the active region and the cavity have delivered a doubling of differential gain, a 30 percent cut in threshold carrier density, and a 20 percent increase in the optical confinement factor. Carrier transport has also improved, while the diffusion capacitance has decreased, as demonstrated by Chalmers.
A better DBR has resulted from the new design. Differential resistance has fallen by 25 percent in the top mirror, while free carrier absorption has showed little, if any, increase. Meanwhile, in the bottom mirror the switch to the pairing of AlAs and Al0.12
Ga0.88 As has delivered a 40 percent hike in thermal
conductivity. Parasitic capacitance has decreased between 30 and 40 percent via the addition of extra oxide layers.
Due to all of these changes, VCSEL speeds have been catapulted to beyond 50 Gbit/s (see Figure 4; from Electronics Letters, August 2013) and to such an extent that the optical interconnect speed is largely limited by the speed of the drive electronics and the optical receiver. Recent work at IBM (Yorktown Heights, NY), using Chalmer’s 850 nm VCSELs and drive and receiver electronics developed at IBM, has enabled optical interconnects operating at a channel rate of 64 Gbit/s (Optical Fiber Communication Conference, 2014, paper Th3C.2), which is a world record for VCSEL-based interconnects. IBM has also demonstrated transmission at
51.56 Gbit/s (the InfiniBand HDR data rate) with the VCSEL- based transmitter held at a high temperature of 90°C.
Measurements of bit-error-rate and eye diagrams for 850 nm VCSEL-based optical interconnects at bit rates from 50 Gbit/s to 57 Gbit/s are shown in Fig. 5. The bit-error-ratio is a measure of the probability that a data bit is erroneously detected, with 10-12
typically considered ‘error-free’ for short-
reach optical fibre interconnects. An eye diagram is constructed by overlaying sweeps of different segments of the digital data stream, thus creating an ‘eye’ displaying the on and off-levels and the transitions between these.
Boosting efficiency Fast speeds are highly commendable, but they are not the only metric that matters: the energy dissipation per bit is probably still more critical, for it is this that has the larger influence on the power drawn by the datacentres. This is exactly the focus
Figure 4. Data transmission over a VCSEL-based optical interconnect at 50, 55 and 57 Gbit/s.
Copyright Compound Semiconductor October 2014
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