industry VCSELs
Figure 1, and eye diagrams for the 25 Gbit/s laser are shown in Figure 5. The focus has been on the laser’s intrinsic parameters; there is still room to increase the parasitic bandwidth of our device, so better performance is expected in the final design.
Our lasers could have a big role to play in the growth of deployment of optical interconnects, which have been touted for many years as the next technological breakthrough. Many believe that there will come a time when various electrical interconnects are replaced by those operating in the optical domain because data rates will become too high for copper to handle effectively. It may be that the optical interconnects within an integrated circuit will have to wait for ‘silicon photonics’ to yield an efficient, more mature light source that is readily implemented on silicon.
However, for optical interconnects between ICs and among line cards, nothing can surpass the VCSEL as the premier light source. It wins thanks to its small size, its high efficiency that translates into low power consumption, its low manufacturing cost, and because it is readily made into one- or two- dimensional arrays flip-chip mountable on silicon, IC carriers, and PCBs alike. Now that the VCSEL has finally approached the critical data rate for optics to dislodge copper, the ‘flood gate’ is opening.
To our delight, even before we can put the finishing touches to our new device, many researchers have been getting in touch and asking us if they can try this VCSEL in their new interconnect transceivers. We have fulfilled some of these wishes, and our VCSELs, plus corresponding photodetectors, have been grouped together to provide data transmission down novel multicore fibres (see Figure 6) and used to build a four-channel receiver (see Figure 7).
Have we now reached the limit of what is possible? Or can data rates go even higher, to 35 Gbit/s, 40 Gbit/s, or even 50 Gbit/s? In our view they can, but it will be about revolution, rather than evolution. It will require some help from the driver, such as pre- emphasis, and the introduction of new modulation schemes with higher bandwidth efficiency.
Implementing these technologies may be tricky, but there doesn’t seem to be an insurmountable barrier to ultra-fast optical interconnects. That’s good news, because their electrical equivalents are clearly running out of steam.
The authors would like to thank Fuad Doany, Dan Kuchta, Ben Lee, and Clint Schow from IBM Thomas J. Watson Research Center for their collaboration and feedback. Our gratitude also goes to Chun Lei from Emcore Corp. for her instrumental contributions to our VCSEL program in the past few years. Many thanks go to the excellent team of the Albuquerque VCSEL FAB who made everything happen.
© 2012 Angel Business Communications. Permission required.
Fig.6.(a) Transceiver schematic and (b) selected images showing the VCSEL and photodiode arrays viewed through the silicon carrier’s optical vias from the underside of the package
Fig.7.A 4 x 25 Gbit/s transceiver made with precursory version of 25 Gbit/s VCSELs (see
www.fujitsu.com/global/news/pr/archives/month/ 2012/
20120531-01.html for more details)
Further reading N Li et al. 2012 Proc. SPIE 8276 827603 A V Rylyakov et al. 2012 OFC 2012, Los Angeles, CA OTh1E.1 B G Lee et al. 2012 J. Lightwave Tech. 30 886 L A Coldren et al. Diode Lasers and Photonic Integrated Circuits, Chapter 5, John Wiley & Sons 1995
October 2012
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