FEATURE OPTICAL COMPONENTS


Figure 1: Sales of integrated optics are expected to reach $1 billion by 2021


$6,000


Discrete Integrated


$4,000 Another promise of silicon photonics – although


still to be demonstrated – is the combination of optics and electronics in one package. Such next-generation 3D packaging, if successful, could change things more dramatically than LightCounting currently anticipates, according to Kozlov. STMicroelectronics is using 3D packaging to


$2,000 $ 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021


passively, we can’t truly break out of the cost structure of today’s optics.’ However, he is confident that passive alignment is coming. Wafer-scale testing remains another challenge.


Grating couplers are one way that designs can be tested while still on the silicon wafer. Tese allow the whole circuit to be tested, but not individual components: either it works or it doesn’t. ‘If you are going to mimic the successes of electronics, you need to test more comprehensibly than that,’ said Reed. His group has developed an erasable grating that


can be placed either side of an individual component to test it. Tese gratings can then be removed from the final circuit using laser annealing. Another issue is the emergence of silicon


photonics foundries. Chip volumes in silicon photonics manufacturing are several orders of magnitude lower than those in electronics. ‘Tis keeps the top foundries from diving in and, until that happens, you won’t see the cost reductions and volume increases people are hoping for,’ said Doerr.


Opportunities Te industry’s resolve to overcome these challenges will only benefit from the growing market opportunities for silicon photonics. One market driver is the growing demand of the


large-scale data centre players, such as Microsoſt, for higher-speed interfaces. ‘Teir appetite increases as the industry is making progress,’ said Kozlov. ‘Six months ago they were happy with 100 gigabit; now they are really focused on 400 gigabit.’ Going to 400Gb/s interfaces will require


four-level pulse-amplitude modulation (PAM-4) transmitters, sparking new competition between indium phosphide, VCSELs and silicon photonics, he says. Silicon photonics may even have an edge


according to results from Cisco, which show that its silicon photonics-based modulators work well with PAM-4 designs. Silicon photonics could even take the market lead here, says Kozlov, for 400-gigabit designs that require multiple PAM-4 transmitters on a single chip.


16 FIBRE SYSTEMS Issue 12 • Summer 2016


create its 4x25Gb/s PSM4 transceiver design using two chips. Te optical engine, implemented in silicon photonics, integrates four modulators, four detectors and the grating couplers used to interface to the fibre. Te second device, a 55nm BiCMOS chip, houses the transceiver’s associated electronics. ‘Te 3D packaging consists of two dies, one


copper-pillar bonded to the other,’ said Benetti. ‘It is a dramatic simplification of the mounting process of an optical module.’ However, Andrew Rickman, CEO and founder


of UK start-up Rockley Photonics, believes silicon photonics cannot achieve its full potential by going aſter the market for 100G and 400G interconnects for the data centre. Instead, Rickman believes silicon photonics should be viewed as one element – albeit an important one – in an array of technologies used to tackle system-level issues. ‘If you take control of enough of the system


problem, and you are not dictated to in terms of what Multi-source agreement [MSA] or what standard that component must fit into, and you are not competing in this brutal transceiver market, then that is when you can optimise the utilisation of silicon photonics,’ said Rickman. ‘And that is what we are doing.’


@fibresystemsmag | www.fibre-systems.com


Rockley has yet to launch its product, but has said


it is designing a silicon photonics switch chip along with a controller ASIC and interconnect protocol to tackle a core problem in mega data centres – how to connect more and more servers in a cost-effective and scalable way. Te start-up is a fabless chip company and will


not be building systems. Instead, Rockley will supply its chipset-based reference design, its architecture IP and soſtware stack to its customers


Another promise of silicon photonics is the combination of optics and electronics in one package


to create custom line cards and fabric cards. Te resulting system is designed as a drop-in replacement for the large-scale data centre players’ switches they already deploy, yet will be cheaper, more compact and consume less power, according to Rickman. Acacia can be viewed as another player taking


control of the coherent transmission system in that it develops its own low-power coherent digital signal processor ASIC as well as the coherent transceiver PIC. Acacia may not be competing in the fiercely competitive intra-data centre market, but it is up against optical module players developing


Ultrafast photomodulation spectroscopy, developed by researchers at the University of Southampton, could help engineers to test silicon photonic chips more easily


Sales ($ millions)


University of Southampton


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