By Steve Bush 14th August 2019
Rockley Electronics is a silicon photonics start-up that recently raised $52m in a funding round, bringing its total to $165m.
With all this investor interest, it was time to find out exactly what the company is offering, so Electronics Weekly spoke to its founder and CEO Andrew Rickman – and industry veteran with some history in photonics.
In 1988, Rickman founded pioneering silicon photonics firm Bookham Technology – which still exists and is now called ‘Oclaro’ – and he was also chairman of Kotura – yet again a silicon photonics company, that is now part of Mellanox.
Having almost the entire chip industry built around it, and being transparent to a range of infra-red wavelengths, makes silicon an attractive photonic substrate – clad it in silicon dioxide, for example, and it can form waveguides.
However, making such structures efficient, work over temperature and tolerate process variation is tough – and III-V sub-components have to be added to a silicon device if light emission is required, and some way has to be found to attach optical fibres, and…. … it all gets very complicated.
What is unique about Rockley, Rickman told Electronics Weekly, is that it has a complete process flow to integrate all the components – silicon and III-V – necessary to create silicon photonic devices at the wafer level.
“You have got to solve all process problems. You can’t just connect things together with wires. You have to solve all of the precision alignment problems on a wafer scale, otherwise it is only a partial solution – Bookham and Kotura weren’t complete solutions,” he said. “We have complete wafer-scale flow that solves all problems – how to attach fibre, how to make 9μm fibre core match smaller waveguides on-chip and how to make it insensitive to temperature.”
Founded in 2013, the company is fabless and its activities are based around its own custom process flow that, Rickman is at pains to point out, was designed for photonics on a clean sheet of paper. “It has no MEMS heritage and no CMOS heritage, but we leverage knowledge of both,” he said.
Did Rockley have to licence any intellectual property to complete its process? “No licencing has been needed,” said Rickman.
However specialised, the process runs on conventional semiconductor manufacturing equipment, although that requires a unique combination of equipment that is not typically found under one roof: “We had to persuade our foundries to have a full set of kit,” said Rickman, who is keeping quiet about their identities. He would only say: “We have development foundries and production foundries.”
As well as process design, Rockley in-house expertise includes electro-optic silicon die design, optical silicon design, III-V electro-optical die design, and both standard CMOS and bi-CMOS die design for control chips – for modulation or tuning, for example.
While III-V die tend to be mounted on the silicon die, to combine silicon photonics with CMOS control chips the firm has 2.5D and 3D packaging options.
To keep heat down – most of the heat comes from the control chip rather than the electro-optical chip, according to Rickman – the latest CMOS nodes have to be exploited, so the firm’s designers can work with what he describes as leading-edge analogue CMOS.
As such, the output voltage swing of the CMOS control chips is limited, so the electro-optical components have been designed to operate with low control voltages – a volt or so.
“For our devices, process and design are very closely related,” he said, “We have built up an intellectual property portfolio that includes process, design and applications.”
Although silicon cannot generate light and has poor electro-optic characteristics, it can still be used in many applications.
For example, detectors for telecoms or datacoms can be created if adding germanium is an option, said Rickman.
Silicon modulators are also possible. “Silicon is inefficient, so the structure has to be be larger, or it will be temperature sensitive. III-V modulators are much more compact, and far faster for modulating either amplitude or phase. We have both Si and III-V modulators in the portfolio,” he said, adding that a III-V modulator need only be tens of microns across, while a laser might only be 250μm square – and can be flip-chipped onto the silicon below.
Multiplexers and de-multiplexers are possible in silicon, as are couplers to match an on-die waveguide to an optical fibre inserted from the outside.
For waveguides, silicon is the preferred answer, and the firm advocates wide waveguides – perhaps 3μm across, for example, as these can be tolerant to process variation. It has a single-mode design for which the firm claims: automatic suppression of higher-order modes, high light confinement, low propagation loss and polarisation independence – all over microns of wavelength bandwidth.
It’s a bigger waveguide, said Rickman, but circuits can be made small because the waveguides tolerate tight bends – he cited an example of 44 180° bends in a row with a loss of <0.02dB.
Silicon can be used at wavelengths between 1.2 and 3μm with the Rockley process – outside which the SiO2 waveguide cladding starts to become absorbing. That said, the firm has techniques to widen the bandwidth outside these wavelengths somewhat.
Performance-wise, Rickman points to the 0.8nm wavelength spacing of 100GHz DWDM signals. “Phase errors need to be low. For a dense wavelength division de-multiplexing filter, we have variation of <0.1nm.”
Where there is a choice of optical processing in the silicon substrate, or adding a III-V die to the substrate, Rickman sums up thus: “III-V is expensive to process, but tiny in area and does a great job.”
Using silicon as a dumb material, the firm has developed a micro-machined groove for aligning in-coming optical fibres directly with an on-chip adiabatic taper interface to a waveguide.
The design aim was to cut the cost of fibre insertion. According to Rickman, equipment of similar accuracy (~1μm) to that needed to align fibres in optical fibre connectors is all that is needed. “If you had to use a lens, the tolerances would not allow low-cost assembly,” he said. Below 1dB insertion loss is claimed.
Another light-coupling option in the tool kit, for adjacent chips on a PCB, is edge-to-edge transmission using free-space optics (left).
As an example of what the firm can do, in 2018 Rockley announced a concept which moved optical transceivers from the front of an equipment rack to the inside.
In a single package, it surrounded a network switch asic (bumped onto the package substrate) on four sides with optical fibre transceivers.
Each transceiver included a multi-fibre optical ribbon cable ‘pig-tail’ for its light connections, and dropped into its own miniature electrical socket.
Laser sources were separate – “not one per asic, not one per transceiver, but in between”, said Rickman.
Which transceivers? “Our transceiver focused on 100G and 400G,” he said.
Rockley’s only announced customer is Hengtong Optic-Electric, to which it sells chip-sets for 400G DR4 transceivers – Hengtong is also an investor in Rockley.
“Hengtong and Rockley are getting through qualification now with customers, and orders could be placed by the end of the year,” said Rickman, adding that other customers exist, some in the sensing field, but have yet to be announced.
Another application of interest is lidar for range sensing and object analysis in cars robots and drones.
For this, Rockley is proposing a 1.55μm coherent system where the outgoing beam acts as a local oscillator for detecting incoming reflected signals – using FMCW (frequency-modulated continuous-wave), for example, which potentially offers a lower power solution that today’s amplitude-modulated time-of-flight systems, said Rickman. “You are actually building giant interferometer,” he added. This would need a “significant number” of narrow line-width lasers, modulators and mixers.
“The silicon platform also has the potential of integrating solid state beam steering and receiving solutions, such as an optical phase array,” according to Rockley.
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