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​Examples of silicon photonics chips made by VTT, which include various wavelength multiplexers and long waveguide spirals.​​

Small is big: growth and new opportunities in silicon photonics

Teemu Simola | 4.6.2015

​There is a growing need for data transfer. As a result, electronic data transfer is being replaced by optical transfer. 

At the heart of the future development of data transfer lies a tiny particle, the photon, an energy package in which electromagnetic radiation travels. Combine a photon with silicon and you get silicon photonics, which offers completely new possibilities. Typical applications include various transmitter and receiver modules, routers and sensor and measurement technologies. 

– Electronic microcircuits form the technological basis of modern society and are the ‘brains’ of many of the electrical devices we use. They are made up of active and passive electronic components printed on semiconductor chips, says Timo Aalto, the winner of the VTT Award 2014 and head of the related research team at VTT. 

A modern processor can include billions of transistors. However, silicon photonics involves the development of optical microcircuits in which light is transferred through waveguides etched into the silicon. In addition to these, an optical microcircuit has active and passive photonic components which are used for the generation, modulation and reception of light. 

Whereas visible light cannot penetrate silicon, silicon photonics is based on the use of infra red light, with a typical wavelength somewhere between 1.2 and 4 micrometres (µm). In optical data transfer, the 1.3 and 1.55 micrometre wavelength ranges are the key bands, says Aalto.

Until recently, VTT manufactured only passive and thermo-optical circuits for silicon chips. All so-called active components, for example lasers, optical amplifiers, fast modulators and light detectors were separate chips produced elsewhere and connected to the silicon chips.

– We developed world-class technology for this purpose as well, but still needed so-called monolithic integration directly onto the chip to enable cost-effective mass production. This is something that we are now developing in collaboration with our customers and partners. The most important of these is Rockley Photonics, with whom we began cooperating in 2014, says Aalto.

 

Rockley aiming for mass production

The British Rockley Photonics has expanded to Finland and will begin the production of data transfer products in Espoo in collaboration with VTT. The company’s head office is located close to London and has a product development unit with around 20 staff in Pasadena, California. VTT’s expertise was one of the main factors that drew the company to Finland. Other features of Finland’s technology sector, such as its firms and university activities, provide a strong basis for the related business activities.  

Now that we have completed the initial phase of collaboration with VTT, it is time to move onto the second phase and the creation of actual products in Micronova’s clean room. We’ll enter the market with a pilot product this year and aim to move onto mass production next year, says Markku Hirvonen, a member of Rockley Photonics’ Board of Directors.

The currently large number of data centres is set to continue growing. Rockley’s forthcoming solutions are intended to boost internal transfers of the huge data masses lying in these centres. 

The idea is to produce silicon photonic products in Finland with the world’s highest performance but lowest power consumption. This is a unique, exciting project, since we are the only ones in the world engaging in this kind of product design, Hirvonen adds. 

The first staff have already been recruited for Rockley’s Finnish unit and the number of employees will grow as the project progresses. 

We will need sales and marketing experts, as well as product development specialists. This means the full range of business know-how. The time now seems to be ripe for the actual production of silicon photonics, says Hirvonen. 

Cooperation with VTT has therefore paid off and silicon photonic products offer a huge range of possibilities. Timo Aalto agrees. 

Through active component development, we aim to produce optical modulators and light detectors which have a speed of at least 25 Gb/s (25 billion bits per second). This would enable the transfer of a Full HD movie recorded on a Blu-ray disc in just a few seconds, for example. 

Silicon photonics has a vast range of possible applications. 

– Growth in optical data transfer capacity will open up a huge number of opportunities, says Timo Aalto. 

The future therefore seems bright, or at least fast.  


 

A 110-channel wavelength selector developed for the European Space Agency, InP-based 10 and 11-channel optical semiconductor amplifiers and a photodiode have been added to the silicon chip.

 

 

Faster at lower cost with greater power

The key current application is short and middle-distance optical data transfers, such as in data centres and local data transfer networks. Data transfer distances in such applications range from a few metres to around 40 kilometres. 

– Silicon photonics can be used in transmitter and receiver modules in which electronic signals are converted into optical signals and vice versa, and in the combination and separation of signals moving at various wavelengths within the same optical fibres. Silicon photonics can also be used to route data packages optically through network nodes, which is cheaper, faster and more energy-efficient than the current electrical routing, comments Aalto. ​​

Longer wavelengths can also prove useful in various measurement applications. Silicon photonics can be used to create small and highly precise microspectrometers for the identification of gases, liquids and biological samples on the basis of their typical transmission spectra. 

Silicon photonics has already been used in the production of optical gyroscopes. Future applications include silicon photonic chips that combine the low cost and small size of micromechanical gyroscopes with the huge measurement precision of the much more expensive fibre optic gyroscopes. 

– Silicon photonics can also prove useful in imaging based on infra-red light. The Spanish company Medlumics manufactures medical imaging devices based on optical coherence tomography (OCT). Such devices are intended for the early detection of conditions such as skin cancer. A silicon photonics chip developed by Medlumics and VTT lies at the heart of this kind of OCT device. The chip was made in VTT’s clean room in Micronova. A range of new applications for silicon photonics, such as optical calculation and quantum computers, are sure to be found in the future. These are at the very earliest stages of development, states Aalto.  ​


 

Illustration (left), microscope image and a scanning electron microscope image of waveguide curves and mirrors developed by VTT, with bending radius down to one micrometre, which is around a thousand times smaller than traditional curves.

From passive to active optical circuits 

VTT has been developing silicon photonics since 1997, when Timo Aalto made his M.Sc. thesis on silicon photonics as an intern at VTT. In 2004, he completed a Ph.D on the same topic. 

– The development of silicon photonics began with the design of thermo-optical switches, followed by the development of silicon waveguides and the related manufacturing technology for optical components in VTT’s clean room. The results of this internationally ground-breaking work resulted in innovations such as a thermo-optical 2 x 2 switch which switches its state in under a millionth of a second (~700 ns), explains Aalto.

This remains a world record for this kind of waveguide switch. Simultaneously, world-class technology was developed for the fabrication of passive silicon waveguide components. 

– An extremely low-loss (0.1 dB/cm) silicon waveguide with a curve radius reaching one micrometre can be used to create very densely integrated optical circuits. Waveguides even a metre in length can be etched on a silicon chip of around one square centimetre. This is not yet possible with any of the competing technologies, says Aalto. 

The technology developed by VTT includes special features such as a waveguide thickness of around three micrometres and a combination of rectangular and rib waveguides. 

– Using these, it is possible to simultaneously minimise optical confinement, polarisation dependency, and the price and chip size, while achieving so-called single-mode function with an extremely wide wavelength band. In competing technologies, this is typically restricted to a narrow wavelength band. Achieving a single-mode waveguide is necessary in most applications such as ultra fast optical data transfer and precise spectrum measurements, Aalto explains. 


 

A test chip used in the development of an optical memory, in which several test components, wavelength filters and optical semiconductor amplifiers are integrated with a silicon chip.


 

A test chip, in which light travelling through a 3 µm-thick silicon waveguide is either amplified or suppressed using a GaAs-based semiconductor amplifier.​

 

What is photonics?

Photonics is a general term for the generation, modulation and use of light, and the related components and equipment. Photon, the name for the elementary light particle, comes from the Greek word ‘fos’ which means light. 

Light consists of a flow of individual photons just as an electrical current consists of a flow of electrons. Photonics and electronics are therefore corresponding terms, although people are much more familiar with ‘electronics’. 

While the electronic microcircuits integrated into silicon chips are one of the key technological pillars of our information society, their optical equivalents are still rising to this position. On the other hand, photonics-based lasers and optical fibres have long been technologically indispensable, even if most people are relatively unaware of their existence. Without them, we would have no Internet, smartphones or HD television.

The widely known term ‘optics’ principally refers to sight and the related devices, such as lenses and microscopes. Photonics is a more descriptive term when it comes to the interaction between light and matter, the combination of light with electronics (as in optoelectronics), semiconductor components related to light, or the use of light for the fast transfer of optical data or other signals. 

Photonics exploits the wave and particle characteristics of light, which Albert Einstein was the first to combine in 1905. We now know that photons are massless fundamental particles that move at the speed of light and whose energy, momentum and wavelength are determined by their oscillation frequency. Although all electromagnetic radiation, from radio waves to gamma rays, is made of photons, ‘photonics’ is usually used to refer to only ultraviolet, visible and infrared light.

The term photonics has entered the public consciousness due to the annual international Day of Photonics (http://day-of-photonics.org). The Day of Photonics was last celebrated in October 2014, when VTT arranged public events showcasing photonics. In 2015, the UN will lead the celebration of the International Year of Light, which will showcase the uses of light-based science and the related applications. ​

www.light2015.org


Photos: VTT

 

 

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