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Ask the expert: silicon photonics

01 Feb 2008

Our new "Ask the expert" feature aims to provide readers with an insight into an emerging field of optics. First up, Intel's Mario Paniccia talks about the latest advances in silicon photonics.

As director of the Photonic Technology Lab at Intel Corporation, Mario Paniccia can offer an unrivalled perspective on recent research into silicon-based photonic devices. His team's pioneering activities led to the first silicon modulator with a bandwidth greater than 1 GHz in 2004 and the first continuous-wave silicon laser in 2005. Paniccia was recognized as one of the top 50 researchers working in silicon photonics by Scientific American in 2004.

Can you summarize how silicon lasers emit light and what has been achieved to date?
Silicon is an indirect band-gap semiconductor with very low light emission efficiency. This means that it is not possible to make a silicon laser in the same way as direct band-gap semiconductor lasers that are based on materials such as InP and GaAs. There have been many attempts to create a silicon laser by altering its band-gap structure. To date, just two types of silicon laser have been demonstrated successfully – a silicon Raman laser and a hybrid silicon laser.

A silicon Raman laser is made of pure crystalline silicon with no modifications and is optically pumped in the same way as the first ruby laser. The laser uses stimulated Raman scattering to generate optical gain in silicon.

The hybrid silicon laser uses InP for light generation and gain. Light is generated through population inversion within the material when it is electrically pumped. The laser cavity is defined by the silicon waveguide, and the wavelength is determined by the gratings etched into the surface of the waveguide.

The laser works when light generated in the InP is evanescently coupled into the silicon waveguide and then amplified as it bounces back and forth in the cavity. Bonding the two materials together avoids problems associated with lattice mismatch, and also allows for many lasers to be developed with one single bond.

Why is it important to pursue the development of this technology?
Silicon is the material of choice for electronics. For the last 40 years, Moore's law and the ability to integrate more and more transistors on silicon has been driving integrated circuits (ICs) into the mass market. If we look at optical technology today, it is still based on discrete components that are hand assembled with very little integration, and is therefore very expensive.

The goal of silicon photonics is to build optical devices from the same processing technologies used in the IC industry today. Taking advantage of integration and scaling to produce low-cost, high-volume optical chips will help drive optical technology into many new areas.

To date, many of the building blocks needed for integrated photonics chips have been demonstrated including high-speed modulators, SiGe-based photodetectors, splitters and couplers. The silicon laser is considered the holy grail in silicon photonics. It is the last remaining technology needed in order to realize integrated photonics chips.

What are the main applications of silicon lasers, and when do you expect them to occur?
The silicon Raman laser is useful for biomedical and sensing applications. This technology can generate wavelengths that are not easily accessible with conventional III-V lasers, particularly in the mid- and far-infrared regions (2–4  µm and 5–10 µm respectively). The hybrid silicon laser is valuable for interconnects and communications since it is electrically pumped. What's more, many lasers of different colours and wavelengths can be created on a single chip. These technologies are currently in the research phase, but one hopes to see them commercialized in the next 5–7 years.

What is the most important recent advance in your field?
There have been many recent breakthroughs. Many of the silicon photonic building blocks have been demonstrated to operate at 40 Gbit/s, including a high-speed silicon modulator and SiGe-based photodetector. Wavelength conversion and amplification in silicon have also been demonstrated at 40 Gbit/s. Finally, a hybrid silicon laser was demonstrated in 2006. We now have many of the key building blocks needed for building integrated photonics chips.

What are the key challenges left to overcome in your field?
The next major challenge will be integrating these individual building blocks. The effort here should not be underestimated as device integration will require process changes. But the value and benefit of integrated silicon photonics creates new form factors and functionality that cannot be developed with discrete components. All of our focus will be on making integrated photonics chips with a silicon laser a reality.

What do you think the next big breakthrough will be?
I can imagine the demonstration of 500 Gbit/s or 1 Tbit/s optical chips in the next 3–5 years. Other than this, I am not sure that there are many breakthroughs left apart from demonstrating integration.

• This article originally appeared in the January 2008 issue of Optics & Laser Europe magazine.

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