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Technical Insight

Hybridization brings optical components together

As the market continues to demand lower cost optical communications, component makers are turning to integration. Streamlined bonding and alignment will bridge the gap between discrete and integrated components write Robert Green and Stuart Morgan of Bookham Technology.
Current market conditions demand low-cost network solutions for today s optical communication systems. The consumer wants competitive pricing from the service provider, which then pushes network operators, infrastructure designers and optical component vendors to reduce costs.

To meet these costs targets, the integration of active and passive optical components onto a single chip is key. Integrated components are inherently easier to mass-produce and therefore cheaper than discrete components that have to be installed and then calibrated separately by the infrastructure provider.

Functionality is also important. Network designers need multifunctional solutions from component vendors and favor components that can perform more functions at a lower cost. An integrated package of components means that the network designer not only gets interoperability, but also gets more functionality from fewer vendors. This also leads the way to long-standing and large order-volume customer-vendor relationships.

Network operators have to respond quickly and cheaply to today s expanding networks and changing consumer needs. This means that as well as being cheap and interoperable, the network infrastructure has to be reconfigurable and more flexible to the end-user s needs.

The never-ending demand for increased bandwidth, as well as the cost and space savings that can be made through miniaturization, are the final factors that firmly place integrated components at the top of a network designer s wish list. So what exactly is integration and how do component makers achieve truly integrated optical components?

Actives and passives

Integration involves two categories of optical components: active and passive. Each part of an optical network employs different types of active and passive components to create and condition light, and channel this optical data to its destination.

Active optical components such as laser diodes and fiber amplifiers generate and amplify light. Many of these devices are based on compound semiconductor technologies such as InP and GaAs.

Passive devices, on the other hand, do not generate light. They generally route signals based on wavelength, intensity or polarization. Arrayed-waveguide gratings and power splitters are examples of components with passive functions.

Monolithic vs hybrid

There are two approaches to integrating actives and passives onto a single chip, which will give either a monolithic or hybrid device. Monolithic devices are fabricated from one material and contain all the essential functions within the substrate and the epilayers. In contrast, a hybrid device contains different materials and technologies that have been combined without the need for optical fiber interconnects.

A truly monolithic device can lead to huge cost savings during manufacturing and assembly, but in practice the properties and strengths of different materials generally dictate a hybrid approach. Using hybridization opens up the use of different materials and technologies optimized for different applications, which is why Bookham Technology has chosen this integration route.

Choosing a platform

To integrate high-performance components via hybridization, a device maker needs a platform to accommodate the actives and passives that are typically based on different materials and manufactured via different processes. While in the past many companies have favored silica, today s material of choice for the platform is silicon.

The immediate benefit of adopting silicon as an optical integration platform for hybridization is the wealth of knowledge accumulated by the semiconductor industry over the last 40 years. Techniques such as flip-chip bonding can easily be modified to suit optical component manufacturing.

Mature fabrication techniques also mean that silicon has a relatively low customization cost compared with other substrates, and gives a high integration yield. Optical integrated device makers can exploit these benefits to help make devices at volume production levels. Silicon also has a relatively high thermal conductivity, which means that heat from active components such as laser diodes is easily dissipated.

Like competing InP and GaAs substrates, silicon offers an intrinsic way of implementing passive optical functions. For example, device makers can easily etch waveguides into the substrate using established photolithography techniques. Cladding layers can be deposited and interconnections added to the silicon substrate using standard epitaxy and lithography techniques. Silicon waveguides can also act as other passive components such as multiplexers.

As well as providing a building block for passive devices, silicon also provides an optical substrate that is easily formed into a hybrid component receptacle for active devices fabricated from materials such as GaAs and InP. Bookham has all three technology capabilities: silicon, GaAs and InP.

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