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Planar integration of optical components

Posted: 16 May 2002 ?? ?Print Version ?Bookmark and Share

Keywords:optoelectronics? optocoupler? dwdm? vcsel? oled?

Planar integration is the concept that optical circuits can be constructed out of waveguides fabricated on silicon wafers, using tools and techniques already developed by the semiconductor industry.

Equipment vendors demand that the components they use stretch the boundaries of achievable performance. For many applications, PLC-based devices match or exceed the performance of discrete components. PLCs must deliver on the promise of lower cost and size through increased integration. The first generation of highly integrated products, for example, 40 channel VOA multiplexers, are now on the market. A 40-channel VOA multiplexer replaces 40 separate discrete VOAs and 39 filters and their associated piece parts, clearly demonstrating the benefits of integration.

Arrayed Waveguide Grating (AWG) perform MUX and DMUX functions in DWDM systems in parallel on one chip. Over the last two years, AWGs have gained a significant percentage of the total MUX/DMUX market. Because a single AWG chip in a single package can multiplex or demultiplex 40 channels in parallel, the production costs for high channel count devices are much lower than for the established competing technologies. To implement the equivalent of an AWG in thin film filters (TFF) or fiber Bragg gratings (FBG) requires 39 designs, 39 separate production runs and 39 separate packaging operations. Because AWGs process all channels in parallel, they have always had a significant advantage in insertion loss for high channel count applications. Essentially, the insertion loss of an AWG is not much different whether there are 4, 40 or 80 channels. Discrete devices, however, must be used in series, and the losses accumulate. The insertion loss of a typical AWG has now been further reduced.

Another key performance parameter is isolation. Isolation is a measure of how much light from an unwanted channel leaks into the passband of an active channel. For discrete filters, such as TFFs and FBGs, isolation is determined by nearest neighbor crosstalk; in other words, the amount of light that leaks into one channel from its immediate neighbors. For integrated devices such as AWGs, however, there is another contribution to total isolation. Instead of continuing to drop away as in a TFF, the transmission curve of an AWG levels off in what is called the "noise floor." This noise floor is due to the fact that the AWG works in parallel, with all of the channels in one chip. Light scattered or lost due to imperfections in one channel might scatter into any other channel. Eighteen months ago, the noise floor for a good AWG was around 35dB, while adjacent channel isolation for both AWGs and TFFs was around 25dB. When the contribution from the noise floor from the other 37 channels was summed together, the total cumulative isolation of a typical AWG was only 19dB. Through application of strict process control and focused improvement programs, top AWG vendors now readily achieve adjacent channel isolations of 30dB, and the noise floor has been lowered to >50dB.

Integration promise

The promise of planar integration has always been much greater than just MUX/DMUX. Next generation networks require active components that allow automatic, dynamic adaptation by system. The first generation of dynamic PLC products is now on the market. Many companies, including NEL and Lightwave Microsystems, are now shipping 40 channel VOA multiplexers based on silica thermo-optic (TO) VOAs and AWGs. These integrated devices are attractive not only through reduced production costs, reducing 79 separate packages to 1, but also through reduced assembly costs.

Other active functions can also be performed by TO elements. Several implementations of PLC-based Dynamic Gain Equalizers (DGE) are now being introduced, where the filter function is actively controlled by TO elements. NEL offers an 8x8 TO cross connect and an array of 8 2x2 TO switches. Several companies are developing integrated versions of reconfigurable optical add/drop multiplexers (ROADM) based on PLC TO switches and AWGs.

Design divergence

It is at this stage that the development of planar integrated optics significantly diverges from the analogy of semiconductor IC integration. In simplified form, ICs get their power from the ability to integrate millions of transistor gates, which basically all perform the same function in the same material. In optical networks, many different functions must be performed and these functions require different optimized materials. Therefore, planar integration of optics will resemble microwave hybrids more closely, where many smaller scale integrated components are assembled on a common platform. A strong candidate for that integration platform in optics is silica on Silicon PLCs. This will result in a PLC platform, which will provide both active and passive optical functions in close proximity with electronic control. It will be neither strictly monolithic nor hybrid but a combination. There has been great progress in silica on Silicon planar integration products. First generation PLCs, such as AWGs, now equal or exceed the performance of discrete products, at a fraction of the cost.

Integrated products, such as VOA multiplexers, are gaining market acceptance due to their performance and cost advantages. At the same time, PLC-based hybrid transceivers, which combine waveguide chips with active lasers and photodetectors, have been developed by several companies. The combination of these two ends of the PLC spectrum holds great promise for future multi-function highly integrated products.

? G. Ferris Lipscomb

Marketing Vice President

Marc Stiller

Product Marketing Director

Lightwave Microsystems

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