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MEMS light valves manage wavelengths for D-WDM

Posted: 01 Nov 2002 ?? ?Print Version ?Bookmark and Share

Keywords:grating light valve? glv? dwdm? optical network? mems?

The recent business downturn has put significant pressure on makers of optical gear to reduce costs while simultaneously improving performance. Fiber-optic networks are also becoming more complex and dynamic with higher data rates, increased spectral density, and changing bandwidth demands. This has lead to the development of numerous dynamic and tunable components that directly address these issues.

A significant technology trend for tunable optical components is MEMS. The reason is simple: MEMS technology enables the development of optical devices that are configurable in-situ, as opposed to the static optical devices historically used in optical networks. Additionally, MEMS share many desirable traits with semiconductors: they are small, fast, low-cost, and reliable.

One type of optical MEMS, known as the grating light valve (GLV), is going into next-generation dynamic optical modules for use in optical networks. The GLV spatial light modulator is an addressable, dynamic diffraction grating array, fabricated using a standard CMOS process flow from the integrated-circuit industry.

The fundamental advantages of the GLV device are its precise analog attenuation, spatially seamless nature, high reliability, and the ability to integrate up to several thousands of diffracting elements into a single device.

The GLV MEMS device has been used commercially for several years in high-performance imaging applications. When used for imaging, the GLV device acts as a spatial light modular, or a one-dimensional array of imaging pixels. GLV devices are routinely manufactured with over one thousand pixels.

To create useful applications for optical networks, the GLV device is built in the form of long one-dimensional arrays of diffraction elements. A DWDM signal is spatially dispersed so that the individual channels are separated and then projected onto the surface of the GLV array. Since different DWDM channels land at different points on the GLV device, the signal can be adjusted on a tunable basis across the spectrum. This leads to many useful dynamic applications for optical networks.

One such application is dynamic gain equalization, in which the GLV device acts as a one-dimensional array of variable optical attenuators to improve the performance of optical amplifiers by equalizing D-WDM channels in long-haul networks. Long-haul and ultra long-haul optical networks are characterized by a cascaded series of optical amplifiers.

The most commonly used optical amplifiers are erbium-doped fiber amplifiers (EDFAs). The implementation of EDFAs has revolutionized fiber optics, as they enable WDM data transport over thousands of kilometers. However, EDFAs do not inherently have a flat gain spectrum - causing optical channels to experience uneven gain as they traverse the cascade of amplifiers. This undesired power imbalance poses serious problems for telecoms carriers - limiting the distance that optical networks can transmit data, increasing cost, and decreasing performance.

To solve the power balance problem inherent to optical amplifiers, network designers install gain equalization elements into the network. Since most long-haul EDFAs are actually multi-stage amplifiers, the gain equalization element is usually inserted into the mid-stage of multi-stage amplifiers.

In the past, gain equalization was performed with static gain flattening filters (GFF) that were based on thin film interference filter technology. While GFFs do perform the function of flattening the spectrum, their correction function is fixed to a pre-determined shape. Static GFFs were effective for static network, but as service providers started to demand flexible networks, the static nature of GFFs has become a significant disadvantage.

Dynamic gain equalization enables network designers to change the gain correction function to adapt to changes in the network. Furthermore, the high-accuracy and high-resolution of the GLV device equalizes the D-WDM spectrum with the lowest possible residual ripple, while the spectrally "seamless" nature can support any channel spacing.

Network designers are increasingly looking to MEMS-based dynamic gain equalizers (DGE) to increase the performance and cost-effectiveness of their fiber-optic systems. In terms of performance, DGEs provide for longer transmission distances with a superior optical SNR.

Using the GLV MEMS device to dynamically adjust the D-WDM spectrum is a powerful concept, but gain equalization is only the first of several possible applications. By adapting the same basic functionality, diffractive MEMS can be the core for other useful dynamic components, including dynamic channel equalization (DCE), reconfigurable channel filtering/blocking, or wavelength-selectable switching.

As system providers and sub-system OEMs develop the next-generation of fiber-optic communications systems, they will look to dynamic optoelectronic components. The GLV technology provides a flexible basis for optical components that enable dynamic operation of these future networks, and surpasses other light-modulation technologies in terms of speed, accuracy, reliability, and manufacturability - making it an optimal solution for helping system providers address evolving environments and customer demands.

Figure 3: Because different D-WDM channels land at different points on the GLV device, the signal can be adjusted on a tunable basis across the spectrum.

Figure 4: Using dynamic gain equalization, the gain correction function can be adapted to changes in the network.

- Robert J. Monteverde

Product Marketing Manager

Silicon Light Machines

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