Electronic Design

Highly Accurate Dynamic Gain Equalizer Controls Optical Power Precisely

Based on Grating Light-Valve technology, this versatile MEMS product provides speed, accuracy, and reliability advantages to communication system designers.

An optical dynamic-gain equalizer (DGE) for dense wave-division multiplexing (DWDM) communication systems sets unprecedented performance levels for precision, dynamic range, and reliability. This product, essentially a seamless one-dimensional array of variable optical attenuators (VOAs), is the first such device to employ diffractive microelectromechanical system (MEMS) technology with ultra high-spectral resolution. Its maker, Silicon Light Machines, a subsidiary of Cypress Semiconductor Corp., uses a patented MEMS Grating Light-Valve (GLV) technology developed at Stanford University in California, and now optimized for telecommunications ap-plications.

Essentially, a GLV is a diffraction-type spatial light modulator that's fabricated in a standard CMOS fabrication facility. The model 2200 DGE delivers power accuracy, or residual ripple of ±0.1 dB, with a minimum attenuation step of 1.0 ms. This allows system designers to very accurately attenuate optical power levels and squeeze out any ripple.

Conventional DGEs use a variety of technologies for tight power-level control. Acousto-optic, liquid-crystal, Faraday rotator filter, waveguide, and thermo-optic methods deliver roughly ±0.3 to ±0.5 dB of optical residual ripple. In DWDM systems, subscribers are added or dropped on demand via add/drop multiplexers. Subsequently, power spectrum shapes can change. So, there's a crucial need to save every bit of power gain to flatten out the power signal as much as possible. Even a fraction of a decibel can make a big difference.

Traditionally, gain flattening was done with static devices. Every additional communication channel that's used affects the shape of the power spectrum. These issues are magnified in dynamically changing DWDM systems.

This is where the model 2200 shines. Designed for C-band operation in the 1550-nm spectral region, it independently attenuates optical power over multiple spectral regions, using a single dynamic module instead of a static approach. Future versions are being considered for operation in the L and S bands.

The 2200 is really a subsystem made up of the drive electronics on a board, an optics module that contains the GLV device, and an optical circulator (Fig. 1). This subsystem fits at the mid-stage of a user's gain block--all monitored optically by an external device. Such a setup provides a low-cost-per-channel solution for high-channel-count systems. The DGE's spectral seamlessness means that it's agnostic to channel count.

Aimed at optical system suppliers like Ciena, Lucent Technologies, Nortel, and Alcatel for long-haul and ultra-long-haul communication systems, the 2200 delivers a dynamic range of more than 15 dB. Other impressive specifications of the model 2200 include spectral power ripple within ±0.1 dB, a correction slope of more than 4 dB/nm, less than 6 dB of excess insertion loss, less than 0.25 dB of polarization-dependent loss, and -40 dB of return loss. Equalization time is under 1.0 ms (Fig. 2).

The subsystem dissipates less than 5 W of power and complies with Telecordia's GR-1221 specifications (GR-63 for the control electronics). Moreover, it's designed to operate over a temperature range of -5°C to 70°C.

Moving Ribbons
Key to the model 2200's operation is an array of parallel aluminum-covered silicon-nitride (SiN2) microribbons suspended above an air gap (Fig. 3a). The ribbons are built in a sacrificial layer and configured as alternate static and dynamic elements spaced less than 0.5 µm apart. Each ribbon is 200 to 300 nm thick, about 500 µm long, and about 10 µm wide.

Located above a ground plane, the ribbons form an air gap. These films provide the spring-like restoration force that counterbalances the electrostatic bias voltage applied to the dynamic ribbons. Multiple GLV elements are ganged together to create a linear array of 2, 10, 100, or more than 1000 elements to form the complete GLV device.

When a bias voltage of about 10 to 20 V is applied to the dynamic ribbons, they're deflected electrostatically. Meanwhile, the static ribbons hold taut and aren't deflected. The bias voltage is applied via drive electronics, and the user communicates through a dual-port RAM control interface.

In the normal state, when no bias is applied, all the mirrors are undeflected, and incoming incident light reflects off the aluminum surfaces. This is known as the specular state.

After the bias voltage is applied to the dynamic ribbons, incident light is diffracted in direct proportion to the amount of applied voltage, and thus the amount of ribbon deflection. This is called the diffraction state.

This analog control of the ribbons' positions enables fine position control and more than 40 dB of contrast levels. Ribbon spacing design is such that 1/4 wavelength (1/4 λ) is obtained upon deflection for maximum specular reflection (Fig. 3b).

The possibility of building large arrays of inline GLV elements provides a seamless design that can be used to actuate and control light signals in a DWDM system with no "blind spots." Because the ribbons are low-mass elements that are deflected very small distances, submicrosecond speeds are possible. Hundreds and thousands of these GLV arrays can be produced on standard semiconductor wafers (Fig. 4). This means they can be mass produced at low cost.

Conventional ICs need not worry about surfaces sticking together, as there are simply no moving parts. But in MEMS ICs, stiction (the sticking together of a moving surface with another moving or stationary surface) is a major problem that affects the reliability of MEMS ICs.

Although he won't disclose the details, Bob Monteverde, product marketing manager of Silicon Light Machines, says, "We have developed a proprietary step in our CMOS manufacturing process that minimizes stiction problems. This is key in the final step of releasing the MEMS ribbons above the sacrificial layer. Fatigue and wear problems also are minimized." The company is so sure about the reliability of the GLV elements that it reports individual units have undergone testing of more than 6 × 1012 switching cycles with no failure mechanisms.

Other Applications
This proprietary GLV technology has already been demonstrated for high-resolution displays and computer-to-plate print applications. Other potential applications include high-speed, small 1-by-2 optical switches (to replace today's mechanical devices) and modulators of low-bandwidth signals (typically 100 kHz to 1 MHz) on top of a DWDM signal.

GLV technology is presently in use in high-resolution display and imaging systems where its high efficiency, wide dynamic range, precise analog attenuation, rapid switching speeds, and high reliability attributes are crucial. Sony Corp., a licensee of the technology, is presently using it in high-definition TV (HDTV) and e-cinema applications. And, Evans & Sutherland uses GLV technology for high-end flight-simulator applications.

Price & Availability
The 2200 DGE subsystem, including the control electronics, optics module, and optical circulator, costs roughly $10,000, depending on volume and performance requirements. First samples are available now.

Silicon Light Machines, 385 Moffet Park Dr., Sunnyvale, CA 94089; Bob Monteverde, (408) 541-4996; www.siliconlight.com.

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