New 1550-nm tunable lasers to cut DWDM systems cost

It has been more than two years since short-wavelength vertical-cavity surface-emitting lasers (VCSELs) made their debut as an alternative light source for high-speed premises applications. Since then, VCSEL technology has continued to make steady progress, with companies such as W.L. Gore & Associates announcing success with long-wavelength (1300-nm) VCSELs for use in large-bandwidth, long-distance applications. Gore also alluded to work on VCSELs in the 1550-nm range for future dense wavelength-division multiplexing (DWDM) applications.

The future has arrived, according to Parviz Tayebati, chief executive at CoreTek Inc., a Burlington, MA, manufacturer of tunable filters and lasers for fiber-optic communications. CoreTek recently announced the successful demonstration of not just a 1550-nm VCSEL, but one that is tunable over 50 nm using micro-electromechanical system (MEMS) technology.

MEMS technology is rapidly emerging as an enabling technology for producing all-optical switching equipment that eliminates the need for optical-to-electronic conversion in network systems. MEMS research usually involves a combination of a tiny movable mirror and an actuating device. Few such products have hit the marketplace to date, and there's an almost feverish race to see which companies can produce reliable products first.

At CoreTek, the new laser technology is expected to solve manufacturing bottlenecks in high-channel-count DWDM system development, enabling equipment for cost-effective optical networks. The company plans to enter the DWDM market with a new generation of enabling components that include the tunable VCSELs, which will join the company's tunable filters that have been available for several months.

The platform technology for both products consists of depositing the MEMS apparatus on top of product-specific material. Combining the MEMS with silicon enables the tunable filter. The same MEMS deposited on indium gallium arsenide phosphide (InGaAsP) material lies at the heart of the tunable VCSEL.

"It is important to note that both the tunable filters and tunable VCSELs are based on the same MEMS technology," says Tayebati. "It's a 50-micron mirror deposited on a little membrane. That part of the structure is identical, so it makes it easier for us in the manufacturing stage to make both devices at the same time. This is the first commercialized MEMS technology, so we've pushed the technology pretty hard for the last year and a half."

Generally, VCSELs operating at 1550 nm are extremely difficult to achieve, says Tayebati. The best result for VCSELs thus far has been less than half a milliwatt of power and shorter wavelengths such as 1300 nm. Achieving any appreciable power at 1550 nm has been an incredibly difficult task. CoreTek says its tunable VCSEL provides 2 mW of continuous-wave power and a 50-nm tuning range over the 1520- to 1570-nm band. It requires "negligible" electrical current as compared to most tunable semiconductor lasers relying on thermal tuning or electrical injection, according to the company.

CoreTek plans to extend the performance of the product in the future. "We're pushing toward 10 mW and higher right now," reports Tayebati. "We can make the same laser for use at different wavelengths. In a DWDM system, you need different wavelengths. You can make different lasers for different wavelengths, but a much better approach is to make one type of laser that can be dialed into any different wavelength you want by applying a small voltage."

The MEMS in CoreTek's products uses a tiny mirror that allows the user to select a wavelength by adjusting a voltage onto the mirror, pushing it toward the substrate. By adjusting the distance between the mirror and the substrate, the wavelength can be adjusted as needed. The advantage is a much more functional component that can be used at a very low cost in cost-sensitive applications. It also lowers the cost in other ways.

"The reason tunability is so important is because when you're working with a 100-channel system, the end user has to store all those different channels on the shelf in case any one of them might go bad," says Tayebati. "With a tunable product, you can use one, two, or maybe three backup lasers on the shelf and tune them into any wavelength in case something goes bad. So it allows the network service providers to use wider bandwidth closer to home."

CoreTek's tunable VCSEL, called the MEM-SEL, should be commercially available by year-end. The company expects to get evaluation samples to customers within a few months. Although nondisclosure agreements prohibit CoreTek from announcing specific customers, Tayebati says the customers most interested in this technology include most of the industry's large telecommunications equipment manufacturers. The company has its sights set on reaching the L band with similar tunable VCSEL products in the near future.

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