Wavelength-tunable semiconductor lasers are on their way to becoming an essential part of wavelength-division-multiplexed (WDM) all-optical networks. Last year at the 2000 Optical Fiber Communications Conference (OFC; Baltimore, MD), CoreTek (Wilmington, MA) announced its version of a tunable laser—an optically pumped vertical-cavity surface-emitting laser (VCSEL) tuned by a microelectromechanical-systems (MEMS) cavity mirror. (CoreTek has since been acquired by Nortel Networks; Brampton, Ont., Canada.) The device allowed early evaluation of the technology, but was not integrated into a single package: for example, the wavelength locker and the pump laser were separate and connected to the laser package via single-mode fiber.
Now, a year later at OFC 2001 (Anaheim, CA), Nortel has unveiled an integrated version of the MEMS VCSEL that integrates the locker and the pump laser with the VCSEL itself. With all its components in a single package, the laser is smaller, easier to install, and more reliable than its predecessor. The challenge in developing the new laser was largely in the packaging, according to Tom Dudley, vice president of marketing and business development at the former CoreTek. In particular, although such devices can be built and aligned by hand with little problem, hand assembly is ill-suited for mass-manufacture. Only automation allows the devices to be produced consistently in large quantities. Nortel engineers, such as the one shown, are intensively developing an automated manufacturing process that relies on robotics and image processing to speed assembly and make it more reliable. "You can't buy this sort of equipment off the shelf," says Dudley. "You have to build or at least specify it yourself."
The device produces 20 mW of optical power, stable to 0.5 dB when locked. Tuning range reaches 32 nm, encompassing 80 channels, with a locking accuracy of ±2.5 GHz and a typical linewidth of 5 MHz. A side-mode suppression ratio of 50 dB makes the laser useful as a light source for 40-Gbit/s systems. The package itself is 15 x 45 mm in size. The integrated VCSEL carries over the advantages of its predecessor, such as the ability to adjust laser output to level the power across WDM channels without producing noise-inducing chirp, which decreases achievable span length.
Nortel is pursuing even higher integration of the device, with full transmitters and even transponders possible, says Dudley. Laser driver electronics inside the package would coexist with alarms that would check for loss of power, laser aging, and improper wavelength tuning and locking.
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.