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New Structure Improves Telecom Lasers

Photonics Spectra
May 2004
Breck Hitz

Many researchers are aggressively developing wavelength-tunable vertical-cavity surface-emitting lasers (VCSELs) based on microelectromechanical systems (MEMS) as optical sources for fiber optic telecommunications systems. Although most work until now has focused on cantilevered structures, an engineering group at Bandwidth9 in Fremont, Calif., recently fabricated a MEMS VCSEL with a bridge structure and observed that its mechanical characteristics are superior to those of cantilevered lasers.

New Structure Improves Telecom Lasers
Figure 1. The mechanical characteristics of microelectromechanical- systems-based vertical-cavity surface-emitting lasers with a bridge structure (top) are superior to those of the older cantilever structure (bottom).

The older cantilevered structure and the new bridge structure are shown in Figure 1. Both lasers have a top distributed Bragg reflector (DBR) suspended above the body of the laser, which comprises a gain region and a bottom DBR. Their wavelength is tuned by applying an electrostatic force between the suspended DBR and the body of the laser. The electrostatic force pulls the top DBR toward the laser body, shortening the air gap and changing the length of the resonator, thereby tuning the wavelength. Although the cantilever design is relatively straightforward to fabricate, the rest position of the cantilever is unstable and often varies over time. This causes instability in the laser's initial wavelength and in its tuning range.

The Bandwidth9 engineers designed the bridge structure to avoid this problem and to make it insensitive to mechanical vibration. The bridge structure in Figure 1 is 495 µm long, 7 µm thick and 10 µm wide. The air gap between the top DBR and the body of the laser is 2.35 µm, and the suspended DBR droops by approximately 0.1 µm because of built-in stress. This is much smaller than the corresponding droop in a cantilevered structure.

Because vibrational characteristics are crucial to reliable MEMS operation, the engineers measured their laser's resonant frequency by sweeping a radio-frequency signal across the tuning voltage applied between the DBR and the laser body. They found its resonant frequency at approximately 80 kHz, which is twice that of a cantilevered laser whose cantilever is half the length of the bridge. They also tested the laser's mechanical reliability, tuning the bridge over its full range 1 million times without observing any degradation.

New Structure Improves Telecom Lasers
Figure 2. The output of the bridge-structure laser was tunable from 1543 to 1565 nm, with a maximum power output of approximately 1.3 mW. ©2004 IEEE.

The tuning characteristics of the laser are shown in Figure 2. The laser could be tuned over most of the C-band, from 1543 to 1565 nm, with a tuning voltage of 46 V. The wavelength was relatively linear with voltage, and the maximum output of approximately 1.3 mW was obtained toward the upper end of the tuning range. However, the laser was not truly single-mode across the entire range: The side-mode-suppression ratio varied from 10 to 40 dB as the laser was tuned.

For testing in a realistic telecom system, the engineers mounted the laser in a butterfly package along with a semiconductor optical amplifier and an electro-absorption modulator chip. They coupled the package output into 300 km of standard single-mode fiber and conducted transmission tests at 2.5 Gb/s. They subjected the laser package to vibrations of up to 10 kHz at accelerations of up to 10 g, and found that they suffered less than 1-dB power penalty after transmission through the 300 km of fiber.

GLOSSARY
microelectromechanical systems (mems)
Refers to micron-size complex machines that have physical dimensions suitable for the fabrication of optical switches for use in state-of-the-art communications networks.
Communicationsfiber optic telecommunications systemsfiber opticslasersmicroelectromechanical systems (MEMS)Research & TechnologyTech Pulsewavelength-tunable vertical-cavity surface-emitting lasers (VCSELs)

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