May 27, 2004 -- Bandwidth9 of the US has developed what it calls a "MEMS bridge structure" to form the basis of a mechanically stable tuning platform for its 1550 nm vertical-cavity surface-emitting lasers (VCSELs) - low-power devices that are being lined up for metro network applications such as dynamic tuning or optical add and drop.
MEMS-based tunable VCSELs commonly exploit the movement of a cantilever structure to vary the length of the laser cavity and thus control the emission wavelength. Although such devices are easy to make and operate, the cantilevers are often highly sensitive to mechanical vibrations and material stress.
Bandwidth9 employs its MEMS bridge in an InP-based VCSEL, which comprises an active region sandwiched between two distributed Bragg reflectors (DBRs). The 495 x 10 x 7 μ m bridge is integrated into the upper DBR and suspended above a 2.35 μ m air gap. The bridge is supported at both ends, which ensures that the VCSEL offers enhanced tuning repeatability and a wider tuning range than cantilever-based versions.
"The bridge structure provides a more stable tuning point, and more resistance to mechanical vibration and stress changes in the material," explains researcher Decai Sun (now working at US LED manufacturer Lumileds). "It also reduces sticking of the tuning structure and increases processing yield."
The VCSEL is tuned by applying a DC voltage bias between the bridge and the ground, using electrostatic attraction to move the bridge within the air gap. With a maximum DC bias of 46 V, the VCSEL tunes across a wavelength range of 22 nm (1565-1543 nm). The peak continuous-wave output power (observed around 1560 nm) is 1.3 mW at 15 ° C.
Bandwidth9 tested the device's operation under vibration--a key metric for any MEMS-based device. For vibration frequencies of up to 10 kHz and acceleration amplitudes of up to 10 Gbits, the VCSEL exhibits a power penalty of less than 0.1 dB, for both back-to-back detection and 300-km transmission of a 2.5-Gbit/sec signal. The VCSEL also shows a low level of bit-error-rate degradation at this degree of mechanical vibration.
• This article originally appeared in FibreSystems Europe in association with LIGHTWAVE Europe, April 2004 p7.
Tami Freeman is deputy editor, FibreSystems Europe in association with LIGHTWAVE Europe.