Novel laser design cuts optical power penalties

Edited by the Electronicstalk editorial team Mar 25, 2002

Zarlink Semiconductor has designed a novel large-aperture vertical cavity surface-emitting laser that increases the performance and reliability of multigigabit VSR optical interconnect equipment.

Zarlink Semiconductor has designed and demonstrated a patent-pending, large-aperture vertical cavity surface-emitting laser (VCSEL) that increases the performance and reliability of multigigabit VSR optical interconnect equipment.

In laboratory tests, the 12-micron aperture VCSEL achieved 10Gbit/s transmission at 80C over 300m of high-bandwidth fibre, with a power penalty of only 0.5dB.

Zarlink's large-aperture 850nm VCSEL is also capable of transmitting light at even higher speeds.

"This success may result in a highly reliable, large-aperture 10Gbit/s VCSEL that transmits farther than the industry-standard 300m, at datarates extending beyond 10Gbit/s", said Jitesh Vadhia, vice president and general manager, Optical Systems division, Zarlink Semiconductor.

Today, VCSELs are widely used for high-speed, short-reach optical interconnections in Gigabit Ethernet, Fibre Channel, and central office equipment.

With rising demand for terabit-class networking equipment to meet Internet-driven bandwidth requirements, a new generation of single- wavelength 850nm VCSELs is under development for 10Gbit/s (OC-192 and 10 Gigabit Ethernet) and 40Gbit/s (OC-768) optical interconnect.

Zarlink large-aperture VCSEL achieves dramatic reduction in power penalty A key challenge in designing 10Gbit/s VCSELs is to increase the density of information-carrying photons passing through the aperture of the laser.

Higher photon density equates with higher bandwidth, higher modulation efficiency and lower noise.

Designers must also meet power penalty specifications.

In an optical transmitter-receiver link, factors such as noise, attenuation, dispersion and jitter combine to degrade the quality of the light signal as it travels along the fibre.

This degradation - called the power penalty - is measured in optical decibels.

For 10 Gigabit Ethernet over 300m, the IEEE 802.3ae standard specifies that the power penalty may not exceed 4.7dB.

Zarlink bettered the industry's power penalty requirement by designing an innovative 12-micron aperture 10Gbit/s VCSEL.

Large-aperture VCSELs are more reliable and deliver longer device life span because they operate with lower current density than small-aperture designs.

However, large-aperture VCSELs also produce several unwanted, higher-order modes that degrade signal integrity.

Zarlink met this challenge by altering the structure of a large-aperture VCSEL to eliminate higher-order modes.

This was achieved with a patent- pending innovation that deliberately introduced losses for higher-order lateral modes, and that also decreased the power penalty.

As a result, in laboratory tests Zarlink successfully demonstrated 10Gbit/s transmission with a power penalty of only 0.5dB, over 300m of high bandwidth LaserWave fibre from OFS, at an industry-standard 80C.

Zarlink's large-aperture VCSEL thus provides a 4.2dB margin in the optical link, resulting in potentially better link performance, longer reach and lower cost, plus increased link reliability.

(This was Electronicstalk's Top Story on 22 March 2002).