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Bookham's optical components set new records

At the Optical Fiber Communication Conference and Exposition (OFC), March 25-28, Bookham will be reporting some impressive technical achievements that we can expect to see incorporated into its products in the near future.
At OFC, Engineers from Bookham Technology will present, the industry’s first description of a directly modulated DFB laser that combines ultrawide bandwidth with very high linearity over the wide temperature range of 25°C – 85°C, but without cooling. At the same meeting, Bookham will detail the record breaking performance recently achieved from its 40 Gbit/s electroabsorption modulator.

New laser saves power and space

The work on the uncooled DFB laser has been performed jointly with researchers from Cambridge University in the UK. Such devices are crucial to the realization of the direct transport of analogue microwave/RF signals over fibre for inbuilding distributed antenna systems in cellular networks and for the burgeoning market in wireless LANs, such as the IEEE 802.11-series standards and HiperLAN.

“The big advantage of going to uncooled directly modulated lasers is they are cheaper, more compact and more efficient,” says Kenton White, advisor with modulated sources technology at Bookham Technology, and paper joint author. “Typically, you can take up to a watt of power consumption off, and, in terms of packaging, I would say there is a factor of two in area reduction in removing the cooler — perhaps even more. And, of course, you don’t need an expensive, and potentially complex, external modulator taking up more space and power. This would really help optics to move into the wireless area, for example, by solving the big problem of providing low-cost fiber-fed radio access points.”

The Cambridge University group is focusing on developing new techniques allowing full RF transmission over multimode fibre using uncooled lasers. The paper also shows that the laser diodes developed by Bookham for datacom applications also exhibit record linearities up to 20 GHz. Broadband spurious-free dynamic ranges of over 100 dB.Hz2/3 are demonstrated over this range, sufficient not only for wireless, but also for radar applications.

The paper describes an InP-based compressively-strained MQW DFB laser, which incorporates a unique semi-insulating buried heterostructure for enhanced thermal performance. Bookham already uses this design of buried heterostructure in its reliable digital-communications lasers, where it has operated at over 100°C - among the highest operating temperatures ever achieved in such applications.

For more details see, Wide-frequency-range operation of a high-linearity uncooled DFB laser for next-generation radio-over-fiber by J Ingham et al. in the OFC 2003 Conference Proceedings.

Modulator sets new standards

A further paper to be given at OFC describes a 40 Gbit/s electro-absorption modulator (EAM) module with unequalled performance. Based on the Franz–Keldysh effect (FKE), the EAM device has the lowest unamplified insertion loss ever reported for an EAM, the lowest polarisation-dependent loss (PDL) over a wide range of wavelengths, and the highest bandwidth for a lumped-element FKE-EAM. All these performance parameters are achieved simultaneously, with a manufacturable design, while maintaining the practical requirement of over 10 dB extinction with a voltage swing of under 2.5V.

“We now have the technology to build the world’s best EAM for high-speed systems, and this is a complete, single package integrating all the necessary 40 Gbit/s RF functions, which is a hard problem,” said Kelvin Prosyk, senior researcher at Bookham Technology and joint author of the OFC paper.

The InP-based EAM device has a bulk active absorption core, which is inherently simple to fabricate and consequently gives high yields. This design also gives the near-zero chirp needed for the dispersion compensation of 40 Gbit/s operation on standard single-mode-fiber links with spans of over about 8 km. This attribute is relevant to other applications of high-speed EAMs, such as the data gating of a train of pulses. In this case, some pulse clipping will occur with anything other than an ideal square gate, thus making low chirp a necessity.

Integrated spot-size converters (SSCs), also used in Bookham’s InP Mach–Zehnder modulators, are a key aspect of the design, as they enable tight device integration and small package size by improving the laser–modulator optical coupling and consequently power throughput. The SSCs are designed to enable low loss, low PDL and good alignment tolerance when coupled to microlenses.

For more details see “Record unamplified loss 40GHz electro-absorption modulator module” by K.Prosyk, et al. in the OFC 2003 Conference Proceedings.

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