Uncooled 1310nm DFB laser breaks 'temperature barrier'

10 March 2003 -- Bookham Technology and Cambridge University's Department of Engineering will present at OFC 2003, the industry's "first" description of an uncooled, directly modulated DFB laser giving ultrawide bandwidth over the temperature range of 25-85°C.

10 March 2003 -- Bookham Technology and Cambridge University's Department of Engineering will present at OFC 2003, the industry's "first" description of an uncooled, directly modulated DFB laser giving ultrawide bandwidth over the temperature range of 25-85°C.

Such devices are crucial to the realisation 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, Modulated Sources Technology, 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 fibre-fed radio access points."

"Cost is key in RF-over-fibre applications in cellular and wireless-LAN distribution," says Prof. Ian White of the Photonics Group, Cambridge University Department of Engineering. "Hence Cambridge University is focusing on developing new techniques allowing full RF transmission over multimode fibre using uncooled lasers.

This paper shows that laser diodes developed by Bookham for datacommunications applications also exhibit record linearities up to 20GHz frequencies. Broadband spurious-free dynamic ranges of over 100 dB.Hz2/3 are demonstrated over this entire range, sufficient not only for wireless but, indeed, also for radar applications."

The design uses an InP-based compressively-strained multiple-quantum-well 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.

Bookham's gain-coupled DFB grating technology further enhances the performance of the design by placing the grating in the middle of the laser active region, rather than displaced from it as in most other DFB lasers. The central location gives a much stronger interaction between the laser light and the grating, resulting in a very fast laser with superior direct-modulation performance and lower spurious-frequency generation, and so improved linearity.

At 25°C the laser has a spurious-free dynamic range (SFDR) of over 103dB-Hz2/3 in the 1-20GHz range, while still maintaining an SFDR of over 90dB-Hz2/3 in the 1-10 GHz range at 85°C. Third-order intermodulation products are below 50dB (typically 60dB) over the entire 1-20GHz range at 25°C.

Excellent linearity performance like this is highly valuable for the 2¿3GHz signals of 3GPP UMTS and IEEE 802.11b and the higher data-rate standards such as IEEE 802.11b, which use carrier frequencies beyond 5GHz. The performance easily exceeds the requirements of fibre-fed distributed antenna systems.

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