XFP modules reach for 40 km

4 April 2003 Atlanta, GA Lightwave -- Module vendors at OFC last week agreed that 10-Gbit/sec XFP transceivers appear the eventual winner in applications now served by 300-pin transponders. To hasten that event, a few developers showcased several approaches toward supporting the 40-km distances common for metro applications.

4 April 2003 Atlanta, GA Lightwave -- Module vendors at OFC last week agreed that 10-Gbit/sec XFP transceivers appear the eventual winner in applications now served by 300-pin transponders. To hasten that event, a few developers showcased several approaches toward supporting the 40-km distances common for metro applications.

For example, Ignis Optics demonstrated a 40-km XFP powered by the10T101 uncooled 1310-nm externally modulated laser (EML) from ASIP. The company touted the device for both 10-Gigabit Ethernet and SONET/SDH applications. Meanwhile, Finisar demonstrated a 1550-nm device using a cooled EML with an integrated avalanche photodiode (APD). A company spokesman said that Finisar believes the integration of a thermo-electric cooler and APD along with the laser and modulator within the XFP form factor constitutes a significant engineering achievement, particularly since the modules dissipates less than 3.5 W. The company also showcased a 40-km version of its 1310-nm device that dissipates less than 2.5 W.

Cheaper, farther

While the use of laser technology common to 300-pin transponders should be expected in the first XFPs for intermediate reach, the form-factor's overall goal of lowest possible cost would imply a desire for cheaper technology. Both Infineon and Picolight Inc. announced at OFC the development of 1310-nm vertical-cavity, surface-emitting lasers (VCSELs), as well as their desire to incorporate their new technology into XFP transceivers. However, VCSEL devices face the obstacle of comparatively low output power.

Newly developed electronic dispersion compensation technology may provide a way around the power problem. For example, AMCC showed the effectiveness of its Niagara S19208 device in conjunction with its S3094 receiver chip in extending the reach of a 1310-nm XFP module supplied by Ignis Optics. The two chips combine forward error correction with electronic dispersion compensation. While the XFP laser used in the demonstration was a DFB, the technology also should be applicable to VCSEL devices.

Santel Networks also demonstrated a new two-chip approach to electronic dispersion compensation, as well as clock and data recovery (CDR). The Santel S44501/S44003 OC-192 chip set provides the ability to recover up to two-thirds of the Optical Signal-to-Noise Ratio (OSNR) penalty caused by chromatic dispersion (CD), according to the company. Typical post-compensation residual OSNR penalty is on the order of 2 dB with 100 km of singlemode fiber, compared to 6 dB or higher penalty in a typical uncompensated link. It was demonstrated with a Harris Corp. transponder at the show.

Of perhaps even more interest to XFP developers, Phyworks Ltd. unveiled an electronic dispersion compensation chip that measures only 5x5 mm -- small enough to fit within an XFP module. The PHY1060 chip, built in CMOS, offers serial-to-serial power consumption of 0.5 W and serial-to-parallel consumption of 0.75 W. The device, which performs analog processing and contains an integrated limiting amplifier, would reside after the transimpedance amplifier within the module and would interface with the line card transceiver chip. The second revision of the device should reach the sampling stage by this August or September.

Electronic dispersion compensation has sparked the interest of XFP developers. For example, Bob Pierce, vice president, marketing, at E20 Communications, said he saw this technology as a viable approach as his company works on its 40-km, 1310-nm XFP. In particular, he said electronic dispersion compensation would enable use of XFPs in a wide range of longer-reach applications outside of traditional telecommunications. -- Stephen Hardy

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