Chirped Bragg Grating Covers the C-Band

Daniel C. McCarthy

Twice in as many years, researchers at 3M have claimed breakthroughs in their fabrication of chirped fiber Bragg gratings. This year's breakthrough, however, appears to be the one that counts: the fabrication of long-length gratings that cover the full C-band. The gratings also demonstrated good performance in a 10-Gb/s optical communications system 40 km long.

Single-channel chirped fiber Bragg gratings are already in place in some networks, but 3M's goal has been to fabricate multichannel gratings capable of compensating for chromatic dispersion for the entire C-band in individual fibers. The challenge to this is largely in the manufacturing process.

Controlling error

Fabrication errors show up in network receivers as gain ripple, which is the disparity in phase from channel to channel. Network engineers are asking grating manufacturers for delay ripple thresholds of ±20 ps/nm², according to James Brennan, the senior researcher for 3M's dispersion compensation products. Lower ripple improves a network's signal-to-noise ratio and requires less power to compensate for a high noise floor.

Most of 3M's fabrication process is deceptively straightforward. An excimer laser writes the gratings in a fiber coiled around a rotating drum. This enables production of continuous gratings more than 10 m long – the breakthrough that was announced in 1999.

Last year's breakthrough was the development of special fibers with improved mechanical integrity to better withstand the manufacturing process.

Improved fabrication

However, it wasn't until this year that 3M was able to translate fibers with better than 1-ppm velocity control, which made commercial applications of the gratings feasible.

To do this, Brennan and his group had to ensure that the fiber rotated at a constant speed during the writing process, a task that required them to micromachine the drum's surface to optical quality 250-nm precision and the motors to <1-µm tolerances.

Improved fabrication enabled 3M to demonstrate a grating about 2 m long with over 30 nm of bandwidth, insertion loss of about 1 dB and a delay slope of -1.1 ps/nm².