OFC '99 Tech Briefs: Erbium/Ytterbium Co-doped Fiber Amplifies Signal in the L-band
Dual amplifier module incorporating erbium-doped fiber amplifier and erbium/ytterbium co-doped fiber amplifier produces 23 dBm of output power in the long wavelength region.
By: Kristin Lewotsky
To increase bandwidth by upping wavelength-division multiplexing (WDM) channel counts, systems designers are forced to move outside of the conventional erbium-doped fiber amplifier (EDFA) window (C-band) to the so-called long-wavelength window (L-band; 1575 nm to 1605 nm). AL though erbium-doped fiber amplifier technology can be used in this band, L-band amplifiers must incorporate up to five times as much erbium fiber as conventional components. The fiber must be well-optimized for background losses, the erbium doping levels are significantly higher, and passive components in the amplifiers must be revised to operate at the longer wavelengths. Meanwhile, the units are only able to generate about 18.3 dBm of total output.
By combining an EDFA pre-amplifier with a double-clad erbium/ytterbium (Er/Yt) co-doped fiber amplifier, engineers at Pirelli were able to generate 23 dBm of output in the L-band.1 This output would support transmission spans as long as 130 km.
The design
The erbium-doped preamplifier consists of a 100-m-long silica-based alumina co-doped fiber with an erbium peak loss of approximately 6.5 dB/m. The device is bidirectionally pumped by lasers operating at 980 nm and 1480 nm.
The Er/Yt co-doped power amplifier consisted of a 12-m-long fiber with 65-µm-dia. inner cladding and 5-µm-dia. core. The core composition consisted of SiO2/P2O5/AL2O3 co-doped with erbium and ytterbium at a ratio of 1:20. A pair of 400-mW, multimode Lasers operating at 920 nm pumped the fiber, which was modified to permit singlemode transmission at the signal wavelength and multimode propagation at the pump wavelength so that the pump beam coupled into the inner cladding, where it was absorbed by the active medium.
Tested at 1575 nm and 1602 nm, the combined amplifier unit produced an output of about 23 dBm, which agreed with theoretical calculations. With optimization and the application of higher pump powers, the researchers believe that the design could produce as much as 26 dBm.
References
F. DiPasquale et. al., "23 dBm output power Er/Yb co-doped fiber amplifier for WDM signals in the 1575-1605 nm wavelength region," OFC '99 Technical Digest, paper #WA2-1, (1999).