Gain-shifted thulium-amplifier efficiency rivals that of EDFAs
Tadashi Kasamatsu and others at NEC Corp. (Nakahara-ku, Kawasaki, Japan) developed a fiber amplifier for the 1480- to 1510-nm band with efficiency of 29%, which rivals that of erbium-doped fiber amplifiers (EDFAs) in the C- and L-bands.1 This increase in efficiency may allow the gain-shifted thulium-doped fiber amplifier to be used in practical WDM networks.
Other gain-shifted thulium amplifiers have been reported, but in addition to having low optical-to-optical conversion efficiencies of around 9%, they also require a solid-state pump laser emitting at 1.05 µm. These pump lasers, typically Yb:YLF or Yb fiber, are larger and less efficient than laser diodes.
The thulium amplifier is a laser-diode (LD) pumped system using an optimized two-stage configuration. Instead of pumping the material at 1.05 µm, it is pumped by laser diodes at 1.4 and 1.56 µm.
This pumping scheme changes the way a population inversion is created in such a way that less energy is lost,and the overall efficiency increases. In a conventional upconversion pumping process, two 1.05-µm photons are needed to push a thulium ion into the excited state, from which it can release a coherent photon; in both cases there is energy loss because the exciting photons push the ion slightly above the necessary energy level. In the new design, the 1.56-µm light pumps the thulium ions to the lower state with less energy loss. The 1.4-µm light populates the upper state and creates a population inversion (necessary for lasing) between the upper and lower states.
The researchers constructed a two-stage, LD-pumped, gain-shifted thulium-doped fiber amplifier using a 40-m fiber (see figure). The first stage is forward-pumped by dual wavelengths to obtain low noise, while the second stage is bidirectionally pumped at 1.4 µm and forward-pumped at 1.56 µm to obtain high efficiency and high output power. In this case, the 1.56-µm pump source was a distributed-feedback (DFB) laser diode, with output power amplified by an EDFA; however, the maximum power was less than 60 mW for all the experiments, leading the researchers to believe that the DFB/EDFA system can be replaced with a simple laser diode.
The output power is 21.5 dBm with a total pump power of 480 mW, gain of more than 20 dB, and a noise figure of less than 7 dB.
For more information, contact Tadashi Kasamatsu at firstname.lastname@example.org.
- T. Kasamatsu, Y. Yano, and T. Ono, IEEE Phot. Tech. Lett. 13, 433 (May 2001).
Yvonne Carts-Powell is a science and technology writer based in Belmont, MA.