Birefringence allows EDFA gain flattening
Researchers Shenping Li, Kin-Seng Chiang, and W. Alexander Gambling of the City University of Hong Kong developed a flexible method for flattening the gain profile of an erbium-doped fiber amplifier (EDFA) using a high-birefringence fiber-loop mirror (HiBi-FLM).1 The device contains a number of high-birefringence fiber sections and polarization controllers. The reflection spectrum of the loop mirror can be adjusted to compensate for the variations in the gain profile. The method is easy to implement, said the researchers.
The experimental setup included an EDFA connected to a HiBi-FLM, which consisted of a number of HiBi fiber sections, and the same number of polarization controllers arranged in a loop (see figure). The input beam was split by a 50:50 coupler into two counterpropagating beams. As they traveled through a number of HiBi fiber sections (which are effectively a number of polarimetric interferometers in series), each beam decomposed into a number of beams.
In general, this design with n sections of HiBi fiber can produce a spectrum of a 2n beam interferometer. The polarization controllers adjust the polarization states of light entering each HiBi section. The intensity of the output light depends on the phase differences among the interfering beams and the polarization states of the light entering these interferometers. In other words, the output depends on the birefringence lengths of the HiBi fiber sections and the settings of the polarization controllers. When the lengths of the HiBi fiber sections are chosen property, the reflection spectrum of the HiBi-FLM can be made to match the gain profiles of the EDFA under different operating conditions by controlling the polarization controllers.
The researchers made one- and two-section devices. The polarization-dependent loss was measured to be less than 0.1 dB for both the HiBi-FLMs. The polarization-mode dispersion caused by the two-section HiBi fiber was about 0.39 ps, and the total length of the HiBi fibers was 18.5 cm. The normalized birefringence of the HiBi fiber was 5.8 x 10-4. Using a two-section HiBi-FLM, the researchers were able to flatten the gain of an EDFA to within ±0.9 dB over a bandwidth of 33 nm, with the center wavelength at 1543 nm, under several EDFA operating conditions. They believe that to limit the insertion loss and complexity of control, the number of HiBi fiber sections should be limited to two or three.
For more information, contact Shenping Li, who now works at Corning Incorporated, at LiS2@corning.com.
- S. Li, K. S. Chiang, and W. A. Gambling, IEEE Photon. Tech. Lett. 13 (9), 942 (September 2001).