Accompanying the development of multimedia networks, communication traffic demands are dramatically increasing. In communication systems of today's multimedia society, erbium doped fiber amplifiers (EDFA) using EDF an amplifying medium and wavelength division multiplexing (WDM) transmission systems performing wavelength multiplexing of signal light play an important role.
In WDM transmission systems, for example, arrayed waveguide grating (AWG) transmits wavelength-multiplexed signal light and an EDFA along the transmission path amplifies the signal light. In addition to signal components, the signal light amplified by EDFA includes amplified spontaneous emission (ASE) generated across the entire band by the EDFA.
On the other hand, recently, WDM transmission systems are being actively introduced in urban metro core networks attaching importance on cost and size. Accordingly, optical add drop multiplexers (OADM) are being increasingly introduced. OADMs perform wavelength demultiplexing with respect to received WDM signal light using AMG to add, transmit, or drop resulting demultiplexed signal lights.
Signal lights added or transmitted are subjected to the power adjustment by a variable optical attenuator (VOA) and are wavelength-multiplexed by AWG. The wavelength-multiplexed (WDM) signal light is amplified by an EDFA and then sent to a transmission path. In the WDM transmission system, the number of signal lights wavelength-multiplexed in the signal light to be transmitted (hereinafter, simply “wavelength count”) varies as needed (e.g., 1 to 40).
When the wavelength count of the WDM signal light is small, ASE alone is transmitted by a channel that does not include a signal component. However, since the deterioration of S/ASE ratio increases after the multi-span transmission, it is difficult to determine whether a signal component is included in the respective channels. In this regard, a technique of constraining ASE by maximizing an attenuation amount of a VOA corresponding to a channel that does not include the signal component has been disclosed (see, e.g., Japanese Laid-Open Patent Publication No. 2003-163641).
In such OADMs, the wavelength count frequently changes due to the adding and the dropping of signal components. Therefore, it is preferable for the characteristics of the EDFA to be such that the excitation light power may be controlled quickly to achieve a constant gain even when the wavelength count changes. This is implemented by a known EDFA configuration that includes two EDFs sandwiching a gain equalizer (GEQ) (see, e.g., Japanese Laid-Open Patent Publication No. 2005-116884).
By calculating the current gain error from monitor values of input/output powers to control the excitation light power of LD (Laser Diode) such that the gain error is reduced, this configuration performs control of constant gain quickly in response to changes in the wavelength count. The configuration further performs constant gain control while maintaining the ratio of the upstream EDF excitation power to the downstream EDF excitation power (excitation ratio).
However, in the above conventional technology, when the wavelength count is small, the polarized state of the signal light is not averaged, resulting in a problem of increased polarization of the signal light. If the degree of polarization of the signal light increases, polarization hole burning (PHB) generated in the EDF increases, deteriorating the quality of the signal light (e.g., optical signal noise ratio (OSNR)).
In particular, when ASE (not polarized) is constrained by VOA control as in the technology disclosed in Japanese Laid-Open Patent Publication No. 2003-163641, the degree of polarization of the signal light increases more since the polarized state of the signal light of each channel is not averaged. When the wavelength count of the signal light is large, the degree of polarization of the signal light decreases since the polarized state of the signal light of each channel is averaged.
PHB is a phenomenon of a gain reduction in polarized light parallel to signal light when high power signal light is input to an EDF. The gain of the signal light and the gain of polarized ASE parallel to the signal light are reduced. On the other hand, since polarized ASE orthogonal to the signal is not affected by PHB, the gain is not reduced. Therefore, the polarized ASE orthogonal to the signal increases and OSNR of the signal light deteriorates.