In order to support different span lengths between Optical Amplifiers in a transmission link, as well as to provide for aging of the link, it is desirable for an optical amplifier to support as large as possible dynamic gain range, i.e. to allow the gain to be dynamically configured over a wide range. For a single channel optical amplifier, this can be easily be achieved, and most single stage amplifiers support a large dynamic gain range. For WDM multi-channel amplifiers the situation is much more complicated due to the need to maintain a flat gain for all gain values.
Thus, most low-end WDM amplifiers are designed as fixed gain amplifier, i.e. they provide flat gain for only a given pre-designed gain value. To support different gain values, system designers may either use a number of fixed gain amplifiers with different pre-set gain values, or place a Variable Optical Attenuator (VOA) before the fixed gain amplifier. The former solution requires different part numbers for the various amplifiers, and therefore complicates operational issues such as inventory control and sparing, while the latter solution leads to a large deterioration in OSNR, and therefore is not suitable for links longer than 200 to 300 km. These drawbacks can be addressed with a Variable Gain EDFA.
Variable Gain EDFA is typically designed to have flat gain at the top of the required gain range, using an appropriate Gain Flatness Filter (GFF), while a VOA is used to attenuate all the channels uniformly in order to achieve a range of gain values. If the VOA is placed at the EDFA input, then a large deterioration in Noise Figure (NF) will occur. On the other hand, if the VOA is placed at the EDFA output then high pump power will be required, thus increasing the cost of the EDFA amplifier. Therefore it is necessary to place the VOA between two amplification sub-stages. The figure below shows a basic design of WDM Variable Gain EDFA, where the VOA is placed between two gain sub-stages in order to improve NF.
The two gain stages are typically pumped by a single pump where the pump power is split between the stages, and the control loop controls the gain of the entire EDFA amplifier (both gain sub-stages and VOA).
By carefully designing the Erbium-Doped Fiber (EDF) length and designated pump power for each sub-stage, it is possible to achieve very good NF performance over a range of gain values, with only a moderate increase in total pump power compared to a fixed gain EDFA. The typical NF performance of a variable gain EDFA is shown in the figure below as compared to a comparable fixed gain EDFA with a VOA placed beforehand.
As can be seen, the NF increases only slightly at the low gain range, representing a significant performance advantage compared to the alternative fixed gain EDFA with a VOA placed before. The variable gain EDFA provides much better NF performance over the entire gain range.