The optical gain e.g. from a fiber amplifier has some dependence on the wavelength, which can be disturbing in certain applications. For example, in optical fiber communications with wavelength division multiplexing, the wavelength dependence of the gain can unbalance the optical powers in the transmitted channels. Therefore, it is common to apply methods for gain equalization, also called gain flattening. There are various technological options:
- For a given kind of active fiber, the shape of the gain spectrum can often be optimized to some extent simply by adjusting the average inversion level of the laser-active ions. This can be done e.g. by varying the pump power or the length of the active fiber.
- The material composition of the fiber core can be optimized. For example, silica fibers can be optimized with various codopants, and fluoride fibers can offer a fairly flat gain spectrum.
- Amplifier fibers of different glass compositions can be combined in an amplifier chain to obtain a wideband hybrid amplifier.
- Another approach is a split-band amplifier, where a wavelength-dependent splitter distributes the signal content over two or more different fiber amplifiers, and another wavelength-dependent fiber coupler serves to recombine the spectral components.
- One can use a Raman amplifier, where the gain spectrum can be tailored with a proper combination of pump sources at different wavelengths .
- Optical filters (gain flattening filters) can be used, which have higher losses in wavelength regions where the gain is higher. Such filters are often based on fiber Bragg gratings (long-period gratings, slanted gratings or chirped gratings), although various other types of filters have been demonstrated.
The optimization of a multi-stage amplifier with flattening filters is a complex task, since it is not obvious, e.g., which combination of amplifiers and filters gives the best results in terms of noise figure and power efficiency. A typical solution for a two-stage amplifier based on doped silica fibers would include an optical filter between the two stages.
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