定义：
使光学放大器装置的增益谱在一定频率范围内趋于平坦的装置。

光纤放大器的光学增益在某种程度上依赖于光波的波长，这是比较让人烦恼的。例如，在光纤通信中应用的波分复用器，它随波长变化的增益会使各传输通道的功率不平衡。因此，有比较采取办法使增益均衡，或者也成为增益平坦。有几种技术可以实现： 

• 对于给定的掺杂活性光纤，增益谱的形状可以通过调整激光活性离子的平均反转能级来进行优化。可以通过改变泵浦功率或者活性光纤的长度来调整。 
• 也可以优化光纤纤芯中材料的成分。例如，二氧化硅光纤中可以加入各种掺杂物质，氟化物光纤可以提供相对平坦的增益谱。 
• 光纤玻璃具有不同组分的光纤放大器可以组成一个放大器链来得到宽带的杂化放大器。这种装置也会包含拉曼放大器。 
• 另外一个方法是采用分频放大器，采用一个依赖于波长的分束器将信号光分配到不同的光纤放大器中，然后采用另外一个依赖于波长的耦合器再将这些光谱成分组合一起。 
• 可以采用光学滤波器（增益平坦滤波器），它在增益较高的波长区域的损耗也较大。这些滤波器通常是采用光纤布拉格光栅（长周期光栅，倾斜光栅或者啁啾光栅），当然也存在许多其他类型的滤波器。 

采用平坦滤波器优化多级放大器是非常复杂的，因为放大器和滤波器的结合可以得到很好的噪声系数和泵浦效率。典型的解决两级掺杂二氧化硅光纤放大器的方案是在两级之间添加一个光学滤波器。 

拉曼放大器中使增益谱平坦可以采用多个能级平衡的泵浦光束的方法来进行。

Definition: a technique for making the gain spectrum of an optical amplifier device flatter over a certain optical frequency range

Alternative term: gain flattening

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 [7].
• 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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