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Gain Equalization

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

Alternative term: gain flattening

Category: optical amplifiers

How to cite the article; suggest additional literature


The optical gain e.g. from a fiber amplifier has some dependence on the wavelength, which can be disturbing. For example, in optical fiber communications with wavelength division multiplexing, the wavelength dependence of the gain can unbalance the powers in the transmitted channels. Therefore, it is common to apply methods for gain equalization, also called gain flattening. There are various technological options:

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.

The gain spectrum of a Raman amplifier can be flattened by using multiple pump beams [7] with well-balanced pump power levels.


The RP Photonics Buyer's Guide contains 2 suppliers for gain equalization filters.

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[1]K. Inoue et al., “Tunable gain-equalization using a Mach–Zehnder optical filter in multistage amplifiers”, IEEE Photon. Technol. Lett. 3, 718 (1991), doi:10.1109/68.84463
[2]S. F. Su et al., “Gain equalization in multiwavelength lightwave systems using acousto-optic tunable filters”, IEEE Photon. Technol. Lett. 4, 269 (1992), doi:10.1109/68.122389
[3]R. Kashyap et al., “Wavelength flattened saturated erbium amplifier using multiple side-tap Bragg gratings”, Electron. Lett. 29 (11), 1025 (1993), doi:10.1049/el:19930685
[4]B. Clesca et al., “1.5 μm fluoride-based fiber amplifiers for wideband multichannel transport networks”, Opt. Fiber Technol. 1, 135 (1995), doi:10.1006/ofte.1995.1004
[5]A. M. Vengsarkar et al., “Long-period fiber-grating-based gain equalizers”, Opt. Lett. 21 (5), 336 (1996), doi:10.1364/OL.21.000336
[6]N. Park et al., “High-power Er–Yb-doped fiber amplifier with multichannel gain flatness within 0.2 dB over 14 nm”, IEEE Photon. Technol. Lett. 8, 1148 (1996), doi:10.1109/68.531818
[7]Y. Emori et al., “100 nm bandwidth flat-gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM laser diode unit”, Electron. Lett. 35, 1355 (1999), doi:10.1109/OFC.1999.766052
[8]M. Harurnoto et al., “Gain-flattening filter using long-period fiber gratings”, J. Lightwave Technol. 20 (6), 1027 (2002), doi:10.1109/JLT.2002.1018814
[9]Ik-Bu Sohn et al., “Gain flattened and improved EDFA using microbending long-period fibre gratings”, Electron. Lett. 38 (22), 1324 (2002), doi:10.1049/el:20020915

(Suggest additional literature!)

See also: gain, fiber amplifiers, tunable optical filters, wavelength division multiplexing
and other articles in the category optical amplifiers


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