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Amplifier Noise

Definition: noise introduced to a signal in an amplifier device

German: Verstärkerrauschen

Categories: optical amplifiers, lightwave communications


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Apart from amplifying the input signal, every optical amplifier also adds some excess noise to the output. This is often quantified with the so-called noise figure.

For phase-insensitive amplifiers (i.e., those where the gain does not depend on the phase of the input), quantum optics dictates some minimum quantum noise level of the added noise. For example, for high gain applied to a shot-noise limited input (a coherent state), the output noise power of the intensity noise must be at least twice that of a hypothetical noiseless amplifier, i.e., the minimum noise figure is 2, corresponding to 3 dB. The phase noise is also increased.

Origins of Amplifier Noise

For laser amplifiers (based on stimulated emission, e.g. fiber amplifiers), the unavoidable excess noise can be regarded as arising from spontaneous emission of the gain medium into the amplified mode. For a four-level gain medium with a low-noise pump, the excess noise can approach the minimum quantum-mechanically allowed level. (Note that the gain medium acts as an energy reservoir, effectively damping the influence of high-frequency pump noise.) The use of quasi-three-level gain media leads to additional excess noise because for a given gain, the upper-state population (and thus the spontaneous emission) has to be higher to compensate for the signal reabsorption. The additional increase in noise in such situations can be quantified with a spontaneous emission factor, which decreases for increasing excitation levels.

Raman amplifiers behave similarly, if operated at not too high temperatures. They can have fairly low noise figures, if the propagation losses are small.

In a non-degenerate optical parametric amplifier, the excess noise comes from vacuum fluctuations entering the idler port, and possibly also from the pump wave. A degenerate parametric amplifier does not need to add excess noise (it has no idler!), but its amplification is phase-sensitive.

In addition to quantum noise, classical fluctuations of the pump source can also cause excess noise.

Minimization of Amplifier Noise

Various methods may be employed for reducing amplifier noise:

  • If a stronger input signal can be supplied, such that less amplifier gain is needed for a certain signal output power, that will reduce the amplifier noise.
  • In the case of a quasi-three-level amplifier, it is important to design it such that it operates with a high excitation level of the laser-active ions near the input end.
  • As the amplifier noise depends on the number of modes involved, using a single-mode fiber is advantageous.
  • Low-frequency excess noise may be caused by pump noise; a low-noise pump source can thus help.


[1]H. Heffner, “The fundamental noise limit of linear amplifiers”, Proc. IRE 50, 1604 (1962), DOI:10.1109/JRPROC.1962.288130
[2]H. Kogelnik and A. Yariv, “Considerations of noise and schemes for its reduction in laser amplifiers”, Proc. IEEE 52 (2), 165 (1964), DOI:10.1109/PROC.1964.2805
[3]C. M. Caves, “Quantum limits on noise in linear amplifiers”, Phys. Rev. D 26 (8), 1817 (1982), DOI:10.1103/PhysRevD.26.1817
[4]Y. Yamamoto, “AM and FM quantum noise in semiconductor lasers - Part I: Theoretical analysis”, IEEE J. Quantum Electron.19 (1), 34 (1983), DOI: 10.1109/JQE.1983.1071726
[5]R. Loudon, “Theory of noise accumulation in linear optical-amplifier chains”, IEEE J. Quantum Electron.21 (7), 766 (1985), DOI:10.1109/JQE.1985.1072735
[6]C. R. Giles and E. Desurvire, “Propagation of signal and noise in concatenated erbium-doped fiber optical amplifiers”, IEEE J. Lightwave Technol. 9 (2), 147 (1991), DOI:10.1109/50.65871
[7]E. Desurvire et al., “Characterization and modeling of amplified spontaneous emission in unsaturated erbium-doped fiber amplifiers”, IEEE Photon. Technol. Lett. 3 (2), 127 (1991), DOI:10.1109/68.76863

See also: noise figure, spontaneous emission, quantum noise, amplified spontaneous emission, optical amplifiers, laser noise, spotlight 2006-07-16

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