Glossary entry

Amplified spontaneous emission (ASE)

Broadband incoherent optical power produced by spontaneous emission events that are subsequently amplified by stimulated emission in an optical amplifier. The fundamental noise source in optical amplifiers.

In any optical amplifier with population inversion, spontaneous emission events occur continuously throughout the gain medium. Spontaneous photons emitted into the guided mode of the amplifier are amplified by stimulated emission over their remaining transit through the gain region — producing broadband incoherent output called amplified spontaneous emission.

ASE has the spectral profile of the gain medium (the spontaneous emission spectrum, weighted by frequency-dependent gain) and accumulates randomly in phase, intensity, and polarization. It is the fundamental source of optical noise added by every amplifier.

ASE power spectral density at the amplifier output, into the supported mode count $M$ :

S_\text{ASE}(\nu) \;=\; M \, n_\text{sp} \, h\nu \, (G - 1),

where $n_\text{sp}$ is the spontaneous emission factor (related to inversion completeness), $G$ is the amplifier gain, and $M$ is the number of supported modes (typically $M = 2$ for two polarizations in single-mode fiber).

Total ASE power in an optical bandwidth $B_o$ :

P_\text{ASE} \;=\; S_\text{ASE} \cdot B_o.

For a typical EDFA with $G = 30$ dB and $n_\text{sp} = 1.5$ : ASE power density at 1550 nm is $\sim 6 \times 10^{-14}$ W/Hz, or about $-72$ dBm in a 0.1 nm reference bandwidth.

Why ASE matters.

Effect	Mechanism
OSNR degradation	ASE adds to the noise floor, reducing optical signal-to-noise ratio at each amplifier stage
Signal power competition	In saturated amplifiers, ASE shares gain medium population with the signal — reducing per-channel gain
Cascaded amplifier accumulation	ASE grows as $\sum G_i$ along an amplifier chain — long links accumulate substantial ASE
EDFA gain pinning	At high signal power, signal stimulated emission depletes inversion and ASE is suppressed; at low signal power, ASE dominates

ASE management. Telecom amplifier design and operation involves several techniques to control ASE:

Technique	Effect
Inter-stage isolators	Block back-propagating ASE that would otherwise re-amplify itself
Optical filtering	Suppress out-of-band ASE before the next amplifier or receiver
Gain flattening filters	Equalize gain across the band, indirectly reducing ASE accumulation in lower-gain channels
Coherent detection	DSP rejects ASE outside the signal modulation bandwidth
Distributed Raman amplification	Lower effective noise figure spreads ASE over the fiber length

ASE is also exploited deliberately in broadband light sources:

Superluminescent diodes (SLDs) — semiconductor amplifiers with high ASE that emit broadband light for OCT, fiber gyros, and FBG interrogation
ASE-source-based optical reflectometry — broadband incoherent source enables low-coherence ranging

The ASE noise floor at an amplifier output is what makes OSNR a finite quantity in any amplified system — without ASE, OSNR would be limited only by other noise sources and would not degrade with amplification.