Amplified Spontaneous Emission is the spontaneous emission exhibited by the pumped laser gain medium even when there is no light signal being amplified. Spontaneous emission is a process in which an atom in an excited state spontaneously emits a photon without any external stimulation. This process occurs randomly and without any specific direction or coherence. However, in the presence of a gain medium, this spontaneous emission can be amplified through a feedback mechanism.

Figure 1: ASE spectra in a forward-pumped fiber amplifier

When a laser gain medium is pumped to achieve a population inversion, ASE is produced. For low-gain dyes in a dye laser, it takes a longer time to build up lasing in the oscillator, so if the pumping is done earlier, it results in the amplification of spontaneous emission from the oscillator. Forward and backward ASE in a fiber amplifier is shown in figure 1.

For a fiber laser or fiber amplifier, the amplified spontaneous emission traveling through the fiber can be significantly stronger than the luminescent emission that occurs in all directions. If the amplified spontaneous emission (ASE) does not have enough strength to produce significant power, it contributes to the amplified signal's noise level. The ASE also limits the noise figure of a laser amplifier. 

ASE is stronger for quasi-three-level laser gain media than for four-level laser media. The output ASE spectrum of a rare-earth-doped fiber amplifier is different from the fluorescence emitted by the fiber in a transverse direction. The reason behind this dissimilarity is the wavelength-dependent amplification and reabsorption. Also, the forward and backward ASE powers are different. In general, the ASE tends to be stronger in the direction opposite to that of pumping. The spectral shape of the ASE may also rely on the intensity level of the pump as shown in figure 2.

Figure 2: Spectra of backward ASE calculated for different pump power levels

From figure 2, it is clear that the spectrum shifts toward shorter wavelengths where gain is higher with increasing power and becomes narrower.

Properties of ASE

Like a laser beam, ASE light is somewhat directional. ASE has broader optical bandwidth than laser emission, a lower degree of polarization, and a different temporal profile. For pulsed dye lasers, ASE is strong because these lasers have high laser gain and the dyes can emit broad optical bandwidth. ASE has the demerit of reducing the laser beam output pulse energy but extracting some of the stored energy from the wanted laser beam. If ASE is not reduced, it may contaminate the laser beam and have effects on the applications of the laser light.

Applications

One of the most common applications of ASE is in fiber amplifiers, which are used to amplify optical signals in fiber optic communication systems. In these amplifiers, the gain medium is typically a doped fiber, such as erbium-doped fiber, which is excited by a pump laser. As the excited atoms in the fiber spontaneously emit photons, the emitted radiation is amplified due to the feedback mechanism. This allows for the amplification of weak optical signals without the need for costly and complex amplification systems.

ASE is used in a variety of other applications, such as in laser cooling, where it can be used to cool atoms to very low temperatures by exploiting the recoil effect of spontaneously emitted photons. ASE is also used in the development of high-power lasers, where it can be used to improve the beam quality and reduce the threshold for lasing.