1. What is the ASE light source?
The ASE light source outputs the the amplified spontaneous emission light and ASE stands for Amplified Spontaneous Emission. It is the wide-band, low-coherence light source with high-brightness and excellent output stability, and is sometimes called a broadband light source or low-coherence light source. The light is generated inside the optical fiber core, and therefore can be incident to a single mode fiber with low coupling loss. The ASE light sources are used in various optical mesurement and sensing systems including FBG sensing, OCT, fiber optic gyroscopes, gas sensing, measurement of optical components, and so on.
2. Emission is caused by rare earth ions
Just like fiber lasers and optical fiber amplifiers, rare earth ions are used for light emission of ASE light sources. As shown in Fig.1, the rare earth ion absorbs the pump light and emits the light at a different wavelength from the pump light. Though this emission light called “spontaneous emission” is weak, it turns to strong emission called “stimulated emission”, when the intensity of the pump light is increased enough. The stimulated emission requires the strong excitation enough to cause the situation that the number of electrons at the excited level exceeds the number of electrons at the ground state as shown in Fig.2. All of ASE light sources, fiber lasers, and optical fiber amplifiers utilize the stimulated emission.
Fig.1. Rare earth ion emits the light at a different wavelength from the pump light.
Fig.2. Spontaneous emission & stimulated emission.
3. Optical fibers effectively cause the stimulated emission
The stimulated emission needs extremely strong pump light, but it can be easily caused with optical fibers. It is possible to confine the pump light in a narrow core in an optical fiber, and therefore the extremely high optical power density can be easily achieved within the fiber core. Thus, RE-doped optical fibers are used to obtain the stimulated emission in ASE light sources, fiber lasers, and optical fiber amplifiers.
4. Light becomes stronger by amplification in the optical fiber
ASE light sources can emit strong light, because the spontaneous emission light is amplified during the light travels through the fiber. Since the strong pump light exists along the longitudinal direction of the fiber, the stimulated emission occurs one after another as the light advances through the fiber as shown in Fig.3, and high intensity light is outputted from the ASE light source..
Fig.3. Spontaneous emission is amplified by stimulated emission before the light is outputted. Though the arrows in the figure (ASE light and pump light) are drawn outside the core, the both lights pass through the core.
5. The wavelength of the emitted light varies depending on the kind of rare earth elements and the fiber materials
While the wavelength of the emitted light basically depends on the kind of rare earth ion, it is also related to the optical fiber material. Each kind of rare earth ion emits the light at specific wavelengths corresponding to its specific energy diagram. On the other hand, the difference caused by fiber materials is based on how easily the energy of excited electrons turns into thermal energy. Figure 4 shows the emission wavelengths in a fluoride fiber and a silica fiber. Since the ratio of conversion into thermal energy is smaller in a fluoride fiber, a fluoride fiber can emit the light at more various wavelengths as compared with a silica fiber.
Fig.4. Emission wavelengths in fluoride fibers and silica fibers.
Another unique feature of a fluoride fiber is capability to emit the light at shorter wavelengths than the pump light wavelength. This is because the electrons excited to the first excited level is further excited to the second excited level as shown in Fig.5. This phenomenon is also attributable to the nature of fluoride fibers that enrgy of the excited electron is less likely to be converted into thermal energy.
FiberLabs offers an ASE light source of a variety of wavelength bands by using the rare earth-doped fluoride fiber. The relationship between the emission wavelengths and the energy diagram is shown in Fig.6.
Fig.5. Two step excitation in fluoride fibers.
ASE light sources
Fig.6. Relationship between energy diagrams and emission wavelengths of ASE light sources.