Working principle of ASE broadband light source

ASE broadband light generated by erbium-doped fiber is amplified spontaneous emission light generated by short-wavelength laser pumping erbium-doped fiber. As shown in the following diagram, the pumped rare earth ions transition between upper and lower energy levels to generate spontaneous emission light, which is amplified in the stimulated emission process. This process is repeated continuously, and even a fairly high output power can be achieved under sufficient pumping conditions. (ASE=Amplified Spontaneous Emission, amplified spontaneous emission light)

ASE radiation is commonly present as noise light in fiber lasers and fiber amplifiers. ASE light usually competes with the signal wavelength laser for gain, causing the effective power of the laser wavelength to decrease, the laser signal-to-noise ratio to decrease, and the degree of polarization to decrease. Therefore, it is hoped that ASE light will be minimized in fiber lasers and fiber amplifiers. When designing fiber lasers and amplifiers, the power share of ASE light will be reduced as much as possible by optimizing the optical path structure. However, ASE light as a light source itself also has some characteristics, such as wide spectral range, spectral flatness, low coherence, low degree of polarization, etc., which are not possessed by laser light sources. Moreover, since the fiber ASE light source is generated in a single-mode erbium fiber, it can be coupled with an ordinary single-mode fiber almost losslessly. Therefore, this ASE broadband light source also has important applications in some occasions, such as fiber gyroscopes, fiber sensing, OCT imaging, and optical communication channel power compensation. In actual C+L band fiber ASE broadband light source products, erbium-doped quartz fiber is usually used as the working material, and a 980nm band semiconductor laser is used as an excitation pump to generate spontaneous radiation in the C band, which is amplified by the stimulated radiation process. Since the erbium-doped fiber has a certain length, the C-band radiation light is absorbed by other erbium ions and re-radiated during transmission, so that the radiation wavelength is shifted to a longer band. These radiations will be amplified by stimulated emission again, and finally the ASE broadband spectrum covering the C-band or L-band is obtained. Through different optical path structure designs, ASE light source products with different band spectra such as C, L, C+L can be obtained.

The above is the optical path for testing the ASE radiation spectrum of Er-doped fiber. Isolator is a fiber isolator, and WDM is a 980/1550nm fiber wavelength divider. The 974nm single-mode pump LD provides pump laser, which excites a section of single-mode Er-doped fiber through WDM coupling. The emitted backward ASE light and forward ASE light are output through two isolators respectively. Under the same pump power, the measured backward ASE spectrum (green line) and forward ASE spectrum (blue line) are shown in the figure. It can be seen that the ASE radiation spectrum directly emitted by the erbium-doped fiber is not flat, and the power density difference between different wavelengths can reach more than 10dB. The ASE spectra in the two directions are not exactly the same. Therefore, in the fiber ASE broadband light source product, spectrum flattening technology (filter) is required to achieve a flat output spectrum.