Saturable absorption in erbium-doped fiber is presented through absorption measurements using a tunable laser and variable power. Its consequent optical bistability is also observed in erbium-doped fiber ring laser with long lengths of the fiber. Higher absorption at shorter wavelengths enables a wider bistable range.
The structure of combining two segments of erbium-doped fiber (EDF) together—one EDF acting as an amplifier and the other as an absorber—has a special gain characteristic curve which has a maximum gain with an increase in the input signal power. This special gain characteristic curve can lead to an optical bistability (OB) phenomenon in a fiber-ring laser employing an EDF of this structure. By adjusting the pump light power injected into the second segment of the EDF, we can change the behavior of the absorber and can, therefore, control the range of the bistability region. A method is proposed to calculate the tunable optical bistability, and the calculation result is in accordance with our experiment.
Resonance properties of the all-fiber ring cavity filled with nonlinear material - saturable rare-earth-doped fiber are analyzed and experimentally investigated. Unlike the earlier investigated erbium-doped fiber at 1550nm where the optical absorption photo-induced change (saturation) is observed only, the ytterbium-doped fiber at 1064nm demonstrates the saturation of the refractive index mainly. For this configuration we report experimental observation of the optical bistability and hysteresis in the transmitted output light at the 10mW-scale incident light power. The experimental results are in qualitative agreement with the theoretical analysis that takes into account the saturation of both parameters: the optical absorption and the refractive index of the doped fiber. The reported results seem promising for applications in high-sensitivity interferometric configurations at 1064nm operation wavelength.
Optical bistability derived from the negative nonlinear absorption effect was investigated in 470 ppm erbium-doped fibers using a 1.5 µm laser diode. Optical bistability was observed in a laser intensity range of 0.3 W/cm2 to 2.5 kW/cm2 at a fiber length of 6 m. The transmitted laser intensity (I out), propagating through the fiber, decreased exponentially in accordance with the well-known exponential law. However, the amplitude of the optical bistability (ΔI out) increased logarithmically as the fiber length was increased. It was clarified that ΔI out was proportional to the product of exp (-L) and log (L) as a function of the fiber length (L). The optical bistability can be explained by considering an enhanced absorption model for a four-level system of the Er3+ ion.
The authors investigate the optical bistability in an L-band dual-wavelength erbium-doped fiber (EDF) laser with overlapping cavities. The bistable input-output hysteresis behavior of the two lasing wavelengths evolves in antiphase. The bistable region is tens of milliwatts wide. The bistability is essentially due to the homogeneous saturable absorption of EDF. The cross-gain saturation causes the bistabilities of the two lasing lines to evolve in antiphase with respect to each other. The gain clamping effect helps to widen the bistable region, which may be controlled by the cavity loss and by the EDF length.
Optical bistability phenomenon in a single fiber ring laser employing erbium-doped fiber (EDF) as gain medium is observed in our experiment. In the EDF amplifiers, the photons of pump light cannot completely transfer into that of signal light because of various attenuation factors. This part of loss (useless pump loss) and active spontaneous emission (ASE) can both lower the small-signal gain of the EDF, and can eventually result in the bistability phenomenon. The range of this bistability has a complicated relationship with the length of the EDF, the erbium-ion doping concentration, the cavity loss and the useless loss coefficient of the pump light.