Photodarkening as a heat source in ytterbium doped fiber amplifiers

Abstract

Theoretical and experimental evaluation of the photodarkening effect as a heat source in ytterbium doped fibers is presented. An additional non-radiative decay channel that opens after photodarkening the fiber is identified via fluorescence lifetime reduction and as an additional heat source proportional to inversion. It is included in the heat source model which was tested on a core-pumped fiber amplifiers. High temperature elevation at low pump powers shows potential heat-related problems in high inversion systems that are more susceptible to photodarkening.

Theoretical Study of the Thermal Distribution in Yb-Doped Double-Clad Fiber Laser by Considering Different Heat Sources

“…For the first heat source the quantum defect is introduced as the main source of heat production [56]. The quantum defect heat arises from the energy difference between the pump and signal photons or spontaneously emitted photon [4]. On the other hand, the difference between pump and signal (lasing) energy causes Table 2.…”

“…The photon wavelength of the laser is longer than the pump photon wavelength, so this shift is called as Stokes shift [57]. The second source of heat is photodarkening loss which can increase the temperature [4,56,58], which depends on the pump and signal wavelengths, seed power, fiber core size, etc. [56].…”

“…Doped fiber amplifiers and lasers have been considered more than that of other solid state lasers [1], because of high-quality, high-performance, gain stability, small quantum defects (QD), flexibility, low transmission loss and cavity noise, low cost, also compactness, no thermal lensing, small footprint, saturation energy, and stability to the environmental effects [2][3][4].…”

“…The ratio of the QD at lasers is typically less than 16% [17], and the lowest value for QD is about 0.6% for the single clad doped fiber [18]. Several phenomena appear in the active medium by heat generation, such as thermal lens [19], mechanical stresses [9], change in the refractive indices [4,20], and consequently birefringent of the active medium [21], which causes some limitation at the output of lasers and amplifiers. In the solidstate lasers with large cross-sections, heat creates a thermal lens and causes mode instability in the fiber lasers and amplifiers [4,22].…”

“…Several phenomena appear in the active medium by heat generation, such as thermal lens [19], mechanical stresses [9], change in the refractive indices [4,20], and consequently birefringent of the active medium [21], which causes some limitation at the output of lasers and amplifiers. In the solidstate lasers with large cross-sections, heat creates a thermal lens and causes mode instability in the fiber lasers and amplifiers [4,22]. Study and verification of the heat distribution in the fiber lasers is the first step to prediction and control of the several effects in high power fiber lasers and amplifiers.…”