750-W double-clad ytterbium tapered fiber laser with nearly theoretically limited efficiency

Abstract

We report the theoretical and experimental study of the properties of an active tapered double clad fiber (T-DCF). Optimization of the most important parameters of T-DCF such as tapering ratio, longitudinal profile, core/clad ratio and absorption have been considered. Using optimized T-DCF design we have demonstrated a high-power (750W) and highly efficient (80%) ytterbium fiber laser.

Actively Q-switched ytterbium tapered fiber laser

“…part of the spontaneous emission leaks out of the core thus mitigating the detrimental effect of inversion depletion. This effect becomes more pronounced with increasing tapering ratio [7][8][9] . Since the intensity of spontaneous emission is proportional to the number of propagating modes, counter-propagating ASE in a taper (from narrow to wide end) is also weaker compared to a cylindrical uniform DCF with similar average core size due to mode selection in the small-core section of the tapered fiber 4 .…”

“…As it was shown earlier, the light amplification in the T-DCF is non-reciprocal and dependent on the propagation direction [7][8][9] . The spontaneous emission propagating from the wide to narrow end of the taper initially emitted under angles NA/T<α<NA, where NA is the numerical aperture and T is the tapering ratio, leaks from the fiber core to the cladding after relatively short propagation distance [7][8][9] . As a result, the ASE induced inversion depletion is low compared to equivalent uniform LMA fiber.…”

Demonstration of 3 kW-Level Nearly Single Mode Monolithic Fiber Amplifier Emitting at 1050 Nm Employing Tapered Yb-Doped Fiber

“…In 2008, researchers from Tampere University of Technology in Finland reported a fiber laser based on a tapered double-clad fiber and realized a single mode output power of 84 W [10]. Since then, the tapered Yb-doped fiber (T-YDF) with a longitudinally varying core diameter has been developed and extensively applied in high-power lasers fields [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The T-YDF can control the number of supported modes in the small-core section, which can improve laser beam quality and mitigate the transverse mode instability (TMI) [11], [12], [13].…”

“…The large-core section can suppress the intensity of nonlinear effects, such as SRS [14], [15]. Thanks to the nonuniform longitudinal geometry, the tapered fiber itself also has strong ASE suppression capability [10], [16], [17]. In 2018, Fedotov et al applied a tapered fiber with a core diameter of 96 µm to construct a 1040 nm linearly polarized continuous wave (CW) laser with an output power of 70 W [18].…”