Higher-Order Mode Pumping in Erbium-Doped Fiber Amplifiers

Erbium-doped fiber designs are currently constrained by the general requirement to guide only one mode at the pump and signal wavelengths. Parameters such as the core diameter and numerical aperture must be carefully controlled to maintain single-mode operation. This limits the flexibility in the doped fiber design, and the small mode field diameter reduces the power handling limit and increases the splice losses to standard single-mode fibers. Relaxing this constraint to permit higher-order mode propagation at the pump wavelength would enable the development of new doped fiber designs with enhanced properties. However, the consequences of permitting higher-order pump modes to propagate within the amplifier fiber remain largely unexplored. Here, we present gain and noise figure measurements for a pure LPII pump mode that is usually not excited and compare the results to standard <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LP}_{01}$</tex> pumping. The setup shown in Fig. 1 is used to measure the gain and noise figure on 8 m of an OFS LRXL erbium-doped fiber. Simulations and cut-off wavelength measurements show that the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LP}_{01}$</tex> and LPII modes are supported at the pump wavelength. The fiber is pumped by a 976 nm laser diode that is combined with the signal using a wavelength division multiplexer (WDM). A long-period fiber grating (LPG) is thermally inscribed on a fiber [1] and converts the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LP}_{01}$</tex> mode into an LPII mode at the pump wavelength with a conversion efficiency of 99 %. The signal wavelengths pass through the LPG and remain in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LP}_{01}$</tex> mode, which is the only mode supported by the fiber at those wavelengths. By replacing the LPG fiber with an identical fiber without any inscriptions, the pump mode can be changed to an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LP}_{01}$</tex> mode.