The power scaling of mid-infrared (mid-IR) fluoride fiber lasers is difficult to realize because of the low melting temperature of fluoride glass and immature fabrication techniques of mid-IR fiber devices. Mid-IR laser output powers of 30.5 W and 41.6 W at 2.94 μm and 2.82 μm based on single-end and dual-end pumping configurations (Table 1), respectively, have been previously reported. Although fiber Bragg gratings (FBGs) written in fluoride fibers have been developed as cavity mirrors in mid-IR fiber oscillators, the thermal degeneration of grating reflectivity, mismatch of the reflective wavelength, and severe splicing losses with silica fiber create new problems for the power scaling of FBG-based mid-IR fiber lasers. This study introduces a high-power erbium-doped fiber laser at 2.8 μm based on a single-end pumping scheme, where a high-reflective (HR) mirror and Fresnel reflection from the end face of a homemade end cap are used to provide cavity feedback.
Figure 1 shows the experimental setup of the high-power 2.8 μm erbium-doped fiber laser. The active fiber has an erbium dopant mole fraction of 7%, core and inner/outer cladding diameters of 15 μm and ~250 μm/290 μm, and core and inner cladding numerical apertures of 0.12/0.45; the fiber is butt-coupled to an HR mirror on one end and spliced to a 500-μm-long AlF3 fiber end cap on the other end. An end cap with core and cladding diameters of 200 μm and 240 μm, respectively, can efficiently decrease the power density of the high-power mid-IR laser while isolating the water vapor diffusion from the end face of the active fiber, which is essential for high-power laser output. A multimode laser diode with a wavelength stabilized at 976 nm and a maximum output power of 128 W is coupled to the inner cladding of the fluoride fiber using two lenses with optimized focal lengths, where one is for pump light alignment and the other is for focusing light into the fiber. A dichroic mirror, which transmits the pump laser and reflects the back-propagation mid-IR laser, is inserted between the two lenses. Water cooling is applied to the pump coupling end of the erbium-doped fiber for appropriate heat dissipation under a high laser power, while the rest of the optical system is passively cooled.
When the output power is less than 10 W, the laser slope efficiency is 32.8% with respect to the launched pump power, and the lasing wavelength is red-shifted from 2800 nm to 2818 nm with increasing laser power. The slope efficiency decreases gradually as the laser power increases because of the pump excited-state absorption (4I11/2→4F7/2). A maximum output power of 33.8 W is achieved at a launched pump power of 128 W, and dual-wavelength lasing at 2865 nm and 2871 nm is obtained owing to the wide reflection band of the HR mirror and high laser gain of the cavity. Because no damage is observed from the AlF3 end cap at the maximum output power, further power scaling of this fiber laser is limited by the available pump power delivered from a 105 μm/125 μm multimode fiber.
With a homemade AlF3 end cap, a 33.8 W erbium-doped fluoride fiber laser operating at 2.8 μm is demonstrated under the appropriate thermal management of the fluoride fiber. A high optical-optical conversion efficiency of 26.4% is achieved at the maximum output power using high-precision spatial pump coupling. To the best of our knowledge, this is the highest output power achieved using a single-end pumping mid-infrared erbium-doped fiber laser.
