1.7 W holmium-doped fluoroindate fiber laser at 3920 nm.

A monolithic fiber laser emitting 1.7 W at 3920 nm is experimentally demonstrated in a Ho3+:InF3 fiber. The cavity comprises a pair of highly reflective fiber Bragg gratings written in the active fiber with the femtosecond phase-mask scanning technique and is spliced to the pump diode with a robust silica-to-fluoride fiber splice. This work is an important step toward high-power all-fiber laser operating in the vicinity of 4 µm.

Femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss at wavelengths beyond 4 µm.

We report on the fabrication of, to the best of our knowledge, the first highly reflective fiber Bragg gratings for the 4 µm wavelength range. A second-order grating with a coupling coefficient (κ) of 230m-1, losses <0.25dB/cm, and a bandwidth of approximately 3 nm was inscribed into the core of a passive indium fluoride (InF3) fiber using a femtosecond (fs) laser. Thermal annealing of this grating at a temperature of 150°C for 90 min resulted in the enhancement of κ to 275m-1. Further, we show that InF3 fibers respond very differently to irradiation with fs laser pulses as compared to ZBLAN fibers and that this difference manifests itself in a significantly larger process window for inscription and in the formation of a more complex refractive index profile that is believed to be caused by the larger nonlinearity of InF3. This Letter paves the way to the development of new wavelength stabilized all-fiber mid-infrared lasers beyond 4 µm.

Emission beyond 4 µm and mid-infrared lasing in a dysprosium-doped indium fluoride (InF3) fiber.

Optical emission from rare-earth-doped fluoride fibers has thus far been limited to less than 4 µm. We extend emission beyond this limit by employing an indium fluoride (InF3) glass fiber as the host, which exhibits an increased infrared transparency over commonly used zirconium fluoride (ZBLAN). Near-infrared pumping of a dysprosium-doped InF3 fiber results in broad emission centered around 4.3 µm, representing the longest emission yet achieved from a fluoride fiber. The first laser emission in an InF3 fiber is also demonstrated from the 3 µm dysprosium transition. Finally, a frequency domain excited state lifetime measurement comparison between fluoride hosts suggests that multiphonon effects are significantly reduced in indium fluoride fiber, paving the way to more efficient, longer wavelength lasers compared to ZBLAN fibers.

Mid-IR supercontinuum from 2.4 to 5.4 µm in a low-loss fluoroindate fiber.

A mid-infrared supercontinuum extending up to 5.4 µm is generated in a low-loss fluoroindate fiber. It is pumped with an erbium-doped fluoride fiber amplifier seeded with 400 ps pulses at 2.75 µm. Both fibers are fusion spliced to increase the robustness and long-term stability of the system. With more than 82% of the total power beyond 3 µm, this approach is promising for efficient mid-IR light generation.