Fiber Laser Produces 2.5 W in the 3-µm Region

Breck Hitz

Lasers that operate in the 3-µm region of the mid-infrared have many applications in remote sensing, spectroscopy, medicine and military countermeasures, and fiber lasers seem likely candidates to provide high powers in this spectral range.

The two most promising fiber approaches use either the ~2.75-µm transition in the erbium ion or the ~2.84-µm transition in the holmium ion, but each has its inherent drawbacks. Stuart D. Jackson at the University of Sydney in Australia has demonstrated a technique to overcome the most serious drawbacks of Ho-doped fiber lasers, obtaining a maximum output of 2.5 W in single-transverse mode at 2.86 µm.

The lack of powerful semiconductor lasers at the 1155-nm peak absorption wavelength of Ho was one obstacle to obtaining high power at 3 µm from Ho-doped fiber lasers. Laser designers were forced to resort to complex and inefficient pumping schemes to obtain laser output. Jackson, however, showed that if the Ho were doped into a fluoride-glass fiber, there was sufficient residual absorption to pump the laser with a double-clad ytterbium-doped fiber laser at ~1100 nm.

An additional advantage of pumping with a relatively high-beam-quality fiber laser is that the Ho laser could be core-pumped, eliminating the need for a double-clad, fluoride-glass fiber, which is both expensive and fragile. The absence of an outer cladding also facilitates conduction cooling, which is important in a laser with a quantum defect of greater than 60 percent.

Figure 1. The energy levels of Ho and Pr show the
resonant energy-transfer mechanism that drains energy out of the lower laser level of Ho. The quantum defect of the system is greater than 60 percent.

A second obstacle to high-power, 3-µm output was the long lifetime (~3 ms) of the lower laser level, which results in self-saturation of the laser gain as population piles up in the lower level. Others have addressed this issue by stimulating a second, "cascaded" lasing transition from the lower laser level or by deactivating the lower level with another mechanism. Jackson effectively depopulated the lower laser level by codoping the fiber with praseodymium, which provided a resonant cross-excitation path to drain population (Figure 1).

He coupled the 1100-nm pump radiation from an Yb-fiber laser into the 15-µm core of the Ho-Pr fiber laser with a dichroic mirror (Figure 2). The 2.86-µm output of the codoped laser was limited by the available pump power, and it reached 2.5 W in a single-transverse mode from a pump power of ~9 W. Laser threshold was 30 mW, and the laser's slope efficiency was 29 percent.

Figure 2. The Ho-Pr fiber laser was efficiently core-pumped with a double-clad Yb fiber laser.

Jackson believes that this is the highest output achieved in the 3-µm region by a fiber laser and that the Ho-Pr codoped laser is the most effective method yet demonstrated for generating 3-³m output from a fiber laser.