Co-Doping Drives 2-µm Fiber Laser
A group of scientists at the University of Sydney in Australia has demonstrated the use of ytterbium as a sensitizer in a holmium-doped fiber laser. Although Yb3+-Ho3+ sensitizing-lasing systems have been explored in crystalline YAG hosts, the research team is the first to have done so in a silica (fiber) host, and the laser has produced more 2-µm power than any previous Ho3+-doped fiber laser.
Lasers operating in the eye-safe 2-µm wavelength region are important for a number of spectroscopic, remote sensing and military applications. Triply ionized holmium, which can be doped into crystalline and amorphous hosts, has a strong transition at 2.1 µm, but it lacks strong absorption at common diode-pump wavelengths. For this reason, sensitizing agents, such as thulium, frequently are co-doped into the host to absorb the optical pump energy and to pass it to the holmium, thereby creating a population inversion.
There are two distinct advantages to using silica as the host for a Yb3+-Ho3+ system. First, the amorphous host can support a higher concentration of ytterbium than can most crystal ones. Second, it allows a long spontaneous lifetime of the upper laser level (5I7) in Ho3+.
The holmium doping in the 1-m-long fiber in the Australian laser was approximately 1 percent by weight, and the ytterbium was approximately 0.1 percent. A dichroic mirror on one end of the D-shaped fiber coupled the 975-nm pump radiation into the cavity and reflected the 2.1-µm laser radiation. An outer cladding of fluoropolymer enhanced the coupling, and Fresnel reflection at the far end of the fiber served as the output mirror.
Figure 1. The fiber laser produced 850 mW of 2.1-µm radiation for a launched pump power of 10.9 W. At pump powers greater than 7 W, the slope efficiency decreased from 12.5 percent to 7.6 percent, which the researchers attributed to deterioration of the fiber's outer cladding at the higher pump powers.
The laser produced a maximum output of 850 mW when 10.9 W of pump power was launched into the cladding (Figure 1). The low slope efficiency and high threshold indicated that a significant amount of the pump energy went astray before it reached the population inversion in the holmium.
The scientists identified a number of processes by which energy can escape the ideal pathway (Figure 2). Excited-state absorption is one, which occurs when a populated level absorbs a laser photon and rises to a higher level. In the diagram, this is shown as Ho3+ ions in the pump level excited to s1. Another process is energy-transfer up-conversion, which occurs when energy transferred from the Yb3+ excites an Ho3+ ion that is not in the ground state. In the diagram, this is shown as Ho3+ ions in the pump level excited to s2.
Figure 2. Two energy levels of Yb3+, designated the excited and ground states, are on the left. Three levels of lowest energy are shown for Ho3+, as well as two higher excited levels, designated s1 and s2, which indicate processes by which the energy can escape. Ideally, the Yb3+ ions absorb the pump photons and rise to the excited state level. The energy is transferred from the Yb3+ to the Ho3+ in an up-conversion process so that the Ho3+ is boosted from the ground state to the pump level, and the Yb3+ returns to its ground state. The Ho3+ subsequently decays by a multiphoton process to its upper laser level, where it becomes part of the population inversion.
Observing fluorescence from the laser that corresponded to the spontaneous decay of the various levels excited by excited-state absorption and by energy-transfer up-conversion confirmed the existence of these loss mechanisms. The most serious loss, however, results from the energy mismatch between the excited Yb3+ level and the Ho3+ pump level, which the group confirmed by observing fluorescence from the Yb3+ that corresponded to spontaneous decay of its excited state.
The researchers are experimenting with altering the concentrations of Yb3+ and Ho3+ in the fiber to reduce the efficiencies of these loss mechanisms.
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