Upconversion Fiber Lasers

Over the past two years, the uniform, high-intensity pumping advantages of the single-mode fiber geometry have been realized in demonstrating several new visible upconversion laser transitions. To date 11 different CW, room-temperature visible upconversion laser/pumping combinations have been demonstrated with visible laser powers of up to 200 mW. Perhaps the most dramatic demonstration of an upconversion laser to date was the CW, room-temperature operation of a Pr3+-doped ZBLAN fiber at 5 visible wavelengths1. An additional advantage of the fiber geometry for upconversion lasers is that the fiber laser is the only laser geometry where the pump spot size and resonator length are decoupled. This extra degree of freedom is extremely important in the upconversion process where the ground-state absorption (GSA) and one or more excited-state absorptions (ESA) must be balanced in order to efficiently populate the upper laser level. For example, the three-photon pumped Tm3+ upconversion laser discussed in this paper would not be possible in a Tm3+-doped crystal due to the extremely weak GSA. The long resonator lengths possible in fiber lasers also allow the minimization of concentration quenching effects while distributing the absorption to facilitate heat removal.