For most lasers, the laser wavelength is longer than the pump wavelength. This is unavoidable for all pumping schemes where one pump photon can generate one laser photon. However, there are upconversion pumping schemes, where two or more pump photons are used to transfer a laser ion to a highly excited level, so that the laser photon energy can be larger than that of the pump source. For example, there are visible lasers pumped with infrared light. (Sometimes, the label upconversion laser is used when the pumping process involves multiple photons, even when the laser wavelength is not shorter than the pump wavelength.)
Early upconversion lasers were demonstrated with bulk crystals, which often had to be cooled to very low temperatures, making the systems impractical. However, significant advances have been achieved e.g. with Er:YLiF4 and Er:LiLuF4 crystals, generating relatively high-power green light [2, 14, 15] at room temperature.
Most currently used upconversion lasers are based on glass fibers, where high pump intensities can be maintained over long lengths, so that the laser threshold can be reached even under otherwise difficult conditions. In most cases, silica glass is unsuitable due to its high phonon energies, which cause strong multi-phonon transitions and thus lead to too short metastable level lifetimes. Therefore, it is often necessary to use heavy-metal fluoride glasses (e.g. ZBLAN), which tend to be fragile and expensive.
Specific examples of upconversion fiber lasers are:
- thulium-doped blue upconversion lasers [5, 11, 12], pumped e.g. around 1120–1140 nm and emitting around 480 nm
- erbium-doped green upconversion lasers [1, 4], pumped e.g. around 980 nm and lasing around 546 nm
- praseodymium/ytterbium-doped upconversion lasers [7, 8], pumped e.g. around 850 nm (using an avalanche process) or via ytterbium as sensitizer, and emitting at one of various visible wavelengths in the red (635 nm), orange (605 nm), green (520 nm), or blue (491 nm) spectral region
Alternative methods for obtaining shorter laser wavelengths are often based on nonlinear frequency conversion.
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|||R. Paschotta, case study on a thulium-doped upconversion laser|