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These are the newest type of lasers being used in time-correlated single photon counting systems. These remarkable devices can provide pulses that are 100 femtoseconds in width. Special optics can be used to increase the pulse width to picoseconds. The pump source (a laser that drives the Ti:Sapphire laser) is a continuous, not mode-locked, argon-ion laser. In addition to being simpler without the need of a mode-locker, the output of the argon-ion laser is ten to fifteen times greater in the continuous mode. Recently, Ti:Sapphire lasers have also been pumped with solid-state diode pumped lasers, similar to Nd:YAG lasers.

One favorable feature of Ti:Sapphire systems is that they automatically switch to mode-locked operation, giving 100 femtosecond pulses. This is due to the Kerr effect, giving a free-running mode-locked state.

Since it is a solid state device, there are no dyes flowing through that need to be replaced (as in dye-lasers) and the Ti:Sapphire crystal seems to have an indefinite life span. The only disadvantage is that Ti:Sapphire lasers have generally long-wavelength output (720-1000 nanometers). Even after frequency doubling, the shortest accessible wavelengths are 360 to 500 nanometers. This can be overcome using frequency tripling or harmonic generation, which is difficult because it requires overlapping the second-harmonic and fundamental beams in the second crystal. Since one is dealing with femtosecond pulses, overlapping the beams in both time and space can be tricky.

Because of the long wavelengths Ti:Sapphire lasers are good sources for multiple photon excitation spectroscopy. The intense emission of the laser can be used to stimulate fluorophores by the simultaneous absorption of two photons.

Source: Fluorescence Spectroscopy, Joseph R. Lakowicz

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