Wavelength-Tunable Ultrashort Pulses

Up to now we have only discussed femtosecond lasers that can deliver short pulses with a fixed wavelength. For time-resolved spectroscopy the tunability of the wavelength provides many advantages, and so much effort has been expended in the development of sources of ultrashort pulses with a wide tuning range. There are several ways to realize them. 

Similar to cw lasers, active media with a broad spectral gain profile can be used for pulsed lasers. With wavelength-selective optical elements inside the laser resonator, the laser wavelength can be tuned across the whole gain profile. However, the drawback is the widening of the pulse length AT with decreasing spectral width Av due to the principal Fourier limit AT 27t/Av. 

Therefore, a new principle was developed for the generation of ultrashort tunable pulses which is based on parametric oscillators and amplifiers (see Vol. 1, Sect. 5.8.8). 

In the parametric process (Fig. 6.42a), a pump photon Ikwp is split in a nonlinear optical crystal into a signal photon and an idler photon ha i, such that energy and momentum are conserved  

In the parametric amplifier two laser beams are focused into the nonlinear optical crystal the seed laser beam with frequency co and a strong pump beam with cop. Due to the parametric process in the nonlinear crystal, a new wave is generated with a difference frequency coi = cop- cog, (Fig. 6.42b). Most of the pump photons split up into a signal and an idler photon where the signal photons superimpose the seed photons and amplify the weak seed beam. 

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P Baum, E. Riedle, M. Greve, H.R. Telle, Phase-locked ultrashort pulse trains at separate and independently tunable wavelengths. Opt. Lett. 30, 2028 (2005)... 

Impressive progress has been achieved with femtosecond optical parametric amplifiers, which can be used as ultrashort pulse generators with wavelengths tunable over a wide spectral range. They will be discussed in Vol. 2, Chap. 6. 

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