Generation of Femtosecond Pulses

6 Time-Resolved Laser Spectroscopy active mode locking of cw lasers 

Technique Mode locker Laser Typical pulse duration Typical pulse energy 

Active mode Acousto-optic modulator Argon, cw 300 ps 10 nJ 

Colliding pulse mode locking CPM Passive mode locking and eventual synchronous pumping Ring dye laser 100 fs 1 nJ 

Kerr lens mode locking Optical Kerr effect Ti sapphire 10 fs 10-100 nJ 

In the last sections pumping allows the Recently some new pulses. The shortest At X = 600 nm this light We will now 

For this situation the time separation Ar = Tjl between the passage of successive pulses through the amplifier achieves the maximum value of one-half of the round-trip time T, This means that the amplifying medium has a maximum time to recover its inversion after it was depleted by the previous pulse. 

The total pulse intensity in the absorber where the two pulses collide is twice that of a single pulse. This means larger saturation and less absorption. Both effects lead to a maximum net gain if the two pulses collide in the absorber jet. 

At a proper choice of the amplifying gain and the absorption losses, this situation will automatically be realized in the passively mode-locked ring dye laser. It leads to an energetically favorable stable operation, which is called colliding-pulse mode (CPM) locking, and the whole system is termed a CPM laser. This mode of operation results in particularly short pulses down to 50 fs. There are several reasons for this pulse shortening  

In principle, the lower limit ATmin of the pulse width is given by the Fourier limit ATniin — ajhv, where a is a constant that depends on the time profile of the pulse (Sect. 11.2.2). The larger the spectral width of the gain profile is, the smaller ATmin becomes. In reality, however, the dispersion effects, which increase with Sv, become more and more important and pre- 

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For a long time dye solutions were the favorite gain medium for the generation of femtosecond pulses because of their broad spectral gain region. Meanwhile, different solid state gain materials have been found with very broad fluorescence band-widths, which allow, in combination with new nonlinear phenomena, the realization of light pulses down to 5 fs. 

Fig. 6.30 Experimental arrangement for the generation of femtosecond pulses by self-phase-mod-ulation with subsequent pulse compression by a grating pair [686]...

J. Heling, J. Kuhl, Generation of femtosecond pulses by traveUing-wave amplified spontaneous emission. Opt. Lett. 14, 278 (1991)... 

Fig. 1. General scheme for generation of femtosecond pulses, amplification and time-resolved spectroscopy (F filter, L lens, T test pulse, R reference pulse, S sample, C continuum).

Fig. 3.7. General scheme of the technique for studying the dynamics of the transition state I, generator of femtosecond pulses 2, optical light amplifier ...

C. P. J., Wilson, K. R., Muller, M. and Brakenhoff, G. J. (1998) Characterization of femtosecond pulses focused with high numerical aperture optics using interferometric surface-third-harmonic generation. Opt. Commun., 147, 153-156. 

A. M. Weiner, A. M. Kan an, and D. E. Leaird, High-efficiency blue generation by fi equency doubling of femtosecond pulses in a thick nonlinear crystal. Optics Letters 23(18), 1441-1443 (1998). 

D. Gunzun, Y. Li, and M. Xiao, Blue light generation in single-pass frequency doubling of femtosecond pulses in KNb03, Optics Communications 180, 367-371 (2000). 

S. Yu and A.M. Weiner, Phase-matching temperature shifts in blue generation by frequency doubling of femtosecond pulses in KNb03, Journal of the Optical Society of Americas 16(8), 1300-1304 (1999). 

LA. Begishev, M. Kalashnikov, V. Karpov, P. Nickles, H. Schonnagel, LA. Kulagin, and T. Usmanov, Limitation of second-harmonic generation of femtosecond Ti sapphire laser pulses, Journal of the Optical Society of America B 21(2), 318-322 (2004). 

Petrov, G. L, Yakovlev, V. V., and Minkovski, N. I. 2003. Near-infrared continuum generation of femtosecond and picosecond pulses in doped optical fibers. Appl. Phys. B 77 219-25. 

Although the idea behind these femtosecond experiments is simple, the experimental setup to carry out the experiments is not at all trivial. In brief, it consists of a pulsed laser source capable of generating two femtosecond pulses of various duration (at the moment the world s shortest pulse is 6 femtosecond) a high vacuum chamber where a beam of molecules is flowing, and a sophisticated detector. A start laser flash called the pump pulse intercepts an isolated molecule in a molecular jet and sets the experimental clock at zero. A second flash called the probe pulse suitably delayed with respect to the pump pulse hits the same molecule and captures a photograph of the reaction in progress at that particular instant. Like the cameras in Muybridge s experiment, a femtosecond camera takes snapshots at different delay times to... 

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

Vaidmanis J A, Fork R L and Gordon J P 1985 Generation of optical pulses as short as 27 femtoseconds directly from a laser balancing self phase modulation, group velocity dispersion, saturable absorption and saturable gain Opt. Lett. 10 131-3... 

For more experimental details and special experimental setups for the generation of femtosecond and attosecond pulses, the reader is referred to the literature [749-751, 753-755],... 

G. Sucha, D.S. Chenla, Rhohertz-rate continuum generation by amphfication of femtosecond pulses near 1.5 pm. Opt. Lett. 16,1177 (1991)... 

This positive development is partly based on new experimental techniques, such as improvements of existing lasers and the invention of new laser types, the realization of optical parametric oscillators and amplifiers in the femtosecond range, the generation of attosecond pulses, the revolution in the measurements of absolute optical frequencies and phases of optical waves using the optical Ifequency comb, or the different methods developed for the generation of Bose-Einstein condensates of atoms and molecules and the demonstration of atom lasers as a particle equivalent to photon lasers. 

Generation, Storage, and Detection of Small Mass-Selected Silver Molecules and Clusters. In addition to the source of femtosecond pulses for the NeNePo investigations, negatively charged molecules and clusters (here silver) are necessary. The vacuum system is described in detail in [271]. 

J.A. Valdmanis, R.L. Fork, and J.P. Gordon, Generation of Optical Pulses as Short as 27 Femtoseconds Directly from Laser Balancing Self-Phcise Modulation, Group-Velocity Dispersion, Saturable Absorption, and Saturable Gain , Opt. Lett. 10, 131 (1985). 

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