Generation of High-Power Ultrashort Pulses

Even shorter pulses below 1 ps can be achieved, if the phase-front of the pump pulse is tilted by reflection from a grating (Fig. 11.27b). In the dye cell now an interference pattern is produced with a travelling intensity distribution. This causes by constructive interference of the Bragg-reflected partial waves at Ajj in the dye cell a travelling-wave packet, which propagates synchroneously with the interference pattern if the angle 6 is chosen properly. 

Amplification of such short DFDL pulses in excimer laser pumped amplifer cells yields intense pulses with peak powers above 10 y and pulse durations down to 30 fs [11.82]. Optical frequency-doubled dye-laser pulses with a wavelength matched to the gain profile of the excimer laser can also be amplified in excimer-gas discharges [11.79]. 

A serious limitation is the low repetition rate of most pump lasers used for the amplifier chain. Although the input pulse rate of the pico- or femtosecond pulses from mode-locked lasers may be many megahertz, most solid-state lasers used for pumping only allow repetition rates below 1 kHz. Copper-vapor lasers can be operated up to 20 kHz. Recently, a multi-kilohertz 

Ti Al203 amplifier for high-power femtosecond pulses at A = 764nm has been reported [11.90]. 

Over the past ten years new concepts have been developed that have increased the peak power of short pulses by more than four orders of magnitude, reaching the terawatt (10 W) or even the petawatt (10 W) regime [11.89,11.90-11.95]. One of these methods is based on chirped pulse amplification, which works as follows (Fig. 11.36)  

We will now discuss the different components of this process in more detail. The oscillator consists of one of the femtosecond devices discussed previously. The pulse stretcher uses a grating pair, where the two gratings, however, are not parallel as for pulse compression, but are tilted against each other (Fig. 11.36b). This increases the path difference between the blue and the red components in the pulse and stretches the pulse length. An aberration-free pulse stretcher with two curved mirrors and a grating is described in [11.96] and is depicted in Fig. 11.37. 

The peak powers of ultrashort light pulses, which are generated by the techniques discussed in the previous section, are for many applications not high enough. Examples where higher powers are required are nonlinear optics and the generation of 

If a laser beam with intensity Iq passes through an amplifier cell of length L and with a gain coefficient —a a 0), the output intensity becomes 

With increasing intensity, saturation starts and the gain coefficient decreases to 

The higher saturation intensity h is, the larger becomes the maximum output intensity. The amplified intensity therefore depends on the incident intensity Im, the small signal gain Go, the saturated gain G and the saturation intensity h. If the amplifying medium is completely saturated, the gain drops to G = 1 and /out is limited to the maximum value 

In order to achieve a larger amplification, several amplifier stages are necessary. 

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