Cuantitativos

The output from the KML is directed into the Dazzler, as shown in fig- ure 4.1. The Dazzler is a type of acousto-optic modulator (AOM), called an acousto-optic programmable dispersive filter (AO-PDF), a device which

Pump High reflector Prism Prism Lens Fold mirror Output coupler Ti:Sapphire crystal

Figure 4.2: Schematic of the optical configuration of the Kapteyn-Murnane Laboratories (KML) Ti:sapphire Oscillator

can be used to shape the phase and amplitude profile of an ultrashort pulse [113].

The optical geometry and electrical connections to the AO-PDF are shown schematically in figure 4.3. An acoustic wave is launched into the 2.5 cm long birefringent paratellurite TeO2 crystal by a piezoelectric trans- ducer excited by an RF signal from a function generator controlled by a laptop computer. The acoustic wave travels along thez-axis of the crys- tal, reproducing the temporal shape of the generated RF signal. A 1kHz synchronization signal from the Evolution laser power supply is passed through a Stanford Research Systems Digital Delay Generator and is used to ensure that the arrival of the acoustic wave into the AO-PDF coincides with the arrival of the KML laser pulse that will be subsequently selected to seed the regenerative amplifier (see section 4.2.3). The Dazzler is not able to operate at the full 96 MHz repetition rate of the KML laser because of the acoustic wave velocity and this synchronization ensures that each pulse that is to be amplified is modified identically by the acoustic wave.

acoustic waveform acoustic wave Transducer input laser pulse modified laser pulse TeO crystal2 Sync signal (1 KHz) Laptop Function Generator

Figure 4.3: The acoustic wave is launched into a TeO2 crystal by a trans-

ducer from an RF function generator controlled by a laptop computer. The crystal is cut such that the acoustic wave reflects from the end face and co- propagates with the input optical wave. The acoustic and optical waves are carefully synchronized to a 1kHz reference signal. Adapted from refer- ence [114].

the crystal has polarization parallel to that axis, with a propagation direc- tion collinear with the propagation direction of the acoustic wave. The op- tical wave can interact with the acoustic wave, causing an optical wave to be coupled into the extraordinary axis with polarization parallel to the ex- traordinary axis (figure 4.4). For a specific frequency of the acoustic signal, phase-matching between the acoustic and optical signal can result in a cou- pling of the ordinary and extraordinary axes. At any one spatial frequency, in the acoustic signal, only one optical frequency can be diffracted at a po- sitionz. Each optical frequencyω travels a distancezbefore encountering the phase matched frequency and being diffracted into the extraordinary mode. Since the refractive index of each of these modes is different, each frequency will see time delay with respect to one another. The relative am- plitude of the output pulse depends on the acoustic intensity at the position

z(ω)where it is diffracted.

acoustic wave

fast acoustic axis (mode 1)

slow extraordinary axis (mode 2)

chirped pulse

compressed pulse

Figure 4.4: The individual frequency components of an input pulse orig- inally incident along the ordinary axis of the crystal may be selectively diffracted into the extraordinary axis at a position along the crystal axis. In this way, the path length of each frequency component in the ordinary and extraordinary axes, which have different refractive indices, can be con- trolled. (Based on reference [113].)

the stretcher matches well the negative dispersion of the compressor, other optical components in the amplifier introduce GVD, higher order disper- sion and nonlinear dispersion which must be compensated for. While it is possible to introduce prism pairs or dispersion compensating mirrors that will do this, they are not programmable and are limited to the first or- ders [114]. By using an AO-PDF, such as the Dazzler, we are able to address two key limitations of the CPA systems. We are able to correct for the GVD introduced by optical elements in the beam path and also to correct for any high order (third order or greater) dispersion introduced by the stretcher and compressor.

The Dazzler can make arbitrary programmable changes to the spectrum and phase of the pulse, for example, we could use it to create a hole in the centre of the spectrum (figure 4.5) which might be useful to overcome non-uniform narrowing of the amplified gain medium which can lead to a narrowing of the spectral bandwidth. It is possible to use the AO-PDF to correct for this by programming a filter where the transmission at the

740

760

780

800

820

840

wavelength / nm

Figure 4.5: The KML laser spectrum before (dashed) and after (solid) the Dazzler has been programmed to give a spectral hole at 787 nm (FWHM 1.6 nm).

wings is greater than in the center.

In this way, the Dazzler can be used to produce an ideal pulse shape, or more frequently, can be used to optimize the pulse for a particular appli- cation, that is, optimization to produce high order harmonics. In chapter 5 we will look at the way we have used the Dazzler to frequency shift the high harmonics.