xit window

Entrance window

Card

z-direction _{Interaction (+) region}

Crosses’

Shadow 2 w ire crosses

HeNe laser beam

F ig u re 6.4a illustration o f the laser beam alignm ent through the interaction cham ber C. W hen the shadows o f the tw o crosses overlap as seen on the card the alignm ent with the interaction region (+) is achieved.

Lens L2 is mounted on a motorised translation stage (M-MFN25CC), also driven by the motion controller, located on a plate that allows for fine elevation adjustment. In addition, the lens L2 can be rotated about, and translated along, horizontal and vertical axes that are perpendicular to the laser beam i.e. x and y-axes. These movements are employed to align the optical axis of the lens L2 with the laser beam. Moving lens L2 along x and y directions brings the transmitted laser beam to the centre of iris 110, whereas rotating lens L2 about these axes brings the reflected laser beam from the inner surface of the lens L2 to the centre of iris 19. Therefore the principal plane of lens L2 is perpendicular to the laser beam and this ensures that the focused laser spot is not skewed.

Alignment of the translation stage with the laser beam is the most sensitive part as it is required that as the lens L2 is translated along the z-direction by the motorised translation stage the focused spot does not move in the x and y directions. To check for the stability of the laser spot in x and y directions lens L3 {f= 500 mm) focuses the laser spot in the interaction region into a far field CCD camera (FFC). The optical axis of lens L3 must also be aligned with the laser beam by centring the reflections of the inner and the outer surface, of the incoming non-focused HeNe laser beam, of L3 on iris 110. After the alignment of lens L3 mirror M6 is fixed and only mirror M7 is used to bring the image of the laser spot onto the centre of the FFC as viewed on the crossed screen of the FFC’s monitor. The mirrors (M6 and M7) are introduced in the arrangement as shown in figure 6.3a to gain flexibility in steering the laser beam. Now if the z translation of the lens L2 in not parallel to the laser beam the focused spot will shift position on the crossed screen of the FFC. Horizontal and vertical shifts are corrected by swivelling (rotating) the translation stage and by tilting

the plate that holds the translation stage respectively. However, after each of these corrections the lens L2 must be realigned before it is translated and the process is repeated till the focused spot is nearly stationary in both x and y directions. The error in these movements is estimated to be ± 5 pm, which is still much smaller than the pinhole diameter of 500 pm of the front plate.

The fused silica window reflects 4% of the laser beam that converges due to focusing by lens L2, which might itself be damaged by the image of reflection of the focused spot. Therefore, an extension is added to move the fused silica window towards lens L2. The overall extended distance of the fused silica window from the interaction region is ~ 230 mm. Using lens L2 with a focal length o f 250 mm on the translation stage whose translation span is 25 mm means that the closest distance of the lens from the fused silica window is ~ 7.5 mm and the furthest distance is 32.5 mm. These are very safe distances, as the reflected laser beam from the fused silica window images the focused spot far behind lens L2. Whereas if the distance between the fused silica window and lens L2 is half the focal length then the reflected laser beam from the fused silica window will be focused onto the lens L2 and damage it.

Once the laser beam path is defined by the irises, NFC and FFC are mounted and lenses and translation stage are aligned with the aid of the HeNe laser beam the mirror M ’ is moved back then the shutter that blocks the laser from the laser control room is opened. Goggles must be worn as long as the laser from the laser control room fires in the target area 1. The alignment o f the femtosecond laser pulses are checked and corrected with the aid of the mirrors and beam splitters. Since the wavelength o f ~ 800 nm of the laser pulses is near the infrared region they are not visible to the unaided human eye and therefore a fluorescent card that glows is used in the aligning process.

6.5 Summary

The experimental arrangements for the z-scan technique were described in detail. The experimental chambers employed were described with the gas line and pumping system. The ion extraction system of the interaction region was tested with a Simion simulation to ensure detection of all extracted ions. The principles behind the Ti.S laser system that is housed in the control room were briefly reviewed.

Components employed in the laser setup for the z-scan were listed and described. Finally procedures o f alignment of the laser beam and the optics were explained.