ABUSO DEL PROCESO Y GARANTÍA DEL DEBIDO PROCESO

the visible power incident on to and the

photomultiplier monitoring the generated UV reflected off the quartz plate. The crystal is placed extracavity by inserting a suitable coumarin 102 output coupler (CRL 903-060-00) 9 which in this case has a radius of curvature

of 15m and reflects 1.3% over the range 468nm to 482nm. Although this is not the full lasing range of the dye, it is satisfactory for this purpose and was used in obtaining the original power curves of Fig 3—9, with which

comparison can be made. The mirror, designated R g , is placed between the biréfringent filter and R , and forms a tunable cavity -R^-Rg ? whose output beam then passes to the crystal (Fig 3-14b).

Providing that only Rg is moved in adjusting this shorter cavity for maximum power, the beam will pass through the crystal in exactly the same orientation as when the

crystal was inside the cavity R^ to R^. Furthermore, since the photodiode monitors the power incident on to Rg , a direct power comparison may be made between the two

arrangements.

Placing the second output coupler Rg as close as possible to Rg , within the mechanical constraints of the Z-fold, also ensures that the divergence of the beam incident on to Rg is approximately the same as when the crystal is intracavity. This is necessary because otherwise the size of the focused spot inside the crystal will be

significantly different and the UV generation will not be optimised. The region between Rg and Rg in Fig 3-14a is one where the intracavity beam should be approximately parallel [133; likewise between R^ and Rg. Thus the

insertion of a virtually plane mirror Rg should have

negligible effect on the shape of the beam reaching Rg in the second arrangement.

However, we have found that when Rg is in place, the cavity R^-Rg-Rg is rather badly optimised with regard to the position of R2 , and that unless R2 is moved, the

cavity is almost unstable and certainly does not produced very much power. Thus mirror R2 was moved slightly away

from the jet causing the beam on to Rg (and thus also on to Rg) to converge more. This means that the size of the focused spot in the crystal is now much smaller than

before. As shown earlier (Chapter 2, Sec 2-1-6-1) the SH power depends not only on the fundamental power but also on the area of the focused spot in the crystal. We would therefore expect in this case that the generated SH power will be very much less than before, even allowing for the different pumping power, unless the beam is refocused to its original spot size by replacing R2 with a mirror of

longer focal length. This would then involve altering the whole geometry of the Z-fold arrangement, introducing further difficulties into the comparison. As then

expected, the UV powers observed in the arrangement of Fig 3-14a are more than an order of magnitude less than might be expected in the optimum case. This comparison between intracavity and extracavity frequency doubling is thus inconclusive.

3-4 Optimised intracavitv frequency doubling

laser, containing a biréfringent filter but no frequency doubling elements, is typically 400mW broad band when

using a 3W pump laser. The output coupler would normally transmit around 5% so there would be a circulating power of about 8W in the cavity.

In our frequency doubled laser, the intracavity power is about 1.25W, again with a 3W pump. This much lower power is due to the rather low gain of C102 combined with the extra losses due to the extra mirrors, the crystal and the crystal cell. However, even these intracavity powers are over three times higher than those available extracavity, and thus offer more than an order of magnitude improvement in the UV powers generated.

In order to obtain higher intracavity powers, the various sources of loss must be considered. Firstly, the mirror coatings, while designed for normal incidence beams at their quoted wavelengths, are not tailored to maximum reflectance when incident angles of 15*^ or more are

involved, as in the case of the mirrors focusing into the crystal, Rg and R^. Secondly, with any crystal which cannot be temperature tuned to obtain phasematching, the crystal must be tilted off-axis for UV generation at all but a narrow band of wavelengths. Such tilting

introduces further losses and severely reduces the

intracavity power. Thus ideally, a crystal should only be used over a narrow wavelength range and further

crystals obtained for use outside this range. Thirdly, the use of an additional pair of Brewster windows on the crystal cell introduces further losses, though these are small, since unless the refractivities of the crystal and

cell windows are the same, Brewster's angle is not the same for both and a compromise must be reached. The only reasonable solution to this problem is to use an index matching fluid between the crystal and quartz. This is naturally only possible if the fluid does not attack the surface of the crystal, and, of course, it must be UV transparent. These conditions are difficult to achieve simultaneously, since most matching fluids that are safe for use on water-based crystals also degrade in

transparency under UV irradiation.

Finally, and most significantly, is the bulk optical properties of the LFM itself. We have observed

considerable scattering within the crystal and at the surfaces, and consider that this aspect of the laser is the most important one in which improvements must be made.

If the losses described can be reduced significantly, then it is reasonable to expect intracavity powers of 3W to 6W with a single tuning element and a 3W pump. If these figures are achieved, then UV powers of between 400pW and 1.6mW can be generated, and even with the 30% output

coupling used here, the smaller of these figures

represents an improvement by a factor of 50 in the best powers that can be obtained by extracavity doubling.

3-5 Single frequency operation as a ring laser.

For high resolution spectroscopic applications, single frequency operation in the UV is essential. Considerable work has been carried out on this aspect of laser

development [5,6,12] and the trend has been towards the use of travelling-wave rather than standing-wave systems.

The advantages of the travelling-wave systems are that single frequency operation is generally easier to obtain and control, and the circulating powers can be much

higher, as spatial hole burning effects are eliminated. The disadvantage is that because of additional mirrors and other optical elements, the threshold pump power is much higher.

We have set the laser into a travelling-wave configuration as shown in Fig 3-15. The only changes that are required in the conversion from a standing-wave laser are that R-j must be moved to approximately its own focal length away from the dye jet and then tilted upwards at Ry which must be at about the same height above the laser base as R g , and that Rg must reflect the beam from the crystal region not back at R^ but slightly below it to Ry. Ideally, as shown by Wagstaff et al [13] and applied to the ring laser by Wagstaff and Dunn [6], the geometry of the Z-fold is unchanged in the conversion, and the angles between the beams in the jet region should be slightly modified. In practice these changes are small enough to be ignored without serious loss of laser power, and we have not attempted any such changes to the jet region.

Ry is a plane mirror, and in our case it it identical to R g , and thus the photodiode which monitors the intracavity power can intercept either of the faint beams transmitted by these mirrors. In practice, the beam alignment in the cavity prevented the beam being seen beyond R g , and so the

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