5.3. MANEJO DE LOS RIESGOS DEL CIANURO EN LA INDUSTRIA MINERA
5.3.1. MEDIDAS DE SEGURIDAD
f
I
o 1.50 Rep. Rate - 1Hz, 22C Spectralon pimping chamber 1.25 1.00 0 .7 5 0 .5 0 Œgi = 2.35% 0 .2 5 0.00 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 + Rod No. 1 O Rod No.2Electrical Input Energy, J pulse'*
F igure 2 .1 0 Comparison o f output energies obtained from CTHiYAG rods having differing dimensions, operated under otherwise identical pump configuration.
Although further work may reveal the relative importance o f the factors^^ described in (i) - (v) ab ove, it w as acceptab le, for the aim s o f this study, to co n clu d e that
further attention should be directed tow ards the longer, thinner crystals due to a higher laser efficiency and an expected improvement to beam quality.
2 .2 .4 R esonator optim isation
The geometry o f the resonator determines the transverse mode structure o f the beam. The effects can be observed as changes to the transverse spatial profile o f the beam in the near field (w ithin the R ayleigh R ange), m easured w ith scanning apertures or CCD camera systems. In the far field the profile is changed too, indicating a change in the divergence properties o f the beam emerging from the resonator. The geometry o f a laser resonator is characterised by the g-param eters w hich , for a resonator containing a thin lens o f focal length f, is given by*^ g^ = 1-L/R^-d^/f, where the subscripts can be 1 or 2 but n ^ m and identify the com ponents. Here, L is the total physical length o f the resonator and d^ is the distance between the rod centre and the mirror having a radius o f curvature R^.
It is reasonable to assume that this equation holds when the focal length o f the rod is long compared to the resonator length and then the critical dim ensions are referenced to the centre o f the rod, as shown in Figure 2 .1 1 .
F ocal Length f
Radius R1 Radius R 2
L
Figure 2.11 Geometry o f a stable laser resonator containing an intra-cavity lens.
T he resonator is critically stable when the product gjg, = 1, representing the set o f conditions when only the lowest order transverse mode is supported in the resonator. For gjg^ > 1, the resonator is unstable; for gjg^ < 1, the resonator is stable and the laser w ill support a number o f transverse modes. The stability regions are displayed graphically in Figure 2 .1 2 .
1
4 2 O -2 - 4 g 1 g 2 = 1 Stability Region - 4 - 2 0 2 4 g parameter,Figure 2 .1 2 Stability diagram for laser resonators.
Changes in the lensing within the resonator cause changes in the value o f gjg^ which cause its locus to progress around the diagram. The resonator may becom e m ore or le ss stable as a result o f changes in the thermal len sin g . T his e ffe c t is apparent from changes in the position o f gjg^ on the Figure. The efficien cy with w h ich stored en erg y is extracted from a la ser rod d ep en d s on the num ber o f transverse modes oscillating in the resonator. Geometries w ell within the boundaries o f stability, where a greater number o f transverse modes are able to oscillate, w ill, therefore, extract more energy than less stable versions.
T he e f f e c t o f ch a n g es in the reson ator g eo m etry , and co n se q u e n tly the stability co n d itio n s, w ere in vestigated for resonators con tain in g 3"x5mm<f> and 4"x4mm(^ rods (Nos. 1 and 4). Both rods were pumped by a single xenon flashlamp although, due to a v ailab ility o f com ponents the 3"x5mm</> rod w as operated in a ceramic pumping chamber while a BaSO^ chamber was used with the 4"x4mm(^ rod. A resonator was formed between an 80% reflecting plane output coupler and a 5m radius o f curvature rear mirror. The pumping chamber was situated so that the centre o f th e rod la y ab o u t 2 0 0 m m and 2 5 5 m m from the o u tp u t and rear o p t ic s , respectively. The stability conditions were varied by changing the amount o f heat input to the crystal so as to change the amount o f rod lensing which occurred. To achieve this, the pulse energy was kept constant while the pump pulse repetition rate was varied betw een IH z and 6H z. T he output energy w as m onitored, using the
calibrated calorimeter to determine the effect o f the changing resonator geometry oh the efficiency with which the transverse modes extracted energy stored in the rod.
F ig u r e 2 .1 3 sh o w s that th ere is a n o tic e a b le ch a n g e in the la ser ou tp ut energies for both crystals, over the range o f pulse repetition rates. At an electrical input o f 102 J p u lse'\ the resonator containing the 4mm diameter rod is observed to reach a maximum output energy at 4H z, being 18% greater than the energy output obtained at IHz, Figure 2.13 (a).
E lectrical D ischarge Energy 1.00
1
1 0 2 J 0 .7 5I
B ' 0 .5 0 O J 0 .2 5 8 3 J Rod No. 2 BaSO^ Pumping Chamber 0.00 O 1 2 3 4 5 6 7 Repetition Rate, Hz (a) 0 .7 0 0 .6 0 0 .5 0 A 0 .4 0 00I
I
0 .3 0 Rod No. 4 Ceramic Pumping Chamber 0.00 O 1 2 3 4 5 6 7 Electrical D ischarge Energy + 1 0 2 J o 8 3 J Repetition Rate, Hz (b)F ig u re 2 ,1 3 E ffect o f repetition rate on laser output energy at two different pump p u lse le v e ls in a 455m m lo n g la ser reson ator c o n ta in in g a) 4" x4m m d ia m eter CTHiYAG rod and b) a 3"x5mm diameter CTH:YAG rod.
In contrast, the resonator containing the 5min diameter rod starts at a maximum at I H z ., thereafter reducing its output until, at 6H z and at m axim um pum ping, the energy is only about 75% o f its level at IHz, Figure 2 .13 (b).
These facts imply a greater change in the lensing o f the 5mm diameter crystal over an eq u ivalent pum ping range, in keeping with the practical observations o f Hamlin et aF^ and Becker et al^^, but, again, in contradiction with the theoretical lin ea r re la tio n sh ip b etw een the fo ca l len gth and the rod rad iu s. H o w ev er, this discrepancy with theory can be explained by the fact that the theoretical approach (Appendix 4) does not consider the way in which the absorbed fraction o f energy changes with rod radius, nor can the theory take account o f the different transfer e ffic ie n c y o f each pum ping chamber type. U nfortunately it w as not p o ssib le to com pare the perform ance o f the two rods in id en tical pum ping cham bers w hich w ould have elim inated the second o f th ese factors. H ow ever, the work o f both H am lin et al and Becker et al show s that, under circu m stan ces w here identical pumping chamber materials are used, larger rods are still found to lens more strongly than smaller diameter rods, in keeping with the indications from this work.
The strong dependence o f the laser output energy on the pulse repetition rate has been reported e lse w h e r e by others^^’^^’^^’^^. H o w ev er , in cr e a sin g therm al population o f the lower laser level rather than thermally induced lensing has been cited as the cause o f the reduction by some^^. To determ ine the magnitude o f the effect thermal lensing had on the reduction in the laser performance, the experiment described above was repeated at fixed input energies o f 83 J pulse'* and 102 J pulse'* and at a fixed repetition rate o f 5Hz. The rear leg o f the resonator was then increased and the output energy recorded for each position o f the rear mirror. The experiment was carried out for both 3" and 4" long laser rods. However, in the case o f the latter, fo c u sin g w ith in the reson ator resu lted in d am age to the rear m irror, and the experim ent was abandoned. The results for the 3"x5mmm<^ rod, shown in Figure 2 .1 4 , indicate that the ro ll-o ff in output energy with increasing length o f the rear resonator leg is rapid. It can also be seen in the figure that, at increasing resonator lengths, the output energy obtained for the highest pump pulse energy is less than the en ergy at lo w er drive le v e ls. This too in d icates strong therm al len sin g w hich is sufficient, at the maximum average drive level o f 509W , to produce lensing in the rod strong enough to cause the resonator to becom e unstable, but which, at the lower average drive level o f 413W, is still weak enough not to have a major effect on the efficiency o f energy extraction.
in th is ex p erim en t, the a v er a g e input p o w er is a co n sta n t, and, th er efo re , the additional heat causing this thermal population can only com e from the energy deficit resulting from changes in the stability conditions. W ithout the changes in stability there would be no energy deficit and therefore it can be concluded that changes in the resonator geom etry are the main, cases o f the reduction in output energy, and not thermal population changes.
I
}
0.60 0.50 0.40 0.30 0.20 0.10 0.00 Rod No. 4 Ceramic Pumping Chamber E lectrical D ischarge Energy + 1 0 2J o 83J 2 00 2 50 3 00 350 400 450 500Length o f Rear resonator Leg, mm
(Total length = rear leg length + 150mm)
F ig u re 2 .1 4 Variation in laser output energy with extension o f the resonator length for a resonator containing a 3"x5mm diameter rod, the pump pulse energy remaining constant.
2 .2 .5 C onclusions
A sp ects effe c tin g the perform ance o f a laser based around the CTH: YAG crystal have been investigated. Particular attention has been paid to the com ponents o f the system which are usually specified before the system is operated. Firstly, the relative performance o f different pumping chamber materials has shown that, to achieve high output effic ien cies, w hile maintaining long periods between servicing, chambers m ade from BaSO^ are superior to either Spectralon or ceram ic. T he form er w as found to degrade rapidly with use, resulting in reduced efficiency. The latter was less efficient over all operating conditions, a feature attributed to the relatively large grain s iz e w h ich is form ed during the firing p rocess and w hich results in le ss e ffic ie n t scattering o f shorter wavelengths.
P reviou s work has suggested that silvered pump cham bers may b e m ore effectiv e in transfering pump light to the CTH: YAG crystal than diffuse chambers using BaSO^^\ Recently, though, Hamlin et a P have demonstrated slope efficiencies up to 5.5% with a new design o f BaSO^ chamber, compared to the 5.1 % reported in reference [31], indicating the sensitivity o f performance ta chamber design, even when the 'reflector' material remains unchanged.
Sim ilarly, it has been shown here that the output energy at a g iv en pump le v e l, is strongly dependent on the reflectiv ity o f the output coupler o p tic. This im plies that a system has low gain and, consequently, that the differences between the slope efficiencies reported in references [31] and [36] may also be attributable to this factor. C om parisons perform ed betw een 60% , 80% and 90% reflectin g output m irrors have shown here that a 90% op tic produces the greatest laser effic ien cy , although the associated increase in intra-cavity power density increases the potential for optical damage to an unacceptable limit.
Further justification for selecting the 80% reflecting optic for further work is based on con sid ering the rod d im en sion s. Sm aller diam eter rods are known to produce higher gain due to the greater density o f energy deposition thus suggesting, from considering the behaviour o f system s with the Rigrod analysis, that a low er reflectivity optic would be nearer the optimum than for the larger diameter rods.
The laser performances o f two rods, one 3"x5mm(^, the other 4"x4mm<^, were compared under near identical operating conditions to determine which was the m ost e ffic ie n t. At IH z, the 4" long rod w as found to produce about 13% m ore e n e r g y than the 3" v e r s io n d e s p ite h a v in g a to ta l rod v o lu m e s m a lle r by approxim ately 17% than the 3" rod. Additionally, at increasing repetition rates the output energy from the resonator containing the 4"x4m m0 rod was found to increase, peaking at 4 H z. In contrast, the output from the 3" x5m m 0 rod in an identical reso n a to r g e o m etry w as found to red u ce w ith in c r ea sin g rep etitio n rate. T he dom inant cau se o f this e ffe c t w as dem onstrated to be therm ally induced len sin g , w hich, for the experim ental configuration used, appeared to be greater at a given av erage input pow er for the 5mn\<f> rod than the 4mm<^ rod. T his result was in contradiction with theoretical considerations but in keeping with the results o f others and easily explained by the variations in absorbed energy between the two systems. T h e len sin g o f both crystals was shown to a ffect the geom etry o f the resonators leading to changes in the g-parameters for each resonator in such a way as to change the energy extraction efficiency.