Performance Optimisation

To optim ise the perform ance o f a laser, it is first necessary to identify the variables within the system. Next, it is necessary to determine which o f the variables are truly variable throughout the characterisation process, that is, which aspects o f the design can be varied throughout the study and which need to be fixed early on because o f the

difficulty and expense involved in altering them later on. Finally, it is essential to sp ecify what param eter is being op tim ised . Preceding w ork has concentrated on increasing the room temperature laser efficien cy and it is the aim o f this work to extend the understanding o f each o f the key variables on this aspect o f the laser performance. It is also the object o f this work to investigate the reasons why, under som e conditions, this performance cannot alw ays be obtained. Outside the scope o f the variables considered here are the dopant concentrations which, being the subject o f much work by other groups, has lead to a standard set o f dopant concentrations b ein g o ffe r e d by the m ain su p p liers o f CTH: YAG c r y s ta ls . A d d itio n a lly the in v estig a tio n s o f the e ffe c t o f pump p u lse duration b eyond the lim its o f standard c a p a c ito r d is c h a r g e c ir c u itr y w a s n o t c o n s id e r e d b e c a u s e o f th e n e c e s s a r y development o f the power supply which would be beyond the practical scope o f this work. The follow ing subsections detail the optim isation and characterisation o f the CTH:YAG laser with the 'once only' variables.

2 .2 .1 Pum ping cham ber and flashlam ps

The object o f the pumping chamber is to transfer the flashlamp pump light to the crystal in order to create a population in version w ithin the crystal. Ideally, light w h ich d o e s not co n trib u te to the p um ping p ro ce ss sh ou ld not b e tran sferred . H ow ever, attem pts to sele c tiv ely filter out unwanted pump lig h t, w ithout lo ss o f overall laser efficiency, have, so far, proven unsuccessful^^. Quarles et al have used a silvered ellipse as a pumping chamber with the rod and flashlamp lying at each o f the fo c i to ensure the m axim um light transfer. H ow ever such cham bers do not stay h ig h ly e f f ic ie n t due to tarn ish in g o f the r e fle c to r su rfa ce s w h ich o c cu rs w ith continued use. An alternative is to use a clo sely coupled chamber made from , for example, alumina ceramic which, despite being less efficient than the silvered ellipse, d o e s not d eg ra d e w ith u se and can be e x p ec te d to o p era te w ith o u t sig n s o f d ete rio r a tio n for in e x c e s s o f thirty years^*. O ther p rop rietary m a terials are becoming available, one o f which is Spectralon (SRM -99LG, Labsphere In c., North Sutton, N H , U SA ). However, there exists only sales literature on the performance o f this material and no long term testing to establish its durability.

Recently, pumping chambers in which the 'reflector' is tightly packed BaSO^ have become more widely available. The BaSO^ is protected from the cooling water, which flows around the rod, by an oval section Pyrex tube which allows the powder, behind the tube, to be as close as possible to both the rod and the flashlamp. With the ex cep tio n o f the silvered e llip se the cham bers detailed above represent a typical cro ss-sec tio n o f cham bers known as 'clo se-co u p led d iffu se' and w hich gen erally

provide a more homogeneous distribution o f pump light in the rod due to the scattering which takes place in the material.

D espite the excellent results obtained by Quarles et aP^ using the silvered pumping chamber only chambers o f a the close-coupled type were considered for use in the laser system . This was because o f the lifetim e problem s associated with the metallic surface o f the ellipse. Appendix 1 details the experimental work to determine the most efficient pumping chamber for use with the CTH:YAG crystal. The results o f this work showed that the Spectralon chamber gave approximately 10% more laser output than the BaSO^ chamber, under identical pump configurations, and that both Spectralon and BaSO^ cham bers produced over tw ice the output that could be obtained from the ceram ic chamber. However, rapid degradation o f the Spectralon m aterial under the test pum p co n d itio n s resu lted in the BaSO^ cham ber b ein g preferred for use with the subsequent tests.

For optimum energy transfer between the discharge circuit and the flashlamp, it is necessary to ca lcu la te the appropriate lam p m atching param eters using the equations derived by Emmett and Markiewicz^^. Appendix 2 shows the results o f these calculations and the design o f the flashlamp circuit used with the laser.

T he c h o ice o f gas used in the flashlam ps is determ ined, for a g iven laser crystal, by the excitation bands o f the laser ion. Chromium is added to thulium and holmium to provide strong absorption in the visib le portion o f the optical spectrum which overlaps w ell with the em ission from xenon filled flashlam ps, Figure 2 .2 . Consequently, publications in which CTH: YAG is operated in pulsed mode report on ly the use o f xenon filled flashlam ps. T he 5.1% slo p e e ffic ie n c y perform ance reported by Quarles et al^‘ was obtained with a xenon filled lam p having an arc length o f 63.5m m and an internal bore (arc diameter) o f 4mm. The gas fill pressure used was 630 Torr. M ost lamp manufacturers supply a standard fill pressure o f 4 50 Torr w h ich rep resen ts a p ra ctica l le v e l to en su re r e lia b le lam p triggering^®. Consequently, it was appropriate to investigate whether the lamp gas pressure had an effect on the overall laser efficiency. This was done by comparing the output energy obtained over a range o f pump energies with lamps identical but for the gas pressure.

A sim p le resonator w as form ed b etw een a plane output cou p ler and a 5m radius o f curvature rear mirror. T he mirror r e fle ctiv ities w ere 80% and 100% respectively. The rod (No. 2) was placed in a BaSO^ pumping chamber (IR Sources, NH, USA) located in the centre o f the resonator. Water flowed over the crystal at an input temperature o f 2 0 ° C. Energy was discharged from the capacitor bank, at a

frequency o f IH z, into a xenon filled flashlamp. The arc length o f the lamp was 92m m and the internal bore 4mm diameter. Fill pressures o f 630 Torr and 45 0 Torr were used for comparison. The resonator was aligned by adjusting the rear mirror to m a x im ise the output, m easured using a PbS p h otod iod e (G raseby Infrared L td ., N ew m arket, Suffolk) and integrating circuit. The output energy was determined by measuring the power with a calibrated calorimeter (Model 20, Laser Instrumentation, C h ertsey , E n glan d ) and the rep etitio n rate u sin g a P h ilip s P M 6 6 6 5 freq u en cy m on itor. F ig u r e 2 .5 sh o w s that, at 5 H z o p era tio n , the lam p fille d to 4 5 0 Torr p rod u ced m a rg in a lly b etter e f f ic ie n c y than the 6 3 0 Torr v e rsio n alth o u g h the difference was not larger than the error in the measurement. D ue to the availability o f the flashlamps filled to 450 Torr as a standard item, and the marginally superior laser performance obtained, all further work was conducted with lamps filled with xenon to a gas pressure o f 450 Torr.

1.20 1.00 % 3_o. Flashlamp 0 .8 0 4 5 0 ^ 0 .6 0 3 e 6

!

6 3 0 0 .4 0 0.20 Rod No. 2. 450mm long resonator 0.00 4 0 5 0 6 0 7 0 8 0 9 0 100 1 10 6 3 0 Torr

Electrical Input Energy, J pulse'*

Figure 2 .5 Laser output energy obtained from a 4 ”x4mm diameter rod excited with the pulsed emission from xenon flashlamps filled to 450 Torr and 630 Torr, the pump geometry remaining otherwise constant.

2 .2 .2 T he optim um laser output coupling reflectivity

T h e reso n a to r form ed around a la ser cry sta l in c lu d e s a h ig h ly r e fle c tin g o p tic ( R « 100%) at one end and a partially transmitting optic at the other end, through which the laser output is obtained. The reflectivity o f the output coupling optic affects the power which can be extracted from the resonator. U sing the Rigrod analysis, Siegman^* shows that the output power, at a high pump energy, is a function o f both

the front and rear mirror r eflectiv ities. To m axim ise the output, the rear mirror reflectivity should alw ays be as high as possible w hile the single pass gain o f the crystal determ ines the optimum output mirror reflectivity. In general, for low gain systems, high reflectivity optics are required to maximise the output. Figure 2 .6 (a), w hile, for high gain system s, the output is not so sensitive to the reflectivity o f the output coupler. Figure 2 .6 (b).

- = 0 .9 9 s in g le -p a s s p o w e r gain 0 .9 5 0 .9 q 0 .5 o .s q 3 a 3 O CD ) O

a) G „= 3, low gain example

a 0 -9 9 0 .9 5 0 .9 0 0 .5 0 .8 0 100 8 0 4 0 6 0 20 0 o u tp u t m irro r tr a n s m is s io n , (% )

b) Gq= 30, high gain example

F igure 2 .6 E ffect o f output coupler reflectivity on laser output - from the Rigrod Analy sis (After Siegman^*)

For exam ple, for CW Nd: YAG lasers, which operate near threshold conditions and are consequently low gain, typical output coupler reflectivities range between 80% and 98%^^. In contrast, pulse-pumped Nd: YAG lasers operate with relatively high gain and, typically, there is little difference in the output energy obtained for output mirror reflectivities between 30% and 50%*^.

Determination o f the optimum output coupler reflectivity for a given laser can proceed via a number o f routes. One is to apply the analysis derived by Koechner*^ using the data co llected from m easuring the threshold en erg ies for a num ber o f known output couplings. There exists a practical restriction on many researchers that the large number o f optics required to complete this task leads to prohibitive expense. Consequently, alternative techniques are required. An alternative would be to apply the Rigrod analysis. How ever, according to Siegm an, this would require obtaining solutions to a transcendental equation and would be beyond the practical scope o f this w o rk . A m ore com m on ap p roach , used by o t h e r s ^ i s to u se a s e le c tio n o f a v a ila b le o p tic s to d eterm in e e m p ir ic a lly the m ost su ita b le ou tp ut co u p ler. By comparing the output energy from a laser under identical operating conditions other than the output coupler reflectiv ity , it is often p o ssib le to 'best gu ess' the m ost suitable reflectivity.

An experimental resonator was constructed to determine the optimum output cou p ler reflectivity. A 3"x5mm<^ rod (N o .4 ) was housed in a BaSO^ pum ping chamber and pumped by a single xenon flashlamp at a repetition rate o f IH z. The reso n a to r w as form ed b etw een a rear m irror, h avin g a 5m c o n c a v e radius o f curvature, and a plane output coupler. Three output coupler reflectivities were available for analysis: 60% , 80% and 90% reflecting. The resonator was 455m m long with the rod lying approximately at the geom etrical centre. Water flow ed over the rod at 22°C to cool it.

The output energies obtained for each output coupler reflectivity, over a range input energies to the flashlamps, are shown in Figure 2 .7.

The highest output energies obtained for any given input energy were obtained for the 90% reflecting optic follow ed by the 80% optic and lastly the 60% optic. Figure 2 .8 shows the data from Figure 2 .7 , recast to show the changes in output energy for fixed pump energies over the range o f reflectivities. Included in the figure are data points for 0 and 100% r e fle c tiv itie s w here, although not exp erim en tally measured, the output would be zero^f

1.00 Mirror R eflectiv ity i + 9 0 % 0 .8 0

t

S 0 .6 0

I

8 0 % 0 .4 0 0

1

j 0.20 6 0 % 0.Ô0 4 0 5 0 6 0 7 0 8 0 9 0 100 110

Electrical Input Energy, J pulse*'

F igure 2 .7 Laser output energy obtained over a range o f pump pulse energies for different output coupler reflectivities. The pump configuration remaining constant.

1.00

Rod No. 4. Rep. R a te - 1Hz,

R esonator length = 455mm Electrical Input Energy

0 .8 0 1 0 1 .8 0 .6 0

Î

0 .4 0 3 o 92.1 8 2 .6 7 3 .7

Î

0.20 0.0 0^ O 20 4 0 6 0 8 0 100 Output Coupling Transmission, %

Figure 2 .8 Effect o f output coupling mirror reflectivity on laser output energy for a number o f fixed pump pulse energies.

From the general shape o f the curve linking the data points, it is easy to conclude that the system is most sim ilar to the low gain exam ples given by Siegm an, being very sensitive to the output coupler reflectivity. This result, although qualitative, is also in agreement with the results obtained by Bowman et who measured a small signal gain o f 0.1 8 cm * ' (G^ ~ 1 .2 and 1 .7 for 3" and 4" lo n g rods r e s p e c tiv e ly ) at

comparable pump energy densities.

From the experimental results, it can be concluded that the maximum output energy for a given pump energy would be obtained for output coupling optics having reflectivities o f 9 0 + 5 % . However, an increasing amount o f power circulating within the resonator as a result o f higher reflectivity mirrors can, in some cases, lead to the peak pow er dam age threshold o f the optical co atin gs b ein g ex ce ed ed , leading to catastrophic damage. Koechner shows that*^ the intensity o f circulating power. I, incident on the resonator mirrors is given by

out AI(1-R,)R,

-'A

(2)

where A is the cross-sectional area o f the rod, Rj is the reflectivity o f the output cou p lin g mirror and is the output pow er o f the laser. T hus, from the output energy and pulse duration, it is possible to determine the power densities within the c a v ity for each o f the m irror r e fle c tiv itie s . F ig u r e 2 .9 sh o w s the c a lcu la ted intracavity power density, derived from the measured output energy at the maximum pump level, for each mirror reflectivity.

200 ^ 175

â

150

i

Q 125

I

O 100 ^ 7 5

Rod # 4 . Rep JRate - 1Hz, 2 2 Resonator length = 455mm

5 0 2 5

4 0 5 0 6 0 7 0 8 0 9 0 100

Output Coupling Reflectivity, %

Figure 2 .9 Effect o f output coupling mirror reflectivity on intracavity power density.

T h e in tra -ca v ity p o w e r d en sity is show n to in c r e a se w ith r e fle c tiv ity , increasing rapidly at higher reflectivities, such that, for an increm ent o f only 10%, betw een 80% and 90% the pow er density is increased by alm ost a factor three. Although no reliable data exists on the damage threshold o f optics operated over

prolonged periods o f tim e at 2.1/xm, the 80% reflecting optic was selected for all future work so as to reduce the risk o f optical damage, while maintaining reasonable output energies.

2 .2 .3 T he effect o f rod dim ensions

The dimensions o f the rod influence a great many factors but an in-depth discussion on the re la tiv e m erits o f d iffe r e n t sized rods is not w arranted h ere. G eneral observations can be made as follows:

i) Light from the flashlamp, which travels to the walls o f the pumping chamber before being reflected towards, and absorbed by, the laser crystal, is reduced in intensity due to the im perfect reflection properties o f the pumping chamber. Consequently more efficient energy transfer can be obtained if the rod receives more o f the pump radiation directly from the flashlam p. Larger rods present larger capture a n g les, and, th erefore, are g en erally pum ped by a greater

In document Development and characterisation of holmium and erbium lasers for the ablation of biological tissue (Page 45-54)