EFFECT OF THE RATIO OF ROD AND LAMP RADII IN CW AND Q-SWITCHED PERFORMANCE OF LAMP PUMPED ND:YAG LASER FOR ULTRA HARD MATERIAL PROCESSING

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1 Jounal of Optoelectonics and Advanced Mateials Vol. 7, No. 3, June 2005, p EFFECT OF THE ATIO OF OD AND AMP ADII IN CW AND Q-SWITCHED PEFOMANCE OF AMP PUMPED ND:YAG ASE FO UTA HAD MATEIA POCESSING S. K. Sudhee *, V. P. Mahadevan Pillai, V. U. Naya Depatment of Optoelectonics, Univesity of Keala, Kaiavattom, Thiuvananthapuam , Keala, India We discuss the fabication of a lamp pumped Q-Switched Nd:YAG lase and the effect of the atio of od and lamp adii in CW and Q-switched pefomance of lamp pumped Nd:YAG lase fo ulta had mateial pocessing. The pefomance of the lase in CW opeation with polaized and unpolaised configuations in both multimode and fundamental modes ae compaed. The expeiments ae epeated in Q-Switched opeation by changing the epetition ate fo diffeent od adii. The vaiation of slope efficiency and theshold pump powe in the above cases ae studied. (eceived Mach 3, 2005 afte evision Apil 26, 2005; accepted May 26, 2005) Keywods: Nd:YAG ase, CW opeation, Q-Switching, single elliptical cavity, Geometical tansfe function coefficient 1. Intoduction All applications of the lase in mateial pocessing ae based mainly on the local convesion of the adiation enegy into heat [1]. The impact of lase technology in the pocessing of natually gown single and poly cystalline diamonds has evolutionized the field in many espects [2,3]. In the vapoization cutting technique of ulta had mateials using Nd:YAG lase equies intensity in the ange W/cm 2. In this scheme, the focused lase beam heats up the suface to the boiling point and geneates a keyhole. In ode to get extemely high enegy density at the ulta had wok piece, the lase beam has to be focused to a vey small spot. The focused spot size depends on many factos of lase esonato. The atio of od and lamp adii is one impotant paamete which detemines the focusability of a lase beam. So it is impotant to chaacteize an optical pump system fo a ulta had mateial pocessing lase by. Side pumping of solid state lase cystals using ac lamps is an efficient technique of optical excitation. The most widely used aangement to collect and concentate adiation emitted by an ac lamp into the lase medium is an enclosue compising of a highly eflective elliptical cylinde with the lase od and pump lamp at each focus. The pump souce is usually assumed to be a cylindical adiato having a ambetian adiation patten and this implies that it appeas as a souce having constant bightness acoss its diamete when viewed fom any point. The lase cystal is usually in the fom of cylindical od. In the pesent investigation the CW and Q-switched pefomance of a lamp pumped Nd:YAG lase is chaacteized by the atio of the od and lamp adii. * Coesponding autho: sudheesk@yahoo.com

2 1594 S. K. Sudhee, V. P. Mahadevan Pillai, V. U. Naya 2. Expeimental Output Couple (80%) Modulated F signal geneato with caie signal at MHz and modulating fequency ange (1 KHz-100 KHz) Tansduce Q-Switch-Compessional mode Mode limiting Apetue [1 mm] ea Mio (100%) Diffacted ight Mode limiting apetue [1 mm] Nd:YAG od mm/ mm Scatteing medium Bewste Window 860 mm Fig. 1. Expeimental aangement of lamp pumped Q-Switched Nd:YAG lase. The expeimental aangement used in the pesent investigation is shown in Fig. 1. The expeimental aangement consists of the optical esonato, cooling system, powe supply and F dive. Nd:YAG ods of dimensions 3φ 108 mm with thee diffeent Nd doping concentation 0.7 at.% have been used fo both CW and Q-switched opeations. In ode to study the vaiation of change in od diamete 4φ 108 mm od with Nd doping concentation 0.7 at.% is also used. An inta cavity Bewste window is used fo getting polaized output. The lase od is pumped by a CW Kypton filled ac lamp having maximum electical input powe of 6 kw. The lase od and ac lamp fo pumping ae contained in a highly polished, gold-plated elliptical pump chambe. An acoustooptic Q-Switch is used fo the geneation of lase pulses with high peak powe. Two apetues ae used fo limiting the highe ode modes. A plane paallel esonato configuation [4] opeating in the stable egion of stability diagam fo the lase is developed. In ode to geneate extemely good quality TEM 00 beam which is equied fo geneating high enegy density, a low value of Fesnel numbe N is selected. The Fesnel numbe detemines the numbe of modes oscillating inside the esonato and is defined as N 2 a = (1) λ whee a is the apetue size of the inta cavity mode limiting apetue, is the esonato length and is the wavelength of lase [5]. Fo a fixed esonato length of 860 mm, if we assume a Fesnel numbe equal to unity, the apetue size can be calculated to be mm. Because of easiness in mechanical fabication and availability, an apetue size of 1mm is used though out the studies fo which the Fesnel numbe is The lase od epesents a limiting apetue fo an incoming beam. Diffaction effects at the edges of the amplifie od will give ise to Fesnel ings which can stongly distub the beam unifomity. Single elliptical cavity is used fo pumping which is vey efficient. Cooling of the od and lamp is accomplished by ciculating DI wate in flow tubes which suound the cystal and the lamp. The ac lamp emits light in a boad spectal ange containing UV, visible and I egions. The absoption bands of Nd:YAG cystal lie with a cental wavelength of 808 nm and so the unwanted UV adiation fom ac lamp has to be filteed out. If UV adiation fom the ac lamp falls continuously on the od, the optical popeties of od will be degaded which is called solaization. This will educe the life of the od. To ovecome in the pesent design, Samaium doped (10%) Pyex glass fo the od flow tube is used fo the od. Samaium doped flow tube has the popety of absobing adiation in the UV egion of boad band emission fom ac lamp and e-emit in the visible and nea infaed egion whee Nd:YAG has maximum absoption. Thus the pumping efficiency has been enhanced. It has been obseved [6,7,8] that the line spectum fom Kypton is a bette match to Nd:YAG than the line spectum of Xenon, since two of its stongest emission lines (760 nm and 811 nm) ae stongly absobed by the lase cystal. This

3 Effect of the atio of od and lamp adii in CW and Q-switched pefomance of lamp pumped 1595 fact justifies the choice of Kypton ac lamp fo the pesent studies. The Kypton ac lamp used has a cold-fill pessue of 2 atmosphees and it opeates at a maximum lamp cuent of 32 A and a voltage of 180 V. The mateial of lamp is fused silica doped with Ceium which will absob unwanted UV emission and e-emit in the nea infaed egion so that the adiation will be absobed by the Nd:YAG cystal. A mechanical shutte is used fo stopping lase oscillations tempoaily without tuning off the ac lamp. A Bewste window made of BK7 mateial with high damage theshold (20 J/cm 2 ) is used geneating the polaized lase output. An acousto-optic Q-switch is used fo pulsating the CW lase output. The Q-Switch used in the pesent study is Model QS27-3C-S (Gooch and Housego, UK) with a caie fequency of MHz which is intended fo use in linealy polaized lamp pumped Nd:YAG lases opeating at 1064 nm. The inteaction medium of Q-Switch is coated with antieflection coating (had multilaye dielectic) with eflectivity 0.2% pe suface. The damage theshold is geate than 500 MW/cm 2. The Voltage Standing Wave atio (VSW) is 1.2:1 and maximum CW dive powe is 100 W. De-ionized wate cooling is povided at the ate of 190 cc/min. In ode to study the effect of the atio of adii of od and ac lamp expeiments ae conducted using two Nd:YAG ods with diamete 3 mm and 4mm having equal length and same Nd doping concentation of 0.7 at.%. The CW lase output powe is measued at diffeent input pump powe fo ods with 3 mm and 4 mm diamete. The expeiment is epeated fo unpolaisedmultimode and polaized-tem 00 modes. The lase is opeated in the Q-Switched mode using both mm and mm ods. Q-switched lase output powe at diffeent fequencies ae measued and slope efficiency cuves ae plotted as shown in Fig esults and discussion 80 Upol-MM-3mm UPol-MM-4mm Pol-TEMoo-3mm Pol-TEMoo-4mm 70 CW output powe(w) Input electical pump powe(w) Fig. 2. Slope efficiency cuves with mm and mm ods with 0.7% doping concentation and apetue size 1mm. The slope efficiency cuves ae plotted and ae shown in Fig. 2. The slope efficiency and theshold pump powe ae calculated fom the cuves and given in Table 1. Table 1. Vaiation of slope efficiency and theshold pump powe with mm and mm ods with 0.7% doping concentation and apetue size 1mm. No mode of opeation 1 unpolaized multimode 2 Polaized- TEM 00 mode slope efficiency(%) theshold pump powe(w) mm mm mm mm

4 1596 S. K. Sudhee, V. P. Mahadevan Pillai, V. U. Naya The slope efficiency in both unpolaised- multimode and polaized- TEM 00 mode ae highe fo 3φ 108 mm od fo which =0.5 compaed to 4φ 108 mm od whee =0.67. The theshold pump powe is lowe fo 4φ 108 mm od whee as the maximum unpolaised-multimode powe is highe. This behavio is explained by consideing the geometical tansfe function coefficient η ge fo both the ods. The geometical tansfe function coefficient η ge is calculated by consideing what faction of the enegy adiated by the lamp into an angle d α is tapped by the cystal. The lamp is assumed to be a cylindical adiato having a ambetian patten. This implies that it appeas as a souce having constant bightness acoss its diamete when viewed fom any point. Integation ove all angles will give us an expession fo the above coefficient [9,10]. whee 1 η ge = α0 + Θ0 (2) π is the atio of od adius to that of lamp,α 0 is the angle measued fom the lamp axis, and Θ 0 is the angle measued fom the lase od diamete and sin Θ0 = sinα 0 It is epoted that the tansfe efficiency η ge is inceasing as the atio (3) inceases [11]. Moe unpolaised multimode powe geneated in the case of 4φ 108 mm od may be due to the highe value of η ge. The lowe polaized-tem 00 output powe in the case of 4φ 108 mm od may be due to the fact that the same mode limiting apetue (1 mm) is used fo both 4φ 108 mm and 3φ 108 mm ods. The ac lamp used is also the same in both cases. Theefoe the sepaation between od and lamp sufaces is less fo 4φ 108 mm od compaed to the othe. The pumping by ac lamp can be consideed to be side pumping by a beam with a top-hat (o flat-top) intensity pofile. As a esult the pump enegy absobed by the peipheal cylindical laye of od is moe than that by the cental potion since the doping concentation is unifom though out the volume. As the apetue size is only 1 mm, moe unpolaised-multimode powe is cut off fo selecting the polaized- TEM 00 mode which is eally geneated less in 4φ 108 mm od compaed to that in 3φ 108 mm od. This also explains the lowe theshold pump powe fo 4φ 108 mm od. Thee may be chances of highe polaized-tem 00 output powe if an apetue size lage than 1 mm is used fo selecting TEM 00 mode in 4φ 108 mm od. The vaiation of the atio of polaized-tem 00 powe to polaizedmultimode powe (η T ) with input ac lamp cuent is plotted fo 3φ 108 mm and 4φ 108 mm ods and is shown in Fig. 3. atio of polaised TEM00 powe to polaised multimode powe x108mm 4x108mm Input pump powe(w) Fig. 3. Vaiation of the atio of TEM 00 powe to polaized multimode powe(η T )with pump powe with diffeent diamete of Nd:YAG od.

5 Effect of the atio of od and lamp adii in CW and Q-switched pefomance of lamp pumped 1597 The atio η T vaies almost linealy in the case of 3φ 108 mm od whee as it fluctuates in the case of 4φ 108 mm od. These fluctuations may be due to the mismatch of size of od and apetue. The slope efficiency and theshold pump powe ae calculated and shown in Table 2. Table 2 Vaiation of slope efficiency and theshold pump powe in Q-switched opeation with mm and mm ods with 0.7% doping concentation and apetue size 1 mm. Slope Efficiency (%) Theshold pump powe(w) od od od diamete-3 mm diamete-4 mm diamete-3 mm Q-Switch fequency(khz) od diamete-4 mm Q-Switched ase Output Powe(W) mm-4KHz 4mm-4KHz 3mm-6KHz 4mm-6KHz 3mm-8KHz 4mm-8KHz 3mm-10KHz 4mm-10KHz 3mm-12KHz 4mm-12KHz Input Electical Powe(W) Fig. 4. The slope efficiency cuves in Q-switched opeation with 3φ 108 mm and 4φ 108 mm ods with 0.7% doping concentation and apetue size 1mm. The vaiation of slope efficiency and theshold pump powe ae plotted and shown in Figs. 5 and 6 espectively od Diamete-3mm od diamete-4mm Slope Efficiency Q-Switch fequency(khz) Fig. 5. Vaiation of Slope efficiency with mm and mm ods with 0.7% doping concentation and apetue size 1mm.

6 1598 S. K. Sudhee, V. P. Mahadevan Pillai, V. U. Naya 2500 od Diamete-3mm od diamete-4mm 2450 Theshold electical Pump Powe(W) Q-Switch Fequency(KHz) Fig. 6. Vaiation of theshold pump powe with mm and mm ods with 0.7% doping concentation and apetue size 1mm. It is obseved that the slope efficiency is highe fo 3φ 108 mm od compaed to the othe. This may be due to the pumping of 4φ 108 mm od in the peipheal volume of cylindical geomety and the eduction in polaized-tem 00 opeation due to the smalle size of the apetue. The slope efficiency is not vaying much with Q-Switch fequency in both the ods. The theshold pump powe is fluctuating almost in the same fashion in both the ods. Howeve it is obseved that at lowe fequencies the theshold pump powe is highe fo 3φ 108 mm od whee as fo 4φ 108 mm od it is highe at highe Q-Switch fequencies. 4. Conclusions and futue pospects It is obseved that in CW opeation, the slope efficiency in both unpolaised-multimode and polaized- TEM 00 mode ae highe fo 3φ 108 mm od compaed to 4φ 108 mm od. The theshold pump powe is lowe fo 4φ 108 mm od whee as the maximum unpolaised-multimode powe is highe. The atio η T vaies almost linealy in the case of 3φ 108 mm od whee as fluctuations ae obseved in the case of 4φ 108 mm od. It is noted that in Q-Switched opeation, the slope efficiency is highe fo 3φ 108 mm od compaed to the othe. The slope efficiency is not vaying much with Q-Switch fequency in both the ods. The theshold pump powe is fluctuating almost in the same fashion in both the ods. All these esults indicates the suitability of 3φ 108 mm Nd:YAG od fo lase system fabication fo ulta had mateial pocessing applications whee one equie good powe stability. The authos assume that the use of lase diodes at wavelength 808 nm fo pumping Nd:YAG ods can esult bette solutions fo the above application whee thee is a scope fo the studies on the effect of the atio of pump beam waist to cavity beam waist. efeences [1] G. Hezige, Feinwektechnik Meβtechnik 91, 156 (1983). [2] M. Coope, ase Technology in the diamond industy, Chapte 6, Poc. of Intenational Diamond Technical Symposium, (1991). [3] S. Davis, ase Sawing, Chapte 7, Poc. of Intenational Diamond Technical Symposium, (1991). [4] A. G. Fox, T. i, Bell Syst. Tech. J40, 453 (1961). [5]. B. Chesle, D. Maydan, J. Appl. Phys. 43, 2254 (1972). [6] B. Nowell, J. D. O Bian, IEEE J. QE-4, 291 (1968). [7] N.. Yeamans, J. E. Ceedon, ong-life high epetition ate flash tubes, Techn. ept. ECOM-3043, US. Amy Elect. Comm, Ft. Monmouth, NJ (1968). [8] B. ead, Appl. Phys. ett. 9, 342 (1966). [9] S. B. Schuldt,.. Aagad, Appl. Opt. 2, 509 (1963). [10] C. Bowness, Appl. Opt. 4, 103 (1965). [11] Yu. A. Kalinin, A. A. Mak, Opt. Tech. 37, 129 (1970).