LiNbO 3 Pockels cell for Q-switch of Er:YAG laser

Transcription

1 LiNbO 3 Pockels cell for Q-swich of Er:YAG laser Jan Šulc* a, Helena Jelínková a, Per Koranda a, Michal Němec a, Miroslav Čech a a Czech Technical Universiy, Faculy of Nuclear Sciences and Physical Engineering, Břehová 7, 5 9 Prague, Czech Republic ABSTRACT Generaion of a gian pulse of Er:YAG laser is complicaed mainly due o he properies of he Er:YAG acive medium iself. is caused by he shor lifeime of he Er:YAG crysal upper laser level and a small gain for one pass of he radiaion hrough he acive medium. n our case, a specially designed LiNbO 3 elecroopic shuer was used for Q- swiching of Er:YAG laser. Brewser angles were employed a he LiNbO 3 crysal faces o avoid he inclusion of a polarizer ino he resonaor. Even if Er:YAG crysal emission is naurally unpolarized we have found ha polarizaion sensiive reflecions a wo Brewser-cu ends of Pockels cell are sufficien o reach an eincion raio necessary for gian pulse generaion. By help of heoreical analysis based on Jones calculus was found he dependency of Pockels cell radiaion ransmission on applied volage. Calculaed ransmission of Brewser-Brewser LiNbO 3 Pockels cell operaing in quarer-wave regime was 3% in closed sae. Theoreically and eperimenally was found, ha for 5 mm long LiNbO 3 crysal, volage. kv is sufficien for Q-swiching. The sable running Q-swiched Er:YAG laser sysem was realized wih above described Pockels cell. The generaed gian pulse lengh and energy was 7 ns and 6 mj, respecively. Keywords: solid sae lasers, Er:YAG, Q-swiching, LiNbO 3 Pockels cell.. NTRODUCTON Generaion of a gian pulse of Er:YAG laser has many complicaions mainly due o properies of he Er:YAG acive medium iself. is caused by he shor lifeime of he upper laser level of Er:YAG crysal ( ~ µs) and a small gain for one pass of he radiaion hrough he acive medium. Oher difficulies follow from radiaion absorpion of his wavelengh (.9 µm) in he rouine opical componens. The echniques so far used for Q-swiching of Er:YAG laser are: mechanical swiching by roaing mirror [,, 3], elecro-opic Q-swiching wih Pockels or Kerr cell [, 3,, 5], acouso-opic Q-swiching [6], a echnique which using waer as saurable absorber [7], or oal inernal reflecion FTR modulaor [8]. Considering ha he elecro-opic Q-swiching has many advanages in comparison o oher mehods, we concenrae on his echnique and eend our previous invesigaions [3]. The main problem in using of he elecro-opic Q-swich for Er:YAG laser is he realiy ha he generaed radiaion is unpolarized and up o now any ideal polarizer for he wavelengh round 3 µm did no eis. For ha reason we decided o use he acive Pockels cell crysal wih Brewser angle cu faces, which serve as parial polarizers in he boh ends of acive crysal. The quesion appeared if one or wo ends of Pockels cell mus be cu under Brewser angle. The goal of our work was a heoreical analysis of boh above menioned cases and eperimenal verificaion of our hypohesis ha LiNbO 3 elecro-opic shuer wih wo Brewser cu ends can serves as Pockels cell and polarizer simulaneously and i gives us possibiliy of using lower quarer volage as i is used regularly in λ/ arrangemen.. ELECTRO-OPTCS EFFECT N LiNbO 3 For he descripion of he radiaion ransmission hrough Pockels cell a Jones calculus (mari) and Fresnel s relaions can be used [9,]. For his purpose he opical elemen of our assumed Pockels cell can be divide on hree pars: inpu par wih Brewser face (B - Fig.) (for he simpliciy we suppose ha a second boundary face of his elemen has anireflecion coaing (AR)), a phase reardaor (EO - Fig.) conrolled by ouer elecric field (his face are supposed also wih he AR) and las par consiss of Brewser oupu face (B - Fig.) and also AR boundary. *sulc@roja.fjfi.cvu.cz; phone: ; fa:

2 Fig.. LiNbO 3 crysal wih Brewser angle cu faces, radiaion ransmission and basic elemens spliing (B, EO)... Descripion of radiaion single passage hrough Pockels cell Jones vecors for horizonal "p" and verical"s" linearly polarized radiaion are and, respecively. Radiaion of "p" polarizaion pass hrough he inpu (and also oupu) par wih Brewser face wihou any losses, herefore Jones vecor for radiaion of "p" polarizaion passed hrough he pars wih Brewser face is. Transmission of verically polarized radiaion hrough he inerface of medium wih differen indices of refracion n and n, respecively, can be evaluaed from Fresnel s relaions for radiaion inensiy ransmiance: T s n cosθ = s, () ncosθ where s is radiaion ransmiance of ampliude: s = n + n () For a special case of par wih Brewser face i follows: ( ) cosθ = cos 9 θ = cos9 cosθ + sin9 sinθ = sinθ (3) sinθ n anθ cosθ = = () n and herefore he radiaion inensiy ransmiance is evaluaed o be: T s n = n s (5) Jones vecor of verically polarized radiaion passed hrough he par wih Brewser face can be deermined: (6) where is calculaed o be = T. s The Jones mari for elecro-opical acive par of Pockels cell (phase rearder roaed of 5 in y ais sysem and driven by eernal elecric field) is: (7)

3 EO ( ) ( ) T R T R ep( i) = 5 5 = and afer simple mahemaical ransformaion can be wrien ( is phase reardaion): T EO cos i sin = isin cos i ( ) e (8) (9) Phase reardaion value for invesigaed Pockels cell was calculaed o be: n r λ 3 π. o. L.. U d = () n indices of refracion for LiNbO 3 crysal and radiaion λ =.9 µm is n =.68 r elecro-opic coefficien for LiNbO 3 is r = 3. m/ V L lengh of he LiNbO 3 crysal is L = 6 mm λ ransmied radiaion wavelengh for Er:YAG laser is λ =.9 µm () d disance of Pockels cell elecrodes is d = 7.5 mm U volage applied on Pockels cell (up o 3.5 kv was invesigaed) The half-wave volage U π (caused he half-wave reardaion = π) for eamined Pockels cell was calculaed o be U π = 9 V and eac volage for λ/ arrangemen of elecro-opic Q-swiching was U λ/ = 67 V. Combining he Jones vecor of radiaion wih various polarizaion passed hrough he par wih Brewser face and Jones mari of elecro-opic par (phase rearder) described above we obained he Jones vecor for radiaion ampliude afer one passage of he radiaion hrough he Pockels cell: a) horizonally polarized "p" radiaion in he inpu of Pockels cell: B EO cos isin B cos i sin () b) verically polarized "s" radiaion in he inpu of Pockels cell: B EO i sin cos B i sin cos (3) and radiaion inensiy can be formulaed in erms of summary vecors : cos = sin P or sin = cos for horizonally and verically polarized radiaion in he inpu, resp. ().. Descripion of radiaion double passage hrough Pockels cell Radiaion passes he Pockels cell wo imes in he same way in each resonaor circulaion because of oal reflecion of radiaion in rear mirror. The radiaion ransmission hrough he invesigaed Pockels cell afer double passage of he

4 radiaion can be derived in he same way as in () and (3). The final radiaion inensiy can be also formulaed in erms of summary vecors as in (): cos sin = ( + ) sin P or ( + ) sin = cos sin for horizonally and verically polarized radiaion in he inpu, respecively. The firs elemen in summary vecors () and (5) is relaed o inensiy of horizonally polarized radiaion, while he second elemen in summary vecors is relaed o inensiy of verically polarized radiaion. means ha inensiy of oupu radiaions can be summarized in following ables: (5) Polarizaion of inpu radiaion (horizonal) y (verical) + y E (horizonal) cos / sin / cos / + sin / E y (verical) sin / cos / sin / + cos / Table : Transmission of Pockels cell wih Brewser angle cu faces for one passage. Polarizaion of inpu radiaion (horizonal) y (verical) + y E (horizonal) cos sin ( + ) sin cos sin + ( + ) sin E y (verical) ( + ) sin cos sin ( ) + sin + cos sin Table : Transmission of Pockels cell wih Brewser angle cu faces for double passage and oal reflecion from rear resonaor mirror. From obained relaion follows ha afer passage of linear polarized radiaion hrough Pockels cell wih Brewser angle cu faces, he oupu radiaion will be generally an ellipical polarized ligh. Only in a special case, when Brewser angle would work like real polarizer, han in he oupu would be he horizonal polarized radiaion (of course depending on phase shif ). n his case he Pockels cell would work like classic Pockels cell in λ/ arrangemen. The ransmissions of he radiaion hrough he Pockels cell wih Brewser angle cu faces afer double passage hrough Pockels cell and oal reflecion from rear resonaor mirror are illusraed in Fig., Fig.3 and radiaion inensiies are summarized in Table. From illusraed graphs i is seen ha Pockels cell will no be compleely closed for coming radiaion. is also eviden ha minimum losses for ransmiing beam eis for open sae of Pockels cell, i means for ( = kπ, k=,,,...) and for he horizonal polarizaion E in he inpu. The ransmission for verical polarizaion is order only. n he case of Pockels cell from LiNbO 3 he ransmission is ~ %.

5 Fig. Double passage of horizonally polarized radiaion hrough Pockels cell, inensiy ransmiance. Fig.3 Double passage of verically polarized radiaion hrough Pockels cell, inensiy ransmiance..3. Pockels cell wih one Brewser angle cu end One of he desired resul of our work was o evaluae wheher Pockels cell crysal wih boh Brewser angle cu faces is suiable for elecro-opic Q-swiching in comparison wih Pockels cell wih only one Brewser angle cu end. nensiy ransmiance of his heoreical Pockels cell crysal can be derived in he same way as in secion.. nensiy ransmiances afer radiaion double passage hrough Pockels cell wih only one Brewser angle cu face and oal reflecion from rear resonaor mirror for various polarizaion of he inpu radiaion are calculaed in Table 3 and illusraed in following graphs (Fig. and Fig.5). From ransmission marices and corresponding graphs i is seen ha he ransmission afer double passage hrough Pockels cell wih only he one Brewser angle is he same as he ransmission afer one passage hrough Pockels cell wih wo Brewser angle. The difference is only in applied volage (for Pockels cell wih he one Brewser end he volage is half). This is quie equivalen o λ/ and λ/ arrangemen. However, for double passage, he Pockels cell wih wo Brewser angle ehibi essenially beer heoreic record (Fig. and Fig.3): Polarizaion of inpu radiaion (horizonal) y (verical) + y E (horizonal) cos sin cos + sin E y (verical) sin cos sin + cos Table 3: Transmission of Pockels cell wih only one Brewser angle cu face for double passage and oal reflecion from rear resonaor mirror. Fig. Double passage of horizonally polarized radiaion hrough Pockels cell wih only one Brewser angle cu face, inensiy ransmiance. Fig.5 Double passage of verically polarized radiaion hrough Pockels cell wih only one Brewser angle cu face, inensiy ransmiance

6 3. EXPERMENT SETUP 3.. Er:YAG laser oscillaor Er:YAG laser was buil o verify he funcion of LiNbO 3 Pockels cell wih Brewser angle cu ends as elecro-opic shuer for Q-swiching. Er:YAG caviy was composed of he Er:YAG crysal wih a diameer of mm and a lengh of 9 mm. The crysal along wih a enon flashlamp was placed ino a diffused ceramic pumping LM caviy. A planparallel opical resonaor was formed by a dielecric % refleciviy mirror and a dielecric mirror wih 87 % refleciviy, which serving as an oupu coupler. Laser repeiion rae was.5 Hz. The resonaor s physical lengh was 3 cm. An elecro-opic shuer for Q-swiching was consruced on he basis of ransverse linear Pockels effec. As a nonlinear medium LiNbO 3 crysal (cross-secion 78 mm, lengh 6 mm) wih Brewser angle cu ends was used. This crysal wih elecrodes and a housing, on which he elecronic swiching circui was mouned, was placed inside he opical resonaor of Er:YAG laser beween he rear mirror and Er:YAG acive crysal (Fig.6). Sysem was working on λ/ arrangemen (Brewser angle cu faces serve as polarizers), i.e. polarizaion plane of laser beam was urned abou 9, when beam passed wice non-linear crysal wih applied U λ/ volage on is elecrodes. When populaion inversion inside acive maerial reached maimum value, specially designed delay circui provided swiching Pockels cell. Swiching ime was less hen ns. During his ime applied volage on non-linear crysal decreased. Therefore polarizaion of passing laser beam was no changed and minimum horizonal polarizaion radiaion losses were reached, hereby gian pulse from resonaor could be generaed. The sysem was invesigaed from he poin of view of generaed gian pulse energy and lengh. Dependence of he oupu gian pulse parameers on he inpu parameers (high volage applied on Pockels cell, pumping energy, delay beween swiching of flashlamp and Q-swich circui) was invesigaed. Fig.6 Schemaic layou of Er:YAG laser oscillaor. 3.. Measuring insrumens For measuremen he oupu laser radiaion energy a compuer-operaed wo channel Molecron JD Joulemeer/ Raiomeer wih hermal deecors Gen-Tec (ED- LA (.8 V/J)) and Molecron (J5 (8.59 V/J)) was used. To invesigae he lengh of generaed gian pulse a wo channel Tekroni (TDS 35, 5 MHz) oscilloscope wih he pyroelecric probe Molecron (P5-) was uilized. High volage, applied on he Pockels cell, was moniored wih high volage probe Tekroni (P65A) and Tekroni oscilloscope. Fig.7 Schemaic layou of Q-swiched Er:YAG laser arrangemen and measuremen.

7 .. Er:YAG laser sysem in free-running regime. RESULTS Consruced Er:YAG laser oscillaor was a firs sep checked in free running regime. When LiNbO 3 crysal was no inside he resonaor, he maimum generaed energy was ~ 5 mj and he lengh of pulse was measured o be 5 µs (FWHM). LiNbO 3 Pockels cell wih Brewser angle cu ends brough inheren passive losses for verical polarizaion radiaion inside he resonaor and herefore he laser reshhold was rised and slope-efficiency was lowered. Consequenly he maimum of generaed energy in free running pulse was 38 mj. The lengh of flashlamp pulse was measured o be 57 µs (FWHM)... Er:YAG laser sysem in Q-swiching regime The opimum parameers for Q-swiched Er:YAG laser sysem signify only one gian generaed laser pulse wih maimum energy and minimum lengh. Simulaneously minimum applied high volage value sufficien o closing Pockels cell was required. For Q-swiching operaion, a specially designed delay circui ensured he precise ime of Pockels cell elecronics swiching. enables o measure he eac delay beween flashlamp pulse rigger and saring poin of he Pockels cell opening. Firsly, he opimalizaion of he inpu parameers, which can have an impac on he oupu pulse characerisic, was done. To follow our heoreical calculaion, he measuremen of he generaed gian pulse energy dependency on he delay of Pockels cell swiching for various high volage value applied on he Pockels cell was carried ou. This high volage was changed from. kv up o 3.5 kv and for each value of his volage he delay of Pockels cell swiching was changed inside he inerval of 3 µs up o 6 µs. From his measuremen he maimum energy for one generaed oupu gian pulse was derived. The graph of dependency of he pulse energy on he Pockels cell swiching delay (volage applied on PC as parameer) is in Fig.8. From group of poins in his Fig.8 is seen ha he maimum generaed energy is obained for applied high volage value from. kv up o.6 kv which is in good agreemen wih our heoreical calculaions (secion.). The opimal delay value was 5 µs. For his delay 5 µs and applied high volage. kv single gian pulse wih energy mj was generaed. n case of lower delay value muliple pulses were generaed and oupu energy was higher. All hese measuremens were done for pumping energy J. Fig.8 Gian pulse energy vs. Pockels cell swiching delay for various high volage applied on Pockels cell. For opimum delay value 5 µs and applied high volage. kv he dependence of he generaed gian pulse lengh and energy on pumping energy was measured (Fig.9). Dependence of gian pulse saring poin delay afer Pockels cell swiching ime on pumping energy was also invesigaed (Fig.). Oscillogram of generaed gian pulse is in Fig..

8 Fig.9 Gian pulse energy and lengh dependence on pumping energy. Fig. Delay of gian pulse generaion vs. pumping energy. Fig. Oscillogram of gian pulse generaed wih maimum pumping energy.

9 5. DSSCUSSON This repor is based on previous sudy [], where general heoreical descripion of Pockels cell ransmission was done. From our new resuls i can be deduced ha:. General losses in enclosure sae (summary for boh polarizaions) are in he case of LiNbO 3 Pockels cell wih wo Brewser angle approimaely 6 %. is double value as compared o Pockels cell wih one Brewser angle. Cu off volage is a he same ime only lile higher han appropriae λ/ volage.. Theoreic losses in opening sae are for preferred polarizaion near zero. To achieve he same value wih he one Brewser Pockels cell, he second end of his cell could have he AR a he perpendicular end. The AR layers are always more sensiive o he damage especially in he case of he gian pulse generaion. 3. Using of wo parallel Brewser angled faces is followed by he compensaion of dispersive properies of he opical maerial used (he Pockels cell wih one Brewser face will ac like dispersive prism). Furhermore, his geomery reained he parallel direcion of he inpu and oupu laser beams. Using of Pockels cell wih he one Brewser angle would lead o consrucion of frequency and angular sensiive resonaor, whose mirror one anoher are no parallel. 6. CONCLUSON Brewser angles was employed a he Pockels cell LiNbO 3 crysal faces o avoid he inclusion of a polarizer ino he resonaor. n our sysem, his one opical elemen serves as Pockels cell and polarizer for Q-swiching simulaneously. Even if he Er:YAG crysal is isoropic and is emission is naurally unpolarized we found ha polarizaion sensiive reflecions a wo Brewser-cu ends of Pockels cell are sufficien o reach eincion raio necessary for gian pulse generaion wihou prelasing. By help of he deailed heoreical analysis based on Jones calculus was found he dependency of Pockels cell radiaion ransmission on applied volage. n closed sae, he ransmission of Brewser- Brewser LiNbO 3 Pockels cell operaing in quarer-wave regime was only 3 %. Theoreically and eperimenally was found, ha for 6 mm long LiNbO 3 crysal and Er:YAG oscillaor volage. kv is sufficien o avoid he laser oscillaion. Wih his Pockels cell a sable running Q-swiched Er:YAG laser sysem was realized. The generaed gian pulse lengh and energy was 69 ns and 6 mj, respecively. 7. ACKNOWLEDGEMENTS This research has been suppored by he Gran of he Czech Minisry of Educaion No.CEZ: JO/98: Laser Sysems and Their Applicaions. 8. REFERENCES. Wannop N.M., Dickinson M.R., Charlon A., King T.A. (99) Q-swiching he erbium-yag laser, J. Modern Op., vol., no., p Wannop N. M., Dickinson M.R., King T. A. (993). An erbium:yag oscillaor-amplifier laser sysem, Op. Commun., vol.3, pp Jelínková H., Němec M., Šulc J., Čech M., Ozolinsh M.,Er:YAG Laser Gian Pulse Generaion, Florence. Ozolinsh M., Eichler H.J., Liu B., Zhu Q. (999). PLZT Elecroopic Q-Swich for Er:Cr:YSGG Laser, Proc.SPE Vol. 3863, p Ozolinsh M., Sock K., Hibs R., Seiner R., (996). PLZT ceramics elecroopic modulaors for infrared solid sae Nd:YAG and Er:YAG lasers, Prod c, SAF96, Eas Brunswick, NJ, p., p Schnell, Osroumov V.G., Breque J., Luhy W.A.R., Weber H.P., Scherbakobv.A. (99). Acousoopic Q- swiching of Erbium lasers, EEE J.Quanum Elecron., vol.6, p Vodopyanov K. L., Kulevsky L. A., Pashinin P.P., M.Prokhorov A. (98). Waer and ehanol as bleachable absorbers of radiaion in an yrium-erbium-aluminium garne laser (λ=,9 µm), ZETF, T.8, 6, p Eichler H.J., Liu B., Khomenko S.. (996). Er:YAG-laser a.9 um Q-swiched by a FTR-shuer wih silicon oupu coupler and polarizer, Op. Ma., vol.5, no., p Smih F. G., King T. A., Opics and Phoonics: An nroducion, Wiley,.. Jelínková H., Šulc J., Koranda P., Němec M., Čech M., Jelínek M., Škoda V, (). LiNbO3 Pockels cell for Q- swich of Er:YAG, will be published Laser Phys. Le.