Lithuanian Journal of Physics article / Lietuvos fizikos žurnalo straipsnis

ON THE USE OF PHOTOELASTIC EFFECT AND PLANE STRAIN OR PLANE STRESS APPROXIMATIONS FOR THE DESCRIPTION OF THERMAL LENSING
Dominykas Bričkus and Aleksandr S. Dement'ev
Center for Physical Sciences and Technology, Savanorių 231, LT-02300 Vilnius, Lithuania
E-mail: aldement@ktl.mii.lt

Received 10 May 2015; revised 19 January 2016; accepted 25 March 2016

Correct use of the photoelastic effect for the description of thermally induced refractive index change is discussed and the analytical relations between thermo-optic coefficients at zero stresses and zero strains are found for all classes of cubic crystals. These relations may be useful for the investigation of thermal effects in very promising sesquioxide class m3 laser crystals. An accepted set of elasto-optical coefficients of the YAG crystal and an alternative one found in the literature were used in numerical simulations. Significant differences in the calculated thermo-optic coefficients and induced birefringence are found using different sets of these coefficients. Misunderstandings related with the so-called photoelastic coefficients are resolved and new expressions for these coefficients are found. It is shown that the incorrect use of these coefficients for different pump beam distributions can lead to significant discrepancies for thermally induced birefringence. It is also shown that common use of the generalized thermo-optic coefficients significantly overestimates the values of optical power of thermal lenses when they are applied to the laser rods with lengths several times longer than their diameter.

Keywords: solid-state laser, thermal effect, photoelastic effect, plane strain, plane stress
PACS: 42.25.Lc, 42.55.Xi, 78.20.hb

FOTOELASTINIO EFEKTO, PLOKŠČIŲ ĮTEMPIŲ IR DEFORMACIJŲ ARTINIŲ NAUDOJIMAS APRAŠYTI ŠILUMINĮ FOKUSAVIMĄ

Dominykas Bričkus, Aleksandr S. Dementjev
Fizinių ir technologijos mokslų centro Lazerinių technologijų skyrius, Vilnius, Lietuva

Naudojant tikslias išraiškas, aprašančias pjezo optinius ir elasto optinius efektus visų klasių kubiniuose kristaluose, surasti analitiniai ryšiai tarp lūžio rodiklio temperatūrinių išvestinių esant nuliniam įtempiui ir nulinei deformacijai. Šie ryšiai gali būti naudingi tirti šiluminius efektus perspektyviuose m3 klasės tipo lazerio kristaluose. Skaičiavimuose naudojami dažnai cituojami standartinis ir literatūroje rastas alternatyvus YAG elasto optinių koeficientų rinkiniai. Parodyta, kad termo optinių koeficientų vertės randamos naudojant šiuos rinkinius ženkliai skiriasi. Rastos radialinio ir tangentinio šiluminio lūžio rodiklio pokyčio analitinės išraiškos YAG tipo mechaniškai izotropiniams kristalams, naudojant plokščių įtempių bei plokščių deformacijų artinius. Aptartos šiluminio lūžio rodiklio pokyčiui aprašyti literatūroje dažnai naudojamos vadinamosios termo optinės konstantos. Pašalinti nesusipratimai, susiję su fotoelastiniais koeficientais, ir rastos naujos jų išraiškos. Parodyta, kad šių koeficientų panaudojimas nėra toks naudingas, kaip dažnai manoma, ir kad esant skirtingiems kaupinimo pluoštams tai gali atvesti prie didelių šiluma indukuoto dvejopo lūžio skirtumų, palyginti su nuosekliai naudojamu plokščių deformacijų artiniu. Taip pat parodyta, kad šiluminio lęšio optinio stiprio vertės gali būti labai pervertinamos, kai plokščių įtempių formulės yra taikomos lazerio strypams su ilgiu, kelis kartus didesniu nei jų diametras.

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