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A new method for measuring electro-optical coefficients of the optically active crystals
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Optics & Laser Technology 36 (2004) 591 – 593 www.elsevier.com/locate/optlastec
A new method for measuring electro-optical coe(cients of the optically active crystals Xin Yin ∗ , Jiyang Wang, Huai-jin Zhang, Shao jun Zhang, hai-kuai Kong National Key Laboratory of Crystal Materials, Department of Materials, Shandong University, Jinan 250100, People’s Republic of China Received 8 August 2003; received in revised form 22 December 2003; accepted 5 January 2004
Abstract A new method has been proposed to measure the electro-optical coe(cients of the crystals which have optical activity. By using this method, the half-wave voltage and the electro-optical coe(cients of the optically active crystals La3 Ga5 SiO14 (LGS), La3 Ga5::5 Nb0:5 O14 (LGN), La3 Ga5:5 Ta0:5 Si14 (LGT) have been determined. The resultants obtained are: 11 (LGS) = 1:9 pm=V, 11 (LGN) = 0:75 pm=V, 11 (LGT) = 0:57 pm=V. This method can also be used to simulate the on–o@ state of the electro-optical Q switch in the laser cavity, whether the switch is made of the ordinary crystals or the optically active crystals. ? 2004 Elsevier Ltd. All rights reserved. PACS: 42.62.Ky; 42.55.Rz Keywords: Electro-optic coe(cient; Optical activity; La3 Ga5 SiO14 (LGS); La3 Ga5::5 Nb0:5 O14 (LGN); La3 Ga5:5 Ta0:5 Si14 (LGT)
1. Introduction According to the conditional point of view, it is complicated to make electro-optic devices by using the crystals which are optical activity. For example, “Crystals of quartz are optically active, and this complicates use of the material as an electro-optic modulator” [1]. Recently, the practical electro-optic Q switch has been made in our laboratory by using La3 Ga5 SiO14 (LGS) crystal which has an optical activity. In this paper, we report a new method for measuring the electro-optic coe(cients of the optically active crystals.
back by an all-reMection mirror and propagates through the optically active crystal again, the polarization plane rotates by an angle − around the wave vector −k. Thus the total rotated angle of the polarization plane is zero, while the polarized light travels through the optically active crystal back and forth for two times. The polarization direction of the analyzer is adjusted to be perpendicular to that of the polarizer. When the half-wave voltage V is applied to the sample, the signal of the optical intensity varies from minimum to maximum. V is the voltage which causes the phase di@erence of the two beams to change by =2.
2. Experiment setup
3. Measurement principle and measurement results
The experiment setup for measuring the electro-optic coe(cients of the optically active crystals is shown in Fig. 1. A polarized He–Ne laser is divided into two beams by a beam splitter. One beam is reMected out by the beam splitter. The other propagates through the beam-splitter and the optically active crystal, then the polarization plane rotates by an angle of around the wave vector k. After it is reMected ∗ Corresponding author. Tel.: +86-531-856-5174; fax: +86-531856-5403. E-mail address: [email protected] (X. Yin).
0030-3992/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.01.011
LGS, LGN, LGT crystals belongs to 32 point group [2,3]. Each has two electro-optic coe(cients [2,3], 11 = −21 = −62 ; 41 = −52 . The half-voltage V can be measured by applying an electric Peld parallel to the x- or y-axis, and by propagating light along the z-axis. The electro-optic coe(cient 11 can be calculated from [4] 11 =
2n3O V (2l=d)
;
(1)
where ; nO ; l and d are the wavelength of the He–Ne laser, the refractive index of the O light, the length along the
592
X. Yin et al. / Optics & Laser Technology 36 (2004) 591 – 593
4. LGS electro-optic Q switch
High voltage Light beam-splitter
source
He-Ne laser All-reflection
Polarizer
Sample
Analizer
Galvanometer Photodiod
Fig. 1. The experiment setup for measuring the electro-optic coe(cients of the optically active crystals.
Table 1 Electro-optic coe(cients 11 of LGS, LGT and LGN crystals
Crystal
Dimensions of the sample x × y × z (mm3 )
V (l=d = 1:1) (kV)
11 (pm/V)
LGS LGN LGT
6:2 × 6:2 × 40:3 4:5 × 8:4 × 42:4 4:3 × 10:4 × 39:1
24.7 56.6 76.0
1.9 0.75 0.57
optical path and the thickness in the direction of the electric Peld, respectively. In determining the electro-optic coe(cients of LGS, LGN and LGT crystal, a pair of the z faces of the sample is polished, and the x surfaces are deposited with an Au Plm as electrodes. The half-voltage V and the electro-optic coe(cient 11 of every crystal obtained are listed in Table 1. For wavelength 0:6328 m of the He–Ne laser, the refractive indices of O light are: nO (LGS) = 1:9116; nO (LGN) = 1:9821 and nO (LGT) = 1:9672, respectively [5]. Another electro-optic coe(cient 41 of LGS crystal has been determined by the interferrometric method [6]. The result obtained is 1:8 pm=V. We also tried to measure the electro-optic coe(cient 11 of Sr 3 TaGa3 Si2 O14 (STGS) crystal [7]. More than 4 kV voltage was applied to the sample with dimensions of 4:5 × 9:4 × 34:8 mm3 along x-, y- and z-axis, respectively. Unfortunately, the electro-optic e@ect has not been observed. 11 of STGS crystal may approximately be zero. It must be pointed out that the electro-optic coe(cients measured by using this method are di@erent from those reported in Ref. [5]. Because this setup can be used to simulate the on–o@ state of the electro-optical Q switch in the laser cavity, the results obtained are more reliable for practical application than others. From Table 1, it can be seen that only the electro-optic coe(cient of LGS is large enough for using the traverses electro-optic e@ect to designe the Q switch. LGN and LGT cannot be used to fabricate the practical Q switch, because the electro-optic coe(cients are too small.
LGS single crystal can be conPgured as a Pockels cell placed in the laser cavity. A schematic diagram of the laser cavity is shown in Fig. 2. As mentioned above, LGS single crystal is optically active in the direction of the optical axis. When a polarized wave in the laser cavity propagates through the Pockels cell back and forth for two times, the total rotated angle of the polarization plane is zero. Thus LGS single crystal can be placed in the laser cavity as a electro-optic Q switch, as those crystals which have no optical activity. A block of LGS single crystal was cut and polished to form a Pockels cell with the dimensions of 10 × 10 × 42 mm3 (x; y; z). The Pockels cell was placed in a Nd 3+ YAG laser cavity. When the voltage applied was 3750 V, the Q switch is e@ective with a pulse output energy of 350 mJ and a pulse width of 7:8 ns at repetition rates of 1, 10 , 20 times=s. The spot burnt by the laser pulse is shown in Fig. 3. The optical threshold of LGS single crystal has been determined using an Nd 3+ : YAG laser. Although it is less than that of DKDP(about 13 ), it is about 10 times of that of LiNbO3 . Thus LGS single crystal can be conPgured as an electro-optical Q switch by using the horizontal electro-optic e@ect. The Q switch is particularly suitable for use in lasers
Fig. 2. Schematic diagram of the laser cavity.
Fig. 3. The spot burnt by the laser pulse with LGS electro-optic Q switch.
X. Yin et al. / Optics & Laser Technology 36 (2004) 591 – 593
of intermediate power. It may partially replace the Q switch by DKDP and LiNbO3 crystals in the commercial lasers.
[3]
Acknowledgements This work was supported by the Doctor Point Foundation of National Education Committee of China (200204 007).
[4] [5]
References [1] Walter G, Vaughan W. Polarization. In: Jean M, Bennett HE, editors. Handbook of optics. Washington: Opt Soc Am; 1978. p. 146 [chapter 10]. [2] Bohm J, Heimann RB, Hengst M, Roewer R, Schindler J. Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3 Ga5 SiO14 (LGS), La3 Ga5::5 Nb0:5 O14 (LGN)
[6] [7]
593
and La3 Ga5:5 Ta0:5 Si14 (LGT), Part I. J Cryst Growth 1999;204: 128–36. Bohm J, Chill E, Flannery C, Frohlich HJ, Hauke T, Heimann RB, Hengst M, Straube U. Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3 Ga5 SiO14 (LGS), La3 Ga5::5 Nb0:5 O14 (LGN) and La3 Ga5:5 Ta0:5 Si14 (LGT), Part II. J Cryst Growth 2000;216:293–8. Minhua J. Crystal phasics. Jinan: Shandong Science and Technology Press; 1982. p. 348, 367 [in Chinese]. Stade J, Bohaty L, Hengst M, Heimann RB. Electro-optic, piezoelectric and dielectric properties of Langasite (La3 Ga5 SiO14 ), Langanite (La3 Ga5::5 Nb0:5 O14 ) and Langatite (La3 Ga5:5 Ta0:5 Si14 ). Cryst Res Technol 2002;37:1113–20. Yin Xin, Wang Jiyang, Wei Jing-qian, Gang Huai-dong. Measurement of electro-optic coe(cients of low-symmetry crystal Yb Ca4 O(BO3 )3 . Opt Lasers Engrg 2002;37:643–9. Takeda H, Sato J, Kato T, Kawasak K, Morikoshi H, Shimamura K, Fukuda T. Synthesis and characterization of Sr 3 TaGa3 Si2 O14 single crystals. Mater Res Bull 2000;35:245–52.
Optics & Laser Technology 36 (2004) 591 – 593 www.elsevier.com/locate/optlastec
A new method for measuring electro-optical coe(cients of the optically active crystals Xin Yin ∗ , Jiyang Wang, Huai-jin Zhang, Shao jun Zhang, hai-kuai Kong National Key Laboratory of Crystal Materials, Department of Materials, Shandong University, Jinan 250100, People’s Republic of China Received 8 August 2003; received in revised form 22 December 2003; accepted 5 January 2004
Abstract A new method has been proposed to measure the electro-optical coe(cients of the crystals which have optical activity. By using this method, the half-wave voltage and the electro-optical coe(cients of the optically active crystals La3 Ga5 SiO14 (LGS), La3 Ga5::5 Nb0:5 O14 (LGN), La3 Ga5:5 Ta0:5 Si14 (LGT) have been determined. The resultants obtained are: 11 (LGS) = 1:9 pm=V, 11 (LGN) = 0:75 pm=V, 11 (LGT) = 0:57 pm=V. This method can also be used to simulate the on–o@ state of the electro-optical Q switch in the laser cavity, whether the switch is made of the ordinary crystals or the optically active crystals. ? 2004 Elsevier Ltd. All rights reserved. PACS: 42.62.Ky; 42.55.Rz Keywords: Electro-optic coe(cient; Optical activity; La3 Ga5 SiO14 (LGS); La3 Ga5::5 Nb0:5 O14 (LGN); La3 Ga5:5 Ta0:5 Si14 (LGT)
1. Introduction According to the conditional point of view, it is complicated to make electro-optic devices by using the crystals which are optical activity. For example, “Crystals of quartz are optically active, and this complicates use of the material as an electro-optic modulator” [1]. Recently, the practical electro-optic Q switch has been made in our laboratory by using La3 Ga5 SiO14 (LGS) crystal which has an optical activity. In this paper, we report a new method for measuring the electro-optic coe(cients of the optically active crystals.
back by an all-reMection mirror and propagates through the optically active crystal again, the polarization plane rotates by an angle − around the wave vector −k. Thus the total rotated angle of the polarization plane is zero, while the polarized light travels through the optically active crystal back and forth for two times. The polarization direction of the analyzer is adjusted to be perpendicular to that of the polarizer. When the half-wave voltage V is applied to the sample, the signal of the optical intensity varies from minimum to maximum. V is the voltage which causes the phase di@erence of the two beams to change by =2.
2. Experiment setup
3. Measurement principle and measurement results
The experiment setup for measuring the electro-optic coe(cients of the optically active crystals is shown in Fig. 1. A polarized He–Ne laser is divided into two beams by a beam splitter. One beam is reMected out by the beam splitter. The other propagates through the beam-splitter and the optically active crystal, then the polarization plane rotates by an angle of around the wave vector k. After it is reMected ∗ Corresponding author. Tel.: +86-531-856-5174; fax: +86-531856-5403. E-mail address: [email protected] (X. Yin).
0030-3992/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.01.011
LGS, LGN, LGT crystals belongs to 32 point group [2,3]. Each has two electro-optic coe(cients [2,3], 11 = −21 = −62 ; 41 = −52 . The half-voltage V can be measured by applying an electric Peld parallel to the x- or y-axis, and by propagating light along the z-axis. The electro-optic coe(cient 11 can be calculated from [4] 11 =
2n3O V (2l=d)
;
(1)
where ; nO ; l and d are the wavelength of the He–Ne laser, the refractive index of the O light, the length along the
592
X. Yin et al. / Optics & Laser Technology 36 (2004) 591 – 593
4. LGS electro-optic Q switch
High voltage Light beam-splitter
source
He-Ne laser All-reflection
Polarizer
Sample
Analizer
Galvanometer Photodiod
Fig. 1. The experiment setup for measuring the electro-optic coe(cients of the optically active crystals.
Table 1 Electro-optic coe(cients 11 of LGS, LGT and LGN crystals
Crystal
Dimensions of the sample x × y × z (mm3 )
V (l=d = 1:1) (kV)
11 (pm/V)
LGS LGN LGT
6:2 × 6:2 × 40:3 4:5 × 8:4 × 42:4 4:3 × 10:4 × 39:1
24.7 56.6 76.0
1.9 0.75 0.57
optical path and the thickness in the direction of the electric Peld, respectively. In determining the electro-optic coe(cients of LGS, LGN and LGT crystal, a pair of the z faces of the sample is polished, and the x surfaces are deposited with an Au Plm as electrodes. The half-voltage V and the electro-optic coe(cient 11 of every crystal obtained are listed in Table 1. For wavelength 0:6328 m of the He–Ne laser, the refractive indices of O light are: nO (LGS) = 1:9116; nO (LGN) = 1:9821 and nO (LGT) = 1:9672, respectively [5]. Another electro-optic coe(cient 41 of LGS crystal has been determined by the interferrometric method [6]. The result obtained is 1:8 pm=V. We also tried to measure the electro-optic coe(cient 11 of Sr 3 TaGa3 Si2 O14 (STGS) crystal [7]. More than 4 kV voltage was applied to the sample with dimensions of 4:5 × 9:4 × 34:8 mm3 along x-, y- and z-axis, respectively. Unfortunately, the electro-optic e@ect has not been observed. 11 of STGS crystal may approximately be zero. It must be pointed out that the electro-optic coe(cients measured by using this method are di@erent from those reported in Ref. [5]. Because this setup can be used to simulate the on–o@ state of the electro-optical Q switch in the laser cavity, the results obtained are more reliable for practical application than others. From Table 1, it can be seen that only the electro-optic coe(cient of LGS is large enough for using the traverses electro-optic e@ect to designe the Q switch. LGN and LGT cannot be used to fabricate the practical Q switch, because the electro-optic coe(cients are too small.
LGS single crystal can be conPgured as a Pockels cell placed in the laser cavity. A schematic diagram of the laser cavity is shown in Fig. 2. As mentioned above, LGS single crystal is optically active in the direction of the optical axis. When a polarized wave in the laser cavity propagates through the Pockels cell back and forth for two times, the total rotated angle of the polarization plane is zero. Thus LGS single crystal can be placed in the laser cavity as a electro-optic Q switch, as those crystals which have no optical activity. A block of LGS single crystal was cut and polished to form a Pockels cell with the dimensions of 10 × 10 × 42 mm3 (x; y; z). The Pockels cell was placed in a Nd 3+ YAG laser cavity. When the voltage applied was 3750 V, the Q switch is e@ective with a pulse output energy of 350 mJ and a pulse width of 7:8 ns at repetition rates of 1, 10 , 20 times=s. The spot burnt by the laser pulse is shown in Fig. 3. The optical threshold of LGS single crystal has been determined using an Nd 3+ : YAG laser. Although it is less than that of DKDP(about 13 ), it is about 10 times of that of LiNbO3 . Thus LGS single crystal can be conPgured as an electro-optical Q switch by using the horizontal electro-optic e@ect. The Q switch is particularly suitable for use in lasers
Fig. 2. Schematic diagram of the laser cavity.
Fig. 3. The spot burnt by the laser pulse with LGS electro-optic Q switch.
X. Yin et al. / Optics & Laser Technology 36 (2004) 591 – 593
of intermediate power. It may partially replace the Q switch by DKDP and LiNbO3 crystals in the commercial lasers.
[3]
Acknowledgements This work was supported by the Doctor Point Foundation of National Education Committee of China (200204 007).
[4] [5]
References [1] Walter G, Vaughan W. Polarization. In: Jean M, Bennett HE, editors. Handbook of optics. Washington: Opt Soc Am; 1978. p. 146 [chapter 10]. [2] Bohm J, Heimann RB, Hengst M, Roewer R, Schindler J. Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3 Ga5 SiO14 (LGS), La3 Ga5::5 Nb0:5 O14 (LGN)
[6] [7]
593
and La3 Ga5:5 Ta0:5 Si14 (LGT), Part I. J Cryst Growth 1999;204: 128–36. Bohm J, Chill E, Flannery C, Frohlich HJ, Hauke T, Heimann RB, Hengst M, Straube U. Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3 Ga5 SiO14 (LGS), La3 Ga5::5 Nb0:5 O14 (LGN) and La3 Ga5:5 Ta0:5 Si14 (LGT), Part II. J Cryst Growth 2000;216:293–8. Minhua J. Crystal phasics. Jinan: Shandong Science and Technology Press; 1982. p. 348, 367 [in Chinese]. Stade J, Bohaty L, Hengst M, Heimann RB. Electro-optic, piezoelectric and dielectric properties of Langasite (La3 Ga5 SiO14 ), Langanite (La3 Ga5::5 Nb0:5 O14 ) and Langatite (La3 Ga5:5 Ta0:5 Si14 ). Cryst Res Technol 2002;37:1113–20. Yin Xin, Wang Jiyang, Wei Jing-qian, Gang Huai-dong. Measurement of electro-optic coe(cients of low-symmetry crystal Yb Ca4 O(BO3 )3 . Opt Lasers Engrg 2002;37:643–9. Takeda H, Sato J, Kato T, Kawasak K, Morikoshi H, Shimamura K, Fukuda T. Synthesis and characterization of Sr 3 TaGa3 Si2 O14 single crystals. Mater Res Bull 2000;35:245–52.