Refractive Index Measurements of Barium Titanate From.4 to 5.0 Microns and Implications for Periodically Poled Frequency
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537 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
EXPERIMENT The refractive indices of barium titanate were measured from 0.4 to 5 microns using the minimum deviation method on a prism fabricated from a boule grown from a top seeded solution. The prism was oriented and polished using standard techniques. The apex angle was measured optically and was 40.015 ±.0015 degrees. A Gaertner L-124 precision spectrometer was used for the minimum deviation measurements. The spectrometer was calibrated by measuring indices of a calcium fluoride prism several times and determining the standard deviation. Our data were within 2 X 10-5 of the published values 12 . An Oriel mercury-xenon lamp source was coupled to a Digikrom L 240 monochromator to permit selection of wavelength at which the index
was to be measured. The near IR measurements were made using an Electrophysics hand held IR viewer. For measurements from 2.5 to 5 microns the quartz optics of the spectrometer were replaced by a single ZnSe collimating lens. An infrared imaging camera (Cincinnati Electronics IRRIS 160 LN) was used in place of the imaging optics to detect the refracted beam. Five separate runs were made throughout the entire spectrum and the -4
standard deviation of the data at any wavelength was less than 2X10 ". The temperature at which the measurements were taken was 210 C. RESULTS The refractive indices as a function of wavelength are shown in figure 1. 2.6
2.5
2.4 'S PC
2.3
2.2
.I.II .
2.1 0
1
. . I I I I . . I . . . . I
2
3
Wavelength (Microns)
4
. . . . I . . . .
6
Figure 1. Ordinary and extraordinary refractive indices of barium titanate as a function of wavelength. Dotted lines represent the Sellmeir fit to the data.
538
The experimental error was less than .0002 over the entire transmission range of the material. The data were fit to a Sellmeir equation of the form' 3 2 2 2 n2= A + B X / (X -C) + D X using a modified Levenburg-Marquardt algorithm. The difference between the data and the predicted refractive index at any wavelength was less than 2X10-4 which is within the error in the experiment. The Sellmeir coefficients are shown in Table I for both ordinary and extraordinary refractive indices and the dotted lines through the data points in figure 1 represent fits to the Sellmeir equation. The Sellmeir coefficients derived from the data were used in all subsequent calculations.
Coeff
Ne
No
A B C D
3.0584 2.27326 .074090 -.02428
3.02479 2.14062 .067007 -.02169
Table I. Sellmeier Coefficients for Barium Titanate
The combination of birefringence and dispersion characteristics of BaTiO 3 make it unsuitable for conventional phase matched frequency conversion. Extensive calculations for frequency doubling of 1.06 gi and optical parametric oscillation utilizing a 1.06 gt pump showed that birefringent phase matching is impossible for these processes. However, success of periodic poling in lithium niobate has led to the suggestion that parametric oscillation might be possible for BaTiO 3 albeit with
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