Infrared corrected sellmeier coefficients for congruently grown lithium niobate and 5 mol% magnesium oxide-doped lithium niobate

Zelmon et al. Vol. 14, No. 12 / December 1997 / J. Opt. Soc. Am. B Infrared corrected Sellmeier coefficients for
congruently grown lithium niobate
and 5 mol. % magnesium oxide–doped
David E. Zelmon and David L. Small
Materials Directorate, Wright Laboratory (WL/MLPO), Wright-Patterson Air Force Base, Ohio 45433-7707 Dieter Jundt
Crystal Technologies Corporation, 1040 East Meadow Circle, Palo Alto, California 94303 Received May 30, 1997 The growth in the uses of lithium niobate for infrared applications has created a need for knowledge of itsoptical characteristics in the infrared spectral region for the purpose of designing phase-matched or quasi-phase-matched devices.
We report measurements of the refractive indices of congruently grown lithium nio- bate and lithium niobate doped with 5 mol. % magnesium oxide.
We use these results to predict the tuning curve of a room-temperature multigrating optical parametric oscillator in each material. 1997 Optical So-ciety of America [S0740-3224(97)00612-7] Lithium niobate (LiNbO3) has been used extensively in a We measured dispersion for both pure and MgO-doped wide variety of optical frequency-conversion devices.1–5 LiNbO3 using the minimum-deviation technique. Crys- Design of these devices and prediction of their behavior tal prisms were manufactured from poled Czochralski- depend critically on precise knowledge of the indices of re- grown boules of approximate dimensions 80 mm in diam- fraction throughout the spectral region of interest.
eter by 45 mm in length.
The growth direction was the though the refractive indices of LiNbO3 have been studied ferroelectric Z axis. The melt composition that was used for many years,6–12 most of the data have been limited to to grow the undoped crystal was congruent (48.38 mol. % wavelengths in the UV, visible, and near-IR spectral re- lithium oxide),24 resulting in a crystal of the same compo- LiNbO3 is now being considered for infrared sition. The MgO-doped crystal was pulled from a melt of applications,13–19 so there is a need for improved knowl- congruent Li/Nb ratio with MgO added to achieve 5.0 edge of its refractive indices at wavelengths throughout mol. % MgO in the melt.25 The melt fraction crystalized the transmission range of this material.
was below 30% in both cases, ensuring high-purity and sured the refractive indices of congruently grown LiNbO3 strain-free optical-quality material.
from 0.4 to 5.0 mm and calculated new Sellmeier coeffi- The prisms were x-ray oriented, cut, and polished by cients that accurately predict refractive indices in this standard techniques. The base faces were Z faces with a spectral range. With the improved Sellmeier coefficients, 2 min. tolerance. The polished faces were perpendicular we have been able to predict the signal and idler wave- to these Z faces.
When the prism was aligned for mini- lengths for an optical parametric oscillator (OPO) fabri- mum deviation, the light propagation was along the X di- cated from periodically poled LiNbO3 that conforms well rection, permitting the measurement of both ne and to experimental observations throughout the spectral re- no . The optical apertures were 15 mm by 13.7 mm (Z).
gion of interest.
The apex angle was measured optically and was 44.941° LiNbO3 is subject to photorefractive changes in its op- for the undoped prism and 44.980° for the MgO-doped tical properties, which is intolerable in many applica- Devices have therefore been fabricated with A Gaertner L-124 precision spectrometer was used for LiNbO3 doped with various amounts of magnesium oxide the minimum-deviation measurements.
Only limited refractive-index data of any kind the spectrometer by measuring indices of a calcium fluo- are available for this material, and the doping level of the ride prism several times and determining the standard samples and the spectral range over which the data exist Our data were within 2 3 1025 of the pub- vary widely.20–23 We have measured the refractive indi- lished values.26 An Oriel Hg-Xe lamp source was coupled ces of LiNbO3 doped with 5 mol. % MgO from 0.4 to 5.0 to a Digikrom L 240 monochromator to permit selection of mm and have calculated new Sellmeier coefficients for this wavelength at which the index was to be measured. The near-IR measurements were made with an Electrophysics 1997 Optical Society of America J. Opt. Soc. Am. B / Vol. 14, No. 12 / December 1997 Zelmon et al. hand-held IR viewer.
For measurements from 2.5 to 5 Table 1. Sellmeier Coefficients for Congruently
mm the quartz optics of the spectrometer were replaced by Grown LiNbO3
a single ZnSe collimating lens.
An IR imaging camera (Cincinnati Electronics IRRIS 160 LN) was used in place of the imaging optics to detect the refracted beam.
separate runs were made throughout the entire spectrum, and the standard deviation of the data at any wavelength was less than 2 3 1024. The temperature at which the measurements were taken was 21 °C.
3. RESULTS
Table 2. Sellmeier Coefficients for Congruently
The refractive indices of undoped congruently grown Grown LiNbO3 Doped with 5-mol. % MgO
LiNbO3 are shown in Fig. 1, in which the refractive indexfor both ordinary (n o) and extraordinary ( n e) waves is plotted versus wavelength. We also measured the refrac- tive indices of congruently grown LiNbO3 that had been doped with 5-mol. % MgO. The results of these measure- ments are shown in Fig. 2, in which the refractive index for both ordinary and extraordinary waves is plotted ver- sus wavelength.
In order to use the refractive index for phase-matching or quasi-phase-matching calculations, the index data are fit to a Sellmeier equation to predict refractive indices atthe wavelengths pertinent to the optical process that wasbeing investigated.
In most cases a single oscillator of the form Al2/(l2 2 B) was used, and we incorporatedother oscillators into the Sellmeier equation by assumingthat their wavelengths were sufficiently far from the spec-tral region of interest that they could be approximated bya constant (far-UV) and a term proportional to l2 (IR).
However, it has been pointed out27 that these approxima-tions lead to significant deviations from the refractive in-dices found experimentally, even in the short-wavelengthend of the visible spectrum in LiNbO3, and we have foundthat similar difficulties arise as one approaches the edgeof the transmission range in the IR. As a result, we havechosen to use a three-oscillator model to represent our in-dex data.
Refractive indices of undoped congruently grown LiNbO3 Multioscillator models for the refractive index of oxide as a function of wavelength.
The solid curves represent the ferroelectrics such as LiNbO three-oscillator Sellmeier fit to the data.
3 have been discussed exten- sively in the literature.11,12,27,28 On the basis of UV re- flectance data27,29 Uchida27 proposed a model for oxideferroelectrics using two UV oscillators corresponding totransitions from the oxygen 2p valence band to the d«and dg transitions at approximately 5 and 10 eV, respec-tively.
Schlarb and Betzler11,12 proposed a different model in which they used two closely spaced oscillators inthe near UV, which they attributed to the defect structureof the LiNbO3, and a far-UV oscillator, which they ap-proximated as a constant term because the oscillatorwavelength was far away from the spectral regions of in-terest.
They used the usual restrahl approximation for the behavior of the refractive index in the IR region. Wehave found that, because of the wide spectral range of ourdata, neither of these models is adequate to reproduce thedata within the limits of our experimental error.
used a three-oscillator Sellmeier equation of the form30 Refractive indices of congruently grown LiNbO3 doped n2 2 1 5 Al2/ l2 2 B! 1 Cl2/ l2 2 D! with 5-mol. % MgO as a function of wavelength.
curves represent the three-oscillator Sellmeier fit to the data.
1 El2/ l2 2 F!.
Zelmon et al. Vol. 14, No. 12 / December 1997 / J. Opt. Soc. Am. B This equation incorporates two UV oscillators and also anIR oscillator to account for the refractive-index behaviornear the IR edge of the transmission range.31,32 curve fit parameters are shown in Tables 1 and 2 for theundoped congruent and 5% MgO-doped materials, respec-tively. The fit that we obtained predicts the data withinexperimental error throughout the spectral range.
wavelengths of the UV oscillators calculated by the fittingroutine are consistent with the transitions observed byUchida and Mamedov.26,28 However, the position of the UV oscillators should be considered only approximate, asour data do not extend into the UV region. In the IR theoscillator wavelength and strength are fitting parametersmade necessary by the closeness of our data to the IRtransmission edge of the material.
The results of the Sellmeier fits for the undoped mate- rial are shown in Table 1, and those for the doped mate-rial are shown in Table 2.
The fit of the calculated indi- Signal and idler wavelengths of a 1.064-mm-pumped OPO in periodically poled LiNbO3 as a function of grating period.
differences of less than 2 3 1024 for both ordinary andextraordinary indices at all wavelengths.
meier coefficients were used in all the subsequent calcu-lations.
4. DISCUSSION
To test the validity of the new Sellmeier equations, we
performed calculations to predict the results of a room-
temperature frequency-conversion experiment performed
on a congruently grown LiNbO3 device. We simulated an
experiment performed by Myers et al.16 in which a room-
temperature (25 °C) multigrating OPO, fabricated from
periodically poled LiNbO3, was pumped at 1.064 mm and
the signal was tuned from 1.35 to 1.92 mm by translating
the pumping beam over a series of gratings whose periods
ranged from 26 to 32 mm. We predicted the tuning curve
for phase matching of this process, and the results are
shown in Fig. 3, in which we plot the output wavelengths
Predicted signal and idler wavelengths of a 1.064-mm- of the OPO, using our Sellmeier equations, and compare pumped OPO in 5-mol. % MgO-doped periodically poled LiNbO3 them with the experimental observations of Ref. 16. The as a function of grating period.
agreement is excellent between the theoretical predic-tions and the experimental results throughout the spec- Sellmeier equation used by them is different from the one The largest deviations occur near the de- that we used it is not obvious how to apply their thermo- generacy point of the OPO because of the extreme sen- optic data to our Sellmeier equation.
In the absence of accurate thermo-optic data in the IR, we calculated the (,1 3 1024) changes in the refractive index.
grating periods required for a device similar to the one de- There have been few frequency-conversion experiments scribed in Ref. 16 but fabricated from 5% MgO-doped con- gruently grown LiNbO3.
The results of these calcula- tions are shown in Fig. 4. The shape of the tuning curve 3, and most of those have involved doubling of near-IR wavelengths.33 Although our data predict the is similar to that from Ref. 16, but the grating periods are results of these experiments well, our study emphasizes all smaller by ;4 mm.
the IR spectral region, and almost all the frequency-conversion experiments on MgO-doped LiNbO3 have beenperformed at an elevated temperature.
We have measured the refractive indices of congruently MgO-doped LiNbO3 are available,15 they are limited grown LiNbO3 and 5-mol. % MgO-doped LiNbO3 from 0.4 mostly to wavelengths in the visible where thermo-optic to 5 mm. The modified Sellmeier equations derived from data are strongly wavelength dependent, making it im- these data give an accurate prediction of experimental re- possible to extrapolate to longer wavelengths without sig- sults for IR frequency-conversion processes in undoped nificant errors.
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