Quantitatively measured photorefractivesensitivity of proton-exchanged lithium niobate,proton-exchanged magnesium oxide-dopedlithium niobate, and ion-exchanged potassiumtitanyl phosphate waveguides

Yukiko Kondo, Satoshi Miyaguchi, Atsushi Onoe, and Yoichi Fujii

The photorefractive sensitivities of proton-exchanged lithium niobate waveguides and Rb-ion-exchangedpotassium titanyl phosphate waveguides are quantitatively measured, and their influence on waveguideapplications is estimated.

1. Introduction

Recently there has been a great demand for coherentblue or green light as the source for optical datastorage and laser printing so the techniques of gener-ating coherent blue or green light, such as second-harmonic generation (SHG), are remarkably devel-oped. 1-3

Many methods, such as Cerenkov radiations andquasi-phase matching,1 2 (QPM) have been proposedfor the purpose of obtaining a high conversion effi-ciency.

Lithium niobate (LN) and potassium titanyl phos-phate (KTP) are attractive materials for SHG devicesbecause their nonlinear coefficients are large.

The optical waveguides are easily fabricated onboth LN and KTP crystals by the proton-exchangeand the ion-exchange methods. The waveguide-typedevices are suitable for obtaining high conversionefficiency because of their good light confinement.

The conversion efficiency of SHG devices is actuallylimited by the photorefractive effect (optical damage)of the material, so quantitative estimation of thephotorefractive effect of the material becomes neces-

Y. Kondo and Y. Fujii are with the Institute of Industrial Science,University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106,Japan. S. Miyaguchi and A. Onoe are with the Corporate Re-search and Development Laboratory, Pioneer Electronic Co., Ltd.,Fujimi 6-1-1, Tsurugashima-city, Iruma-gun, Saitama, Japan.

Received 15 April 1993; revision received 2 September 1993.

0003-6935/94/163348-05$06.00/0.© 1994 Optical Society of America.

sary when the material with high conversion effi-ciency for the SHG devices is selected.

In this paper we quantitatively measure the pho-torefractive sensitivity of various types of proton-exchanged waveguide. The proton-exchanged wave-guides for measurement are fabricated on both LNand MgO-doped LN crystals.

The photorefractive sensitivity of Rb-ion-exchangedwaveguides on KTP crystals is first measured quanti-tatively with the diffraction-grating method.

The Rb-ion-exchanged KTP waveguides are provedto be promising materials for SHG devices becausetheir photorefractive sensitivity is almost the same asor smaller than that of the annealed proton-ex-changed waveguides.

2. Fabrication of the Waveguides

The optical planar waveguides were fabricated onz-cut LN and z-cut MgO-doped LN crystals by theproton-exchange method shown in Fig. 1. The tem-peratures of the proton exchange ranged from 200 to230 0C, and the proton-exchange time ranged from 20min to 5 h. After the proton-exchange process, thewaveguides were annealed at 350 C for several hours,as shown in Table 1.

Samples A, B1, B2, and B3 shown in Table 1 arefabricated on the undoped LN crystals, whereassamples MO, MI, and M2 are fabricated on theMgO-doped crystals.

On samples A and MO, a 2-pLm-deep proton-exchanged layer is formed and is not annealed,whereas on samples B1, B2, B3, MI, and M2, a thin

3348 APPLIED OPTICS / Vol. 33, No. 16 / 1 June 1994

Fig. 1. Proton-exchange method.

Table 1. Fabrication Condition of the LN Waveguides

Initial Depth of AnnealingWaveguide Proton-Exchanged Conditions

Material Sample Layer (pum) at 350 0C (h)

LN (z cut) A 2.1 No annealingB1 0.32 2B2 0.31 4B3 0.25 4

MgO-doped LN MO 1.9 No annealing(z cut) Ml 0.47 2

M2 0.35 4

Table 2. Fabrication Condition of the KTP Waveguides

DiffusionTemperature Time Ba(NO3)2/RbNO3

Sample (IC) (min) (mol. %)

K1 320 45 3K2 320 10 15

melting point of the salt.5is shown in Table 2.

The fabrication condition

3. Experimental Method

Among the various methods, the photorefractive-grating method6 is the most sensitive to small refrac-tive-index changes induced by the photorefractiveeffect. Moreover, the photorefractive effects in-duced by the light with polarization extraordinary toLN and the light with polarization ordinary to LN canbe separately measured by this method.

In our experiment, the photorefractive diffraction-grating method shown in Fig. 3 was used to measurethe photorefractive sensitivity. The Ar-ion laserbeam (514.5 nm) is split into two beams and illumi-nated on a rutile prism. The grating that is due tophotorefraction is formed by the interference fringesof the Ar-ion laser on the LN waveguides.

The refractive-index change An that is due tophotorefraction can be obtained by the measurementof the diffraction efficiency of the grating, as shown inthe following equation:

= sin[nTAnL/(X cos 0)]12,

proton-exchanged layer is formed and annealed for 2or4hat350 C.

The optical planar waveguides were also fabricatedon z-cut KTP crystals by the ion-exchange methodshown in Fig. 2.4 The Rb ions were diffused onto thesubstrate for 10-45 min at 320 C. Ba2+ ions wereadded to the salt for the purpose of controlling thediffusion rate of the Rb+ ions and reducing the

(1)

where L is the length of the interference (4 mm), 0 isthe Bragg angle of the grating, and X is the wave-length of the He-Ne laser (632.8 nm).

The photorefractive sensitivity is defined as

S = An/E, (2)

where E is the energy density in the waveguide.

4. Results of the Experiment

The diffraction efficiency of the proton-exchanged LNwaveguides as a function of the duration of theillumination of the Ar laser is shown in Figs. 4-7.

As the annealing time increases, the waveguidesbecome gradually susceptible to optical damage. Asshown in Fig. 4, the annealed waveguides are moresusceptible to the photorefraction than the wave-guides that are not annealed. In the case of theannealed waveguides, the diffraction efficiency of theHe-Ne laser is saturated for a short duration of the

Fig. 2. Fabrication of the KTP waveguides. Fig. 3. Experimental setup.

1 June 1994 / Vol. 33, No. 16 / APPLIED OPTICS 3349

5

3 -- .................

i5 3 r ...~~~~~~~~~~........................

C)I

CI-

0

0 10 20

Fig. 4. Diffraction efficiency ofguides.

30 40 50time(s)

the proton-exchanged LN wave-

r.

-4)

4

0oC_e)

600 700 800

time(s)

Fig. 7. Diffraction efficiency of the proton-exchanged MgO-dopedLN waveguides (no annealing).

Ar laser with a power density of less than 1.3 x 103(W/cm 2 ).

The waveguide without annealing is not suscep-tible to photorefraction, and a large power density isneeded. The diffraction efficiency is saturated afterthe waveguides are illuminated for 700 s with a powerdensity of 1.3 x 104 (W/cm 2 ) (Figs. 4 and 5).

6

.0

I-

5

4

3

2

1

00 100 200 300 400 500 600 700

time(s)

Fig. 5. Diffraction efficiency of the proton-exchanged LN wave-guides (no annealing).

10

i?

C-.4)

._

C.-.

0 5 10 15 20 25 30time(s)

Fig. 6. Diffraction efficiency of the proton-exchanged MgO-dopedLN waveguides.

The proton-exchanged waveguides formed on theMgO-doped LN crystals also become susceptible tooptical damage as the annealing time increases. Thediffraction efficiency is saturated after the wave-guides are illuminated for a short time with a powerdensity of less than 1.8 x 103 (W/cm 2

) (Fig. 6).In the case of the MgO-doped LN waveguide that is

not annealed, the photorefraction needs a large powerdensity. The diffraction efficiency is saturated afterthe waveguides are illuminated for 800 s with a powerdensity of 1.8 x 104 (W/cm 2 ) (Figs. 6 and 7).

In both the undoped crystal and the MgO-dopedcrystal, the effect on the photorefraction of annealingthe waveguides is almost the same (Figs. 4-7).

The diffraction efficiency of the KTP waveguides isshown in Fig. 8. The Rb-ion-exchanged waveguidewith 15 mol. % of Ba(NO3)2 seems to be moresusceptible to the optical damage than the waveguidewith 3 mol. % of Ba(NO3 )2.

The photorefractive sensitivity was calculated fromthe measured data of diffraction efficiency and isshown in Table 3.

The photorefractive sensitivity S of the proton-exchanged layer, which is 2 Am deep and is notannealed, is approximately 10-11-10-12 cm2 /J. The

CsS4)

'4-4.

S0C)

0!5I-

5-

4 .. .............. .. 50 K 2; 1.2 X1 ) -

3 ..... . .; .......................... . ............ .............

2 ..... ............ ........... ............ . .......... ........... .............

/: K 1 4.d xIOA2W/cI. ............ ............ ............. ............ ............. .............................

0 10 20 30 40 50 60 70 80 90time(s)

Fig. 8. Diffraction efficiency of the ion exchanged KTP wavecguides.

3350 APPLIED OPTICS / Vol. 33, No. 16 / 1 June 1994

60

50 ......... ............... .. ... .. .................... ....... ....... .. .... .. ..

30 .. .. .. .. ............. ... .... ....................... ..

/M0 18X 10 (W/c

30

-0 100 200 300 400 500

Table 3. Parameters and Photorefractive Sensitivity of the Waveguides

Diffusion Energy DiffractionWaveguide Depth Density Efficiency An/EMaterial Sample (pim) (J/cm 2 ) -r(%) (cm2 /J)

LN(z cut) A 2.1 1.1 x 106 0.74 3.8 x 10-12Bi 2.1 4.0 x 103 0.62 0.98 x 10-9B2 1.8 1.0 x 103 1.0 5.0 x 10-9B3 3.7 9.9 x 102 1.0 5.2 x 10-9

MgO-doped MO 1.9 2.7 x 105 0.9 1.8 x 10-11LN (z cut) Ml 1.9 2.7 x 103 1.5 2.3 x 10-9

M2 4.7 3.6 x 102 1.0 1.4 x 10-8KTP (z cut) Ki 13 6.7 x 103 0.5 5.2 x 10-10

K2 11 7.4 x 102 1.0 6.8 x 10-9

photorefractive sensitivity S of the annealed proton-exchanged waveguides, however, is approximately10-8-10-9 cm2/J, which means that they are 10-2-10-3 times weaker from the optical damage thanthose without annealing.

The photorefractive sensitivity S of proton-ex-changed waveguides without annealing is 1O-4times as small as that of Ti-indiffused ones.

From the above results we can conclude that theproton-exchanged waveguides become weak from op-tical damage by annealing.

The photorefractive sensitivity S of the Rb-ion-exchanged KTP waveguides is approximately 10-9 1010 cm2/J and is almost the same as or less thanthat in proton-exchanged waveguides annealed at350 °C for several hours.

5. Discussion

According to the mechanism of optical damage (pho-torefractive effect) proposed by Chen, 7 it is likely thatphotorefractive sensitivity depends on both the elec-tro-optic constant and the photoconductivity of thematerial, and photorefraction seems to be propor-tional to the electro-optic constant and inverselyproportional to photoconductivity.8

It is reported that the MgO-doped LN bulk crystalsare more resistible to optical damage than the un-doped LN crystals because photoconductivity of thecrystal is greatly improved by doping MgO.8 In thecase of proton-exchanged LN waveguides, however,the effect of doping MgO is small, and the protonsseem to play a more dominant role than the Mg2+ions.

Several papers,91 0 including one of our own, 1 havealready reported that the electro-optic constant of theLN crystal is reduced by the proton exchange but isrecovered by annealing.

Minakata et al. reported that the degradation of theelectro-optic effect was caused by the structural changeof the LN crystal during the proton-exchange pro-cess.12 As the LN crystal is changed into a symmet-ric structure by proton exchange, it is likely that theproton-exchanged waveguide is reversed into an asym-metric structure by annealing. Therefore the in-creased photorefractive sensitivity in annealed proton-exchanged waveguides can be explained by the

recovery of the electro-optic constant caused by thelarge structural change of the crystal.

6. Conclusion

A quantitative estimation of photorefraction has beenmade for proton-exchanged waveguides fabricated onLN, and MgO-doped LN, and Rb-ion-exchanged KTPwaveguides, and the conclusion of our experiment isas follows:

(1) Although the photorefractive sensitivity in-creases as the annealing time increases, the photore-fractive sensitivity of the annealed proton-exchangedwaveguides is still much smaller than that of Ti-indiffused waveguides.

(2) The source of increased photorefractive sensi-tivity in annealed proton-exchanged waveguides canbe explained mainly by the recovery of the electro-optic constant by annealing.

(3) The annealed proton-exchanged waveguidescan be used for SHG devices, especially for the QPMapplication, because they are still resistible to opticaldamage in spite of the reduction of the resistance byannealing, and the periodic structure of the QPMdevices is also said to be resistible to optical damage. 13

The KTP waveguide proved to be a very attractivematerial for SHG devices, such as balanced phase-matching devices, because it is resistant to opticaldamage if the fabrication condition is properly cho-sen.

We are grateful to T. Tohma of Pioneer ElectronicCo., Ltd., for useful discussions and great help.

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