Light-induced refractive index changes in singlemode channel waveguides in KTiOPO4

1 October 1995

OPTICS

COMMUNICATIONS ELSEVIER

Optics Communications

120 (1995) 47-54

Light-induced refractive index changes in singlemode channel waveguides in KTiOP04 J.-P. Ruske, 14. Rottschalk, S. Steinberg Institute of Applied Physics, Friedrich-Schiller-University Jena, Mar- Wien-Platz 1, D-07743 Jena, Germany Received 28

February 1995

Abstract Light-induced changes of the effective refractive index in Rb * K ion-exchanged singlemode channel waveguides in KTiOPO, been investigated in the visible wavelength region dependent on time, guided optical mode intensity and temperature. A hypothesis for the explanation of the light-induced effects is suggested. Thermooptic and pyroelectric effects are discussed. The light-induced refractive index changes do not restrict the function of integrated-optic components in the visible.

have

1. Introduction Because of its high linear electrooptic and nonlinear coefficients potassium titanyl phosphate ( KTiOPO, or KTP) is a promising substrate material for applications in the field of integrated-optic devices, e.g. phase- and Mach-Zehnder-interferometer modulators [ 1,2] Fabrication processes using the ion exchange technology [ 3-61 and ion implantation [7] were developed leading to low-loss optical waveguides with a. high photorefractive threshold [ 1,3]. Because of a strong diffusion anisotropy the Rb ++ K ion exchange process irr KTP mainly occurs in the z-direction of the c:rystal [4]. As a result, integrated-optic devices requiring well-defined channels or barriers will be possible., such as singlemode channel waveguide modulators in z-cut KTP at short visible wavelengths. Furthermore, the suitability of these waveguides and devices essentially depends on the amount of phase instabilities ibat is mainly of light-induced refractive index changes. We fabricated channel waveguides in KTP with singlemode operation in the whole visible wavelength region [ 81. Contrary to the bulk photorefractive effect, 0030-4018/95/$09.50 0 1995 Elsevier Science B.V. All ri,:hts reserved SSDIOO30-4018(95)00370-3

e.g. described for LiNbO, [9], we present the investigation of light-induced changes of the effective refractive index in singlemode KTP channel waveguides at A=0.4880 pm, h=0.5145 km and A=0.6328 km. The influence of pyroelectric and thermooptic phase changes as well as the possible origin of the lightinduced effects are discussed.

2. Sample preparation The channel waveguides were fabricated on the ( + z)-face of z-cut KTP substrate material (delivered by Crystal Technology, xyz dimensions: (5 X 25 X 1) mm3) in the y-propagation direction. We performed a rubidium * potassium ion exchange in a mixed melt of [65 mol% RbN03/32 mol% KN03/3 mol% Ba( NO,),] _Such mixed melts are the only possibility to obtain the small refractive index changes at the surface [ 51, which are necessary for singlemode operation at short visible wavelengths, without a subsequent annealing procedure, and reproducibility at the same time. The mixed melts also permitted us to carry out

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the ion exchange at the relatively low temperature 310°C and, therefore, with relatively long exchange times from 0.3 up to 3 hours. The addition of the divalent barium ions to the melt homogenizes the ion exchange process, reduces the variations in substrate conductivity and raises the diffusion velocity [ 41. This all results in only small differences between the refractive index profiles of planar waveguides on the ( + z) and ( -z) -faces as we could show by means of m-line spectroscopy measurements and WKB profile approximation. We defined the channel waveguides by using sputtered chromium/nickel masks. The channel widths varied between 2.0 and 5.5 pm corresponding to the expected singlemode waveguiding from the blue up to the red. After exchange the samples were endface-polished. We measured the waveguide attenuation by end-fire coupling of a polarized optical fiber output (YORK HB 450). Considering the coupling efficiency between fiber and waveguide as well as the Fresnel-losses due to the reflection at the endfaces typical attenuation values of 2.0 dB/cm for TM-polarization and 1.5 dB/cm for T&polarization were determined.

3. Experimental

set-up

A special interferometric set-up based on a two-beam interference of the output beams of two simultaneously excited neighbouring channel waveguides [ 81 and improved with regard to a former publication [ lo] was used for the investigation of the light-induced changes of the effective refractive index (commonly known as the photorefractive effect). The experimental arrangement is shown in Fig. 1. For the observation of light-induced refractive index changes an interference pattern is generated with a low power beam of one wavelength (Ll ) . The half-wedged glass plates enable the light to be focused in two points at the input endface of the sample, on the one hand, and to be brought to interference in the plane near the magnifying objective in front of the CCD device, on the other hand. The refractive index change in the channel waveguide region is caused by a second wavelength beam (L2) of considerably higher power, which is coupled into only one of the two channel waveguides. The light-induced phase shift and the change of the effective channel waveguide index, respectively, are

120 (1995) 47-54

Fig. 1. Schematic arrangement of the experimental set-up for the measurement of light-induced optical phase changes in channel waveguides. Ll, L2: laser beams, P: prism, BS: beam splitter, M: mirror, MO: microscope objective, PD: photodiode, CCD: CCD device, F: filter plate, SGP: special glass plate (half-wedged sheet).

determined by a special PC program. The sign of the index change is obtained by simple consideration about the direction of the shift of the interference fringes at the CCD device. In other words, the sign of the interference pattern shift is unambiguously connected with the sign of the effective refractive index change in the channel waveguide because of the geometry of the experimental set-up that is due to the number and position of the microscope objectives and prisms. The guided optical power at the second wavelength is detected by a photodiode. A filter plate (F) prevents the laser beam L2 to be detected by the CCD device. The polarizations of the beams can be chosen depending on the experimental purpose that is for a TM-polarized high power beam we can measure the phase shift of TM- or TE-light and vice versa. The advantages of this experimental arrangement are the high power densities in the channel waveguides as well as the independence of temperature changes because of nearly equal optical path lengths of the two beams. Furthermore, in comparison with photorefractive measureMach-Zehnder integrated-optic ments using interferometer devices [ 1 l] our channel waveguide arrangement guarantees a set of unambiguous parameters for the evaluation of the experimental results. The accuracy of the determination of the light-induced changes of the effective refractive index is about 5 X lo-’ for typical sample lengths of about 1 cm due to the mechanical and thermal noise of the experimental set-up.

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the KTP channel waveguide show a characteristic time dependence which can be described in good approximation by the exponential expressions AN(r)=ANsX[l-exp(

-t/~~)]

(1)

for the build-up process and AN(t) = AN, X exp( - t/rR)

-6

-4

2

0

-2

distance [ pm

4

6

]

Fig. 2. Measured nearlield distributions in the lateral and in the depth directions for a channel width of 4 km, at TM-polarization and A=O.5145 km.

For the calculation of the intensity from the guided optical power we assumed the optical mode field distribution as an elliptical area which is uniformly illuminated and whose axes are equal to the full width of half maximum (FWHM) of the mode intensity distribution. The illuminated elliptical area is between 5 p,m* and 10 km2 (compare Fig. 2). The maximum guided optical power at a given wavelength was in the order of 1 mW.

4. Experimental

results

We have investigated the change of the effective refractive index No0 (in the following written as ,V) of the fundamental channel waveguide mode as a function of time, guided optical mode intensity, wavelength and temperature in singlemode KTP channel waveguides whose typical optical nearfield distribution is shown in Fig. 2. Because of the waveguide characteristics such as refractive index profile and mode intensity distribution, the change of the effective refractive index in the channel waveguide is not equal to the change Iofthe refractive index in the bulk material at the same i ntensity. The effective refractive index decreases if the waveguide has been irradiated. Fig. 3 shows the typical time dependence of the effective refractive index change AN(r) due to irradiation with TM-polarized light at A = 0.5 145 p.m. The interference pattenl was generated with light at h = 0.6328 p,rn and TM-polarization. Both the build-up process and the relaxat ion in

(2)

for the relaxation, respectively, with the saturation value AN, and the time constants TVand TV.The time constants are about 30 s for the build-up process and 100 s for the relaxation, respectively, while AN, reaches approximately - 6.0 X 10e6. In addition to this “normal” light-induced effect, an interesting and unexpected behavior can be observed if a completely relaxed (or “virgin” that is asexchanged) KTP channel waveguide is initially irradiated. First, we obtain a light-induced phase shift and effective refractive index change ( - 1.6 X 10P5), respectively, which is some times greater than that of a channel waveguide already illuminated (Fig. 4, around t = 0). However, this phase shift decreases during the irradiation until a new (also saturated) state is reached after more than 10 minutes. This time is much longer than the time constants above mentioned. By the way, the “normal” light-induced effect we see in the right part of Fig. 4. The magnitude of the initial phase shift only restores after some days. All lightinduced refractive index changes in all figures with the exception of the left part of Fig. 4 have been measured after the initial light-induced effect.

0

5

10

15

time [ min ]

Fig. 3. Time dependence of the light-induced change of the effective refractive index in KTP channel waveguides; the interference pattern is generated with 0.6328 Frn TM-polarized light; the irradiation of the signal waveguide is performed with 0.5145 km TM-polarized light.

J.-P. Rude et al. /Optics

rl 0

5

10 time

15

20

Communications

25

[ min ]

Fig. 4. Time dependence of the light-induced refractive changes of a completely relaxed (or as-exchanged) KTP

index

channel

waveguideunder initial irradiation.

intensity

[ W/cm’ ]

Fig. 5. Light-induced refractive index changes in a singlemode KTP channel waveguide versus guided optical mode intensity for three wavelengths in the visible; dashed curves: TE-light; solid curves: TM-light.

intensity

[ W/cm2 1

Fig. 6. Time constants of the light-induced change of the effective refractive index versus guided optical mode intensity.

120 (1995) 47-54

The change of the effective refractive index in saturation, in all cases induced by TM-polarized light, is plotted in Fig. 5 as a function of the guided optical mode intensity. The curves show only a small gradient for both the detected TM-light and m-light (please note the logarithmic scale division), while the maximum amount of the refractive index changes is not higher than 1 X 10m5. Moreover, the light-induced refractive index changes at h = 0.5 145 Frn are greater than those at the other two wavelengths A = 0.4880 Frn and A = 0.6328 pm, respectively. The measured change of the effective refractive index N, (TM-light that is polarization in z-direction of the crystal, solid curves) is about three times higher than that of N, (TE-light, xdirection, dashed curves) for the very same guided optical mode intensity and TM-polarization of the irradiation beam. For irradiation with TE-polarized light similar results were obtained. The dependence of the time constants on the intensity is only small. The time constants for both the build-up process and the relaxation are to be seen in Fig. 6 as a function of the guided optical mode intensity, using TM-light for irradiation and measurement in this case. The temperature dependence of the amount as well as the time constants of the refractive index changes was investigated between 25°C and 50°C. After the sample had been heated the phase shift was determined in the thermally stable state for the very same guided optical mode intensity (5600 W/cm*). Fig. 7 shows that the values of AN, and the time constants, respectively, decrease with increasing temperature (TM-light for irradiation and measurement). The ratio that A Ns bears to the time constants is nearly constant for all

T[Kl Fig. 7. Time constants and amount of the light-induced change of the effective refractive index versus the absolute temperature.

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J.-P. Ruske et al. /C’ptics Communications I20 (1995) 47-54

Fig. 8. Light-induced refractive index changes in an annealed protonexchanged channel waveguide fabricated in congruent LiNb03 and in Rb t) K ion-exchanged channel waveguides in KTP versus guided optical mode intensity for two wavelengths in the visible.

tcm~ratures. For tem~ratmcs higher than 55°C no light-induced phase shifts could be detected. After cooling of a waveguide which had been irradiated at higher temperatures the initial value of AN, restored after some days. All our meas~emen~ indicate that the light-inl~uced refractive index changes in KTP channel waveg;uides are about two orders of magnitude smaller than those in annealed proton-exchanged (APE) channel waveguides in LiNb03, however, at least one order of magnitude definitely. This is valid for same guided optical mode intensity and wavelength in the visible, as to be seen in Fig. 8, where the measured light-inducedrefractive index changes occuring in an APE channel waveguide fabricated in congruent x-cut LiNbO, (TE-light for i~adiation and measurement) are pictured together with the curves for AN, in the KTP channel waveguides (compare Fig. 5). In the APE channel waveguicles no photorefractive maximum can be observed at h = 0.5 145 pm.

5. Discussion

We assume that the light-induct refractive index changes in KTP channel waveguides are caused by a combination of a change carrier drift and the elearooptic effect, similar to the model of the photorefractive effect described by Glass et al. for bulk LiNb03 [ 91. The linear electrooptic effect in z-cut KTP (ypropagation) can be written as

An, = - @+,,E,

(TM-light),

(3)

An,= - $n$-,3Ez

( TE-light),

(4)

where n, and n, are the refractive indices in z- and xdirection, respectively, r3, and r13the electrooptic coefficients and E, an electric field in z-direction (compare [4] ). In KTP only z-directed electric fields cause a refractive index change. As already mentioned, the measured change of the effective refractive index for TM-light was about three times greater than that for TE-light if the same polarization and guided optical mode intensity of the irradiation beam was used. This corresponds to the ratio between r,, X n,” in (3) and r13Xn: in (4). The origin of the formation of a static electric field can be a drift of charge carriers or a dielectric polarization dependent on the optical intensity which is a X‘*‘-effect. However, such polarization effects show very short time constants in comparison with those measured by us. Thus, it is highly probable that the irradiation causes a directed separation of charge carriers by means of a photovol~ic process leading to a current density (5)

j,=&(I),

which depends on the irradiation intensity I. This current density produces an electric field E, which causes a current density again, however, directed against jp. The magnitude of this countercurrent density depends on the conductivity of the material which is devided into the dark conductivity ad and the phot~onductivity @ph.Because of (3) and (4) only the z-componentsj,, and E, are relevant for refractive index changes. The saturation is determined by j, =Jpz + ( fld

+ ~,)Es,z

=O

(6)

and is characterized by an electric saturation field Es,z. index change depends on the intensity dependence of jp,, and (Tphrrespectively. With The intensity dependence of the refractive

j&)

(7)

= &cy,I

weobtainfrom

(3), (6) and (7)

cFd+cph(z)

(8)

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for the intensity dependence of the refractive index change in saturation for z-polarized light, where k, is the characteristic photovoltaic constant for TM-light and (Y,the absorption constant. For TE-light we have to write nXr r,3, k, and cu, in (8). The refractive index change An, is assumed to be nearly the same also for the effective refractive index in the singlemode channel waveguide, that is An, = AN,,, = AN; = AN. With this, for the intensity dependence of the phase shift we can write

120 (1995) 47-54

-18 ;.19 g-20 lg-21 -22 -233.10

3.15

3.20

3.25

3.30

3.35

1/l[10-31K]

where L is the channel waveguide length. The fact that our light-induced effective refractive index change has a negative value is in good agreement with observations concerning second-harmonic generation (SHG) in KTP by Jongerius et al. [ 131. They explained the measured degradation in the SHG conversion efficiency and the observed spectral broadening by assuming an increase of the dispersion of the effective refractive indices [ n,,( 2~) - n,rf( w) ] due to optical damage. Following our results, especially the light of the frequency 2w causes a decrease of the refractive index in the waveguide region. The change of the effective refractive index depends on the mode dispersion. Since n,rr( 20) is far from the cut off point the decrease of n,,( o) is greater than that of n,& 2w), tantamount to an increase of the index dispersion above mentioned. Because of the same sign of the photovohaic and electrooptic coefficients on both z-faces due to the directed charge carrier movement the sign of the light-induced refractive index change should be the same on the ( + z) - and ( - z) -faces. However, to our knowledge the photovoltaic coefficients in KTP have not yet been measured so far. The saturation time depends on the conductivity. The time constant T of such diffusion processes of charge carriers can be described by the relations

%I(0 =

l0

(build-up

o,+(+,tl(0

process)

(10)

and rR = EEO (relaxation) (+d

,

(11)

Fig. 9. Relationship between ln( aT) and 1/T calculated from the temperature dependence of the time constants in Fig. 7.

respectively, as a function of the permittivity E and the conductivity. By means of Fig. 6 we obtain a relatively small dark conductivity of about lo- I4 (0 cm) - ’ in comparison with the ionic conductivity of bulk KTP which is approximately lo-” (am) -’ [4]. The amount of the dark conductivity has the same order of magnitude as the photoconductivity. For guided optical mode intensities of about 300 W/cm2 we obtain equal conductivities from Fig. 6 by using ( 10) and ( 11) . We expect a saturaration of the refractive index change for guided optical mode intensities higher than approximately lo4 W/cm*,because of the higher photoconductivity in comparison with the dark conductivity. The time constants of the diffusion process depend on the absolute temperature T. The relation between the conductivity and the diffusion constant D is given by the Nemst-Einstein relation u,=Nq*D/k,T,

(12)

where N is the concentration of charge carriers, q their charge and kB the Boltzmann’s constant. By means of the Arrhenius relation D = Do exp( - E,,,Ik,T)

(13)

we obtain Nq=Do

ud= -ex k,T

(14)

for the dark conductivity with the activation energy of the diffusion process E,, [ 121. If ln( crd7) is plotted against l/T the slope of the straight line is - E,,,/k,. In Fig. 9 the corresponding curves are plotted if TMlight was used for irradiation and measurement. We

J.-P. Rude et al. /Optics Communications 120 (1995) 47-54 obtain an activation energy of about 1.1 eV. Our measurements indicate a constant ratio of the refractive index change and the time constants with temperature, which confirms with the Eqs. (8)) ( 10) and ( 11). Since to our knowledge the photovoltaic coefficients in KTP have not yet been measured, it is difficult to compare experiment and theory. Therefore, further investigations will be done to clarify the behavior of both the conductivity and the photovoltaic effect of the Rb @ K ion-exchanged KTP crystal by using an experimental method already described by Goring et al. [141.

6. Thermooptic

and pyroelectric

behavior

In addition to the described photovoltaic process in combination with the linear electrooptic effect thermal processes take place and have to take into consideration for the explanation of the refractive index changes in the channel waveguide region. The absorption CYof the guided optical power causes an increasing temperature T within the channel waveguide. The change 01: temperature in the thermally stable state can be written in the form AT = a.PiLRti,

(15)

where Pi is the incident power, L the channel waveguide length and R, the thermal resistivity of the heat transfer problem of the channel waveguide concerning the KTP substrate and the air. Since the heat transmission resistance waveguide+air is much greater than the temperature lag waveguide + substrate we have R,=

1 InrfR -7rL A,

(16)

for the thermal resistivity of our problem. As sketched in Fig. 10, R denotes the radius of the active heating area that is of the channel waveguide and r the shortest distance between waveguide and bottom of the substrate, in good approximation the thickness of the sample, and A,, is the thermal conductivity of KTP. Using (15) and (16), we obtain

AT=&;!@. - “A*

(17)

With h,i,z = 3.3 W K-’ m-’ [4] and the absorption of 2.6 m-’ at A=05145 pm [ 151, R=2 pm and r= 1

53

channel waveguide

:.j “-..,

KTP

r ./

,/ ..,I

Fig. 10. Schematic sketch of the heat transfer problem in lightabsorbing channel waveguides (cross-section).

mm we obtain AT=2.8X 10m3 K for the maximum guided optical power of about 1 mW. An increasing temperature changes the refractive index of the crystalline material KTP due to thermooptic and the pyroelectric effects. With the thermooptic coefficient dn,ldT= 1.6 X low5 K-i [4] we obtain a thermooptically induced refracive index change An, = +4.5 X lOpa for the maximum guided optical power above mentioned. The pyroelectrically induced polarization change AP is defined by AP=~AT=EE,AE.

(18)

With (3) and (18) the pyroelectric change for TM-light is

A,,

z

=--ln3r

2 z 33

p33AT -

refractive

index

(19)

83380

Using the pyroelectric coefficientp,, = - 7 X lo-’ As m-‘K-l [4,16] ande33= 17.5 [4],r3,=36.3pmV-’ [ 41, n, = 1.8957 at A = 0.5 145 pm and AT above calculated we obtain a pyroelectrically induced refractive index change An,= + 1.5 X lo-‘. Thus, the magnitudes of both the thermooptically and pyroelectrically induced refractive index changes in our Rb c, K ion exchanged KTP channel waveguides can be neglected in comparison with the light-induced refractive index changes up to a guided optical power of about 1 mW at A = 0.5 145 pm, on the one hand, and in comparison with the mechanical and thermal noise of the experimental set-up which is about 5 X lo-‘, on the other hand. If a guided optical power greater than 1 mW would be used one can separate the light-induced effects from both the other effects because of their different signs.

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7. Conclusions We have demonstrated the behavior of low-loss Rb @ K ion-exchanged singlemode channel waveguides in KTP concerning light-induced refractive index changes in the visible wavelength region. The amount of the measured light-induced decrease of the effective refractive channel waveguide index is not larger than 1 X lo-’ for guided optical mode intensities up to 5 X lo4 W/cm*. This is at least one, mostly two orders of magnitude smaller than that we obtained in APE channel waveguides in LiNbO, under comparable conditions. The corresponding phase shifts do not restrict the function of integrated-optic devices which are based on singlemode KTP channel waveguides, also in the short visible wavelength region. For instance, at A = 0.4880 pm we obtain a light-induced phase shift of only rr/4 from 1AN, 1 = 2.0 X lop6 for a channel waveguide length of 1 cm and 1 mW guided optical power (about 1 X lo4 W/cm2 guided optical mode intensity). The effective refractive index changes show the characteristicexponential behavior of charge carrier diffusion processes. We have measured the amount as well as the time constants of the refractive index changes as a function of the guided optical mode intensity, wavelength, temperature and polarization. The sign of the light-induced refractive index changes on the ( + z) -face of KTP is negative. Our measurements indicate that the light-induced refractive index changes are due to a combination of the photovoltaic and the linear electrooptic effect in the Rb * K ion-exchanged crystal. Thermooptic and pyroelectric refractive index changes are negligible up to a guided optical power of 1 mW.

120 (I995) 47-54

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