15 April 2000

Ž .Optics Communications 177 2000 189–193www.elsevier.comrlocateroptcom

Optical waveguide formation by MeV Hq implanted intoLiNbO crystal3

Hui Hu a,), Fei Lu a, Feng Chen a, Feng-Xiang Wang a, Jian-Hua Zhang a,Xiang-Dong Liu a, Ke-Ming Wang a, Bo-Rong Shi a,b

a Department of Physics, Shandong UniÕersity, Jinan 250100, Shandong, Chinab Department of Physics, UniÕersity of Osnabruck, D-49069 Osnabruck, Germany¨ ¨

Received 8 December 1999; accepted 19 January 2000

Abstract

A MeV Hq ion-implanted waveguide was formed on an LiNbO substrate. The dose of implanted Hq ions was 2=10163

ionsrcm2 with an energy of 1.0 MeV at room temperature. The dark modes were measured using the prism couplingtechnique. The refractive index profile was analyzed using the reflectivity calculation method. The fiber probe technique wasused to measure the attenuation of the waveguide. The lattice damage in the guide region caused by Hq ion implantationwas investigated using the RBSrchanneling technique. q 2000 Elsevier Science B.V. All rights reserved.

PACS: 42.79.Gn; 42.82.EtKeywords: Optical waveguide; Ion implantation; Waveguide loss

1. Introduction

Ž .Lithium niobate LiNbO is characterized by3

large pyroelectric, piezoelectric, electro-optic, andw xphoto-elastic coefficients 1 . It is a centrally impor-

tant material in integrated and guided wave optics.Several waveguide fabrication techniques have beendeveloped to achieve waveguide structures inLiNbO , such as diffusion, ion exchange and ion3

implantation. An exciting feature of the ion implanta-tion technique is the ability to produce multiple

w xoptical barrier layers using different ion energies 2 .MeV He ions have been extensively used to form

) Corresponding author. E-mail: phshibr@sdu.edu.cn

w xion-implanted LiNbO waveguides 3,4 . Compared3

with He ions, H ions can penetrate deeper into thew xLiNbO crystal with the same energy 5,6 , so there3

may be some advantages in using H ions to formdouble structure waveguides.

In the present work, we report the waveguideformation in LiNbO crystal by MeV Hq implanta-3

tion. The refractive index profiles and loss measure-ments of the waveguide before and after annealingwere investigated.

2. Experimental

The optically polished x-cut, y-propagationLiNbO sample with the size of 20=10=1 mm3

0030-4018r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0030-4018 00 00526-5

( )H. Hu et al.rOptics Communications 177 2000 189–193190

was implanted with H ions. Half of the sample wasshielded with a metal mask. At room temperature,1.0 MeV Hq ion beam with the dose of 2=1016

ionsrcm2 was scanned to ensure a uniform implan-tation over the sample. During implantation, thenormal to the top face of the LiNbO sample was3

tilted 78 off the beam direction to minimize thechanneling effect.

The dark modes of the waveguide were measuredwith a Metricon 2010 prism coupler. A He–Ne laserbeam at 633 nm struck the base of a rutile prism,which was brought into contact with the waveguide.The reflected beam was detected by a silicon pho-todetector. The angle of incident laser beam could bevaried by means of a rotary table upon which theprism, waveguide, photodetector were mounted. Theintensity of light striking the photodetector was plot-ted as a function of the incident angle, at which asharp drop in the intensity profile would correspondto a propagation mode. The measurement systemwas controlled with a computer.

The fiber probe technique was used to measurew xwaveguide loss 7 . When a waveguide mode was

excited with a prism using a 633 nm He–Ne laserbeam, a fiber probe scanned down the length of thepropagating streak to detect the exponential decay oflight. The loss of each mode could be measuredseparately by this method. The light intensity de-tected was plotted as a function of the propagationdistance. It was assumed that the intensity of thelight scattered out of the waveguide was proportionalto the transmitted power in the waveguide. Thewaveguide loss was determined by least-square fit-ting of the experimental data to the function: 10

Ž .log I x s10 log I y a x, where I is the initial0 0Ž .power, I x is the transmitted power through the

waveguide at the distance x, and a is the attenua-w xtion of a waveguide mode 8 . The whole measure-

ment was done with the Metricon 2010 prism cou-pler.

When Hq ions were implanted into LiNbO crys-3

tal, some point defects could be produced in thesurface layer due to ionization and excitation. Chan-neling has shown to be a useful tool in the investiga-tion of the lattice disorder at the crystal surface. Thewaveguide was analyzed by the RBSrchannelingtechnique with 2.1 MeV He2q ions from the 1.7 MVtandem accelerator of Shandong University. The

sample was mounted in a three-dimensional go-niometer with an accuracy of 0.018. The backscatter-ing particles were detected at a scattering angle of1658 with a surface barrier detector. The implantedarea and the shielded area of the LiNbO sample3

were analyzed separately for comparison.

3. Results and discussion

During H ion implantation, a low index opticalbarrier was built up at the end of the ions’ track dueto elastic energy deposition from ions to the LiNbO3

crystal lattice. The region between this barrier andthe surface was therefore surrounded by regions oflow index and was able to act as a waveguide. Fig. 1shows the measured light intensity reflected from theprism versus the effective refractive index of theincident light. More than sixteen dips can be foundin this graph. The first several sharp dips with largeeffective refractive index maybe correspond to theguide modes. This means that the light in thesemodes is confined well by the optical barrier. Whenmore and more light is tunneling out of the barrierby decreasing the effective refractive index of theincident light, the dips in Fig. 1 become broader and

w xbroader 8 .The refractive index profile of the waveguide was

obtained using the reflectivity calculation method

Fig. 1. Relative intensity of reflected light versus the effectiverefractive index of incident light.

( )H. Hu et al.rOptics Communications 177 2000 189–193 191

Ž . w xRCM 9 . The refractive index distribution wasapproximated by two half Gaussians. For this givenset of index parameters, a numerical routine wasused to calculate the corresponding waveguide modeindices. A least-squared fitting program was thenused to adjust the parameters of the index profilesuch that the sum of the square of the errors betweenmeasured and calculated mode indices is minimized.This method is suitable to characterize the ion im-plantation waveguides because the observed sub-strate dark mode lines are contained in the calcula-tion to give more precise refractive index profiles.

Ž .Fig. 2 shows the refractive index profiles n of theo

waveguide before and after annealing for 30 min at2008C. About a 1.4% index decrease was found inthe optical barrier with Hq ion implantation, and theindex decrease was about 0.9% after post-implanta-tion annealing at 2008C for 30 min. Even after the

Ž .waveguide was annealed for a long time about 4 h ,the optical barrier shape did not show any change.This indicates that the Hq ion-implanted waveguidecan survive for a long time at 2008C.

It seems reasonable that the lattice damage pro-duced by ion implantation is the main reason forrefractive index change in LiNbO crystal. The lat-3

tice damage leads to a decrease in physical density

Fig. 2. Comparison of refractive index of n profiles before ando

after annealing for 1.0 MeV hydrogen ion-implanted waveguide inLiNbO to a dose of 2=1016 ionsrcm2 at room temperature.3

Solid line: before annealing; dotted line: after annealing at 2008Cfor 30 min.

Fig. 3. The vacancy density verses the depth in LiNbO crystal3

induced by 1.0 MeV Hq ion implantation to a dose of 2=1016

ionsrcm2.

w xand hence to a reduced refractive index 10 . In orderto get some information on the lattice damage, we

Ž .used TRIM’98 code Transport of Ions in Matter tow xsimulate the process of ion implantation 11 . Fig. 3

represents the vacancy distribution in LiNbO crystal3

by 1.0 MeV Hq ion implantation. It can be seen thatthe shape of damage distribution and that of refrac-tive index distribution are similar. The peaks of thedamage profile and refractive index profile are at9.84 and 10.4 mm, respectively. We attribute thisdifference to the simulation error of the computercode.

Fig. 4 shows the loss measurements of two wave-guide modes at 633 nm wavelength for the Hq

ion-implanted waveguide before and after annealing.We can see that for the mode with effective refrac-tive index 2.2830, the loss is 8.73 dBrcm. Foranother mode with effective refractive index 2.2817,the loss is 4.26 dBrcm. The high loss can beattributed to light leakage. As we know, the light inan ion-implanted waveguide is confined with a nar-row low index optical barrier. The light can partiallyleak through the barrier into the substrate because ofthe limited barrier thickness. After the waveguidewas annealed for 30 min at 2008C, the losses of themodes mentioned above were reduced to 4.07 and2.55 dBrcm, respectively. The reasons for improvedloss are probably due to the light scattering fromdefects formed by ion implantation process. Anneal-

( )H. Hu et al.rOptics Communications 177 2000 189–193192

Ž .Fig. 4. Logarithm of the scattered intensity versus propagation distance along the waveguide with two excited modes: a before annealingŽ . Ž . Ž .of mode 2.2816; b after annealing of mode 2.2816; c before annealing of mode 2.2830; and d after annealing of mode 2.2830. The full

squares represent experimental data. The lines represent the fitted values.

ing at 2008C can remove the defects in waveguidelayer effectively and at the same time has littleinfluence on the width of the optical barrier, asindicated in Fig. 2.

In order to study the properties of the latticedamage in the guiding region caused by Hq ionimplantation process, the backscattering spectra ofthe waveguide in the channeling and non-channelingdirections were measured by using RBSrchanneling

technique. In Fig. 5, dashed and solid lines representthe random and channeling spectra for virgin LiNbO3

crystal. Open circles represent the aligned spectrumfor Hq ion-implanted LiNbO waveguide. The re-3

sults show that there is hardly any detectable differ-ence of lattice disorder between the ion implantationand virgion region. Although Hq ions lose theirenergy in the guide region mainly by electronicenergy loss according to the calculation of TRIM’98,

( )H. Hu et al.rOptics Communications 177 2000 189–193 193

Fig. 5. Rutherford backscatteringrchanneling spectra for LiNbO3

waveguide. Dashed and solid lines represent random and alignedspectra for virgin LiNbO . Open circles represent the aligned3

spectrum for Hq ion-implanted LiNbO waveguide.3

this kind of energy loss has little attribution tocrystal lattice damage.

4. Summary

LiNbO waveguide was formed by MeV Hq ion3

implantation. The modes were measured by using theprism coupling method. The refractive index profilewas obtained by RCM. It is found that the height ofoptical barrier decreased a little on annealing at2008C for 30 min, but the position and the width ofthe barrier remained unchanged. The loss measure-ments using fiber probe technique were preformedwhen two modes were excited separately. The resultsshow that the loss in the mode with effective refrac-tive index 2.2830 is larger than the loss in the mode

with effective refractive index 2.2817. Annealing at2008C for 30 min can reduce the loss of the wave-guide. The Rutherford backscatteringrchannelingtechnique indicates that there is no obvious crystaldamage induced by ion implantation procession onthe guiding area.

Acknowledgements

This work was supported by the National NaturalŽ .Science Foundations of China grant No. 19875032 ,

Natural Science Foundation of Shandong province,and the Laboratory of Heavy Ion Physics, PekingUniversity. The authors would like to thank Prof.Xue-yao Zhang and Ji-Tian Liu of Shandong Univer-sity, and Prof. Ding-Yu Shen and Xue-Mei Wang ofPeking University for their help.

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