Optical channel waveguides by copper ion exchange in glass

Optical channel waveguides by copper ion-exchange in glass D. Salazar, H. Porte, and H. Ma´rquez

Optical channel waveguides have been obtained by electric-field-assisted diffusion of copper films on glass substrates. The mode indices of these channel waveguides were determined with the prism-coupling technique, and the refractive-index profile of the waveguide was reconstructed from measurements of the near-field intensity distribution. © 1997 Optical Society of America

1. Introduction

2. Experimental Setup

Actually, a variety of ion-exchange processes have been developed for fabricating different kinds of glass channel waveguides.1– 4 The ion-exchange technique is considered to be an important method for fabricating passive integrated optical devices. Ionexchange glass waveguides are typically fabricated with molten salts or metallic films as ion sources. In the case of electric-field-assisted diffusion, silver films are extensively used as ion sources. However, there is limited information related to copper ionexchange in glass,5– 8 and, to the best of our knowledge, copper channel waveguides have not been investigated. In early work, planar optical waveguides were obtained by Cu1–Na1 exchange on glass showing highindex change and low birefringence.7 As a starting point in the development of copper-integrated optics circuits, our first step was to fabricate straight channel waveguides. In this paper we present the modal characterization by the prism-coupling technique and near-field measurements of a channel waveguide obtained by copper ion-exchange in glass.

Sets of ten-channel waveguides were fabricated by electric-field-assisted Cu1–Na1 ion exchange in soda–lime glass substrates ~n 5 1.514!. A standard photolithography technique was used to make the mask openings in a titanium layer on a 1.0-mm-thick glass substrate. The mask design contains lines 4 cm long and 5 mm wide, and the space between adjacent lines was 100 mm. With these dimensions a long propagation path was created and lateral diffusion effects were smaller than the line space. Figure 1 shows the experimental setup used for the diffusion process. A copper film 1.0 mm thick was deposited on the line pattern. To ensure a good electrical contact and prevent oxidation and flaking of the copper during the diffusion process, a 0.5-mm gold film was deposited on the top of the copper film and similarly on the backsurface of the sustrate. The diffusion process was carried out at 350 °C with an electric field of 10 Vymm for 30 min. The electric current flow was monitored during the diffusion process by a digital multimeter in series with the voltage source. During this diffusion process a 42-mA constant current was detected, corresponding to a density current of 100 mAycm2 ~measurements were taken at intervals of 1 min! in agreement with experimental results obtained in copper ionexchanged planar waveguides.7 After the diffusion procedure, by using chemical etchings, we eliminated metallic layers. Then, the end faces were optically polished to facilitate light coupling into the channel waveguides.

´ pD. Salazar and H. Ma´rquez are with the Departamento de O tica, Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada, Kilo´metro 107, Carretera Tijuana-Ensenada, Ensenada Baja California, Me´xico. H. Porte is with the Laboratoire d’Optique P. M. Duffieux, Universite´ de Franche-Compte´ UFR des Sciences et Techniques, 16 Route de Gray, 25030, Bensancon, Cedex, France. Received 11 February 1997; revised manuscript received 7 July 1997. 0003-6935y97y348987-05$10.00y0 © 1997 Optical Society of America

3. Results and Discussion A.

Effective Refractive-Index Measurements

Sets of ten copper ion-exchange channel waveguides were analyzed by a Metricon Model 2010 prism– 1 December 1997 y Vol. 36, No. 34 y APPLIED OPTICS

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Fig. 1. Schematic representation of copper ion-exchange process.

coupler system. The mean values of the effective refractive indices of guided modes are shown in Table 1; indice values were obtained with an accuracy of 61 3 1024 with measurements of the coupling-mode angles of the prism coupler. The values of the refractive indices for different channels are similar, and the maximum index change obtained was Dn 5 0.052 at l 5 632.8 nm. B.

Channel Waveguide Diffusion Process

Ion exchange in glass is a well-known technique that has been used for manufacturing graded-index optical waveguides in glass substrates.4 The evolution of the ion-concentration distribution in the glass during the ion-exchange process has been interpreted by diffusion theories. Diffusion theory has been developed by some authors; in particular, a general equation ~Fick’s first law! of ion exchange with an external electric field was derived by Tervonen et al.2: D¹2C D~M 2 1!~¹C!2 1 MJ0¹C ]C , 5 2 ]t C~M 2 1! 1 1 @C~M 2 1! 1 1#2

(1)

which represents a change of concentration with time in the electric-field-assisted diffusion process. Here M is the ratio of the self-diffusion constant from the exchange ions, D is the diffusion coefficient of incoming ions, C is the relative concentration, and J0 is the electric current density. Solving Eq. ~1! is not easy. To obtain accurate results, one must take into consideration that diffusion coefficients depend on the concentration, the boundary conditions are time dependent, and ions have different mobilities. If electric-field-assisted diffusion is used to fabricate channel waveguides, the presence of a metallic Table 1. Effective Mode Indices of a Typical Cu-Diffused Channel Waveguides at 632.8 nm

Mode order

n

Standard Deviation

00 01 02

1.5667 1.5437 1.5186

0.0035 0.0051 0.0060

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Fig. 2. Numerically calculated constant concentration contour of copper ion. Contour corresponds to a 0.1 change in concentration from 0.9 to 0.1.

mask induces a lateral electric field, and lateral diffusion also originates. Concentration profiles are determined by lines of electrical flux. Mask presence modifies the ionic current distribution, and it is necessary to include boundary conditions on each step when Eq. ~1! is solved by numerical methods. To model the channel waveguide fabrication process by the ion-exchange process where Cu1 ions are introduced in glass, we calculated a change of concentration with a finite-difference algorithm ~Ionex, Optonex Ltd., Integrated Optics Design Software! to solve Eq. ~1!. Our experimental values used in this work were the following: Cu diffusion coefficient, D 5 5.39 3 1016 m2ys; voltage, 10 V; temperature, T 5 350 °C; ion source strip width, W 5 5 mm; Cu thin-film thickness, 1 mm; M 5 0.19; current density, 100 mAycm2; time diffusion, t 5 1800 s. Figure 2 shows the calculations of the copper ionic diffusion in two dimensions. The solid curves are constant concentration contours of copper ions. It is well known that there is a close correlation between the index profile and the glass waveguides chemical composition. The change in refractive index is taken to be proportional to the concentration of the Cu1 dopant ion introduced in the glass.7 The refractive-index distribution function n~x, y! for channel waveguides, with two-dimensional diffusion from a long narrow strip of source material on the surface of the substrate, is given by10

S D SD

n~x, y! 5 ns 1 ~Dn!exp

2x2 y , 2 erfc dx dy

(2)

where ns is the substrate index, Dn is the maximum surface index change, and dx and dy are the lateral diffusion distance and diffusion depth of the channel waveguide, respectively. C.

Near-Field Measurements

The near-field method has been used to measure the refractive-index distribution in channel waveguides.

Fig. 3. Near-field experimental setup.

This method is based on the relation between the refractive-index profile and the field distribution of a guided mode. We know that the electric field E~x, y! of the guided mode is described by the Helmholtz equation11: ¹t2E~x, y! 5 @k02n2~x, y! 2 b2#E~x, y! 5 0,

tives, imaging the waveguide end onto a CCD camera, and then converted to digital data. From this information, the second derivative of intensity is computed numerically. Refractive-index profiles for channel waveguides have been determined by measuring the near-field intensity with a CCD camera used to image the nearfield pattern in a frame grabber to capture it pixel by pixel. This technique presents some advantages, because a linear response of the system exists with respect to the detected signal. Figure 4 shows a typical intensity contour of an output light obtained from the channel waveguide made from a 5-mm-width mask opening. Knowing the magnification of the optical imaging system ~determination of magnification was made with a microreticle properly focused and microscope objective–CCD camera distance fixed!, we determine the dimensions

(3)

where b is the guide propagation constant, k0 is the wave number in the air, and n~x, y! is the refractiveindex profile of the waveguide. From Eq. ~3! we can derive an expression for the refractive-index profile: n2~x, y! 5

b2 ¹t2@I~x, y!1y2# 2 k02 k02@I~x, y!#1y2

(4)

and calculate n~x, y! for a given copper channel waveguide from their near-field intensity pattern, I~x, y!. The intensity is proportional to the square of E~x, y!, and the constant term does not affect the near-field distribution. The experimental setup used in this work is shown in Fig. 3. The samples were aligned in a micromechanical mount to obtain efficient coupling. Light from an He–Ne laser ~l 5 0.6328 mm! is coupled into the waveguide through a 403 microscope objective. After propagation in the guide, the output light from the waveguide is collected by other microscope objec-

Fig. 4. Contours of output intensity of a typical channel waveguide.

Fig. 5. Near-field results: ~a! width intensity smooth profile ~normalized!, ~b! width profile index reconstruction, ~c! depth intensity smooth profile ~normalized!, and ~d! depth profile index reconstruction. 1 December 1997 y Vol. 36, No. 34 y APPLIED OPTICS

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index distribution for copper channel waveguides with an equation derived from two-dimensional diffusion theory. Index profiles, obtained from nearfield measurements and theoretical calculations, have a similar shape, but index reconstruction from near-field intensity shows experimental noise. Copper ion-exchanged channel waveguides have an appearance similar to planar waveguides,7 which do not show coloration as a result of possible metallic copper droplets. It is well known that metallic particles in channel waveguides can give rise to absorption bands and scattering losses. By simple inspection it is difficult to appreciate scattered light from waveguides when end coupling is realized. Regarding this, we believe that there is little loss but loss measurements have not been taken. 4. Conclusions

Fig. 6. Refractive-index profile distribution of copper channel waveguides: ~a! width direction and ~b! depth direction.

of the channel waveguide by scaling our images with the calibration reference. One can determine the refractive-index distribution by using Eq. ~4!. This calculation must be made carefully; if we use direct CCD data, results will not be clear, because derivatives are sensitive to experimental noise and it is necessary to adjust the experimental data. We adjusted the data through averaging each point i with its neighbors, i 2 1 and i 1 1. Slices of the measured field intensity were taken to index reconstruction. One is a parallel slice closer to the edge, and the other is a transversal cut at the center of the channel waveguide giving the following dimensions: 8.18 mm, width, depth diffusion, 3.01 mm; Dn 5 0.058. In Fig. 5, we present a graphic sequence of the results: ~a! width intensity of the smooth profile, ~b! width profile of the index reconstruction, ~c! depth of the smooth intensity profile, and ~d! depth profile of the index reconstruction. In Fig. 6, we show a simulation of the refractive index for the copper channel waveguides, using Eq. ~2!, the theoretical index distribution to the diffusion channel waveguide. We obtained the following values: ns 5 1.514, Dn 5 0.058, dy 5 3.01 6 0.45 mm ~from the near-field images!, and dx 5 5.0 mm ~the mask width!. By using these values, we calculated the refractive8990

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Graded-index channel waveguides have been obtained by means of Cu1–Na1 ion exchange on glass substrates with mask openings of 5 mm. The diffusion process was assisted by an external electric field. The maximum index change found by the prismcoupling technique was Dnmax 5 0.052 at 632.8-nm wavelength. Refractive-index distributions were reconstructed from CCD images from the near-field patterns of waveguide output; the maximum index change was Dn 5 0.058, and from the same image we determined that the waveguide was 8.18 6 0.78 mm wide and 3.01 6 0.45 mm deep. The maximum values of the index change obtained from prism-coupling and near-field techniques are closer. We calculated a theoretical index distribution with measured parameters from the near-field pattern ~Dn 5 0.058 and 3.0-mm depth! and the mask width ~5.0 mm!. The shapes of the index distribution in the lateral direction are in good agreement with near-field results, but in the depth direction there are differences. These differences can be due to the presence of the metallic mask in the diffusion process. Finally, our results show that copper channel waveguides are suitable as an alternative process for the fabrication of passive integrated optics circuits. We are grateful to Javier Da´valos for assistance in preparing the samples. The authors were supported by Consejo National de Investigaciones Cientificas y Tecnicas-Centre National de la Recherche Scientifique cooperation program E.130.1918. References 1. A. Miliou, H. Zhenguang, H. C. Cheng, R. Srivastava, and R. V. Ramaswamy, “Fiber-compatible K1–Na1 ion-exchanged channel waveguides–fabrication and characterization,” IEEE J. Quantum Electron. 25, 1889 –1897 ~1989!. 2. A. Tervonen, S. Honkanen, and M. Leppihalme, “Control of ion-exchanged waveguides profiles with Ag thin films sources,” J. Appl. Phys. 62, 759 –763 ~1987!. 3. T. Possng, R. Goring, and C. Kaps, “Index gradient fabrication by ion exchange,” in Vision Science and Its Applications, Vol. 2 of 1994 OSA Technical Digest Series ~Optical Society of America, Washington, D.C., 1994!, pp. 74 –77.

4. S. I. Najafi, Introduction to Glass Integrated Optics ~Artech House, Norwood, Mass., 1992!. 5. S. Gevorgyan, “Single step buried waveguides in glass by fieldassisted ion-exchange,” Electron. Lett. 26, 38 –39 ~1990!. 6. S. Saka, K. Kamiya, and K. Kato, “Incorporation of copper into glass by the Cu–Na ion exchange,” J. Non-Cryst. Solids 52, 77–90 ~1982!. 7. H. Marque´z, D. Salazar, A. Villalobos, G. Paez, and J. M. Rinco´n, “Experimental study of Cu1–Na1 exchanged glass waveguides,” Appl. Opt. 34, 5817–5822 ~1995!. 8. F. Gonella, “Characterization of Cu-Na ion-exchanged glass waveguides,” Appl. Phys. Lett. 69, 314 –315 ~1996!.

9. J. M. White and P. F. Heidrich, “Optical waveguide refractive index profile determined from measurements of mode indices: a simple analysis,” Appl. Opt. 15, 151–155 ~1976!. 10. M. N. Weiss and R. Srivastava, “Determination of ionexchanged channel waveguide profile parameters by mode index measurements,” Appl. Opt. 34, 455– 458 ~1995!. 11. K. Moroshita, “Index profiling of three dimensional optical waveguides by the propagation-mode near-field method,” IEEE J. Lightwave Technol. 4, 1120 –1122 ~1986!. 12. G. L. Yip, P. C. Noutsios, and L. Chen, “Improved propagationmode near-field method for refractive-index profiling of optical waveguides,” Appl. Opt. 35, 2060 –2067 ~1996!.

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