Mode conversion in a thin film waveguide by a two-stage coupling process G. A. Teh and G. I. Stegeman

University of Toronto, Physics Department, Toronto, Ontario M5S 1A7. Received 11 August 1978. 0003-6935/79/020145-02$00.50/0. © 1979 Optical Society of America. A guided mode in a thin film waveguide can be converted

to another mode of a different order propagating in a separate waveguide by directional coupling.1-4 The concept of tapered block coupling,4 by virtue of its simplicity, is particularly suited for such an application. However, there may be oc­casions when one wishes to retain the converted mode in the original waveguide. A scheme to accomplish this is via the use of a so-called branching waveguide,5 where, by a careful deposition technique, a thin film is divided into two sublayers having a carefully shaped contour. A TE0-TE1 mode con­version has been achieved in such a structure. We have ap­plied a double coupling process, which can perform the above task in a simpler manner.

The basic principle is related to a technique previously described,4 in which mode conversion, via high efficiency ta­pered velocity coupling, was obtained between two separate waveguides. In our present experiments, a design is chosen so that the coupled power in the secondary waveguide is recoupled to another mode in the original waveguide. As the theory of tapered velocity coupling has been discussed3,6 and experimentally established previously,3,4,7 we will proceed to describe our experimental results.

The schematic representation is shown in Fig. 1. Guide 1 and guide 2 are two silver ion-diffused waveguides fabricated by a well known technique.8 The silver nitrate melt that we used was kept at a temperature of 250°C. With an immersion time of 4 min, guide 1 supports only the TE0 mode (mode index (β/k = 1.5440). For guide 2, a two-stage dipping (2 min followed by partial immersion for 1 h) resulted in a tapered structure which supports four TE modes (with β/k values of 1.5791, 1.5569, 1.5375, and 1.5208) in the heavily diffused section. In the lightly diffused region, the guide supports only the TE0 mode (β/k = 1.5264).

With reference to Fig. 2, the operation of our device can be neatly followed. From the figure it would appear that tapered velocity coupling applies to only the TE0 and TE1 modes of guide 2. As expected, excitation of the TE0 mode in guide 2 results in it being coupled to the TE0 mode of guide 1 at the taper.4 For the TE2 mode, the situation is slightly more complicated. At the first crossover of β's the TE1 mode of guide 2 is coupled to the TE0 mode of guide 1. This TE0 mode is recoupled to the TE0 mode of guide 2 when the two corre­sponding β's next intersect each other. Thus we notice that a TE1 mode is converted into a TE0 mode within the same waveguide.

Using a wedge, by simply pressing guide 1 against the taper of guide 2, we observed a mode conversion efficiency of about 50%. This figure is improved to around 75% when an index

matching liquid (n = 1.515) was applied between the two waveguides. There is, in principle, no reason why close to 100% conversion efficiency cannot be obtained. This is be­cause one can always trade a longer tapered interaction length for coupling efficiency.3 However, our ability to control the taper length is limited by the diffusion process used to fabri­cate our waveguides. A sputtering system3 would provide a better means of controlling the taper.

The use of a separate and detachable waveguide (guide 1 in our example) has the practical advantage that it allows guide 2 to be used independently if desired. The design is fairly straightforward. We only need to arrange for at least two modes of guide 2 to have mode indices higher than the mode in guide 1 and to make sure that after the taper the TE0 mode in guide 2 has a lower mode index than guide 1. The

Fig. 1. Schematic diagram of mode converter.

Fig. 2. Variation of mode indices β/k with distance z.

particular set of waveguides described above was arrived at by a trial-and-error basis. Various immersion times were employed to fabricate the ion-diffused waveguides. For those waveguides which did not satisfy the design criteria just de­scribed, negligible mode conversion was observed. This is a further confirmation of the validity of our design.

We demonstrated that a simple two-stage coupling process can be used to achieve TE1-TE0 mode conversion in the same waveguide. The results encourage endeavor in applying tapered coupling techniques to problems encountered in in­tegrated optics.

G. A. Teh is on leave from Nanyang University, Physics Department, Republic of Singapore.

15 January 1979 / Vol. 18, No. 2 / APPLIED OPTICS 145

References 1. M. G. F. Wilson and G. A. Teh, in Digest of Topical Meeting on

Integrated Optics (Optical Society of America, Washington, D.C., 1974); paper WB9-1.

2. F. Auracher, Opt. Commun. 11, 187 (1974). 3. M. G. F. Wilson and G. A. Teh, IEEE Trans. Microwave Theory

Tech. MTT-23, 85 (1975). • 4. G. A. Teh, V. So, and G. I. Stegeman, in Digest of Topical Meeting

on Integrated and Guided Wave Optics (Optical Society of America, Washington, D.C., 1978), paper WD1-1.

5. H. Yajima, Appl. Phys. Lett. 22, 647 (1973). 6. R. B. Smith, J. Opt. Soc. Am. 66, 882 (1976). 7. D. G. Dalgoutte, R. B. Smith, G. Achutaramaya, and J. H. Harris,

Appl. Opt. 12, 1860 (1975). 8. T. G. Giallorenzi, E. J. West, R. Kirk, R. Ginther, and R. A. An­

drews, Appl. Opt. 12, 1240 (1973).

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