Coupling from a thin film to an optical fiber G. A. Teh and G. I. Stegeman

University of Toronto, Physics Department, Toronto, Ontario M5S 1A7. Received 3 April 1978. 0003-6935/78/0815-2483$0.50/0. © 1978 Optical Society of America. Many applications of integrated optics, especially those in

optical communication, involve the need of efficient coupling between thin film optical circuits and fibers. There are two basic approaches: direct endfire excitation1,2 and surface coupling via evanescent fields.3,4 The former requires careful alignment and dimensional precision. We have recently shown that efficiently coupling two thin films on separate substrates is possible using the tapered velocity mechanism.5

This method is simple and free from the usual tolerance constraints.

Our experiment is a logical extension of an earlier work,6

and the basic principle is related to a technique previously described4 in which 70% coupling efficiency has been achieved from a thin film to a specially designed externally mounted rectangular fiber.

The schematic representation of our experimental ar­rangement is shown in Fig. 1. The coupling was achieved from a Corning 7059 film on fused quartz substrate to an un­clad 60-μm diam fiber drawn from soda-lime glass. The fiber was pressed against the tapered section of the thin film waveguide by a chip of fused quartz. The results are shown in Fig. 2. The fiber end appears erroneously as a series of filaments since it was out of focus for the photograph. For the region of geometrical overlap of the fiber with the guided light beam in the film, we estimate that at least 90% of the light has been coupled out of the film.

It must be appreciated that the experiments were con­ducted with a fiber that supports tens of thousands of modes. Effectively this means that the fiber serves like an output prism coupler7 and is, in principle, 100% efficient. Therefore, one would expect that as long as the mode index β/k in the film is less than the refractive index of the glass fiber, efficient coupling, even at a uniform section of the film, could occur. This was in fact found to be so in our experiments for the TE2

Fig. 1. Schematic diagram of film-to-fiber coupling.

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Fig. 2. Light is coupled into the glass film through the prism coupler on the right; at the taper, the guided light is transferred to the fiber.

mode (β/k = 1.5048) and the TE 3 mode (β/k = 1.4692), which have mode indices less than the refractive index (n = 1.5124 at λ = 6328 Å) of the soda-lime glass fiber. This phenomenon is similar to the results reported by Laybourn et al.3 On the other hand, the TE 0 mode (β/k = 1.5512) and the TE1 mode (β/k = 1.5332) are both not coupled at the uniform section of the film. However, all four modes are efficiently coupled at the taper, indicating that the power transfer involves a tapered velocity mechanism.

The Corning 7059 glass film used in our experiment has a refractive index of 1.557 and a thickness of 2.1 μm at the uni­form section. The effective tapered section for the TE0 mode is about 4 mm. We emphasize that while the use of tapered velocity coupling is not significantly advantageous with the very overmoded fiber that we have employed, it is, we believe, intuitively clear that the method lends itself far more favor­ably with most commercially available cladded fibers. We are of the opinion that it is especially suited for single mode fiber if evanescent field coupling were to be employed in fi­bers/integrated optics interconnections.

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

References 1. L. P. Bovin, Appl. Opt, 13, 391 (1974). 2. P. K. Tien, G. Smolinsky, and R. J. Martin, IEEE Trans. Micro­

wave Theory Tech. MTT-23, 79 (1975). 3. P. J. Laybourn, C. A. Millar, G. Stewart, and C. P. W. Wilkinson,

Electron. Lett. 11,2 (1975). 4. D. G. Dalgoutte, R. B. Smith, G. Achutaramayya, and J. H. Harris,

Appl. Opt. 14, 1860 (1975). 5. 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 WDl-1.

6. M. G. F. Wilson and G. A. Teh, IEEE Trans. Microwave Theory Tech. MTT-23, 85 (1975).

7. P. K. Tien, Appl. Opt. 10, 2395 (1971).

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