Ultrasmall waveguide bends - CiteSeerX

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Ultrasmall waveguide bends: the corner mirrors of the future? L.H. Spiekman Y.S. Oei E.G. Metaal F.H. Groen P. Demeester M.K. Smit

Indexing ferms: Optical waveguides, Corner mirrors

Abstract: Curved waveguides with strong lateral modal confinement have been fabricated on InGaAsP/InP by etching through the waveguide core. This has resulted in extremely sharp bends (radii as small as 30 Im) with negligible bend loss, enabling fabrication of very low loss 90" turns on a chip area as small as that of a corner mirror. 1

components (f30 x 30 pm), at the cost of a higher loss (typically 1 dB/90"). The purpose of this paper is to present the fabrication of improved waveguide bends, which combine the low loss of previously known bends with the extremely small size of corner mirror structures. For comparison, first some aspects of corner mirror and curved waveguide fabrication are discussed. Then, design and fabrication of the improved waveguide bends are treated, followed by the measurement results and a short conclusion.

Introduction

2

Over the years, in optoelectronic component research many high quality components have been developed, both passive (3 dB couplers, wavelength demultiplexers, polarisation splitters and converters) and active (lasers, photodetectors, phase and intensity modulators, switches). One of the targets of integrated optics is to integrate a variety of these components on the same substrate. This must lead to low cost devices which are suitable for large bandwidth communications. Optical waveguides will serve as interconnects between the components of such a circuit, requiring the ability to change the propagation direction of the guided light. This is not a trivial problem, and it has received due attention in the literature. The proposed solutions can be subdivided into two categories: those that employ curved waveguides which smoothly change the propagation direction [1-61, and those that use a corner mirror structure for abrupt directional change [7-113. originally proposed by Benson in 1984 [12]. Each of these approaches has its own specific advantages and disadvantages. Curved waveguides can be made with extremely low (radiation) loss, but only for large bending radii. (As far as we know, no bending radii smaller than 100 pm have been reported.) The mirror solution, on the other hand, results in extremely small 0IEE, 1995 Paper 1664J (E3, E13), first received 29th June and in revised form 19th September 1994 L.H. Spiekman, Y.S. Oei and M.K. Smit are with the Delft University of Technology, Department of Electrical Engineering, Laboratory of Telecommunication and Remote Sensing Technology, PO Box 5031, NL-2600 GA Delft, The Netherlands E.G. Metaal is with PTT Research, Leidschendam, The Netherlands F.H. Groen is with Delft University of Technology, Department of Applied Physics, Optics Research Group, The Netherlands P. Demeester is with the University of Gent, Department of Information Technology, Belgium I E E Proc.-Optoelectron., Vol. 142, No. 1 , February I995

Corner mirrors

Corner mirrors on semiconductor material are typically fabricated in a self-aligned two-step process with a geometry as shown in Fig. 1. The waveguides are patterned in the first masking step, which also defines the

1 1

Fig. 1

waveguide mask deep etch m a s h

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Typical geometry of a corner mirror

position of the mirror, and then etched to the desired depth. The second masking step protects the entire chip, except for an etching window left at the position where the mirror is to be formed by a deep etching step. There are a number of critical parameters that make it difficult to attain low loss corner mirrors: 1. It is very important that the etched mirror facet is at right angles with the chip surface. A slight tilt, as shown in Fig. 1, will reflect part of the light into the substrate instead of into the output waveguide. Therefore, an etching process optimised for facet verticality must be used, and even then a deviation from verticality of only 2" will cause between 0.5 and 1 dB loss [lo]. 2. Misalignment of the mirror facet will cause an offset between the reflected beam and the output waveguide [ll]. Such a mismatch can occur in a self-aligned fabricaThe authors thank J.J.G.M. van der To1 for helpful discussions. 61

tion process due to mask erosion during etching. Also, a tilt of the facet will influence the position of the mirror. 3. Finally, the facet roughness induced by imperfections in the mask or the etching process will introduce additional mirror loss, which can be in the order of 0.5 to 1 dB [ll]. Thus, the mirror etch process must be optimised for high anisotropy (facet verticality), low mask erosion and low roughness etching - conditions which are difficult to combine. 3

4

Design of deeply etched CUNed waveguides

The radiation loss suffered at a certain bending radius can be reduced by increasing the mode confinement in the bend, i.e. by increasing the refractive index contrast. For ridge waveguides, this means increasing the etching depth. As Fig. 4 indicates, the result is that curved waveguides with smaller radii of curvature can be used.

CuNed waveguides

Directional change by means of curved waveguides is relatively easy to accomplish. The bends are defined in the same masking step as the other waveguides, and only one etching step is needed. There are, however, some other aspects that need consideration: 1. Curved waveguides have radiation loss, the more so the smaller the radius of curvature. In Fig. 2, this effect is 301.

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i Our approach in this work is to increase the index contrast by etching through the guiding layer, resulting in a waveguide structure as shown in Fig. 5. As the limit-

bending radius. prn Fig. 2 Radiation loss in a curved waueguide 000 measurementsfrom Reference 2 calculationon the basis ofEIM and a conformal transformationCl61

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demonstrated. Typically, the radiation loss is negligible for larger radii R, while for smaller R it rises exponentially to unacceptable values. The transition between these regions can be shifted to lower radii by applying a larger refractive index contrast. In the case shown, 200 pm is a usable radius. This implies usage of a rather large chip area. 2. In a curved waveguide, the optical mode shifts outward. This shift is larger for smaller bending radii, as Fig. 3 shows, and leads to coupling loss at the connection

Fig. 3 Mode profile for a straight waveguide and for curved waveguides with decreasing radii of curvature Inscl shows oEset naded for low loss coupling ofslraighl to curved waveguides

between a straight waveguide and a bend. The solution to this problem is the application of a well optimised inward offset at a straight-to-bend junction, which can reduce this loss to 0.01-0.1 dB. Waveguide bends thus allow directional change with extremely low loss, at the cost of using a larger chip area. 62

600 nrn

InP substrate

Fig. 5

Deeply etched Waveguide structure used throughout this work lnGaAsP wre with 2, = 1.3 p has a thickness of 600nm: InP cladding layer is 3oonmthick

ing case of maximum optical confinement in a ridge waveguide, that is expected to enable minimal bending radii with low radiation loss. This has already been established theoretically by Benson, Kendall and coworkers, who have derived exact formulae for the curvature loss from any solution known on a cylinder of reference in a layered medium [13, 141. They have applied these results to the rib waveguide and predicted that the curvature loss would drop dramatically when the outer slab waveguide of the layered structure is cut off. This theoretical prediction has been confirmed by microwave simulation [15]. In our work, the outer slab of the curved waveguides is indeed cut off, and the qualitative aspects of this theory are shown to be strongly supported by our results. In view of the complicated formulas involved, we here use only the effective index method (EIM), and make no attempt to calculate numerical values for the bending loss. Because the EIM is based on a separation of the modal fields in the x- and y-directions (first treating the lateral regions of the waveguide as slab waveguides, and IEE Proc.-Optoelectron., Vol. 142, No. I , February 1995

then solving the two-dimensional problem with the thus obtained effective indices), the cut off nature of the slabs next to the waveguide poses some problems. However, directly inserting Ne,, = 1 in the second step of the EIM seems to give reasonable results. A comparison with the results of a finite element straight waveguide mode solver (Fig. 6) indicates good agreement for widths 2 2 pm.

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ing gallery mode', i.e. a mode that is only guided by the outer waveguide ridge and is not influenced by the inner ridge. Apparently, the mode shape then changes more than with narrower waveguides (in which the inner ridge contributes to the modal confinement), leading to increased coupling loss. On the other hand, small waveguides are not desirable from the perspeajve of scattering loss. We therefore applied 3 pm-wide 'straight waveguides in the straight sections. They were connected to narrower bends using tapers. The bend width was chosen as W = 1.4 pm, for at this width the fundamental mode does not suffer substrate leakage, while the first-order mode is cut off. With these parameters, for radii of curvature of 100, 70, 50 and 30pm, a set of U-bend structures was designed with different curved waveguide path lengths, to assess bend loss per 90". No offsets were used in the straight-to-bend junctions, because these would be smaller than 0.1 pm even for the 30 pm bends (Fig. 8).

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Fig. 6 Comparison of effective index of deeply etched straight waveguides, as calculated with EIM (line)and with afinite element method 2-D mode solver (points)

For modelling curved waveguides with the EIM, a conformal transformation technique is used as described by Reference 16. According to the EIM, for bending radii in the order of the waveguide width, radiation loss is still negligible. The modal effective index decreases with decreasing radius. As soon as it becomes lower than the substrate refractive index, substrate leakage will occur. This effect helps to maintain monomodality, but it also imposes a lower bound to the range of possible bending radii [17]. In our waveguide structure, substrate leakage for the fundamental mode will occur at R z 26 pm for W = 1 pm, and at smaller radii for larger W . Another factor influencing the range of usable bending radii is the mode profile (Fig. 3), which is not only shifted, but also changed in shape. The latter effect cannot be cured with an offset. Fig. 7 shows the straight-to-bend

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and without (----) offset, for Fig. 8 Junction loss with )-( straight-to-bend junctions of 1.4 pm-wide waveguides Optimal offset varies from 0.01 pm at R = 200 p to 0.1 p at R = 25 p

In our design, we use deep etching on the entire chip. This is not at all a prerequisite for using deeply etched bends: they can be made with the same process as corner mirrors, as shown in Fig. 9. The etching depth of the

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6 1 1.5 2 25 3 3.5 waveguide width, prn Fig. 7 Minimal straight-to-bend coupling loss for diffment radii and waveguide widths 05

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coupling loss (with optimal offset) for different radii and waveguide widths. Clearly, very wide bends are not a good choice. The light will then propagate as a 'whisperIEE Proc.-Optoelectron., Vol. 142, N o . I , February 1995

Fig. 9 Configuration for combining deeply etched bends with shallowly etched waveguides, employing an etching window 63

waveguide outside the etching window is freely choosable. The field mismatch at the discontinuities can be reduced below 0.1 dB by adapting the waveguide width at both sides of the junction. 5

Fabrication

The devices were made on an SI-InP substrate with MOVPE-grown undoped epitaxial layers [18], the thickness of which slightly deviated from the design values: 660 nm InGaAsP (A, = 1.3 pm) and 320 nm InP. The devices were defined in photoresist with projection lithography, and transferred into a 140 nm-thick RFsputtered SiO, masking layer with CHF, reactive ion etching. Device dimensions turned out to be 0.3 fim smaller than the design values. Subsequently, the chip was etched with C H n , RIE, employing an alternate etching/O,-descumming process to reduce polymer deposition induced roughness. This process produces extremely smooth sidewalls and correspondingly low propagation losses [19], which is a matter of major concern for high index contrast waveguides. 6

Measurement results

Straight waveguide propagation loss was measured both with Fabry-Perot (FP) contrast measurements and cutback transmission measurements. Other losses were measured with transmission measurements as excess losses relative to the straight waveguide loss. For the FP measurements, the wavelength was swept between 1533 and 1535nm using a temperature tuned DFB laser. The transmission measurements were performed with a 1508nm Fabry-Perot laser. Light was coupled in and out of the chip with AR coated microscope objectives ( N A = 0.65) and projected onto a Gedetector through a pinhole. All measurements were performed with the chip uncoated. Cutback measurements revealed a propagation loss through the 1.1 pm straight waveguides of 6.6 dB/cm for TE and 5.0 dB/cm for TM (the TE/TM difference reflects the better lateral confinement of the TM mode), in good agreement with the FP results. For the (multimode) 2.7 pm straight waveguides, losses of 0.6 dB/cm were measured, both for TE and TM polarisation. FP measurements gave a higher value of 2dB/cm, but are not very reliable for multimode waveguides. Waveguides incorporating series of tapers with lengths varying from 10 to 50 p n were measured, and showed an excess loss, in the order of the statistical measurement error of the set-up, of about 0.2 dB. This indicates that the loss introduced by individual tapers was negligible for all tested taper lengths. Fig. 10 shows the excess loss of the U-bend structures for TE polarisation (the TM results are comparable). For each of four bending radii, four U-bends with different total curvatures were measured. The points missing from the Figure are due to provable defects in the corresponding waveguides. It is seen that there is no significant increase in loss with increasing total bend angle, indicating that radiation loss and substrate leakage are negligible. Mainly, the loss is due to the constant contribution at the straight-tobend and bend-to-bend junctions. It is seen, for example, that the loss of the R = 30 pm U-bends of around 0.6 dB can be explained by this junction contribution (indicated by the horizontal lines on the left of Fig. 10) of 0.52 dB, 64

as predicted by modal overlap calculations. Even for the anomalous value of 1.1 dB for a 270" bend (which we believe to be due to a defect so small as not to be found

test structure bends

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90

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total bend angle,degrees

Fig. 10 Excess loss of U-bend structures for T E polarisation Estimated measurement error is indicated. Losses plotted as a function of amount of total bend angle the light has to propagate through. A calculated value for Constant loss contribution of four straight-to-bmd and two bend-to-bend junctions indicated by straight l i n s on the left

with a microscope), the radiation loss must still be below 0.2 dB/90". 7

Conclusion

It has been shown that, with deeply etched ridge waveguides, extremely sharp waveguide bends with negligible bend loss can be made. In the experiments, the main loss contribution came from the junctions. Radiation losses are smaller than 0.2dB/9Oo even for bending radii as small as 30 pm. With the configuration proposed in Fig. 9, a 90" change of direction can be made on an area with a size comparable to that of a corner mirror, with an identical two-step maskindetching process, but without the need to carefully control the etching process for sidewall verticality, and with considerably lower loss. 8

References

1 MARCATILI, E.A.J.: 'Bends in optical dielectric guides', Bell Syst. Tech J., 1969.48, pp. 2103-2132 2 VERBEEK, E.H.,PENNINGS, E.C.M., VAN UFFELEN, J.W.M., and THIJS. PJ.A.: 'Fabrication and analvsis of low-loss InGaAsP/ InP optical waveguides with extremely small bends'. Proceedings of 15th European Conference on Optical communications (ECOC '89). Gothenburg, 10th-14th September 1989, Postdeadline papers, pp. 78-81 3 SINGH, J., HENNING, I., HARLOW, M., and COLE, S.: 'Singlemode low-loss buried optical waveguide bends in GaInAsPpnP fabricated by dry etching', Electron. Lett., 1989.25, pp. 899-900 4 VAN BRUG, H., GROEN, F.H., OEI, Y.S.. PEDERSEN, J.W., PENNINGS, E.C.M., DOEKSEN, D.K., and VAN DER TOL, J.J.G.M.: 'Low-loss straight and curved ridge waveguides in LPEgrown GaInAsP, Electron. Lett., 1989,25, pp. 1330-1332 5 PENNINGS, E.C.M.: 'Bends in optical ridge waveguides, modeling and experiments'. PhD thesis, Delft University of Technology, Delft, The Netherlands, 1990. ISBN 90-wO3413-7 6 SMIT, M.K., PENNINGS, E.C.M., and BLOK, H.: 'A normalized approach to the design of optimal optical waveguide bends', J . Lightwave Technol., 1993,11, pp. 1737-1742 I NIGGEBRUGGE, u., ALBRECHT, P., DOLDISSEN, w., NOLTING, H.P., and SCHMID, H.: ‘Self-slimed low-loss totally reflecting waveguide mirrors in InGaAsPpnP. Proceedings of 3rd Conference on Integrated optics, ECIO '87, 28th-30th March 1987, pp. 90-93 IEE Proc.-Optoelectron., Vol. 142, No. I , February I995

8 ALBRECHT, P., DOLDISSEN, W., NIGGEGRUGGE, U,, NOLTING, H.P., and SCHMID, H.: ‘Waveguide mirror components in InCaAsPflnP. Proceedings of 14th European Conference on Optical communications (ECOC ’88),Brighton, 1988,pp. 235-238 9 GINI, E., GUEKOS, G., and MELCHIOR, H.: ‘Low loss comer mirrors with 45” deflection angle for integrated optics’, Electron. Lett., 1992,28, pp. 499-501 10 BURNESS, A.L., LOSEMORE, P.H., JUDGE, S.N., HENNING, I.D., HICKS, SE., DOUGHTY, G.F., ASGHARI, M., and WHITE, I.: ‘Low loss mirrors for InP/InGaAsP waveguides’, Electron. Lett., 1993,29,pp. 520-521 1 1 VAN ROIJEN, R., VAN DER HOFSTAD, G.L.A., GROTEN, M., VAN DER HEYDEN, J.M.M., THIJS, P.J.A., and VERBEEK, B.H.: ’Fabrication of low-loss integrated optical corner mirrors’, Appl. Opt., 1993,32,pp. 3246-3248 12 BENSON, T.M.: ‘Etched-wall bent-guide structure lor integrated optics in the 111-V semiconductors’, J . Lightwave Technol., 1984,2, pp. 31-34 13 KENDALL, P.C., ROBSON, P.N., and SITCH, J.E.: ’Rib waveguide curvature loss: the scalar problem’, IEE Proc. J , 1985, 132, pp. 140-145

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14 KENDALL, P.C., STERN, M.S., and ROBSON, P.N.: ‘A new curvature loss formula of Huygens-type for rib waveguides’, IEE Proc. J , 1988,135,pp. 11-16 15 BENSON, T.M., KENDALL, P.C., and STERN, M.S.: ‘Microwave simulation of optoelectronic bending loss in presence of dielectric discontinuity’, IEE Proc. J , 1988,135, pp. 325-331 16 HEIBLUM, M., and HARRIS, J.H.: ‘Analysis of curved optical waveguides by conformal transformation’, IEEE J . Quantum Electron., 1975,QEll, pp. 75-83;correction, QE-12, p. 313 17 P E N N I N G , E.C.M.,\DERI, R.J., and HAWKINS, R.J.: ‘Simple method for estimating usable bend radii of deeply etched optical rib waveguides’, Electron. Lett., 1991,27, pp. 1532-1533 18 MOERMAN, I., COUDENYS, G., DEMEESTER, P., TURNER, B., and CRAWLEY, J.: ‘Influence of gas mixing on the lateral uniformity in horizontal MOVPE reactors’, J . Cryst. Growth, 1991,107, pp. 175-180 19 SPIEKMAN, L.H., VAN HAM, F.P.G.M., KROONWIJK, M., OEI, Y.S., VAN DER TOL, J.J.G.M., GROEN, F.H., and COUDENYS, G.: ‘A new fabrication process for very low-loss narrowwidth InGaAsP/InP waveguides’. Proceedings of 6th Conference on Integrated optics, ECIO ’93, Neuchltel, Switzerland, 18th-22nd April 1993,pp. 230-231

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