Abstract

One dominant issue for micro-resonator filters has been polarization sensitivity due to the form asymmetry in nanophotonic waveguides. Differences in the filter’s transmission intensity for TE and TM polarizations is attributed to the polarization dependent coupling. Complete power transfer in ultra-small directional couplers is demonstrated in agreement with simulations. Polarization dependence is simulated for waveguides of various widths, showing the presence of a critical width at which the propagation constants are equal for TE and TM modes. A design for polarization-independent and single-mode waveguides is discussed, along with implications for the applications of micro-resonators in general.

© 2003 Optical Society of America

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References

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IEEE J. Lightwave Technol. (1)

B. Little, G.S. T. Chi, H. Haus, J. Foresi, and J. P. Laine, �??Micro-ring resonator channel dropping filters,�?? IEEE J. Lightwave Technol. 15, 998 (1997).
[CrossRef]

IEEE J. Quantum Electron. (1)

R. A. Forber and E. Marom, �??Symmetric directional coupler switches,�?? IEEE J. Quantum Electron. QE-22, 911 (1986).
[CrossRef]

IEEE Photon. Technol. Lett. (2)

M. K. Chin, C. Youtsey, W. Zhao, T. Pierson, Z. Ren, S. L. Wu, L. Wang, Y. G. Zhou, and S. T. Ho, �??GaAs microcavity channel-dropping filter based on a race-track resonator,�?? IEEE Photon. Technol. Lett. 11, 1620-1622 (1999).
[CrossRef]

Dominik G. Rabus, and Michael Hamacher, �??MMI-coupled ring resonators in GaInAsP-InP,�?? IEEE Photon. Technol. Lett. 13, 812-814 (2001).
[CrossRef]

J. Lightwave Technol. (2)

Other (2)

Optical Waveguide Mode Solver (OWMS), Apollo Photonics, Waterloo, Canada.

K. Okamoto, Fundamentals of Optical Waveguides, (Academic. Press, 2000) p. 238.

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Figures (7)

Fig. 1.
Fig. 1.

(a) Schematic, and (b) SEM image, of a race-track shaped resonator.

Fig. 2.
Fig. 2.

The output spectrum for the TM polarization, measured at the transmission port of a race-track resonator. The FSR is 20 nm. The output is normalized by the input power. The variation in peak height is primarily due to the variation in power coupling with wavelength.

Fig. 3.
Fig. 3.

Calculated coupling length as a function of the gap size and the waveguide width (w), for both TE (dotted) and TM (solid) polarizations

Fig. 4.
Fig. 4.

(a) SEM image of a 3-µm coupler, (b) measured results of power coupling fraction as a function of the coupler’s length.

Fig. 5.
Fig. 5.

Polarization-independent coupler: beam propagation simulation of an ultra-small directional coupler with l c=18 µm for TE and TM modes. The numbers shown are the powers at the two output waveguides. About 2.5 dB is lost to radiation.

Fig. 6.
Fig. 6.

Calculated effective indices for quasi-TE and TM modes as a function of ridge waveguide width. The insets show mode profiles at various ridge widths, and the waveguide structure. D, the waveguide core thickness, is 0.5 µm.

Fig. 7.
Fig. 7.

Polarization independent single-mode ridge waveguides for different bending radii. The waveguides are defined by the critical width and the etching depth, and satisfy the criteria βXY for fundamental Ex mode and Ey modes, and less than 1dB/mm loss for the fundamental mode and at least 10x larger loss for the first-order mode.

Equations (1)

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P c = sin 2 ( π 2 l l c ) ,

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