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Photonic crystal directional coupler switch with small switching length and wide bandwidth

Noritsugu Yamamoto, Toru Ogawa, and Kazuhiro Komori

Open Access Open Access

Abstract

A directional coupler switch structure capable of short switching length and wide bandwidth is proposed. The switching length and bandwidth have a trade-off relationship in conventional directional coupler switches. Dispersion curves that avoid this trade-off are derived, and a two-dimensional photonic crystal structure that achieves these dispersion curves is presented. Numerical calculations show that the switching length of the proposed structure is 7.1% of that for the conventional structure, while the bandwidth is 2.17 times larger.

©2006 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. Schematics of our considering ring buffer device. It consists of an directional coupler switch as an input- and output-port and an ring shape waveguide to store the optical pulses.

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Fig. 2.
Fig. 2. Schematic of directional coupler switch. In white region of which length is L fix, optical parameters are fixed. In gray region of which length is L sw, the wavenumbers of each eigen modes will be changed from k e and k o to k′e and k′o respectively by switching operation such as change of refractive index.

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Fig. 3.
Fig. 3. Schematic of ideal dispersion curve for small switching length and wide bandwidth. Blue lines are even modes and red lines are odd modes. Solid lines and dashed lines are the dispersion curves at switch-off and switch-on condition, respectively. f n is the operating frequency which should be between the flat frequency region of even modes before and after the switching operation.

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Fig. 4.
Fig. 4. Dispersion curves and energy distributions of parallel photonic crystal waveguides consisting of uniform holes arranged in a triangular lattice. The field distribution of even mode around the wavenumber marked by circle is spread into center hole array. On the other hand, the field of another are concentrated nearby waveguides.

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Fig. 5.
Fig. 5. (a) The directional coupler structure to realize flat frequency region. The radiuses of the air holes of center and outside of waveguides are enlarged to 0.445a and 0.33a respectively. (The radiuses of the another air holes are 0.29a.) And the position of the air holes of outside of waveguides are shifted 0.213a towards the center of the structure. (b) The dispersion curves of the structure shown in Fig. 5(a). The even mode have flat frequency region.

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Fig. 6.
Fig. 6. Switching lengths and bandwidths of directional coupler switch: (a) proposed structure and (b) conventional structure. Solid lines denote switching length, and dashed lines denote bandwidth.

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Tables (1)

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Table 1. Minimum switching lengths and bandwidths for various refractive index changes

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Equations (8)

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( k e − k o ) L c = ( 2 n + 1 ) π
( k e , fix − k o , fix ) L fix + ( k e , off − k o , off ) L sw = ( 2 n + 1 ) π , and
( k e , fix − k o , fix ) L fix + ( k e , on − k o , on ) L sw = 2 mπ ,
L sw = ( 2 n + 1 ) π ( k e , on − k e , off ) − ( k o , on − k o , off )
dk = − 1 ∂ ω ∂ k ∂ ω ∂ n dn .
L sw ≃ ( 2 n + 1 ) π − 1 ∂ ω e ∂ k ∂ ω e ∂ n dn + 1 ∂ ω o ∂ k ∂ ω o ∂ n dn
≃ ( 2 n + 1 ) π ( − 1 ∂ ω e ∂ k + 1 ∂ ω o ∂ k ) ∂ ω ∂ n dn .
Δ ω = Δϕ ( 1 ∂ ω e , fix ∂ k − 1 ∂ ω o , fix ∂ k ) L fix + ( 1 ∂ ω e , off ∂ k − 1 ∂ ω o , off ∂ k ) L sw .
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