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Continuously tunable devices based on electrical control of dual-frequency liquid crystal filled photonic bandgap fibers

Lara Scolari, Thomas Tanggaard Alkeskjold, Jesper Riishede, Anders Bjarklev, David Sparre Hermann, Anawati, Martin Dybendal Nielsen, and Paolo Bassi

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Abstract

We present an electrically controlled photonic bandgap fiber device obtained by infiltrating the air holes of a photonic crystal fiber (PCF) with a dual-frequency liquid crystal (LC) with pre-tilted molecules. Compared to previously demonstrated devices of this kind, the main new feature of this one is its continuous tunability due to the fact that the used LC does not exhibit reverse tilt domain defects and threshold effects. Furthermore, the dual-frequency features of the LC enables electrical control of the spectral position of the bandgaps towards both shorter and longer wavelengths in the same device. We investigate the dynamics of this device and demonstrate a birefringence controller based on this principle.

©2005 Optical Society of America

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Figures (16)
Fig. 1.
Fig. 1. Formation of a defect in a planar aligned nematic LC caused by the presence of a reverse tilt domain.

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Fig. 2.
Fig. 2. Cross section of a triangular PCF filled with LC and placed between two electrodes. The most important parameters of a LC filled PCF used in the following are illustrated as well: hole size (d), inter-hole distance (Λ), diameter of the fiber (L), relative dielectric permittivity of the LC (εLC) and relative dielectric permittivity of the material surrounding the fiber (εB).

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Fig. 3.
Fig. 3. Example of simulated E-field distribution normalized to V/L, field in the homogeneous structure. The PCF parameters are: d = 5 μm, A =10 μm, L = 125 μm, εLC = 10.6, εB = 3.91.

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Fig. 4.
Fig. 4. Example of normalized average value of the electric field in the rings as a function of the relative dielectric permittivity of the background material. The inset shows how the rings are defined and labeled. The PCF parameters are the same of Fig. 3, but with εB varying.

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Fig. 5.
Fig. 5. Maximum deviation of the electric field in each ring of holes as function of the relative permittivity of the external material.

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Fig. 6.
Fig. 6. Polarized micrograph of a silica capillary infiltrated with the dual-frequency LC and schematic drawing of the LC alignment in the capillary.

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Fig. 7.
Fig. 7. Dielectric anisotropy ∆ε as a function of the frequency of the electric field applied to the LC. The dielectric constant was measured by measuring the capacitance of a both planar and homeotropic aligned cell and calculating the dielectric constant from the capacitance.

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Fig. 8.
Fig. 8. Depending on the sign of the dielectric permittivity, the induced polarization P gives a dielectric torque to the molecules, turning the director towards being parallel (a) or perpendicular (b) to the field direction.

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Fig. 9.
Fig. 9. Reorientation of the LC when a 1 kHz voltage (a) and a 50 kHz voltage (b) are applied to the LC MDA-00-3969.

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Fig. 10.
Fig. 10. Transmission spectrum of the LMA-15 filled with the dual-frequency LC and coupled with a white light source.

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Fig. 11.
Fig. 11. (a) Positive shift (towards longer wavelengths) of the bandgaps when a 1 kHz E-field is applied to the LCPCF. (b) Negative shift (towards shorter wavelengths) of the bandgaps at 50 kHz. (c) Functional dependence of the shift on voltage for 1 kHz (right part of the x-axis) and 50 kHz (left part of the x-axis).

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Fig. 12.
Fig. 12. Photodiode voltage when a 1 kHz sine wave with a 96 Vrms voltage and amplitude modulated by a 10 Hz square signal is applied to the electrodes.

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Fig. 13.
Fig. 13. Measured rise and decay time of the LCPCF as a function of the applied voltage.

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Fig. 14.
Fig. 14. Experimental setup for measuring the birefringence.

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Fig. 15.
Fig. 15. Phase shift on the Poincaré sphere when (a) 18 Vrms, (b) 35 Vrms, (c) 55 Vrms, (d) 82 Vrms are applied to the LCPCF device.

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Fig. 16.
Fig. 16. Plot of the relative change in birefringence as a function of the applied voltage

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Equations (2)

Equations on this page are rendered with MathJax. Learn more.

λ m = 2 d m + 1 / 2 n 2 2 ( E ) − n 1 2 where m = 1,2 , . .
Δ ϕ = 2 π ( Δ n − Δ n ' ) L / λ
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