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All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers

Thomas Tanggaard Alkeskjold, Jesper Lægsgaard, Anders Bjarklev, David Sparre Hermann, Anawati, Jes Broeng, Jun Li, and Shin-Tson Wu

Open Access Open Access

Abstract

Photonic crystal fibers (PCFs) have attracted significant attention during the last years and much research has been devoted to develop fiber designs for various applications, hereunder tunable fiber devices. Recently, thermally and electrically tunable PCF devices based on liquid crystals (LCs) have been demonstrated. However, optical tuning of the LC PCF has until now not been demonstrated. Here we demonstrate an all-optical modulator, which utilizes a pulsed 532nm laser to modulate the spectral position of the bandgaps in a photonic crystal fiber infiltrated with a dye-doped nematic liquid crystal. We demonstrate a modulation frequency of 2kHz for a moderate pump power of 2–3mW and describe two pump pulse regimes in which there is an order of magnitude difference between the decay times.

©2004 Optical Society of America

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Figures (15)
Fig. 1.
Fig. 1. (a) Optical micrograph of the end facet of the PCF used in the experiment. (b) Polarized micrographs of a silica capillary (inner diameter=5µm) infiltrated with the nematic LC E7. Capillary is angled at 0 degrees (top right) and at 45 degrees (bottom right) relative to the axis of the polarizer.

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Fig. 2.
Fig. 2. Molecular structure of the compounds used in nematic LC E7.

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Fig. 3.
Fig. 3. Ordinary and extra-ordinary refractive indices as function of wavelength of nematic LC E7. The experimental points have been measured using a “Multi-wavelength Abbe refractometer” (Atago model DR-M4) and the theoretical curves have been calculated using the extended Cauchy equations for anisotropic materials.

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Fig. 4.
Fig. 4. Ordinary and extraordinary refractive indices as function of temperature of nematic LC E7. The experimental points have been measured using a “Multi-wavelength Abbe refractometer” (Atago model DR-M4) and the theoretical curves have been calculated using the four parameter model.

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Fig. 5.
Fig. 5. Calculated average refractive index at 67°C (isotropic phase) of nematic LC E7. The refractive index has been calculated from the experimental data in the nematic phase using both the extended Cauchy equations and the four-parameter model.

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Fig. 6.
Fig. 6. Transmission spectrum of the LCPBG fiber measured at 67°C (solid black line), which is the isotropic phase of the LC. The PCF was infiltrated for 10mm of the length with the nematic LC E7. The spectral position of the long-wavelength side of the bandgap edges was calculated using a simple analytical expression for the cut-off wavelengths. These are marked with the vertical red dotted lines.

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Fig. 7.
Fig. 7. Transmission spectrum of the LCPBG fiber measured at 40°C (solid black line), which is in the nematic phase of the LC. The PCF was infiltrated for 10mm of the length with E7. The spectral position of the long-wavelength side of the bandgap edges was calculated numerically using a simple cut-off approach for an anisotropic cylindrical waveguide. These are marked as the vertical red dotted lines.

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Fig. 8.
Fig. 8. Thermal tuning of a bandgap located around 545nm. The spectral position of the bandgap is tuned by varying the temperature from 44°C to 56°C, which is just below the clearing point of the LC (58°C). The tuning sensitivity is mainly linked to the refractive index gradient of the ordinary refractive index no. Around room temperature, this gradient is close to zero and only a small tuning sensitivity is observed.

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Fig. 9.
Fig. 9. Thermal tuning of a bandgap located around 1400nm. The spectral position of the bandgap is tuned by varying the temperature from 44°C to 56°C, which is just below the clearing point of the LC (58°C). The tuning sensitivity is mainly linked to the refractive index gradient of the ordinary refractive index no. Around room temperature, this gradient is close to zero and only a small tuning sensitivity is observed.

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Fig. 10.
Fig. 10. Molecular structure of Disperse Red 1.

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Fig. 11.
Fig. 11. Doping the LC with the dye Disperse Red 1 (DR1), enhances the absorption around 500nm, but does not alter the alignment and the transmission above the absorption wavelengths of the dye. Transmission spectra with an un-doped LC (top), and a doped LC (bottom).

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Fig. 12.
Fig. 12. Experimental setup for measuring the dynamics of the dye-doped LCPBG fiber. The same setup is used for measuring the polarization sensitivity of the bandgaps.

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Fig. 13.
Fig. 13. Oscilloscope traces of the time-domain response of the 1620nm CW polarized laser source, which is coupled into the dye-doped LCPBG fiber. The traces shows the response of the 1620nm CW source when the pump laser is square-wave modulated with f=100Hz (a), f=1kHz (b) and f=2kHz (c). The estimated pump power was 2–3mW.

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Fig. 14.
Fig. 14. Polarization sensitivity of the transmission at the edge of the bandgap centered around 1400nm in Fig. 7. Maximizing and minimizing the transmission at 1620nm identified polarization 1 and Polarization 2, respectively.

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Fig. 15.
Fig. 15. Differential Group Delay (DGD) measured on the LCPBG fiber with the polarization sensitive bandgap edges shown on Fig. 14. The DGD was measured using the HP/Agilent 8509 Polarization analyzer and the HP 8168C tunable laser. A relatively small temperature sensitive DGD is measured on the edge of the bandgap, but due to the short length of the LCPBG fiber, this translates into a rather high birefringence on the order of 10-3.

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Equations (8)

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

n e , o ( λ ) = A ′ e , o + B ′ e . o λ 2 + C ′ e , o λ 4
n e ( T ) = A − BT + 2 ( Δ n ) o 3 ( 1 − T T c ) β
n o ( T ) = A − BT − ( Δ n ) o 3 ( 1 − T T c ) β
〈 n ( T ) 〉 = A − BT
Δ n ( T ) = ( Δ n ) o ( 1 − T T c ) β
λ m = 2 d m + 1 2 n 2 2 − n 1 2
λ d ≤ 2 n 2 2 − n 1 2
τ TD = C p ρ λ T L 2
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