Photonic crystal fiber with a dual-frequency addressable liquid crystal: behavior in the visible wavelength range

All-fiber tunable devices based on high-index photonic crystal fibers filled with liquid crystals

In this paper we present all-in fiber tunable devices based on specially designed and optimized high-index photonic crystal fibers filled with nematic liquid crystals. A special host microstructured optical fibers have been designed and manufactured to ensure low-loss index guiding and mode field diameter matching to SMF-28 fiber, ensuring low losses on interconnections with leading in-out FC/PC connectorized pigtails. We present four types of tunable all-fiber devices: tunable retarders with tuning range as high as 20 λ, tunable polarizers with variable axis of polarization and continuously tunable polarization dependent losses, tunable and fully controllable polarization controller and finally indeterministic depolarizer in which depolarization is caused by random thermodynamic process. We also present a cost-effective method to achieve change in the direction of the steering electric field, which was controlled by custom-made programable controllers. Finally, we present a method for effective packaging for the proposed devices.

Ferroelectric Liquid Crystals in Microcapillaries: Observation of Different Electro-optic Switching Mechanisms

Ferroelectric liquid crystals (FLCs) can provide fast electro-optic switching effects in photonic crystal fibers, which are promising for developing integrated optical modulators to be used for optical telecommunication. The present study describes experiments on capillaries filled with FLCs, which pave the way toward application of FLCs in state-of-the-art solid core microstructured optical fibers. A method to achieve uniform alignment of the helix axis and the appearance of two different switching modes with switching times in the submillisecond range are found. An additional slow relaxation that was reported recently can be attributed to space charge fields, which may appear due to the migration of ions.

Asymmetric band gap shift in electrically addressed blue phase photonic crystal fibers

In this paper, we present electrooptic experiments on photonic crystal fibers filled with a liquid crystalline blue phase. These fibers guide light via photonic band gaps (PBGs). The blue phase is isotropic in the field-off state but becomes birefringent under an electric field. This leads to a polarization dependent shift of the PBGs. Interestingly, the effect on the PBGs is asymmetrical: while the short wavelength edges of the PBGs shift, the long wavelength edges are almost unaffected. By performing band gap and modal analyses via the finite element simulations, we find that the asymmetric shift is the result of the mixed polarization of the involved photonic bands. Finally, we use the band gap shifts to calculate effective Kerr constants of the blue phase.

Analytical description of 2D magnetic Freedericksz transition in a rectangular cell of a nematic liquid crystal

We study the Freedericksz transition induced by a magnetic field in a rectangular cell filled with a nematic liquid crystal. In the initial state the director of the nematic liquid crystal is uniformly aligned in the cross section plane of the cell with rigid anchoring of the director at cell walls: planar on the top and bottom walls, and homeotropic on the left and right ones. The magnetic field is directed perpendicular to the cell cross section plane. We consider two-dimensional (2D) orientational deformations of the nematic liquid crystal in the rectangular cell and determine the critical value of the Freedericksz transition field above which these orientational deformations occur. The 2D expression for the director alignment profile above the threshold of Freedericksz transition is analytically found and the profile shapes as functions of cell sizes, values of the Frank elastic constants of the nematic liquid crystal and the magnetic field are studied.

Measurement of group velocity dispersion in a solid-core photonic crystal fiber filled with a nematic liquid crystal

Liquid crystal-filled photonic crystal fibers (PCFs) are promising candidates for electrically tunable integrated photonic devices. In this Letter, we present group velocity measurements on such fibers. A large mode area PCF, LMA8, was infiltrated with the liquid crystal mixture, E7. The measurements were performed with an interferometric setup. The fiber exhibits several spectral transmission windows in the visible wavelength regime that originate from the bandgap guiding mechanism. The dispersion of these windows is very unusual compared to typical fibers. Our measurements show that it can change from -2500 ps km(-1) nm(-1) to +2500 ps km(-1) nm(-1) within a spectral range of only 15 nm. This leads to multiple zero dispersion wavelengths in the visible wavelength range.

Dual-band electro-optic polarization switch based on dual-core liquid-crystal photonic crystal fibers

Compact voltage-controlled all-in-fiber polarization switches are designed and investigated based on dual-core photonic crystal fibers, by selectively infiltrating one of the fiber's cores with a nematic liquid crystal. The electro-optical control of the liquid crystal core's optical properties allows for the splitting of the two orthogonal polarizations, showing crosstalk values lower than -20 dB in a 40 nm window at 1550 nm, for an ultracompact length less than 0.6 mm. With proper selection of the control voltage and the component length, dual-band operation with a crosstalk lower than -20 dB is also demonstrated for the 1300 and 1550 nm telecom bands.

Blue-phase liquid crystal cored optical fiber array with photonic bandgaps and nonlinear transmission properties

Blue-phase liquid crystal (BPLC) is introduced into the pores of capillary arrays to fabricate fiber arrays. Owing to the photonic-crystals like properties of BPLC, these fiber arrays exhibit temperature dependent photonic bandgaps in the visible spectrum. With the cores maintained in isotropic as well as the Blue phases, the fiber arrays allow high quality image transmission when inserted in the focal plane of a 1x telescope. Nonlinear transmission and optical limiting action on a cw white-light continuum laser is also observed and is attributed to laser induced self-defocusing and propagation modes changing effects caused by some finite absorption of the broadband laser at the short wavelength regime. These nonlinear and other known electro-optical properties of BPLC, in conjunction with their fabrication ease make these fiber arrays highly promising for imaging, electro-optical or all-optical modulation, switching and passive optical limiting applications.

Guided-wave liquid-crystal photonics

In this paper we review the state of the art in the field of liquid-crystal tunable guided-wave photonic devices, a unique type of fill-once, molecular-level actuated, optofluidic systems. These have recently attracted significant research interest as potential candidates for low-cost, highly functional photonic elements. We cover a full range of structures, which span from micromachined liquid-crystal on silicon devices to periodic structures and liquid-crystal infiltrated photonic crystal fibers, with focus on key-applications for photonics. Various approaches on the control of the LC molecular orientation are assessed, including electro-, thermo- and all-optical switching. Special attention is paid to practical issues regarding liquid-crystal infiltration, molecular alignment and actuation, low-power operation, as well as their integrability in chip-scale or fiber-based devices.

All-optically controllable dye-doped liquid crystal infiltrated photonic crystal fiber

A novel demonstration of an all-optically controllable dye-doped liquid crystal infiltrated photonic crystal fiber (DDLCIPCF) is presented. Overall spectral transmittance of the DDLCIPCF can decrease and then increase with a concomitant red-shift of the spectrum curve with increasing irradiation time of one UV beam. Continuing irradiation of one green beam following UV illumination on the DDLCIPCF can cause the transmission spectrum to recover completely. The reversible all-optical controllability of the photonic band structure of the fiber is attributable to the isothermal planar nematic (PN)→scattering (S)→isotropic (I) and I→S→PN state transitions of the LCs via the UV-beam-induced trans→cis and green-beam-induced cis→trans back isomerizations of the azo-dye, respectively, in the cladding of the DDLCIPCF. The photoinduced appearance of the S state and the variation of the index modulation between the core and the cladding of the fiber result in the variation of overall spectral transmittance and the shift of transmission spectrum, respectively.

Loss-reduced photonic liquid-crystal fiber by using photoalignment method

We present a loss-reduced photonic liquid-crystal fiber (PLCF) using the noncontact photoalignment method. The photoexcited and adsorbed azo dye on the capillary surface of a PLCF induces uniform and highly ordered orientation of the liquid crystal (LC). The anchoring force of the photoalignment effect is combined with that generated by surface boundary conditions of the photonic crystal fiber (PCF). Transmission loss resulting from LC scattering can be reduced from -2.8 to -1.3 db/cm within 10 min. This photoinduced alignment yields a permanent boundary for the LC in the PCF that reduces scattering loss and can be further modulated by electrical fields. The electrical tunable effect and fast dynamic response of the photoaligned PLCF are also presented. This low-loss PLCF can be applied conveniently in various PLCF devices.

Switchable waveguiding in two liquid-crystal-filled photonic crystal fibers

Switchable waveguiding is investigated in two liquid-crystal-filled photonic crystal fibers with a solid core using the nematic liquid-crystal mixture E7 under planar and homeotropic anchoring conditions. Addressing experiments using ac voltages show polarization-dependent and -independent effects with response times down to a few ms. It is shown that the attenuation spectra of the two liquid-crystal-filled photonic crystal fibers can be changed dramatically by just varying the boundary conditions. Electromagnetic field simulations are presented, which are in good agreement with the experimental findings.

Liquid crystal parameter analysis for tunable photonic bandgap fiber devices

We investigate the tunability of splay-aligned liquid crystals for the use in solid core photonic crystal fibers. Finite element simulations are used to obtain the alignment of the liquid crystals subject to an external electric field. By means of the liquid crystal director field the optical permittivity is calculated and used in finite element mode simulations. The suitability of liquid crystal photonic bandgap fiber devices for filters, waveplates or sensors is highly dependent on the tunability of the transmission spectrum. In this contribution we investigate how the bandgap tenability is determined by the parameters of the liquid crystals. This enables us to identify suitable liquid crystals for tunable photonic bandgap fiber devices.

Infiltrated photonic crystal fiber: experiments and liquid crystal scattering model

Experimental results obtained by means of a cut-back technique indicate low attenuations (< 1 dB x cm(-1)) for a solid core photonic crystal fiber filled with the nematic liquid crystal E7. These results observed in the visible wavelength range are compared with electromagnetic field simulations. The latter are carried out with a full vectorial finite element algorithm. Based on the modal properties under the condition of perpendicular anchoring of the liquid crystal molecules, the wavelength dependent attenuation is estimated using a power loss model considering the turbidity of the nematic liquid crystal. The results indicate that the scattering properties of this type of materials make them extremely interesting for fiber optical filters in the visible wavelength range and that filling materials with a relatively high turbidity are in general potentially useful as filling materials for solid core photonic crystal fibers.

Photo and electrical tunable effects in photonic liquid crystal fiber

This work demonstrates photo alignment and electrical tuning effects in photonic liquid crystal fiber (PLCF). Applying voltages of 0 approximately 130V and 250 approximately 400V shifts the short and long wavelength edges of the transmission bands by about 45 nm and 74 nm toward longer wavelengths, respectively. An electro-tunable notch filter is formed in the PLCF without the use of gratings. The range of tunability of the notch filter is around 180 nm with an applied voltage of 140 approximately 240 V. This photo-induced alignment yields a permanently tilted LC structure in PCF, which reduces the threshold voltage, and can be further modulated by electric fields. The polarization dependent loss and fast response time of photo-aligned PLCF is also demonstrated. The finite-difference frequency-domain method is adopted to analyze the shift of the transmission bandgap, and the simulation results are found to correlate well with experimental data.

Low-loss propagation and continuously tunable birefringence in high-index photonic crystal fibers filled with nematic liquid crystals

Experimental investigations of microstructured fibers filled with liquid crystals (LCs) have so far been performed only by using host fibers made of the silica glass. In this paper, the host photonic crystal fiber (PCF) was made of the PBG08 high-refractive index glass (approximately 1.95) that is much higher than silica glass index (approximately 1.46) and also higher then both ordinary and extraordinary refractive indices of the majority of LCs. As a result, low-loss and index-guiding propagation is observed regardless of the LC molecules orientation. Attenuation of the host PCF was measured to be approximately 0.15 dB/cm and for the PCF infiltrated with 5CB LC was slightly higher (approximately 0.19 dB/cm), resulting in a significant reduction to approximately 0.04 dB/cm of the scattering losses caused by the LC. Moreover, an external transverse electric field applied to the effective photonic liquid crystal fiber (PLCF) allowed for continuous phase birefringence tuning from 0 to 2.10(-4).