Alkeskjold, Thomas Tanggaard

Bjarklev, Anders Overgaard

Broeng, Jes

Hermann, D.S.

We describe a new class of hybrid photonic crystal fibers, which are liquid crystal infiltrated fibers. Using these fibers, we demonstrate 'distributed' tunable filter and switching functionalities operating by the photonic bandgap effect

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Distributed optical fibre devices based on liquid crystal infiltrated photonic crystalfibersAlkeskjold, Thomas Tanggaard; Broeng, Jes; Hermann, D.S.; Bjarklev, Anders OvergaardPublished in:Optical Fiber Communication Conference, 2004. OFC 2004Link to article, DOI:10.1109/OFC.2004.1362053Publication date:2004Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Alkeskjold, T. T., Broeng, J., Hermann, D. S., & Bjarklev, A. O. (2004). Distributed optical fibre devices based onliquid crystal infiltrated photonic crystal fibers. In Optical Fiber Communication Conference, 2004. OFC 2004IEEE. DOI: 10.1109/OFC.2004.1362053ThA3 Distributed Optical Fiber Devices Based on Liquid Crystal Infiltrated Photonic Crystal Fibers. Jes Broeng Cryslul Fibre N S ,  Blokkua 84, DK~3460  Bfrkw#d Drnrrwrk Abstract: We describe a new class of hybrid photonic crystal fibers, which are liquid crystal infiltrated fibers. Using these fibers, we demonstrate 'distributed' tunable filter and switching functionalities operating by the photonic bandgap effect. D 2003 Optical Swiety of America OCIS codas: (060.2310) Fiber Optics: (230.3990) Microrrructure Devices: (230,3720) Liquid Crystal Devices Introduction Photonic Crystal Fibers (PCFs) have appeared as a new class of optical waveguides, which have attracted large scientific and commercial interest during the last years. PCFs are microstructured silica waveguides with a large number of air holes located in the cladding region of the fiber [I]. The size and location o f  these air holes openes up for a large degree of design freedom within optical waveguide design, and PCFs with properties tailored for fiber lasers, airguiding fibers, nonlinear fibers, hybrid fibers etc. have been demonstrated 12-51, Further, the existence o f  air holes in the PCF gives access close to the fiber core and by introducing new materials into the air holes, a high interaction between light and hole material can be obtained, while maintaining the microstruclure o f  the waveguide. I n  this paper, we describe what we call Liquid Crystal Photonic Bandgap Fibers, which are PCFs infiltrated with Liquid Crystals (LCs) i n  order to obtain increased fiber functionality. We describe a thermo-optic fiber switch with an extinction ratio of 80dB and tunable PBGs using thermo-optic tuning of various LC phases, and demonstrate the significance of the actual L C  phase. Liquid Crystal Inf i l t rated Photonic Crystal Fibers Liquid crystals are organic materials consisting o f  geometrically anisotropic molecules, leading to long-range orientational [6 ]  order and a number o f  mesophases, which are thermodynamic phases with physical properties intermediate between those of pure liquids and pure solids. Examples o f  mesophases are shown i n  Fig. 1. left. Nemarics have only long-range orientational order, while smecrics A and C i n  addition have long-range positional order in one dimension, resulting in a structure of thin (2-5 nm) layers; I n  case o f  chiral molecules, a l l  phases become noncentrosymmetric, leading to a helical superstructure i n  chiral nematic (N*=cholesteric) and chiral Smectic C (SmC*), but not in chiral smectic A (SmA*). Infiltrating the LCs into the holes o f  the PCF, gives rise to high-index inclusions, which refractive index properties arc highly dependent on the molecular alignment o f  the LC. The periodic structure o f  the PCF combined with these inclusions gives rise to bandgaps in the PCF, which can be modulated by changing the optical properties OF the LCs. The principle of this device i s  illustrated i n  Fig. I, right, which illustrates a PCF, filled with a L C  and coated with a thin conducting layer, which forms a resistive microheater. Below i s  shown polarized micrographs o f  SmA* and N liquid crystals inside a PCF hole and describes the alignment of the L C  inside the hole. The waveguiding principle of this device i s  illustrated on Fig. 2, left, which shows mode indices o f  allowed states in a PCF with high-index inclusions. Bandgaps occur below the silica line (n=1.45) around a normalized wavelength o f  0.3 and 0.55. In the bandgaps, a guided mode i s  supported, which i s  plotted as insets together with two cladding modes as illustration. From this figure i t  i s  clear that the mode in the bandgap around 0.55 i s  tightly confined, while the mode in the 0.3 bandgap i s  weakly confined since light also propagates in the high-index inclusions. Fig. 2, right, shows a micrograph o f  the end-facet o f  a PCF infiltrated with a cholesteric LC, which after infiltration only supports a finite number o f  guided modes. I n  this case, a grcen guidcd mode i s  supported in the visible range of the spectra. Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 10, 2010 at 11:08 from IEEE Xplore.  Restrictions apply. ~ ThA3 v -. Nematic Smectic A Smectic C . _ -  Cholesteric (N*) Smectic C* Fig. I .  Lrfr: Examples of phases of ihenmrropic liquid crysrals: Nrnwric. Siileciic A and Sinrciic C phairs (non-chiral a~oleculcsi. uppwing in ttial order upon cooling. if ths s a ~ m  nwlctivl porrcsacr all of IIICSC plmcs. If llrc IIIOBCUI~S =IC chinl. we instead have N' (=cholralericl. SnA* and SnC'. These phases a11 lack nirmr planes. A liclicvl rupsnlrurlurc due 10 rhe niolecula~ chiralily appears in Ihe N* and SsC* phases (ihownl. bur no1 in rhe SaA* phase (not showni. Righr. rop: Device principle of Liquid Cryscll Infiliraad Fibers. Infdrracrd seclion is  cmrrd wirh a resisiivr iiii~c~lirairr by which the ieinpervrure of llir IC can be controlled. Right, bottom: Crossed polarizer nicrogruphs of LC insidc K P  void angled at different angles k i w r e n  fiber and p o l a i m  : a i  SmA* LCIO". hi SmA* ILm45". c i  N IKc@0". di N L C 1 4 Y  i5 Fig. 2. Lefl: M a l e  indices of t lk Tzsl 300 iiinlcr in a Iri;m&r silica PCF wirli B hi@-index iimtrrial (n=1.59l in Ihe holcr. Inset shows IWO clsditing ~ l t a l c s  and two guided rilodrs in [he fin7 and second handpup. Thc guidcd riaxlc in bandgnp 2 is poorly conlimed since liglii is slso guided in ilir inclusions. Righi. Micrognph of the end kcel of B PCF f k d  B cholerretic liquid ccystrll. wIYcIi acts as thigh-index inclusions and P green g~idcil  s lalc is supporled by rlir PRF effect. Tunable Photonic Bandgap Fiber Devices LC infiltrated PCFs havc bccn tcstcd using an all-silica triangular PCF with a pitch of  7pm and a holcsizc of 3.5pm. We have used 3 different LCs lor our experiments: I )  a Cholesteric (MDA-00-1445, Merck), which is a nematic with a chiral dopanl, 2) The nematic host (MDA-00-1444, Merck) used i n  the cholesteric and 3 )  a SmA* liquid crystal (TM216. BDH). Fig. 3 ,  top left, shows the transmission spectrum o l  a LC PCF infiltrated with the cholesteric and with the nematic, which was used as host in the cholesteric. The major part of the molecules in these compositions is the same, namely the nematic host, but by adding a chiral dopant, the nemalic(MDA-OO-l444) changes into a cholesleric(MDA-00-1445) and thereby changes the alignment of the LC within the PCF hole. As shown on Fig. 3, this changes the transmission spectrum considerably. Fig. 3, Top right, shows micrographs of the guided modes at 4 tcrnperatures < f a  PCF filled with the cholesteric LC. 2 Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 10, 2010 at 11:08 from IEEE Xplore.  Restrictions apply. ThA3 Fig. 3. Top. left: Trunmission specss of a PCF f ~ d  Wilh a chirat nenwir  LC and will> ilir corresponding nciiuric hori LC. Spccrruiii i s  shown for LC f e i i l p ~ r r l ~ r r ~  of 25-C and 45'C. Top. "ht: Micmgraphs of ihc guided nmler of r U1 filled PCF. 31 green@T=77T hl yrtlow@T=89'C c )  off rrare@T=9l'C dl h lur@T=94T Borrora, lefr: Trmsnirsion specrn recorded at 3 closely spaced fcnlperal~rr~ of a F'CF filled IOnun wiili P S I I L ~ *  LC (TM2161. Insst shows a nicrognph of the end facer of lhe PCF. Botlou right: Transiiussion speclruili of a sample (as in top left) compand to the sprcwiii S weeks tarer. Insec shows extinction ratio of 3 saiilples with 3 differrni infdimrion lengths. Exrincrion ratio w a  mawred ai 978nn1 and 1580nar By utilizing the molecular reorientation at the SmA* to N* phase transition of TM216, which occurs at 26.2"C, i t  is possible to obtain a highly scnsitivc switching functionality in the fiber [7 ] ,  which switches between a transmission (SmA* phase) and a scattering (NI phase) state within 0.4"C and has an extinction ratio of up to 80dB, while maintaining a low insertion loss of  approx. IdB. This is illustrated on Fig. 3, bottom, which shows transmission spectra of the filled fiber together with measured extinctions ratios as function of infiltration length. Also shown is the transmission spectrum for a sample tested after infiltration and tested again 5 weeks after infiltration. After 5 weeks, the spcctrum shows similar shape as the first test, but more ripples and transmission dips occurs due tn unknown reasons. Conclusion In this paper we have described a new type of hybrid Photonic Crystal Fibers, which have been infiltrated with Liquid Crystals in order to transform the transmission fiber inlo a fiber with build-in signal processing capabilities. W e  demonstrates Ihermo-optic tuning of Photonic Bandgaps, which arises when Photonic Crystal Fibers are filled with Liquid Crystals and thermo-optic switching of  bandgaps with a very high extinction ratio of 80dB. Refer en e es ( I ]  P. Russell "Photonic crystal fibers'' Science 299,358-362 (2003). I21 I. Luilprn ct uf.. "High-power ai-chd Iqe-amic-ureu photonic crystal f i k r  Iase~", 0pric.v E+e.m 11,818 (2003). (31 R. P. Cregan et al.. "Single-nldc photonic band gap guidance of light in air", Sciwce 285,1537-1539 (1999). 141 K. P. Hmsen er ab. "Highly nonlineur photonic crystal fiber with zero-dispcnion ar t . S t p i ' ,  In Pmcrrdinp ofqricuf Fihw Communication Confrrmcr PDFA9 (2002). [SI B.J. Eggleton. C. Krrbagc. P.S. Wealhmk. R.Wbdcler & A. Hale. "Micrortructurcd o[)ticat Rbcr devices". 0jxic.v .5rprrrr 9,698-713(2001). I61 P.G de Grnnes and 1. Prosr, "Tlv Pfwsiw qtliquid crysml.~",  2nd edition. (Clamdon Press. Oxford, 1993). I71 T. 7. Larsen, D. S .  Hrmenn. J. Broeng, and A. Bjarklev. "A novel photonic crystal fibre rwiich." in Pmcsnfin~,r  ,!/CLEO Europe. post- dradllnr pnpcr CEPL-9-THU (2003). 3 Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 10, 2010 at 11:08 from IEEE Xplore.  Restrictions apply. 

Distributed optical fibre devices based on liquid crystal infiltrated photonic crystal fibers

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Distributed optical fibre devices based on liquid crystal infiltrated photonic crystal fibers

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