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Jull, EI and Gleeson, HF orcid.org/0000-0002-7494-2100 (2017) Tuneable and switchable liquid crystal laser protection system. Applied Optics, 56 (29). pp. 8061-8066. ISSN 1559-128X

https://doi.org/10.1364/AO.56.008061

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!uneableandswitchableliquidcrystallaser

protectionsystem

ETHANI.L.JULL*ANDHELENF.GLEESON

SchoolofPhysicsandAstronomy,UniversityofLeeds,Leeds,LS29JT,England

*Correspondingauthor:py12ej@leeds.ac.uk

TheuseofaliquidcrystalLyotfilterasasimpleandcompactswitchablelaserprotectionsystemisdemonstrated.

ThesystemOFFstateexhibitsawavelength-independenttransmissionandswitchestoanONstate,whichrejectsa

selected wavelength. The response time of the switchable system is <110 ms, depending on the rejected

wavelength,with the ability for faster switchingof <5mswhenusing a lower-order rejectionband.A rejection

tuning range between 480 and 640 nm is demonstrated, and the potential to operate outside of the visible

spectrum is discussed. In theON state, the transmission at the rejectedwavelengthwas found to be effectively

limitedbythepolarizerextinctionratio,whiletransmissionatotherwavelengthsallowsforpartialobservations

throughthesystemevenwheninprotectionmode.

1.Introduction

High-poweredhandheldlaserwithhighintensityandcoherent

radiationcandazzleandcausepermanentdamagetosensorand

surveillancesystems.Suchdevicesarebecomingincreasinglyeasyto

acquire;therefore,thereisanurgentdemandforprotectivesystems.

Successfulsystemsneedtoeffectivelyrejectspecificlaserwavelengths,

thusprovidingmuchneededprotectionwhilepermittingcontinuous

operation.

TuneableliquidcrystalLyotfilterscanselectaspecificwavelength

byapplyingawavelength-dependentretardancetolightpassing

throughalignedpolarizers.Lyotfilterswerefirstdevelopedin1933by

Lyot[1],andtheirinitialimplementationintoastronomical

observationswaswidespread[2].Itwasnotuntil1990thataliquid

crystalcomponentwasimplementedasthebirefringentmaterial.This

wasfirstachievedbyMiller[3]in1990,allowingradiometricand

photometricstandardstobelinked.Theabilitytodirectlycontrolthe

transmissionspectrumthroughapplicationofanelectricormagnetic

fieldtotheliquidcrystalelementledtomanyfurtherdevelopments

andapplicationsoftheLyotfiltertoastronomicalspectralobservations

[4-8],remotesensing[9,10],lasertuning[11],andbiologicalimaging

[12,13].Inrecentyears,thefocushasshiftedtowardtheoptimization

ofthetransmissionspectrumforspecificapplications.These

optimizationsincludeincreasingthetunability[14],freespectralrange

[15].Switchingspeed[16],contrastratio[17],andspectralresolution

[16]andalsoreducingthepassbandsize[14]ortheappearanceofside

lobes[18].

Thedesignsuseavarietyofarrangements,occasionallyincluding

multipleLyotfilterstages,creatingnarrowtransmissionwindows,and

ideallyremovingtransmissionatallotherwavelengths.However,in

theapplicationdescribedinthispaper,theoppositeeffectisrequired

toachieveprotectionofdevicesfromlaserradiation.Thus,ourdesign

focusesonsufficientextinctionataspecific(laser)wavelengthwhile

retainingasmuchtransmissionaspossibleatotherwavelengths.This

meansthatprotectionfromthelaserradiationisachieved,while

partialobservationsarestillpossibleusingotherwavelengthswhile

thesensoris“underthreat”.

Foralaserprotectionsystemtobeeffectivelyimplementeditmust

bepossibletoswitchfromafullwavelength-independenttransmission

mode,withtransmissionacrosstheentirevisiblespectrum,toa

specificwavelengthrejectionmode.Thus,thesensororcameracan

operatewithoutcompromisewhenitisnot“underthreat”bylaser

radiation.However,when“underthreat,”thesystemwillswitchto

rejectthespecificlaserwavelength,whileallowingcontinuedsensor

operationviathepartialtransmissionatotherwavelengths.Theabilityfortheprotectionsystemtobetunedtorejectarangeofwavelengths

isalsoimportant,asmanydifferentlasersmaybeusedbyapotential

attacker.Thesystemshouldalsohave:afastresponsetime,to

minimizethelaserpowerincidentonthesensor;acompactdesign,to

avoidunnecessaryopticalcompromises;andideally,lowoperating

voltagesandpowerconsumption.

TherearefewreportsoftuneableliquidcrystalLyotfiltersbeing

implementedinsensorprotectionagainstlaserradiation[19,20].

AlthoughthedesignsimplementedbyReesandStaromlynska[19]and

WuandWu[20]performedwellfortheapplicationsconsidered,they

wereunabletodemonstrateswitchingbetweenafulltransmissionand

awavelength-dependentrejectionmode.Thewavelength-

independenttransmissionmodeis,however,quiteclearlyrealized

whenavoltagemuchhigherthanthresholdisappliedtoaliquid

crystaldevice.Further,inonecasetheresponsetimeoftheliquid

crystalLyotfilterwasnotquantified[19];intheother,thedesignwas

bulkyandusedaprism[20].

Accepted for publication in Applied Optics, September 2017

Inthispaper,weclearlydescribethedesignandevaluationofa

simple,compact,tuneableliquidcrystalLyotfilterforlaserprotection.

Wedemonstrateswitchingfromawavelength-independent

transmissionmode,allowingoperationacrosstheentirevisible

spectrum,toarejectionmodeatachosenspecificwavelength.The

abilitytotunetoaspecificwavelengthrejectionisimportantfor

protectionagainstthewidevarietyofpowerfulhandheldlasersnow

available.Importantly,theresponsetimesforswitchingbetween

variousrejectionstatesareexperimentallyevaluatedandshowntobe

sufficientlyfasttoallowprotectionagainstlaserattack.Thus,although

Lyotfiltersarewell-known,wedemonstrateacombinationof

desirablefeaturesthatwouldallowthemtobeusedinanew,

increasinglyimportantapplication.

ALiquidCrystalLyotFilter

Asingle-stageliquidcrystalLyotfilteriscomprisedofaliquidcrystal

variableretardersandwichedbetweentwolinearpolarizers,as

depictedinFig.1(a).Thelinearpolarizersareuncrossed,andtheliquid

crystaldeviceisorientedformaximumretardance.Theliquidcrystal

devicehasacellgapdandavoltage-dependenteffectivebirefringence

Δn(V),resultinginaretardanceofΓ=2πΔn(V)d/λ,whereλisthewavelengthoflight[15].Wavelengthsthatsatisfythehalf-wave-plate

condition(λ=dΔn/(m-½),misaninteger)willbeblockedbytheanalyzerandarethusrejected;thiseffectisutilizedforlaserprotection.

TheparametersdandΔncanbeinitiallyvariedoverrangesoftypically2-20µmand0.05to0.25,respectively,allowinginitialcalibrationof

therejectionwavelength.Inthispaper,weselectacellgapof~5µm

andtheliquidcrystal4-pentyl-4’-cyanobiphenyl(5CB)withabirefringenceof~0.2,givinganinitialrejectionbandat~667nmfor

m=2.Thevoltage-dependencyoftheeffectivebirefringence“observed”byplanepolarizedlightallowstunability,asdiscussedlaterinthe

paper.

Thepercentagetransmission,T,isgivenbyEq.(1)[15]:

! ! cos!!!! ! cos

!!!! ! !!! , (1)

wherepolarizerandFresnellosses(~55%forunpolarisedlight)

havebeenneglected.

Thewavelength-dependenceofthebirefringencehasbeen

consideredviatheextendedCauchymodel[21],whichisusedto

determinethebirefringencedispersionfor5CBatatemperatureof

25.1°C.Thisallowsforaccuratemodelingofthetransmission

spectrum.

B.LiquidCrystals

Theuseofliquidcrystalsindisplaysandotherelectro-opticdevices

iswellknown,andtheLyotfilterapplicationmakesuseofliquidcrystal

properties[22].Intheliquidcrystaldevice,anematicliquidcrystal

withpositivedielectricanisotropyiscontainedinaplanar-aligned

geometrysuchthattheaveragemoleculardirection(thedirector,!)

liesinasingledirectionparalleltotheglassplatesofthecell.

Applicationofavoltagecausesreorientationofthedirectortowardthe

fieldlines(perpendiculartotheglassplates)andhenceareductionin

theeffectivebirefringenceofthedevice,anexampleofwhichisshown

inFig.1(b).Thisisacontinuousprocessaboveamaterial-dependent

thresholdvoltage,Vth,givingthedirectcontrolovertheretardanceof

thedeviceandhencethetransmissionspectrum.

Theswitchingspeedoftheliquidcrystaldeviceisgovernedbythe

initialstate,magnitudeoftheappliedfield,andtheelasticrestoring

forces.Equation(2)describestherisetimeforthedirectortoreorient

inresponsetoanappliedvoltage,V>Vth.Equation(3)describesthefall

timeassociatedwithchangingtheappliedvoltagetoalowervalue,Vb.Themajorityofliquidcrystaldevicesoperatesuchthattheapplied

voltageiscompletelyremoved,Vb=0.However,inthisdeviceoperation

ismorecomplexandabiasvoltageisneededtoswitchtolaser

protectionmode.

!!∀#∃ ! !!!!!!!!!!!! !!!!

!! (2)

!!∀## ! !!!! !!!!!!!! !!!!

! ! (3)

whereηistherotationviscosity,Δεisthedielectricanisotropy,andk11isthesplayelasticconstant[23].

Fig. 1. (a) Schematic of a liquid crystal Lyot filter is shown. The

polarizerandanalyzer,inthexyplane,areat45°tothexaxis,whiletheliquidcrystaldirectorisalignedwiththexaxis.ImageredrawnfromYangetal.[18](b)Voltagedependencyofbirefringencefor5CBhasbeen theoretically calculated to demonstrate the ability to tune the

bulkbirefringence.

2.ExperimentalMethods

Theliquidcrystaldevicewasmadefromtwopiecesofglass,coated

withindiumtinoxideelectrodes.Aliquidcrystalalignmentlayerof

0.5%poly(vinylalcohol)solutioninwaterwasspin-coatedontothe

glasssubstrateandrubbedforunidirectionalalignment.UVgluemixed

with5µmspacerbeadswasusedtoconstructacell,withthecellgapof

thedevicemeasuredtobe(6.0±0.2)µmusingreflectionspectroscopy

[24].

TransmissionspectraoftheLyotfilterweremeasured,between400

and800nm,usingamodifiedOlympusBH-2microscopecoupledtoan

OceanOpticsHR4000high-resolutionspectrometer,whichis

describedindetailelsewhere[24].Thetransmissionwasnormalized

withrespecttotheexperimentallosses,asdescribedlater.Whilethe

spectraweremeasured,theliquidcrystaldevicewasheldataconstant

temperatureof(24.0±0.1)°CusingaLinkamTHMS600hotstageand

LinkamTMS91controller.

Forswitchingandtuningsinusoidalvoltagesupto~30Vrms,ata

frequencyof1kHz,wereappliedtothedeviceusingtowaveform

generatorscoupledwithahigh-voltagewidebandamplifier.This

configurationallowsswitchingbetweenspecificrejectionwavelengths;

thefirstwaveformgeneratorappliedthestartingvoltageandthen

triggerstheapplicationofthesecondvoltage,providingafast,smooth

transitionbetweenvoltages.Theintensitywasmeasuredusinga

photodiode,andthe10%-90%intensityvaluesweremeasured.The

timebetweenthesetwopointswastakenastheresponsetime,which

wasmeasuredforspecificwavelengthsbycouplingalowerpower

laserdiode(λ=532nm)intoapolarizingmicroscope.

Theuseofpolarizersinthistypeoffilterinherentlyreducesthelight

transmissionbyapproximately50%.Fresnelreflectionsattheglassair

interfacesresultinafurther~8%loss,leadingtoamaximum

theoreticaltransmissionof~42%.Theexperimentalnormalization

procedureremovesthepolarizerlossesonly(50%).Therefore,100%

transmissioninthedatacorrespondsto50%transmissionwhenthe

filterisimplemented.TheFresnellosses,materialabsorptions,orany

otheropticallossesarenotaccountedforinnormalizationand

thereforewillappearinthedata.

Fig.2.(a)Theoretical[Eq.(1)]andexperimentalLyotfiltertransmissionasafunctionofwavelength.(b)Transmissionspectraforasingle-stage

liquidcrystalLyotfilterfortheOFFstate(30Vrmsapplied)withuniformtransmissionacrossthevisiblespectrumandtheONstate(0Vrmsapplied),

which rejects650nm light. Images taken ineach stateusingaNikonD7100camerademonstrate theability forpartialobservationswhen in

protectionmode.(c)Transmissionspectraofasingle-stage5CBLyotfilterasafunctionofappliedvoltage.Therejectionwavelengthshiftstoward

theblueendofthespectrumwithincreasingvoltage.(d)Rejectedwavelengthisfoundtoapproximatelyvarylinearlywithappliedvoltage.The

originaldataassociatedwiththisworkareavailablefromtheResearchDataLeedsrepositoryunderaCC-BYlicenseathttp://doi.org/10.5518/222.

3.ResultsandDiscussion

ThetransmissionoftheliquidcrystalLyotfiltermeasuredIthe

absenceofafieldisshowninFig.2(a),whereexperimentaldataand

thetheoreticalcalculations[Eq.(1)]areseentobeinexcellent

agreement.ThespectruminFig.2(a)isdefinedastheONstate,with

theliquidcrystaldeviceexhibitingmaximumbirefringenceand

rejectingaspecificwavelength(655nm),withlessthan1.0%

transmissionbetween648and665nm.Foraclass4redlaser

(500mW),theincidentpowerwouldbereducedtoapproximately5

mW,correspondingtoaclass3Rlowrisklaser.Inpractice,this

minimumintransmissionislimitedbytheextinctionratioofthe

polarizers.Ifthepolarizerswereideal,thentheresultanttransmission

wouldapproachthetheoreticalminimumofzero.Fabry-Perot

interferencefringesarealsoobservedinalltransmissionspectra,as

canbeseenfromthesmall,wavelength-dependentfluctuations.These

fringesarearesultoftheinterferencethatoccurswhenthelightis

reflectedbetweentwoplaneparallelplates.

Onapplicationofavoltageof30Vrms,Fig.2(b),thebirefringenceof

theliquidcrystaldeviceapproacheszero,andtheOFF(transmitting)

stateisachieved.ItisimportanttonotethattheOFFstatecorresponds

tothehigh-voltageapplicationandisdenotedOFFtorefertothe

opticalstateofthesystem.Thisstatetransmitswavelengthsalmost

uniformlyacrossthevisiblespectrum.Figure2(b)demonstratesthe

abilitytoswitchthesystemfrommaximumtransmissionatall

wavelengths,standardoperation,torejectionofspecificwavelengths,

protectionmode.Therelativelyhighvoltagerequiredforfull

transmissionwouldnotimpedetheapplicationofthissystemfor

sensororcameraprotectionasthepowerrequirementsofliquid

crystaldevicesareextremelylow.ItcanalsobeseeninFig.2(b)that

theoperationofacamera,whileinprotectionmode,isstillpossible.

Tunabilityisachievedoverawidespectralrangebyvaryingthe

appliedvoltageandhencetheeffectivebirefringence.Figure2©shows

theeffectofchangingthevoltagebetween0.67and1.06Vrms.Asthe

voltageisincreasedthetransmissionspectrumblueshifts.The

minimumtransmissionwavelength(correspondingtothehalf-wave

condition)wasdeterminedasafunctionofvoltage,andthe

relationshipisshowninFig.2(d)tobeapproximatelylinear.The

rejectionwavelengthhasawidetuneablerangespanningthevisible

spectrum,i.e.from475to650nm.Indeed,inprinciple,amuchlarger

wavelengthrangecanbeaccessedifthehigher-orderhalf-wave-plate

conditionswereutilized(increasingtheintegervaluem),providedthatanyabsorptionbandsassociatedwiththeliquidcrystalmaterialand

substratesareavoided.

Equation(4)givesthefittedlinearfunction,applicableforthe

demonstratedvoltagerange,whichcanbeusedasacalibrationforthis

specificprotectionsystem.Forinstance,giventhataspecific

wavelengthlaser(λ=532nm)isdazzlingasensorthecalibrationequationcanbesuedtodeterminethevoltagerequiredtorejectthis

wavelength.Inthiscase,avoltageof(0.943±0.003)Vrmswouldbe

requiredtoalterthebirefringencesuchthatahalf-wave-plate

conditionismet:

!!∀#!��! ! 646 !!∀#

!!!!!!∀!. (4)

Bycouplingtheswitchingabilitywiththistuning,thesystemcanbe

switchedfromtheOFFstate(~30Vrms)toaspecificONstate,withthe

responsetimeforthisswitchlimitedbythedetectionsystemandthe

responsespeedoftheliquidcrystaldevice(typicallymilliseconds).To

testtheresponsespeedsofthefilteritself,agreenlasermodule(λ=532nm)wascoupledtothesystem.Theblockingvoltagerequiredfor

thislaserwavelengthis(0.943±0.003)Vrms.Aphotodiodewasusedto

measurehowtheintensityvarieswithtimewhenswitchingfromthe

OFFtotheONstate,asdemonstratedinFIG.3(a).

Theresponsetimeofthesystemisclearlyanimportantfactorinits

designfortherejectionoflaserwavelengths.Schwarzetal.[25]showedthatthelongertheexposuretimethelowerthelaserpower

requiredtodamagethesensorsystem.WhentheLyotfilterissuedto

protectagainstthegreenlaser,theintensityisreducedto(4.7±0.1)%

ofthemaximumintensitywhenthefilterisswitchedfromthe

transmissionOFFstatetotherejectionONstate.TheONswitching

mechanismfortheliquidcrystalisarelaxationratherthanbeinga

drivenresponseviaincreasedvoltageandisthereforepredictedusing

literaturevaluesfor5CB[26-28],inEq.(3),tobe59ms.Theswitch

speedwasexperimentallymeasuredtobeτON=(60±10)ms,which

comparesextremelywellwiththepredictedvalue.Switchingbackto

theOFFstateresultedinarisetimeofτOFF=(45±5)µs.Thefirst

minimuminintensity,seeninFig.3(a),correspondstothelowerorder

half-wave-platecondition(m=1)passingthroughλ=532nm;inFigs.2(a)and2(b)theminimacorrespondtothesecondordercondition

(m=2).Inthiscase,thefirst-orderminimumcouldpotentiallybesuedtoincreasetheswitchingspeedofthedevicetoapproximately5ms;

however,thefirstorderconditionhasthedisadvantageofalargefull-

width-half-maximum,meaningthatpartialobservationsarenotas

clearasinthesecond-ordercase.

Theabovetesthasdemonstratedthevalidityofthecalibration

equationandhowthissystemcanbeimplementedasalaser

protectionelement,withtheabilitytobeappliedtodifferent

protectionschemes.Iffasterswitchingisrequired,thelower-order

minimacanbeutilized,butifpartialobservationsaremoreimportant

andthesensorismoreresilient,ahigher-orderconditionandtherefore

longerswitchtimecanbeused.

Fig. 3. (a) Change in intensity measured as a function of time for

switchingasingle-stageLyotfilterfromtheOFFstate(30Vrms)tothe

ONstateforλ=532nm(0.940Vrms).Changeinintensityisclear,andtheresponsetime isτON=(60±10)ms.Datapointswereaveraged

using the adjacent-averagingmethod. Image inserts are of the laser

spotinthefarfield,takenusingadeltaPixcamera.(b)Responsetimes

associatedwith switchingbetweendifferentprotection schemesare

shown. Error for values in themillisecond range is ± 10ms; in the

microsecondrange,itis±5µs.

Tofurthertestswitchingspeedsbetweenstates,thethreemost

currentlyaccessiblehandheldlaserwavelengthsweretakenasλ=650nm(red),λ=532nm(green),andλ=405nm(blue).FromEq.(4)blockingvoltagesof0.69Vrms(red),0.94Vrms(green),and1.21Vrms

(blue)werecalculated.Theswitchingtimesbetweenallpossiblestates

areshowninFig.3(b).

InswitchingfromtheOFFstatetoaspecificrejectionstate,response

timesof40-110mswerefound.Ifswitchingbetweenspecificwavelengthblockingstates,theresponsetimesrangebetween40and

230ms.

Wecanconsidertheeffectivenessofoursysteminprotectingsensor

systemfromlaserradiationbyputtingourresultsincontextwith

existingliterature,whichevaluatesdamagetocamerascausedby

lasers.Schwarzetal.founditdifficulttocausedamagetoaCCDcamerawithacontinuouslaserwhentheexposuretimeoftheCCDcamerato

thelaserwaskeptbelow250ms[25].Specifically,nodamageoccurred

belowapowerdensityof159kWcm-2.Assumingabeamdiameterof6

mm,thispowerdensitycorrespondstoahugepowerof~20W.Thus,

theswitchingspeedsofourfilter,allfasterthan110ms,shouldprovide

excellentprotectionforaCCDcameraagainstlaserswithpowersof

≤20W.

4.Conclusion

Aswitchable,tuneable,liquidcrystalLyotfilterwasdesignedand

testedasalaserprotectionsystemusingbothbroadbandand

monochromaticlightsources.Atuneablerejectionrangeof475-650

nmwasexperimentallydemonstratedusinglowvoltagesfrom0.6to

1.05Vrms.Theoreticallythetuningrangeismuchlargerduetothe

extendedtransmissionspectrumofaLyotfilter,namely,the

higher/lowerorderhalf-wave-plateconditions.Acalibrationequation

wasextractedfromthetuningdata,allowingspecificwavelength

rejectionvoltagestobededuced.Switchingbetweenafull

transmissionmodeandaselectivewavelengthrejectionmodewas

demonstrated,usingabroadbandlightsourceandahigh-resolution

spectrometer.Thiswasachievedbyswitchingtheappliedvoltageto

theliquidcrystalcellfrom30to0Vrms,withaminimumin

transmissionof1.0%attherejectionwavelength.Thesystemwas

testedusingalasersource(λ=532nm)andwasshowntoswitchbetweenatransmissionOFFstatetoanONstatein(60±10)ms,using

voltagesof30and0.94Vrms,respectively.

TheimplementationoftheLyotfilterasaprotectionsystemwould

beincrediblysimple,withstraightforwardcalibrationtestsallowing

foranytemperaturedependenceoftheliquidcrystal’sbirefringenceto

beaccountedfor.Thresholdvoltageeffectscouldpotentiallybe

avoidedthroughtheuseofacopolymernetworkliquidcrystal,which

wouldallowforcontinuoustuning[29],althoughthismayresultin

scatteringeffects.Bycouplingthislaserfilterwithawarningsystem,

thesystemcanswitchrapidlyfromfulltransmissiontoablocking

state,thusprovidingmuchneededprotectionfromdazzlingand

permanentdamage.Thesystemshowspromiseasacompact,rapid,

andtuneablelaserprotectiondevicethatcouldbeimplementedwith

easeintheprotectionofsensorsorsurveillancecameras.

Funding Information. EngineeringandPhysicalSciencesResearch

Council(EP/N509681/1)

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