MOGlabs CEL002 User manual

Cateye External Cavity Diode Laser
Model CEL002
Revision1.10

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ityarisingoutoftheuseoftheinformationcontainedwithinthis
manual. Thisdocumentmaycontainorreferenceinformationand
productsprotectedbycopyrightsorpatentsanddoesnotconvey
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others. MOGLabswillnotbeliableforanydefectinhardwareor
softwareorlossorinadequacyofdataofanykind,orforanydirect,
indirect,incidental,orconsequentialdamagesinconnectionswith
orarisingoutoftheperformanceoruseofanyofitsproducts.The
foregoinglimitationofliabilityshallbeequallyapplicabletoany
serviceprovidedbyMOGLabs.
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Nopartofthispublicationmaybereproduced,storedinaretrieval
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Contact
Forfurtherinformation,pleasecontact:
MOGLaboratoriesP/L
49UniversitySt
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AUSTRALIA
[phone]
[email protected]
www.moglabs.com
MOGLabsUSALLC
41914thSt
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USA
[phone]
[email protected]
www.moglabsusa.com
MOGLabsEurope
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10627Berlin
Germany
[phone]
[email protected]

Preface
Diodelaserscanbewonderfulthings: theyareefficient,compact,
lowcost,highpower,lownoise,tunable,andcoveralargerangeof
wavelengths.Theycanalsobeobstreperous,sensitive,andtempera-
mental,particularlyexternalcavitydiodelasers(ECDLs).Withexter-
nalcavityfeedbackandadvancedelectronicssuchastheMOGLabs
DLC external cavity diode laser controller, a simple $10 100mW
AlGaAsdiodecanbecomearesearch-qualitynarrow-linewidthtun-
ablelaser.TheCEL“cateye”laserdescribedhereprovidesarobust,
stable, acousticallyinert, lowlinewidthandhighlytunable laser
alternative.
Wewouldliketothankthemanypeoplethathavecontributedtheir
hardwork,ideas,andinspiration,inparticularDanielThompson,
SebastianSalibaandMichaelVentura.
WehopethatyouenjoyusingtheMOGLabsCEL.Pleaseletusknow
ifyouhaveanysuggestionsforimprovementinthelaserorinthis
document,sothatwecanmakelifeinthelaserlabeasierforall,
andcheckourwebsitefromtimetotimeforupdatedinformation.
MOGLabs,Melbourne,Australia
www.moglabs.com
i

ii

Safety Precautions
Safeandeffectiveuseofthisproductisveryimportant.Pleaseread
thefollowinglasersafetyinformationbeforeattemptingtooperate
thelaser. Alsopleasenoteseveralspecificandunusualcaution-
arynotesbeforeusingMOGLabslasers,inadditiontothesafety
precautionsthatarestandardforanyelectronicequipmentorfor
laser-relatedinstrumentation.
CAUTION–USEOFCONTROLSORADJUSTMENTSOR
PERFORMANCEOFPROCEDURESOTHERTHANTHOSE
SPECIFIEDHEREINMAYRESULTINHAZARDOUS
RADIATIONEXPOSURE
LaseroutputfromtheCEL canbedangerous. Pleaseensurethat
youimplementtheappropriatehazardminimisationsforyourenvi-
ronment,suchaslasersafetygoggles,beamblocks,anddoorinter-
locks. MOGLabstakesnoresponsibilityforsafeconfigurationand
useofthelaser.Please:
•Avoiddirectexposuretothebeam.
•Avoidlookingdirectlyintothebeam.
•Notethesafetylabels(examplesshowninfigurebelow)and
heedtheirwarnings.
•Whenthelaserisswitchedon,therewillbeashortdelayof
twosecondsbeforetheemissionoflaserradiation,mandated
byEuropeanlasersafetyregulations(IEC60825-1).
•TheSTANDBY/RUN keyswitchmustbeturnedtoRUN before
thelasercanbeswitchedon. Thelaserwillnotoperateif
iii

iv thekeyswitchisintheSTANDBYposition.Thekeycannotbe
removedfromthecontrollerwhenitisintheclockwise(RUN)
position.
•Tocompletelyshutoffpowertotheunit,turnthekeyswitch
anti-clockwise (STANDBY position), switch the mains power
switchatrearofunittoOFF,andunplugtheunit.
•WhentheSTANDBY/RUNkeyswitchisonSTANDBY,therecan-
notbepowertothelaserdiode,butpowerisstillbeingsup-
pliedtothelaserheadfortemperaturecontrol.
WARNING Theinternalcircuitboardandpiezoelectrictransducersareat
highvoltageduringoperation.Theunitshouldnotbeoperated
withcoversremoved.
CAUTION AlthoughtheCEL isdesignedandpricedwiththeexpecta-
tionthattheend-usercanreplacethediodeandchangethe
alignment,somecomponentsarefragile.Inparticularthefilter,
piezoactuator,andoutputcouplerareveryeasilydamaged.
Pleasetakecareoftheseitemswhenworkinginsidethelaser.
The filter and output coupler are hard-coated and can be
cleanedbutgreatcareisneededaswithanyintracavitylaser
optics.
NOTE MOGLabsproductsaredesignedforuseinscientificresearch
laboratories.Theyshouldnotbeusedforconsumerormedical
applications.
Label identification
TheInternationalElectrotechnicalCommissionlasersafetystandard
IEC60825-1:2007mandateswarninglabelsthatprovideinformation
onthewavelengthandpowerofemittedlaserradiation,andwhich
showtheaperturewherelaserradiationisemitted.Figures1and2
showsexamplesoftheselabelsandtheirlocationontheCELlaser.

v
Model number:
CEL002
Serial number:
A30013011504-01
Manufactured:
APRIL 2015
Complieswith21 CFR 1040.10,and 1040.11 exceptfor
deviationspursuantto Laser NoticeNo.50,dated24June2007
MOG Laboratories Pty Ltd, 18 Boase St
Brunswick VIC 3056, AUSTRALIA
IEC 60825-1:2007
AS/NZS 2211.5:2006
INVISIBLE LASER RADIATION
AVOID EXPOSURE TO BEAM
CLASS 3B LASER PRODUCT
Wavelength
750 – 790 nm
Max Power
200 mW
Model number: CEL002
Serial number: A30013011504-01
Manufactured: APRIL 2015
Complieswith21CFR 1040.10,and1040.11 exceptfor
deviations pursuantto Laser NoticeNo.50,dated24June 2007
MOG Laboratories Pty Ltd, 18 Boase St
Brunswick VIC 3056, AUSTRALIA
IEC 60825-1:2007
AS/NZS 2211.5:2006
INVISIBLE LASER RADIATION
AVOID EXPOSURETO BEAM
CLASS 3B LASER PRODUCT
Wavelength
750 – 790 nm
Max Power
200 mW
Emission indicator
Figure 1: Schematic showing location of laser warning labels compli-
antwithInternationalElectrotechnicalCommissionstandardIEC60825-
1:2007,andUSFDAcompliancelabel. Aperturelabelengravedonthe
frontoftheCELlaserneartheexitaperture;warningadvisorylabelonthe
rearandcompliancelabelonside.

vi
AVOID EXPOSURE
LASER RADIATION IS
EMITTED FROM THIS APERTURE Aperture label engraving
Warning and advisory label
Class 3B
US FDA compliance
Model number: CEL002
Serial number: A30013011504-01
Manufactured: APRIL 2015
Complieswith21 CFR1040.10, and1040.11 exceptfor
deviations pursuantto Laser NoticeNo.50,dated24 June2007
MOG Laboratories Pty Ltd, 18 Boase St
Brunswick VIC 3056, AUSTRALIA
IEC 60825-1:2007
AS/NZS 2211.5:2006
INVISIBLE LASER RADIATION
AVOID EXPOSURE TO BEAM
CLASS 3B LASER PRODUCT
Wavelength
750 – 790 nm
Max Power
200 mW
Figure 2: WarningadvisoryandUSFDAcompliancelabels.

Protection Features
MOGLabslasersincludesanumberoffeaturestoprotectyouand
yourlaser.
Protection relay Whenthepowerisoff,orifthelaserisoff,thelaserdiode
isshortedviaanormally-closedsolid-staterelayatthelaser
headboard.
Emission indicator TheMOGLabscontrollerwillilluminatetheemissionwarn-
ingindicatorLEDimmediatelywhenthelaserisswitchedon.
Therewillthenbeadelayofatleast2secondsbeforeactual
laseremission.
Interlock Itisassumedthatthelaserpowersupplyiskeyedandinter-
lockedforsafety.Thelaserheadboardalsoprovidesconnec-
tionforaninterlock(seeappendixB),ifusedwithapower
supplywhichdoesnotincludesuchaninterlock.
vii

RoHS Certification of
Conformance
MOGLaboratoriesPtyLtdcertifiesthattheMOGLabsExternalCav-
ityDiodeLaserdoesnotfallunderthescopedefinedinRoHS Di-
rective 2002/95/EC,andisnotsubjecttocompliance,inaccordance
withDIRECTIVE 2002/95/EC Out of Scope; Electronics related;
Intended application is for Monitoring and Control or Medical In-
strumentation.
MOGLaboratoriesPty Ltdmakesnoclaimsorinferencesof the
compliancestatusofitsproductsifusedotherthanfortheirintended
purpose.
viii

Contents
Preface i
Safety iii
Protection Features vii
RoHS Certification of Conformance viii
1 Introduction 1
1.1 Externalcavity...................... 2
1.2 Modecompetition. . . . . . . . . . . . . . . . . . . . . 3
1.3 Piezo-electricfrequencycontrol . . . . . . . . . . . . 3
1.4 Temperatureandcurrent. . . . . . . . . . . . . . . . . 4
2 First light 5
2.1 Temperature........................ 6
2.2 Current........................... 7
3 Operation 9
3.1 Wavelength........................ 9
3.2 Scanning.......................... 11
3.3 Externalmodulation . . . . . . . . . . . . . . . . . . . 12
4 Alignment 13
4.1 Pre-alignmentoflenstubeanddiode . . . . . . . . . 13
4.2 Initialdiodetest . . . . . . . . . . . . . . . . . . . . . 15
4.3 Orientationandpolarisationoftheoutputbeam. . . 15
4.4 Cateyereflector...................... 16
A Specifications 19
A.1 RFresponse........................ 21
ix

xContents
A.2Mechanical........................ 22
B Laser head board 23
B.1 B1045/1046headboard . . . . . . . . . . . . . . . . . 24
B.2 B1047/B1240headboards. . . . . . . . . . . . . . . . 25
B.2.1 SMAinput..................... 26
B.3 Laserconnection . . . . . . . . . . . . . . . . . . . . . 27
B.4RFcoupling........................ 27
References 32

1. Introduction
Semiconductorlaserdiodesarecompact,efficientandlow-cost,but
usuallyhavepoorwavelengthcontrol,linewidthandstability. The
addition of an external frequency-selective cavity allows control
oftheoperatingwavelengthoverafewnmrange,withsub-MHz
linewidthandstability.TheMOGLabsCEL(seeFig.1.1)ismachined
fromasolidaluminiumblock, sothatthelaseris stable, robust,
andinsensitivetoacousticdisturbances.Thecavityishermetically
sealedforadditionalsuppressionofenvironmentalfluctuationsand
drift.
TheMOGLabsCELisa“cat-eye”design(seeFig.1.2),inwhichan
externalcavityisformedbetweentherearreflectingsurfaceofthe
semiconductordiode,andacat-eyereflectoratseveralcentimetres
fromthediode[1–3]. Ratherthanthecustomarydiffractiongrating
ofLittrow-configurationECDLs,ahighefficiencyultranarrowfilter
Figure 1.1: InsidetheMOGLabsCELcateyelaser.
1

2Chapter 1. Introduction
isusedtoselectasingleexternalcavitymode. Withouttheneed
forilluminatingalargeareaofagratingforfeedback,acat-eye
retroreflectorandpartiallytransmittingoutputcouplercanbeused
toformtheexternalcavity. Thecateyereflectorisinherentlyself-
aligning, so thatthe laserisextremelyinsensitivetomechanical
disturbance,andalsoensureshighfeedbackcouplingefficiencyand
consequentlynarrowlinewidth.
DIODE FILTER LENS LENS
LENS PZT
OC
Figure 1.2: Schematicofacateyeexternalcavitydiodelaser(ECDL).The
externalcavity,formedbytherearfacetofthelaserdiodeandtheoutput
coupler,determinesthelaserfrequency. Onelongitudinalcavitymodeis
selectedbyanultranarrowintracavitybandpassfilter. Acateyereflector
isformedbytheoutputcoupler(OC)andintracavitylens,andthelightis
recollimatedbytheextracavityoutputlens.
Theoutputbeamfromalaserdiodeiscollimatedwithahighnumeri-
calaperture(NA)lensandincidentonthefilter.Thefiltertransmis-
sionwavelengthdependsontherotationangle.Transmittedlightis
back-reflectedbythecateyelens/output-couplercombinationwhich
efficientlycoupleslightbackintothelaserdiode.Moredetailscan
befoundinthereferences[1–3].
1.1 External cavity
Semiconductorlaserdiodesnormallyhaveahighreflectivityrear
facetandafrontfacetwithreflectivityofonlyafewpercent. The
diodecavityiscalledtheintrinsicorinternalcavity. Theexternal
cavityisformedbythecateyeandthedioderearfacet,andwhen
theexternalfeedbackisgreaterthanthatofthefrontfacet,theex-

1.2 Mode competition 3
ternalcavitydeterminesthelasingwavelength.Theexternalcavity
istypicallyaround40mmlongfromrearfacetofsemiconductorto
outputcoupler,givingacavitymodespacing(FSR)ofc/2L=3to
4GHz.
Thelaserdiodeandcollimatinglensareheldrigidlyinafocusing
tube. Thefilterisfixedtoabearing-mountedrotationassembly
withfineactuatorscrewstoadjusttheangle. Thespring-loaded
screws operate in a push-pull arrangement which can be locked
againsteachothertofurtherreducetheeffectsofmechanicalvibra-
tion.Variationofthefilterangleisusedforcoarseselectionofthe
wavelength,withinthegainbandwidthofthelaserdiode.
1.2 Mode competition
Asthewavelengthisvaried,competitionbetweenthefrequencyde-
terminedbytheinternalandexternalcavities,andthefiltertrans-
missionwindow,leadstomode hops.Fromfigure1.3itcanbeseen
thatthenetgain(combinedproductofsemiconductorgain, filter
loss,internalandexternalcavityinterference)canbeverysimilar
atadjacentexternalcavitymodes. Asmallchangeintheinternal
cavitymode,orthefilterangle,canleadtotheoverallgainbeing
greateratamodeadjacenttothemodeinwhichthelaserisos-
cillating,andthelaserthenhopstothathigher-gainmode. See
Ref.[4]foradetaileddiscussion.
1.3 Piezo-electric frequency control
Smallchangestothelaserfrequencyareachievedbycontrolling
theexternalcavitylengthwithapiezoelectricactuator. Forthe
MOGLabsCEL,theoutputcouplerismountedtoamultilayerpiezo-
electric“stack”.Thecavitylengthvariationisoforder10nmpervolt,
producingafrequencyshiftof70MHz/Vwitharangeof10GHzfor
150Vdrivevoltage. Thebandwidthislimitedbymechanicalreso-
nances,typically25kHz.

4Chapter 1. Introduction
384.0 384.1 384.2 384.3 384.4
Frequency (THz)
External cavity
Diode cavity
Diode gain
COMBINED
Filter
Figure 1.3: Schematicrepresentationforthevariousfrequency-dependent
factorsofanECDL,adaptedfromRef.[4],forwavelengthλ=780nmand
externalcavitylengthLext=15mm.
1.4 Temperature and current
Thelaserfrequencyisalsodependentontemperatureandinjection
current;thesensitivitiesaretypically3MHz/µAand30GHz/K[5].
Thus,low-noisestableelectronics,suchastheMOGLabsDLC ex-
ternalcavitydiode lasercontroller, areessential (seeRef.[6])to
achievesub-MHzlinewidthandstability.
AcriticalaspectofanECDListemperaturecontrolofthecavity,since
thelaserfrequencydependsonthecavitylengthandhenceonthe
thermalexpansioncoefficientofthecavitymaterial[4]. Thecavity
canbemachinedfrommaterialswithlowthermalexpansioncoeffi-
cientbuteventhenthepassivestabilityisinadequateforresearch
applications. Activefeedbackofthecavitytemperaturecombined
withcavitylengthcontrolprovideaflexibleandstableapproach.
TheMOGLabsCEL uses a negativetemperaturecoefficient(NTC)
thermistortosensethecavitytemperatureandPeltierthermoelec-
triccooler(TEC)toheatandcoolthecavitymaterial.

2. First light
Initialinstallationofthelaseristypicallyamatterofmountingit
toanopticaltableandconnectingtoaMOGLabscontroller. The
lasercanbemountedtopostsusingtheM3threadedholesonthe
base,orbyremovingthecoverandscrewingdirectlytotheoptical
tableusingtheM6x25socketheadcapscrewsprovided. Thehole
spacingalsoallowsdirectmountingtoimperialtablesfornon-metric
countries(Burma,LiberiaandtheUSA).
Thelaserincludesawatercoolingchannelforlaseroperationat
unusuallyhighorlowtemperatures,orinlaboratorieswithhighor
unstableairtemperature. Formostapplications,watercoolingis
notrequired;dissipationtotheairand/oropticaltableissufficient.
Theperformanceofanexternalcavitydiodelaserisstronglyde-
pendentontheexternalenvironment,andinparticularacousticvi-
brations. Verysmallchangesintheexternalcavitylengthhavea
largeeffectonthelaserfrequency,typically25MHzpernanometre
lengthchange. ThemonolithicblockconstructionoftheMOGLabs
CELreducestheinfluenceofvibrationsonthecavitylength,butsome
elasticityremains. Acousticdisturbancesintheairgapalsoaffect
thefrequency.Activefeedbacktothelaserfrequencyreducesthese
influences,butsomesimplemeasurestominimisecouplingtoenvi-
ronmentalvariationsandvibrationsourcesmaybewarranted. For
example,asurroundingboxtoreduceairmovementandaccidental
bumpingofthelaser;mountingthelasertoaheavysupport,andiso-
lationfromtheopticaltablewithanintermediarybreadboardwhich
isseparatedfromthemainopticaltablewithviscoelasticpolymer
(e.g.SorbothaneTM).
Oncethelaserismountedappropriately,thelasercanbeswitched
on. Pleaserefertothesuppliedtestdatafornominaltemperature
andcurrentsettings, andin particularbeawareof themaximum
currentlimit. 5

6Chapter 2. First light
ItisassumedthataMOGLabsDLCcontrollerwillbeusedtodrive
thelaser. Ifanalternativesupplyisused,notethat+5Vmustbe
providedonpin15oftheheadboardconnectortoopentheprotective
relay.SeesectionBforconnectiondetails.Alsopleaserefertothe
lasertestdataforthemaximumsafeoperatingcurrent.
2.1 Temperature
Thepreferreddiodetemperaturewilldependonthediode,there-
quiredwavelength,andtheambientroomtemperature. Forexam-
ple,typicalAlGaAsdiodesusedfordatastorageapplications(CD-R
burners)haveanominalwavelengthofλ=784nmat25◦C,with
adλ/dT slopeof−0.3nm/◦C,implyinganoptimumtemperatureof
about12◦. Dependingonthehumidity,lowtemperaturesmayin-
ducecondensationonthediodeandcollimationlens.Thefilterwill
determinethefinalwavelength,andthefeedbackisgenerallysuffi-
cientto“pull”thewavelengthby±5nm,andthusinthisexamplea
sensiblesettemperaturewouldbeabout17to18◦C.

2.2 Current 7
2.2 Current
Theoutputofsemiconductorlaserdiodesfollowanominallylinear
powervs. currentrelationship,oncethecurrentisaboveadevice-
specificthreshold(seeFig.2.1). Initiallythecurrentshouldbeset
above threshold, but well below the nominal maximum operating
current,untilthelaserisfullyaligned.
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140 160180 200
Power (mW)
Injecon current(mA)
Bare 150mW diode
780.243nm
Extracavity esmate
Figure 2.1: Samplelaserdiodepower-currentcharacteristiccurves,with
andwithoutanexternalcavity. Theoutputforadiodewithgoodanti-
reflectioncoatingisnegligible. Thestepsshowthatforhighercurrents,
someofthelightfromthediodeisnottransmittedbythefilter,typically
becausetheexternalcavitymodefrequencyisnotperfectlyalignedwith
thefiltertransmissionfrequency.

8Chapter 2. First light