2 Chapter 1. Introduction
TheMOGLabsCEL/CEX/CEFisa “cat-eye” design(seefigure1.2),in which
an external cavity is formed between the rear reflecting surface of the
semiconductordiode,andacat-eyereflectoratseveralcentimetresfrom
the diode[1–3]. Ratherthanthecustomarydiffraction grating ofLittrow-
configurationECDLs, ahigh efficiencyultranarrowfilterisusedtoselect a
singleexternalcavitymode.Withouttheneedforilluminatingalargearea
ofagratingforfeedback,acat-eyeretroreflectorandpartiallytransmitting
outputcoupler(OC)canbeusedtoformtheexternal cavity. Thecateye
reflectorisinherentlyself-aligning,sothatthelaserisextremely insen-
sitivetomechanicaldisturbance,andalsoensureshigh feedbackcoupling
efficiency and consequently narrow linewidth.
DIODE
FILTER L2 L3
L1 PZTOC
Figure 1.2: Schematicofacateyeexternalcavitydiodelaser(ECDL).Theex-
ternal cavity,formedbytherearfacetofthelaserdiodeandtheoutputcoupler
(OC),determinesthelaserfrequency. One longitudinalcavitymodeisselected
by anultranarrowintracavitybandpassfilter.A cateye reflectoris formedby the
output coupler and intracavity lens L2.
Theoutputbeamfromalaserdiodeiscollimatedwithahighnumerical
aperture(NA)lensL1andthenincidentonthefilter. Thefiltertrans-
missionwavelengthdependsontherotationangle. Transmittedlightis
back-reflectedbythecateyelens/output-couplercombinationwhicheffi-
cientlycoupleslightbackintothelaserdiode. LensL2focuseslighton
theOCandlensL3re-collimatesthelaserbeam. Moredetailscanbe
found in references [1–3].
1.1 External cavity
Semiconductorlaser diodesnormally have ahigh reflectivity rearfacet
and afrontfacetwith reflectivityofonlyafew percent. The diodecavity
iscalled the intrinsic or internal cavity. The external cavity is formed
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