Specifications

2 Chapter 1. Introduction

isusedtoselectasingleexternalcavitymode. Withouttheneed

forilluminatingalargeareaofagratingforfeedback,acat-eye

retroreﬂector and partially transmitting output coupler can be used

toformtheexternalcavity. Thecateyereﬂectorisinherentlyself-

aligning, sothatthelaseris extremelyinsensitivetomechanical

disturbance,andalsoensureshighfeedbackcouplingeﬃciencyand

consequently narrow linewidth.

DIODE FILTER LENS LENS

LENS

PZT

Figure 1.2: Schematicofacateyeexternalcavitydiodelaser(ECDL).The

external cavity, formed bythe rear facet of the laser diode and the output

coupler,determinesthelaserfrequency. Onelongitudinalcavitymodeis

selectedbyanultranarrow intracavitybandpassﬁlter. Acateyereﬂector

is formedby the output coupler(OC) and intracavitylens, and the lightis

recollimated by the extracavity output lens.

Theoutputbeamfromalaserdiodeiscollimatedwithahighnumeri-

calaperture(NA)lensandincidentontheﬁlter.Theﬁltertransmis-

sionwavelengthdepends ontherotationangle.Transmitted lightis

back-reﬂectedbythecateyelens/output-couplercombinationwhich

eﬃciently couples lightback into thelaser diode.More details can

be found in the references [1–3].

1.1 External cavity

Semiconductorlaserdiodesnormallyhaveahighreﬂectivityrear

facetandafrontfacetwithreﬂectivityofonlyafewpercent. The

diodecavityiscalledtheintrinsicorinternalcavity. Theexternal

cavityisformedbythecateyeandthedioderearfacet,andwhen

the external feedback is greater than that of the front facet, theex-