3.3 Mode-hops 11
Asthelaserwavelengthisvaried,usuallybychangingthecavitylength
with apiezo, competitionbetween thewavelength determinedbythe dif-
ferentwavelength-dependentcavityelementscanleadtoamode hop: a
jumpinlaserwavelengthtoa differentexternal cavitymode.Wavelength-
dependentelementsincludetheexternalcavity,thelaserdiodeinternal
cavitybetweentherearandfrontfacetsofthediode,thefilter,andthe
gain bandwidth of the laser diode.
Thedifferentwavelength-dependentcharacteristicsareshownschemati-
callyinfigure3.2.Thenetgainistheproductofsemiconductorgain,filter
response, andinternal andexternal cavity interference. The netgain can
beverysimilaratadjacentexternalcavitymodes. Asmallchange inthe
laser cavityoptical pathlength, thediodeinternal cavitymode frequency,
orthefilterangle,canleadtotheoverallgainbeinggreateratamode
adjacent tothe modein whichthelaser isoscillating,andthe laserthen
hops to that higher-gain mode.See Ref. [6] for a detailed discussion.
384.0 384.1 384.2 384.3 384.4
Frequency (THz)
External cavity
Diode cavity
Diode gain
COMBINED
Filter
Figure 3.2: Schematicrepresentationforthevariousfrequency-dependentfactors
of anECDL, adaptedfrom Ref. [6], forwavelength λ =780nm and externalcavity
length L
ext
=15mm.
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