Specifications

1.2 Mode competition 3
ternal cavity determines the lasing wavelength.The external cavity
istypicallyaround40mmlongfromrearfacetofsemiconductorto
outputcoupler,givingacavitymodespacing(FSR)ofc/2L = 3to
4GHz.
The laserdiodeandcollimatinglensareheldrigidlyinafocusing
tube. Thefilterisfixedtoabearing-mountedrotationassembly
withfineactuatorscrewstoadjusttheangle. Thespring-loaded
screws operate in a push-pull arrangement which can be locked
againsteach othertofurtherreduce theeffectsofmechanical vibra-
tion.Variation of the filter angle is usedforcoarseselectionof the
wavelength, within the gain bandwidth of the laser diode.
1.2 Mode competition
Asthe wavelengthisvaried, competitionbetweenthefrequency de-
termined bytheinternaland externalcavities,andthefiltertrans-
missionwindow, leadsto mode hops.Fromfigure1.3 itcanbe seen
thatthenetgain(combinedproductofsemiconductorgain, filter
loss,internalandexternalcavityinterference)canbeverysimilar
atadjacentexternalcavitymodes. Asmallchangeintheinternal
cavitymode,orthefilterangle,canleadtotheoverallgainbeing
greateratamodeadjacenttothemodeinwhichthelaserisos-
cillating,andthelaserthenhopstothathigher-gainmode. See
Ref. [4] for a detailed discussion.
1.3 Piezo-electric frequency control
Smallchangestothelaserfrequencyareachievedbycontrolling
theexternalcavitylengthwithapiezoelectricactuator. Forthe
MOGLabs CEL,the outputcoupler ismounted toa multilayer piezo-
electric“stack”.Thecavitylengthvariationisoforder10nmpervolt,
producinga frequencyshiftof70MHz/Vwitha range of10GHz for
150Vdrivevoltage. The bandwidthislimitedbymechanicalreso-
nances, typically 25kHz.