3.5 Faraday isolator 13
Time (0.4 sec/div)
Frequency (THz)
Figure 3.4: Laserfrequencyvs. timeforalaserwitha4Hzpiezoramprate,
givingabout10GHzofcavityfrequencychangewhenscanningmode-hop-free
properly (left) and with a mode-hop at one edge (right).
Figure 3.5: Extendedchassis CEFoption,whichaddstwocouplingmirrorsand
fibreoutputtotheextendedchassisoptionCEX.ThechasissmountedFaraday
isolator and beam shaping are available with either CEFor CEXoption.
isolators, and alsothe additionof couplingmirrorsandafibrecoupler
(option CEF, see figure 3.5).
3.5.1 Faraday isolator alignment
Faradayisolatorsarecriticaltothestabilityofanexternalcavitydiode
laser:evenveryweakreflectionsfromexternalopticscanhaveasignificant
effectonthelaserfrequency.Atleast30dBofisolationisneeded;thatis,
the optical feedback into the ECDLshould be less than 0.1% of the output
power.Double-stage isolatorsprovide 60dB ormore ofisolation whichis
necessary if locking to a high-finesse optical cavity.
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