18 Chapter 4. Troubleshooting
4.1.1 BIAS optimisation
Ideally the frequencies of the external cavity mode andthe intrinsic laser
diodemodearesynchronisedasthe laserfrequencyis varied(seefig-
ure3.2). Theexternalmodefrequencyiscontrolledbythepiezo. The
intrinsicdiodemodefrequencycanbecontrolledbyadjustingthelaser
diode current.
The“feed-forward”orcurrentbias“automatically”adjuststhediodein-
jectioncurrent asthepiezovoltageischanged.Feed-forwardcurrentbias
adjustmentisafeatureofMOGLabsDLCcontrollers. Eachlaserrequires
adifferentchangeindiodecurrentforagivenchangeinpiezovoltage,
and the ratio (and sign) can be adjusted withthe BIAStrimpot on the DLC
controller.
Optimisationisstraightforward. Withthelaserfrequencyscanning,the
BIAS controlisadjusteduntilthemaximummode-hop-freescanrangeis
observed. Smallchanges tothe injectioncurrent optimisethe scanrange
nearthenominalcentrefrequency. AfastFizeauwavemeter,anatomic
absorptionspectroscopysignal, oraFabry-Perotcavitycanbeusedto
monitorthelaser frequencywhilevaryingthedifferentcontrolparameters.
1. Ensure that BIASis enabled (DIPswitch 4).
2. AdjustSPANwhile monitoringCHANAFreqandCHANBCurrentona
dual-channeloscilloscope. Twosawtoothsignalsshouldbeappar-
ent. FirstsetCHANAFreqtomatchthepeak-to-peak voltage value
(andsign)specifiedinthelasertestreport,thenconfirmthatthe
peak-to-peakvoltage(andsign)onCHANB Current matchesthelaser
testreport. AdjusttheBIAS trimpotuntilCHANBCurrent matchesif
necessary.
3. ReduceSPAN tozero. AdjustthelaserdiodeCURRENT tofindthe
required laser wavelength and approximate output power.
4. Ifthewavelengthisclosebutnotquitecorrect,smalladjustments
ofeitherCURRENTorFREQUENCY mayberequiredtofindabetter
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