MOGlabs CEX - 3.4 Laser Scanning Control

12 Chapter 3. Operation
3.4 Scanning
Theexternalcavitylengthiscontrolledbyapiezoactuatormovingthe
externalcavitymirror.Thecavitylengthchangeswithpiezovoltage,and
foralargechange, thelaserwillusuallyhoptoaneighbouringcavity
mode. Figure3.3isaschematicofthenetgainvariationwithlaserfre-
quency,showingtwoadjacentmodesofverysimilargain. Figure3.4is
ameasurementofthefrequencyofalaserscanningproperly,andwitha
mode-hop at one edge of the scan.
Frequency (GHz)
Relative Gain
1
0
0
100-100-200 200
Figure 3.3: Combinedgainforanexternalcavitydiodelaser,includingtheinter-
nal and external modes, the diode laser gain, and the filter response. The broad
featureisthefrequencyselectivityofthefilter,andthesmallerpeaksarethe
externalcavitymodes (seefigure3.2).Thelaserwill easilyhopbetweenthetwo
highest external modes with similar net gain.
Themode-hop-freerange(MHFR)canbeoptimisedbycarefuladjustment
of theinjection current,which affects theoptical pathlength ofthe diode
and hence the frequency of the cavity mode.
3.5 Faraday isolator
Thelasercanbesuppliedinaverycompactform,orwithoptionalextended
chassis(optionCEX orCEF)whichallowsinternalmountingofFaraday