Specifications

12 Chapter 3. Alignment
E
From diode
Filter
θ
Figure 3.3: Orientationofthediodelaserbeamellipsewithrespecttothe
filter rotation, for TE polarised diode, oriented with p-plane polarisation.
fromthefilter. Inthatcase,fortheMOGLabsCEL,thepolarisation
shouldbehorizontalandtheellipseshouldbewithlongaxisvertical.
Some diodes, particularly around 750 to 820nm, are TM polarised,
withpolarisationparalleltothelongaxisoftheellipse. Forthese
diodes, the ellipse should be horizontal.
Thefilterdependenceonpolarisationisweakandinmostcasesany
rotation of the diode will work acceptably well.
3.4 Cateye reflector
Lightreflectedfromacateyelens/mirrorcombinationwillbeparallel
to theincident radiation, regardless of the incidentangle.Thus the
cateyereflectorisself-aligning:thelightisalwaysreflectedbackto
the diode, evenif the beamis not wellcollimated. The effect relies
on the lens-mirror distance matching the focal length of the lens.
Thereareseveralmethodsforachievingoptimumfocus;probablythe
easiestis toadjustthe focussoas tominimisethe lasingthreshold.
Mountthediodecollimationlenstubeandcateyereflectorassem-