22 Chapter 4. Troubleshooting
The lasing threshold,atwhichthe overall gainexceeds losses, shouldbe
as low as possible to maximise the output power and also the scan range
andfrequencystability. Thelowestthresholdisachievedbyoptimising
thefocusofthelaserdiodecollimationlensandofthecateyelens(see
figure4.1).Thelensfocusiswavelengthdependent,sowheneverthelaser
wavelengthischangedsubstantially,werecommendalsooptimisingthe
threshold.
Smallchangestothefocusadjustmentcanbeachievedbysimplyadjusting
thediodefine-focusadjustontherearoftheCEL barrel.Suchadjustments
(nomorethanonetenthofaturn)aresuitablewhenthresholdoptimisation
dueonlytoawavelengthshiftisrequired.Ifalargeradjustmentofthefine-
focusisneeded,theintracavitycateyelensshouldinsteadbeoptimised
as below using the provided focus adjust tool (see figure 4.1).
Focus adjust tool
Cateye OCFilterDiode LensFocus Recollimator
Figure 4.1: Arrangement oflens tubeandcateye reflector, andfocus adjusttool.
The sequence is as follows:
1. Ensure the DLCis off or on standby.
2. Take note of the filter angle by marking on the laser barrel the
alignment of the filter assembly notch (the mark on the spindle, not
themarkonthebrassclamp)relativetothelaserbarrelnotch.Take
a photo from above, so that youcan restore the original filter angle
if needed.
3. Undo thespindle-clamplockscrew afew turns,andundothespring
plungerand wavelength adjust screwsuntil they are clearofthe
circular bore.
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