[Laser] Laser diode observations

[laser@mailman.qth.net]

Hello All

Belated Happy New Year

Firstly, over the Christmas period I managed to find a clear night in between 
days of English rain and made a QSO over 22.4km with G8LSD.

The path was a hilltop to hilltop and used two laser pointer modules (1mW and 
3mW). Both G8LSD and I learned a lot about required pointing accuracy and I 
now have some rebuilding to do in the next few weeks. Allan also has PWM 
voice and this was successful one way.

I've now completed a circuit for my RLT6620G  20mW laser diode purchased from 
Roithner in Austria. The circuit is working well and for the present, I've 
limited the output to a nominal 10mW.  Curiously, the characteristics of the 
5mW and the 20mW diodes purchased from them have been outside the limits 
shown on the data sheets. - Which called for very careful adjustment while 
setting up.

For example. On the 20mW device, ( $55 + tax / shipping) the lasing threshold 
on the datasheet is given as anywhere between 25 to 75mA. However on the 
'real thing' it was 18mA. An advantage (if you're careful in setting up) as 
the overall dissipation and heating was less than anticipated.

On the 5mW devices, the laser operated as expected but the photodiode current 
was only 20% of the expected figure?

I wonder if that spread in characteristics is normal, or are Roithner devices 
not quite as they appear?
Whatever the case, the lesson seems to be to raise the current very slowly 
and very carefully check the characteristics for yourself.

Vy 73

David   G0MRF






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