[Laser] Laser Characteristics.- A steep learning curve.

[laser@mailman.qth.net]

Dear   Shyam, Jim, Alberto, Rob and John.

Thank you all for the many informative comments passed over the reflector in 
the past few days. A steep learning curve indeed. Who would have thought that 
in trying to be so very careful with my laser diode that the most dangerous 
thing I have done is to cool it with the aerosol freezer!

As a result of your e-mails I have now stopped my testing and I'm now 
rebuilding the circuit to run from positive ground and a negative supply rail 
as this will allow much improved heatsinking. P channel fets and a PNP 
darlington arrived this morning.

The 20mW diode was supplied by Roithner Lasertechnik in Austria as part 
number RTL6620G  (660nm 20mW 9mm package)  I imagine that this is not the 
original manufacturers part number but a local stock number created in 
Vienna. There are no identifying markings on the device but a single page 
data sheet is available.

Initially (Sorry Alberto) I had decided not to use the internal photodiode as 
part of the current regulator. I could foresee a situation where a 
combination of light reduction due to heating could lead to the circuit 
driving excessive current into the laser diode. I still believe this is a 
small risk, but I think the photodiode could be used as a final limiting 
element to control the tendency for the laser to exceed the maximum light 
output power rating as temperature falls.

Anyway, the circuit has now been revised to include transient protection, 
better heatsinking, a meter to monitor power O/P via the photodiode and 
crystal derived 1:1 square wave drive.( for FFT detection using Alberto's 
Argo software) I still need to work out a method of limiting using the 
photodiode and then hopefully I can make a few QSOs this winter.

I'll post results at www.g0mrf.freeserve.co.uk/laser.htm
The matching reciver is already working very well.  ( /laser301.htm )

Thanks chaps, your e-mails have saved me $$$.

73

David


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