[Laser] Laser diode characteristics.

[Rob Barris]

These sites have a lot to say on the subject:

http://www.repairfaq.org/sam/laserfaq.htm

http://www.repairfaq.org/sam/laserdps.htm#dpstoc

"The following must be achieved to properly drive a laser diode and not 
ruin it in short order:
*	Absolute current limiting. This includes immunity to power line 
transients as well as those that may occur during power-on and 
power-off cycling. The parameters of many electronic components like 
ICs are rarely specified during periods of changing input power. 
Special laser diode drive chips are available which meet these 
requirements but a common op-amp may not be suitable without extreme 
care in circuit design - if at all.
*	Current regulation. Efficiency and optical power output of a laser 
diode goes up with decreasing temperature. This means that without 
optical feedback, a laser diode switched on and adjusted at room 
temperature will have reduced output once it warms up. Conversely, if 
the current is set up after the laser diode has warmed up, it will 
likely blow out the next time it is switched on at room temperature if 
there is no optical feedback based regulation.
Note that the damage from improper drive is not only due to thermal 
effects (though overheating is also possible) but due to exceeding the 
maximum optical power density (E/M field gradients?) at one of the end 
facets (mirrors) - and thus the nearly instantaneous nature of the risk.

The optical output of a laser diode also declines as it heats up. This 
is reversible as long as no actual thermal damage has taken place. 
However, facet damage due to exceeding the optical output 
specifications is permanent. The result may be an expensive LED or 
(possibly greatly) reduced laser emission.
"


(there's a lot more...)

How many pins on your expensive laser diode ?  One of them might be for 
output power feedback for power supply control.

Rob

On Sunday, November 3, 2002, at 02:43 AM, G0MRF@aol.com wrote:

> A small update......
>
> The current at which the lasing threshold occurs does indeed increase 
> with
> increasing temperature of the laser diode.
>
> I watched the light output diminish as the device warmed up. Then 
> applied a
> quick squirt of aerosol freezer. Hey presto......Instant increase in 
> output
> power.
>
> Conclusion:   More heatsinking required.
>
> So now it's a question of how much higher than the threshold can I 
> safely go?
> I looked at the data on the internal photodiode and it says:
>  Min 50uA / mW
>  Max 500uA / mW
>
> So, not much use there..................
>
> 73
>
> David
>