[Laser] Mod Power to several lasers?

Sun, 31 Aug 2003 14:03:52 -0400

Each diode will have a different lasing threshold current and a
different damage threshold current - they each need an individual driver
circuit to accomodate them, since even on a single piece of silicon the
individual diodes will vary greatly - so if these came out of different
CD players, they probably vary greatly, both in their current transfer
function characteristics and their degrees of aging.

I usually recommend against the K3PGP TX circuit for bare laser diodes.
It is a dangerous/risky circuit because it does not electronically
account for the feedback diode of the laser. The whole purpose for
having that monitor diode there is to provide feedback to the circuit,
close the loop and *eliminate* the possibility of a toasted diode. 

The K3PGP circuit relies on human feedback (using the multimeter and
adjusting the drive current) in order to optimize the diode's output.
There are several problems with this:

1) Once the circuit is "tuned" for a specific drive current, it doesn't
change based on changes outside the circuit. Remember that the damage
threshold of a laser diode is not related to the heat it is dissipating
but rather to the optical power density at the "mirrors" on each end of
the silicon. SO now tune up your diode indoors (at 74 degrees F) and
take it outside on a 50-degree night. Your diode suddently gets more
efficient because of the cooler ambient temperature, and the optical
power generated per milliamp of drive current increases, pushing you
over the damage threshold and destroying the diode. A properly-designed
feedback-based driver circuit could have prevented this. 

2) Most experimenters don't want to have to tune-up the drive current
each time they power up the laser, using an optical power meter to
measure the actual output of the diode as they increase the drive. They
just want to tune it up once and then be able to turn it on and off in
the future. But if you do this with the K3PGP circuit, you'll roast your
diode the 2nd time you turn it on. The diode will perform much better at
turn on than when it's warmed up; same reason as in #1. So even though
the circuit provides a soft start, this is a misnomer; it starts up much
too quickly to allow the brass casing and substrate mount to come up to
temperature, and will easily blow out the diode due to the diode's
higher efficiency when cold. A proper warm-up takes several minutes
(especially in cold ambient temperatures) and the feedback-based circuit
can account for this, pushing the diode to maximum optical power output
over a wide range of efficiences.

3) The K3PGP circuit doesn't account for light reflected back into the
laser diode. When you tune up the drive current, it's done with the bare
diode and no collimation optics (unless you know the exact amount of
power lost due to edge shadows) with the meter right up against the
diode's bore. *After* tune-up, the optics are usually put in place. This
inevitable reflects a small amount of light back into the diode, which
contributes to the optical power density on the mirrors. If this
additional light is seen by the monitor diode, the driver circuit can
drop the output current slightly to keep the diode running at exactly
the right spot. (This effect can also cause problems in the OPPOSITE way
with feedback-based drivers, so it's important never to retune a
feedback-based circuit to a higher power once collimation optics are in
place. Otherwise the driver would increase the drive current if the
opics were removed)

All in all, it's a very good idea to build a proper,
monitor-current-feedback-based diode driver circuit. It's easy to
modulate a circuit like this because you just have to inject additional
monitor current and it'll drop the laser's output power (one of many
ways to do it). You can then apply the same (buffered) modulation signal
to several drivers and modulate the whole array.

I can try to pull together some good circuits for feedback-based laser
drive. But there's also an IC made by IC-haus (Germany) that will do the
job nicely for you, at a wide range of output currents, with modulation
(though not TTL-level). They're $5-$6 apiece at sample quantities (<100)
from the IC-Haus US distribution (see IC-Haus website).

73
Dave K2THZ

On Sun, 2003-08-31 at 00:48, John Matz wrote:
> Have you just tried putting the diodes in series from one higher voltage
> source ... that way they all have the same current and get the same
> modulation ... but you would need aq higher source voltage etc to mrun them
> all.
> John Matz KB9II
> ----- Original Message -----
> From: "Richard Burrows" <[email protected]>
> To: <[email protected]>
> Sent: Saturday, August 30, 2003 11:36 AM
> Subject: [Laser] Mod Power to several lasers?
> 
> 
> > Hi
> >
> > I wonder if anyone has an idea how to run six 5mw lasers from a circuit
> similar to K3PGP's Tx modulator. I can get one diode running fine but then
> as you add more diodes the current is shared unequaly between the diodes
> (they take what they need)and then they blow!!!!!
> > I could build a seperate K3PGP's Tx modulator for each diode and run them
> in parallel, but driven from common mod source, but that  seems over
> complicated.
> > Has anyone any ideas or circuits they could point me to please.
> >
> > To explain my application -  6 IR  ex-CD lasers each behind its own 1inch
> collimator lens all driven from same modulator, this  setup to be used for
> cloud bounce comms. I got 4 working  all aligned to one spot , but current
> was up very high and weakest one blew taking rest with it. Back to the
> drawing board!
> >
> >
> >                             Richard  G8BYI
> >
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