We found a diagram on the Internet which transmits sound waves over
the beam of a laser pointer by using the output of an audio source to
modulate the intensity of the laser.  The sound is "recovered" (quite
well actually) by shining the beam onto a photo cell which is
connected to the input of a small audio amplifier.  Alternately, a
photo resistor and battery source could be used instead of the photo
cell for sound recovery.

The diagram we used is here:
http://www.scitoys.com/scitoys/scitoys/light/light.html#laser_communicator

We have partially killed three laser pointers (fortunately we found a
bunch of them at a dollar store for $2.00 (USD $1.28).  The strange
part is the *partial* killing of the laser and this leads to the
question.

We began by soldering a wire to the neg of the laser board (see above
Internet link for picture).  Then another alligator clip was used to
connect to the pos.  Then those wires were soldered to the transformer
and a battery pack was added in series with this side of the circuit. 
The strange thing is that the laser pointer will only shine at only
about 1/10 its original intensity from this point on.

The circuit still works but the beam is very dim (actuall, it may be
more like 1/100 the original intensity but the way the human eye
adjusts for intensity it appears to be more like 1/10).  The
question(s) is(are): "Why does the laser pointer dim?  If it is being
damaged, why does it not behave like most other semiconductive
circuits and simply die?  Why does it not return to its original
intensity when removed from the circuit?"

I guess all that can be summed up by asking, "What's going on?"

There is no need to explain any of the theory behind the actual
circuit (since that's well understood here) unless that theory is
vital to the dimming of the laser.  Our "guess" is that there is a
voltage spike (probably caused by making or breaking the circuit) that
is somehow being amplified by the matching transformer and allowed to
pass through the laser circuitry.

We're using a soldering pencil so magnetism is not as high as with a
gun; we have been careful with input voltages, etc... What else can I
add...?  I can't think of anything more at this point.

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Request for Question Clarification by sgtcory-ga on 09 Nov 2002 10:53 PST

Hello avonlea,

Do you have the ability to take measurements on the boards? (voltage)
You can check the voltages (base bias) for Q1 and Q2 in an operational
state, and compare it to one of the damaged ones.

Let me know if you would like an answer based on steps you could take,
as pin-pointing the actual problem is *hard* without having the
circuit in hand. (Or even a schematic of the small circuit in it's
whole form.)

Thanks,
SgtCory

---

Clarification of Question by avonlea-ga on 12 Nov 2002 10:32 PST

The diode (and yes, it's a laser generating diode -- that's what's
used in laser pointers costing less than $100.00) appears to have died
either before connecting the power (again; we've been careful on
voltage) or the moment the power was connected.

As stated in my first note, when the laser is removed from the circuit
it does not recover.

The puzzling point is that the pointer only dims, it does not die
completely.  That's what I'm wanting to know: "Why?"

---

Request for Question Clarification by sgtcory-ga on 12 Nov 2002 12:11 PST

Excerpt from Diode Lasers
"This is an interaction between the temperature of the facet and its
optical
absorption.  When the temperature of the facet grows, the absorption
can also
grow which feeds back positively to the temperature and the
temperature "runs
away" until it is physically damaged."

It's my guess that the facet inside the diode is getting overspiked
(current). The diode does not blow completely as seen in typical
diodes, rather a portion is damaged. You are then only able to get a
fixed amount of current through the diode, and current is directly
proportional to the amount of lasing that you are seeing. (Or lack of
lasing I should say)

Let me know if this is along the right track.

Thanks,
SgtCory


http://www.technology.niagarac.on.ca/people/mcsele/lasers/LasersDiode.htm

avonlea,
more info needed, for e.g. what kind of light generating source is
used by the 2.00 laser pointer?  my suspicion is that the "laser"
source is more like a special diode (it got partially fried when you
upped the power to the whole circuit to support extra circuitry to
control the audio signals, so when you removed the extra circuits,
there is no way the diode could recover).

maybe am wrong, but perhaps u could just experiment connecting the
extra power without the audio circuitry and see if you get the same
problem.  Might be as simple as that 8-)

as  it stands the circuit seems "too" simple. also considering when u
turn on a Radio. there might be a "thump" on the output. which while
maybe be shortlived. or even inaudible will definately be damaging to
the diode circuitry as IT will be amplifieed by the tranformer
the simplest solution To THAT problem would be to have back to back
6.8 V zener diodes in series with a 5 k resister  across the out put
of the transformer> something like this.
http://first.scriptmania.com/test/laser.gif
Again this is only a solution If ure haveing this problem . but we
can't be sure u are.

See my clarifications.

If you haven't already; try a clipping circuit across the primary
and/or the secondary of the matching transformer. ie a pair of
parallel silicon power diodes anode to cathode will absorb voltage
spikes exceeding + or - 0.6 volts. A third winding on the matching
transformer will match the impedence of the clipping circuit if
neither the primary nor secondary impedence is suitable. The dc
resistance of the matching tranformer may be reducing the current to
the lasar diode to account for the dimming. If so brightess will
return if you short the transformer temporarilly.  Neil

Ya don't s'pose ya got NOISY POWER, do ya? :)

I feel that the answer to the question is that you have overdriven the
laser LED so much
that its light emitting efficiency is reduced.  LED's typically have a
11year half intensity
life(means if the LED is operated for 11years continuosly within the
rated power, the
intesity reduces by half).  It is not much different for a Laser LED
diode.  I feel that by
intensional/unintensional overdrive, you've reduced the life of the
LED, which makes
it to look dim.  It is more similar to the accelared aging of
electronic parts(like buring-in
a board of components, or simply operating a component such as an
electrolytic
condensor at its maximum allowed voltage at high humidity and
temperature).  In this
case, just the excess current from the circuit aged the LED.