Part 1 - Modulation

[marconi]

This is a start of a series of mods and circuits to come
it was decided by Spec and I to start some new faqs and links
as many of the old ones are non-existant or too lengthy to find anything.
I still suggest going to sams laserfaq for the gospel on lasers..
because he is the man,..and has spent a good part of his dedicated time
making that info available to us all..Forever I will be greatful to him.
Bare with me as I am an engineer not a linguist or typist , and may not convay everything correctly..
Altho I am a college grad my head keeps getting filled
with too much useless information as the years go by and I guess this lets out all the good stuff I did learn right out the window..
I am not the "laser expert " these are things I have learned and built
and they work for me.
This is here for us to share and learn by and perhaps improvements will be made.
If you see anything out of order please chime in.
so without farther ado...


Modulation...
There are several ways to skin a cat , but my favorite is TTL blanking.
Some like to use shutters and fancy flags or solenoids..
But we will stick to using electronics to do this for us.
What you use will depend on your application as some type of lasers and diodes cannot be modulated ...

Now the amount of modulation is important..
Depending on your diode..the amount of depth or cutoff current is important.
You do not want to completely turn off the diode during modulation
as this will cause an early death to your diode..
What we want to do is set the cut-off point just above threshold of lasing.
altho you may still see the beam during a standing still beam..
you wont when it is moving..This is the only way to do it properly ..
The reason for this is because we do not want to subject the diode to too many changes during its operation..
that is..going from lasing to not lasing..or putting too much current out ....
this is the area where most damage can accure.
What happens is if we spike the diode with overshoots or too little of current the facet (face of the diode ) will be damaged.
so we must control the area where we turn on and off the amount of light from the diode without damage...

Of course the use of a very fast shutter can be used..but this would require some fancy machanical arrangments and syncronization which I have never had any luck with.
This of course is easier on the diode and in some cases the only way you can
cut-off the beam...
I do suggeste to always use a powered to open shutter as a control device for a BEAM-OFF condition.



if you have a MaXYZmodules RED or Green laser this will pertain to you
if not it still can be useful info as I will be building up to more info on how to modulate your diodes without destroying them. its almost a guarded secret to some. As if it is not done correctly..you will pop your diode.

A typical circuit I use is:
(pix coming soon)
Using a 4n33 darlington opto-isolator or similar as an input device ,Then
all you would need is a 100 to 330 ohm input resistor and a 1k ohm pull-up
resistor to the supply voltage on the output ,Depending what kind of driver circuit you are using this may not be needed...
maybe put a 5k pot on the input to control the amount of blanking needed.
This is similar to what our green and red modules have...
If you want analog modulation the use of a different opto (a/c input type)
would do it.
Also, If you are using an analog control device (ramping the input voltage) to control the amount of brightness , Consider the use of a voltage divider (pot) on the input, also to correct for lineararity (voltage range) a few diodes in series with the output of the opto may be needed..!!!

I'll add to this some more as time permits
Hope this helps some

EOT *^_^*

[marconi]

The use of OPTO-ISOLATORS as the input device has a few advantages.

First, It only needs a little over a volt to turn it on.
So with a simple audio amp or just a signal generator,.
you could modulate it easily.
How much modulation could be controlled by a pot as a voltage divider
on the input.

Second, since there is a small voltage knee range in-between
the on and off condition..
By added a pull-up resistor to the input , just enough to lightly bias it
you can voice or music modulate it..
altho I havnt tried music yet..but I believe it would work ...
The particular Opto I use is good to over 10khz so it should have pretty good range..
The opto you choose to use will depend on its use.
One type I mostly use for TTL is a Darlington output (4n33 or LTV-8141s)
If you want to use your laser for music or voice you may want to use a
Non-darlington type or just a standard Opto-isolator, the a/c input ones seem to work the best as both halves of the waveform is passed on..
as some dont clamp quite as hard and allow for more analog range.
experimentation on your part is in order here.,
Of course all this depends on how much bypassing and capacitance
is inside the driver circiut you are using,,.This is what limits the frequency range...so remember that when you cant get the blanking you want!!!
I try and use very very little bypassing on the active side of the current driver
to the diode. By-passing is mostly done at the regulators...but.
One should be very careful here as improper board lay-out and component
values can induce oscillations of the op-amp.
Selective by-passing is another neat chore...and proper placement is essential.

Third, They can be directly driven by TTL giving smooth control of the
backtrace lines in lasershow use.

Ok..now that you have some of my "inside" info..
Go ahead and start experimenting...
Adding the blanking/modulation control to a laser is a pretty neat thing
to do to your laser.
You will see, that there is more you can do with your set-up..

By the use of LM-317's a few pots to control the voltage and a couple of dc motors (pc fans) and glue some mirrors offset slightly .
A Lm-555 as a square-wave generator with pots to control pulse length and frequency.
You can make a modulated lissajou (spirograph) lasershow or a liquid sky generator and keep the kids entertained for a while.

EOT *^_^*