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'[EE]: lasers..'
2002\01\13@174459 by andy n1yew

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hey all,

can someone tell me what the maximum frequency that one can amplitude modulate a regular red pointer laser at?

i think it would be as fast as you can switch the laser on and off, right?

thanks,
andy

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2002\01\13@182956 by Jinx

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> Can someone tell me what the maximum frequency
> that one can amplitude modulate a regular red pointer laser at?

Sounds like a similar question to the 4066 query - the capacitance
of the diode and driver circuit in general would probably determine
the modulation speed

Also, if it uses a constant current source there may be some small
delay to get full power back into the diode after an "off"

Best thing you could do is look up the diode and PSU and get
some specs. You should be able to experimentally determine
what you need to know by looking at the output of the receiver
or the effect you're trying create. There'll be a point where the
degradation will be unacceptable

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2002\01\13@184244 by Jinx

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> I  think it would be as fast as you can switch the laser on and off

BTW, meant to add this to previous post

I've got a couple of lasers described as "modulating" lasers. That
is, I think they are purpose-built for modulating, unlike a continuous
wave pointer laser. These particular lasers can be modulated from
100Hz to 50MHz at 90% depth max, using either digital or analogue
signals. I'm guessing that the max modulation frequency for a laser
pointer would be pretty high but probably not 50MHz

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2002\01\14@024112 by Vasile Surducan

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Well, I wish to modulate up to 50MHz a commercial laser pointer. ( those
with 3...4 $ ) but the maximum frequency I've got ( stable and just
modulate it using power supply modulation ) was 10KHz no more.

Vasile


On Mon, 14 Jan 2002, Jinx wrote:

{Quote hidden}

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2002\01\14@034425 by Jinx

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part 1 827 bytes content-type:text/plain; (decoded 7bit)

> Well, I wish to modulate up to 50MHz a commercial laser pointer.
> ( those with 3...4 $ ) but the maximum frequency I've got ( stable
> and just modulate it using power supply modulation ) was 10KHz
> no more

Hi Vasile, try this. Strip the laser out of the pointer and hook it up
to this constant current modulator. You might be lucky and get a
part number off the diode so you can look up the specs for it. I
use this one on a 5mW IR laser with no problems, but I've no idea
what the maximum modulation frequency is. Perhaps someone
can have a stab at it.

You can lower the BC548's base resistor to 4.7R if you need up
to 180mA. I wonder if ZTX transistors were used this could improve
the switching frequency. Would their lower saturation voltages
mean a major re-design ?


part 2 4707 bytes content-type:image/gif; (decode)


part 3 131 bytes
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2002\01\14@040624 by Vasile Surducan

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Interested schematic Joe, but I want to keep the internal constant current
generator, this make things too complicated.
I have also doubts about using BD139 at such high frequencies ( MHz
)Even the driver ( CMOS 4093 ) will go only to up 8...10MHz
. My ideea was to use a high frequency operational amplifier ( a cheap
NE5539 from Philips ) as controlled current generator and a good comutation
output transistor.
I've guess the schematic you've point could go at max. 50KHz. Can you
measure the output modulation with a PIN photodiode ( or a standard
photodiode if you haven't a PIN one ) and a scope?

Vasile

On Mon, 14 Jan 2002, Jinx wrote:

{Quote hidden}

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2002\01\14@051639 by Jinx

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> I've guess the schematic you've point could go at max. 50KHz.
> Can you measure the output modulation with a PIN photodiode
> ( or a standard photodiode if you haven't a PIN one ) and a scope?

Suppose I could when I have a little time

As you say, the 4093 will limit the frequency, although a 74F132
would be OK to 50MHz, if you really need to use a gate at all on
the bypass' base

Is the BD139 too slow ? > 200MHz ? Anyway, there are plenty
of HF power transistors around if you don't like the BD139

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2002\01\14@075711 by Roman Black

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Hi Vasile! Hi Jinx, I saw your 100mA constant
current laser modulator circuit. I think you CAN get
good high freq modulation from a cheap laser but it
might mean a slight re-design in a more "agricultural"
fashion. ;o)

Suggestions:
* power driver/cc MUST be very low capacitance
* power driver must be simple
* decoupling and layout like RF power stage;
* short thick tracks
* attention to ground planes
* parts close together (and away from others)
* power decoupling with RC
* NON inductive resistors

To try and get these objectives i'd try a totally
different approach:

+24v --------R--------------------------------------
(reg?)            |        |           |
                 |        R           |
  (RC decouple!) |        |           E
                 C        |---------B    fast PNP
                 |        |           C
                 |        R           |
                 |        |           |
                 |        C           R  constant current
                 |  in- B    NPN      |   resistor
                 |        E           |
                 |        |           | +
                 |        |          LED  (laser)(C?)
                 |        |           | -
gnd  -------------------------------------------------

The small amount of current regulation you lose
by changing to just a 24v source and resistor is
minimal, and you can expect brightness/freq
illinearities in transmitter, receiver, atmostphere
etc, so a couple of percent really won't matter.
Depending on your needs the 24v could be reduced
as needed, reductions will reduce the R C(laser)
issue, even though a good laser should have quite
a low capacitance.

BUT the big gains in simplicity and very low circuit
capacitance will give good HF performance. Hopefully.
The power stage needs to be driven hard to give the
speed, so expect high base currents into both the
transistors.

Also I think real attention needs to be paid to the
power decoupling, both to provide fast current pulses
to the load and to stop it chewing up your PSU
rails, obviously using a 24v rail and high freq
decoupling will help heaps. The entire power circuit
as shown should be almost circular with the decouple
cap and PNP,R,LED in a close ring. I'm sure the RF
guys will add some gems of wisdom to my crude
ramblings. :o)
-Roman


Jinx wrote:
{Quote hidden}

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2002\01\14@172901 by Jinx

face picon face
> +24v --------R--------------------------------------
> (reg?)            |        |           |
>                   |        R           |
>    (RC decouple!) |        |           E
>                   C        |---------B    fast PNP
>                   |        |           C
>                   |        R           |
>                   |        |           |
>                   |        C           R  constant current
>                   |  in- B    NPN      |   resistor
>                   |        E           |
>                   |        |           | +
>                   |        |          LED  (laser)(C?)
>                   |        |           | -
> gnd  -------------------------------------------------

Hi, what do you mean by "constant current resistor" ? Is this
a simple resistor or an active c-c, like a damn fast LM317 ?
If a simple R, then getting one with low H would be desirable

For the output pnp I was thinking along the lines of a ZTX751,
2A, 140MHz, and better for pulse switching than the normal
bipolar. Similarly something like a ZTX651 for the npn. Or do
you have "proper" HF or RF devices in mind ?

I'm very conscious of over-driving laser LEDs, they're more
sensitive than ordinary LEDs and any experimenter would
have to be quite careful. The crucial part of the circuit is the
current-limiting. Get that wrong and it's

"Welcome to Knackeredville. Population - your LED"

Vasile, what's your application ? Video link or something ?

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2002\01\15@053248 by Vasile Surducan

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part 1 919 bytes content-type:TEXT/PLAIN; charset=US-ASCII

On Mon, 14 Jan 2002, Roman Black wrote:

{Quote hidden}

 Hi Joe and Roman !

Ok, I think I have a solution using a 350MHz bandwith operational
amplifier and 1GHz power transistor as a switching constant current
source ( gif. attached )
All operational compensations and in+;in- resistors are not shown here.
It requires bipolar supply but negative one is small and low current one.
I think you know what C1,D1,D2 are doing, the rectangular ( or not )
pulse is applied between IN and GND.
All Roman's suggestion will be respected !

Best, Vasile


part 2 2764 bytes content-type:IMAGE/GIF; name="laser.gif" (decode)

part 3 136 bytes
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2002\01\16@040841 by Vasile Surducan

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On Tue, 15 Jan 2002, Jinx wrote:

> Vasile, what's your application ? Video link or something ?

No video link, its mostly a physical experiment where I need to
modulate a laser from low to very high frequency ( different shapes )
and to excite a living green plant ( and probably a human tissue). The
plant response will be probably different at some power levels and
frequency modulations.

Vasile

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2002\01\17@114136 by Roman Black

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Hi Vasile, there's not much I like about
your circuit, sorry! :o)

The 350MHz op amp and -ve supply rail can
probably be replaced with a cheap 200MHz
transistor giving the same overall circuit
performance, and what's with C1 and diodes? Are
you compensating the slow turn-on of the diodes?

I really think it's worth making the thing as
dc-coupled as you can, you are driving it with
a sharp edged square wave, so just think of it
as a very fast output driver.

Unless the laser has a significantly high
capacitance you can do the constant current
with just a resistor and a decent supply rail.
This gives benefits in price, reliability,
board layout etc etc. Driving 100mA at this freq
I think the most work will be in the output
stage, ie transistor selection, board layout,
psu decoupling etc etc. Any RF type circuits need
to start their life as simple as possible I feel.
Complications will come soon enough. :o)
-Roman



Vasile Surducan wrote:
{Quote hidden}

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2002\01\18@040101 by Vasile Surducan

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On Fri, 18 Jan 2002, Roman Black wrote:

> Hi Vasile, there's not much I like about
> your circuit, sorry! :o)
>

Because probably is not too good for laser driving.
Correct, but it works ok with blue leds modulated
with different signal shape that rectangular one.



> The 350MHz op amp and -ve supply rail can
> probably be replaced with a cheap 200MHz
> transistor giving the same overall circuit
> performance, and what's with C1 and diodes? Are
> you compensating the slow turn-on of the diodes?


No, I compensate the slow turn off of the transistor !
The capacitor is charging at about 1V...1.4V and
discharging at off command with reverse potential on Vbe
allowing fast turn off.
The problem with a switching transistor is not the turning on
time but the turning off, and the self storage time.
>
> I really think it's worth making the thing as
> dc-coupled as you can, you are driving it with
> a sharp edged square wave, so just think of it
> as a very fast output driver.

you have right, but if I don't want a square wave
but a controlled shape and  amplitude
by the input signal ? See my schematic from this
point of view, and as a simple alternative to
change from LEDs to laser.

{Quote hidden}

 I'm agree with you here... except that all serious
laser driver hasn't a simple resistor for constant
current but a specialised chip soldered very close to
the laser. That's because of high current treshold
( greater than 7Amps ) needed to lasser effect begun.
I'm talking now about high power lasser diode which are
not working in CW, but also about 3mW CW laser diode
from CD-rom drive, both have a specialised circuit.


Best regards, Vasile



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2002\01\19@094355 by Roman Black

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Vasile Surducan wrote:

> > The 350MHz op amp and -ve supply rail can
> > probably be replaced with a cheap 200MHz
> > transistor giving the same overall circuit
> > performance, and what's with C1 and diodes? Are
> > you compensating the slow turn-on of the diodes?
>
> No, I compensate the slow turn off of the transistor !
> The capacitor is charging at about 1V...1.4V and
> discharging at off command with reverse potential on Vbe
> allowing fast turn off.
> The problem with a switching transistor is not the turning on
> time but the turning off, and the self storage time.

I understand. :o)

{Quote hidden}

Ouch! I thought it was for simple on/off modulation.
My circuit is of no use to you. :o)


>   I'm agree with you here... except that all serious
> laser driver hasn't a simple resistor for constant
> current but a specialised chip soldered very close to
> the laser. That's because of high current treshold
> ( greater than 7Amps ) needed to lasser effect begun.
> I'm talking now about high power lasser diode which are
> not working in CW, but also about 3mW CW laser diode
> from CD-rom drive, both have a specialised circuit.

I don't understand this, i've driven 5mW red lasers
fine with a 100mA constant current supply. You are saying
they need 7A to start? Is that to get them turned on
quickly enough, ie effects of the internal R and C
of the laser?
-Roman

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2002\01\21@023915 by Vasile Surducan

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On Sun, 20 Jan 2002, Roman Black wrote:

>
> >   I'm agree with you here... except that all serious
> > laser driver hasn't a simple resistor for constant
> > current but a specialised chip soldered very close to
> > the laser. That's because of high current treshold
> > ( greater than 7Amps ) needed to lasser effect begun.
> > I'm talking now about high power lasser diode which are
> > not working in CW, but also about 3mW CW laser diode
> > from CD-rom drive, both have a specialised circuit.
>
> I don't understand this, i've driven 5mW red lasers
> fine with a 100mA constant current supply. You are saying
> they need 7A to start? Is that to get them turned on
> quickly enough, ie effects of the internal R and C
> of the laser?

 Oh my damn english !
 Small power CW lasers are working ok with your solution.\
 High power pulsed lasers have a thresold of about 7 to 10
 amperes ( even if an IR 10W laser diode looks exectly like
 an ordinary 5mm led ) and need specialised driving IC/circuit, else
 the laser effect will not take place ( at less then current thresold
 will work like an ordinary IR led )

Vasile

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