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Pulsed drivers debate!

mfo

mfo

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Ok. I am posting this thread as a legitimate question. I will ask a question, give my answer to it and then I want all of your answers. Here goes.

Why do some lasers have pulsed output instead of a steady output? I hear it is to prevent the diode from getting to hot. However when you first initiate an electronic device (Let's say a 100W light bulb for arguments sake), isn't the initial power consumption spike a lot higher than the steady power consumption? Like when you initially turn on a 100W light bulb, it will use 1000W of electricity for a split second, then stable out at 100W. (I know it isn't necessarily 1000W, I am just saying 1000W just so you all know what I am getting at. I do know it is a factor of many times its power consumption).

I would imagine that pulsed would in essence make the circuit hotter, since it is constantly getting that start up spike. I guess I am wrong however since this is used by a decent amount of lasers? Can any electronics experts (Cough Jerry cough) elaborate on this? Thanks!
 





lasersbee

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Well.... I;m far from an expert.... but I'll give it a try...

You can't compare a 100 watt light bulb "turn on" characteristics with
an LED or LD...
You can't completely turn a 100 watt light bulb on and off very fast before
the heated filament can no longer cool off to show OFF times of the applied
pulsing signal... That's why cheap light dimmers on 60hz work with incandescent
bulbs and you can barely see it pulse...

LEDs and LDs use much less power to energize... and they turn on and off completely
with every pulse of the applied signal..

There is very low initial "turn on" current generated compare to a 100 watt bulb.
I've read data sheets that state Pulsed operations at twice the rated CW ratings...
but the pulse width at 50% duty cycles are 100ns.

50% of the time the LD die is heating up and 50% of the time it is cooling down...
but almost instantly.... there is little thermal mass...

Anyone else can chime in at any time.....


Jerry
 
Hemlock_Mike

Hemlock_Mike

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An incandesant light has a fillament which changes resistance related to heat. That's why the inrush current. LEDs and LDs are a different light source.

Mike
 
mfo

mfo

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So basically due to the fact that LED's and LD's are more efficient, they put out much less heat? As opposed to less efficient electronic devices such as an incandescent light bulb?
 
lasersbee

lasersbee

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Basically...
LEDs and LDs don't have that large inrush current...

Jerry
 
Jimmymcjimthejim

Jimmymcjimthejim

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The reason why some DPSS lasers (especially higher powered ones) are pulsed is because if they are ran continuous then there is not enough energy to pump the crystal at full efficiency. The shorter pulses allow for a higher powered pump, which makes more power to meet the crystal's needs.
 
mfo

mfo

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Jimmymcjimthejim said:
The reason why some DPSS lasers (especially higher powered ones) are pulsed is because if they are ran continuous then there is not enough energy to pump the crystal at full efficiency. The shorter pulses allow for a higher powered pump, which makes more power to meet the crystal's needs.
Click to expand...

Interesting, I wonder what property of the crystal calls for this? I especially noticed this in DPSS blue lasers (473nm).
 
R

r691175002

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Alright let me explain this properly.

Pulsing a bare laser diode will not make it brighter or more powerful. If you can run a laser diode at 100mA, you can pulse it at 200mA for 50% of the time (Actually, you probably can't). This trade off will not appear brighter nor dimmer to the eye, but might produce a dashed line depending on the frequency of the pulses. As the frequency of pulses becomes reasonably fast the photoreceptors in your eyes won't have time to depolarize before the next wave of photons hit.

To put it simply, a pulsed driver for a bare laser diode is a dumb idea. Making a constant current circuit that can output precisely timed pulses is much harder and provides no real benefit.

The reason why many blue/yellow pointers are pulsed is because the efficiency is a non-linear crystal is proportional to the power density squared. By providing short but more powerful bursts, the crystals become more efficient and thus produce more power. When using crystals, 100mW of CW power will produce less light than 200mW pulses at a 50% duty cycle (which would produce less light than 1000mW at 10%).

In short, if using crystals go for it. Otherwise a pulsed driver is a waste of time.
 
mfo

mfo

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r691175002 said:
Alright let me explain this properly.

Pulsing a bare laser diode will not make it brighter or more powerful. If you can run a laser diode at 100mA, you can pulse it at 200mA for 50% of the time (Actually, you probably can't). This trade off will not appear brighter nor dimmer to the eye, but might produce a dashed line depending on the frequency of the pulses. As the frequency of pulses becomes reasonably fast the photoreceptors in your eyes won't have time to depolarize before the next wave of photons hit.

To put it simply, a pulsed driver for a bare laser diode is a dumb idea. Making a constant current circuit that can output precisely timed pulses is much harder and provides no real benefit.

The reason why many blue/yellow pointers are pulsed is because the efficiency is a non-linear crystal is proportional to the power density squared. By providing short but more powerful bursts, the crystals become more efficient and thus produce more power. When using crystals, 100mW of CW power will produce less light than 200mW pulses at a 50% duty cycle (which would produce less light than 1000mW at 10%).

In short, if using crystals go for it. Otherwise a pulsed driver is a waste of time.
Click to expand...

Makes perfect sense, thanks. Strange...one post and such an elaborate answer. Are you a vet hiding yourself? If not, welcome to LPF...
 
P

pappu

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The right answer to your query is that if you switch on any device may it be light bulb or laser or any resistive load at zero crossing, means at the point where the current starts to increase from zero value, the sudden inrush of current which may be 10-20 larger than its steady state value, may be avoided. In this way the life of the device may be increased manyfold. The same funda is used in mains synchronous triac driver or Pulse Width Modulation drivers.

pappu
Senior Scientist
Photonics Division
 
R

r691175002

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I was 691175002 until I forgot my password. The password recovery isn't sending me any emails so it was easiest to just create a new account.

I have no idea what pappu is saying but its wrong. To begin with, a laser diode driver is specifically designed to never provide more current than it has been set to. There are never any "sudden inrushes" of current:
Scopecap1.jpg


Secondly, turning on and off would not increase the life of the laser diode. In fact, TTL laser drivers are designed to provide threshold current even when "off" to keep the laser diode just barely lasing (so its still on, but dim).

Pappu's post states that "at the point where current starts to increase" - in other words, when you turn it on... "may be avoided".
This is utterly ridiculous considering the entire point of pwm is to turn the device on and off over and over.
 
D

Deleted member 8382

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I thought a pulsed laser at 100mW peaks would look brighter to the eye than for instance 80mW of continous power. I know this is true with LEDs, why not with lasers then?

Yours,
Albert
 
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Things

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Hallucynogenyc said:
I thought a pulsed laser at 100mW peaks would look brighter to the eye than for instance 80mW of continous power. I know this is true with LEDs, why not with lasers then?

Yours,
Albert
Click to expand...

This isn't true in 2 senses ...

1. Your eye's brightness sensitivity isn't linear, so it takes about 4x the power to make it look 2x as bright. The difference between 100mW and 80mW would be barely noticeable to the eye.

2. Because the laser is saturating your eye, it is not going to get any brighter whether it's pulsed or CW, since if it's pulsed fast enough, you wouldn't even know it. Your eye's reaction time might be struggling with the pulses ...

If you were to get a 10W pulsed laser and a 80mW CW laser, THEN you will see a difference, but with such small power difference it's not obvious.
 
R

r691175002

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Hallucynogenyc said:
I thought a pulsed laser at 100mW peaks would look brighter to the eye than for instance 80mW of continous power. I know this is true with LEDs, why not with lasers then?

Yours,
Albert
Click to expand...

You bring up a good point.

There are a few differences between an LED and a pulsed laser diode. First, imagine if the pulses were applied at 1Hz. Although you would be able to see the individual flashes, the LED would look much brighter while it is on, and that would reflect itself in your perception of the amount of light it is emitting.

As the frequency of the PWM increases this effect becomes less pronounced. As most laser diodes run at frequencies in the kHz range or higher while software PWM might be 60Hz, a laser diode does not get as much of a gain.

More importantly, a laser diode that runs 100mA CW wont run 200mA at 50%. The real numbers are probably closer to 170mA 30% but you would of course have to check the datasheet.
 
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