current limiting resistor in laser diodes?

[Albert]

Hi all,

According to Laser Sams FAQ at:

http://www.repairfaq.org/sam/laserdps.htm#dpslpnr

Some laser diodes are designed to be operated with a curent limiting
resistor as a driver. Sam points out that he tested some diodes that
had a very low threshold current and had a wide margin between max
output abd threshold.

While larger laser diodes operate fine like this, I've never seen a
reputable laser diode manufacturer spec his diodes to be operated in
this manner.

Yes, I know it's common practice in the Asian laser pointer biz to do
this, but has anyone ever hear of a laser diode manufacturer
specifying his doides to operate in this manner?

Also, is there any chance that the diodes that operate with a limiting
resistor only might be a VCSEL diode? VCSEL's are much more stable and
do tend to lase at much lower values while maintaining alot of
headroom to ramp up to max power out. Even the lowest powered VCSEL's
don't need photodiodes and run with limiting resistor drivers.

Any comments appreciated.

Al

 

[Sam Goldwasser]

While larger laser diodes operate fine like this, I've never seen a
reputable laser diode manufacturer spec his diodes to be operated in
this manner.

Yes, I know it's common practice in the Asian laser pointer biz to do
this, but has anyone ever hear of a laser diode manufacturer
specifying his doides to operate in this manner?
No.

Also, is there any chance that the diodes that operate with a limiting
resistor only might be a VCSEL diode? VCSEL's are much more stable and
do tend to lase at much lower values while maintaining alot of
headroom to ramp up to max power out. Even the lowest powered VCSEL's
don't need photodiodes and run with limiting resistor drivers.

No, the beam characteristics look like a normal edge emitter and the
mounting as shown in one of the pics in the Laser FAQ is of an edge
emitter.

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[Albert]

No.

OK Sam, thanks. I have never seen anything in print regarding this
either, although the Asian world has their own products for their own
use and much of it is not available here, except in assembeled form.

Here's my problem.....

A friend has a set of Ramsey Laser Communicator modules, which is a
receiver and a transmitter. The transmitter is a PWM unit operating
around 18 Khz. The transmitter has blown up 4 lasers (so far). It uses
a laser pointer mounted on the transmitter board with wires coming out
to hook onto 5 volts regulated voltage.

When the laser pointers were taken apart, all of them had a single
current limiting resistor and no other electronics. There were no
photodiode connections on the laser module either.

The lasers are very cheap looking standard laser pointers that put out
1 milliwatt visible red, exact lambda unknown.

So, I'm trying to figure out whether this is a valid and proper way to
operate laser diodes or whether it's improper and whether we need to
work on another driver circuit for him.

Thanks for any light that you or anyone else can shed on this.

Al

 

[Sam Goldwasser]

OK Sam, thanks. I have never seen anything in print regarding this
either, although the Asian world has their own products for their own
use and much of it is not available here, except in assembeled form.

Here's my problem.....

A friend has a set of Ramsey Laser Communicator modules, which is a
receiver and a transmitter. The transmitter is a PWM unit operating
around 18 Khz. The transmitter has blown up 4 lasers (so far). It uses
a laser pointer mounted on the transmitter board with wires coming out
to hook onto 5 volts regulated voltage.

When the laser pointers were taken apart, all of them had a single
current limiting resistor and no other electronics. There were no
photodiode connections on the laser module either.

The lasers are very cheap looking standard laser pointers that put out
1 milliwatt visible red, exact lambda unknown.

So, I'm trying to figure out whether this is a valid and proper way to
operate laser diodes or whether it's improper and whether we need to
work on another driver circuit for him.

Thanks for any light that you or anyone else can shed on this.

Have you tried contacting the manufacturer or is that not even a
reasonable option?

If they use those Far East laser diodes, then it could be valid, though
I've never heard anything good about the reliability of those pointers
in general. So, it's possible the diodes are just trash-period.

My inclination would be to say, yes, you should have a proper laser
diode driver there. At 18 kHz, it doesn't need to be very sophisticated.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header is ignored.
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[Albert]

Thanks Sam,

Others with the same units report no diode failure although these
other users might not be putting as much run time on the lasers.

I'll say this, and it's a long shot:>:

He lives in Mexico and the air is very dry and clear, I suspected
static damage since the switch is open leaving one lead to the laser
completely isolated and prone to static damage.

I can't think of anything else, other than the fact that the diodes
shouldn't be run like that to start with.

Have you tried contacting the manufacturer or is that not even a
reasonable option?

Ramsey people didn't even know the difference between a phododiode and
a phototransistor when I called them looking for advice about making
the receiver more sensitive. My guess is that support from the
manufacturer is not an option.

If they use those Far East laser diodes, then it could be valid, though
I've never heard anything good about the reliability of those pointers
in general. So, it's possible the diodes are just trash-period.

Well, I see some laser pointers selling for 3 dollars. How can they
make money at those prices?? Maybe they knowingly put second rate
laser diodes in becasue it will work the first 10 or 20 uses!?

My inclination would be to say, yes, you should have a proper laser
diode driver there. At 18 kHz, it doesn't need to be very sophisticated.

This was my inclination too, a very strong one. But, there are many of
these laser pointers in use and some of them must run ok. Then, I saw
your newer comments in the FAQ about this topic, and started to
wonder. If the lasing threshold is 15 ma and the operating current is
35 ma, this seems like an unusually wide margin to me too. So, maybe
it is ok to run them with single current limiting resistors.

But, no reputable vendor suggests this as an acceptable method of
powering a laser??? I can't believe that...if it was really ok to
power a laser diode like this, those new process laser diodes would be
everywhere, not just in laser pointers from Asia.

I'll suggest to my friend that he continues researching for a proper
photodiode based APC type laser diode driver.

Thank you Sam.

Al

 

[Sam Goldwasser]

Others with the same units report no diode failure although these
other users might not be putting as much run time on the lasers.

I'll say this, and it's a long shot:>:

He lives in Mexico and the air is very dry and clear, I suspected
static damage since the switch is open leaving one lead to the laser
completely isolated and prone to static damage.

I can't think of anything else, other than the fact that the diodes
shouldn't be run like that to start with.

Might also be worth putting a small capacitor directly across the diode. Just
make sure the RC time constant is much less than the minimum period of
the pulse width modulation.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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[Lasernerd]

OK Sam, thanks. I have never seen anything in print regarding this
either, although the Asian world has their own products for their own
use and much of it is not available here, except in assembeled form.

Here's my problem.....

A friend has a set of Ramsey Laser Communicator modules, which is a
receiver and a transmitter. The transmitter is a PWM unit operating
around 18 Khz. The transmitter has blown up 4 lasers (so far). It uses
a laser pointer mounted on the transmitter board with wires coming out
to hook onto 5 volts regulated voltage.

When the laser pointers were taken apart, all of them had a single
current limiting resistor and no other electronics. There were no
photodiode connections on the laser module either.

The lasers are very cheap looking standard laser pointers that put out
1 milliwatt visible red, exact lambda unknown.

So, I'm trying to figure out whether this is a valid and proper way to
operate laser diodes or whether it's improper and whether we need to
work on another driver circuit for him.

Thanks for any light that you or anyone else can shed on this.

Al

E-Mail this guy shawohwa(AT)ms7.hinet.net try his product ask him for
a sample of his modules 640nm diode modules he will surely want to
hear a report on his product

 

[Scrim]

OK Sam, thanks. I have never seen anything in print regarding this
either, although the Asian world has their own products for their own
use and much of it is not available here, except in assembeled form.

Here's my problem.....

A friend has a set of Ramsey Laser Communicator modules, which is a
receiver and a transmitter. The transmitter is a PWM unit operating
around 18 Khz. The transmitter has blown up 4 lasers (so far). It uses
a laser pointer mounted on the transmitter board with wires coming out
to hook onto 5 volts regulated voltage.

When the laser pointers were taken apart, all of them had a single
current limiting resistor and no other electronics. There were no
photodiode connections on the laser module either.

The lasers are very cheap looking standard laser pointers that put out
1 milliwatt visible red, exact lambda unknown.

So, I'm trying to figure out whether this is a valid and proper way to
operate laser diodes or whether it's improper and whether we need to
work on another driver circuit for him.

Thanks for any light that you or anyone else can shed on this.

Al

E-Mail this guy shawohwa(AT)ms7.hinet.net try his product ask him for
a sample of his modules 640nm diode modules he will surely want to
hear a report on his product[/QUOTE]

IMHO: A simple resistor and constant voltage source should work to provide a
fairly constant current to a laser diode, but it is better suited to a
battery source since any spikes on the voltage source will cause current
spikes to which laser diodes are very sensitive (data sheets warn about
spikes as short as nanseconds if I remember correctly). Even if the voltage
source is normally well behaved, spikes can occur as the supply is switched
on or off. In the past I converted a laser pointer for use with an external
dc supply and just in case I added an RC filter between the psu and laser
pointer - it never complained. Using a large C and small R gives minimal
voltage drop but a slight delay in reaction from the laser when the psu
switches on or off. Since such short spikes can be a problem you need to
have an RF type approach where even wires are inductors which can cause
ringing on fast switching edges and remember that a large C might have
significant impedance at high frequency.
I guess in your case the power supply is modulated at 18KHz so there is a
continuous source of potential trouble from the switching waveform as well
as the main psu. Perhaps there is a small amount of ringing on the waveform
reaching the current limiting resistor and this is significantly reducing
the diode life (scope)? A small capacitor before the current limiting
resistor could help with this. Sometimes a capacitor is placed directly
across the diode too, since even the short wires to it are inductive I
guess, but this will be less effective since the diode is a pretty much a
constant voltage device.
In my experience cheap laser pointers are often unreliable due to a poor
electrical contact between the PCB and the metal diode housing. Careful
application of epoxy to prevent movement and silver conductive paint solves
this.
Hope this is of some help,

Scrim

 

[Sam Goldwasser]

IMHO: A simple resistor and constant voltage source should work to provide a
fairly constant current to a laser diode, but it is better suited to a
battery source since any spikes on the voltage source will cause current

The problem with this simple approach with most low power visible laser
diodes is that the difference between threshold current and maximum
current is usually very small and the actual current for a particular
output power changes with temperature. So, if it is set up for the
desired output power, next time you turn it on it may not even lase
or may be destroyed depending on temperature!

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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[Helmut Wabnig]

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header is ignored.
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Hi Sam, does your "reply form" work or not?

I have tha HP5001A Manual, mail me directly [email protected]
without the x-es.

w.

 

[Tom B.]

I've wondered about this and concluded that for mass-marketed pointers,
it might be cheapest to use diodes with relatively high power handling
capacity, perhaps running rejected 50 mW DVD laser diodes at drastically
derated power. The slight increase of diode chip cost would be small
compared to the savings on the driver. With automated manufacturing,
it's probably also possible to autoselect the appropriate resistor for a
given diode, thus taking care of component variation issues.

I just checked my favorite cheapo, which measured 3 mW at 25C (without
collimator). After cooling it to -10C in my freezer, it put out perhaps
6 mW. Less of an increase than I expected, but it did not burn out.
Another trick occurs to me - using negative temperature coefficient
resistors would also allow simple resistor drive over a wide temperature
range. Resistance would increase at lower temperatures, reducing current
to compensate for increased diode efficiency.

 

[Sam Goldwasser]

Hi Sam, does your "reply form" work or not?

I have tha HP5001A Manual, mail me directly [email protected]
without the x-es.

Yes, the feedback form should work. I did reply to your offer.

Thanks.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the feedback form on the S.E.R FAQ Web sites.

 

[Sam Goldwasser]

I've wondered about this and concluded that for mass-marketed
pointers, it might be cheapest to use diodes with relatively high
power handling capacity, perhaps running rejected 50 mW DVD laser
diodes at drastically derated power. The slight increase of diode chip
cost would be small compared to the savings on the driver. With
automated manufacturing, it's probably also possible to autoselect the
appropriate resistor for a given diode, thus taking care of component
variation issues.

I just checked my favorite cheapo, which measured 3 mW at 25C (without
collimator). After cooling it to -10C in my freezer, it put out
perhaps 6 mW. Less of an increase than I expected, but it did not burn
out. Another trick occurs to me - using negative temperature
coefficient resistors would also allow simple resistor drive over a
wide temperature range. Resistance would increase at lower
temperatures, reducing current to compensate for increased diode
efficiency.

Higher power laser diodes are still expensive even in chip form. And,
that doesn't necessarily get you much as the difference between Ithresh and
Imax is still a limited range and you don't want some to come out of
the factory emitting 25 mW!

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

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