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modulateable laser diode drivers

M

Mike Noone

Jan 1, 1970
0
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!

-Mike
 
L

Ludwig Weise

Jan 1, 1970
0
Mike said:
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!

-Mike

It is standard task in CD and DVD drives to modulate laser diodes. When
asked for something specific, you can look at Sam's laser faq:

http://www.repairfaq.org/sam/laserdps.htm#dpsicd0

I have used the IC_Haus chips for low power applications and the IXYS
MOSFETs and gate drivers if more power is required.

Richard
 
R

Rich Grise

Jan 1, 1970
0
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!


Well, at least this year they're asking their homework questions at the
beginning of the semester, rather than at the beginning of finals week. ;-)

Cheers!
Rich
 
Mike said:
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!

-Mike

The rise time is a function of the bandwidth. The old rule of thumb is
0.35/bandwidth.

Are you going to pulse the laser, or do the sine method?

Is this hacking, or a project for money? Hopefully it is hacking as I
would like to see the results of what you come up with.
 
J

Joerg

Jan 1, 1970
0
Hello Mike,

Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!

Analog Devices offers laser diode driver chips. ADN2841 and things like
that. Most are under $20 but if that's too much you'd have to roll your
own. Basically a current source where current is steered into ground
until you want a laser pulse. Now the steering transistor lets go for a
few nanoseconds or whatever pulse length you'd like, then closes again.

As you have recognized the current through the laser needs to be limited
and regulated. There is a photodiode feedback on the laser diodes for
that, usually. Stabilization is not a trivial tasks if you are dealing
with occasional pulsing. Best to read up on that stuff before a few
hundred Dollars go poof.

Usually you'd also need a controller for the thermo-electric cooler. AD
and others carry TEC chips for that, same price range. Unfortunetaly it
all goes towards PWM which isn't so cool should phase noise be critical
in your case (but can be done).
 
J

Jamie

Jan 1, 1970
0
Mike said:
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!

-Mike
hmm.
we have a system at work that uses an inline high frequency
transformer to modulate the HV line going to the laser.
its a simple basic laser and the HV supply is pasted through
a round core a few turns, on the other side is a pulse amplifier
we use to TX a signal.. it only modulates the laser at a small
percentage due to the fact the simple laser unit wasn't designed
for a large modulated signal like this in this manner.
at the other end, it simply goes into a detector with slow AGC
so that the AM signal can be detected.
it wasn't designed by us but it works great.
i don't think a 100 mhz could be done using the
technique above, well not on the lasers we are using..
 
M

Mike Noone

Jan 1, 1970
0
Rich said:
Well, at least this year they're asking their homework questions at the
beginning of the semester, rather than at the beginning of finals week. ;-)

Cheers!
Rich

Well, at least in this post you're jumping to incorrect assumptions
immediately, instead of asking questions first to find out the right
answer.

-Mike
 
W

Winfield Hill

Jan 1, 1970
0
Mike Noone wrote...
Hi - can anybody lend me any guidance as to how to build a laser
diode driver that can be modulated? I would like to be able to
modulate it at 1-100Mhz or so. Most importantly, I need it's
rising edge to be as sharp as possible. Are there any
inexpensive ICs that can do this?

You are talking about modulation, and not 100% on/off driving,
right? The way I do that is quite simple. I use a standard
moderate-speed light-sensor feedback laser-diode current-source
controller, and add a modulation network at the laser diode.
I operate the laser at about half current, as appropriate.

The laser impedance is very low at these currents, a few ohms.
The low impedance allows me to treat this node like a summing
junction, so that adding a 50-ohm resistor in series with an
ac-coupling cap from the coax cable carrying the RF modulation
provides both the RF signal to the diode, and well terminates
the 50-ohm coax. Using small dimensions, I have been able to
modulate low-cost CD-ROM lasers up to 200MHz in this fashion,
and done a bit differently in a quick lash up, up to 1.5GHz
(I may have modulated the laser successfully well above that,
but the photodiode I was using wimped out).

From my Jan 10 2005 s.e.d. post,
"The 664nm red DVD laser was an Hitachi HL6504FM and my detector
was an Optek OPF480 PIN diode, biased at -100V, measured into a
25-ohm load (double-end termination) with an HP network analyzer.

"Components were 1206 SMT hand-soldered with zero-distance
spacing. Bias-tees were 12GHz-bandwidth Picosecond Pulse Labs.
A first attempt at a PCB stripline replacement only goes to
600MHz so far."

.. bias-tee laser
.. ________| |______,---------,
.. ________|-||-+--|______(-50R-|>|-'
.. 50-ohm |____X__| \\ mirror PD bias
.. coax | \\| optics 100V
.. 50R //| & etc |
.. | // __|____
.. laser // ________| X |___50-ohm
.. current ,-|>|----+-)_______|--+-||-|____ coax
.. supply +-||-50R-' | coax |_______| term.
.. '----------' bias-tee

Jamie Morken answered, "Cool! That looks like a neat circuit, I
am buying that same photodiode from Digikey (part#: 365-1029-ND)"

Comment: In the latter approach, the bias-T isolates the 50-ohm
coax RF signal from the low-frequency current source, all on the
50-ohm line. But, making a good bias-T can be tricky, and using
the low-Z at the laser diode instead should be an easier approach,
perhaps not good to 1.5GHz, but certainly to 200-400MHz.
 
M

Mike Noone

Jan 1, 1970
0
Winfield said:
You are talking about modulation, and not 100% on/off driving,
right?

Hi Mr. Hill - Ideally, I would like to be able to drive the diode with
any waveform. For now, my plan is to pulse it with a square wave at
about 20mhz, with approximately a 2 or 4% duty cycle (pulse width = 1
or 2ns). This signal will be going into an Acam TDC (Time to Digital
Converter) chip, probabaly the TDC-GPX
(http://www.acam-usa.com/Content/English/gpx/gpx_1.html). The idea is
to measure the time between the rising edge of this pulse and the
risinge edge coming from a photodetector that has optics filtering all
but the wavelength of the laser, to get a distance measurement. Thus
response time of the circuit is unimportant, but I do think it is
fairly important to get a very clean rising edge on the pulse going to
the laser driver.
The way I do that is quite simple. I use a standard
moderate-speed light-sensor feedback laser-diode current-source
controller, and add a modulation network at the laser diode.
I operate the laser at about half current, as appropriate.

The laser impedance is very low at these currents, a few ohms.
The low impedance allows me to treat this node like a summing
junction, so that adding a 50-ohm resistor in series with an
ac-coupling cap from the coax cable carrying the RF modulation
provides both the RF signal to the diode, and well terminates
the 50-ohm coax. Using small dimensions, I have been able to
modulate low-cost CD-ROM lasers up to 200MHz in this fashion,
and done a bit differently in a quick lash up, up to 1.5GHz
(I may have modulated the laser successfully well above that,
but the photodiode I was using wimped out).

From my Jan 10 2005 s.e.d. post,
"The 664nm red DVD laser was an Hitachi HL6504FM and my detector
was an Optek OPF480 PIN diode, biased at -100V, measured into a
25-ohm load (double-end termination) with an HP network analyzer.

"Components were 1206 SMT hand-soldered with zero-distance
spacing. Bias-tees were 12GHz-bandwidth Picosecond Pulse Labs.
A first attempt at a PCB stripline replacement only goes to
600MHz so far."

. bias-tee laser
. ________| |______,---------,
. ________|-||-+--|______(-50R-|>|-'
. 50-ohm |____X__| \\ mirror PD bias
. coax | \\| optics 100V
. 50R //| & etc |
. | // __|____
. laser // ________| X |___50-ohm
. current ,-|>|----+-)_______|--+-||-|____ coax
. supply +-||-50R-' | coax |_______| term.
. '----------' bias-tee

Jamie Morken answered, "Cool! That looks like a neat circuit, I
am buying that same photodiode from Digikey (part#: 365-1029-ND)"

Comment: In the latter approach, the bias-T isolates the 50-ohm
coax RF signal from the low-frequency current source, all on the
50-ohm line. But, making a good bias-T can be tricky, and using
the low-Z at the laser diode instead should be an easier approach,
perhaps not good to 1.5GHz, but certainly to 200-400MHz.

Do you think your circuit would be well suited for my application? I
must admit that I am not as good at analog circuit design as I would
like to be. (strength is really in digital embedded systems)

Thanks,

-Mike
 
M

Mike Noone

Jan 1, 1970
0
The rise time is a function of the bandwidth. The old rule of thumb is
0.35/bandwidth.

Are you going to pulse the laser, or do the sine method?

Is this hacking, or a project for money? Hopefully it is hacking as I
would like to see the results of what you come up with.

I am currently planning on pulsing it with a 1-2ns pulse, every 50ns or
so. These numbers can be changed if necessary, but the important thing
is the rising edge time, as it will be starting a time to digital
converter and then the returning pulse will be stopping it.

The project is for a robot I've been working on for the last 15 months
or so. I want to give it the ability to avoid obstacles and map out
areas. If you're interested, there are some pictures, schematics
(outdated and with some mistakes), and some really old videos of it
here: https://netfiles.uiuc.edu/mnoone/www/.

Thanks,

-Mike
 
M

Mike Noone

Jan 1, 1970
0
Joerg said:
Hello Mike,
Analog Devices offers laser diode driver chips. ADN2841 and things like
that. Most are under $20 but if that's too much you'd have to roll your
own. Basically a current source where current is steered into ground
until you want a laser pulse. Now the steering transistor lets go for a
few nanoseconds or whatever pulse length you'd like, then closes again.

Analog's offerings look quite good. I especially like the ADN2525, with
a 24ps rise time! Unfortunately, I can't find it for sale in single
quantites, nor do they seem to sample it. I plan on contacting them on
Monday - but being a student that probabaly won't go too far. Some of
the others look pretty good too - I especially like the ADN2870, which
is available for about $10 in single quantities which is completely
reasonable.
As you have recognized the current through the laser needs to be limited
and regulated. There is a photodiode feedback on the laser diodes for
that, usually. Stabilization is not a trivial tasks if you are dealing
with occasional pulsing. Best to read up on that stuff before a few
hundred Dollars go poof.

Usually you'd also need a controller for the thermo-electric cooler. AD
and others carry TEC chips for that, same price range. Unfortunetaly it
all goes towards PWM which isn't so cool should phase noise be critical
in your case (but can be done).

I have been thinking about going the TEC route. I think initially I
will skip that as it is added cost and complexity, and I don't think it
will be terribly important as I don't particuarly care about having the
same power. Does that sound like a logical approach?

Thanks!

-Mike
 
C

colin

Jan 1, 1970
0
Mike Noone said:
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.

For moderate speed on off switching a common way is to just use a transistor
to drive it or possibly a few very fast cmos gates in parallel.

You need to limit the current of course, a fixed resistor is simple but a
bit limited, an easy way to do this at high speed is to provide a constant
current source then divert this away from the Laser with a diode conected to
the cmos gates or a transistor.

If you have low duty rate im not sure how you would implement optical power
feedback.

For high speed sinewave modulation as Win has sugested its a good idea to
use a DC bias source and ac couple the modulation signal.

Colin =^.^=
 
C

colin

Jan 1, 1970
0
Winfield Hill said:
From my Jan 10 2005 s.e.d. post,
"The 664nm red DVD laser was an Hitachi HL6504FM and my detector
was an Optek OPF480 PIN diode, biased at -100V, measured into a
25-ohm load (double-end termination) with an HP network analyzer.

"Components were 1206 SMT hand-soldered with zero-distance
spacing. Bias-tees were 12GHz-bandwidth Picosecond Pulse Labs.
A first attempt at a PCB stripline replacement only goes to
600MHz so far."

. bias-tee laser
. ________| |______,---------,
. ________|-||-+--|______(-50R-|>|-'
. 50-ohm |____X__| \\ mirror PD bias
. coax | \\| optics 100V
. 50R //| & etc |
. | // __|____
. laser // ________| X |___50-ohm
. current ,-|>|----+-)_______|--+-||-|____ coax
. supply +-||-50R-' | coax |_______| term.
. '----------' bias-tee

Looks nice, did you ever measure the capacitance of the laser diode ?
The data sheet I found for that diode didnt mention capacitance.

Ive managed to modulate a cheap pointer type laser diode @ 1ghz with a
simple single transistor driver, originaly designed for 10mhz, although it
was quite easy to get it to work at 100mhz, it required an ever increasing
pull up current to get it to 1ghz, one problem is the capacitance of the
diode im using, rlt6305, wich measures at over 50pf wich is surprisingly
high, coupled with the lead inductance even with the diode next to the
driver transistor must be quite a few nh.

When I finally got it to work at 1ghz I measured the RF voltage accros the
diode at the ends of the lead as >3vp-p but at the base of the diode case it
was less than 1vp-p.

My next layout im cropping off the laser diode leads and soldering the case
directly to the ground plane,
Im now also using a dc bias and AC coupling the RF, using a chain of MMIC
and a big Gaas Fet.

I try and avoid transmision lines if at all possible as its often hard to
wideband match the terminations without lossy pads etc. The oscillator is at
the start of the MMIC chain and part of the voltage accross the laser is
easily fed back to the PLL via transmision line.

Im not sure if the limiting factor is the capacitance or the internal speed
of the Laser wich makes it necessary to drive the laser with a larger p-p
voltage at these speeds, wich then makes the capacitance more of a problem,
maybe driving the diode off harder doesnt speed up the laser much and I just
need a better laser.

Anyone know of any faster lasers (5mw visible)?

Colin =^.^=
 
W

Winfield Hill

Jan 1, 1970
0
colin wrote...
Winfield Hill wrote ...

Looks nice, did you ever measure the capacitance of the laser diode ?
The data sheet I found for that diode didnt mention capacitance.

Well, the above 50-ohm setup went to 1.5GHz, so that works out to
no more than 2pF. The laser dies are very small, and fragile, too.
I try and avoid transmision lines if at all possible as its often
hard to wideband match the terminations without lossy pads etc.

Hmm, I suppose that can work if the laser is mounted right there.
 
W

Winfield Hill

Jan 1, 1970
0
Mike Noone wrote...
Ideally, I would like to be able to drive the diode with any waveform.
For now, my plan is to pulse it with a square wave at about 20MHz,
with approximately a 2 or 4% duty cycle (pulse width = 1 or 2ns).
This signal will be going into an Acam TDC (Time to Digital Converter)
chip, probabaly the TDC-GPX
(http://www.acam-usa.com/Content/English/gpx/gpx_1.html).
Do you think your circuit would be well suited for my application?

If I had presented my circuit with a 0.5ns-risetime pulse, I imagine
it would have responded just fine. But remember, I was prebiasing
the laser, which is important. I think you can bias it at low light
levels, say 10 or 20% of its nominal output level, but for fastest
response I'd guess it has to be above the lasing threshold.
 
C

colin

Jan 1, 1970
0
Winfield Hill said:
colin wrote...

Well, the above 50-ohm setup went to 1.5GHz, so that works out to
no more than 2pF. The laser dies are very small, and fragile, too.

50R and 2pf = 1.5ghz but if you take into account the estimated dynamic
resistance of the diode at 1R in parallel it comes out at about 100pf.

Ive measured 3 different types of laser diode and they come out at 30-60pf,
I measured a varactor diode too just to make sure my LCR meter could measure
this accuratly and it was spot on.
I found it quite surprising it was so high and still not sure if I beleive
it as it can work to 1ghz.

I found a data sheet for the diode you mention from alldatasheet.com wich
shows the modulation response and its dropping very fast at 1.5ghz, but they
dont specify the test set up.

Im wondering if I should switch to a vcesl as ive heard people mention a
capacitance as low as 2pf, and theyr specd at 2.5gb/s but there doesnt seem
to be any visible ones readily available.
Hmm, I suppose that can work if the laser is mounted right there.

It helped with reducing inteference too, wich is mainly why I did it in the
first place as my received light level is very low.

Colin =^.^=
 
W

Winfield Hill

Jan 1, 1970
0
colin wrote...
50R and 2pf = 1.5ghz but if you take into account the estimated dynamic
resistance of the diode at 1R in parallel it comes out at about 100pf.

That's correct, I shouldn't have used 50 ohms, sorry. In fact
the dynamic resistance could be considerably under 1 ohm.
Ive measured 3 different types of laser diode and they come out at
30-60pf, I measured a varactor diode too just to make sure my LCR
meter could measure this accuratly and it was spot on. I found it
quite surprising it was so high and still not sure if I beleive
it as it can work to 1ghz.

If you measured the capacitance at 0V or with a reverse bias,
you probably got a lower value than it has when operating.
I found a data sheet for the diode you mention which shows the
modulation response and its dropping very fast at 1.5ghz, but
they dont specify the test set up.

My measurements were so much faster than the datasheet, about 3x,
I suspect they in fact were suffering from slow detector response.
 
C

colin

Jan 1, 1970
0
Mike Noone said:
Hi - can anybody lend me any guidance as to how to build a laser diode
driver that can be modulated? I would like to be able to modulate it at
1-100Mhz or so. Most importantly, I need it's rising edge to be as
sharp as possible. Are there any inexpensive ICs that can do this? If
not, can anybody lend me any guidance as to how to go about creating
such a device? My understanding is that the driver needs to be a
current source with over-current limiting so as not to burn out the
diode, but I'm unsure as to how to go about creating such a circuit.
Thanks!

just found a few here
http://www.roithner-laser.com/Drivers.htm

some go upto 155mhz but some of the datasheets with internal circuits give
you ideas on how to build your own.

Colin =^.^=
 
Mike said:
I am currently planning on pulsing it with a 1-2ns pulse, every 50ns or
so. These numbers can be changed if necessary, but the important thing
is the rising edge time, as it will be starting a time to digital
converter and then the returning pulse will be stopping it.

The project is for a robot I've been working on for the last 15 months
or so. I want to give it the ability to avoid obstacles and map out
areas. If you're interested, there are some pictures, schematics
(outdated and with some mistakes), and some really old videos of it
here: https://netfiles.uiuc.edu/mnoone/www/.

Thanks,

-Mike

You are going to need to sweep the laser, much like a bar code reader.
My gut feeling is you are going to have difficulty measuring short
distances with a pulse technique. Have you investigated clock jitter?
Worse yet, from the avi, I wouldn't call that smooth motion, so the
crystal will have microphonic tendancies. That is, vibrating the
crystal will modulate it. You can average out the readings I suppose to
get around the crystal vibration.
 
M

Mike Noone

Jan 1, 1970
0
You are going to need to sweep the laser, much like a bar code reader.
My gut feeling is you are going to have difficulty measuring short
distances with a pulse technique. Have you investigated clock jitter?
Worse yet, from the avi, I wouldn't call that smooth motion, so the
crystal will have microphonic tendancies. That is, vibrating the
crystal will modulate it. You can average out the readings I suppose to
get around the crystal vibration.

Videos are of the robot's first steps. There were countless bugs in the
code at that point, and motion was sloppy. If you look carefully in the
video you can actually see that one of the legs turns off during
certain movements - just becomes a complete gimp leg. Was a major
software bug that I became aware of after taking that video.

Anyways - I was planning on having the robot stop, and then scan, then
start again. What clock jitter are you referring to?

Thanks,

-Mike
 
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