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Laser diode driver

I am trying to modify a laser-scanner from a laser printer into a blue/
violet photoplotter. I need some kind of driver for my blue/violet
laser. I've seen the schematics on Sam's laser faq, and looked at
various ic's from maxim, analog devices, and ic-haus. They all seem to
have trouble with optical-power-correction. Most of the integrated
circuits force the average power to be constant, which might work for
data streams, but for images, it is not good. I need this to work at
frequencies from DC to 100Mhz. The circuits on sams don't seem to have
this problem, but the OPC causes spikes in intensity when going from
low to high level because the photodiode current is, correctly, way to
low.

I've considered using the equivalent of two diode drivers where my
modulator can switch between them, this way, there will be 2
capacitors for storing the photodiode level. I think this would work,
but there are lots of parts. And i don't think my breadboard will work
at the 100Mhz i need.

I am a little confused as to the bandwidth of OPC circuits. Does the
photodiode output really vary at khz or mhz? Is it possible to just
calibrate the machine every time it is run? Is there any circuits or
chips that do something like what i need?

Thanks in advance,
Jon Pry
 
J

Jamie

Jan 1, 1970
0
I am trying to modify a laser-scanner from a laser printer into a blue/
violet photoplotter. I need some kind of driver for my blue/violet
laser. I've seen the schematics on Sam's laser faq, and looked at
various ic's from maxim, analog devices, and ic-haus. They all seem to
have trouble with optical-power-correction. Most of the integrated
circuits force the average power to be constant, which might work for
data streams, but for images, it is not good. I need this to work at
frequencies from DC to 100Mhz. The circuits on sams don't seem to have
this problem, but the OPC causes spikes in intensity when going from
low to high level because the photodiode current is, correctly, way to
low.

I've considered using the equivalent of two diode drivers where my
modulator can switch between them, this way, there will be 2
capacitors for storing the photodiode level. I think this would work,
but there are lots of parts. And i don't think my breadboard will work
at the 100Mhz i need.

I am a little confused as to the bandwidth of OPC circuits. Does the
photodiode output really vary at khz or mhz? Is it possible to just
calibrate the machine every time it is run? Is there any circuits or
chips that do something like what i need?

Thanks in advance,
Jon Pry
Most systems i've seen use some kind of feed back to insure the
output is where you want it.
You'll see diodes with a third leg which is a PD for monitoring the
Laser output. This is used to balance the driving circuit of the Laser.

Don't know if that is what you have.
 
Hi, I'm one of the guys that helps write sam's faq. I do a lot of
photonics for a university.
You need to answer a heck of a lot of questions for us to help you.

First off , why are you trying for 100 mhz for a photoplotter, to the
best of my knowledge the fastest blue laser photoprinter in the world
has a 20 mhz wide ecl input and gets there with a AOM, not direct
modulation, is printing on the order of 20-200 pages per second and is
roomsized. What the heck kind of emulsion do you have that is sensive
enough for a blue diode and that high a speed? Seriously, 100 Mhz
square wave drive?

Second of all, the new blue diodes are not characterized for that kind
of speed. You'd need to characterize them as a load.
You will need some form of process feedback to compensate for
temperature varaiations etc.

Third of all laser diodes are "soft" they do not handle spiking etc
very well, which is why the optical feedback is used, 100 mhz is RF,
and thus you need impedance matching to the device, a straight current
source chip is not going to cut it. To achive 100 mhz, first you need
a pile of diodes to blow up, and a expensive network analyser to shape
the matching networks etc. You need to learn to use microstrip and
other RF techniques for the cabling. Thats why you wont see that in
the FAQ, its almost never asked for. You'll need a DC turnon voltage
and a "DC block / Bias Tee" to inject the RF current. Such things are
available from companies like New Focus or Thorlabs, but are
expensive. the blue diodes are very fragile, they dont stand up to
well this kind of abuse. Its NOT a driver chip ,in a production
quanity device, you modulate the RF and then inject RF over the
cosntant current DC bias. You probably need a fast adjustable RF
limiter too, A fast diode driver from one of the companies that
specializes in pulsing diode device is what you need to start the
charterization with.

Your looking for companies like AVtech, www.avtechpulse.com

Here is a bias Tee

http://www.thorlabs.com/newgrouppage9.cfm?objectGroup_ID=1593

Steve Roberts
 
Hi, I'm one of the guys that helps write sam's faq. I do a lot of
photonics for a university.
You need to answer a heck of a lot of questions for us to help you.

First off , why are you trying for 100 mhz for a photoplotter, to the
best of my knowledge the fastest bluelaserphotoprinter in the world
has a 20 mhz wide ecl input and gets there with a AOM, not direct
modulation, is printing on the order of 20-200 pages per second and is
roomsized. What the heck kind of emulsion do you have that is sensive
enough for a blue diode and that high a speed? Seriously, 100 Mhz
square wave drive?

My polygonal mirror rotates scan the beam at about 2000 passes/second.
Say i am trying to write 100,000 pixels per scan, this comes out to
100Mhz for the worst case of alternating black and white pixels. The
laser scanner will be moved over the plot at a rate of about 5 seconds/
inch. So not quite 200 prints per second, but much better resolution.
Just about all emulsion is sensitive to 405nm. For one thing, gelatin/
chromium emulsion certainly are. It is difficult to estimate the
effect of reprocity, but as a rough calculation, the film will be
exposed to about 5 minutes of 5mw radiation / sq meter. Which is more
than enough for any film i know of.

Also, square wave is not so important. From what i gather the laser
should be modulated between threshold and some high value. The scan
speed/filtering need to be adjusted to make this work for the film.
Second of all, the new blue diodes are not characterized for that kind
of speed. You'd need to characterize them as a load.
You will need some form of process feedback to compensate for
temperature varaiations etc.

There are blu-ray burners, and 1x blu-ray is ~30MB a second, so say
240Mhz, probably higher to deal with reed-soloman, 8/10b encoding and
such.
Third of alllaserdiodes are "soft" they do not handle spiking etc
very well, which is why the optical feedback is used, 100 mhz is RF,
and thus you need impedance matching to the device, a straight current
source chip is not going to cut it. To achive 100 mhz, first you need
a pile of diodes to blow up, and a expensive network analyser to shape
the matching networks etc. You need to learn to use microstrip and
other RF techniques for the cabling. Thats why you wont see that in
the FAQ, its almost never asked for. You'll need a DC turnon voltage
and a "DC block / Bias Tee" to inject the RF current. Such things are
available from companies like New Focus or Thorlabs, but are
expensive. the blue diodes are very fragile, they dont stand up to
well this kind of abuse. Its NOT a driver chip ,in a production
quanity device, you modulate the RF and then inject RF over the
cosntant current DC bias. You probably need a fast adjustable RF
limiter too, A fast diode driver from one of the companies that
specializes in pulsing diode device is what you need to start the
charterization with.

I think the problem is not so much the RF as getting it to work from
RF all the way down to DC.
Your looking for companies like AVtech, www.avtechpulse.com

Here is a bias Tee

http://www.thorlabs.com/newgrouppage9.cfm?objectGroup_ID=1593

Steve Roberts

I've come up with a possibly stupid idea for how to do this.


+9v---------------------------------------------------/\/\/\/-----
100 /
|/
-------------|
| |\
| \
| |
| |
________ |
| | ___
/ / / \ /
|/ |/ |/ ___
Mod--/\/\/\---| --| --| |
1k |\ | |\ | |\ |
\ | \ | \ |
______________________________________|______|____________________
| |
| |
| |
DAC0 ----/\/\/\/----- |
50k |
|
|
DAC1 ---/\/\/\/-------------
50k


The PNP transistor is a MPSH81, the 3 npn's are MPS5179D75Z

The idea is that there are a 5 DAC external to this that is able to
set both the bias and modulation current. My plan to do the output
correction is to calibrate the system during the end of a scan. After
the scan is done there is like 10us of dead zone. During which i can
set the laser to both high and low state, digitize the output of the
photodiode and update the DAC. I assume the temperature of the laser
diode can't change much in the 500us of single scan.

Thanks. You seem to know what you are talking about.

Jon
 
D

Don Klipstein

Jan 1, 1970
0
In <7899e2ca-3755-46d8-b0de-dc7725c68108@s36g2000prg.googlegroups.com>,
My polygonal mirror rotates scan the beam at about 2000 passes/second.
Say i am trying to write 100,000 pixels per scan, this comes out to
100Mhz for the worst case of alternating black and white pixels. The
laser scanner will be moved over the plot at a rate of about 5 seconds/
inch. So not quite 200 prints per second, but much better resolution.
Just about all emulsion is sensitive to 405nm. For one thing, gelatin/
chromium emulsion certainly are.

Believe me, many, probably most photographic emulsions do very well at
405 nm. I don't consider that the higgest obstacle.
It is difficult to estimate the effect of reprocity, but as a rough
calculation, the film will be exposed to about 5 minutes of 5mw radiation
/ sq meter. Which is more than enough for any film i know of.

Also, square wave is not so important. From what i gather the laser
should be modulated between threshold and some high value. The scan
speed/filtering need to be adjusted to make this work for the film.

You could only need to pass a sine wave of 100 MHz with frequency
response at least somewhat flat from DC to somewhat over 100 MHz, and not
too much ringing at any frequencies at or near where gain starts varying
heavily inversely with frequency.
There are blu-ray burners, and 1x blu-ray is ~30MB a second, so say
240Mhz, probably higher to deal with reed-soloman, 8/10b encoding and
such.


I think the problem is not so much the RF as getting it to work from
RF all the way down to DC.

<<SNIP from here to edit for space>>

I see the big challenge making a broadband RF circuit that works from DC
to a little past 100 MHz. This touches heavily upon such things as stray
inductance, stray capacitance, and within that distributed inductance,
distributed capacitance, stripline PCB principles, knowledge of a
stripline transmission line, and principles of transmission lines such as
characteristic impedance and need for matching load impedance to
characteristic impedance to flatten frequency response, and need to match
source impedance to characteristic impedance to minimize resending any
reflections from the load if the load mismatches the transmission line.

This gets a bit easier if you understand characteristic impedance,
impedance matching to characteristic impedance, use of the Smith chart to
determine theoretically effects (and degree thereof) of for mismatches,
what the characteristic impedance of a coax cable and a twinlead cable are
as a function of dimensions and dielectric constant of insulating material
involved, "velocity factor" and relationship to dielectric constant of
insulating material exposed to electric field, and principles of
characteristic impedance and velocity factor of a stripline transmission
line.

Heck - video amplifiers, which only need to work from DC to about 4 or 5
MHz or so, get to be a slight bit of a challenge! If you want practice,
try making a video amplifier set and a pad-down to gain and pad back down
a signal set for an SVGA monitor. (No, I have not done that!) If you can
do this without horizontal blurring or ghosting, then you are probably
most of the way there to amplifying or buffering a signal of DC to 100
MHz. I do suggest this as practice!

One more note - if a transistor has Ft of 300 MHz, then it's "beta" or
Hfe at 100 MHz is close to 3!
If you use a transistor good for microwave frequencies, then your
circuit needs to be well-behaved at frequencies up to a goodly fraction of
the transistor's Ft. Watch for parasitic oscillations! Know
common-collector and common-base as well as common-emitter transistor
amplifier circuit theory! One classic oscillator is the Colpitts, which
uses a transistor in common-base mode! Know the treatments for parasitic
oscillations, and I suspect they are best-published for different
frequencies and different applicable impedances than those you will run
into! Know how to translate component values to different frequencies and
different "working impedances", and hope that you are treating the same
oscillation mode that the treatment is for, hope for lack of 2nd and 3rd
or whatever order complications in your application, etc!

Also, you might be exploring into territory that has not all of its
charts well-published, which means you could be a prime victim for
Murphy's law finding a way to effectively supersede Ohm's law!

Oh, one more thing - if you have a bipolar transistor in a Class A
amplifier (better know what that is), if the transistor is a fast one then
design for lowish supply voltages preferably 30 volts or less, with the
transistor having average collector-emitter voltage while conducting
around or under 20 volts. For that matter, in Classes B and AB (better
know what those are), if the transistor is conducting close to half the
time its average C-E voltage could easily need to be under 25 volts or so
if the transistor is a fast one.
One key here: "SOA" or "Safe Operating Area", and that is a graph of
allowable combinations of C-E voltage and collector current. It is common
for bipolar transistors to have allowable power dissipation decreased when
C-E voltage is more than somewhere around 30 volts, and this gets worse
for faster ones. The principle here is "forward bias second breakdown",
where a hotspot develops in a localized region within the transistor
"chip" / die.

I have made my living doing stuff other than this for the past 23 years,
and I have this little bit of knowledge of this that could be "dangerous"
here... I suspect a possible hazard is need to kick in a few dozen hours
of extra work time on weekly salary or per-diem time or unbillable hourly
as "continuing education", and that gets to be hell when deadlines make
you spend an extra 15 or 50 hours fixing something in a matter of days to
a week!

Best Regards,

- Don Klipstein ([email protected])
 
P

Phil Hobbs

Jan 1, 1970
0
My polygonal mirror rotates scan the beam at about 2000 passes/second.
Say i am trying to write 100,000 pixels per scan, this comes out to
100Mhz for the worst case of alternating black and white pixels. The
laser scanner will be moved over the plot at a rate of about 5 seconds/
inch. So not quite 200 prints per second, but much better resolution.
Just about all emulsion is sensitive to 405nm. For one thing, gelatin/
chromium emulsion certainly are. It is difficult to estimate the
effect of reprocity, but as a rough calculation, the film will be
exposed to about 5 minutes of 5mw radiation / sq meter. Which is more
than enough for any film i know of.

Also, square wave is not so important. From what i gather the laser
should be modulated between threshold and some high value. The scan
speed/filtering need to be adjusted to make this work for the film.

There are blu-ray burners, and 1x blu-ray is ~30MB a second, so say
240Mhz, probably higher to deal with reed-soloman, 8/10b encoding and
such.


I think the problem is not so much the RF as getting it to work from
RF all the way down to DC.


I've come up with a possibly stupid idea for how to do this.


+9v---------------------------------------------------/\/\/\/-----
100 /
|/
-------------|
| |\
| \
| |
| |
________ |
| | ___
/ / / \ /
|/ |/ |/ ___
Mod--/\/\/\---| --| --| |
1k |\ | |\ | |\ |
\ | \ | \ |
______________________________________|______|____________________
| |
| |
| |
DAC0 ----/\/\/\/----- |
50k |
|
|
DAC1 ---/\/\/\/-------------
50k


The PNP transistor is a MPSH81, the 3 npn's are MPS5179D75Z

The idea is that there are a 5 DAC external to this that is able to
set both the bias and modulation current. My plan to do the output
correction is to calibrate the system during the end of a scan. After
the scan is done there is like 10us of dead zone. During which i can
set the laser to both high and low state, digitize the output of the
photodiode and update the DAC. I assume the temperature of the laser
diode can't change much in the 500us of single scan.

Thanks. You seem to know what you are talking about.

Jon
I did some work ten years or so ago on improving printer and scanner
technology by combining holographic (hologon) scanners with analogue
current-modulated diode lasers. Since the whole laser goes on at once
(except for a few possible mode hop funnies during turn-on), you can go
quite a bit faster than with an AOM.

The other useful trick is use the chirp of the laser diode to get a
factor of between 10 and 40 improvement in the attainable scan rate with
lowish-speed hologons. You use a couple of diffraction gratings in
series, coming in near normal and coming out near grazing on both
gratings. This turns the long skinny stripe from the DL into a more or
less circular beam, and makes it steer in angle with drive current.

So instead of scanning a single spot, you effectively scan a whole
stripe of 10 to 40 pixels at once. You adjust the exposure dose by
changing the dwell time on each spot.

My preliminary scanner demo worked great (minus the hologon--it was just
the stripe part), but my management changed before I got the whole thing
working. It was also capable of measuring the range to an uncertainty
of about 1 part in 10**5 of the distance to the object, by using the
tuning trick with a solid interferometer. The current tuning produced
an optical frequency shift (i.e. the chirp) that became a baseband
modulation of the interferometer's output.

It was intended to be a pocket-sized scanner and coordinate measuring
machine, fast enough to give good results handheld, and could have sold
for a few hundred bucks. It never got finished, unfortunately, so I
don't know if it would have worked as I expected. The new part, namely
the chirp-scanned X-Z measurement, worked great at about 40 Mpels.

So there are several ways of getting beyond 20 Mpel/s.

Cheers,

Phil Hobbs
 
One, get a copy of Phil Hobb's book on electro-optical system design,
its excellent

Two HP's (now Agilent) Appcad freeware has much of what you need
for the board layout in terms of transmission line and matching.
http://www.hp.woodshot.com/ you'll need to know the E sub r of
your board material

Three, minicircuits.com

Four, start looking at patents

such as 6667661

and here:

by Marc Thompson

http://members.aol.com/marctt/CV/Abstracts/eleclet.htm

Mr Thompson is missing some spike protection circuitry, and startup
circuitry, that would need to be there as well as some impedance
matching details

In my experience anything from Shuji Nakamora's family of blue
devices (led or laser) looks more like a SCR then a Led when driven,
they tend to act like 4 level devices. (PNPN) Rather expensive
device to put on a curve tracer, but....
Start with the nichia blue led as a test load, then scale to the
lasers, no guarentee they are that similar, but the led is a heck of a
lot more rugged.

look here:

A 10Gb/s SiGe compact laser diode driver using push-pull emitter
followers and miller compensated output switch
Maxim, A.
Solid-State Circuits Conference, 2003. ESSCIRC apos;03. Proceedings of
the 29th European
Volume , Issue , 16-18 Sept. 2003 Page(s): 557 - 560


here:

take a look at Analog Devices AD9661A diode driver. (usually do free
samples)

Good Luck,

Steve
 
P

Phil Hobbs

Jan 1, 1970
0
In my experience anything from Shuji Nakamora's family of blue
devices (led or laser) looks more like a SCR then a Led when driven,
they tend to act like 4 level devices. (PNPN) Rather expensive
device to put on a curve tracer, but....

Steve,

_That_ is interesting. Do you have a curve handy? I'd love to put that
in the second edition. Talk about a gotcha.

Cheers,

Phil Hobbs
 
P

Phil Hobbs

Jan 1, 1970
0
how about I send you a part? I've lost access to the tracer.

Steve

Steve,

If you have a spare that isn't too precious, that would be wonderful. I
have a recently-repaired HP4145B semiconductor parameter analyzer I can
try it on. Just so I don't blow it up, can you give me a ballpark idea
of how big the negative resistance is, and whether it's fast or slow? I
don't want to wind up with the poor thing turning into a
transmission-line oscillator, because it might not survive.

(I haven't priced those Nichia things in a couple of years, but last
time I looked they were $2k or so.)

Thanks,

Phil Hobbs

M/S 08-122,
IBM T. J. Watson Research Center
1101 Kitchawan Rd,
Yorktown Heights NY 10598.
 
J

JosephKK

Jan 1, 1970
0
[email protected] [email protected] posted to sci.electronics.design:
My polygonal mirror rotates scan the beam at about 2000
passes/second. Say i am trying to write 100,000 pixels per scan,
this comes out to 100Mhz for the worst case of alternating black and
white pixels. The laser scanner will be moved over the plot at a
rate of about 5 seconds/ inch. So not quite 200 prints per second,
but much better resolution. Just about all emulsion is sensitive to
405nm. For one thing, gelatin/ chromium emulsion certainly are. It
is difficult to estimate the effect of reprocity, but as a rough
calculation, the film will be exposed to about 5 minutes of 5mw
radiation / sq meter. Which is more than enough for any film i know
of.

Also, square wave is not so important. From what i gather the laser
should be modulated between threshold and some high value. The scan
speed/filtering need to be adjusted to make this work for the film.

There are blu-ray burners, and 1x blu-ray is ~30MB a second, so say
240Mhz, probably higher to deal with reed-soloman, 8/10b encoding
and such.


I think the problem is not so much the RF as getting it to work from
RF all the way down to DC.


I've come up with a possibly stupid idea for how to do this.


+9v---------------------------------------------------/\/\/\/-----
100 /
|/
-------------|
| |\
| \
| |
| |
________ |
| | ___
/ / / \ /
|/ |/ |/ ___
Mod--/\/\/\---| --| --| |
1k |\ | |\ | |\ |
\ | \ | \ |
______________________________________|______|____________________
| |
| |
| |
DAC0 ----/\/\/\/----- |
50k |
|
|
DAC1 ---/\/\/\/-------------
50k


The PNP transistor is a MPSH81, the 3 npn's are MPS5179D75Z

The idea is that there are a 5 DAC external to this that is able to
set both the bias and modulation current. My plan to do the output
correction is to calibrate the system during the end of a scan.
After the scan is done there is like 10us of dead zone. During which
i can set the laser to both high and low state, digitize the output
of the photodiode and update the DAC. I assume the temperature of
the laser diode can't change much in the 500us of single scan.

Thanks. You seem to know what you are talking about.

Jon

I like the plan some, but for better tracking use an optical splitter
during the scan and compare optical output versus intended value
continuously. 500 uS is well into thermal response time.
 
J

JosephKK

Jan 1, 1970
0
Phil Hobbs [email protected] posted to
sci.electronics.design:
Steve,

_That_ is interesting. Do you have a curve handy? I'd love to put
that
in the second edition. Talk about a gotcha.

Cheers,

Phil Hobbs

That would even be worthy of an obscure electronics item.
 
I like the plan some, but for better tracking use an optical splitter
during the scan and compare optical output versus intended value
continuously. 500 uS is well into thermal response time.

The laser diode has an integrated photodiode, i don't know if this is
any better or worse than an optical splitter. Leaving that aside,
comparing the output to the intended value continuously is probably
impossible. Say there is a random stream of bits of ~50% duty cycle.
Then one would need to use an integrator on both the photodiode and
the incoming signal. Then we could say the integral over some period
was like 5000 units when we should have had 6000. Then the question
is, is the bias current of the modulation current to low. This is not
too easily solvable. There will be situations or time periods where
the duty cycle is significantly more/less than 50% allowing better
guesses on what to adjust, but these cannot be guaranteed. Really, i
don't think the integrator will work either. It will absorb so much
data from transients that the average will be totally screwed.

On another note, i've gotten a hold of some MAX3701 ic's. I've
designed a prototype board that is being produced. Whole thing has
come to about $300 so far. I'm using a 100mhz LVPECL link from an fpga
to the laser module for the data, and the laser diode will be directly
soldered to the driver board. I hope to get rid of most of the RF
design problems by keeping things smaller than lamba/10.
 
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