# modulateable laser diode drivers

M

#### [email protected]

Jan 1, 1970
0
Mike said:
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

I figured that your timing was based on a clock, but maybe with the
pulse with to voltage scheme, jitter won't be a problem. That is,
whatever initiates the timing pulse will have jitter, especially if it
is based on a crystal clock (and what isn't). However, the exact start
time of the pulse shouldn't matter, just the pulse width.

J

#### Joerg

Jan 1, 1970
0
Hello Mike,

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. When I was a student I often rolled my own (and then sometimes spent the saved money in the pub...). If you have the time this provides a learning opportunity far beyond what a university can offer. The design, that is, not the pub ;-) However, be careful. Saving$10 and then frying a \$300 laser diode
because of a slight circuit bug would not be such a good experience.
Happens easily. Something bursting into oscillation, loop instability etc.
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?

Study the diode's data sheet carefully. Many laser diodes cannot be run
without cooling, at least not for too many milliseconds. Plus it will
complicate the diode current regulation if your temperature is all over
the place.

J

#### Joerg

Jan 1, 1970
0
Hello Win,

... 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).

Even with a common base transistor up front from the transimpedance amp?

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

Maybe the photodiode would benefit from a strict current coupling for
the amp (like into the emitter of an RF transistor) instead of a 50ohm
link. Guess if that was more than 10 years ago the whole amp would have
to have been discrete as well.

J

#### joseph2k

Jan 1, 1970
0
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

Mike, important question, are you going to turn the laser diode on and off
or just modulate it within the laser emission region? The diode properties
make a difference in the required properties of the drive circuit.

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