Current Controller for Laser Diode

I have a laser diode whose output wavelength is proportional to input
current.

I need to build a circuit which allows me to control the current from 0
to 10mA. This will allow me to drive my laser over its required range
of output.

Can someone recommend a circuit to do this function. Note, current
control must be very precise (~0.01mA).

Thanks,
Marek

 

[Rene Tschaggelar]

marek.krzeminski@gmail.com wrote:

I have a laser diode whose output wavelength is proportional to input
current.

I need to build a circuit which allows me to control the current from 0
to 10mA. This will allow me to drive my laser over its required range
of output.

Can someone recommend a circuit to do this function. Note, current
control must be very precise (~0.01mA).

A precision current source can be built from an operational
amplifier, by measuring the current from a shunt resistor to the
inverting input. On the non-inverting input, you have the voltage preset
from a DAC. A standard circuit really.
You're lucky that your speed is close to DC.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Tim Wescott]

marek.krzeminski@gmail.com wrote:

I have a laser diode whose output wavelength is proportional to input
current.

I need to build a circuit which allows me to control the current from 0
to 10mA. This will allow me to drive my laser over its required range
of output.

Can someone recommend a circuit to do this function. Note, current
control must be very precise (~0.01mA).

Thanks,
Marek

Any other constraints? Does the diode need to have one terminal

grounded or can it float? Any frequency response issues?

If frequency response isn't critical and if the diode can have both
terminals floating here's an easy circuit:


VCC
Vref +
+ |
| |\ |
'----|+\ '----------
Vcntl ___ | >-----. to laser
------|___|--o---|-/ | .----------
R3 | |/ .-. |
| R1 | | |
| | | |
| C1 '-' ||-+
| || | ||<- Q1
o----||------o---||-+
| || |
| .-----o
| ___ | .-.
'--|___|------' | |
| | Rc
R2 '-'
|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Q1 is a garden-variety power MOSFET. R1 and C1 maintain stability even
though Q1's gate is capacitive. Rc, R2 and R3 set the gain from
(Vref-Vcntl) to the current through the laser diode. Figuring out the
details is left as an exercise to the reader.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Winfield Hill]

Tim Wescott wrote...

VCC
Vref +
+ |
| |\ |
'----|+\ '----------
Vcntl ___ | >-----. to laser
------|___|--o---|-/ | .----------
R3 | |/ .-. |
| R1 | | |
| | | |
| C1 '-' ||-+
| || | ||<- Q1
o----||------o---||-+
| || |
| .-----o
| ___ | .-.
'--|___|------' | |
| | Rc
R2 '-'
|
|
===
GND

Q1 is a garden-variety power MOSFET. R1 and C1 maintain stability
even though Q1's gate is capacitive. Rc, R2 and R3 set the gain
from (Vref-Vcntl) to the current through the laser diode. Figuring
out the details is left as an exercise to the reader.

Move C1 from the FET's gate to the opamp's output.


--
Thanks,
- Win

 

[Ken Smith]

In article <hhqk1196em0gnu33vn99upbqb0t6qvjfs9@4ax.com>,
John Larkin <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:
[...]

But the laser sounds unusual. I've never heard of a laser whose
wavelength was proportional to current, or one that would lase below
roughly 10% of max rated current.

The wavelength is more like Lamda = A + B*I

This is true for most/all semiconductor lasers. The "N" of a
semiconductor increases as the carrier density increases. Since the
cavity is formed by mirrors on the ends of a chunk of semiconductor the
wavelength tends to decrease as the current increases.

It is handy that the wavelength also increases as the temperature
increases so the increase in temperature that goes with the increase in
current has an effect in the same direction. If this wasn't true "line
locking" a laser to an absobtion line would be very troublesome.

--
--
kensmith@rahul.net forging knowledge

 

[Larry Brasfield]

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote
in message news:hhqk1196em0gnu33vn99upbqb0t6qvjfs9@4ax.com...

But the laser sounds unusual. I've never heard of a laser whose
wavelength was proportional to current, or one that would lase below
roughly 10% of max rated current.

The OP slightly misstated that feature. What semiconductor
lasers will do is lase at a wavelength that varies approximately
linearly with current, provided you remove an offset term.
The effect arises mainly from thermal modulation of cavity
dimension. It can be used for interesting purposes when the
limited wavelength range spans particular spectral lines one
might wish to examine.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Ken Smith]

In article <nyASd.117$Xk.7291@news.uswest.net>,
Larry Brasfield <donotspam_larry_brasfield@hotmail.com> wrote:
[....]

The OP slightly misstated that feature. What semiconductor
lasers will do is lase at a wavelength that varies approximately
linearly with current, provided you remove an offset term.
The effect arises mainly from thermal modulation of cavity
dimension.

This is true in the long term. For the very short term, the carrier
density in the junction area dominates.

It can be used for interesting purposes when the
limited wavelength range spans particular spectral lines one
might wish to examine.

Take a look at NISTs "chip scale atomic clock" project for a good example.


--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <421b4bc9$0$3397$5402220f@news.sunrise.ch>,
Rene Tschaggelar <none@none.net> wrote:

Ken Smith wrote:
[....]
The wavelength is more like Lamda = A + B*I
[...]
This is true but rather unreliable since the diodelaser cavity
is too short.

The NIST program is using a vertical cavity laser in exactly this way. It
works quite well.

There are just too many modes in it. A better
approach would be to antireflex the laser diode and have a
controlled external cavity with a grating or fabry perot.

This is better in terms of performance but way too much money for many
projects.
--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <421b5997$0$3401$5402220f@news.sunrise.ch>,
Rene Tschaggelar <none@none.net> wrote:
[...]

A grating and a lens doesn't cost much and are quickly mounted.
Movement either by a piezo, or an RC servo. Yes, the antireflex
coat is not for free but doesn't cost that much either.
At least you can achieve single line emission under the
gain profile of the few nm.

You've got my interest. How much for the needed hardware?

NIST is trying for a atomic clock that competes with the OCXOs. They've
got quite a ways to go yet on the development.

BTW: in the NIST system, they have to modulate the light at the 9GHz
frequency because they are using first order coherent population trapping.
Making a 9GHz cavity that small would be trouble some. Doubly so because
you really want a wide peak on it so you don't get pulling.

--
--
kensmith@rahul.net forging knowledge

 

[Rene Tschaggelar]

Ken Smith wrote:

In article <421c566f$0$3408$5402220f@news.sunrise.ch>,
Rene Tschaggelar <none@none.net> wrote:
[....]

The hardware is fairly cheap.

Define "fairly cheap". Looking at what you've listed below, it looks to
me like the laser system will cost over $1000US. I was hoping you had
come up with something that the group in the land down under had missed.
It looks like you've suggest the same basic kit as them. I guess that is
reasonable as it is most likely the best way to do it if you don't have
the new vertical cavity technology. The one thing they stressed was the
need for very good mechanical stability in all of the parts.

Well, an aspherical lens, a few squaremillimeters of AR coat,
a squarecentimeter of grating is doable for a couple of dollars,
it is the mounting and handling that is time consuming.
With some automation, in numbers, the price should come down.
Yes, the mechanical stability is important. Possibly thermally
stabilized. But a micro setup should be doable.

You need the laser diode of
yours, but on the backside, where usually the monitor is,
you need an antireflex coat. A lambda quarter of falcium
floride or such. Then you need some optics to expand the beam,
An achromat or a microscope lens. Having the beam widened up,
it goes as moreless parallel beam to a grating. 30$ or so
at Edmund Scientific. The grating retroreflects the wanted
wavelengths back. The selection of the wavelength is the angle
of the grating. This job is mainly mechanical, setting up
the lot on a sturdy plate, adjusting the angles, remove
hysteresis ...
Note that laser gain equation have now the lenses and the
grating in it as losses. This means the lens system should
accomodate for the large NA.
A longer laser cavity has less longitudinal modes and the
grating is selective amongst them.


NIST is trying for a atomic clock that competes with the OCXOs. They've
got quite a ways to go yet on the development.

I read some articles about that. Considering that I get an OCXO
in less than half a cubic inch, running between 0 and 50degC, at
less than a watt, for 500$, that is quite a task.

They seem to be inching towards actually doing it. The laser production
yeld is still a bit of an issue. The vertical cavity laser has to run at
the 894 line. Only a small cross sectioned ring of them on the wafer end
up wanting to run there. The rest are either too high or too low. This
is a silicon growing issue that they have to get a handle on.

Running the light through the body of the cell twice by using a mirror on
the far side solves one of the big mechanical issues by forcing the light
to average to parallel. Switching to CPT turns the 9GHz from RF cavity to
modulator issues.

One of the folks who is working on this told me that he can see a
production price of something like $100 in the future. There is nothing
in the system that is by its nature expensive other than perhaps the laser
its self.

Ah, they are locking to maximum/minimum absorption with
the 9GHz sidebands ? That is not that trivial. Nor
immediately to be made small. With some custon microwave
chips though...

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Ken Smith]

In article <421f1cdc$0$3402$5402220f@news.sunrise.ch>,
Rene Tschaggelar <none@none.net> wrote:
[...]

It is the machine parts that worry me most with optics too.
For those that are used to work with inch bolts, the
Thorlabs mounts may appear workable, but when the
expectations are somewhat higher, there is only custom
parts. Yes, I can can spend a day or so behind the milling
machine but the results are, well, on the border to be
useable.

I give drawings to someone who can turn them into metal parts. That way,
I get to keep all of my fingers (so far).

[...]

a very good magnetic shield and "C coil" around it. In the proto-type the
cell is suspended inside another glass chamber using Kapton tape. None of
this is done with normal IC processing stuff.

Well, these are laboratory solutions for one-of.

Actually, they intend to end up with a many off. There is a lot of
government demand for exact time and exact position.


--
--
kensmith@rahul.net forging knowledge