Laser stability issues

[AnAppleADay]

Hi all

I'm fairly new to working with lasers and so please forgive me any stupid questions. I am currently working on a device measuring the extinction of a laser diode module beam through mist. Thereto, a reference signal is taken at the beginning and then compared to the measured signal through the mist.

The working environment is quite tough in a test hall with changing temperatures (let's say worst case 0-40 degC), heavy vibrations on the rig, electromagnetic disturbance and a relatively unstable power grid. This resulted in long term drift of the laser module's intensity as well as mode hopping. As you might guess, this is not very beneficial for the quality of the measurements.

So currently, I am looking for a more stable laser diode module. The wavelength is not an issue, but the power output is crucial. We are working with red lasers, but this might be a general problem. Also, for health&safety reasons it needs to be a class II laser.

After skimming through different lasers I remained really uncertain about what I really need. Is it sufficient to look for an APC module? Also, I don't quite understand where price differences between several laser diode modules originate from. Okay, there are some differences in beam divergence etc., but I don't really understand why you can get virtually anything between 50$ and 2000$ without big differences in the specs?

It would be great if you could give me some small advice here. I wish you all a merry festive season!

Cheers
Sascha

 

[DrSid]

As far as I know, red semiconductor single mode diode (aka DVD) should give you best stability and beam quality available (405nm Bluray would be similar), not counting some exotic solutions. I'm not sure if usual DVD diodes lase from 20mW though.

But then there is question of driver .. and those can differ a lot.

As for modules .. well .. it seems some people will simply pay for the quality, or maybe they just don't know how easy is to build one. It would be very cheap, and you would have control over all stages - driver, diode, optics, cooling.

 

[AnAppleADay]

As far as I know, red semiconductor single mode diode (aka DVD) should give you best stability and beam quality available (405nm Bluray would be similar), not counting some exotic solutions. I'm not sure if usual DVD diodes lase from 20mW though.

But then there is question of driver .. and those can differ a lot.

As for modules .. well .. it seems some people will simply pay for the quality, or maybe they just don't know how easy is to build one. It would be very cheap, and you would have control over all stages - driver, diode, optics, cooling.

Yes, so I am probably one of those people who don't know how to build a driver. I think I am quite fine with buying a self-contained module to be ready to go. Yet, I still don't quite see what is crucial for obtaining a stable power output. What am I looking for at this point?

 

[DrSid]

You can always buy some driver, there are some laser specific being sold here, though I don't have experience with building red lasers (and drivers may differ). Drivers we use for handhelds are usually fed with battery, which changes output with temperature and charge, so they should also work fine with net source.
Sure, it all depends how much stable the output must be. For handhelds 1% is perfectly fine, it might not be for your application. Both driver and diode react on temperature and may change output when they get warm by operation, and they will. Still on 20mW this should be not as a big problem like with lets say 2W.

Building the module then would consist of soldering few wires, mounting the diode into some kind of module for optics, and usually adding some heatsinking to the diode module and driver. Any part can be then tested for stability alone (you can measure driver output for example), any part can be replaced, and I bet it's going to be cheaper then pre-made lab module.

As for the output stability .. what level of stability do you need ? What level of instability do you have ?

The instability really can be caused by anything, it can be the diode, it can be the driver, it can be the source (but good driver should fix that), it could even be optics changing shape with temperature (unlikely at 20mw). Also your module might be faulty and have some bad contact somewhere. I'm afraid until you treat laser unit as a black box, we won't know more, except for trying another one.

Btw. generally any laser is sensitive to heavy vibration. Is it important for the experiment to have laser mounted on the rig ? Couldn't it be stationed elsewhere with just a mirror on the rig for example ?

 

[lasersbee]

I saw that you are looking for a stable Class II Laser for your
application. A higher class Laser could be attenuated with
some appropriate optical filters.

I would go with a Laser Module that uses an APC Circuit for
Laser output stability.
Any changing temperatures or voltages within specs should
be compensated by the APC.

The price differences can be attributed to :-

1) A pristine brand new single quantity LD from the Manufacturer

2) A LD ripped out of some electronic consumer device.

3) General Gouging

There are a lot of Laser suppliers on the market.
The trick is to do your homework to find the most suitable
one for your needs..
Google is a charm...


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[AnAppleADay]

Thanks for the input thus far!

@DrSid

I am talking about output power of <1mW, so heating up should not be an issue. However, the surrounding conditions can incorporate large changes in temperature (but still within the specs of most laser diodes).

The drift of the power should be less 1% over a time of about 2 hours or even longer. At the moment, I experience large deviations at power-on of up to 10% and minor deviations after some time of about 3% (I usually allow the laser 10-15 minutes to "settle").

Unfortunately, the laser needs to be mounted on the rig, so detatching is no option.



@ lasersbee

You are absolutely right that there are a lot of lasers out there, but that's the problem at the same time. Oftentimes, it is absolutely not obvious what the differences are between two modules with one costing 50$ and another costing 500$.

So generally speaking, do you think an APC laser diode module will do the trick or do I need to pay attention to something else? Is there a ball park number of price I need to pay at least to get what I want in terms of stability and quality or is this up to personal luck?

Cheers

 

[lasersbee]

Back around 1988 I bought a 5mW Laser for nearly $200.00
that was APC circuit controlled.
It was necessary to build the Laser Diode that way to protect
it since it cost so much.

Today you can find APC protected LDs for a LOT less...
Like I said Google is the place to start...
I just did a 3 second Google search and found this .PDF with
some specs...

Home - Laser Components GmbH

You will need to bite the bullet and chose a LD and do some
of your own testing since I really don't know what your
project application is.


Jerry

You can contact us at any time on our Website: J.BAUER Electronics