Laser noise

Thu Mar 04, 2010 6:15 pm   

I was trying to attempt to measure noise on a 6 watt laser. The main problem
is trying to focus on something without it melting or catching fire.
I was trying to look at the reflective light withg a photodiode.
I guess there migh be some fancy expensive device ??

greg

Thu Mar 04, 2010 6:30 pm   

On Thu, 04 Mar 2010 17:15:05 GMT, zekfrivo_at_zekfrivolous.com
(GregS)wrote:


Indium Gallium Arsenide power wavehead photodetector.

Thu Mar 04, 2010 7:09 pm   

What about using a lens to spread the light?

Thu Mar 04, 2010 7:28 pm   

On Thu, 4 Mar 2010 10:09:27 -0800, "William Sommerwerck"
<grizzledgeezer_at_comcast.net>wrote:


Wouldn't that effect or distort the noise or influence the reading
otherwise?

Thu Mar 04, 2010 7:34 pm   

Well, at the least it would reduce the absolute value of noise read. But if
the light were spread evenly, and you knew the area ratio of spread to
direct light...

Thu Mar 04, 2010 7:50 pm   

On Thu, 4 Mar 2010 10:34:36 -0800, "William Sommerwerck"
<grizzledgeezer_at_comcast.net>wrote:


True if you had the ratio just do the math.

Thu Mar 04, 2010 7:54 pm   

In article <hmouge$19m$1_at_news.eternal-september.org>, "William Sommerwerck" <grizzledgeezer_at_comcast.net> wrote:


Spreading would be fine, as long as it does not spread to my eyes.
But thats a good idea.

greg

Thu Mar 04, 2010 8:55 pm   

On Thu, 04 Mar 2010 17:15:05 GMT, GregS <zekfrivo_at_zekfrivolous.com> wrote:

Like a filter?

Thu Mar 04, 2010 9:15 pm   

On 3/4/2010 12:15 PM, GregS wrote:

Three front-surface reflections from window glass, plus an ordinary
photodiode. Make sure the multiple reflections miss each other.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Thu Mar 04, 2010 11:49 pm   

So you'd have 4% of 4% of 4%, or about .000064 of the original energy,
right?

Fri Mar 05, 2010 12:05 am   

"GregS" <zekfrivo_at_zekfrivolous.com> wrote in message
news:hmopjo$4m6$1_at_usenet01.srv.cis.pitt.edu...

Use a clear optical glass beam splitter. Pick the incident angle to
get as much or less as you want. Terminate the beam into a beam stop
(brick?).

T

Fri Mar 05, 2010 1:00 am   

zekfrivo_at_zekfrivolous.com (GregS) writes:


All sorts of ways of reducing the power density including lenses to spread it
(as someone else mentioned), or a simple beam sampler using a glass plate to
pick up 8 or 10 percent. A couple times and you get down to reasonable power
levels. Or get a proper beamsplitter that can be obtained to reflect
a very low percentage of the beam power. An optical filter or even a mirror
that reflects most of the beam and transmits a small percentage can also be
used, but probably only after the lens to reduce the power density so as not
to damage it.

Then any photodiode will be fine.

Those expensive devices have some means of reducing the power to acceptable
levels for their sensors.

--
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Fri Mar 05, 2010 1:01 am   

Meat Plow writes:


Not as long as the noise relative to the total power into the sensor
is what's measured.

--
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Important: Anything sent to the email address in the message header above is
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Fri Mar 05, 2010 2:11 am   

On Mar 4, 4:01 pm, s...@repairfaq.org (Samuel M. Goldwasser) wrote:

Pre-apologize for the naive question:
Are you measuring amplitude noise and/or spectral noise?
Are these valid questions?

Fri Mar 05, 2010 2:32 am   

On 3/4/2010 5:49 PM, William Sommerwerck wrote:

Depends on the angle, but it's that order of magnitude. Figuring 5% per
bounce, that 6 W comes down to about 750 uW. Keeping the reflections
from overlapping is really key--interference between the different beams
will make the ratio go all over the place.

Ordinary hardware store black paint, e.g. ultra-flat black Krylon, is a
really really good index match to fused quartz, so if you have a
quarter-inch quartz windows lying around, you can come in at a fairly
oblique angle, let it bounce around inside a few times, and pick the
reflection you want. If the angle of incidence is more than, say, 20
degrees, you have to worry about the polarization dependence of the
reflection coefficient.

Laser noise is commonly space- and polarization-dependent--it's a
surprisingly deep subject.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net