I searched around for the filter you were talking about on
edmondoptics.com but I couldn't find the one you mentioned, can you
give me a link? You also mentioned an amplifier, is this something I
will definitely need? If so, how do I go about building one?
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1512&search=1
About 2 screens down the list, glass reference number RG715.
Larger ones are further down.
Can you explain what you mean by "DC accuracy"? How do I go about
modulating the laser? Is there a step by step guide for this online
somewhere?
Measuring steady intensity requires DC stability (constant gain,
constant offset, with low level of low frequency noise). The noise
level of opamps rises as the frequency falls.
Modulating the source at a constant frequency allows you to pass the
signal through a bandpass filter you need constant gain at that
frequency, and low noise only in that band. DC drift doesn't matter
at all, and the only noise you deal with is that in the pass band.
Then you rectify the amplified AC to restore the intensity
information. If you can do synchronous rectification (reversing the
sign of the gain each time the laser changes from off to on and vice
versa)_you have a lock in amplifier that rejects noise that gets
through the band pass filter but is not synchronous with the laser
modulation. (If the noise is random, some gets amplified with one
polarity and some gets amplified with the other polarity, so a bit of
averaging after the demodulation averages it out to near zero.)
This approach may need an adjustable phase shifter before the
demodulation to correct for the phase shift through the system for
maximum output. A true lock in amplifier would demodulate two signals
with 90 degree phase shift and take the square root of the sum of the
squares of the two DC results to find the magnitude of the modulated
signal, regardless of its phase. But that is probably way more
capability than you need, here.
Can you help me select the correct band pass filter to filter out
"eliminating the static background
illumination and the 120 Hz flicker caused by line powered lights?"
What would be the correct frequency?
You want as high a frequency that does not degrade the signal with
either the laser or photo diode response. A few to a few tens of
kilohertz is common. The bandpass filter is usually just an active
filter made with the opamps that amplify the photo diode signal.
A nice little filter design program is available free from Texas
Instruments:
http://focus.ti.com/docs/toolsw/fol...pa_amp_general&HQS=NotApplicable+OT+filterpro
Here is another one from Microchip, though I haven't used it, yet.
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en010007
Okay, if this is the best way to do it, then this is what I want to do.
I want this to work as well as it can. Although, how do I go about
making the band pass filter a lock in amplifier driven by the
modulating signal, so that it amplifies not only the correct frequency,
but when synchronously demodulated, only the correct phase? Is there a
step by step guide somewhere that explains how to do this?
Do you know how to make an amplifier that switches from a positive
gain to a negative gain using a cmos switch? There are several ways,
including just have two amplifiers in parallel, one inverting, and one
with the same gain but non inverting, and selecting back and forth
between them with a SPDT CMOS switch. you get 3 of these in a CD4053.
http://www.ee.washington.edu/stores/DataSheets/cd4000/cd4051.pdf
It can also be done with a single opamp and a SPDT switch like one of
the 4 in a CD4066. (which you can combine to make the double throw
switch, above.)
http://www.fairchildsemi.com/ds/CD/CD4066BC.pdf
You need the modulation pulse signal from the laser available at the
photo diode location to make this work. So it might not be practical
if they are widely separated, without running a long cable.
I am definitely concerned with eye safety, but I've had very little
experience with lasers so I'm not sure what precautions to take. If
I expand the beam diameter (how do I do this?) to keep much of it from
entering the eye's pupil, how do I refocus it back down to the
diameter of the IR receiver, or can I just leave it as a large
diameter?
A pair of lenses (a small concave one to fan the beam out and a larger
convex one to collimate it back to a parallel but larger beam is one
way. There is still some danger, if the beam bounces off a shiny
surface that happens to refocus it, but simply intercepting the wider
beam is a lot less dangerous then looking directly into the
concentrated one. It is also easier to get the wider beam somewhere
on the detector. A single convex focusing lens can be used at the
detector to gather enough of the beam to let it operate. If the beam
is several times the diameter of the collection lens, there should
still be enough signal for a clean recovery, and a lot less problems
with beam aiming. Put the visible blocking filter (if any)right
against the detector to minimize the need for a big filter.
And by the way, how large of a diameter would I need to
increase the beam to keep much of the laser from entering the eye's
pupil?
How expensive is your lawyer? The calculations are pretty
complicated. I'll see if I can find the fat document that defines the
safety standards. But it depends mainly on the power of the laser.
If its power is low enough the raw beam may be safe.
Also, won't this cause diffusion of the beam and prevent me
from projecting the beam 650feet (or 200m)?
You will have a pretty fat beam at 650 feet, even if you try to make
the beam as small as you can. Spreading it properly will just make it
fat all the way over, instead of cone shaped. You may also need a
circularizing lens at the laser to change the fan shaped beam from the
die into a more round beam.
Homework:
http://www.coherent.com/Downloads/LaserDiodeTechNote1.pdf
Adding a cylinder lens to the collimation optics can really improve
the beam collimation.
You may need a stand in visible laser in the same package as the IR
laser to test the optics.
This sounds like a great idea, can you help me select the correct lens?
You don't need a high quality image, just gathering, so a plastic
Fresnel is probably the lowest cost, large but short focal length lens
you can get.
They sell them the size of large screen televisions, but I think a few
inches in diameter might do.
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2039&search=1
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2040&search=1
Doesn't fit in a mouse, then. ;-)
