Re: laser targeting

On Jan 11, 3:14�pm, obviouslydelusional
<obviouslydelusio...@gmail.com> wrote:

On Jan 10, 9:54�pm, "m...@sushi.com" <m...@sushi.com> wrote:

Of course, now you have to wonder if Al Qaeda has a "holy crap I'm
being targeted" device. Who knows, maybe an off the shelf radar/lidar
detector would work, though the laser frequency would be off a bit.
The police lidar is 900 or 950nm, while the military is using
something a bit longer than a micron. So the optical filter and
perhaps sensor would need to be changed.

I don't believe I have any weapons school audio of laser targeting,
but the buzzword is "sparkle."


Those were all good thoughts you had, but you may �be over thinking
things a bit.

What it comes down to is detecting the presence of Nd:YAG 1064 nm
laser light, which most military targeting systems use. �If you're
being swept by a beam of that wavelength, it's time to duck, I don't
care what it's pulse rate is.

Start with a very sensitive photodiode or other sensor that is
sensitive to the 1064 nm wavelength. �Put it behind a 1064 nm laser
line filter, such as the following Newport optical item for $150.
Maybe even include a concave reflector to improve gathering ability:

http://search.newport.com/?sku=10LF10-1064

The laser line filter is essentially a very narrow bandpass filter,
which in this case peaks at 1064. �So what you have is a sensor setup
that will only detect if exposed to 1064 light. �Unless the sensor is
horribly slow, it should respond even to pulses. �I suspect that's how
most police laser detectors work. �And now that I think of it, maybe
just get a police laser detector and swap the original filter for a
laser line filter?

It might not work real well in daylight or in urban areas as there
will be a 1064 nm component to many light sources. �But out around the
ranges where it's dark should be fine.


Most photodiodes peak in infrared, so that is not an issue. The
trouble is they are very very slow. A photodiode has a lot of
capacitance. When you hook it up to the traditional TIA, the feedback
resistor is essentially the impedance presented to the capacitiance,
so you get a very low bandwidth system. [Large R, lare C, implies
large RC product.]  Worse yet, all that capacitance at the front end
make the circuit unstable, so you need a feedback capacitor, which in
turns makes things even slower.

I have a cascode photodiode amp in the works for another project. I
came up with a servo scheme to remove the constant DC (i.e. ambient
light), allowing the sensor to respond to changes in light. The
cascode places a low impedance on the photodiode, neutralizing the
capacitance. Essentially, you still have a large C, but the R is now a
few ohms (Assuming bipolar cascode, R=1/(40*collector current))

Ultimately this kind of circuit needs a detector. Even if you have a
simple low bandwidth amp, even with a filter, you need to detect the
change in intensity, and this will be a highpass filter of some sort.
There is no escaping that. The problem with your scheme is the circuit
is working in the 1/f region of the amplifier. By sticking with higher
frequencies, you can get to a location where the thermal noise is
dominant. Using the servo scheme, you have removed the DC offset at
the first gain stage, which improves the dynamic range of the
circuit.  [A gain stage followed by a highpass means the gain stage
has to swing enough to accommodate the DC. With a servo, the DC is
canceled at the first stage, so the amplifier needs to only swing for
AC signals.]