Laser perimeter sensors using low power IR, exist and are legal. Just CLASS IIIA laser products, no big deal. For a one off on your own property, no one cares if you make it eyesafe. 5 mW of CW IR is not going to do eye damage even if you focus it with eye surgery equipment, which uses as much as 3-5 mW for aiming beams.
I'm sure the Iranians have had beam break for the last 20 years, purchased at Radio Shack which used to sell them. So forget the "Are you USAians?" comments, which are well intended but kinda whacky. The Navy Seals and many cops have neat ways of getting around them, without the fake mirrors used in Hollywood movies, anyways.
Since no one here so far is a optics professional, and has told you what you really need to, I'm going to say this.
Look, you need to collimate a 780 nm laser, which is barely visible on paper at point blank, and 1/3rd so people have genetics that prevent you from seeing it at all. Keep the power at about 5 mW, and then run it into a 10X beam expander. Adding power above a certain threshold just is a waste of money, it is better to use better optics. A Riflescope, backwards, is a poor quality 3X upcollimator.
Modulate the laser, use a 1 to 10 khz modulation with at least 80% modulation depth. This aids in detection. A simple solar cell is too slow for long distance detection, a human does not block the beam long enough for the capacitance of the cell to discharge in most cases. So you need to use a fast detector, and the modulation helps you ignore background, which is considerable. The sky glows at near IR at night, and the bad guys know that too, nothing new there. (Oxygen Skyglow)
How laser systems deal with rain and snow will not be discussed in this post or by pm.
You need about a 10:1 Newtonian collimator.
Go to surplushed.com and get a 3 cm focal length lens and a 20 cm focal length lens. The 20 is going to need to be at least a inch in diameter, if not more. Use the TV camera mentioned below to help you arrange the lenses.
Meredith Instruments sells what you need, diode wise, and will have the initial collimator to insert the diode into.
Or just get a 1 mW hene from Ebay or Meredith. The .1 mRad beam does not need much upcollimation.
Next go over to electrooptical.net, Phil Hobbs web site.
He has a PDF on using bootstrapped photodiodes as sensitive detectors.
You need that, to get enough sensitivity.
The output of the Hobbs circuit goes into a bandpass filter tuned to your modulation frequency, a clipping circuit, and a LM555 missing pulse detector.
This forms a basic lock in amplifier topology, think of it as a tuned radio system for the laser light.
A two to four inch lens with a F1 focal characteristic, ie the focal length is about the same as the lens diameter collects the light.
Get a IR pass visible block filter from Meredith. This makes sure your detector just sees the IR.
You'll need a black and white CCD camera to see the laser for aiming. 25$ from Supercircuits.com. Some camcorders in night mode might just work just fine. Color cameras have filters that often block IR.
Now for the tricky part. Air currents from convective cooling of the soil are going to mess with your beam over very long distances.
So you need to up your possible breakage beams to more then one. You need to fold the beam back and forth a few times, or use a couple of beams spaced vertically. You then need to look for co-incidence, ie more then one beam being broken at one time.
Pro systems get rid of this by using multiple short hops.
Enjoy the false alarms from deer.
You'll need to use 3 point Kinematic mounts to adjust the laser, details at Thorlabs.com. Buy one MM2 or KM2 mirror so you can learn how to make them, they sell the needed 80 pitch screws and taps for the adjustments.
1 watt modulated Leds have been detected to 10 miles or more.
The upcollimated laser record for hobbyists is upwards of 100 Km between mountains, using photomultipliers and less then 50 mW of laser power.
I really suggest cameras with software for this, modern PC based security is pretty good.
Everything I mentioned is off the shelf and googlable. All the circuits are found on OP-AMP and LM555 data sheets.
No, I do not build custom gear for non-corporate or non-educational entities, unless it is for special needs persons, helping charitable causes, or science fair projects. I've gotten burned more then once.
Good luck with your project, but cameras with IR leds for illumination and Passive IR motion sensors work better and are cheaper. Around 21$ a sensor unit. They run off 12 volts and you daisy chain them.
A good camera system will set you back a few hundred, plus recycling a old pc.
The good guys have ways of defeating them, I'm sure. Bad guys don't.
Low cost thermal security cameras exist too, and watching the deer and the bunnies is fun.
I'd go into more details, but my kind of systems are hard to defeat, and I really dont like giving that away.
Laser beam break schematics have been published in old electronics magazines for twenty years. This is not new.
I used thermal motion for my own back yard for years, and it was easy to get at Home Depot. Alarm supply companies sell cheap modules with relay outputs. Phone wire is cheap. Beeper goes off in the bedroom, and 150 watts of floodlights went on. About two to three falses per night, mostly fast moving dogs, and coyotes, even though we're pretty much city folk with a big yard. Soon the beeper got hooked to a switch.
Again, enjoy the false hits.
The system did deter entry, with the lights. Although our problem was a peeping tom and the local kids throwing eggs. Adding a border collie to the system took care of him. Silent, smart, and they nip just right....
Dang eggs can really mess up white al siding when they freeze. Vinyl took care of that.
Mom, God Bless her, with a baseball bat, learned to listen to the sensor and scare the crap out of the kids when she caught up with them.
Hint kiddos, do not stay behind the bushes when the neighbors have a son who does research grade electronics. He can vector mom to an initial point.
Any good alarm technician could have told you this. My point is, skip the laser. Its a pain in the neck outdoors. Its great along perimeter fencing, because people along a fence will break the beam, again, and again. For the amount of time spent developing a beam break system, you can buy something newer and proven.
Real pro systems use something else entirely.
Steve
Just take a look at the various beamshots on this forum. A big part of those are taken outside and I am almost certain no permit was ever apllied for or granted for these situations.