Laser or comparable security proximity detection ideas. Need perimeter coverage

Maybe not looking as much towards a total DIY project as much as I am finding some parts that will get me fixed up. We are having problems with people in my yard and I need to build an invisible fence type perimeter around my place with some type of basic audible alarm in the house.

I have found simple driveway alarms which are just like the garage door units but I need to pull like a 50-100ft beam across the yard and in a few different places and I would rather not be buying several of these not-cheap sensor units with a stack of receivers in the house.

I am open to links to places to buy this stuff at the right price or thoughts on PCB or bread board build ups to roll my own. If this is not as complex as it sounds, I might like to just build my own for the fun of it I guess. I am not sure of all the parts that are involved in a system like this or even how they work. Is this just an IR transmitter and receiver and when the receiver loses signal, it goes off?

If we go DYI on this, let me know what kind of circuit we would be looking at here. I should note that I have seen some circuits for basic sensor systems but we need to build a wireless system and I have never done anything wireless. Not sure what that is all about. So far, it looks like we basically will just be using an IR LED to produce a beam of IR light, reflect that back with a mirror, then receive that with an IR receiver and when that receiver drops out, the circuit is tripped. It also looks like the IR LEDs are pulsed for some reason I do not understand yet.

Why not go video cameras with frame type motion detection. Aim the camera's and select where you want the detection in the image and record them to boot?

I am not familiar with those. where does a guy get something like that and what is the official name? can these operate with super low power? we will not have AC power available and would have to rely on a photocell for battery maint and then simply transmit to a receiver.

You didn't mention the no power issue. Here is an example system for low power recording: Ultra Mini DVR camera with Pinhole Lens - Hide It Yourself (1) / All In One Camera If you have a compusa store nearby, they had a small system in the store.

My state is using similar systems to catch illegal dumping.

I saw this in the London Drugs flyer this morning for $130:

https://www.electro-tech-online.com/custompdfs/2011/01/1456SW361-OBC_OutBackCam_OIG-E_WEB.pdf