Thanks for your suggestions MikeMI.
I want to use two set of sensors at a fixed distance from each other, as it could be seen via image of post#1 and also following link: **broken link removed**
And as you said, I'm in need of "beam break detector".
Both your suggestions are based on IR detection and although they are good at long distance and daylight working, but I'm afraid if they are suitable for my application due to IR wavelength (700 nm – 1 mm), compared to laser (180 – 700 nm) . For my application, after getting signal from sensor, on a micro-controller, I measure the time that I have input (signal). And these timings are very important for further calculations.
regarding your first suggestion, how to put them in one side? same question for your second suggestion. I want sender and emitter in one side. Is using a reflector accurate enough?