Laser break beam sensors are usually kind of cumbersome. They work by detecting an object breaking a laser beam - but you usually have to set up a laser on one end and a sensor on the other. Unless your sensor is big, it can easily get out of alignment.
Laser Break Beam Sensor has a light sensor+lens next to the laser to detect the reflected dot when the beam is broken, so it can sense when someone breaks the beam from the starting location. There's no need for a matching end point sensor! It forms a good strong laser with a sensing distance of up to 1 meter.
Setting up Laser Break Beam Sensor is pretty easy, connect 5VDC to the red wire (you really do need 5V, 3V won’t work!), Ground to the black wire and a pullup resistor on the output pin. You can have the pullup go to any voltage really. When the beam is broken, the output blue wire will pull down to ground. If it's being pulled up, the voltage will be 'high' when there's nothing in the way of the beam. There's a potentiometer which can be used to adjust sensitivity.
Adafruit Web Site: https://www.adafruit.com/products/2122