You'reI think you need to clarify whether you mean a sensor to detect where across the beam profile an obstruction is present, or if you mean a sensor to detect the range from the laser head to some target, or something else.
If you mean the first option, then you're probably looking for a CCD camera. This is essentially an array of photodiodes, giving a voltage corresponding to the amount of light incident on each pixel. By reading out this video you can find the region in which the finger breaks the beam. But if you're just wanting to detect a few different levels, you could just use a few photodiodes stacked in a line and then use a microcontroller to count how many of them are blocked, and therefore infer how far the finger has crossed into the beam.