High power lasers use a contraption, called Faraday cell, or optical isolator, that allows propagation of light in one direction, but not the other.
The device consists of a Faraday rotator and two polarisation filters. The rotator uses Faraday effect, which rotates polarisation of light under magnetic field in suitable medium by angle depending on strength of the magnetic field in direction of the propagation of light, so returning light is rotated in opposite direction. Around the rotator are two polarisation filters rotated by 45° and the rotator is adjusted to turn the light by 45°. In one direction that allows the light to pass, but in the opposite direction the light arrives at the second filter 90° out of plane and is absorbed.
The wikipedia article also describes polarisation-independent variant that uses birefringent wedges instead of filters. 791
In one direction, the light is properly recombined, in the other it is diverged and blocked by a collimator.
In the lasers, it is used to prevent reflections going back to earlier stages and causing additional pulses or even damaging those stages—high power laser is composed of oscillator that creates the initial pulse (say, 0.5 ns long) and several progressively larger amplifiers and the early stages are not designed for the powers at the end of the optical path.
The <PERSON> isolators are also used in optical communications.
The power lasers also use other elements, Pockels cells and Kerr cells. Both are blocks of suitable material that rotates polarisation only when electric field is applied. The cells are again guarded by polarisation filters so the light can pass only if appropriate electric field is applied. The rotation is reciprocal here, so light can pass both ways, but it is used to quickly open and close the optical path.
Usually a <PERSON> cell is used to split the laser cavity until the medium is charged and then connect it so the pulse can start building up and then a <PERSON> cell, which is faster, but needs (much) higher voltage, is used to let the beam out for the desired 0.5–1 ns.
This allows to build the device suggested in DarioP's answer measuring about 1 m. 469