Which type of laser employs the technique of a resonant cavity?

The Fabry-Perot Cavity Laser uses the concept of a resonant cavity to produce laser light. This type of laser is designed with two parallel mirrors that create a reflective cavity, allowing for the amplification of light through stimulated emission. The resonant aspects arise from the cavity's ability to support specific wavelengths of light, which constructively interfere and enhance the light intensity within the cavity.

In a Fabry-Perot cavity, the spacing and alignment of the mirrors are crucial because they determine the specific resonant modes or frequencies that can be supported, effectively allowing only certain wavelengths to build up inside the cavity. This distinct feature is essential to the functioning of the laser, enabling it to produce coherent and monochromatic light.

Other types of lasers, such as the Helium-Neon laser, semiconductor laser, and fiber optic laser, may utilize different methods for light amplification that do not primarily rely on the resonant cavity principle in the same structural way as the Fabry-Perot configuration. Therefore, the Fabry-Perot Cavity Laser stands out for its fundamental use of a resonant cavity to achieve laser action, making it the correct answer to the question.