Low temperature helium 4 is a model system for the study of condensed matter because quantum effects play an important role and because it can be prepared experimentally with remarkable purity. The Quantum Liquid and Solid Group of Laboratoire Kastler Brossel has developed an experimental set-up able to produce metastable states of liquid and solid helium-4 and to measure their density and compressibility. The compressibility is measured via the speed of sound and is done using a Brillouin pump/pulse laser spectroscopy technique. If the frequency of the probe laser and that of the pump laser are shifted by the “Brillouin frequency”, energy from the pump laser is transferred to the probe laser. Till now, the Brillouin measurement is performed with two independent lasers. The purpose of this M2 internship is to develop a Brillouin gain spectrometer based on a single laser taking advantage of the low value of the speed of sound in liquid or solid helium-4 (~ 250 m/s) corresponding to a Brillouin shift being quite small (~350 MHz). This allows, from a single laser, to produce both the pump beam and the probe beam by shifting a part of the laser using an acousto optic modulator. One advantage of this configuration is that the beat note spectral width between the probe and the pump is considerably lowered with respect to one of a Brillouin spectrometer with two independent lasers.