A bi-frequency laser emission system comprising: - a laser, called main laser, comprising an amplifying medium disposed in a laser cavity and a means for pumping said amplifying medium, the main laser being configured for lasering in said cavity and in accordance with an initial laser gain (G ini ( v)) as a function of an optical frequency, the system being characterised in that it further comprises: - a spectral selection device (D ss) comprising: * a gaseous phase molecular or monoatomic medium disposed in said laser cavity; and * a laser, called excitation laser (L ex), having an excitation wavelength ( λ ex) configured for exciting, through optical absorption, an excitation energy level (N ex) of said medium; * said medium being configured to emit, through radiative de-excitation of said excitation energy level to a first (N 1) and a second (N 2) energy level with lower energy, at least one frequency doublet comprising a first frequency ( v 1) associated with said first level (N 1) and a second frequency ( v 2) associated with said second level (N 2), said first and second frequencies each having a value included in said initial laser gain of said amplifying medium and selected so that an associated radiation is not reabsorbed by said medium; - the excitation laser being configured so that the transition between a fundamental level of said medium and said excitation energy level is saturated and the molecular or atomic density being high enough, such that said initial laser gain (G ini ( v) of the main laser is changed into a selective laser gain (G sel ( v)) through local enhancement of said initial laser gain at said frequencies ( v 1, v 2) of said doublet, said main laser then being configured for lasering in phase, simultaneously and only on said frequencies ( v 1, v 2), of said at least one doublet. - a laser, called main laser, comprising an amplifying medium disposed in a laser cavity and a means for pumping said amplifying medium, the main laser being configured for lasering in said cavity and in accordance with an initial laser gain (G ini ( v)) as a function of an optical frequency, the system being characterised in that it further comprises: - a spectral selection device (D ss) comprising: * a gaseous phase molecular or monoatomic medium disposed in said laser cavity; and * a laser, called excitation laser (L ex), having an excitation wavelength ( λ ex) configured for exciting, through optical absorption, an excitation energy level (N ex) of said medium; * said medium being configured to emit, through radiative de-excitation of said excitation energy level to a first (N 1) and a second (N 2) energy level with lower energy, at least one frequency doublet comprising a first frequency ( v 1) associated with said first level (N 1) and a second frequency ( v 2) associated with said second level (N 2), said first and second frequencies each having a value included in said initial laser gain of said amplifying medium and selected so that an associated radiation is not reabsorbed by said medium; * a gaseous phase molecular or monoatomic medium disposed in said laser cavity; and * a laser, called excitation laser (L ex), having an excitation wavelength ( λ ex) configured for exciting, through optical absorption, an excitation energy level (N ex) of said medium; * said medium being configured to emit, through radiative de-excitation of said excitation energy level to a first (N 1) and a second (N 2) energy level with lower energy, at least one frequency doublet comprising a first frequency ( v 1) associated with said first level (N 1) and a second frequency ( v 2) associated with said second level (N 2), said first and second frequencies each having a value included in said initial laser gain of said amplifying medium and selected so that an associated radiation is not reabsorbed by said medium; - the excitation laser being configured so that the transition between a fundamental level of said medium and said excitation energy level is saturated and the molecular or atomic density being high enough, such that said initial laser gain (G ini ( v) of the main laser is changed into a selective laser gain (G sel ( v)) through local enhancement of said initial laser gain at said frequencies ( v 1, v 2) of said doublet, said main laser then being configured for lasering in phase, simultaneously and only on said frequencies ( v 1, v 2), of said at least one doublet.