A high-gain optical amplifier for a wave to be amplified at a wavelength referred to as the emission wavelength, includes: optical pumping elements (4) producing a pump wave at a wavelength referred to as the pump wavelength; a solid amplifying medium (1) that is doped with active ions, the solid amplifying medium being capable of emitting laser radiation at the emission wavelength when the medium is pumped by the pumping elements; cooling elements (2) capable of cooling the solid amplifying medium to a temperature of no higher than 250 Kelvin; and optical multiplexing elements capable of coupling together the pump wave and the wave to be amplified in the amplifying medium. The amplifying medium has Stark sublevels contained within a spectral domain ranging over less than 200 cm−1 (approximately 20 nm, when expressed in wavelength). A laser including a resonant optical cavity and an amplifier are also described.