A device for amplifying a multi-wavelength pulsed laser beam is provided, which comprises: a solid amplifying medium with two planer faces, a front face and a reflecting rear face; and a device for cooling the amplifying medium by the rear face. The front face of the amplifying medium is tilted relative to its rear face by a first non-zero tilt and the device further comprises a trapezoidal prism, with an input face and an output face which form between them a second non-zero tilt, the first and second tilts being such that the beams of each wavelength are parallel to one another at the output of the prism.