Abstract: The 4-pass amplifier comprises an optical isolator (7), a polarizing beamsplitter (2), a gain material (3) and a first and a second reflecting element (5, 6). It further comprises means (4) for modifying the polarization state of a light beam after passing through said gain material for a first time and before passing through said gain material for a second time with respect to two orthogonal axes in a way which is equivalent to exchanging said two orthogonal axes. The polarization-rotating means (4) rotate the polarization of the light by 45° and are preferably a Faraday rotator. The reflecting element (5) is, e.g., a multilayer dielectric mirror. An incident light beam (1) is linearly polarized by the polarizing beamsplitter (2), amplified by the gain material (3), its polarization plane is rotated by the polarization rotator (4) by 45°, and the light beam is reflected by the reflecting element (5). It passes again through the polarization rotator (4) and the gain material (3).