A fluid cooled neodymium phosphate glass impulse laser comprising a passive release circuit. The laser may be designed with reduced size so as to be portable and is mainly used in medicine particularly for eye operations, in spectroscopy, in cosmetics, teaching and microtechnology. The proposed laser comprises a resonator of small size having a fluid cooled laser body and which may be disassembled without detuning the resonator, a closed cooling circuit, a supply unit of small size and low consumption and an electronic circuit. The resonator comprises a thick passive release composed of a F/2 center-colored LiF crystal and having a characteristic passive transmission power of 70%, a high reflection coefficient mirror and a partially reflecting output mirror having a characteristic transmission of 60%. The active material is composed of a phosphate glass bar of small diameter, generally 3mm, having a high concentration of Nd ions, generally from 1.2 to 1.4.times.10.sup.21 ions/cm.sup.3.