A diode-pumped solid-state laser for single mode operation contains a resonator (R) with an input (3) and an output (7) mirror between which an active laser medium (4), a polariser (5) and a double refraction crystal (6) are arranged. The optical axes (Ea, Eo) of the crystal are arranged so that they form an angle of 45 DEG in relation to the direction of polarisation (EL) produced by the polariser (5). The losses for the ordinary and extraordinary beam are set to about the same value. With the aid of a temperature control the temperature of the double refraction crystal (6) is kept at a constant value so that neither the refractive indices nor the length of the crystal change.
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