Laser

A laser comprising:

• a first end mirror and a second end mirror defining an optical cavity therebetween;
• a first gain medium and a second gain medium positioned in the optical cavity;
• at least one radiation source configured to provide pump radiation to the first and second gain media, wherein the pump radiation comprises a first pump beam directed to be incident on the first gain medium and a second pump beam directed to be incident on the second gain medium so as to stimulate emission of radiation from the first and second gain media thereby establishing a laser beam in the optical cavity;
• and
• a control apparatus operable to adjust a property of at least one of the first and second pump beams and thereby control a thermal lens of at least one of the first and second gain media such that an instability zone from the power curve of the laser is substantially removed and the thermal lens of the first gain medium has a different strength to the thermal lens of the second gain medium.