Laser device and method for operating laser device

A method for operating a laser device, the method comprising:

• providing a traveling wave type resonator comprising a first mirror and a second mirror, and a laser medium between the first mirror and the second mirror, wherein the first mirror and the second mirror are disposed such that round-trip light that travels in round trips in the resonator has a focus inside the laser medium;
• causing excitation light to be incident on the resonator such that the excitation light is superimposed on the round-trip light at the focus and narrowed to be thinner than the round-trip light;
• and
• performing control such that:
• a round-trip Gouy phase shift of the resonator has a value excluding 2π×n/m, where m is an integer of less than 15 and n is an integer of equal to or less than m, and
• ZR×α<0.5 is satisfied where a Rayleigh length of the excitation light is ZR and an absorption coefficient of the laser medium with respect to the excitation light is α.