A waveguide laser medium having an optical waveguide, which is used for effecting light amplification or laser oscillation and is formed by performing the ion exchange of alkaline ions contained in a predetermined portion of a laser glass substrate, which includes the alkaline ions and laser active ions, for another kind of ions, which operate to increase the refractive index of the laser glass, thereby making the refractive index of the predetermined portion become higher than that of the other portions of the laser glass substrate. Further, the laser glass substrate is made of, for example, phosphate glass containing neodymium ions as the laser active ions and further containing 0.01˜8.0 mol % of Na.sub.2 O, from which Na.sup.+ ions are obtained as the alkaline ions, and the ion operating to increase the refractive index of the predetermined portion is Ag.sup.+ ions. Such a waveguide laser medium is employed in a waveguide laser. Thereby, the ion exchange can be efficiently performed without the occurrence of a crack in the laser glass substrate and so on. Furthermore, light amplification, as well as laser oscillation, can be efficiently performed.
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