Abstract: A laser amplifier includes a gain medium of a glass fiber, a first exciting light source, a second exciting light source, and another first exciting light source. The gain medium of the glass fiber includes rare earth dopant in a host medium. Ions of the rare earth dopant has an energy level structure including a ground level and a pair of laser upper level and laser lower levels between which a stimulated transition is to be produced after production of a population inversion between the pair of laser upper and laser lower levels. The pair of laser upper and laser lower levels being higher than the ground level. The first exciting light source is coupled to the input side of the glass fiber for introducing first exciting light to the glass fiber to produce the population inversion.