Abstract: The invention is based on the discovery that under certain conditions lasing at room temperature can be achieved in a Pr.sup.3+ -doped fluorozirconate fiber pumped in the infrared at 1.01 .mu.m and 835 nm, the lasing taking place in the blue (491 nm), green (520 nm), orange (605 nm) and red (635 nm and 715 nm). One laser comprises a length of Pr.sup.3+ -doped potical waveguide such as a fiber, and means for exciting the Pr.sup.3+ ions to an energy level in the band (.sup.3 P.sub.2, .sup.1 I.sub.6, .sup.3 P.sub.1, .sup.3 P.sub.0), in which the Pr.sup.3+ concentration is in the range substantially 50 ppm to substantially 10,000 ppm (by weight). The Pr.sup.3+ concentration is preferably in the range substantially 200 ppm to substantially 2,000 ppm (by weight). The optical fiber is preferably a fluorozirconate fiber doped at the foregoing concentrations with Pr.sup.3+ ions. The fiber preferably comprises a doped core clad with a further glass. The excitation means is preferably arranged to excite the Pr.sup.