Abstract

Visible upconversion fiber lasers in the blue spectral range offer an alternative to air cooled argon ion lasers A multiple photon absorption process of near infrared pump photons allows an efficient excitation of high energy levels from which laser transitions in the visible spectral range are possible The praseodymium ytterbium doped ZBLAN-fiber used in our experiments can be excited by a single mode laser diode at a wavelength around 850 nm and yields five laser transitions between 491 nm and 717 nm The blue-green laser line at 491 nm is the most interesting transition because it can directly replace the 488 nm argon ion laser line in many applications However, applications in measurement and medical technology require low amplitude noise Due to the coupling of the upper laser level to the photon field in the laser resonator fiber lasers show a relaxation oscillation This oscillation is usually in the range of a few hundred kilohertz In addition to the relaxation oscillation fiber lasers show noise features due to the nonlinear multi photon pump process, mode competition of longitudinal modes and technical noise from mechanical instabilities of the whole setup To reduce the amplitude noise an active stabilization of the output power has to be applied This technique is well known for single frequency solid state lasers, however, to our knowledge it had never before been applied to fiber lasers operating on several longitudinal modes

© 2000 IEEE