Photosensitivity and bragg grating technology for upconversion all-fiber lasers

Abstract

Fiber lasers are compact lightsources with high intrinsic beam quality and high efficiency. The lasing spectrum ranges from the blue to the near infrared and is achievable using fibers with different dopants and glass compositions. All-fiber lasers are characterized by a high attainable stability and are also available in the pulsed regime. A widespread range of applications for all-fiber lasers exists in the domain of telecommunications as well as in the medical domain. The first part of this work is dedicated to the characterization of fluoride glasses regarding photosensitive effects. Fluoride glasses allow several lasing transitions, which are not available in silicate glasses. Photosensitive effects in fluoride glasses would pave the way to fabricate fiber Bragg gratings in fluoride fibers. Fiber Bragg gratings can act as laser mirrors and are a necessary tool on the way to all-fiber lasers with an output beam inside a fiber tail. In this work, doped and undoped fluoride glasses are exposed to different ultraviolet lasers and induced refractive index changes are investigated by changes of their absorption spectra. Index changes higher than 10-4 have been obtained. The second part is dedicated to demonstrate novel upconversion laser configurations in the visible spectral range using fiber Bragg grating technology in germanosilicate fibers and upconversion in fluoride fibers. Upconversion lasing in the green, orange and red spectral range were demonstrated. Tuning over 8 nm was shown for the orange laser. To minimize intracavity losses, mode field diameter adapter in germanosilicate fibers have been realized. The incorporation of an all-fiber accousto optic phase modulator inside the cavity allows pulsed operation of the upconversion laser. Mode locking is demonstrated at the red wavelength with pulse durations below 100 ps. In conclusion, the potential to fabricate Bragg gratings in Cerium doped and undoped fluoride glasses was demonstrated. Furthermore, green, orange and red lasing of an all-fiber configuration has been shown by a hybrid approach based on fiber Bragg grating technology in germanosilicate fibers and upconversion in fluoride fibers. An all-fiber mode locked upconversion laser in the red spectral range was demonstrated. We believe that such novel upconversion light sources represent a useful tool in different biomedical applications.