Contents 1. INTRODUCTION
Broadband ASE (amplified spontaneous emission) optical fiber sources emitting radiation in the eye-safe range (> 1.5 μm) are commonly used in a wide range of applications such as spectroscopy [1], microscopy [2], sensing [3] or medical diagnosis. It should be noted that short coherence length is direct a consequence of the broad spectral distribution. The commonly used silica fibers have a limitation because of high phonon energy limiting donor-acceptor energy transfer. Moreover, the concentration of RE (rare-earth) is also strongly limited due to clustering and phase separation effects. Novel optical broadband fiber sources of radiation operating in the eye-safe spectral region relay on the co-doped optical fibers. Double or triple doped glass enable to obtain broad emission via the superposition of luminescence bands of Ho3+, Er3+ and Tm3+ ions. A key aspect of obtaining > 1.5 μm luminescence is the phonon energy of glassy matrix. The probability of non-radiative transition is proportional to the phonon energy of glass host. In case of the optical fiber drawing no less important parameter is the thermal stability of glass [4], [5]. Among oxide, low phonon glasses germanium based ones are characterized by rare-earth-ion solubility higher than chalcogenide glass and stronger physicochemical stability than fluoride glass. Moreover, the spectral transmittance is up to 6 um and lower probability of non-radiative relaxation. In addition, OH− content is important to reduce the non-radiative energy transfer from RE to OH− and this problem was overcome by efficient dehydration [6]. Thus, germanate glasses has been widely studied as a potential fiber laser candidate [7]–[9].
