Influence of temperature on the post-irradiation temporal loss evolution in Yb-doped aluminosilicate fibers, photodarkened by 488 nm CW irradiation

Abstract

Photodarkening (PD) in optical fibers is a serious concern for the development and maintenance of the reliable operation of high -power ytterbium fiber lasers (YDFL) , over a long period of time . Formation of colour centres in the silica glass host is often believed to be the reason behind PD [1]. Yb-doped aluminosilicate fiber suffers PD as a permanent excess optical loss (unless optically or thermally photobleached) having a low-energy absorption tail extending into the 0.9-1.0 um absorption band and 1.1 um emission band of YDFLs [2,3]. Our previous studies emphasised on oxygen deficient centres (ODC) as a precursor of PD in Yb-doped aluminosilicate fiber [4] under a 488 om CW pump and also revealed the post-irradiation temporal evolution of the induced loss [5]. To the best of our knowledge, our present report is the first of its kind to show the temperature dependance of the post-irradiation induced excess loss in VIS-NIR in Yb-doped aluminosilicate fiber, even after stopping the 488 nm CW pump . A 488 nm CW laser was used to photodarken a 2 m long sample ofYb-doped aluminosilicate fiber (Yb cone, 17000 ppm-wt, N.A. 0.15, core dia 8.3/lm, OD 125 urn). Drop (-21%) in the throughput power at 488 om was monitored, using a suitable silicon detector and a power meter . In one case, the fiber was photodarkened at room temperature for 90 minutes. Then this photodarkened fiber was immediately set up to monitor the white light transmission (at room temperature), using an Optical Spectrum Analyzer (OSA) , for more than 24 hours . The post-irradiation temporal increase in loss, over 24 hours, was almost saturated to 2.5 dB/m at 633 om. In another case, a fresh fiber was similarly photodarkened under 488 om irradiation and then , while monitoring the white light transmission, it was heated at 120°C for the first 30 minutes just after PD. Then it was kept at room temperature for more than 24 hours. The post-irradiation loss evolution over this 30 minutes of heating was around 2.6 dB/m and this loss, after 24 hours , was almost saturated to 3.1 dB/m , both at 633 nm. These results are shown in Fig. 1 (a) & (b). Also, the influence of external heating on PD, during irradiation, and the postirradiation temporal loss evaluation of such fibers will be presented. This is a clear indication that the post-irradiation temporal evolution of loss, in a photodarkened Yb-doped aluminosilicate fiber, depends on the temperature and this loss dynamics can be accelerated and possibly enhanced by external heating. This kind of spontaneous relaxation processes, involving electron and hole centres , was studied in germanosilicate glass [6]. Further studies in that direction , especially on aluminosilicate host, can be useful for the optimal design and operation of high power fiber lasers, in order to avoid or minimize photodarkening..so • j ust aft er PD: (b) -j us t en er room temp -55 PD:room temp ;< :;;t -60 If> If> o -es • 30 mlns aft er 0 • aft e r po : 30 c PD: room .£. -65 mln s at 120 0 S -7 0 temp deg C E ttl~-70 -e • 24 hou rs .to...