Green up-conversion in Yb³⁺-Tb³⁺ and Yb³⁺-Tm³⁺-Tb³⁺ doped fluoro-germanate bulk glass and fibre

Yb3+–Tb3+ and Yb3+–Tm3+–Tb3+ doped fluoro-germanate glass samples were fabricated in bulk and fibre form. Bright up-conversion has been obtained by exciting the samples with a 975 nm laser source, and the power dependence of the up-conversion intensities with respect to the excitation power has been characterised. Two mechanisms must be taken into account to explain the population of the 5D4 level of the terbium ions in these experiments; namely multi excited state absorption (MESA) and cooperative up-conversion (CU). While in the Yb3+–Tm3+–Tb3+ samples both MESA and CU contribute simultaneously to the 5D4:Tb3+ population, in the Yb3+–Tb3+ case a pure CU mechanism is responsible for the 5D4:Tb3+ population. In the fibre geometry, the re-absorption effect has been observed. The detrimental role of the re-absorption has been explained by characterising the up-conversion spectrum at different fibre lengths

Transient modal instabilities in high power fibre lasers

Transverse modal instability (TMl) [1] is proving to be a major power-scaling limitation in high-power fibre lasers (HPFLs) [2]. The origin of TMI is predominantly thermal and a number of theoretical models have been introduced to explore its characteristics [3-9]. The majority of TMI reports so far deal with fibre amplifiers in MOPA configurations [3-14], and only a small numbers of recent reports refer to TMI in single-cavity HPFLs [15-16]. In addition, most of the experimental TMI work is carried out under steady-state pumping conditions and the transient TMI response is largely overlooked [17]. TMI threshold reduces in the presence of backreflections in amplifiers [18], and, therefore, TMI effects are expected to be more pronounced in single-cavity HPFL oscillators.In this work, we explore for the first time the transient behaviour of power instabilities in single-cavity HPFLs with modulated (ON/OFF) pumping (quasi-CW (QCW) operation). Fig. 1(a) shows the schematic of the tested HPFL. The ends of the Yb-doped signal fibre of a GTWaveTM [19-20] are spliced (S) with singlemode (SM) pigtails with a high reflector (HR) and output coupler (OC) fibre Bragg gratings inscribed in their cores. The fundamental mode (FM) of the active fibre is matched to the SM pigtails. Higher-order modes (HOMs) supported by or thermally induced in the active fibre are scattered into the pigtail cladding at the splices (S), and a cladding-mode stripper (CMS) in the output pigtail is used to strip them out and monitor their power. The combined pump diodes are modulated with variable repetition rates and ON/OFF duty cycles and are launched into the pump fibres of the GTWaveTM. The fundamental mode (FM) at the output and HOMs at the CMS are monitored with high-speed photodetectors

Passive and active whispering gallery mode microresonators in optical engineering

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Using IFN-gamma release assay to confirm tuberculin skin test improves the screening of latent tuberculosis infection in Italian healthcare workers

Background Healthcare workers (HCWs) represent a tuberculosis (TB) risk group for a wide range of tasks in healthcare, even in countries with low TB incidence, like Italy. Latent Tuberculosis Infection (LTBI) screening programs are an important tool for TB prevention in these setting. Methods A retrospective study under a LTBI screening program among HCWs at the Siena University Hospital (Italy), was conducted between September 2011 and July 2015. Tuberculin Skin Test (TST) was used as a first level examination; all TST-positive cases were tested with QuantiFERON-TB Gold In-Tube (QFT-GIT) test, together with a group of TST-negative subjects. Results Among the 2136 HCWs screened, 144 (6.7 %) were TST-positive and therefore tested with QFT-GIT, confirming a positive result in 36 cases (25 %). Agreement between two tests was poor (k = 0.092; 95 %, Confidence Interval [CI]- 0.048–0.136, p = 0.002). Among TST-positive cases, discordant results occurred more frequently in BCG vaccinated than unvaccinated HCWs (86.3 %, p < 0.001). The probability of a QFT-GIT-positive result increased according to the TST diameter (p = 0.001). No putative risk factor was associated with LTBI occurrence. Conclusions The use of QFT-GIT test as a second step in TST-positive cases offers an appropriate tool for LTBI detection, especially among BCG-vaccinated HCWs