Hiroyuki Nasu

Spectroscopic investigation of Nd 3+ -doped ZBLAN glass for solar-pumped lasers

We evaluated the optical properties required for the design of a solar-pumped laser for Nd3+-doped ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass. The quantum efficiency (QE) of near-infrared emission from the 4 F3/2 state of Nd3+ using sunlight as an excitation source was 70%. The product of the stimulated emission cross section and the radiative lifetime (σseτr) of the 4F3/2→4I11/2 was 1.5×10−23 cm2·s. The integrated absorption strength in the 450–900 nm region was the largest among Nd3+-doped fluoride glasses. The high QE, large σseτr product, and large integrated absorption strength indicate that Nd3+-doped ZBLAN is one of the most promising materials for solar-pumped lasers.

Excitation wavelength dependence of quantum efficiencies of Nd-doped glasses for solar pumped fiber lasers

The quantum efficiencies of the emission from the 4F3/2(R) level of Nd doped in tellurite glass were measured with an integrating sphere using natural sunlight(ηns), simulated sunlight (ηns), and 808 nm laser light (η808), respectively. The radiative quantum efficiency (ηr) was estimated from the fluorescence lifetime (τf) and the radiative lifetime calculated by Judd-Ofelt analysis (τr). ηr was almost 100 % for χ ≤ 0.5 mol.%. η808 was 86 % for χ=0.05 mol.% and decreased monotonically with increasing in χ. ηns had a peak at χ=0.5 mol.% and the maximum was 33 %. It is thought absorption of the excitation light by the host glass limits the quantum efficiency of the tellurite glass under sunlight excitation. Therefore, it is important to reduce absorption of the tellurite glass host in order to realize efficient solar-pumped tellurite fiber lasers.

Spectroscopic Investigation of Nd3+-doped ZBLAN Glass for Solar Pumped Lasers

We clarified that Nd3+-doped ZBLAN glass would be a promising material for solar pumped laser applications due to its high quantum efficiency for sunlight, large stimulated emission cross-section, long emission lifetime and broad absorption bands.

Spectroscopic properties of Nd, Er codoped glasses for solar pumped fiber lasers

The absorption and fluorescence characteristics of Er doped and Nd, Er codoped fluoride glasses were investigated under illumination of the simulated sunlight, laser or a monochromatic light filtered from a Xe lamp. Er was used as a sensitizing agent enhancing the energy conversion and the emission efficiency of Nd ions in fluoride glass intended for the sunlight excitation. Er doped fluoride glasses showed four emission peaks under simulated sunlight illumination at the wavelengths of 550, 848, 980, and 1530 nm attributed to the electronic transitions of Er3+ ions. The quantum efficiency of the emission from all of the bands had a peak at x = 0.5 mol. % Er and with the maximum of 73 %. The intensity of each emission band showed different ratios for various ErF3 contents. It is expected that concentration quenching of 4S3/2 state is easy to occur with high concentration of ErF3 compared to the other states. The energy transfer from Er to Nd was studied using a monochromatic light illumination which is absorbed by Er3+ ions only. Strong contribution of Er absorption to the 1.05 μm emission of Nd, Er co-doped fluoride glass was observed. Er was confirmed as a suitable sensitizer for the enhanced energy conversion and emission efficiency of Nd ions in ZBLAN glasses which are proposed for highly efficient solar pumped fiber lasers.

The efficiencies of energy transfer from Cr to Nd ions in silicate glasses

The efficiency of energy transfer from Cr to Nd in silicate glasses has been examined in order to develop a gain medium for high-efficiency solar pumped fiber lasers (SPFLs). The internal quantum efficiency (QE) of the emission from the 4T2 state of Cr in Cr-doped glasses and from the 4F3/2 state of Nd in Nd-doped and Nd,Cr codoped glasses was measured using an integrating sphere. For Cr-doped and Nd,Cr codoped glasses, 650 nm excitation was used. For Nd-doped glasses, 808 nm excitation was used. The QE of Cr-doped glass (ηCr) was 7.5 % for 0.05 mol.% Cr2O3, the QE decreased monotonically with increasing Cr2O3 content. The QE of the Nd-doped glass (ηNd) has a maximum of 43% at 0.2 mol.% Nd2O3. We suggest that absorption of host glass could lower the QE at the low content side of the maximum. The QE of Nd emission in Nd,Cr codoped glass (ηNd,Cr) excited at 650 nm, which excites the 4A2→4T2 transition of Cr was 5.7 % for 0.05 mol.% Cr2O3 and 0.2 mol.% Nd2O3 content. The energy transfer quantum efficiency, ηtr, from Cr to Nd which is defined as the ratio of the ηCr and the ηNd,Cr was calculated from the obtained QEs. The largest ηtr was 13.4 % at 0.01 mol.% Cr2O3, and decreased with increasing Cr2O3 when content of Nd2O3 was fixed by 0.2 mol.%. This tendency is quite similar to the QEs of the Cr emission in Cr-doped glasses. Thus an increase in the QE of Cr maybe essential to increase the ηtr.

Numerical simulation of Nd-fluoride and tellurite solar-pumped fiber lasers

Numerical simulation of Nd-doped fluoride and tellurite fiber lasers pumped directly by sunlight have been performed. Calculations have been done for double clad fibers to achieve both efficient coupling sunlight with a fiber and spatial power condensation. The pumping source was assumed to be initially AM1.5 sunlight and was focused onto a fiber end. The both ends of the fiber were assumed to be transparent for at the pump wavelength and highly reflective at laser wavelength. It was assumed that the launched sunlight propagate in one longitudinal direction in the inner but the laser light propagate in both longitudinal directions in the core. Propagation equations were solved under boundary conditions determined by the reflectivity of the fiber ends by a boundary value problem solver in MATLAB. The calculated sunlight concentration ratio at lasing threshold was about 300 suns for solar pumped fiber lasers with optimized structure. This sunlight concentration can be achieved by conventional optics such as lenses and ellipsoidal mirrors. The slope efficiency was about 0.78 % for a fluoride fiber and 1.32 % for a tellurite fiber. These efficiencies are can be improved by farther optimization of fiber parameters and pumping scheme. Solar pumped fiber lasers will be able to provide alternative means to utilize solar power.