Hisayoshi Toratani

Spectroscopic properties and energy transfers in Tm3+ singly- and Tm3+Ho3+ doubly-doped glasses

Abstract Spectroscopic measurements and analysis of energy transfer processes for Tm3+ singly- and Tm 3+ Ho 3+ doubly-doped glasses pumped with 790 nm diode laser have been performed. The emission cross-sections and the net gain have been determined from the measured absorption spectra for both the ions on the basis of principle of reciprocity. Tm3+ singly-doped fluorozircoaluminate glass, which has a higher fluorescence efficiency at 1.82 μm and a longer upper state lifetime, is a material suitable for Tm laser compared with oxide glasses. The quantum efficiency of Ho3+ fluorescence of 5I75I8 at 2.05 μm for the Tm 3+ Ho 3+ doubly-doped glass is dominated by three energy transfer processes: Tm3+Tm3+ cross-relaxation, net Tm3+Ho3+ energy transfer, and energy transfer upconversion. The first process allows pumping efficiency to approach 2, and the second one contributes to population of the upper 5I7 laser level, while the final process gives rise to a sublinear increase in upper state population with pump power. Optimum doping levels for laser performance are easily predicted based on spectroscopic measurements and are found to be 2–8Tm3+ (1020/cm3) for Tm3+ singly-doped and 2–8Tm3+/0.3–1Ho3+ (1020/cm3) for Tm3+/Ho3+ doubly-doped systems, respectively.

Compositional dependence of nonradiative decay rate in Nd laser glasses

Abstract Variations of nonradiative decay rate from 4 F 3 2 of a neodymium ion with the change of network modifier were determined from the fluorescence lifetime and radiative decay rate, calculated on the basis of Judd-Ofelt theory, for silicate and phosphate glasses. While, on the basis of Miyakawa-Dexters theory, the nonradiative decay rate due to the multiphonon relaxation process was estimated from the electron-phonon coupling strength and phonon energy which were in turn determined from phonon sideband measurements. The latter did not always give the same modifier dependence of the nonradiative decay rate as the former, which suggested that the Judd-Ofelt theory did not always give the precise value of the radiative decay rate. The opposite variations of nonradiative decay rate with alkali modifier were found for silicate and phosphate glasses. This was explained as a result of the difference in the contribution of the electron-phonon coupling strength to the nonradiative decay rate due to the multiphonon relaxation process.

Radiative and nonradiative properties of neodimium doped silicate and phosphate glasses

Abstract Compositional dependence of radiative and nonradiative properties of Nd3+ were investigated in silicate and phosphate glasses. Radiative decay rate is determined by the asymmetry and covalency of ligand field. The large radiative decay rate in phosphate glasses due to the large asymmetry connected with their chain structure. As the cationic field strength Z/a2 of modifier increases, radiative decay rate increases in silicate glasses but decreases in phosphate glasses. This result indicates that in silicate glasses the distortion of ligand field increases with increasing Z/a2, while in phosphate glasses the asymmetry increases with decreasing Z/a2 because the configuration of the ligands becomes more deformable with decreasing Z/a2 of cation connecting the chains. Nonradiative decay rate due to multiphonon relaxation is determined by the electron-phonon coupling strength and phonon energy. The nonradiative decay rate for the change of modifier cations are determined by electron-phonon coupling strength and those for the change of modifier contents are determined by phonon energy.

Mechanisms of upconversion fluorescences in Er3+, Tm3+ codoped fluorozircoaluminate glasses

The upconversion of infrared radiation into green and red fluorescences has been investigated for Er3+ and Tm3+ codoped fluorozircoaluminate glasses under 790 nm excitation. Introduction of Tm3+ into Er3+ doped system preferentially quenches the green upconversion fluorescence from the 4S32 level of Er3+ due to the efficient cross-relaxation of 4I132 → 4I152 (Er): 3H6 → 3H4(Tm) which can significantly reduce the upconversion efficiency from the 4I132 level to the emitting 4S32 level. However, the Tm3+ behaves as a good sensitizer of the red upconversion fluorescence from the 4F92 level of Er3+ which is mainly populated by the cross-relaxation of 3H4 → 3H6 (Tm): 4I112 → 4F92(Er). The excitation energy stored in the 4I112 level of Er3+ migrates through the glass with a diffusion-limited regime. Through this regime, only Er3+ ions which have diffused their energy into the vicinity of Tm3+ ions can transfer the energy to the 3H5 level of Tm3+. At lower Er3+ concentrations (≤ 3 mol%), this diffusion-limited energy migration is not so efficient as to enable energy transfer upconversion from the 4I112 level to the 4F92 level of Er3+.

Dynamics and mechanisms of up-conversion processes in Yb3+ sensitized Tm3+- and Ho3+-doped fluorozircoaluminate glasses

Abstract The efficient upconversion fluorescences corresponding to the 1 G 4 → 3 H 6 (480 nm) and 3 F 4 → 3 H 6 (805 nm) transitions of Tm 3+ and the 5 S 2 → 5 I 8 (545 nm) transition of Ho 3+ were observed in Yb 3+ /Tm 3+ and Yb 3+ /Ho 3+ doubly doped fluorozircoaluminate glass, respectively under 980 nm excitation. The dependences of these upconversion fluorescences upon the pumping intensity and the doping concentrations were interpreted with a rate equation model which takes into account of Yb 3+ ⇆ Tm 3+ (Ho 3+ ) energy transfers. The energy transfers from Yb 3+ to Tm 3+ (Ho 3+ ) only occur within those Yb 3+ ions which are closer to the Tm 3+ (Ho 3+ ) ions, and the more distant Yb 3+ ions must transfer their energy to the closer Yb 3+ ions through the glass with a diffusion-limited regime. The back transfers from Tm 3+ (Ho 3+ ) to Yb 3+ were also demonstrated in both the Yb 3+ /Tm 3+ and Yb 3+ /Ho 3+ systems. They account for the concentration quenching of the up-conversion fluorescences at higher doping concentrations.

Temperature coefficient of electronic polarizability in optical glasses

Abstract The temperature coefficient of refractive index d n /d T and that of volume expansion β have been measured by an interferometric procedure designed to determine the coefficients simultaneously, with one sample. The temperature coefficient of electronic polarizability φ have been calculated from d n /d T and β. φ is expressed as the sum of the contributions of each glass component, φ i · φ i is expressed as follows: φ i = s i γ i , where s i is the thermal change in cation-oxygen distance and γ i is the change in electronic polarizability due to the inter-ionic distance change. φ i decreases as the ionic field strength z / a 2 of the cation increases, which is the same tendency as that of s i . γ i , calculated from φ i and s i , gradually increases as z / a 2 increases. This means that φ i is mainly determined by the elongation of the inter-ionic distance, and it also suggests that the deformation of the electron cloud of oxygen ion due to the inter-ionic distance change, γ i becomes larger, as the electronic polarizing power of the cation increases.

Second order optical nonlinearity of surface crystallized glass with lithium niobate

Significant second harmonic generation was observed for a surface crystallized but transparent glass of 35Li2O⋅30Nb2O5⋅35SiO2 (in mol %), for which the crystalline layer (about 0.35 μm in thickness) was composed of partially c‐axis oriented and connected LiNbO3 fine grains. The effective second order nonlinear optical susceptibility of the film deff reached ∼5.25 pm/V after adjusting the incident angle. The tensor components of the film were d33=6.9 pm/V and d31=d21=d24=d15=1/3d33=2.3 pm/V if the partially c‐axis oriented thin film was described by a space group C∞V.

Phosphate laser glass of absorption loss of 10−4cm−1+

We have experimentally determined loss coefficients of OH as 8.1 × 10−7 cm−1 / ppm and of ionic platinum as 8.6 × 10−7-7cm−1/ppm for LHG-5 type phosphate laser glass without Nd2O3 doping. Glass has been produced with absorption loss of about 1 ×c 10−4cm−1 which is one order of magnitude lower than is presently commercially available.

Pr3+ doped InF3/GaF3 based fluoride glass fibers and Ga–Na–S glass fibers for light amplification around 1.3 μm

Abstract We have developed three types of highly efficient Pr 3+ doped fiber (PDF) amplifiers around 1.3 μm based on non-oxide glasses. A Pr 3+ doped InF 3 /GaF 3 based fluoride glass fiber module (PFDM) has a gain coefficient: 0.29 dB/mW and a signal output power: 15.2 dBm (33 mW) at a pump power of 272 mW. A Pr 3+ doped chalcogenide system: Ga–Na–S (GNS) single-mode fiber has a larger gain coefficient: 0.81 dB/mW at signal wavelength of 1340 nm for signal input of −30 dBm. However, GNS–PDFs are best suited for amplification in the wavelength range of 1325–1350 nm when the input power is > −8 dBm. We propose a hybrid PDF consisting of the above two types of PDFs which has a flatter gain spectrum in the 1.3 μm wavelength region and will be able to extend the range of the wave division multiplexing (WDM) usage.

Radiation resistance of fluorophosphate glasses for high performance optical fiber in the ultraviolet region

Transmission loss characteristics of a fluorophosphate fiber was investigated in 200–800 nm wavelength region. The transmission loss was found to be primarily dominated by extrinsic absorption due to transition metal impurities. The total scattering is the next most important loss factor which is in fact Rayleigh in character exhibiting a λ−4 dependence, and the ultraviolet absorption tail is the least significant loss factor. Radiation resistance characteristics of the bulk glasses corresponding to the core and cladding have also been investigated. Upon exposure to ultraviolet radiation from a 1 kW Hg-Xe lamp, the radiation-induced defect centers leading to additional absorption bands in the ultraviolet region result mainly from the photoionization processes of Fe2++hν=Fe3++e− and 2Cl−+hν=Cl2−+e− for the Cl− doped glasses, and the former one is also responsible for the glasses without doping of Cl−. The electron capture centers in the glasses are the [PO4] and/or [P2O7] groups. If the irradiation is carr...