“Transcending” is such a comfortable word. “Transcending” technology not only contributes to the development in a specific field, but its influence rapidly and widely spreads to peripheral fields. This special issue picks up some technologies that may transcend the current technologies. There is always a new future in “Transcending”, and there are many technological advances on the roadside of the researches that we have dedicated to overcome. We hope you will feel the new movement of optical fiber and optical device technology toward such a new era from this issue.

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We investigated the ultra-low propagation loss of crystallized glass/fiber. We described the analysis and discussed the optical scattering induced by glass crystallization and proposed a novel method to increase the transparency by refractive index matching between crystalline and glassy phases in crystallized glass. This is called optical stoichiometry. We reported high index matching and low propagation loss with 0.6 dB/cm at 1.55 μm in SrO-TiO2-SiO2 crystallized glass.

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Silica glass is the most indispensable material for high-power laser applications and optical communications due to its superior optical properties. However, optical loss remains an outstanding challenge in both fields. More than 80% of the optical loss in silica glass is due to Rayleigh scattering, which can be suppressed by reducing density fluctuations. Previously, reducing fictive temperature had been the only path to reduce density fluctuations. Pressure-quenching is an alternate method that has been recently discovered. Here, I review the finding of the pressure-induced suppression of the optical loss by considering the voids in silica glass. For further reduction of optical scattering, our recent research in compressed glass at higher pressures is presented using both experimental and simulation studies. Other reports of glass treated at various pressures and temperatures are reviewed to examine the possibility of controlling fluctuations in glass.

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As developing laser technology, lasers have come to be used for various applications such as chemical analysis and material processing. In addition, the expectation for higher power laser becomes strong due to expanding the application range. On the other hand, as the output power of the laser becomes higher, higher light resistance is required for the laser crystal, wavelength conversion crystal and fiber which constitute the laser. In this paper we have investigated an approach to increasing the output power using a quasi-phase-matched wavelength conversion element. In particular, special attention was paid to the heat generated in the crystal as output power increased. The maximum output power is limited by the influence of this heat, and due to overcoming it we have developed an exhaust-heat wavelength conversion module that efficiently removes heat. we demonstrated stable 15 W green power in CW single-pass conversion.

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Nonlinear propagation equations and thermal conduction equation are numerically solved to investigate the laser-induced thermal effects on the high-power continuous-wave (CW) second harmonic generation. The absorption coefficients, heating mechanism and thermal performance of modules are evaluated by comparing the numerical and experimental results. The result suggests existence of the green-induced absorption in addition to the linear and two-photon absorption. The green-induced absorption is explained by a polaron and its optical property is discussed.

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The zeolite method enables the uniform dispersion of impurity elements, particularly rare-earth elements, in silica glass. Applications primarily allow more ultra-high concentrations of rare-earth elements to be doped in silica glass than in the conventional method. We proposed a high average-power laser medium and an ultra-short fiber laser by the zeolite method. The zeolite method, which covers uniform dispersion characteristics, can separate the complexity in the measurement results and simplify countermeasures in material development. As an example, we explain an application for photo-darkening suppression in Yb-doped silica fiber and show the possibility of the zeolite method.

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