Jun Akiyama

Laser ceramics with rare-earth-doped anisotropic materials

The fabrication of laser-grade anisotropic ceramics by a conventional sintering process is not possible owing to optical scattering at randomly oriented grain boundaries. In this Letter, we report the first (to our knowledge) realization of transparent anisotropic ceramics by using a new crystal orientation process based on large magnetic anisotropy induced by 4f electrons. By slip casting in a 1.4 T magnetic field and subsequent heat treatments, we could successfully fabricate laser-grade calcium fluorapatite ceramics with a loss coefficient of 1.5 cm(-1).

Laser Demonstration of Diode-Pumped Nd3+-Doped Fluorapatite Anisotropic Ceramics

We report the first demonstration of a diode-pumped anisotropic ceramic laser that uses microdomain-controlled neodymium-doped hexagonal fluorapatite [Nd3+:Ca10(PO4)6F2, Nd:FAP] polycrystalline ceramics as the gain medium, which were fabricated by the rare-earth-assisted magnetic grain-orientation control method, as a step toward achieving giant micro photonics. The laser delivers 1063.10 and 1063.22 nm output beams when pumped with a central wavelength of 807.5 nm and a 2 nm bandwidth diode laser operating in quasi-continuous-wave (QCW) mode. We obtained a maximum QCW peak power of 255 mW with an uncoated 2 at. % Nd:FAP material.

Orientation control of micro-domains in anisotropic laser ceramics

We present theoretical studies on the orientation control of micro-domains in anisotropic laser ceramics, and produce the distribution function of the crystal orientation in micro-domains including anisotropic laser ceramics. Also the improvement in the orientation distribution caused by preferential grain growth is discussed, where our theoretical analyses were applied to several different Nd:FAP ceramics. Detailed XRD analyses based on this distribution function show that the preferential grain growth improved the orientation distribution of the green body that was slip-casted under magnetic field.

Anisotropic Yb:FAP laser ceramics by micro-domain control

We have realized highly transparent Yb:fluorapatite (FAP) ceramics by use of slip casting under rotational magnetic field, even though the main crystal axis become a hard magnetization axis due to the enhancement of magnetic anisotropy by the total angular momentum of 4f electrons in doped rare-earth ions. We confirmed that our Yb:FAP ceramics reached to have a laser-grade quality: it did not interrupt laser oscillation when it was inserted into a lasing cavity. We also evaluate the absorption and the round-trip loss including Fresnel loss of our Yb:FAP ceramics, which were 3.7 cm−1 at 902 nm and 0.26 (11.5% by a single pass) at 1064 nm, respectively.

Fabrication of Rare-Earth Patterned Laser Ceramics by use of Gradient Magnetic Field

New micro-domain orientation controlling and patterning process for anisotropic laser ceramics has been developed. We have successfully obtained Nd:FAP core-clad composite structure by imposition of 10T gradient magnetic field during slip casting.

New Generation of Laser Ceramics with Anisotropic Materials

Transparent anisotropic ceramics with oriented grain structure have been developed by means of advanced electromagnetic processing. We report the first demonstration of RE: FAP (RE: Nd, Yb) ceramics, which have laser-grade transparency.

Novel model on thermal conductivity in laser media: Dependence on rare-earth concentration

Authors have proposed a novel model on thermal conductivity in various laser media. The rare-earth doping effects were also discussed by comparing the measured value in Nd³⁺ and Yb³⁺-doped Y₃Al₅O₁₂, YVO₄, and GdVO₄.

Process design of microdomains with quantum mechanics for giant pulse lasers

The power scaling of laser devices can contribute to the future of humanity. Giant microphotonics have been advocated as a solution to this issue. Among various technologies in giant microphotonics, process control of microdomains with quantum mechanical calculations is expected to increase the optical power extracted per unit volume in gain media. Design of extensive variables influencing the Gibbs energy of controlled microdomains in materials can realize desired properties. Here we estimate the angular momentum quantum number of rare-earth ions in microdomains. Using this process control, we generate kilowatt-level laser output from orientation-controlled microdomains in a laser gain medium. We also consider the limitations of current samples, and discuss the prospects of power scaling and applications of our technology. This work overturns at least three common viewpoints in current advanced technologies, including material processing based on magnetohydrodynamics, grain-size control of transparent polycrystals in fine ceramics, and the crystallographic symmetry of laser ceramics in photonics.

Optical Characterization of Yb Doped Fluorapatite Anisotropic Ceramics

Yb doped FAP ceramics obtained under rotating magnetic field is optically characterized revealing the laser grade of the material. We obtain 0.8cm-1 for cavity loss coefficient.

First demonstration of rare-earth-doped anisotropic ceramic laser

Transparent polycrystalline ceramics have attracted much attention as new laser gain media due to their capability for engineered structure and improved high fracture toughness [1]. On the other hand, rare-earth-doped hexagonal fluorapatite crystals show excellent optical properties for extremely high power laser operation such as large stimulated emission and absorption cross-sections, as well as long fluorescence lifetimes [2]. However, it is difficult to obtain large-size and composite media by using conventional crystal growth method. Therefore, transparent polycrystalline (ceramic) laser media made of anisotropic materials are expected to provide the new generation high power laser oscillators. Recently, we have succeeded in fabricating highly transparent calcium fluorapatite (Ca10(PO4)6F2, FAP) ceramics by applying rare-earth assisted magnetic grain orientation method (imposing 1.4 T magnetic field during slip casthing) and subsquent HIP sintering process [3, 4]. Here, we report about the microdomain-oriented Nd:FAP anisotropic ceramic laser.