The supercontinuum (SC) generations in a birefringent TeO₂-ZnO-Na₂O-Bi₂O₃ (TZNB) tellurite microstructured optical fiber (BTMOF) have been demonstrated by pumping near the zero-dispersion wavelengths (ZDWs) with a tunable picosecond erbium-doped fiber laser.

Outcome：

2024
In this study, the nonlinear refractive index of tellurite glass was measured at a wavelength of 0.8 μm using the Z-scan method, and its effect on supercontinuum (SC) generation was investigated through simulations. TLWMN, TZLB, and TZNL were found to exhibit negative nonlinear refractive indices. It was revealed that spectral broadening due to self-phase modulation and optical wave breaking occurs in the anomalous dispersion region under negative nonlinearity, unlike the previously reported behavior in the normal dispersion region with positive nonlinearity.

2023
In this study, a polarization-maintaining all-solid microstructured optical fiber using tellurite glass was designed and fabricated for mid-infrared spectroscopic applications. A flat-core structure was used to control dispersion and birefringence, and the generation of single-polarization supercontinuum (SC) light was demonstrated under 2.1 μm excitation. High extinction ratio and broadband polarized SC generation were confirmed. A four-rod structure was also designed and fabricated to further optimize dispersion control.

2022
When the pump is polarized parallel with the x-axis of the BTMOF, the SC broadening is governed by the four-wave mixing (FWM), stimulated Raman scattering (SRS) and cross-phase modulation (XPM). The FWM signal can be generated from 1538 to 1293 nm, and the idler can be emitted from 1597 to 1907 nm. Due to the high nonlinear property of the BTMOF, the XPM between the FWM signal and idler can be significantly amplified by the cascaded FWM effect. When the pump is polarized parallel with the y-axis of the BTMOF, the SC is broadened by the Stokes and anti-Stokes SRS. A broadest Stokes SRS band with a 10-dB bandwidth of 232 nm is generated by pumping at 1545 nm with an average pump power of 23.8 dBm. Potentially, widely tunable fiber lasers can be developed by exploring the FWM or SRS in highly nonlinear tellurite fibers with tailored dispersion profiles. The SC evolutions agree well with the numerical simulation.

Here, an all-solid tellurite optical rod with a disordered transverse refractive index profile was first fabricated to study the transverse localization of light.

Outcome：

2024
This study reports the fabrication of a tellurite-air glass transversely disordered optical fiber (TDOF) with a refractive index contrast greater than 1. By stacking 1800 TZNL fibers into a TLWMN tube and controlling drawing temperature, a structure with ~33% air-hole density was achieved. Most holes matched the theoretical size (~1.1 µm), though non-uniform density remained near the fiber core.

2023
We have successfully fabricated an air-tellurite glass transversely disordered fiber (TDOF). The cross-sectional area ratio of air to glass is approximately 30%, which is much higher than that of previously reported TDOFs with air-holes. It has been confirmed that optical confinement at a wavelength of 1.5 μm is possible.

2022
In the late 1980s, it was proposed that an optical wave system in which the refractive index profile is random in the transverse plane but is invariant in the longitudinal direction supported the transverse localization of light. The transport of optical images using transverse localization of light and its potential applications in biological and medical imaging were demonstrated by using a polymer optical fiber. The experimental results showed that after a CW probe beam propagated in our 5-cm-long fabricated tellurite optical rod, the beam became localized. With the potential of tellurite glasses such as high nonlinearity, broad transmission window and the control of the refractive index difference, the performance of image transport through a tellurite disordered optical medium by transverse localization of light is expected to improve in the near future.

Here, a tellurite step-index fiber with large core is fabricated to keep the zero-dispersion wavelength (ZDW) of the fundamental mode close to that of the material dispersion which is located in the near-infrared region.

Outcome：

2022
Four-wave mixing (FWM) in optical fibers is promising for wavelength conversion in optical networks due to its ultrafast response and high bit rate transmission. To further improve the performance of FWM-based wavelength conversion such as its bandwidth and conversion efficiency, the low nonlinearity of silica fibers should be improved by using novel highly nonlinear soft-glass fibers such as lead-silicate, bismuth-oxide, tellurite and chalcogenide fibers. By using a femtosecond pulsed laser pumped in the vicinity of the ZDW, it is expected to broaden and extend the bandwidth of the FWM-based wavelength conversion towards the near-infrared window for potential applications. When the pump wavelength was tuned from 1647 to 1795 nm, the signal was tuned from 1550 to 1434 nm, and the idler was generated from 1757 to 2400 nm. A 17.5 dB maximum signal gain at 1550 nm and +1.1 dB idler conversion efficiency at 1757 nm were obtained. When the pump wavelength was 1795 nm, the signal and generated idler wavelengths were 1434 and 2400 nm, respectively. To the best of our knowledge, this is the first time to demonstrate a FWM-based wavelength conversion performance whose wavelength spacing is ~966 nm (from 1434 to 2400 nm) in a tellurite step index optical fiber as short as 1 m.

We report here the design of a new chalcogenide hybrid microstructured optical fiber (HMOF) with a buffer layer around the core and its potential performance of tailoring chromatic dispersion and supercontinuum (SC) generation.

Outcome：

2022
The new chalcogenide HMOF has an AsSe₂ core. The refractive index difference ∆n between the AsSe₂ core and cladding material is supposed to be 0.3. The fiber microstructure and the ∆n between the core and buffer materials are designed in order to obtain broad anomalous dispersion regimes with near-zero and flattened chromatic dispersion profiles for broadband SC generation. Moreover, the suppression of chromatic dispersion fluctuation caused by fiber transverse geometry variation is investigated. By using the proposed chalcogenide buffer-embed HMOFs, the calculation shows that near-zero and flattened anomalous chromatic dispersion regimes from 4.5 µm can be obtained. When the variation of fiber structure occurs for ±1, ±5 and ±10 %, the chromatic dispersion fluctuation can be greatly suppressed. In addition, the calculation shows that a broad SC spectrum from 2.5 to more than 16.0 μm can be obtained when a 0.9-cm-long section of the new chalcogenide buffer-embed HMOF is pumped at 5.0 μm by a femtosecond laser with 1-kW peak power.

Here, we show a broad and flattened SC spectrum with 5-dB spectral flatness over 1060 nm spectral bandwidth by using a 20-cm-long segment of the fabricated tellurite HMOF.

Outcome：

2023
We succeeded in generating single-polarized mid-infrared SC light by controlling the wavelength dispersion and polarization-maintaining properties of a tellurite all-solid-state microstructured optical fiber with a flat core.

2022
Supercontinuum (SC) generation in optical fibers has been a topic of great interest because it can provide multiwavelength optical sources with high coherence and brightness which are useful for many potential applications such as wavelength division multiplexing transmission, optical frequency combs, spectroscopy and optical coherence tomography. However, SC spectra can also suffer from significant fluctuations in amplitude and it is important that the SC source is generated with low noise and with high spectral flatness. Recently, tellurite glasses have been employed for MOFs instead of silica glasses due to their high nonlinearity, wide transmission in the mid-IR region, low attenuation and high thermal stability. Several efforts have been devoted to obtain tellurite MOFs with flattened dispersion profiles because they can enhance the nonlinear spectral broadening and the spectral flatness of SC generation. But, they require very complex fiber structures which are difficult to be fabricated.
In order to control chromatic dispersion profile of tellurite fibers in higher extent but simplify their fiber structure, our group proposed a tellurite hybrid microstructured optical fiber (HMOF). The fiber was successfully fabricated and its chromatic dispersion was tailored to be near-zero and flattened with three zero-dispersion wavelengths at 1270, 1973 and 3627 nm.

In this work, we study the suppression of chromatic dispersion fluctuation and the performance of FOPA in a tellurite hybrid microstructured optical fiber (HMOF) with buffer layer by taking into account the variation in the core diameter and fiber transverse geometry.

Outcome：

2022
Fiber-based optical parametric amplification (FOPA) is one of typical applications of four-wave-mixing (FWM) in highly nonlinear optical fibers which can transfer energy from one or two strong pump fields to a weak signal field and generate a new idler field. Since FOPA can provide broad gain bandwidths and high signal gain in many spectral bands where other types of optical amplifier cannot reach, it has been exploited for various applications such as signal amplification, wavelength conversion, phase-conjugation, slow and fast lights, optical signal processing and biomedical applications. However, high efficiency of FOPA performance is not easy to be obtained. It requires suitable pump sources, optical fibers with high nonlinearity and appropriate designs of chromatic dispersion which can satisfy the phase-matching condition. One of important technical issues for FOPA performance is that phase-matching condition for FWM in optical fibers is very sensitive to the fluctuation of chromatic dispersion which is caused by the fiber transverse geometry variation. A new fiber structure has been reported to suppress the chromatic dispersion fluctuation for silica fiber on the supposition that only the core diameter varies along the fiber length.
It was shown that tellurite HMOF with buffer layer is more advantageous to suppress the chromatic dispersion fluctuation and to improve the FOPA signal gain and bandwidth than conventional tellurite HMOF without buffer layer. By optimizing the diameters and refractive indices of the core and buffer layer so as to effectively suppress the chromatic dispersion fluctuation, HMOF with buffer layer can be a promising medium to overcome the influence of fiber transverse geometry variation on the performance of FOPA.

Currently, endoscopes that use visible-light imaging fibers are widely used to directly observe the inside of the human body. These tests and surgeries are less invasive for patients. However, the scope of application of endoscopes can be expanded if high-resolution infrared image transmission becomes possible. With infrared imaging, images inside blood vessels and thermography inside the human body can be obtained, leading to new diagnostic and therapeutic methods. Unfortunately, existing image fibers use materials with low infrared transparency, which limits their ability to transmit long-wavelength infrared light with high resolution. Therefore, this research project aims to fabricate an image fiber using a new glass material with high infrared transmittance. The ultimate goal is to achieve high-resolution infrared image transmission.

Outcome：

2025
In this fiscal year, we designed a clover-shaped structure for tellurite-based transversely disordered optical fibers (TDOFs) to suppress cluster formation and optimized the structure using beam propagation simulations. The fibers were then fabricated and evaluated through image transmission experiments. The results demonstrated the potential for improved resolution compared to conventional structures, providing guidelines for the realization of high-performance infrared image transmission fibers .

2024
In this study, a novel random cross-section optical fiber (FLOF: Four-Leaf Optical Fiber) was fabricated using tellurite glass, aiming for high-resolution imaging in the infrared region. Two types of glass with different refractive indices were randomly arranged while suppressing cluster formation. Successful propagation of laser light with a wavelength of 1.55 μm was demonstrated. This structure is expected to exhibit Anderson localization, indicating its potential for applications in high-definition image transmission.

2023
We succeeded in fabricating a chalcogenide multicore fiber. It was revealed that high-resolution image transmission is possible at a wavelength of 9.3 μm.

2022
In 2017, Stone et al. presented a silica multicore fiber with low index contrast image thanks to a special structure where same sized cores would never be neighbors, decreasing significantly the crosstalk between cores. More recently, Tuan et al. successfully created a long (10cm) all-solid tellurite glass disordered fiber (TDOF) with good image transportation at different wavelength (1.4 to 1.6µm). It is, though, known that the problem of disordered fiber is that image transmission can be distorted due to oversized clusters of fibers of same refractive indices. By coupling these two research, this work tries to find how to obtain better results compared to the TDOF previously reported in view of coupling efficiency or image resolution by using a honeycomb-like network with different sized neighbor cores with controlled randomness.

2021
Programmation for beam pattern evolution of a new fiber structure was made and early interesting results started to appear

The importance of real-time current measurement is increasing with the rise in renewable energy generation. Optical fiber current sensors offer advantages such as remote measurement and no risk of electric shock. These sensors use materials with a high Verdet constant and a photoelastic coefficient of zero. Lead silicate glass has been the traditional material of choice, but its use is restricted due to regulations. The objective of this research is to find lead-free tellurite glass that can replace lead silicate glass.

Outcome：

2025
In this fiscal year, we developed tellurite glass optical fibers for current sensing by introducing an overclad structure to suppress cladding modes and achieve single-mode guidance. As a result, core-guided light propagation was confirmed, and a reduction in cladding modes was achieved. These results provide a foundation for realizing high-precision current sensors based on tellurite glass fibers.

2024
With the growing adoption of renewable energy, real-time current monitoring is essential for building smart grids. Optical fiber current sensors, known for their excellent electrical insulation and noise resistance, are attracting attention. However, the use of conventional lead glass is becoming difficult due to environmental regulations. This study focuses on lead-free, tellurite-based low-photoelastic glasses. The Verdet constant—a key magneto-optical property—was measured, and optical fibers were fabricated. Among various compositions, materials with high figures of merit were selected for fiber fabrication and propagation tests. While some issues remain in light confinement, the study demonstrated the potential of these environmentally friendly materials as viable alternatives.

2023
By searching for the glass compositions in TeO₂-ZnO-Li₂O-Bi₂O₃ system glasses, we found glass compositions with lower photorealistic constant than that of conventional glass materials. We fabricated single-mode fibers using these glasses and confirmed the rotation of polarized light by magnetic fields.

The aim of this study is to clarify the wavelength dependence of the nonlinear refractive index n_2 in chalcogenide and tellurite glasses. By systematically evaluating and comparing the nonlinear optical responses of these materials, the study seeks to provide guidelines for selecting suitable materials and optimizing excitation conditions at different wavelengths. In particular, the dispersion characteristics of n_2 will serve as fundamental data for achieving efficient wavelength conversion and supercontinuum generation in the mid-infrared region.

Outcome：

2025
In this fiscal year, we extended our study from chalcogenide glasses to tellurite glasses and carried out a detailed investigation of the effects of thermal processes on the nonlinear refractive index. In particular, we focused on the local temperature rise induced by intense pulsed irradiation and evaluated how its temporal scale and spatial distribution influence the nonlinear optical response. The results revealed that, under high-intensity conditions, contributions from thermal effects such as thermal lensing cannot be neglected in addition to the electronic response, leading to a reduction in the effective nonlinear refractive index and, in some cases, even a negative value. These findings indicate that the nonlinear optical response in tellurite glasses should be understood as a composite phenomenon that includes not only intrinsic electronic nonlinearity but also thermal contributions. Such insights provide important guidelines for the design of mid-infrared devices using high-power light sources and for the evaluation of nonlinear optical materials.

2024
This study evaluated the nonlinear refractive index n_2 of Ge-As-Se chalcogenide glass (ChG) in the mid-infrared region using the Z-scan method. A threefold variation was observed across 2600–5000 nm, with a peak near 3500 nm. Simulations showed that supercontinuum (SC) generation bandwidth is maximized when pumped at this peak. These findings are valuable for mid-IR broadband light source design.

The objective of this study is to explore a new fabrication method for transversely disordered optical fibers (TDOFs), which are attracting attention for infrared image transmission, by utilizing phase-separated glass. Conventional TDOFs require complex and time-consuming fiber drawing processes. This study aims to simplify the fabrication process and achieve high-performance structures by harnessing phase separation phenomena. To this end, the research investigates glass compositions that exhibit phase separation and evaluates their thermal stability and crystallization behavior to identify materials suitable for fiber fabrication.

Outcome：

2025
In this study, we investigated the applicability of phase-separated glasses in the TeO₂-B₂O₃-Ga₂O₃ system for the fabrication of TDOFs for infrared image transmission. The results revealed compositional characteristics that promote phase separation, while also showing that the thermal and mechanical properties of these glasses remain insufficient for fiber drawing. These findings indicate the need for material design that can simultaneously achieve phase separation and suitable fiber-drawing performance.

2024
This study explores a new method for efficiently fabricating transversely disordered optical fibers (TDOFs) for infrared image transmission using phase-separated glass. Focusing on the TeO₂–B₂O₃–Ga₂O₃ system, phase separation was confirmed through striation patterns and compositional analysis. Adjusting the glass composition and melting conditions yielded thermally stable, phase-separated candidate.

The aim of this study is to explore glass compositions based on Ga₂O₃ that exhibit low phonon energy, are suitable for fiber fabrication, and are resistant to surface degradation caused by deliquescence. From the perspective of materials strategy, the study also aims to develop novel compositions that could serve as alternatives to fluoride and tellurite glasses, which contain scarce elements.

Outcome：

2025
In this study, building on the promising compositions identified in the previous year for the Cs₂O–Ga₂O₃ glass system, we further investigated compositional optimization and material properties to improve their suitability as fiber laser host materials. In particular, glasses with ZnO, Nb₂O₅, and Li₂O additives were systematically evaluated in terms of thermal stability (ΔT) and phonon energy. The results showed that all modified compositions exhibited improved resistance to crystallization compared to the base glass, indicating favorable characteristics for fiber drawing. In addition, Raman spectroscopy confirmed that these glasses maintain low phonon energies.
Furthermore, this year we focused on hygroscopicity, which is directly related to practical usability, and examined the time-dependent surface degradation through storage tests. As a result, compositions containing Li₂O, especially CG10Li, exhibited significantly suppressed surface deterioration compared to the conventional composition. These findings demonstrate that it is possible to design gallate glasses that simultaneously achieve low phonon energy, high thermal stability, and improved environmental durability.
Overall, the optimized compositions are promising candidates for fiber laser host materials, and future work will involve fiber fabrication and evaluation of laser performance.

2024
This study explored glass compositions for fiber laser applications using gallium oxide-based glasses. Additives were introduced to the cesium oxide and gallium oxide system to improve surface stability and thermal performance. The following compositions were found to be suitable: 30 mol% cesium oxide–65 mol% gallium oxide–5 mol% zinc oxide (CG5Zn), 30 mol% cesium oxide–65 mol% gallium oxide–5 mol% niobium pentoxide (CG5Nb), and 25 mol% cesium oxide–65 mol% gallium oxide–10 mol% lithium oxide (CG10Li). These glasses exhibited excellent thermal stability and low phonon energy, which are favorable for fiber drawing. Among them, CG10Li showed no visible surface degradation after storage under controlled indoor conditions. These compositions are promising alternatives to fluoride glasses and tellurite glasses as host materials for fiber lasers.