本研究では，全固体PBGFの伝搬特性を光によって動的に制御することで，一本のファイバに対して複数の伝搬特性を持たせ，光波制御応用の可能性を広げることを目的としている．

成果：

2022年度
　単一のコア，クラッド構造から成る従来型光ファイバは，光通信の伝送媒体，ファイバレーザ，光増幅器として幅広い分野で利用されている．一方，フォトニックバンドギャップファイバ(PBGF)は従来の全反射による光導波ではなく，フォトニックバンドギャップによる光導波を行うため，従来型光ファイバにはない様々な特性を持ち，センサや光波制御などに応用されている．特に全固体PBGFは希土類添加ファイバレーザの自然放出光制御，バンドパスフィルター，分散補償ファイバなど多くの応用の可能性があり，盛んに研究が行われている．動的特性制御の手法として，光カー効果による屈折率変化を利用し，非線形屈折率の高いテルライトガラスやカルコゲナイドガラスからなるファイバに関して，動的特性制御の実現可能性に対する検証を行った．コアの伝搬光とは別に，制御光を高屈折率ロッドに入射すると，1kW以上で伝搬波長域のシフトが顕著になることを初めて明らかにした．

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.

成果：

2023年度
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.

Multiple Raman peak properties in a 130 m tellurite fiber is investigated using different pump sources.

成果：

2022年度
When a nanosecond laser operated at ～1545 nm was used as the pump source, second-order Raman shift with multiple Raman peaks was observed. Especially for the first-order Raman shift, six obvious Raman peaks were obtained. To the best of our knowledge, this was the first demonstration of multiple Raman peaks in tellurite fibers. When a picosecond laser operated at ～1064 nm was used as the pump source, second-order Raman shift with only dual Raman peaks was observed, which can be applied to tunable lasers and amplifiers. Furthermore, the influence of fiber length on the evolution of Raman shift was investigated using a 2.5 m tellurite fiber. The reduction in fiber length led to a lower Raman gain, which induced only dual Raman peaks in each order Raman shift.

The effect of third-order dispersion on the coherence properties of supercontinuum spectra has been numerically investigated by solving the generalized nonlinear Schrödinger equation and using the complex degree of coherence theory.

成果：

2023年度
First, the effect of third-order dispersion on the coherence properties of supercontinuum spectra generated by soliton fission and soliton self-frequency shift has been investigated. The results show that if the group velocity dispersion values at the pump wavelength are identical and under the identical pumping conditions, negative third-order dispersion can cause much larger intensity fluctuations and wavelength jitters at the red edge of generated solitons than positive third-order dispersion, resulting in higher coherence degradation. Then the effect of negative third-order dispersion on the coherence properties of supercontinuum spectra generated by soliton fission and soliton self-frequency shift compensation has also been investigated. When at low input laser peak powers, due to the combined effects of modulation instability, negative third-order dispersion and the second zero-dispersion wavelength, the coherence depending on the input laser peak power decreases steeply first and then increases. When at high input laser peak powers, the modulation instability, negative third-order dispersion and soliton collision significantly contribute to the coherence degradation. The second zero-dispersion wavelength cannot stabilize the red-shifted solitons.

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

成果：

2023年度
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.

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.

成果：

2023年度
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.

成果：

2023年度
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.

成果：

2023年度
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.

Here, we realize infrared image transmission fibers using non-silica glasses.

成果：

2022年度
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.

2023年度
Programmation for beam pattern evolution of a new fiber structure was made and early interesting results started to appear

中赤外アップコンバージョン方は感度，応答速度，解像度などの点ですぐれた可視イメージングセンサを用いて，中赤外イメージを得る方法として有望である．しかし，変換後の可視光が微弱であるという課題がある．この課題を解決するために，本研究では中赤外アップコンバージョンゴーストイメージングを提案し，その実現可能性について検討することを目的とする．

成果：

2023年度
数値シミュレーションにより光源として，単一波長コヒーレント光，単一波長インコヒーレント光，多波長インコヒーレント光を用いた場合の中赤外アップコンバージョンゴーストイメージングについて検討した．単一波長コヒーレント光を用いた場合と比較して，単一波長インコヒーレント光を用いた方が，干渉による分解能の劣化が少ないことを明らかにした．また，多波長光源を用いるとアップコンバージョンによる波長ごとの倍率の違いに由来する歪みが生じるが，画像再構成法としては圧縮センシング法を用いることで像の劣化を抑制できることを明らかにした．

再生可能エネルギー源による発電量の増加により，リアルタイムで電流を計測する重要性が増している．光ファイバ電流センサは感電や漏電の危険性がなく，遠隔からの計測が可能という利点を有する．光ファイバによる電流の検出は，電流に付随する磁場が光ファイバ中の直線偏光を回転させることを利用するため，ヴェルデ定数の大きな材料が光ファイバ電流センサに適する．変形や温度変化に起因する応力によって偏光の乱れが生じることから，光弾性係数がゼロである材料を使用する必要がある．これらの条件を満たし，熱的安定性が高い材料として，鉛シリケートガラスが用いられてきた．しかし，近年，各国の規制により鉛の使用は制限されるようになり，光ファイバ電流センサ用の無鉛ガラスの開発が強く望まれている．本研究では，鉛シリケートガラスの代替可能なテルライトガラスを見出すことを目的とする．

成果：

2023年度
TeO₂-ZnO-Li₂O-Bi₂O₃系ガラスの組成探索により，光弾性率が実用材料よりも低いガラス組成を複数見出した．それらのガラスを用いてシングルモードファイバを作製し，磁場による偏光の回転を確認した．