Hiroyasu Kawashima

Mid-infrared supercontinuum generation in a suspended-core As2S3 chalcogenide microstructured optical fiber.

We demonstrate the supercontinuum (SC) generation in a suspended-core As(2)S(3) chalcogenide microstructured optical fiber (MOF). The variation of SC is investigated by changing the fiber length, pump peak power and pump wavelength. In the case of long fibers (20 and 40 cm), the SC ranges are discontinuous and stop at the wavelengths shorter than 3500 nm, due to the absorption of fiber. In the case of short fibers (1.3 and 2.4 cm), the SC ranges are continuous and can extend to the wavelengths longer than 4 μm. The SC broadening is observed when the pump peak power increases from 0.24 to 1.32 kW at 2500 nm. The SC range increases with the pump wavelength changing from 2200 to 2600 nm, corresponding to the dispersion of As(2)S(3) MOF from the normal to anomalous region. The SC generation is simulated by the generalized nonlinear Schrödinger equation. The simulation includes the SC difference between 1.3 and 2.4 cm long fiber by 2500 nm pumping, the variation of SC with pump peak power in 2.4 cm long fiber, and the variation of SC with pump wavelength in 1.3 cm long fiber. The simulation agrees well with the experiment.

Mid-infrared 2000-nm bandwidth supercontinuum generation in suspended-core microstructured Sulfide and Tellurite optical fibers

In this work, we report the experimental observation of supercontinua generation in two kinds of suspended-core microstructured soft-glass optical fibers. Low loss, highly nonlinear, tellurite and As2S3 chalcogenide fibers have been fabricated and pumped close to their zero-dispersion wavelength in the femtosecond regime by means of an optical parametric oscillator pumped by a Ti:Sapphire laser. When coupled into the fibers, the femtosecond pulses result in 2000-nm bandwidth supercontinua reaching the Mid-Infrared region and extending from 750 nm to 2.8 µm in tellurite fibers and 1 µm to 3.2 µm in chalcogenide fibers, respectively.

Ultrabroad supercontinuum generation through filamentation in tellurite glass

Although soft glass such as tellurite or chalcogenide glass is transparent in the range of 3–6 μm, fiber made of it is difficult to generate as a supercontinuum (SC) to that range because of the high loss, the wavelength limit of the pump source, and the challenges in light-coupling. To circumvent these problems, we developed an SC light source by using tellurite bulk glass through filamentation. For this scheme, the optical path length in the glass is very short due to the adopted high pump power, so the negative influence of material loss is reduced greatly. The light-coupling is straightforward, and the coupling efficiency is high. The bulk glass for SC generation is cheap, and can be fabricated easily. We have shown that under suitable pump conditions, the SC generation by filamentation can cover from visible to 6 μm. It is the broadest SC generation by tellurite (including glass and fiber). For suitable pump conditions, the glass was free of optical breakdown. If the interface reflections were deducted, the SC conversion efficiency was 87%; the SC conversion efficiency was stable. To the best of our knowledge, this is the first report on filamentation in tellurite glass which has a comparatively small bandgap.

Five-Octave-Spanning Supercontinuum Generation in Fluoride Glass

An ultra-broadband supercontinuum (SC) covering 0.2–8.0 µm which is more than 5 octaves, is demonstrated in a piece of fluoride glass. The 20 dB bandwidth spans from 0.39 to 7.4 µm. The filaments, bright conic visible emission, and interference fringe pattern are observed. The glass thickness, optical path, and pump conditions are optimized to enable the SC to cover the entire transmission range of the glass, which includes the whole functional group region of chemical bands and groups. The SC conversion efficiency is confirmed to be stable.

Widely tunable second-harmonic generation in a chalcogenide–tellurite hybrid optical fiber

When a chalcogenide-tellurite hybrid optical fiber with a high refractive index difference Δn=0.24 is pumped by an optical parametric oscillator with a pump wavelength from 1700 to 3000 nm, widely tunable second-harmonic generation (SHG) from 850 to 1502 nm is obtained. The observation of SHG is primarily due to the surface nonlinearity polarization at the core-cladding interface and the second-harmonic signal remains stable at the maximal level throughout the laser pulse irradiation.

Fabrication of a Chalcogenide-Tellurite Hybrid Microstructured Optical Fiber for Flattened and Broadband Supercontinuum Generation

A chalcogenide-tellurite hybrid microstructured optical fiber is designed and fabricated by the rod-in-tube drawing technique. The core is made of chalcogenide glass with a composition of 15Ge-3Ga-12Sb-70S (atm%, GGSS) and the cladding is made of tellurite glass with a composition of 78TeO2-5ZnO-12Li2O-5Bi2O3 (mol%, TZLB). The loss is ~1.1 dB/m at ~2.0 μm. Zero dispersion wavelength of the HMOF is shifted down to ~2370 nm from GGSS material dispersion ~4600 nm. Supercontinuum (SC) generation is investigated by pumping a 1-cm-long HMOF with the wavelengths of ~2000, 2300, and 2600 nm from a tunable optical parametric amplifier system. Flattened and broadband SC generation from ~950 to 3350 nm with the intensity flatness of 6 dB bandwidth is obtained with the peak power of ~40 MW at the wavelength of ~2300 nm.

Dark-Square-Pulse Generation in a Ring Cavity With a Tellurite Single-Mode Fiber

We demonstrate the generation of dark square pulses in a ring cavity with a 207-m long tellurite single-mode fiber. The duration of the pulse is tuned by adjusting the linear phase delay bias in the cavity. We obtain the dark-square pulse with a maximum full width at half maximum of 112 ns. The dark pulse is formed by the combined actions of the bright-square-pulse generation, birefringence, and polarizing effect in the cavity. The tellurite fiber can simultaneously provide the high nonlinearity, high birefringence, and large normal dispersion, which dominate the formation of a dark pulse in the cavity.

Chalcogenide-tellurite composite microstructured optical fibre

We report on fabrication a composite microstructured optical fibre composed of highly nonlinear chalcogenide Ge-Ga- Sb-S glass core and tellurite TeO2-ZnO-Li20-Bi2O3 glass clad. We aimed at obtaining more flattened chromatic dispersion for pumping chalcogenide glass based optical fibre by a pulse laser at current telecommunication wavelengths, i.e. λ = 1.35 - 1.7 μm, which is difficult to achieve by using a single material chalcogenide fibers due to their high refractive index (n > 2.1). A fibre design exploiting a composite of two glasses and one ring of the air holes brings similar options for tuning the fibre dispersion such as use of complex multi rings of air-holes approach. A good choice of glasses, allows for fabricating a composite chalcogenide-tellurite optical fibre benefiting from high nonlinearity of chalcogenide core glass but exploiting a tellurite glass technology and fibre drawing. In the paper, we discuss some aspects of CMOF design concerning current chalcogenide and tellurite glass choice. Also, we show the supercontinuum spectra recorded from current chalcogenide-tellurite CMOF pumped with a custom made femtosecond fibre laser at λ = 1.55 μm with the pulse duration of 400 fs.

Filamentation and supercontinuum generation in tellurite glass

Though soft glass such as tellurite or chalcogenide glass is transparent in the range of 3-6 μm, fiber made of it is difficult to generate supercontinuum (SC) to that range because of the high loss, the wavelength limit of pump source, and the challenges in light-coupling. To circumvent these problems, we developed SC light source by using tellurite bulk glass through filamentation. For this scheme, the optical path length in the glass is very short due to the adopted high pump power, so the negative influence of material loss is reduced greatly. The light-coupling is straightforward, and the coupling efficiency is high. The bulk glass for SC generation is cheap, and can be fabricated easily. We adopted a pump wavelength of 1600 nm which is comparatively long. Such a long pump wavelength ensures a large energy ratio of the glass’s bandgap to the incident photon, so the disadvantage of small bandgap of tellurite glass is reduced, and the twophoton absorption is avoided as well. We have shown that under suitable pump condition, the SC generation by filamentation can cover from visible to 6 μm. It is the broadest SC generation by tellurite (including glass and fiber). For the suitable pump condition, the glass was free of optical breakdown. If the interface reflections were deducted, the SC conversion efficiency was 87%. The SC conversion efficiency was stable. To the best of our knowledge, this is the first report on filamentation in tellurite glass which has a comparatively small bandgap.

100-Nanosecond-Level Square-Pulse Generation in a Ring Cavity with a Tellurite Single-Mode Fiber

We demonstrate the square-pulse generation in a ring cavity with a 207 m tellurite single-mode fiber. The stable square pulse with the wide full width at half maximum (FWHM) of 118 ns is formed by the nonlinear polarization switching in the cavity. The high nonlinearity and birefringence of the tellurite fiber play the key role during the square-pulse formation and the net normal dispersion is advantageous for stabilizing the square pulse. When the linear phase delay bias is set close to the polarization switching point, the third-order harmonic pulses and multiple packets are observed under different pump powers.