Kenshiro Nagasaka

Mid-infrared supercontinuum generation spanning 2.0 to 15.1 μm in a chalcogenide step-index fiber.

We experimentally demonstrate mid-infrared (MIR) supercontinuum (SC) generation spanning ∼2.0 to 15.1 μm in a 3 cm-long chalcogenide step-index fiber. The pump source is generated by the difference frequency generation with a pulse width of ∼170  fs, a repetition rate of ∼1000  Hz, and a wavelength range tunable from 2.4 to 11 μm. To the best of our knowledge, this is the broadest MIR SC generation observed so far in optical fibers. It facilitates fiber-based applications in sensing, medical, and biological imaging areas.

Coherent mid-infrared supercontinuum generation in all-solid chalcogenide microstructured fibers with all-normal dispersion

We report the coherent mid-infrared supercontinuum generation in an all-solid chalcogenide microstructured fiber with all-normal dispersion. The chalcogenide microstructured fiber is a four-hole structure with core material of AsSe2 and air holes that are replaced by As2S5 glass rods. Coherent mid-infrared supercontinuum light extended to 3.3 μm is generated in a 2 cm long chalcogenide microstructured fiber pumped by a 2.7 μm laser.

Coherence property of mid-infrared supercontinuum generation in tapered chalcogenide fibers with different structures

We have numerically investigated the coherence property of mid-infrared supercontinuum generation in tapered step-index chalcogenide fibers with different structures. The pump source is a 4 μm laser with pulse width of 500 fs and peak power of 1 kW. The length ratio is the ratio of transition region length near the laser input to the other transition region length near the output. We calculate the bandwidth and the spectrally averaged coherence of the supercontinuum spectra generated in fibers with different length ratios under the same pumping condition. Numerical results show that as the length ratio increases, the bandwidth decreases from 4.84 μm to 4.11 μm while the spectrally averaged coherence increases from 0.53 to 0.9 and then jitters near the maximum. The length ratio within 1–1.5 is preferable to keep a balance between bandwidth and coherence.

Supercontinuum generation in the normal dispersion regime using chalcogenide double-clad fiber

We experimentally demonstrate mid-infrared (MIR) supercontinuum (SC) generation in a 2.8-cm-long chalcogenide double-clad fiber (Ch-DCF). The Ch-DCF made of As2Se3, AsSe2, and As2S5 glasses has near-zero chromatic dispersion in the normal dispersion regime. We pump the Ch-DCF using a 200 fs pulse laser at 10 µm, and obtain MIR SC extending from 2 to 14 µm. Numerical results show that the Ch-DCF has the potential of generating a highly coherent MIR SC light source.

Broadband optical parametric amplifier formed by two pairs of adjacent four-wave mixing sidebands in a tellurite microstructured optical fibre

A broadband fibre-optical parametric amplifier (FOPA) operating at a novel wavelength region that is far from the pump wavelength has been demonstrated by exploiting two pairs of adjacent four-wave mixing (FWM) sidebands generated simultaneously in a tellurite microstructured optical fibre (TMOF). Owing to the large nonlinearity of the TMOF and the high pump peak power provided by a picosecond laser, a maximal average gain of 65.1 dB has been obtained. When the FOPA is operated in a saturated state, a flat-gain amplification from 1424 nm to 1459 nm can be achieved. This broadband and high-gain FOPA operating at new wavelength regions far from the pump offers the prospect of all-optical signal processing.

Highly efficient picosecond degenerate four-wave mixing in a tellurite microstructured optical fiber

Wavelength-tunable picosecond degenerate four-wave mixing was demonstrated in a tellurite microstructured optical fiber (TMOF). The zero-dispersion wavelength of the TMOF was shifted to 1570 nm by introducing a single ring of air holes in the cladding. The anti-Stokes signal sideband can be generated from 1490 to 1500 nm, and the Stokes idler sideband can emit from 1595 to 1645 nm. Because of the high nonlinearity of the TMOF and the large peak power of the picosecond pump, a maximal signal gain of 31.2 dB and an idler conversion efficiency of +35 dB were achieved.

Experimental investigation of mid-infrared supercontinuum generation in chalcogenide step-index optical fibers

We experimentally investigate mid-infrared (MIR) supercontinuum (SC) generation in chalcogenide step-index optical fibers. The pump source is generated by the difference frequency generation with a pulse width of ~170 fs, a repetition rate of ~1000 Hz, and a wavelength range tunable from 2.5 to 11 μm. The wide MIR SC will be applied in sensing, medical and biological imaging areas.

Far-detuned four-wave mixing for mid-infrared wavelength conversion in chalcogenide As2S5 suspended core fiber

The mid-infrared (MIR) range is of great interest because fundamental molecular vibrational absorption bands exist in the MIR range. In the MIR range, typically, lasing can be generated using quantum cascade lasers, cascaded Raman lasers, and optical parametric oscillators (OPOs). Recently, fiber OPOs (FOPOs) in the MIR range have received attention because of their flexibility of the parametric gain curve designing the chromatic dispersion. Chalcogenide glass is the promising candidate for MIR FOPO because of their wide transmission window and high nonlinear coefficient. In the present paper, we design the chromatic dispersion of four-hole As2S5 chalcogenide suspended core fiber (SCF), and demonstrate a far-detuned four-wave mixing (FWM) for MIR FOPO. We design the four-hole As2S5 chalcogenide SCF for far-detuned FWM using a ∼2 μm pump source. A four-hole As2S5 chalcogenide SCF which has a core diameter of 3.25 μm is fabricated using a homemade draw tower. We experimentally observed far-detuned FWM in the four-hole As2S5 chalcogenide SCF. A detuning frequency of over 80 THz is measured in 21 cm long fiber using a 2.7 ps pulse laser at 1.96 μm. The experimental observation was confirmed by numerical demonstration.

Mid-infrared frequency conversion via normal dispersion modulation instability in chalcogenide fibers

Mid-infrared frequency conversion via normal dispersion modulation instability in chalcogenide fibers has been numerically investigated by calculating the phase matching conditions and solving the generalized nonlinear Schrödinger equation. The core material of the chalcogenide fibers is As2Se3 and the cladding material is As2S5. Usually, the larger converted wavelength spacing between the pump and the far-detuned converted signal, the smaller gain. Therefore, the dispersion of the chalcogenide fibers are optimized to balance the gain and the converted signal wavelength spacing. The results show that the converted far-detuned mid-infrared signal can be tuned to 10 μm. The results also show that for a pumping source with the fixed wavelength, the far-detuned frequency conversion can be optimized by controlling the core size of step-index chalcogenide fibers. By using the simple step-index structure and controlling the core size of the chalcogenide fibers, the far-detuned mid-infrared frequency conversion can be achieved.

Modeling of dispersion flattened chalcogenide double clad fibers for mid-infrared light generation

The proper control of the chromatic dispersion in an optical fiber is one of the most important factors to enhance the nonlinear optical effect. Practically, a near-zero flattened chromatic dispersion is necessary to generate broadband supercontinuum (SC) generation and wideband parametric generation. Recent years, many researchers have demonstrated mid-infrared (MIR) SC and parametric generation in a chalcogenide fiber, because MIR light source has many applications, such as optical coherence tomography and metrology [1]. Chalcogenide glasses are the attractive candidate to obtain the MIR light because of a wide transmission window up to around 18 μm [2] and a high nonlinearity. One of the attractive fiber structures is microstructured optical fibers (MOFs) that can tailor chromatic dispersion in the MIR region. However, MOFs require high structural precision in both transverse and longitudinal directions to achieve a desired chromatic dispersion profiles. This will make it more difficult than all-solid fibres to fabricate desired fibers. Another advantage of all solid fibers is an availability of a combination of fiber materials. In this study, to gain the flexibility of structure design, we use chalcogenide double clad fiber (Ch-DCF) which is made of As2Se3, AsSe2, and As2S5 glasses.