Normal Dispersion Profile Research Articles

Efficient mid-infrared dispersive wave generation through soliton breakup and cascaded Raman amplification in an axially varying fluorotellurite fiber

In this paper, we demonstrate efficient mid-infrared dispersive wave (DW) generation through soliton breakup and cascaded Raman amplification in an axially varying fluorotellurite fiber. The input part of the fluorotellurite fiber has two zero-dispersion wavelengths and the remaining part has an all normal dispersion profile. The pump source is a femtosecond fiber laser with an operational wavelength of 1980nm, which is located at the anomalous dispersion region between two zero-dispersion wavelengths and close to the second zero-dispersion wavelength of the fluorotellurite fiber. As the pump light is launched into the fluorotellurite fiber, the pump light (or a higher-order soliton) experiences a temporal breakup and large pulse broadening, which enables nearly complete conversion of input solitonic radiation into resonant nonsolitonic radiation in the DW regime. Meanwhile, the generated DWs are improved by more than two orders of magnitude via cascaded Raman amplification in the fluorotellurite fiber, resulting in the generation of efficient mid-infrared DWs peaked at 2700 nm with an ultrahigh power division ratio of ∼ 85% and a compressible pulse width of ∼ 61 fs. Our work presents a way to realize ultrahigh-efficiency mid-infrared coherent light generation.

Panda-type elliptical-core fiber with flat and low normal-dispersion at 1.5–2.5 µm

The panda-type polarization-maintaining fiber (PMF) with a heavily GeO2-doped elliptical core is proposed for the supercontinuum generation. An elliptical air hole is introduced into the core to tailor the dispersion and birefringence properties. The proposed fiber with an appropriate choice of parameters can exhibit a flat and near-zero normal-dispersion profile at 1.5–2.5 µm, and its birefringence reaches beyond 8.8 × 10−4 due to the combined effects of the stress-applying parts (SAPs) and elliptical core with an air hole. The linearly polarized flattened supercontinuum extending over one octave can be generated when the 0.1-ps linear-polarization pulse with the peak power of 100 kW at 1920 nm is injected into a 1-m-long proposed fiber and polarized along the principal axes.

The large-mode-area multicore fiber with low and flat normal dispersion profile at 1.5–2.3 μm

For the purpose of realizing the large-mode-area (LMA) fibers with low and flat normal group velocity dispersion (GVD) profiles, the approach based on the multicore fiber (MCF) consisting of identical cores is proposed. The contribution of the coupling between two adjacent cores to tailoring the GVD of the MCF can be analyzed by using the dispersive equation, which is evaluated by the coupled mode theory (CMT) and can describe the GVD relation between the MCF and single-core waveguide. Furthermore, the designed 19- and 37-core MCFs with the step-index waveguides exhibit extremely flat normal GVD profiles with values of close to 0 ps/nm·km in the wavelength range of 1500 – 2300 nm, while the effective mode area of the 37-core MCF can reach 440 μm2 at 1900 nm. For the case of the MCF based on the photonic crystal fiber (PCF) structure, the 7-core MCFs can offer the concave-shaped normal GVD profiles in the 1900-nm wavelength region, where the minimum GVD values are around −46 ps/nm·km, and the wavelengths of the dip center can shift from 1880 to 2110 nm by tuning the core spacing. The effective mode areas are increased beyond 170 μm2 at 1900 nm. The proposed LMA normal-dispersion MCFs have potential applications in high-power ultrashort-pulse fiber lasers and amplifiers.

Numerical simulation of supercontinuum generation based on the active nonlinear fiber

The supercontinuum (SC) generation (SCG) based on the active nonlinear fiber has attracted much attention due to its advantages in the energy amplification and spectral broadening of the output pulses. The active step-index single-mode fiber is proposed for the SCG. A flat normal-dispersion profile ranging from 1 240 nm to 2 550 nm can be obtained by tuning the germanium (Ge) concentration and core size, where the values of the dispersion slope are between −0.01 ps/nm2/km and 0.05 ps/nm2/km. Furthermore, the output pulse from the 5 m Er-doped normal-dispersion fiber (NDF) with g0=9 m−1 has a peak power of 2.8 kW and a 20 dB spectral bandwidth of 507 nm ranging from 1 262 nm to 1 769 nm by using the 1 550 nm and 1 ps Gaussian pulse with a peak power of 10 kW as the pumping source.

Investigation of tellurium-based chalcogenide double-clad fiber for coherent mid-infrared supercontinuum generation

Abstract Considerable investigations have been carried out for chromatic dispersion tailoring of chalcogenide glass fiber with various structures and different material compositions. Here we proposed a tellurium-based chalcogenide glass fiber with a dual-clad structure in a W-type refractive-index profile. The structure of the fiber based on Ge-As-Se-Te glasses was numerically analyzed in detail. The simulation results indicate that an all normal dispersion profile can be realized in this tellurium glass fiber with an optimized dual-cladding structure. Then the optimized tellurium chalcogenide fiber with an all-normal dispersion profile was fabricated by multi-extrusion method. Pumping with a 5 μm ultra-short pulsed laser (150 fs, 1 kHz), abroadband supercontinuum (SC) covering 1.5–11 μm can be generated in the 19 cm-long fiber. The coherence of SC was also investigated by numerical simulation. The result shows that a high coherent SC covering 3.5–10.5 μm can be generated in this all-normal dispersion fiber. In conclusion, all-normal dispersion profile can be realized easily in this tellurium-based glass fiber due to their large materials ZDWs.

Supercontinuum generation in chalcogenide double-clad fiber with near zero-flattened normal dispersion profile

We demonstrate supercontinuum (SC) generation in a 2.8-cm-long chalcogenide double-clad fiber (Ch-DCF). The calculated chromatic dispersion of the fundamental mode shows that the Ch-DCF has flattened chromatic dispersion, which is within −10 ± 10 ps/nm/km from 3.8 to 12.6 in the normal dispersion regime. The variation of SC spectra is investigated by changing the pump wavelength and pump peak power. The broadband SC spectra extending from 2 to 14 μm at the −40 dB level is observed when the pump wavelength of 10 μm and pump peak power of 1.3 MW. The SC generation is simulated by the scalar generalized nonlinear Schrödinger equation. The simulation results are mostly the same as the experimental results and show that the supercontinua generated in the Ch-DCF are highly coherent.

Numerical simulation of coherent visible-to-near-infrared supercontinuum generation in the CHCl3-filled photonic crystal fiber with 1.06 μm pump pulses

The hollow-core photonic crystal fiber (PCF) is filled by highly nonlinear liquid chloroform (CHCl3) in the center core. The CHCl3-filled PCF with an appropriate geometric parameters exhibits a normal dispersion profile in the visible-to-near-infrared (NIR) region, where the values of group velocity dispersion (GVD) in the vicinity of 1060-nm pump wavelength can be tuned in the range from −20 to −50ps/nm/km. Furthermore, the nonlinear parameters at wavelengths <1100nm are higher than 0.7W−1m−1, which can lead to a significant enhancement of the spectral broadening, especially on the blue side. The numerical results show that, when 100-fs pulses centered at 1060nm are launched in the 0.01m-long fiber (47kW peak power), the generated supercontinuum (SC) spectra with high degree of coherence can cover near 2 octaves spanning from 340 to 1360nm at −20dB level. The spectral variations are < 5dB in the wavelength range of 365–1315nm.

Extending of flat normal dispersion profile in all-solid soft glass nonlinear photonic crystal fibres

The bandwidth of coherent supercontinuum generated in optical fibres is strongly determined by the all-normal dispersion characteristic of the fibre. We investigate all-normal dispersion limitations in all-solid oxide-based soft glass photonic crystal fibres with various relative inclusion sizes and lattice constants. The influence of material dispersion on fibre dispersion characteristics for a selected pair of glasses is also examined. A relation between the material dispersion of the glasses and the fibre dispersion has been described. We determined the parameters which limit the maximum range of flattened all-normal dispersion profile achievable for the considered pair of heavy-metal-oxide soft glasses.

Heavily Germanium-Doped Silica Fiber With a Flat Normal Dispersion Profile

A heavily germanium-doped (Ge-doped) silica fiber with a four-layer refractive index profile is proposed to obtain all normal flat dispersion property. The waveguide dispersion in the fiber can be modified by adjusting the fiber parameters, including the refractive indices and the core radiuses. As a result, the flat normal dispersion in the fiber can be obtained in the wavelength range of 1540–2600 nm, where the values of the dispersion slope are between $-$ 0.0058 and 0.03 $\hbox{ps/nm}^{2}/\hbox{km}$ . Furthermore, the numerical results show that the flat-top supercontinuum spectrum ranging from 1000 to 2600 nm can be generated by launching pump pulses at the wavelength of 1550 nm in the heavily Ge-doped fiber with a four-layer refractive index profile.

Coherent supercontinuum generation up to 2.3 µm in all-solid soft-glass photonic crystal fibers with flat all-normal dispersion.

Supercontinuum spanning over an octave from 900 - 2300 nm is reported in an all-normal dispersion, soft glass photonic crystal fiber. The all-solid microstructured fiber was engineered to achieve a normal dispersion profile flattened to within -50 to -30 ps/nm/km in the wavelength range of 1100 - 2700 nm. Under pumping with 75 fs pulses centered at 1550 nm, the recorded spectral flatness is 7 dB in the 930 - 2170 nm range, and significantly less if cladding modes present in the uncoated photonic crystal fiber are removed. To the best of our knowledge, this is the first report of an octave-spanning, all-normal dispersion supercontinuum generation in a non-silica microstructured fiber, where the spectrum long-wavelength edge is red-shifted to as far as 2300 nm. This is also an important step in moving the concept of ultrafast coherent supercontinuum generation in all-normal dispersion fibers further towards the mid-infrared spectral region.

Supercontinuum-based 10-GHz flat-topped optical frequency comb generation

The generation of high-repetition-rate optical frequency combs with an ultra-broad, coherent and smooth spectrum is important for many applications in optical communications, radio-frequency photonics and optical arbitrary waveform generation. Usually, nonlinear broadening techniques of comb-based sources do not provide the required flatness over the whole available bandwidth. Here we present a 10-GHz ultra-broadband flat-topped optical frequency comb (> 3.64-THz or 28 nm bandwidth with ~365 spectral lines within 3.5-dB power variation) covering the entire C-band. The key enabling point is the development of a pre-shaping-free directly generated Gaussian comb-based 10-GHz pulse train to seed a highly nonlinear fiber with normal dispersion profile. The combination of the temporal characteristics of the seed pulses with the nonlinear device allows the pulses to enter into the optical wave-breaking regime, thus achieving a smooth flat-topped comb spectral envelope. To further illustrate the high spectral coherence of the comb, we demonstrate high-quality pedestal-free short pulse compression to the transform-limited duration.

Parabolic pulse generation in tapered silicon fibers

Numerical simulations are used to investigate parabolic pulse generation in tapered silicon optical fibers. The simulations are based on a realistic silicon fiber taper with micrometer-sized core dimensions, designed to have a decreasing normal dispersion profile, and include the effects of linear and nonlinear absorption. Evolution to the parabolic regime is quantified by a misfit parameter, and the optimum values for the input pulse and taper length are established. The results identify a distortion-free nonlinear propagation window for the transmission of optical pulses in normal dispersion silicon waveguides.