Numerical investigation of a broadband coherent supercontinuum generation in Ga8Sb32S60 chalcogenide photonic crystal fiber with all-normal dispersion

All normal dispersion (ANDi) and highly nonlinear chalcogenide glass photonic crystal fiber (PCF) is proposed and numerically investigated for a broad, coherent and ultra-flat mid-infrared supercontinuum generation. The proposed PCF consists of a solid core made of Ga₈Sb₃₂S₆₀ glass surrounded by seven rings of air holes arranged in a triangular lattice. We show by employing the finite difference frequency domain (FDFD) method that the Ga₈Sb₃₂S₆₀ PCF dispersion properties can be engineered by carefully adjusting the air holes diameter in the cladding region and ANDi regime is achieved over the entire range of wavelengths with a zero chromatic dispersion around 4.5?μm. Moreover, we demonstrate that injecting 50?fs width and 20?kW peak power laser pulses (corresponding to a pulse energy of 1.06?nJ) at a pump wavelength of 4.5?μm into a 1?cm long ANDi Ga₈Sb₃₂S₆₀ PCF generates a broad, flat-top and perfectly coherent SC spectrum extending from 1.65?μm to 9.24?μm at the 20?dB spectral flatness. These results make the proposed Ga₈Sb₃₂S₆₀ PCF an excellent candidate for various important mid-infrared region applications including mid-infrared spectroscopy, medical imaging, optical coherence tomography and materials characterization.     

A new design of low-loss and ultra-flat zero dispersion photonic crystal fiber using hollow ring defect

New hollow ring defect structure is introduced in photonic crystal fiber design for ultra- flat zero dispersion with very low waveguide losses. The hollow ring defect consisted of a central hole surrounded by a doped silica ring provides highly flexible defect engineering capabilities in photonic crystal fibers to achieve precise control of dispersion value and dispersion slope while independently maintaining low waveguide losses, which was not attainable in previous designs. A nearly flat zero dispersion of D=0±0.51 ps/nm km was obtained in the wavelength range of 1.44–1.61 μm with the maximum slope of ?2.7×10^?2 ps/nm² km. The confinement loss was less than 5.75×10^?8 dB/m along with the bending loss of 2.8×10^?6 dB/m for the radius of 10 mm, and splice loss of less than 1.57 dB to conventional single mode fiber at 1.55 μm.     

Dispersion characteristic of hexagonal and square lattice chalcogenide As2Se3 glass photonic crystal fiber

In this paper, we report a chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) for dispersion compensating application. We have used the improved fully vectorial effective index method (IFVEIM) for comparing the dispersion properties (negative and zero dispersion) and effective area in hexagonal and square lattice of As₂Se₃ glass PCF using different wavelength windows. It has been demonstrated that due to their negative dispersion parameter and negative dispersion slope in wavelength range 1.2-2.5 μm, both lattice structures of As₂Se₃ glass PCFs, with pitch (Λ = 2 μm), can be used as dispersion compensating fibers. Further, design parameters have been obtained to achieve zero dispersion in these fibers. It is also shown that As₂Se₃ glass PCF provides much higher negative dispersion compared to silica PCF of the same structure, in wavelength range 1.25-1.6 μm and hence such PCF have high potential to be used as a dispersion compensating fiber in optical communication systems.     

Highly nonlinear photonic-crystal fibers for the spectral transformation of Cr: forsterite laser pulses

We demonstrate novel photonic-crystal fibers (PCFs) fabricated of a highly nonlinear glass. Dispersion profiles and nonlinearity of these fibers are tailored with an array of submicron holes in the fiber core. With the PCF structure designed to provide a nonlinearity on the order of 10³ W⁻¹ km⁻¹ at the radiation wavelength of 1 μm and a fundamental-mode dispersion profile with zero group-velocity dispersion around 1.19 μm, unamplified femtosecond Cr: forsterite laser pulses are efficiently frequency-converted into the 540-1000-nm wavelength range through solitonic spectral-transformation mechanisms and four-wave mixing.     

Phase-matched four-wave mixing of isolated waveguide modes of high-intensity femtosecond pulses in a hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers with a special dispersion profile are shown to allow phase-matched nonlinear optical interactions of isolated air-guided modes of high-intensity femtosecond laser pulses confined in the hollow fiber core. We present theoretical and experimental studies of the four-wave mixing of fundamental and second-harmonic pulses of a Cr:forsterite laser with an initial pulse duration of about 50 fs and an intensity on the order of 10¹⁴ W/cm² in waveguide modes of a hollow photonic-crystal fiber with a core diameter of about 13μm.     

New nonlinear and dispersion flattened photonic crystal fiber with low confinement loss

A new nonlinear dispersion flattened photonic crystal fiber with low confinement loss is proposed. This fiber has threefold symmetry core. The doped region in the core and the big air-holes in the 1st ring can make high nonlinearity in the PCF. And the small air-holes in the 1st ring and the radial increasing diameters air-holes rings in cladding can be used to achieve the dispersion properties of the PCF. We can achieve the optimized optical properties by carefully selecting the PCFs structure parameters. A PCF with flattened dispersion is obtained. The dispersion is less than 0.8 ps/(nm km) and is larger than −0.7 ps/(nm km) from 1.515 μm to 1.622 μm. The nonlinear coefficient is about 12.6456 W⁻¹ km⁻¹, the fundamental mode area is about 10.2579 μm². The confinement loss is 0.30641 dB/km. This work may be useful for effective design and fabrication of dispersion flattened photonic crystal fibers with high nonlinearities.     

Stimulated four-photon mixing and efficient polychromatic visible light generation in SiO2−based single-mode optical fibers pumped at 1.319 μm

Abstract

The generation of near-infrared and intense visible light through stimulated multi-wave mixing processes in single-mode silica-based optical fibers pumped by a Q-switched and mode-locked Nd:YAG laser operated at 1.319 μm is described. The experimental results show that intense infrared light around 1.2 μm is produced via selp-hase-matched four-photon mixing at the minimum group velocity dispersion region of pure SiO₂?core and P₂O₅?doped silica fibers. In the visible spectral region, from 580 nm to 600 nm, 20 W peak power 100-ps pulses were generated by pumping single spans of single-mode P₂O₅?doped and undoped SiO₂?core fibers with 1.319-μm laser pulses. The signal light generated in such fibers propagated in the LP₀₂ fiber mode and exhibited a threshold power that depended upon the fiber length and a critical length that was power dependent. Also, it exhibited an asymmetrical spectrum of a few nanometers bandwidth, with a long tail toward high frequencies. For GeO₂?doped silica-based fibers, a multiple-wavelength visible signal propagating in several high-order fiber modes was generated.     

Yb3+-doped 200 μm diameter core, gain guided index-antiguided fiber

The lasing in an end-pumped gain guided index-antiguided (GG-IAG) Yb³⁺-doped silicate glass fiber with a 200 μm diameter core is demonstrated. Laser beams with similar beam propagation factors M ² and mode field diameters W ₀ (>160 μm) were observed at the output end of the GG-IAG fibers under different pump powers, which indicated that single mode behavior and excellent beam quality were achieved during propagation. Furthermore, the laser amplifier characteristics in the present Yb³⁺-doped GG-IAG fiber were also evaluated.     

All-glass leakage channel fibers with triangular core for achieving large mode area and low bending loss

A novel fiber design of large mode area (LMA) all-glass leakage channel fibers (LCF) with triangular core is proposed. The effective single mode behavior and bend characteristics of the fiber are numerically investigated. The designed LCFs can achieve LMA larger than 1400 μm², effective single mode operation and low bend loss by carefully choosing the diameters of the fluorine-doped silica rods.     

Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers

We report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to ?1.6 ps² is experimentally studied. Strongly positively chirped pulses from 4.3?ps (0.01?ps²) to 39?ps (2.5?ps²) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8?ps (?0.07 ps²) to 3.9?ps (?1.6 ps²). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped-pulse regimes are discussed.     

Extremely large birefringence and shifting of zero dispersion wavelength of photonic crystal fibers

Three different types of photonic crystal fibers have been investigated which promise very large birefringence. The first type fiber is band gap guiding, the second index guiding, while the third type is index guiding with high refractive index circular and elliptical regions in the innermost ring. The birefringence, group velocity dispersion, modal effective index and mode field area of these fibers have been numerically estimated by employing finite difference time domain method. When elliptical regions are introduced in the first and second rings with the combination of small circular regions, each of these proposed fibers exhibits large birefringence with shifted zero dispersion point. Among these three different types of fibers, the band gap guiding photonic crystal fiber promises the largest birefringence (～5.45×10^?2) reported so far. The value of the birefringence and group velocity dispersion of these fibers can be controlled by controlling the hole pitch. Largest birefringence is achieved with a specific value of hole pitch.     

Designing a graded index depressed clad non-zero dispersion shifted optical fiber for wide band transmission system

Non-zero dispersion shifted fibers (NZ-DSFs) find extensive use in wavelength division multiplexed (WDM) system as it reduces the non-linear effects like four-wave mixing (FWM) generation. A depressed clad graded index fiber with a central dip in the refractive index profile, as well as without dip, has been modeled to perform as an NZ-DSF using the spot size optimization technique. The performance characteristics of the proposed NZ-DSF have been studied by changing different fiber parameters; such as inner core radius (a), relative refractive index differences (Δ⁺), normalized end position of depressed clad (C), depression parameter (ρ), etc. for a given value of Petermann-2 spot size . By suitably adjusting the fiber parameters, the effective core areas (A_eff) as simulated here are very large (80 μm²) so that the effect of non-linearities upon them can be minimized. These NZ-DSFs have also been optimized for WDM transmission system. The dispersion slopes of the proposed fibers with and without dip have been estimated which are comparable with the reported results.     

Novel large negative dispersion and tunable zero dispersion wavelength of photonic crystal fiber

The article describes a novel doped CS₂ core photonic crystal fiber with high negative chromatic dispersion. The proposed design is simulated through a full-vector finite element method and anisotropic perfectly matched layers. The numerical results show that we can achieve a negative dispersion coefficient of ?5600 ps/(nm km) almost at the wavelength of 1.55 μm by carefully adjusting the proposed PCF structure parameters. The proposed PCF may have great potential applications in dispersion compensating, optical parametric amplification, and optical fiber communication.     

Nonlinear absorption of non-chain HF laser radiation in germanium

The transmission of high-power HF(DF) laser radiation through Ge single crystals of different quality has been experimentally and numerically investigated. Based on the experimental data for the lasing spectrum, the two-photon absorption coefficient in Ge is estimated to be K₂ =55±10 cmGW?1 at λ=2.8μm. The transmission dynamics of high-power radiation with λ=2.6?3 μm through germanium crystals is numerically simulated.     

Optimization of optical characteristics of In0.29Ga0.71As0.99N0.01/GaAs straddled nano-heterostructure

Designing of a nanoscale Quantum Well (QW) heterostructure with a well thickness of ～60?Å is critical for many applications and remains a challenge. This paper has a detailed study directed towards designing of In_0.29Ga_0.71As_0.99N_0.01/GaAs straddled nanoscale-heterostructure having a single QW of thickness ～60?Å and optimization of optical and lasing characteristics such as optical and mode gain, differential gain, gain compression, anti-guiding factor, transparency wavelength, relaxation oscillation frequency (ROF), optical power and their mutual variation behavior. The outcomes of the simulation study imply that for the carrier concentration of ～2?×?10¹⁸cm^?3 the optical gain of the nano-heterostructure is of 2100?cm^?1 at the wavelength is of 1.30?μm. Though the obtained gain is almost half of the gain of InGaAlAs/InP heterostructure, but from the wavelength point of view the InGaAsN/GaAs nano-heterostructure is also more desirable because the 1.30?μm wavelength is attractive due to negligible dispersion in the silica based optical fiber. Hence, the InGaAsN/GaAs nano-heterostructure can be very valuable in optical fiber based communication systems.     

New high negative dispersion photonic crystal fiber

A new high negative dispersion photonic crystal fiber is proposed. It has double-core structure. The inner core has a circle germanium-doped region. The outer core is formed by removing the 3rd ring air-holes around the core. There are two ring air-holes between the two cores, Diameter of the 1st ring air holes is bigger than that of the 2nd ring air-holes, this can make mode coupling between inner mode and outer mode and showed that the high negative PCF is the result of this structure characteristics. There are honeycomb photonic lattice in the PCF's cladding. The influence of the structure parameters deviated from the design those on the chromatic dispersion are evaluated. When the structure parameters Λ=1.50 μm, d_core=2.10 μm, d₁=0.90 μm, d₂=0.44 μm and d₃=1.04 μm, the dispersion coefficient D is −1320 ps/(nm·km) at 1550 nm. This is a new kind of chromatic dispersion compensation PCF.     

New values of pion-nucleon scattering lengths and f2

From fixed t dispersion relations we have obtained more accurate values for the pion nucleon coupling constant and s-wave scattering lengths, using our recent low energy πN angular distribution and total cross section data. The results are: f² = 0.0790 ± 0.0010, a₁ ? a₃ = 0.262 ± 0.004μ^?1, a₁ + 2a₃ = ? 0.014 ± 0.005μ^?1.     

Dispersion compensation in optical transmission systems using high negative dispersion chalcogenide/silica hybrid microstructured optical fiber

In this paper, a new hybrid microstructured optical fiber (H-MOF) based upon photonic bandgap (PBG) light guiding mechanism which can be used for dispersion compensation in optical transmission systems is designed and simulated. The H-MOF core is made up of silica glass and the holes in the cladding network are filled with As₂Se₃ chalcogenide glass. By selecting an appropriate geometrical parameters for the structure, the dispersion and confinement losses of the proposed H-MOF at 1.55 µm are calculated to be ?6700 ps/nm/km and 6?×?10^?4 dB/m, respectively. Relative dispersion slope (RDS) of the H-MOF at 1.55 µm is about 0.00347 nm^?1. The proposed H-MOF is suitable for use in wavelength division multiplexing and dispersion compensating systems in optical fiber transmission networks.     

Design and analysis of a photonic crystal fiber with four-hole unit

A novel photonic crystal fiber (PCF) based on a four-hole unit is proposed in order to meet the requirements of high birefringence, negative dispersion and confinement loss in fiber-optic communication. The proposed design has been simulated based on the full vector finite element method (FVFEM) and anisotropic perfectly matched layers (APML). Analysis results show that the proposed PCF can achieve a high birefringence to the order of 10⁻² at the wavelength of 1.55 μm, a large negative dispersion over a wide wavelength range and confinement losses lower than 10⁻⁹ dB/m simultaneously, which has important applications in polarization-maintaining (PM) fibers, single-polarization single-mode (SPSM) fibers, dispersion compensation fibers and so on.     

Measurements of CO2 in a multipass cell and in a hollow-core photonic bandgap fiber at 2 μm

Recently, hollow-core photonic bandgap fibers (HC-PBFs) for use in the 2 μm wavelength region have become available. We have employed tunable diode laser absorption spectroscopy (TDLAS) to quantify CO₂ in nitrogen, injected into a HC-PBF. Our spectrometer contains both an HC-PBF-based absorption cell and an astigmatic Herriott multipass gas cell. The Herriott cell was used for comparison with the HC-PBF cell. The HC-PBF cell’s sensitivity and limit of detection were calculated to be 3.5×10^?4 cm^?1?Hz^?1/2 and 59 ppm?m, respectively. To substantiate the spectrometer performance, a measurement was done in the Herriott cell probing a reference gas mixture with nominal 400 μmol/mol CO₂ in N₂. The spectrometric results were in good agreement with the reference value. The relative standard uncertainty of the spectrometric result was found to be at the ±2 % level.