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.     

Design of single polarization single mode dispersion compensating photonic crystal fiber

In this paper, we present a photonic crystal fiber based on hexagonal structure for improved negative dispersion as well as high birefringence in the telecom wavelength bands. It is demonstrated that it is possible to obtain negative dispersion coefficient of −712 ps/(nm km) and relative dispersion slope (RDS) perfectly match to that of single mode fiber (SMF) of about 0.0036 nm⁻¹ at the operating wavelength 1550 nm. The proposed fiber exhibits high birefringence of the order 2.11 × 10⁻² with nonlinear coefficient about 57.57 W⁻¹ km⁻¹ at 1550 nm. Moreover, it is confirmed that the designed fiber successfully operates as a single mode in the entire band of interest.     

Highly birefrigent photonic crystal fiber with high negative dispersion for broadband dispersion compensation

In this paper, we proposed a dual-enhanced core photonic crystal fiber (PCF) with high birefringence and ultra-high negative dispersion for dispersion compensation in a polarization maintained optical system. Using finite difference time domain (FDTD) method, we presented dispersion compensating PCF (DC-PCF) with negative dispersion between −1650 ps nm⁻¹ km⁻¹ and −2305 ps nm⁻¹ km⁻¹ in C-band and particularly −2108 ps nm⁻¹ km⁻¹ in λ = 1.55 μm wavelength. By this method, we can compensate dispersion in 124 km long span of a conventional single mode fiber (SMF) by 1 km-long of the DC-PCF at λ = 1.55 μm wavelength. Moreover, fundamental mode of the proposed PCF can induce birefringence about 3.5 × 10⁻³ at 1.55 μm wavelength.     

Maintaining single polarization and dispersion compensation with modified rectangular microstructure optical fiber

In this paper, we propose and numerically demonstrate a highly birefringent microstructure optical fiber which shows negative dispersion coefficient of about −288 to −550 ps/(nm km) covering S to L wavelength bands and −425 ps/(nm km) at the excitation wavelength 1550 nm. This proposed design successfully compensate the dispersion covering S to L communication bands ranging from 1460 to 1625 nm along with relative dispersion slope (RDS) perfectly matched to that of single mode fiber of about 0.0036 nm⁻¹. Apart from dispersion compensation, the designed MOF offers high birefringence of 2.94 × 10⁻² at 1550 nm and better compensation ratio with design simplicity due to circular air-holes in the fiber cladding.     

Interface properties of an O2 annealed Au/Ni/n-Al0.18Ga0.82N Schottky contact

We oxidized a Ni/Au metal bi-layer contact fabricated on HVPE Al_0.18Ga_0.82N from 373 K to 573 K in 100 K steps. In the range 1 kHz to 2 MHz, the Capacitance–Voltage–Frequency (C–V–f) measurements reveal a frequency dispersion of the capacitance and the presence of an anomalous peak at 0.4 V owing to the presence of interface states in the as deposited contact system. The dispersion was progressively removed by O₂ anneals from temperatures as low as 373 K. These changes are accompanied by an improvement in the overall quality of the Schottky system: the ideality factor, n, improves from 2.09 to 1.26; the Schottky barrier height (SBH), determined by the Norde [1] method, increases from 0.72 eV to 1.54 eV. From the Nicollian and Goetzberger model [2], we calculated the energy distribution of the density of interface states, N_SS. Around 1 eV above the Al_0.18Ga_0.82N valence band, N_SS, decreases from 2.3×10¹² eV⁻¹ cm⁻² for the un-annealed diodes to 1.3×10¹² eV⁻¹ cm⁻² after the 573 K anneal. Our results suggest the formation of an insulating NiO leading to a MIS structure for the oxidized Au/Ni/Al_0.18Ga_0.82N contact.     

A novel dispersion-shifted single mode optical fiber design with ultra-high-bit-rate and very low loss for long-haul communications

In this paper, a novel dispersion-shifted multi-clad optical fiber with very small bending loss and ultra-high bit-rate applicable for large capacity information transmission systems is presented. To decrease dispersion and higher-order dispersion effects at λ = 1.55 μm, a weighted pulse broadening factor and genetic algorithm (GA) optimization technique is used. Compared to the works reported previously, this method can precisely set the zero-dispersion wavelength. This kind of dispersion-shifted fibers has dispersion, dispersion slope, mode field diameter (MFD), effective area and quality factor within −1.40 × 10⁻⁴ to −8.44 × 10⁻² ps/km nm, 3.06 × 10⁻² to −4.07 × 10⁻² ps/km nm², 5.56−5.85 μm, 119.25−176.42 μm² and 3.49-5.27 at λ = 1.55 μm, respectively. Besides, by applying dispersion at λ = 1.55 μm as the cost function, dispersion of about 1.31 × 10⁻⁸ ps/km nm is obtained. Thus, this novel optical fiber can be used in long-haul high information-transmission capacity communication systems.     

Analysis of dispersions of coupled asymmetric subwavelength-diameter wires

We propose a novel waveguide based on a structure of coupled asymmetric subwavelength-diameter wires to realize a high negative dispersion. The waveguide consists of an optical nanofiber and a GaAs nanowire. The properties of the supermodes are calculated using finite elements method. The results show that the symmetric supermode in the coupled structure can exhibit a giant negative dispersion up to −4.5 × 10⁶ ps/nm/km.     

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.     

Investigation of monolayer dispersion of benzoic acid supported on the surface of H-titanate nanotubes

Benzoic acid (BA) can disperse spontaneously onto the surface of H-titanate nanotubes (HTNTs) in a sub-monolayer state by heating mechanical mixtures method. The structure of BA-HTNTs system has been characterized by X-ray diffraction (XRD), thermogravimetric (TG), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) in detail. The results show that the H-bond association structure among BA molecules collapses and the carboxyl groups react with the surface hydroxyl group of HTNTs to form a salt-like structure on the surface after dispersion. The monolayer dispersion capacity determined by XRD is ca. 0.305 g BA g⁻¹ HTNTs, which is lower than the utmost monolayer dispersion capacity 0.550 g BA g⁻¹ HTNTs calculated according to a model that the benzene ring in BA molecules is perpendicular to the surface of HTNTs. At the same time, the dispersion capacity is also measurement by the fit of C 1s XPS peak at various BA loadings at first time.     

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.     

Highly nonlinear birefringent photonic crystal fiber

We propose an index guiding highly nonlinear birefringent photonic crystal fiber (PCF). Using a full vectorial finite element method (FEM), we investigate the key propagation characteristics of the proposed design. We demonstrate that it is possible to design a simple PCF structure configuration with a birefringence in the order of 10⁻² and a nonlinear coefficient of 49 W⁻¹ km⁻¹ at the wavelength of 1.55 μm. It is demonstrated that two zero dispersion wavelengths can be achieved by the proposed design. Bending analysis and fabrication issues are also discussed thoroughly.     

Optical operation by elastooptical dispersion of Nd-containing oxide crystals

The dispersion of the piezooptical coefficients induced by external light for neodymium-containing NdGaO₃, K₅Nd(MoO₄)₄, α-RbNd(WO₄)₂, and NdAl₃(BO₃)₄ crystals was discovered. As a pumping laser we have used Nd:YAG laser possessing power density varying from 0.2 up to 1 GW/cm², pulse duration 15 ns and frequency repetition 10 Hz. The dispersion of the piezooptical coefficients was evaluated in the spectral range 450-910 nm. We have found that illumination by the laser pulses with power density up to 1 GW/cm² cause substantial changes of the piezooptical coefficients in these crystals. However, behaviour of the obtained dependences is substantially different for all the compounds. The possible origins of the observed effects are considered.     

A broadband ultra flattened chromatic dispersion microstructured fiber for optical communications

A double-cladding microstructured fiber (MF) is proposed in this paper. The inner cladding of this optical fiber is composed of elliptical air holes and silica. The dependence of dispersion on the diameter of the air holes, the pitch, and the axes of the elliptical holes is investigated numerically. The proposed fiber possesses an ultra flattened dispersion curve over a wide wavelength range, and its dispersion value is small. The effective mode area is approaching to 60 μm², and the confinement loss is as low as <0.025 dB/km at 1550 nm. While choosing suitable structure parameters, an ultra dispersion-flattened MF within a broadband from1000 nm to 1900 nm can be achieved. The dispersion fluctuation is 0.6-1.0 ps/(nm·km) in all S, C and L band.     

Design and characterization of single mode circular photonic crystal fiber for broadband dispersion compensation

In this paper, we present a single mode circular photonic crystal fiber (C-PCF) for broadband dispersion compensation covering 1400 to 1610 nm wavelength band over the telecommunication windows. Investigations of guiding properties are carried out using finite element method (FEM) with circular perfectly matched layer boundary condition. Numerical study reveals that a negative dispersion coefficient of about −386.57 to −971.44 ps/(nm km) is possible to obtain over the wavelength ranging from 1400 to 1610 nm with a relative dispersion slope (RDS) of about 0.0036 nm⁻¹ at 1550 nm wavelength. In addition, the single mode behaviour of C-PCF is demonstrated by employing V parameter. According to simulation, it is found that the proposed C-PCF acts as a single mode fiber within 1340 to 1640 nm wavelength. Moreover, effective dispersion, relative dispersion slope, birefringence and confinement loss are also presented and discussed.     

Impact of extinction ratio of single arm sin LiNbO3 Mach-Zehnder modulator on the performance of 10 and 20 Gb/s NRZ optical communication system

We investigate the impact of extinction ratio of single arm sin² LiNbO₃ Mach-Zehnder (MZ) amplitude modulator on the performance of 10 and 20 Gb/s single-channel optical communication system. For different fiber lengths, the system performance has been analyzed with the increase in the extinction ratio. The effect of variation in dispersion parameter has also been illustrated. The impact of extinction ratio (ζ), dispersion parameter and length of the fiber has been further optimized with minimum bit error rate (BER) at optimal decision threshold (10⁻⁹) for 10 and 20 Gb/s bit rate. It is found that the system gives optimum performance at extinction ratio (ζ) value 20 dB. The increase in the transmission distance from 468 km for 10 Gb/s to 532 km for 20 Gb/s has been reported, and 8 dB improvement in the Q value has been observed as the value of ζ is increased from 10 to 20 dB. At 20 Gb/s, the system gives optimum performance for dispersion parameter value only up to 4 ps/nm km; however, at 10 Gb/s the system can operate for dispersion values up to 14.3 ps/nm km. Further we investigate the self-phase modulation (SPM) effect for the increase in the input power. It is observed that the SPM effect is negligible below 3 dB m input power and it increases at higher power levels.     

Design and analysis of novel highly nonlinear photonic crystal fibers with ultra-flattened chromatic dispersion

The present article describes novel highly nonlinear photonic crystal fibers (HN-PCFs) with flattened chromatic dispersion and low confinement losses. The proposed design has been simulated based on the finite-difference method with anisotropic perfectly matched layers absorbing boundary condition. It is proved that the design novel HN-PCFs is obtained a nonlinear coefficient greater than 45 W⁻¹ km⁻¹ and low dispersion slope −0.009 ps/(nm².km) at 1.55 μm wavelength. In addition, results from numerical simulation show that the ultra-flattened dispersion of 0 ± 0.65 ps/(nm.km) can be obtained in a 1.36-1.62 μm wavelength range with confinement losses lower than 10⁻⁷ dB/m in the same wavelength range. Another advantage of the proposed HN-PCFs is that it possessed modest number of design parameters.     

Determination of nonlinear coefficient and group-velocity-dispersion of bismuth-based high nonlinear optical fiber by four-wave-mixing

Nonlinear coefficient and group-velocity-dispersion of bismuth-based nonlinear fibers were determined by four-wave-mixing measurements. The wavelength dependence of refractive index of bulk bismuth-based glasses was also measured to estimate the material dispersion and waveguide dispersion. A newly developed bi-directional four-wave-mixing configuration enabled us to determine all fiber parameters simultaneously. The obtained fiber nonlinearity γ ∼ 1000 W⁻¹ km⁻¹ of bismuth-based nonlinear fiber is the highest one in the step-index fiber made of oxide glasses as expected from a high refractive index at 1550 nm. Dispersion analysis reveals that the both material dispersion and waveguide dispersion affect to the large group-velocity-dispersion of bismuth-based nonlinear fiber.     

Large negative dispersion in dual-concentric-core photonic crystal fiber with hybrid cladding structure based on complete leaky mode coupling

Considering the optical stability of solution, the sugar-solution is infused into the outer core ring of dual-concentric-core photonic crystal fiber (DCCPCF). The influences of structure parameters and solution concentration on the phase and loss matching are comprehensively analyzed. By choosing the appropriate outer core mode to completely couple with the inner core fundamental mode, the large negative dispersion PCF around 1.55 μm is designed, which has the dispersion value of − 39,500 ps/km/nm as well as bandwidth of 7.4 nm and effective mode area of 28.3 μm². The designed PCF with hybrid cladding structure can effectively compensate the positive dispersion of conventional single mode fiber, and suppress the system perturbation caused by a series of nonlinear effects. Considering the mode field mismatching between the DCCPCF and the tapered fiber, the calculated connection loss around 1.55 μm is below 3 dB. In addition, the equivalent propagation constants of two leaky modes are deduced from the coupled-mode theory, and the complete mode coupling case can be well predicted by comparing the real and imaginary parts of propagation constants.     

Polarization maintaining holey fibers for residual dispersion compensation over S + C + L wavelength bands

We report on a highly birefringent holey fiber for broadband dispersion compensation covering the S, C, and L telecommunication bands i.e. wavelength ranging from 1460 to 1625 nm. The finite element method with circular perfectly matched layer boundary condition is used to investigate the guiding properties. Numerical analysis demonstrates that it is possible to obtain negative dispersion coefficient of about −470 to −850 ps/nm/km over S to L-bands and a relative dispersion slope perfectly matched with single mode fiber (SMF) of about 0.0036 nm⁻¹ at 1550 nm. At the same time birefringence of the order 2.53 × 10⁻² is realized at 1550 nm wavelength. Owing to superior optical properties of the proposed holey fiber, this can be a promising candidate for broadband dispersion compensation and sensing applications.     

Integrative optimization of chirped mirrors for intracavity dispersion compensation

A new integrative optimization of three chirped mirrors for precise broadband intracavity group delay dispersion compensation in ring cavity is proposed. Simulation demonstrates that the residual group delay dispersion ripples of these three chirped mirrors are less than 25 fs² for most of the wavelengths from 600 to 1200 nm. Pulse evolution simulation shows that these three matched chirped mirrors compensate the group delay dispersion in the Ti:Sapphire ring cavity laser well to obtain an octave-spanning spectrum.