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.     

Theoretical studies on frequency doubling in glass optical fibers in constant-intensity approximation

Theoretical consideration of the second harmonic generation in the constant-intensity approximation with account of the inverse effect of the excited wave on a stimulation wave through account of the phase change of interaction waves is presented. The behavior of harmonic wave intensity in a fiber for different parameters of the problem has been studied, the self-action of the light both in glass optical fiber and inside the homogeneous medium has been analyzed. In this approximation a decrease of conversion efficiency and a change of parameters of the curves of synchronism versus the pump intensity, in particular, the change of positions of intensity minima that does not take place in the constant-field approximation, are observed. The propagation of a plane wave packet in the homogeneous medium with quadratic nonlinearity and in an optical fiber accompanied by the self-phase modulation and leading to the change of the Gaussian pulse spectrum is analyzed with account of the phase changes of all the interacting waves.     

Tunable microwave photonic filter based on chirped fiber gratings working with a single optical carrier at constant wavelength

This paper presents a tunable transversal filter working with a single optical carrier at constant wavelength. The filter consists of a set of chirped gratings whose time delay is tuned with respect to the emission wavelength of a fiber laser by a piezoelectric actuator; extra lengths of fiber are inserted in the filter arms in order to avoid interferences between signals reflected in different gratings. Two and three taps filters are experimentally demonstrated, the filters transfer function is electronically tuned within the free spectral range.     

Ultra-flat and broadband two-pump optical parametric amplifiers using a single-section highly nonlinear fiber

A broadband two-pump optical parametric amplifier with ultra-flat gain spectra is proposed in a single-section highly nonlinear fiber. By elaborately setting the dispersions and pump wavelength space, a gain over 250 nm with 0.02-dB uniformity is obtained. The pump polarizations and fiber length can be changed, achieving polarization-insensitive or higher gain, while the flatness and bandwidth of gain curve remain the same.     

Impact of chromatic dispersion and group delay ripple on self-phase modulation based optical regenerators

We demonstrate that chromatic dispersion and group delay ripple limit the performance of all-optical regeneration based on self-phase modulation. The level of degradation depends in detail on the type of phase distortions introduced. The regenerator performance is measured using the optical signal to noise ratio (OSNR) margin provided to the system for a given bit error ratio. The addition of chromatic dispersion and group delay ripple induces a significant reduction in this OSNR margin.     

Design and fabrication of an intrinsically gain flattened Erbium doped fiber amplifier

We report design and subsequent fabrication of an intrinsically gain flattened Erbium doped fiber amplifier (EDFA) based on a highly asymmetrical and concentric dual-core fiber, inner core of which was only partially doped. Phase-resonant optical coupling between the two cores was so tailored through optimization of its refractive index profile parameters that the longer wavelengths within the C-band experience relatively higher amplification compared to the shorter wavelengths thereby reducing the difference in the well-known tilt in the gains between the shorter and longer wavelength regions. The fabricated EDFA exhibited a median gain ?28 dB (gain excursion below ±2.2 dB within the C-band) when 16 simultaneous standard signal channels were launched by keeping the I/P level for each at −20 dBm/channel. Such EDFAs should be attractive for deployment in metro networks, where economics is a premium, because it would cut down the cost on gain flattening filter head.     

Residual stress profiles in optical fibers determined by the two-waveplate-compensator method

Stress-induced birefringence impacts the performance of many optical devices. Techniques are needed to measure accurately stress profiles in optical fibers. The two-waveplate-compensator (TWC) method allows the accurate measurement of small retardations. The full-field TWC method is applied here to measure the two-dimensional retardation distribution of single-mode fibers with a spatial resolution of 0.45 μm and a sensitivity of 0.07 nm. Axial stress profiles are hence determined along the axis of the fiber. The stress profiles determined with the TWC method are in good agreement with profiles previously reported in the literature while containing less noise and resolving more details.     

A Modified full-vectorial finite-difference beam propagation method based on H-fields for optical waveguides with step-index profiles

A modified full-vectorial finite-difference beam propagation method based on H-fields in solving the guided-modes for optical waveguides with step-index profiles is described. The propagation is split into two substeps. In the first substep, the field propagates in the absence of the cross-coupling terms, and then they are evaluated and double used in the second substep. An improved six-point finite-difference scheme is constructed to approximate the cross-coupling terms along the transverse directions. By using the imaginary-distance procedure, the field patterns and the normalized propagation constants of the fundamental modes for a buried rectangular waveguide and rib waveguide are presented, and the hybrid nature of the full-vectorial guided-modes is demonstrated. Solutions are in good agreement with the benchmark results from film mode matching method, which tests the validity and utility of the present method.     

Low loss, low dispersion and highly birefringent terahertz porous fibers

We demonstrate that porous fibers in addition to low loss and high confinement, have near zero dispersion for 0.5-1 THz resulting in reduced terahertz signal degradation compared to microwires. We also show for the first time that these new fibers can be designed, introducing asymmetrical sub-wavelength airholes within the core, to achieve high birefringence ≈0.026. This opens up the potential for realization of novel polarization preserving fibers in the terahertz regime.     

Non-intrusive polarization dependent loss monitoring in fiber-optic transmission systems

A method for non-intrusively monitoring the polarization dependent loss (PDL) of an installed fiber-optic transmission system is proposed using live dense wave division multiplexing (DWDM)-based traffic as the probing signal. The method extracts the statistical parameters of system PDL from the measured partial PDL data. Field measurements of PDL were performed on long-haul DWDM systems deployed in Sprint’s network and the results validated our theoretical model.     

Solutions of the full- and quasi-vector wave equations based on H-field for optical waveguides by using mapped Galerkin method

The mapped Galerkin method in solving the full-vector and quasi-vector wave equations in terms of transverse magnetic fields (H-formulation) for optical waveguides with step-index profiles is described. By transforming the whole x-y space onto a unit square and using two-dimensional Fourier series expansion, the modal distributions and propagation constants for optical waveguides are obtained in the absence of boundary truncation. Results for step-index circular fiber, buried rectangular waveguide, and optical rib waveguide are presented. Solutions are good agreed with exact solutions and numerical results by using vector nonlinear iterative method, Fourier operator transform method, and vector beam propagation method.     

Fabrication of gradient refractive index ball lenses using the method of combination of ion exchanging and sagging

Based on the Fick’s diffusion equations, the distribution function of refractive index of a gradient refractive index ball lens (GRIN ball lens/GBL) is derived. Lithium containing silicate glass is fabricated and GRIN ball lenses (GBLs) which diameters are from 0.3 mm to 3.0 mm are made by the method of combination of Ion exchanging and sagging in sodium nitrate. Refractive index profiles of these GBLs are measured by interferometer, and the performances such as effective focal length (EFL), back focal length (BFL) and numerical aperture (NA) between GBLs and homogeneous ball lenses (HBLs) are compared. Results show that the distribution of the index of refraction is parabolic curve and its Δn is about 0.0002, the performances of the former are super to the latter.     

Generalised dispersive phase and its effect on four wave mixing in fibers

A generalised method of calculating the efficiency of four wave mixing products in an optical fiber is presented in this paper. Apart from indicating the insufficiency of the existing theoretical model, this work brings out the importance of calculating the dispersive phase shift more precisely in combination with the nonlinear phase shift to predict the efficiencies of sideband generation. The experimental results for the generation of the first and second order sidebands are analysed. Contrary to expectations, higher powers and longer lengths of fiber do not result in larger number of sidebands. An attempt is made to understand this aspect as well as the experimentally observed fluctuations in the four wave mixing products.     

Design guidelines and characteristics for a kind of four-layer large flattened mode fibers

A theoretical method to analyze a kind of four-layer large flattened mode (LFM) fibers is presented. The properties of the fiber, including the fundamental and higher-order modal fields, effective area and bending loss are discussed by comparison. At the same time, the reasons for the different characteristics are considered. The obtained results indicate that the effective area of the four-layer LFM fiber is about 1.3 times larger than that of the conventional standard step-index fiber and the fiber can suppress the higher-order modes via bending effectively. The four-layer LFM fiber has less efficient bend-induced filtering ability than the conventional step-index fiber; however, it has more efficient filtering ability than the three-layer LFM fiber.     

Mode Exciting Properties of Photonic Crystal Fibres with the Optical Field Incident from a Single Mode Fibre

We numerically investigate the mode exciting properties of photonic crystal fibres by using the beam propagation method when the optical field is input from a traditional single mode fibre. The results show that both the excited mode spectrum and the coupling-efficiency of each excited mode depend on the normalized pitch А/λ and the normalized hole-size А/λ. Furthermore, we obtain the boundary profile of the optimizing coupling-efficiency for the excited fundamental mode： the boundary （А/λ）＊ is linear to the boundary （А/λ）. All of these will pave the way for smoothing applications of photonic-crystal fibres, such as splicing and designing photonic-crystal-fibre functional devices.     

Temporal filtering using time lenses for optical transmission systems

An optical signal processing scheme using time lenses in a 4-f configuration for optical communication systems is proposed. The first time-lens combined with a dispersive element such as an optical fiber produces the Fourier transform of the input signal and the second time lens combined with an optical fiber placed after the temporal filter produces the inverse Fourier transformation. Typically, in an optical signal processing scheme based on space/time-lens, the signal at the output is space/time-reversed because of the direct Fourier transformation after the spatial/temporal filter, which is undesirable for a practical optical communication system. Here, we propose a technique to implement both direct and inverse Fourier transformation using time lenses which has no spatial analogue. As a result, the bit sequence at the output is not time-reversed. Two applications of the proposed scheme, a demultiplexer for wavelength division multiplexing (WDM) systems and a higher-order dispersion compensator, have been discussed and numerically implemented.     

Research on the temperature characteristics of optical fiber refractive index

The delay of optical signal is determined by the refractive index and length of optical fiber, and temperature would have an intense influence on the index. To establish the relationship between refractive index and temperature, the temperature characteristics of refractive index was analyzed and the thermo-optical coefficient equation was derived according to the polarization of the induced electric dipole moment in SiO₂ optical fiber. A measuring system based on optical fiber delay was carried out to measure the index within the temperature range of −30 °C to 70 °C and the experimental result was compared with the theoretical result. The final result shows that the relationship between refractive index and temperature is linear in the temperature range of discussion.     

Optical pulse shaping at moderate power using a twisted-fibre NOLM with single output polarisation selection

We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8-10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.     

Generation of millimeter-wave in optical pulse carrier by using an apodized Moiré fiber grating

A novel scheme is proposed to transform a Gaussian pulse to a millimeter-wave frequency modulation pulse by using an apodized Moiré fiber Bragg grating in radio-over-fiber system. The relation between the input and output pulses is analyzed theoretically by Fourier transformation method and the requirements for the proposed fiber grating are presented. An apodized Moiré fiber Bragg grating is designed and its characteristics are studied. It is shown that the proposed device is feasible, and the new scheme is believed to be an effective solution for the generation of millimeter-wave sub-carrier in future radio-over-fiber systems.     

Determination of the optimal wavelength for temperature independent detection of commercial gasoline with a polymer cladding optical fiber

An influence of temperature variations on transmission of a polymer cladding silica core (PCS) optical fiber was investigated in a wide spectral range covering the first (1710 nm) and the second (1170 nm) vibrational overtone bands of gasoline absorption. Thermo-induced changes of the fiber background transmittance have strong dependence on wavelength. A narrow wavelength band around 1214 nm was found to be almost free from the thermal effects while maintaining sufficient sensitivity for gasoline detection.